2026-08-19 Item 12D BMP Design Manual Update — Encinitas City Council, August 19, 2026 (part 2 of 3)

1. Review Column 1 and identify which of these potential sources of stormwater pollutants apply to your site. Check each box that applies. 2. Review Column 2 and incorporate all of the corresponding applicable BMPs in your project site plan.

3. Review Columns 3 and 4 and incorporate all of the corresponding applicable permanent controls and operational BMPs in a table in your projectspecific stormwater management report. Describe your specific BMPs in an accompanying narrative, and explain any special conditions or situations that required omitting BMPs or substituting alternatives. 2026-08-19 Item #12D Page 276 of 816
Appendix E: BMP Design Fact Sheets
E-5 2023
If These Sources Will Be
on the Project Site … … Then Your SWQMP Shall Consider These Source Control BMPs 1 Potential Sources of
Runoff Pollutants
2 Permanent Controls—Show on
Drawings
3 Permanent Controls—List in Table
and Narrative
4 Operational BMPs—Include in Table
and Narrative
 SC-A. Onsite storm drain
inlets
 Not Applicable
 Locations of inlets.  Mark all inlets with the words “No Dumping! Flows to Bay” or similar.
 Maintain and periodically repaint or replace
inlet markings.
 Provide stormwater pollution prevention
information to new site owners, lessees, or operators.
 See applicable operational BMPs in Fact
Sheet SC-44, “Drainage System Maintenance,” in the CASQA Stormwater Quality Handbooks at www.cabmphandbooks.com.
 Include the following in lease agreements:
“Tenant shall not allow anyone to discharge anything to storm drains or to store or deposit materials so as to create a potential discharge to storm drains.”  SC-B. Interior floor
drains and elevator shaft sump pumps
 Not Applicable
  State that interior floor drains and
elevator shaft sump pumps will be plumbed to sanitary sewer.  Inspect and maintain drains to prevent
blockages and overflow.
 SC-C. Interior
parking garages
 Not Applicable
  State that parking garage floor drains will be plumbed to the sanitary sewer.  Inspect and maintain drains to prevent blockages and overflow. 2026-08-19 Item #12D Page 277 of 816
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 SC-D1. Need for future
indoor & structural pest control
 Not Applicable
  Note building design features that discourage entry of pests.  Provide Integrated Pest Management information to owners, lessees, and operators.

If These Sources Will Be
on the Project Site … … Then Your SWQMP Shall Consider These Source Control BMPs 1 Potential Sources of
Runoff Pollutants
2 Permanent Controls—Show on
Drawings
3 Permanent Controls—List in Table
and Narrative
4 Operational BMPs—Include in Table
and Narrative
 SC-D2. Landscape/
Outdoor Pesticide Use
 Not Applicable
 Show locations of existing trees or areas of
shrubs and ground cover to be undisturbed and retained.  Show self-retaining landscape areas, if any.  Show stormwater treatment facilities.  State that final landscape plans will
accomplish all of the following.
 Preserve existing drought tolerant
trees, shrubs, and ground cover to the maximum extent possible.  Design landscaping to minimize
irrigation and runoff, to promote surface infiltration where appropriate, and to minimize the use of fertilizers and pesticides that can contribute to stormwater pollution.  Where landscaped areas are used to
retain or detain stormwater, specify plants that are tolerant of periodic saturated soil conditions.  Consider using pest-resistant plants,
especially adjacent to hardscape.
 To ensure successful establishment,
select plants appropriate to site soils, slopes, climate, sun, wind, rain, land use, air movement, ecological consistency, and plant interactions.  Maintain landscaping using minimum or
no pesticides.
 See applicable operational BMPs in Fact
Sheet SC-41, “Building and Grounds Maintenance,” in the CASQA Stormwater Quality Handbooks at www.cabmphandbooks.com.
 Provide IPM information to new owners,
lessees and operators.
2026-08-19 Item #12D Page 278 of 816
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 SC-E. Pools, spas,
ponds, decorative fountains, and other
water features.
Not Applicable
Show location of water feature and a sanitary
sewer cleanout in an accessible area within 10 feet.
 If the local municipality requires pools
to be plumbed to the sanitary sewer, place a note on the plans and state in the narrative that this connection will be made according to local requirements.  See applicable operational BMPs in Fact
Sheet SC-72, “Fountain and Pool Maintenance,” in the CASQA Stormwater Quality Handbooks at www.cabmphandbooks.com.

Also show how the designated area will be
protected from wind dispersal.
 Any drains from dumpsters, compactors,
and tallow bin areas shall be connected to a grease removal device before discharge to sanitary sewer.  State how site refuse will be handled
and provide supporting detail to
what is shown on plans.
 State that signs will be posted on or near
dumpsters with the words “Do not dump hazardous materials here” or similar.  State how the following will be
implemented:
• Provide adequate number of
receptacles.
• Inspect receptacles regularly; repair or
replace leaky receptacles.
• Keep receptacles covered.
• Prohibit/prevent dumping of liquid
or hazardous wastes.
• Post “no hazardous materials” signs.

Inspect and pick up litter daily and clean up spills immediately. Keep spill control materials available on- site. See Fact Sheet SC-34, “Waste Handling and Disposal” in the CASQA Stormwater Quality Handbooks at www.cabmphandbooks.com.

If These Sources Will Be
on the Project Site … … Then Your SWQMP Shall Consider These Source Control BMPs 1 Potential Sources of
Runoff Pollutants
2 Permanent Controls—Show on
Drawings
3 Permanent Controls—List in
Table and Narrative
4 Operational BMPs—Include in
Table and Narrative
 SC-H. Industrial
processes.
 Not Applicable
 Show process area.  If industrial processes are to be located onsite, state: “All process activities to be performed indoors. No processes to drain to exterior or to storm drain system.”  See Fact Sheet SC-10, “Non- Stormwater
Discharges” in the CASQA Stormwater Quality Handbooks at www.cabmphandbooks.com.
2026-08-19 Item #12D Page 280 of 816
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 SC-I. Outdoor storage
of equipment or materials. (See rows J and K for source control measures for vehicle cleaning, repair, and maintenance.)  Not Applicable
 Show any outdoor storage areas, including
how materials will be covered. Show how areas will be graded and bermed to prevent run-on or runoff from area and protected from wind dispersal.  Storage of non-hazardous liquids shall be
covered by a roof and/or drain to the sanitary sewer system, and be contained by berms, dikes, liners, or vaults.  Storage of hazardous materials and wastes
must be in compliance with the local hazardous materials ordinance and a Hazardous Materials Management Plan for the site.  Include a detailed description of
materials to be stored, storage areas, and structural features to prevent pollutants from entering storm drains.

Where appropriate, reference
documentation of compliance with the requirements of local Hazardous Materials Programs for:
 Hazardous Waste Generation
 Hazardous Materials
Release Response and
Inventory
 California Accidental
Release Prevention
Program
 Aboveground Storage Tank  Uniform Fire Code
Article 80 Section
103(b) & (c) 1991
 Underground Storage Tank
 See the Fact Sheets SC-31, “Outdoor
Liquid Container Storage” and SC-33, “Outdoor Storage of Raw Materials” in the CASQA Stormwater Quality Handbooks at www.cabmphandbooks.com.

Describe operational measures to implement the following (if applicable):
 Washwater from vehicle and equipment
washing operations shall not be
discharged to the storm drain system.
 Car dealerships and similar may rinse cars
with water only.
 See Fact Sheet SC-21, “Vehicle and
Equipment Cleaning,” in the CASQA Stormwater Quality Handbooks at www.cabmphandbooks.com
If These Sources Will Be
on the Project Site … … Then Your SWQMP Shall Consider These Source Control BMPs 2026-08-19 Item #12D Page 282 of 816
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1 Potential Sources of
Runoff Pollutants
2 Permanent Controls—Show on
Drawings
3 Permanent Controls—List in Table
and Narrative
4 Operational BMPs—Include in Table
and Narrative
 SC-K. Vehicle/Equipment
Repair and Maintenance
 Not Applicable
 Accommodate all vehicle equipment repair
and maintenance indoors. Or designate an outdoor work area and design the area to protect from rainfall, run-on runoff, and wind dispersal.
 Show secondary containment for exterior
work areas where motor oil, brake fluid,
gasoline, diesel fuel, radiator fluid, acidcontaining batteries or other hazardous materials or hazardous wastes are used or stored. Drains shall not be installed within the secondary containment areas.  Add a note on the plans that states either (1)
there are no floor drains, or (2) floor drains are connected to wastewater pretreatment systems prior to discharge to the sanitary sewer and an industrial waste discharge permit will be obtained.  State that no vehicle repair or
maintenance will be done outdoors, or
else describe the required features of the outdoor work area.  State that there are no floor drains or if
there are floor drains, note the agency from which an industrial waste discharge permit will be obtained and that the design meets that agency’s requirements.  State that there are no tanks,
containers or sinks to be used for parts cleaning or rinsing or, if there are, note the agency from which an industrial
waste discharge permit will be
obtained and that the design meets that agency’s requirements.

In the report, note that all of the following restrictions apply to use the site:
 No person shall dispose of, nor permit the
disposal, directly or indirectly of vehicle fluids, hazardous materials, or rinsewater from parts cleaning into storm drains.  No vehicle fluid removal shall be
performed outside a building, nor on asphalt or ground surfaces, whether inside or outside a building, except in such a manner as to ensure that any spilled fluid will be in an area of secondary
containment. Leaking vehicle fluids shall be
contained or drained from the vehicle immediately.
 No person shall leave unattended drip parts
or other open containers containing vehicle fluid, unless such containers are in use or in an area of secondary containment.
2026-08-19 Item #12D Page 283 of 816
Appendix E: BMP Design Fact Sheets
E-12 2023
If These Sources Will Be
on the Project Site … … Then Your SWQMP Shall Consider These Source Control BMPs 1 Potential Sources of
Runoff Pollutants
2 Permanent Controls—Show on
Drawings
3 Potential Sources of Runoff
Pollutants
4 Permanent Controls—Show on
Drawings
 SC-L. Fuel Dispensing
Areas
 Not Applicable
 Fueling areas1 shall have impermeable
floors (i.e., portland cement concrete or equivalent smooth impervious surface) that are (1) graded at the minimum slope necessary to prevent ponding; and (2) separated from the rest of the site by a grade break that prevents run-on of stormwater to the MEP.  Fueling areas shall be covered by a canopy
that extends a minimum of ten feet in each direction from each pump. [Alternative:

The fueling area must be covered and the
cover’s minimum dimensions must be equal to or greater than the area within the grade break or fuel dispensing area1.] The canopy [or cover] shall not drain onto the fueling area.  The property owner shall dry sweep the
fueling area routinely.
 See the Business Guide Sheet,
“Automotive Service—Service Stations” in the CASQA Stormwater Quality Handbooks at www.cabmphandbooks.com.
 SC-M. Loading Docks  Not Applicable
 Show a preliminary design for the loading
dock area, including roofing and drainage. Loading docks shall be covered and/or graded to minimize run-on to and runoff from the loading area. Roof downspouts shall be positioned to direct stormwater away from the loading area. Water from loading dock areas should be drained to the sanitary sewer where feasible. Direct connections to storm drains from depressed loading docks are prohibited.  Loading dock areas draining directly to the
sanitary sewer shall be equipped with a spill control valve or equivalent device, which shall be kept closed during periods of operation.  Move loaded and unloaded items indoors
as soon as possible.
 See Fact Sheet SC-30, “Outdoor Loading
and Unloading,” in the CASQA Stormwater Quality Handbooks at www.cabmphandbooks.com.
2026-08-19 Item #12D Page 284 of 816
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 Provide a roof overhang over the loading area
or install door skirts (cowling) at each bay that enclose the end of the trailer.

If These Sources Will Be
on the Project Site … … Then Your SWQMP Shall Consider These Source Control BMPs 1 Potential Sources of
Runoff Pollutants
2 Permanent Controls—Show on
Drawings
3 Potential Sources of Runoff
Pollutants
4 Permanent Controls—Show on
Drawings
 SC-N. Fire Sprinkler
Test Water
 Not Applicable
 Provide a means to drain fire
sprinkler test water to the sanitary sewer.
 See the note in Fact Sheet SC- 41,
“Building and Grounds Maintenance,” in the CASQA Stormwater Quality Handbooks at www.cabmphandbooks.com.
2026-08-19 Item #12D Page 285 of 816
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SC-O. Miscellaneous Drain or
Wash Water
 Boiler drain lines  Condensate drain lines  Rooftop  equipment  Drainage sumps  Roofing, gutters, and trim
 Not Applicable
 Boiler drain lines shall be directly or
indirectly connected to the sanitary sewer system and may not discharge to the storm drain system.  Condensate drain lines may
discharge to landscaped areas if the flow is small enough that runoff will not occur. Condensate drain lines may not discharge to the storm drain system.  Rooftop mounted equipment with
potential to produce pollutants shall be roofed and/or have secondary containment.  Any drainage sumps onsite shall
feature a sediment sump to reduce the quantity of sediment in pumped water.
 Avoid roofing, gutters, and trim
made of copper or other unprotected metals that may leach into runoff.

If These Sources Will
Be on the Project Site
…
… Then Your SWQMP Shall Consider These Source Control BMPs 1 Potential Sources of
Runoff Pollutants
2 Permanent Controls—Show on
Drawings
3 Potential Sources of Runoff
Pollutants
4 Permanent Controls—Show on
Drawings
2026-08-19 Item #12D Page 286 of 816
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 SC-P Plazas, sidewalks,
and parking lots.
 Not Applicable
 Plazas, sidewalks, and parking lots shall be
swept regularly to prevent the accumulation of litter and debris.  Debris from pressure washing shall be
collected to prevent entry into the storm drain system. Washwater containing any cleaning agent or degreaser shall be collected and discharged to the sanitary sewer and not discharged to a storm drain.

1. The fueling area shall be defined as the area extending a minimum of 6.5 feet from the corner of each fuel dispenser or the length at which the hose and nozzle assembly may be operated plus a minimum of one foot, whichever is greater. 2026-08-19 Item #12D Page 287 of 816
Appendix E: BMP Design Fact Sheets
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E.3SC-Q Large Trash Generating Facilities
Description
Storm water runoff from areas where trash is stored or disposed of can be polluted. In addition, loose trash and debris can be easily transported by water or wind to nearby storm drain inlets, channels, and/or creeks. Trash generating facilities that generate large amounts of trash require special attention to protect trash storage areas from rainfall, run-on, runoff, and wind dispersal. Large trash generating, or trash build-up areas, include but are not limited to restaurants, supermarkets, “big box” retail stores serving food, and pet stores. The City Engineer may designate additional facilities if they are likely to generate or accumulate large quantities of trash.

Example isometric view and plan view of an allowable trash enclosure facility is presented below.

The project applicant may be allowed to use an alternative trash enclosure design that might be more appropriate for a project site if the alternative design is approved by the City Engineer.

MS4 Permit Category
Source Control
BMP Manual Category
Source Control
Applicable Performance
Standard
Source Control
Primary Benefits
Source Control
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Typical Isometric and Plan View of a Trash Enclosure BMP Design Adaptations for Project Goals
Source control BMPs reduce the amount of pollutants that are generated. This fact sheet contains details on the additional measures required to prevent or reduce pollutants in storm water runoff associated with trash storage and handling for large trash generating facilities. The requirements presented here are in addition to the requirements of SC-5 which requires all development projects to protect trash storage areas from rainfall, run-on, runoff, and wind dispersal:
2026-08-19 Item #12D Page 289 of 816
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• Areas where trash containers are stored must be enclosed on four sides to prevent offsite transport of trash. Four-sided trash enclosures typically consist of three walled sides and one gated side. Trash enclosures limit the potential for trash to pollute storm water runoff by limiting mobilization mechanisms (runoff, run-on, and wind dispersal). • Trash enclosures must be covered to minimize direct precipitation and prevent rainfall from entering enclosures. Structural overhead covers are required as container lids are often left open.
• Enclosures must be hydraulically isolated from surrounding areas. Slabs shall be sloped such that any leaked materials will be contained within the enclosure. Drains must be provided that capture and direct potential leaks to the sanitary sewer or appropriate BMPs. Divert runoff from surrounding areas away from the enclosure to prevent contamination and dispersion of collected materials.
• Owner must provide BMP storm water training to employees. Employee participation is required to ensure that enclosures are properly maintained and kept clean.

Design Criteria and Considerations
All trash shall be stored in weather-protected receptacles/bins and recyclable materials shall be protected against adverse weather conditions, which might render the collected materials unmarketable. Trash enclosure dimensions will vary based on projected usage and the following information is offered as an aid in planning new projects. Businesses that use dumpsters must design the enclosure to accommodate 3-yard containers at a minimum. The tenants may use any dumpster size that is appropriate for their needs, but the enclosure must be able to accommodate different tenants with varying waste production, including any recycling requirements. The design of the enclosure must be signed and sealed by a California licensed engineer. Substantiating structural calculations may be required. The location and design of the enclosure will require review and approval by the City Engineer. Building permits may be required.

The following recommendations for typical bin sizes are adopted from the City of Escondido trash enclosure guidelines. The following bin/container measurements are approximate (add 8” to width for side pockets):

Typical Trash Bin Sizes
Size Width Depth Height (front) Height (back)
3 cubic yard 72” bin, 81” plus lid 43” 42” 70”
4 cubic yard 72” bin, 81” plus lid 56” 72” 72”
Filled weight should not exceed 1,000 pounds.

1. Enclosures shall be structurally strong and constructed of reinforced masonry block or wood 2026-08-19 Item #12D Page 290 of 816
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panels/boards. Structural requirements for enclosures are detailed in the City of San Diego specifications for Wood and Masonry Fences.
http://www.sandiego.gov/development-services/pdf/industry/infobulletin/ib223.pdf

2. The enclosure should be constructed to the following minimum inside dimensions to accommodate three cubic-yard dumpsters (larger enclosures may be necessary to accommodate additional trash bins, recycling bins, and accessibility):

No. of Bins Loading Width Depth Height
One Front 8’ 6’ 6’
One Side 7.5’ 8’ 6’
Two Front 16’ 6’ 6’
Two Side 8’ 16’ 6’

3. The enclosure slab should be designed to keep storm water drainage out of the enclosure area, typically sloped at 0.5%. Slab construction specifications will vary according to methods of construction, but should be at least 4 inches of reinforced concrete.

4. Sturdy gates/doors shall be installed on all enclosures. Gates should not be mounted directly onto the block wall or inside of enclosure. The enclosure should include hardware to secure the gate’s doors both open and closed (i.e., cane bolt w/sleeve and latch between doors and sleeve in pavement).

5. To prevent trash enclosures from contributing to storm water runoff pollution, all enclosures must be fitted with a roof deigned to drain into on-site landscape areas (where necessary) and/or to appropriate BMPs. The roof must provide sufficient clearance to allow the dumpster lid to open to the 90 degree position.

6. Enclosure roofs not conforming to City specifications for Patio Covers may require a building permit. Generally, roofs not more than 12 feet in height above grade and constructed with conventional light-frame wood construction are considered acceptable. The use of metal roofs is not recommended as they can act as a source of pollutants. http://www.sandiego.gov/development-services/pdf/industry/infobulletin/ib206.pdf

7. Dumpsters associated with food establishments shall be sized per County Health Department requirements for wash down. Drains shall be connected to the business grease interceptor. 2026-08-19 Item #12D Page 291 of 816
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E.4SC-R Animal Facilities
Description
Animal facilities have an elevated potential for bacterial loading. If animal fecal material comes into contact with storm water, the storm water can become polluted. Animal facilities include but are not limited to animal shelters, dog daycare centers, veterinary clinics, groomers, pet care stores, and breeding, boarding, and training facilities. The City Engineer may designate additional facilities where animal fecal material is likely to be found.

Design Adaptations for Project Goals
Source control BMPs reduce the amount of pollutants that are generated. This fact sheet contains details on the additional measures required to prevent or reduce pollutants in storm water runoff associated with animal facilities. The requirements presented here are in addition to the source control requirements for all projects:
• Dry weather runoff must be controlled. Dry weather runoff from hosed off areas as part of animal facility operations must not drain to the MS4. Dry weather flows should be retained on-site through implementation of BMPs or collected and discharged to the sanitary sewer. • Outdoor activity areas must be identified on site plans. Plan reviewers must be able to ensure that runoff from these areas is either diverted to the sanitary sewer or directed to appropriate treatment BMPs. On-site inspection of facilities, grading, and drainage may be required.
• Trash enclosures within animal facilities must be covered to minimize direct precipitation and prevent rainfall from entering enclosures. Structural overhead covers are required as container lids are often left open.

MS4 Permit Category
Source Control
BMP Manual Category
Source Control
Applicable Performance
Standard
Source Control
Primary Benefits
Source Control
2026-08-19 Item #12D Page 292 of 816
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E.5SC-S Plant Nurseries and Garden Centers
Description
Storm water runoff from plant nurseries and garden centers has an elevated risk of being polluted by organics, nutrients, and/or pesticides. Nurseries and garden centers require special attention to protect against these elevated risks. Plant nurseries and garden centers include but are not limited to commercial facilities that grow, distribute, sell, or store plants and plant material. The City Engineer may designate additional facilities if they are likely to be a source of organics, nutrients or pesticides.

Design Adaptations for Project Goals
Source control BMPs reduce the amount of pollutants that are generated. This fact sheet contains details on the additional measures required to prevent or reduce pollutants in storm water runoff associated with plant nurseries or garden center facilities. The requirements presented here are in addition to the requirements of SC-1 through SC-5 which require all development projects to avoid and reduce pollutants in storm water runoff:
• Owner must provide BMP storm water training to appropriate employees. Employee participation is required to ensure that source controls are properly maintained and behavioral BMPs are followed.
• Eliminate overwatering and overspraying of plants. Overwatering and overspraying of plants increases dry weather flows and pollutant loading, and wastes water. Delivery systems and schedules should account for different plant types and containers. • Discharges from outdoor watering areas must be controlled. Regular runoff from outdoor watering can contribute un-authorized dry weather flows to the MS4 (e.g., runoff from watering the plants at garden centers). Runoff water is also likely to be polluted by potting soil mixes and plants that contain fertilizers and/or pesticides. So, regular runoff should be treated and/or retained on-site through BMPs or discharged to the sanitary sewer.

MS4 Permit Category
Source Control
BMP Manual Category
Source Control
Applicable Performance
Standard
Source Control
Primary Benefits
Source Control
2026-08-19 Item #12D Page 293 of 816
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E.6SC-T Automotive Facilities
Description
Storm water runoff from automotive facilities can pollute storm water runoff with oils and grease, metals, and other pollutants. Pollutants sources can include maintenance and repair activities, outside storage areas, liquid material storage, and others. Automotive facilities require additional measures because of the potential impact of pollutants. Automotive facilities include but are not limited to facilities that perform maintenance or repair of vehicles, vehicle washing facilities, and retail gasoline outlets. The City Engineer may designate additional facilities if they are likely sources of storm water pollutants.

Overhang should be measured from the perimeter of the hydraulically isolated activity area. • Hydraulically isolate activity areas. Activity areas should be protected from run-on that can mobilize pollutants and pollute uncontaminated storm water through the use of grading, berms, or drains. Direct drainage from the hydraulically isolated area to an approved sanitary sewer or a BMP.
• Pave activity areas with hydraulic concrete or appropriately sealed asphalt cement.

Unpaved activity areas could contaminate ground water. So all activity area, including area for MS4 Permit Category
Source Control
BMP Manual Category
Source Control
Applicable Performance
Standard
Source Control
Primary Benefits
Source Control
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fueling vehicles or equipment shall be paved with hydraulic concrete. If the area is already paved with asphalt, apply an asphalt sealant to the pavement surface. Maintain the paved surface to prevent gaps and cracks.
• Provide sedimentation manhole with outlet. Automotive facilities discharging to the sanitary sewer must follow standards set by the City Industrial Wastewater Control Program for the outlet design. See Appendix S: Sump/Clarifier Maintenance Standards found here for the outlet design:
o http://www.sandiego.gov/mwwd/environment/iwcp/other.shtml • Provide appropriate oil controls. All equipment and vehicle washing activity areas should include oil controls. On-site wash recycling systems may be used for oil control if they meet applicable effluent discharge limits for the sanitary sewer. • Identify auto-related usage areas on site plans and describe activities and drainage.

Plan checkers must be satisfied that grading and drainage will prevent contact between pollutants and storm water. Drains within the facilities must be connected to the sanitary sewer or a BMP. Verification may be required.
• Owner must provide BMP storm water training to employees. Employee participation is required to ensure that activity areas are properly maintained and kept clean. 2026-08-19 Item #12D Page 295 of 816
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E.7SD-A Tree Well
Tree Well (Source: County of San Diego LID Manual – EOA, Inc.) Description
Trees planted to intercept rainfall and runoff can be used as storm water management measures that provide additional benefits beyond those typically associated with trees, including energy conservation, air quality improvement, and aesthetic enhancement. Typical storm water management benefits associated with trees include:
• Interception of rainfall – tree surfaces (roots, foliage, bark, and branches) intercept, evaporate, store, or convey precipitation to the soil before it reaches surrounding impervious surfaces
• Reduced erosion – trees protect denuded area by intercepting or reducing the velocity of rain drops as they fall through the tree canopy
• Increased infiltration – soil conditions created by roots and fallen leaves promote infiltration • Treatment of storm water – trees provide treatment through uptake of nutrients and other storm water pollutants (phytoremediation) and support of other biological processes that break down pollutants
MS4 Permit Category
Site Design
Manual Category
Site Design
Applicable Performance
Standard
Site Design
Primary Benefits
Volume Reduction
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Typical tree well system components
include:
• Trees of the appropriate
species for site conditions
and constraints
• Available growing space
based on tree species, soil
type, water availability,
surrounding land uses, and
project goals
• Optional suspended
pavement design to provide
structural support for
adjacent pavement without
requiring compaction of
underlying layers
• Optional root barrier devices as needed; a root barrier is a device installed in the ground, between a tree and the sidewalk, intended to guide roots down and away from the sidewalk in order to prevent sidewalk lifting from tree roots. • Optional tree grates; to be considered to maximize available space for pedestrian circulation and to protect tree roots from compaction related to pedestrian circulation; tree grates are typically made up of porous material that will allow the runoff to soak through.
• Optional shallow surface depression for ponding of excess runoff • Optional planter box drain
Design Adaptations for Project Goals
Site design BMP to provide incidental treatment. Tree wells primarily function as site design BMPs for incidental treatment. Benefits from tree wells are accounted for by adjustment factors presented in Appendix B.2. This credit can apply to other landscaping trees as well (that meet the same criteria).

Storm water pollutant control BMP to provide treatment. Applicants are allowed to design trees as a pollutant control BMP. For this option to be approved by the City Engineer, applicant is required to do infiltration feasibility screening (Appendix C and D) and provide calculations supporting the amount of credit claimed from implementing trees within the project footprint. The City Engineer has the discretion to request additional analysis before approving credits.

Schematic of Tree Well 1
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Design Criteria and Considerations
Tree wells must meet the following design criteria and considerations. Deviations from the below criteria may be approved at the discretion of the City Engineer if it is determined to be appropriate:

Siting and Design Intent/Rationale
□
Tree species is appropriately chosen for the
development (private or public). For public
rights-of-ways, local planning guidelines and
zoning provisions for the permissible species
and placement of trees are consulted. A list of
trees appropriate for site design that can be
used by all county municipalities are provided
in Appendix E.26
Proper tree placement and species
selection minimizes problems such as
pavement damage by surface roots and
poor growth.
□
Location of trees planted along public streets
follows local requirements and guidelines.

Vehicle and pedestrian line of sight are
considered in tree selection and placement.

Unless exemption is granted by the City
Engineer the following minimum tree
separation distance is followed
Improvement
Minimum
distance to
Tree Well
Traffic Signal, Stop sign 20 feet
Underground Utility lines
(except sewer) 5 feet
Sewer Lines 10 feet
Above ground utility structures
(Transformers, Hydrants,
Utility poles, etc.)
10 feet
Driveways 10 feet
Intersections (intersecting curb
lines of two streets) 25 feet
Roadway safety for both vehicular and
pedestrian traffic is a key consideration
for placement along public streets.
□
Underground utilities and overhead wires
are considered in the design and avoided or
circumvented. Underground utilities are routed
around or through the planter in suspended
pavement applications. All underground
Tree growth can damage utilities and
overhead wires resulting in service
interruptions. Protecting utilities routed
through the planter prevents damage and
service interruptions.
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Siting and Design Intent/Rationale
utilities are protected from water and root
penetration.
□
Suspended pavement design was developed
where appropriate to minimize soil compaction
and improve infiltration and filtration
capabilities.

Suspended pavement was constructed with an
approved structural cell.

Suspended pavement designs provide
structural support without compaction
of the underlying layers, thereby
promoting tree growth.

Recommended structural cells include
poured in place concrete columns, Silva
Cells manufactured by Deeproot Green
Infrastructures and Stratacell and
Stratavault systems manufactured by
Citygreen Systems.
□
A minimum soil volume of 2 cubic feet per
square foot of canopy projection volume is
provided for each tree. Canopy projection area
is the ground area beneath the tree, measured
at the drip line.

The minimum soil volume ensures that
there is adequate storage volume to
allow for unrestricted
evapotranspiration.

A lower amount of soil volume may be
allowed at the discretion of the City
Engineer if certified by a landscape
architect or agronomist. The retention
credit from the tree is directly
proportional to the soil volume provided
for the tree.
□
DCV from the tributary area draining to the tree
is equal to or greater than the tree credit volume
The minimum tributary area ensures that
the tree receives enough runoff to fully
utilize the infiltration and
evapotranspiration potential provided. In
cases where the minimum tributary area is
not provided, the tree credit volume must
be reduced proportionately to the actual
tributary area.
□
Inlet opening to the tree that is at least 18
inches wide.

A minimum 2 inch drop in grade from the inlet
to the finish grade of the tree.

Design requirement to ensure that the
runoff from the tributary area is not
bypassed.

Different inlet openings and drops in
grade may be allowed at the discretion of
the City Engineer if calculations are
shown that the diversion flow rate
(Appendix B.1.2) from the tributary area
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Siting and Design Intent/Rationale
Grated inlets are allowed for pedestrian
circulation. Grates need to be ADA compliant
and have sufficient slip resistance.
can be conveyed to the tree. In cases
where the inlet capacity is limiting the
amount of runoff draining to the tree, the
tree credit volume must be reduced
proportionately.

Conceptual Design and Sizing Approach for Site Design

1. Determine the areas where tree wells can be used in the site design to achieve incidental treatment. Tree wells reduce runoff volumes from the site. Refer to Appendix B.2. Document the proposed tree locations in the SWQMP.

2. When trees are proposed as a storm water pollutant control BMP, applicant must complete feasibility analysis in Appendix C and D and submit detailed calculations for the DCV treated by trees. Document the proposed tree locations, feasibility analysis and sizing calculations in the SWQMP. The following calculations should be performed and the smallest of the three should be used as the volume treated by trees:
a. Delineate the DMA (tributary area) to the tree and calculate the associated DCV. b. Calculate the required diversion flow rate using Appendix B.1.2 and size the inlet required to covey this flow rate to the tree. If the proposed inlet cannot convey the diversion flow rate for the entire tributary area, then the DCV that enters the tree should be proportionally reduced.
i. For example, 0.5 acre drains to the tree and the associated DCV is 820 ft3. The required diversion flow rate is 0.10 ft3/s, but only an inlet that can divert 0.05 ft3/s could be installed.
ii. Then the effective DCV draining to the tree = 820 ft3 * (0.05/0.10) = 420 ft3 c. Estimate the amount of storm water treated by the tree by summing the following:
i. Evapotranspiration credit of 0.1 * amount of soil volume installed; and ii. Infiltration credit calculated using sizing procedures in Appendix B.4.

Maintenance Overview
Normal Expected Maintenance. Tree health shall be maintained as part of normal landscape maintenance. Additionally, ensure that storm water runoff can be conveyed into the tree well as designed. That is, the opening that allows storm water runoff to flow into the tree well (e.g., a curb opening, tree grate, or surface depression) shall not be blocked, filled, re-graded, or otherwise changed in a manner that prevents storm water from draining into the tree well. A summary table of standard inspection and maintenance indicators is provided within this Fact Sheet. 2026-08-19 Item #12D Page 300 of 816
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Non-Standard Maintenance or BMP Failure. Trees wells are site design BMPs that normally do not require maintenance actions beyond routine landscape maintenance. The normal expected maintenance described above ensures the BMP functionality. If changes have been made to the tree well entrance / opening such that runoff is prevented from draining into the tree well (e.g., a curb inlet opening is blocked by debris or a grate is clogged causing runoff to flow around instead of into the tree well, or a surface depression has been filled so runoff flows away from the tree well), the BMP is not performing as intended to protect downstream waterways from pollution and/or erosion.

Corrective maintenance will be required to restore drainage into the tree well as designed.

Surface ponding of runoff directed into tree wells is expected to infiltrate/evapotranspire within 24- 96 hours following a storm event. Surface ponding longer than approximately 24 hours following a storm event may be detrimental to vegetation health, and surface ponding longer than approximately 96 hours following a storm event poses a risk of vector (mosquito) breeding. Poor drainage can result from clogging or compaction of the soils surrounding the tree. Loosen or replace the soils to restore drainage.

Other Special Considerations. Site design BMPs, such as tree wells, installed within a new development or redevelopment project are components of an overall storm water management strategy for the project. The presence of site design BMPs within a project is usually a factor in the determination of the amount of runoff to be managed with structural BMPs (i.e., the amount of runoff expected to reach downstream retention or biofiltration basins that process storm water runoff from the project as a whole). When site design BMPs are not maintained or are removed, this can lead to clogging or failure of downstream structural BMPs due to greater delivery of runoff and pollutants than intended for the structural BMP. Therefore, the City Engineer may require confirmation of maintenance of site design BMPs as part of their structural BMP maintenance documentation requirements. Site design BMPs that have been installed as part of the project should not be removed, nor should they be bypassed by re-routing roof drains or re-grading surfaces within the project. If changes are necessary, consult the City Engineer to determine requirements. 2026-08-19 Item #12D Page 301 of 816
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Summary of Standard Inspection and Maintenance
The property owner is responsible to ensure inspection, operation and maintenance of permanent BMPs on their property unless responsibility has been formally transferred to an agency, community facilities district, homeowners association, property owners association, or other special district. Maintenance frequencies listed in this table are average/typical frequencies. Actual maintenance needs are site-specific, and maintenance may be required more frequently. Maintenance must be performed whenever needed, based on maintenance indicators presented in this table.

The BMP owner is responsible for conducting regular inspections to see when maintenance is needed based on the maintenance indicators.

Threshold/Indicator Maintenance Action Typical Maintenance Frequency Tree health Routine actions as necessary to maintain
tree health.
• Inspect monthly.
• Maintain when needed.

Dead or diseased tree Remove dead or diseased tree. Replace per original plans.
• Inspect monthly.
• Maintain when needed.

Standing water in tree well for longer than 24 hours following a storm event Surface ponding longer than approximately 24 hours following a storm event may be detrimental to tree health Loosen or replace soils surrounding the tree to restore drainage. • Inspect monthly and after every 0.5-inch
or larger storm event. If standing water is observed, increase inspection frequency to after every 0.1-inch or larger storm event. • Maintain when needed.

Presence of mosquitos/larvae For images of egg rafts, larva, pupa, and adult mosquitos, see http://www.mosquito.org/biology Disperse any standing water from the tree
well to nearby landscaping. Loosen or
replace soils surrounding the tree to restore drainage (and prevent standing water). • Inspect monthly and after every 0.5-inch
or larger storm event. If mosquitos are observed, increase inspection frequency to after every 0.1-inch or larger storm event. • Maintain when needed
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Threshold/Indicator Maintenance Action Typical Maintenance Frequency Entrance / opening to the tree well is blocked such that storm water will not drain into the tree well (e.g., a curb inlet opening is blocked by debris or a grate is clogged causing runoff to flow around instead of into the tree well; or a surface depression is filled such that runoff drains away from the tree well) Make repairs as appropriate to restore drainage into the tree well. • Inspect monthly.
• Maintain when needed.
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E.8SD-B Impervious Area Dispersion
Photo Credit: Orange County Technical Guidance Document Description
Impervious area dispersion (dispersion) refers to the practice of effectively disconnecting impervious areas from directly draining to the storm drain system by routing runoff from impervious areas such as rooftops (through downspout disconnection), walkways, and driveways onto the surface of adjacent pervious areas. The intent is to slow runoff discharges, and reduce volumes. Dispersion with partial or full infiltration results in significant volume reduction by means of infiltration and evapotranspiration.

Typical dispersion components include:
• An impervious surface from which runoff flows will be routed with minimal piping to limit concentrated inflows
• Splash blocks, flow spreaders, or other means of dispersing concentrated flows and providing energy dissipation as needed
• Dedicated pervious area, typically vegetated, with in-situ soil infiltration capacity for partial or full infiltration
• Optional soil amendments to improve vegetation support, maintain infiltration rates and enhance treatment of routed flows
• Overflow route for excess flows to be conveyed from dispersion area to the storm drain system or discharge point
MS4 Permit Category
Site Design
Manual Category
Site Design
Applicable Performance
Criteria
Site Design
Primary Benefits
Volume Reduction
Peak Flow Attenuation
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Typical plan and section view of an Impervious Area Dispersion BMP 2026-08-19 Item #12D Page 305 of 816
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Design Adaptations for Project Goals
Site design BMP to reduce impervious area and DCV. Impervious area dispersion primarily functions as a site design BMP for reducing the effective imperviousness of a site by providing partial or full infiltration of the flows that are routed to pervious dispersion areas and otherwise slowing down excess flows that eventually reach the storm drain system. This can significantly reduce the DCV for the site.

Design Criteria and Considerations
Dispersion must meet the following design criteria. Deviations from the below criteria may be approved at the discretion of the City Engineer if it is determined to be appropriate:

Siting and Design Intent/Rationale
□
Dispersion is over areas with soil types capable
of supporting or being amended (e.g., with
sand or compost) to support vegetation. Media
amendments must be tested to verify that they
are not a source of pollutants.

Soil must have long-term infiltration
capacity for partial or full infiltration and
be able to support vegetation to provide
runoff treatment. Amendments to
improve plant growth must not have
negative impact on water quality.
□
Dispersion has vegetated sheet flow over a
relatively large distance (minimum 10 feet)
from inflow to overflow route.

Full or partial infiltration requires
relatively large areas to be effective
depending on the permeability of the
underlying soils.
□
Pervious areas should be flat (with less than
5% slopes) and vegetated.

Flat slopes facilitate sheet flows and
minimize velocities, thereby improving
treatment and reducing the likelihood of
erosion.

Inflow velocities
□
Inflow velocities are limited to 3 ft/s or less or
use energy dissipation methods (e.g., riprap,
level spreader) for concentrated inflows.

High inflow velocities can cause erosion,
scour and/or channeling.

Dedication
□
Dispersion areas must be owned by the project
owner and be dedicated for the purposes of
dispersion to the exclusion of other future uses
that might reduce the effectiveness of the
dispersion area.

Dedicated dispersion areas prevent
future conversion to alternate uses and
facilitate continued full and partial
infiltration benefits.

Vegetation
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Siting and Design Intent/Rationale
□
Dispersion typically requires dense and robust
vegetation for proper function. Drought
tolerant species should be selected to minimize
irrigation needs. A plant list to aid in selection
can be found in Appendix E.26.

Vegetation improves resistance to
erosion and aids in runoff treatment.

Conceptual Design and Sizing Approach for Site Design

1. Determine the areas where dispersion can be used in the site design to reduce the DCV for pollutant control sizing.

2. Calculate the DCV for storm water pollutant control per Appendix B.2, taking into account reduced runoff from dispersion.

3. Determine if a DMA is considered “Self-retaining” if the impervious to pervious ratio is:
a. 2:1 when the pervious area is composed of Hydrologic Soil Group A b. 1:1 when the pervious area is composed of Hydrologic Soil Group B Conceptional Design and Sizing Approach for Storm Water Pollutant Treatment and Flow Control DMAs using impervious area dispersion are considered to meet both pollutant control and hydromodification flow control requirements if ALL of the following criteria are met:

1. All impervious area within the DMA discharges to the pervious area before the runoff discharges from the DMA.

2. As a minimum, the top 11 inches of the pervious area uses amended soils in accordance with the SD-F fact sheet and the pervious area also meets the requirements for dispersion (e.g. slope, inflow velocities, etc.) in the SD-B fact sheet.

3. The impervious to pervious area ratio is 1:1 or less.

4. Maintenance Overview

5. Normal Expected Maintenance. Vegetated area shall be maintained as part of normal landscape maintenance. Additionally, ensure that storm water runoff can be conveyed into the vegetated area as designed. That is, the mechanism that allows storm water runoff from impervious area to flow into the pervious area (e.g., a curb cut allows runoff from a parking lot to drain onto adjacent landscaping area, or a roof drain outlet is directed to a lawn) shall not be removed, blocked, filled, or otherwise changed in a manner that prevents storm water from draining into the pervious area. A summary table of standard inspection and maintenance indicators is provided within this Fact Sheet.

6. Non-Standard Maintenance or BMP Failure. Impervious area dispersion is a site design BMP that normally does not require maintenance actions beyond routine landscape 2026-08-19 Item #12D Page 307 of 816
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maintenance. If changes have been made to the area, such as the vegetated area has been replaced with impervious area, or the mechanism that allows storm water runoff from impervious area to flow into the pervious area has been removed (e.g., roof drains previously directed to vegetated area have been directly connected to the street or storm drain system), the BMP is not performing as intended to protect downstream waterways from pollution and/or erosion. Corrective maintenance will be required to restore drainage into the pervious area as designed. If the pervious area has been removed, contact the City Engineer to determine a solution.

7. 8. Runoff directed into vegetated areas is expected to be drained within 24-96 hours following a storm event. Surface ponding longer than approximately 24 hours following a storm event may be detrimental to vegetation health, and surface ponding longer than approximately 96 hours following a storm event poses a risk of vector (mosquito) breeding. Poor drainage can result from clogging or compaction of the soils. Loosen or replace the soils to restore drainage.

Other Special Considerations. Site design BMPs, such as impervious area dispersion, installed within a new development or redevelopment project are components of an overall storm water management strategy for the project. The presence of site design BMPs within a project is usually a factor in the determination of the amount of runoff to be managed with structural BMPs (i.e., the amount of runoff expected to reach downstream retention or biofiltration basins that process storm water runoff from the project as a whole). When site design BMPs are not maintained or are removed, this can lead to clogging or failure of downstream structural BMPs due to greater delivery of runoff and pollutants than intended for the structural BMP. Therefore, the City Engineer may require confirmation of maintenance of site design BMPs as part of their structural BMP maintenance documentation requirements. Site design BMPs that have been installed as part of the project should not be removed, nor should they be bypassed by re-routing roof drains or re-grading surfaces within the project. If changes are necessary, consult the City Engineer to determine requirements. 2026-08-19 Item #12D Page 308 of 816
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Summary of Standard Inspection and Maintenance
The property owner is responsible to ensure inspection, operation and maintenance of permanent BMPs on their property unless responsibility has been formally transferred to an agency, community facilities district, homeowners association, property owners association, or other special district. Maintenance frequencies listed in this table are average/typical frequencies. Actual maintenance needs are site-specific, and maintenance may be required more frequently. Maintenance must be performed whenever needed, based on maintenance indicators presented in this table.

The BMP owner is responsible for conducting regular inspections to see when maintenance is needed based on the maintenance indicators.

Threshold/Indicator Maintenance Action Typical Maintenance Frequency Poor vegetation establishment Re-seed, re-plant, or re-establish vegetation per original plans.
• Inspect monthly.
• Maintain when needed.

Dead or diseased vegetation Remove dead or diseased vegetation, reseed, re-plant, or re-establish vegetation per original plans.
• Inspect monthly.
• Maintain when needed.

Overgrown vegetation Mow or trim as appropriate. • Inspect monthly. • Maintain when needed.

Standing water in vegetated pervious area for longer than 24 hours following a storm event Surface ponding longer than approximately 24 hours following a storm event may be detrimental to vegetation health Disperse any areas of standing water to nearby landscaping (i.e., spread it out to another portion of the pervious area so it drains into the soil). Make appropriate corrective measures such as adjusting irrigation system, or repairing/replacing clogged or compacted soils. • Inspect monthly and after every 0.5-inch
or larger storm event. If standing water is observed, increase inspection frequency to after every 0.1-inch or larger storm event. • Maintain when needed.
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Presence of mosquitos/larvae For images of egg rafts, larva, pupa, and adult mosquitos, see http://www.mosquito.org/biology Disperse any areas of standing water to nearby landscaping (i.e., spread it out to another portion of the pervious area so it drains into the soil). Loosen or replace soils to restore drainage (and prevent standing water)
• Inspect monthly and after every 0.5-inch
or larger storm event. If mosquitos are observed, increase inspection frequency to after every 0.1-inch or larger storm event. • Maintain when needed
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Threshold/Indicator Maintenance Action Typical Maintenance Frequency Entrance / opening to the vegetated pervious area is blocked such that storm water from impervious area will not drain
into the pervious area (e.g., a curb cut opening is blocked by debris or a roof drain outlet has been directly connected to the storm drain system) Make repairs as appropriate to restore drainage into the vegetated pervious area. • Inspect monthly.
• Maintain when needed.
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E.9SD-C Green Roofs
Location: County of San Diego Operations Center, San Diego, California Description
Green roofs are vegetated rooftop systems that reduce runoff volumes and rates, treat storm water pollutants through filtration and plant uptake, provide additional landscape amenity, and create wildlife habitat. Additionally, green roofs reduce the heat island effect and provide acoustical control, air filtration and oxygen production. In terms of building design, they can protect against ultraviolet rays and extend the roof lifetime, as well as increase the building insulation, thereby decreasing heating and cooling costs. There are two primary types of green roofs:
• Extensive – lightweight, low maintenance system with low-profile, drought tolerant type groundcover in shallow growing medium (6 inches or less) • Intensive – heavyweight, high maintenance system with a more garden-like configuration and diverse plantings that may include shrubs or trees in a thicker growing medium (greater than 6 inches)
Typical green roof components include, from top to bottom:
• Vegetation that is appropriate to the type of green roof system, climate, and watering conditions
• Media layer (planting mix or engineered media) capable of supporting vegetation growth • Filter fabric to prevent migration of fines (soils) into the drainage layer MS4 Permit Category
Site Design
Manual Category
Site Design
Applicable Performance
Standard
Site Design
Primary Benefits
Volume Reduction
Peak Flow Attenuation
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• Optional drainage layer to convey excess runoff
• Optional root barrier
• Optional insulation layer
• Waterproof membrane
• Structural roof support capable of withstanding the additional weight of a green roof Typical profile of a Green Roof BMP
Design Adaptations for Project Goals
Site design BMP to provide incidental treatment. Green roofs can be used as a site design feature to reduce the runoff generated from the site through replacing conventional roofing. This can reduce the DCV and flow control requirements for the site.

Design Criteria and Considerations
Green roofs must meet the following design criteria. Deviations from the below criteria may be approved at the discretion of the City Engineer if it is determined to be appropriate:
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Siting and Design Intent/Rationale
□
Roof slope is ≤ 40% (Roofs that are ≤
20% are preferred).

Steep roof slopes increases project complexity
and requires supplemental anchoring.
□
Structural roof capacity design supports
the calculated additional load (lbs/sq. ft)
of the vegetation growing medium and
additional drainage and barrier layers.

Inadequate structural capacity increases the risk
for roof failure and harm to the building and
occupants.
□
Design and construction is planned to be
completed by an experienced green roof
specialist.

A green roof specialist will minimize
complications in implementation and potential
structural issues that are critical to green roof
success.
□
Green roof location and extent must
meet fire safety provisions.

Green roof design must not negatively impact
fire safety.
□
Maintenance access is included in the
green roof design.

Maintenance will facilitate proper functioning
of drainage and irrigation components and
allow for removal of undesirable vegetation
and soil testing, as needed.

Vegetation
□
Vegetation is suitable for the green roof
type, climate and expected watering
conditions. Perennial, self-sowing plants
that are drought-tolerant (e.g., sedums,
succulents) and require little to no
fertilizer, pesticides or herbicides are
recommended. Vegetation pre-grown at
grade may allow plants to establish prior
to facing harsh roof conditions.

Plants suited to the design and expected
growing environment are more likely to
survive.
□
Vegetation is capable of covering ≥ 90%
the roof surface.

Benefits of green roofs are greater with more
surface vegetation.
□
Vegetation is robust and erosion-resistant
in order to withstand the anticipated
rooftop environment (e.g., heat, cold,
high winds).

Weak plants will not survive in extreme
rooftop environments.
□ Vegetation is fire resistant.

Vegetation that will not burn easily decreases
the chance for fire and harm to the building
and occupants.
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Siting and Design Intent/Rationale
□
Vegetation considers roof sun exposure
and shaded areas based on roof slope and
location.

The amount of sunlight the vegetation receives
can inhibit growth therefore the beneficial
effects of a vegetated roof.
□
An irrigation system (e.g., drip irrigation
system) is included as necessary to
maintain vegetation.

Proper watering will increase plant survival,
especially for new plantings.
□
Media is well-drained and is the
appropriate depth required for the green
roof type and vegetation supported.

Unnecessary water retention increases
structural loading. An adequate media depth
increases plant survival.
□
A filter fabric is used to prevent
migration of media fines through the
system.

Migration of media can cause clogging of the
drainage layer.
□
A drainage layer is provided if needed to
convey runoff safely from the roof. The
drainage layer can be comprised of gravel,
perforated sheeting, or other drainage
materials.

Inadequate drainage increases structural
loading and the risk of harm to the building
and occupants.
□
A root barrier comprised of dense
material to inhibit root penetration is
used if the waterproof membrane will not
provide root penetration protection.

Root penetration can decrease the integrity of
the underlying structural roof components and
increase the risk of harm to the building and
occupants.
□
An insulation layer is included as needed
to protect against the water in the
drainage layer from extracting building
heat in the winter and cool air in the
summer.

Regulating thermal impacts of green roofs will
aid in controlling building heating and cooling
costs.
□
A waterproof membrane is used to
prevent the roof runoff from vertically
migrating and damaging the roofing
material. A root barrier may be required
to prevent roots from compromising the
integrity of the membrane.

Water-damaged roof materials increase the risk
of harm to the building and occupants.

Conceptual Design and Sizing Approach for Site Design

1. Determine the areas where green roofs can be used in the site design to replace conventional roofing to reduce the DCV. These green roof areas can be credited toward reducing runoff generated through representation in storm water calculations as pervious, not impervious, areas but are not credited for storm water pollutant control.

2. Calculate the DCV per Appendix B.2.
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Maintenance Overview
Normal Expected Maintenance. A green roof requires routine maintenance to: maintain vegetation health; and maintain integrity of the roof drainage system. A summary table of standard inspection and maintenance indicators is provided within this Fact Sheet.

Non-Standard Maintenance or BMP Failure. Green roofs are site design BMPs that normally do not require maintenance actions beyond the normal maintenance described above. If a roof leak is discovered, it may be an indicator that the waterproof membrane has failed. The waterproof membrane (roof liner) shall be inspected and repaired or replaced as necessary.

Green roof systems normally receive only direct rainfall (not runoff from additional tributary area directed into the system). It is expected to be drained within 24-96 hours following a storm event.

Surface ponding longer than approximately 24 hours following a storm event may be detrimental to vegetation health, and surface ponding longer than approximately 96 hours following a storm event poses a risk of vector (mosquito) breeding, as well as risk of damage to the roof. Poor drainage can result from clogging or compaction of the media, optional drainage layer, or drainage system. The specific cause of the drainage issue must be determined and corrected.

Other Special Considerations. Site design BMPs, such as green roofs, installed within a new development or redevelopment project are components of an overall storm water management strategy for the project. The presence of site design BMPs within a project is usually a factor in the determination of the amount of runoff to be managed with structural BMPs (i.e., the amount of runoff expected to reach downstream retention or biofiltration basins that process storm water runoff from the project as a whole). When site design BMPs are not maintained or are removed, this can lead to clogging or failure of downstream structural BMPs due to greater delivery of runoff and pollutants than intended for the structural BMP. Therefore, the [City Engineer] may require confirmation of maintenance of site design BMPs as part of their structural BMP maintenance documentation requirements. Site design BMPs that have been installed as part of the project should not be removed, nor should they be bypassed by re-routing roof drains or re-grading surfaces within the project. If changes are necessary, consult the [City Engineer] to determine requirements. 2026-08-19 Item #12D Page 316 of 816
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Summary of Standard Inspection and Maintenance
The property owner is responsible to ensure inspection, operation and maintenance of permanent BMPs on their property unless responsibility has been formally transferred to an agency, community facilities district, homeowners association, property owners association, or other special district. Maintenance frequencies listed in this table are average/typical frequencies. Actual maintenance needs are site-specific, and maintenance may be required more frequently. Maintenance must be performed whenever needed, based on maintenance indicators presented in this table.

The BMP owner is responsible for conducting regular inspections to see when maintenance is needed based on the maintenance indicators.

Dead or diseased vegetation Remove dead or diseased vegetation, reseed, re-plant, or re-establish vegetation per original plans.
• Inspect monthly.
• Maintain when needed.

Overgrown vegetation Mow or trim as appropriate. • Inspect monthly. • Maintain when needed.
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Threshold/Indicator Maintenance Action Typical Maintenance Frequency Standing water in BMP for longer than 24 hours following a storm event Surface ponding longer than approximately 24 hours following a storm event may be detrimental to vegetation health Disperse any areas of standing water to nearby landscaping (i.e., spread it out to another portion of the green roof so it drains into the soil). Make appropriate corrective measures such as adjusting irrigation system, clearing underdrains, or repairing/replacing clogged or compacted soils. • Inspect monthly and after every 0.5-inch
or larger storm event. If standing water is observed, increase inspection frequency to after every 0.1-inch or larger storm event. • Maintain when needed.

Presence of mosquitos/larvae For images of egg rafts, larva, pupa, and adult mosquitos, see http://www.mosquito.org/biology Disperse any areas of standing water to nearby landscaping (i.e., spread it out to another portion of the green roof so it drains into the soil). Loosen or replace soils to restore drainage (and prevent standing water).
• Inspect monthly and after every 0.5-inch
or larger storm event. If mosquitos are observed, increase inspection frequency to after every 0.1-inch or larger storm event. • Maintain when needed
Leaks or other damage to waterproof membrane
Repair or replace as applicable. • Inspect membrane if leak is observed. • Maintain when needed.
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E.10 SD-D Permeable Pavement (Site Design
BMP)
Description
Permeable pavement is pavement that allows for
percolation through void spaces in the pavement
surface into subsurface layers. Permeable pavements
reduce runoff volumes and rates and can provide
pollutant control via infiltration, filtration, sorption, sedimentation, and biodegradation processes. When
used as a site design BMP, the subsurface layers are
designed to provide storage of storm water runoff so
that outflow rates can be controlled via infiltration into subgrade soils. Varying levels of storm water treatment and flow control can be provided depending on the size of the permeable pavement system relative to its drainage area and the underlying infiltration rates. As a site design BMP permeable pavement areas are designed to be self-retaining and are designed primarily for direct rainfall. Self-retaining permeable pavement areas have a ratio of total drainage area (including permeable pavement) to area of permeable pavement of 1.5:1 or less.

Permeable pavement surfaces can be constructed from modular paver units or paver blocks, pervious concrete, porous asphalt, and turf pavers. Sites designed with permeable pavements can significantly reduce the impervious area of the project. Reduction in impervious surfaces decreases the DCV and can reduce the footprint of treatment control and flow control BMPs.

MS4 Permit Category
Site Design
Manual Category
Site Design
Applicable Performance Standard
Site Design
Primary Benefits
Photo Credit: San Diego Low Impact Development Design Manual Typical Permeable Pavement
Components (Top to Bottom)
Permeable surface layer
Bedding layer for permeable surface
Aggregate storage layer with optional
underdrain(s)
Optional final filter course layer over
uncompacted existing subgrade
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Design Adaptations for Project Goals
Site design BMP to reduce impervious area and DCV. Permeable pavement without an underdrain can be used as a site design feature to reduce the impervious area of the site by replacing traditional pavements, including roadways, parking lots, emergency access lanes, sidewalks, trails and driveways.

Conceptual Design and Sizing Approach for Site Design

1. Determine the areas where permeable pavements can be used in the site design to replace conventional pavements to reduce the DCV. These areas can be credited toward reducing runoff generated through representation in storm water calculations as pervious, not impervious, areas but are not credited for storm water pollutant control.

2. Calculate the DCV per Appendix B.2, taking into account reduced runoff from permeable pavement areas.

Maintenance Overview
Normal Expected Maintenance. Routine maintenance of permeable pavement includes: removal of materials such as trash and debris accumulated on the paving surface; vacuuming of the paving surface to prevent clogging; and flushing paving and subsurface gravel to remove fine sediment. If the BMP includes underdrains, check and clear underdrains. A summary table of standard inspection and maintenance indicators is provided within this Fact Sheet.

Non-Standard Maintenance or BMP Failure. If the permeable pavement area is not drained between storm events, or if runoff sheet flows across the permeable pavement area and flows off the permeable pavement area during storm events, the BMP is not performing as intended to protect downstream waterways from pollution and/or erosion. During storm events up to the 85th percentile storm event (approximately 0.5 to 1 inch of rainfall in San Diego County), runoff should not flow off the permeable pavement area. The permeable pavement area is expected to have adequate hydraulic conductivity and storage such that rainfall landing on the permeable pavement and runoff from the surrounding drainage area will go directly into the pavement without ponding or overflow (in properly designed systems, the surrounding drainage area is not more than half as large as the permeable pavement area). Following the storm event, there should be no standing water (puddles) on the permeable pavement area.

If storm water is flowing off the permeable pavement during a storm event, or if there is standing water on the permeable pavement surface following a storm event, this is an indicator of clogging somewhere within the system. Poor drainage can result from clogging of the permeable surface layer, any of the subsurface components, or the subgrade soils. The specific cause of the drainage issue must be determined and corrected. Surface or subsurface ponding longer than approximately 96 hours following a storm event poses a risk of vector (mosquito) breeding. Corrective maintenance, increased inspection and maintenance, BMP replacement, or a different BMP type will be required. If poor drainage persists after flushing of the paving, subsurface gravel, and/or underdrain(s) when applicable, 2026-08-19 Item #12D Page 320 of 816
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or if it is determined that the underlying soils do not have the infiltration capacity expected, the [City Engineer] shall be contacted prior to any additional repairs or reconstruction.

Other Special Considerations. Site design BMPs, such as permeable pavement, installed within a new development or redevelopment project are components of an overall storm water management strategy for the project. The presence of site design BMPs within a project is usually a factor in the determination of the amount of runoff to be managed with structural BMPs (i.e., the amount of runoff expected to reach downstream retention or biofiltration basins that process storm water runoff from the project as a whole). When site design BMPs are not maintained or are removed, this can lead to clogging or failure of downstream structural BMPs due to greater delivery of runoff and pollutants than intended for the structural BMP. Therefore, the [City Engineer] may require confirmation of maintenance of site design BMPs as part of their structural BMP maintenance documentation requirements. Site design BMPs that have been installed as part of the project should not be removed, nor should they be bypassed by re-routing roof drains or re-grading surfaces within the project. If changes are necessary, consult the [City Engineer] to determine requirements.

The runoff storage and infiltration surface area in this BMP are not readily accessible because they are subsurface. This means that clogging and poor drainage are not easily corrected. If the tributary area draining to the BMP includes unpaved areas, the sediment load from the tributary drainage area can be too high, reducing BMP function or clogging the BMP. All unpaved areas within the tributary drainage area should be stabilized with vegetation. Other pretreatment components to prevent transport of sediment to the paving surface, such as grass buffer strips, will extend the life of the subsurface components and infiltration surface. Along with proper stabilization measures and pretreatment within the tributary area, routine maintenance, including preventive vacuum/regenerative air street sweeping, is key to preventing clogging. 2026-08-19 Item #12D Page 321 of 816
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Summary of Standard Inspection and Maintenance
The property owner is responsible to ensure inspection, operation and maintenance of permanent BMPs on their property unless responsibility has been formally transferred to an agency, community facilities district, homeowners association, property owners association, or other special district. Maintenance frequencies listed in this table are average/typical frequencies. Actual maintenance needs are site-specific, and maintenance may be required more frequently. Maintenance must be performed whenever needed, based on maintenance indicators presented in this table.

The BMP owner is responsible for conducting regular inspections to see when maintenance is needed based on the maintenance indicators.

Threshold/Indicator Maintenance Action Typical Maintenance Frequency Preventive vacuum/regenerative air street sweeping
Pavement should be swept with a vacuum power or regenerative air street sweeper to maintain infiltration through paving surface • Schedule/perform this preventive action at
least twice per year.

Accumulation of sediment, litter, or debris
on permeable pavement surface
Remove and properly dispose of accumulated materials. Inspect tributary area for exposed soil or other sources of sediment and apply stabilization measures to sediment source areas. Apply source control measures as applicable to sources of litter or debris. • Inspect monthly and after every 0.5-inch
or larger storm event.
•Remove any accumulated materials found
at each inspection.
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Threshold/Indicator Maintenance Action Typical Maintenance Frequency Weeds growing on/through the permeable
pavement surface
Remove weeds and add features as necessary to prevent weed intrusion. Use non-chemical methods (e.g., instead of pesticides, control weeds using mechanical removal, physical barriers, and/or physical changes in the surrounding area adjacent to pavement that will preclude weed intrusion into the pavement). • Inspect monthly.
•Remove any weeds found at each
inspection.

Standing water in permeable paving area following a storm event, or runoff is observed overflowing off the permeable paving surface during a storm event This condition requires investigation of why
infiltration is not occurring. If feasible, corrective action shall be taken to restore infiltration (e.g., pavement should be swept with a vacuum power or regenerative air
street sweeper to restore infiltration rates, clear underdrains if underdrains are present). BMP may require retrofit if infiltration cannot be restored. The [City Engineer] shall be contacted prior to any repairs or reconstruction. • Inspect monthly and after every 0.5-inch
or larger storm event. If standing water is observed, increase inspection frequency to after every 0.1-inch or larger storm event. • Maintain when needed.
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Threshold/Indicator Maintenance Action Typical Maintenance Frequency Presence of mosquitos/larvae For images of egg rafts, larva, pupa, and adult mosquitos, see http://www.mosquito.org/biology If mosquitos/larvae are observed: first, immediately remove any standing water by dispersing to nearby landscaping; second, make corrective measures as applicable to restore BMP drainage to prevent standing water.

If mosquitos persist following corrective measures to remove standing water, or if the BMP design does not meet the 96-hour drawdown criteria because the underlying soils do not have the infiltration capacity expected, the [City Engineer] shall be contacted to determine a solution. A different BMP type, or a Vector Management Plan prepared with concurrence from the County of San Diego Department of Environmental Health, may be required. • Inspect monthly and after every 0.5-inch
or larger storm event. If mosquitos are observed, increase inspection frequency to after every 0.1-inch or larger storm event. • Maintain when needed.

Damage to permeable paving surface (e.g., cracks, settlement, misaligned paver blocks, void spaces between paver blocks need fill
materials replenished)
Repair or replace damaged surface as appropriate.
• Inspect annually.
• Maintain when needed.
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E.11 SD-E Rain Barrels
Description
Rain barrels are containers that can capture rooftop
runoff and store it for future use. With controlled timing and volume release, the captured rainwater can be used
for irrigation or alternative grey water between storm
events, thereby reducing runoff volumes and associated
pollutants to downstream waterbodies. Rain barrels tend to be smaller systems, less than 100 gallons. They have low installation costs. Treatment can be achieved when
rain barrels are used as part of a treatment train along with other BMPs that use captured flows in applications that do not result in discharges into the storm drain system. Rooftops are the ideal tributary areas for rain barrels. Due to San Diego’s arid climate, some rain barrels may fill only a few times each year.

Design Adaptations for Project Goals
Site design BMP to reduce effective impervious area and DCV. Barrels can be used as a site design feature to reduce the effective impervious area of the site by removing roof runoff from the site discharge. This can reduce the DCV and flow control requirements for the site.

Conceptual Design and Sizing Approach for Site Design

1. Determine the areas where rain barrels can be used in the site design to capture roof runoff to reduce the DCV. Rain barrels reduce the effective impervious area of the site by removing roof runoff from the site discharge.

Photo Credit: San Diego Low Impact Development Design
Manual
MS4 Permit Category
Site Design
Manual Category
Site Design
Applicable Performance Standard
Site Design
Primary Benefits
Typical Rain Barrel Components
Storage container, barrel or tank for
holding captured flows
Inlet and associated valves and piping
Outlet and associated valves and piping
Overflow outlet
Optional pump
Optional first flush diverters
Optional roof, supports, foundation,
level indicator, and other accessories
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2. Calculate the DCV per Appendix B.2, taking into account reduced runoff from permeable pavement areas.

Maintenance Overview
Normal Expected Maintenance. Rain barrels can be expected to accumulate some debris that is small enough to pass through the inlet into the storage container. Leaves may accumulate at the inlet.

Ancillary parts including valves, piping, screens, level indicators, and other accessories will wear and require occasional replacement. Maintenance of a rain barrel generally involves: removing accumulated debris from the inlet and storage container on a routine basis; and replacement of ancillary parts on an as-needed basis. A summary table of standard inspection and maintenance indicators is provided within this Fact Sheet. If the system includes a pump, maintenance of the pump shall be based on the manufacturer’s recommended maintenance plan.

Non-Standard Maintenance or BMP Failure. If any of the following scenarios are observed, the BMP is not performing as intended to protect downstream waterways from pollution and/or erosion.

Corrective maintenance, increased inspection and maintenance, BMP replacement, or a different BMP type will be required.
• The inlet is found to be obstructed at every inspection such that storm water bypasses the rain barrel. The rain barrel is not functioning properly if it is not capturing storm water. This would require addition of ancillary features to protect the inlet, such as screens on roof gutters. • The rain barrel is not drained between storm events. If the rain barrel is not drained between storm events, the storage volume will be diminished and the rain barrel will not capture the required volume of storm water from subsequent storms. This would require implementation of practices onsite to drain and use the stored water, or a different BMP if onsite use cannot be reliably sustained.

Other Special Considerations. Site design BMPs, such as rain barrels, installed within a new development or redevelopment project are components of an overall storm water management strategy for the project. The presence of site design BMPs within a project is usually a factor in the determination of the amount of runoff to be managed with structural BMPs (i.e., the amount of runoff expected to reach downstream retention or biofiltration basins that process storm water runoff from the project as a whole). When site design BMPs are not maintained or are removed, this can lead to clogging or failure of downstream structural BMPs due to greater delivery of runoff and pollutants than intended for the structural BMP. Therefore, the [City Engineer] may require confirmation of maintenance of site design BMPs as part of their structural BMP maintenance documentation requirements. Site design BMPs that have been installed as part of the project should not be removed, nor should they be bypassed by re-routing roof drains or re-grading surfaces within the project. If changes are necessary, consult the [City Engineer] to determine requirements. 2026-08-19 Item #12D Page 326 of 816
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Summary of Standard Inspection and Maintenance
The property owner is responsible to ensure inspection, operation and maintenance of permanent BMPs on their property unless responsibility has been formally transferred to an agency, community facilities district, homeowners association, property owners association, or other special district. Maintenance frequencies listed in this table are average/typical frequencies. Actual maintenance needs are site-specific, and maintenance may be required more frequently. Maintenance must be performed whenever needed, based on maintenance indicators presented in this table.

The BMP owner is responsible for conducting regular inspections to see when maintenance is needed based on the maintenance indicators.

Threshold/Indicator Maintenance Action Typical Maintenance Frequency Accumulation of debris at the inlet Remove and properly dispose of accumulated materials.
• Inspect monthly and after every 0.5-inch
or larger storm event.
•Remove any accumulated materials found
at each inspection.

Outlet blocked Clear blockage. • Inspect monthly and after every 0.5-inch or larger storm event.
•Remove any accumulated materials found
at each inspection.

Accumulation of debris in the storage
container
Remove and properly dispose of accumulated materials.
• Inspect twice per year.
• Maintain when needed.

Leaks or other damage to storage container Repair or replace as applicable. • Inspect twice per year. • Maintain when needed.
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Threshold/Indicator Maintenance Action Typical Maintenance Frequency Standing water in storage container between storm events outside of normal use timeframe for the stored water. Normal use timeframe is 36 to 96 hours following a storm event.

Use the water as intended, or disperse to landscaping.
• Inspect monthly and after every 0.5-inch
or larger storm event. If standing water is observed, increase inspection frequency to after every 0.1-inch or larger storm event. • Maintain when needed.

Presence of mosquitos/larvae For images of egg rafts, larva, pupa, and adult mosquitos, see http://www.mosquito.org/biology If mosquitos/larvae are observed: first, immediately remove any standing water by using the water as intended for irrigation or alternative grey water, or by or dispersing to landscaping; second, check outlet for blockage and clear blockage if applicable to restore drainage; third, install barriers such as screens that prevent mosquito access to the storage container. • Inspect monthly and after every 0.5-inch
or larger storm event. If mosquitos are observed, increase inspection frequency to after every 0.1-inch or larger storm event. • Maintain when needed.

Leaks or other damage to ancillary parts including valves, piping, screens, level indicators, and other accessories Repair or replace as applicable. • Inspect twice per year. • Maintain when needed.

Rain barrel leaning or unstable, damage to roof, supports, anchors, or foundation Make repairs as appropriate to correct the problem and stabilize the system. • Inspect twice per year.
• Maintain when needed.
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E.12 SD-F Amended Soils
Photo Credit: Orange County Technical Guidance Document MS4 Permit Category
Site Design
Manual Category
Site Design
Applicable Performance Standard
Site Design
Primary Benefits
Volume Reduction
Peak Flow Attenuation
Description
Amended soils are soils whose physical, chemical, and biological characteristics have been altered from the natural condition to promote beneficial storm water characteristics. Amended soils shall be used as part of SD-B Impervious Area Dispersion, where applicable. Typical storm water management benefits associated with amended soils include:
• Improved hydrologic characteristics—amended soils can promote infiltration, decrease runoff rates and volumes, and more effectively filter pollutants from storm water runoff • Improved vegetation health—amended soils provide greater moisture retention, and altered chemical and biological characteristics that can result in healthier plant growth, reduced irrigation demands, and reduced need for fertilization and maintenance • Reduced erosion—amended soils produce healthier plant growth and reduced runoff which results in reduced soil erosion
Not all amended soils have the same storm water benefits, the soil amendment used should be suited for the design purpose and design period of the amended area.

Design Adaptations for Project Goals
Varying categories of soil amendments have different benefits and applications. Mulch is a soil amendment that is added at grade, rather than mixed into the soil. Mulch reduces evaporation and improves retention. Shavings and compost are common soil amendments that improve biological and chemical properties of the soil. Sand can be used as an amendment to improve the drainage rates of amended soils. Native soil samples may need to be analyzed by a lab to determine the specific soil amendments needed to achieve the desired infiltration, retention, and/or filtration rates. 2026-08-19 Item #12D Page 329 of 816
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Design Criteria and Considerations
Soil amendments must meet the following design criteria and considerations. Deviations from the below criteria may be approved at the discretion of the City Engineer if appropriate:

Siting and Design Intent/Rationale
□
When mulch is used as an amendment, it is
applied at grade over all planting areas to a
depth of 3”.

Mulch should be applied on top and not
mixed into underlying soils
□
When shavings or compost is used as an
amendment, it is rototilled into the native soil
to a minimum depth of 6” (12 inches
preferred).

If soil is not completely mixed the overall
benefit will be reduced.
□ Compost meets the criteria in Appendix F.3.1.2 If poor quality compost is used, it will have negative impact to water quality.
□
Soil amendments are free of stones, stumps,
roots, glass, plastic, metal, and other deleterious
materials.

Large debris in amended soils can cause
localized erosion. Trash/harmful
materials can result in personal injury or
contamination.
□ Mixing of soils are done prior to planting
Soil mixing before planting results in a
more homogeneous mixing and will
reduce the stress on plants.
□
Care is taken around existing trees and shrubs
to prevent root damage during construction
and soil amendment application.

Preservation of existing established
vegetation is an important part of site
design and erosion control.
□
Soil amendments are applied at the end of
construction
Soil amendments applied too soon in the
construction process may become over
compacted reducing effectiveness.
□
Soil amendments are compatible with planned
vegetation
The soil amendments impact the pH and
salinity of the soil. Some plants have
sensitive pH and/or salinity tolerance
ranges.

Conceptual Design and Sizing Approach for Site Design
• When soil amendments are used a runoff factor of 0.1 can be used for DCV calculation for the amended area.
• Amended soils should be used as part of SD-B Impervious Area Dispersion, and to increase the retention volume in other BMPs.
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Maintenance
Annual maintenance may be required to determine reapplication requirements of amended soils.

Amended soils should be regularly inspected for signs of compaction, waterlogging, and unhealthy vegetation.
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E.13 HU-1 Cistern
Photo Credit: Water Environment Research Foundation: WERF.org Description
Cisterns are containers that can capture rooftop runoff and store it for future use. With controlled timing and volume release, the captured rainwater can be used for irrigation or alternative grey water between storm events, thereby reducing runoff volumes and associated pollutants to downstream water bodies. Cisterns are larger systems (generally>100 gallons) that can be self-contained aboveground or below ground systems. Treatment can be achieved when cisterns are used as part of a treatment train along with other BMPs that use captured flows in applications that do not result in discharges into the storm drain system. Rooftops are the ideal tributary areas for cisterns.

Typical cistern components include:
• Storage container, barrel or tank for holding captured flows • Inlet and associated valves and piping
• Outlet and associated valves and piping
• Overflow outlet
• Optional pump
• Optional first flush diverters
MS4 Permit Category
Retention
Manual Category
Harvest and Use
Applicable Performance
Standards
Pollutant Control
Flow Control
Primary Benefits
Volume Reduction
Peak Flow Attenuation
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• Optional roof, supports, foundation, level indicator, and other accessories Source: City of San Diego Storm Water Standards
Design Adaptations for Project Goals
Site design BMP to reduce effective impervious area and DCV. Cisterns can be used as a site design feature to reduce the effective impervious area of the site by removing roof runoff from the site discharge. This can reduce the DCV and flow control requirements for the site.

Harvest and use for storm water pollutant control. Typical uses for captured flows include irrigation, toilet flushing, cooling system makeup, and vehicle and equipment washing.

Integrated storm water flow control and pollutant control configuration. Cisterns provide flow control in the form of volume reduction and/or peak flow attenuation and storm water treatment through elimination of discharges of pollutants. Additional flow control can be achieved by sizing the cistern to include additional detention storage and/or real-time automated flow release controls.

Design Criteria and Considerations
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at the discretion of the City Engineer if it is determined to be appropriate:

Siting and Design Intent/Rationale
□
Cisterns are sized to detain the full DCV of
contributing area and empty within 36 hours.

Draining the cistern makes the storage
volume available to capture the next
storm.

The applicant has an option to use a
different drawdown time up to 96 hours
if the volume of the facility is adjusted
using the percent capture method in
Appendix B.4.2.
□
Cisterns are fitted with a flow control device
such as an orifice or a valve to limit outflow in
accordance with drawdown time requirements.

Flow control provides flow attenuation
benefits and limits cistern discharge to
downstream facilities during storm
events.
□
Cisterns are designed to drain completely,
leaving no standing water, and all entry points
are fitted with traps or screens, or sealed.

Complete drainage and restricted entry
prevents mosquito habitat.
□
Leaf guards and/or screens are provided to
prevent debris from accumulating in the
cistern.

Leaves and organic debris can clog the
outlet of the cistern.
□
Access is provided for maintenance and the
cistern outlets are accessible and designed to
allow easy cleaning.

Properly functioning outlets are needed
to maintain proper flow control in
accordance with drawdown time
requirements.
□
Cisterns must be designed and sited such that
overflow will be conveyed safely overland to
the storm drain system or discharge point.

Safe overflow conveyance prevents
flooding and damage of property.

Conceptual Design and Sizing Approach for Site Design and Storm Water Pollutant Control

1. Calculate the DCV for site design per Appendix B.

2. Determine the locations on the site where cisterns can be located to capture and detain the DCV from roof areas without subsequent discharge to the storm drain system. Cisterns are best located in close proximity to building and other roofed structures to minimize piping.

Cisterns can also be used as part of a treatment train upstream by increasing pollutant control through delayed runoff to infiltration BMPs such as bioretention without underdrain facilities.

3. Use the sizing worksheet in Appendix B.3 to determine if full or partial capture of the DCV is achievable.
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4. The remaining DCV to be treated should be calculated for use in sizing downstream BMP(s).

Conceptual Design and Sizing Approach when Storm Water Flow Control is Applicable Control of flow rates and/or duration will typically require significant cistern volumes, and therefore the following steps should be taken prior to determination of site design and storm water pollutant control. Pre-development and allowable post-project flow rates and durations should be determined as discussed in Chapter 6 of the manual.

1. Verify that cistern siting and design criteria have been met. Design for flow control can be achieved using various design configurations, shapes, and quantities of cisterns.

2. Iteratively determine the cistern storage volume required to provide detention storage to reduce flow rates and durations to allowable limits. Flow rates and durations can be controlled from detention storage by altering outlet structure orifice size(s) and/or water control valve operation.

3. Verify that the cistern is drawdown within 36 hours. The drawdown time can be estimated by dividing the storage volume by the rate of use of harvested water.

4. If the cistern cannot fully provide the flow rate and duration control required by this manual, a downstream structure with additional storage volume or infiltration capacity such as a biofiltration can be used to provide remaining flow control.

Maintenance Overview
Normal Expected Maintenance. Cisterns can be expected to accumulate sediment and debris that is small enough to pass through the inlet into the storage container. Larger debris such as leaves or trash may accumulate at the inlet. While the storage container is generally a permanent structure, ancillary parts including valves, piping, screens, level indicators, and other accessories will wear and require occasional replacement. Maintenance of a cistern generally involves: removing accumulated sediment and debris from the inlet and storage container on a routine basis; and replacement of ancillary parts on an as-needed basis. A summary table of standard inspection and maintenance indicators is provided within this Fact Sheet. If the system as a whole includes a pump or other electrical equipment, maintenance of the equipment shall be based on the manufacturer’s recommended maintenance plan.

Non-Standard Maintenance or BMP Failure. If any of the following scenarios are observed, the BMP is not performing as intended to protect downstream waterways from pollution and/or erosion.

Corrective maintenance, increased inspection and maintenance, BMP replacement, or a different BMP type will be required.
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• The inlet is found to be obstructed at every inspection such that storm water bypasses the cistern. The cistern is not functioning properly if it is not capturing storm water. This would require addition of ancillary features to protect the inlet, or pretreatment measures within the watershed draining to the cistern to intercept larger debris, such as screens on roof gutters, or drainage inserts within catch basins. Increase the frequency of inspection until the issue is resolved.
• Accumulation of sediment within one year is greater than 25% of the volume of the cistern.

This means the sediment load from the tributary drainage area has diminished the storage volume of the cistern and the cistern will not capture the required volume of storm water.

This would require pretreatment measures within the tributary area draining to the cistern to intercept sediment.
• The cistern is not drained between storm events. If the cistern is not drained between storm events, the storage volume will be diminished and the cistern will not capture the required volume of storm water from subsequent storms. This would require implementation of practices onsite to drain and use the stored water, or a different BMP if onsite use cannot be reliably sustained.
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Summary of Standard Inspection and Maintenance
The property owner is responsible to ensure inspection, operation and maintenance of permanent BMPs on their property unless responsibility has been formally transferred to an agency, community facilities district, homeowners association, property owners association, or other special district. Maintenance frequencies listed in this table are average/typical frequencies. Actual maintenance needs are site-specific, and maintenance may be required more frequently. Maintenance must be performed whenever needed, based on maintenance indicators presented in this table.

The BMP owner is responsible for conducting regular inspections to see when maintenance is needed based on the maintenance indicators.

Threshold/Indicator Maintenance Action Typical Inspection and Maintenance Frequency Accumulation of sediment, litter, or debris
at the inlet
Remove and properly dispose of accumulated materials.
• Inspect monthly and after every 0.5-inch or
larger storm event.
•Remove any accumulated materials found at
each inspection.

Outlet blocked Clear blockage. • Inspect monthly and after every 0.5-inch or larger storm event.
•Remove any accumulated materials found at
each inspection.
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Threshold/Indicator Maintenance Action Typical Inspection and Maintenance Frequency Accumulation of sediment, litter, or debris
in the storage container
Remove and properly dispose of accumulated materials.
• Inspect monthly. If the BMP is 25% full* or
more in one month, increase inspection frequency to monthly plus after every 0.1-inch or larger storm event. •Remove materials annually (minimum), or more
frequently when BMP is 25% full* (or at manufacturer threshold if manufacturer threshold is less than 25% full*) in less than one year, or if accumulation blocks outlet Standing water in storage container between storm events outside of normal use timeframe for the stored water. Normal use timeframe is 36 to 96 hours following a storm event depending on the purpose and design of the cistern.

Use the water as intended, or disperse to landscaping. Implement practices onsite to drain and use the stored water. Contact the [City Engineer] to determine a solution if onsite use cannot be reliably sustained. • Inspect monthly and after every 0.5-inch or
larger storm event. If standing water is observed, increase inspection frequency to after every 0.1-inch or larger storm event. • Maintain when needed.

Presence of mosquitos/larvae For images of egg rafts, larva, pupa, and adult mosquitos, see http://www.mosquito.org/biology If mosquitos/larvae are observed: first, immediately remove any standing water by using the water as intended
for irrigation or alternative grey water, or by dispersing to landscaping; second, check cistern outlet for blockage and clear blockage if applicable to restore drainage; third, install barriers such as screens that prevent mosquito access to the storage container. • Inspect monthly and after every 0.5-inch or
larger storm event. If mosquitos are observed, increase inspection frequency to after every 0.1-inch or larger storm event. • Maintain when needed.
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Threshold/Indicator Maintenance Action Typical Inspection and Maintenance Frequency Leaks or other damage to ancillary parts including valves, piping, screens, level indicators, and other accessories Repair or replace as applicable. • Inspect twice per year. • Maintain when needed.

Leaks or other damage to storage container
Repair or replace as applicable. • Inspect twice per year. • Maintain when needed.

Cistern leaning or unstable, damage to roof, supports, anchors, or foundation Make repairs as appropriate to correct the problem and stabilize the system. • Inspect twice per year.
• Maintain when needed.
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E.14 INF-1 Infiltration Basin
Photo Credit: http://www.stormwaterpartners.com/facilities/basin.html Description
An infiltration basin typically consists of an earthen basin with a flat bottom constructed in naturally pervious soils. An infiltration basin retains stormwater and allows it to evaporate and/or percolate into the underlying soils. The bottom of an infiltration basin is typically vegetated with native grasses or turf grass; however other types of vegetation can be used if they can survive periodic inundation and long inter-event dry periods. Treatment is achieved primarily through infiltration, filtration, sedimentation, biochemical processes and plant uptake. Infiltration basins can be constructed as linear trenches or as underground infiltration galleries.

Typical infiltration basin components include:
• Inflow distribution mechanisms (e.g., perimeter flow spreader or filter strips) • Energy dissipation mechanism for concentrated inflows (e.g., splash blocks or riprap) • Forebay to provide pretreatment surface ponding for captured flows • Vegetation selected based on basin use, climate, and ponding depth • Uncompacted native soils at the bottom of the facility • Overflow structure
MS4 Permit Category
Retention
Manual Category
Infiltration
Applicable Performance
Standard
Pollutant Control
Flow Control
Primary Benefits
Volume Reduction
Peak Flow Attenuation
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Typical plan and section view of an Infiltration BMP
Design Adaptations for Project Goals
Full infiltration BMP for stormwater pollutant control. Infiltration basins can be used as a pollutant control BMP, designed to infiltrate runoff from direct rainfall as well as runoff from adjacent areas that are tributary to the BMP. Infiltration basins must be designed with an infiltration storage volume (a function of the surface ponding volume) equal to the full DCV and able to meet drawdown time limitations.
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Integrated stormwater flow control and pollutant control configuration. Infiltration basins can also be designed for flow rate and duration control by providing additional infiltration storage through increasing the surface ponding volume.

Recommended Siting Criteria
Siting Criteria Intent/Rationale
□
Placement observes geotechnical
recommendations regarding potential
hazards (e.g., slope stability, landslides,
liquefaction zones) and setbacks (e.g.,
slopes, foundations, utilities).

Must not negatively impact existing site
geotechnical concerns.
□
Selection and design of basin is based
on infiltration feasibility criteria and
appropriate design infiltration rate (See
Appendix C and D).

Must operate as a full infiltration design and
must be supported by drainage area and in-situ
infiltration rate feasibility findings.

Recommended BMP Component Dimensions
BMP Component Dimension Intent/Rationale
Freeboard ≥ 2 inches Freeboard minimizes risk of
uncontrolled surface discharge.

Ponding Area Side Slopes 3H:1V or shallower
Gentler side slopes are safer, less
prone to erosion, able to establish
vegetation more quickly and easier
to maintain.

Settling Forebay Volume ≥ 25% of facility volume
A forebay to trap sediment can
decrease frequency of required
maintenance. Other pretreatment
devices may be used in accordance
with Appendix B.6.

Design Criteria and Considerations
Infiltration basins must meet the following design criteria. Deviations from the below criteria may be approved at the discretion of the [City Engineer] if it is determined to be appropriate:
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Design Criteria Intent/Rationale
□
Finish grade of the facility is ≤ 2% (0%
recommended).

Flatter surfaces reduce erosion and
channelization with the facility.
□
Infiltration of surface ponding is limited
to a 36-hour drawdown time.

Prolonged surface ponding reduce volume
available to capture subsequent storms.

The applicant has an option to use a different
drawdown time up to 96 hours if the volume
of the facility is adjusted using the percent
capture method in Appendix B.4.2.

Inflow and Overflow Structures
□
Inflow and outflow structures are
accessible by required equipment (e.g.,
vactor truck) for inspection and
maintenance.

Maintenance will prevent clogging and ensure
proper operation of the flow control
structures.
□
Inflow velocities are limited to 3 ft/s or
less or use energy dissipation methods
(e.g., riprap, level spreader) for
concentrated inflows.

High inflow velocities can cause erosion, scour
and/or channeling.
□
Overflow is safely conveyed to a
downstream storm drain system or
discharge point. Size overflow structure
to pass 100-year peak flow for on-line
basins and water quality peak flow for
off-line basins.

Planning for overflow lessens the risk of
property damage due to flooding.

Conceptual Design and Sizing Approach for Storm Water Pollutant Control To design infiltration basins for stormwater pollutant control only (no flow control required), the following steps should be taken:

1. Verify that siting and design criteria have been met, including placement and basin area requirements, forebay volume (not included in infiltration footprint for sizing), and maximum slopes for basin sides and bottom.

2. Calculate the DCV per Appendix B based on expected site design runoff for tributary areas.

3. Use the sizing worksheet (Appendix B.4) to determine if full infiltration of the DCV is achievable based on the infiltration storage volume calculated from the surface ponding area and depth for a maximum 36-hour drawdown time. The drawdown time can be estimated by dividing the average depth of the basin by the design infiltration rate. Appendix D provides guidance on 2026-08-19 Item #12D Page 343 of 816
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evaluating a site’s infiltration rate.

Conceptual Design and Sizing Approach for Storm Water Pollutant Treatment and Flow Control Control of flow rates and/or durations will typically require significant surface ponding volume, and therefore the following steps should be taken prior to determination of stormwater pollutant control design. Pre-development and allowable post-project flow rates and durations should be determined as discussed in Chapter 6 of the manual.

2. Iteratively determine the surface ponding required to provide infiltration storage to reduce flow rates and durations to allowable limits while adhering to the maximum 36-hour drawdown time. Flow rates and durations can be controlled using flow splitters that route the appropriate inflow amounts to the infiltration basin and bypass excess flows to the downstream storm drain system or discharge point.

3. If an infiltration basin cannot fully provide the flow rate and duration control required by this manual, an upstream or downstream structure with appropriate storage volume such as an underground vault can be used to provide additional control.

After the infiltration basin has been designed to meet flow control requirements, calculations must be completed to verify if stormwater pollutant control requirements to treat the DCV have been met.

Maintenance Overview
Normal Expected Maintenance. Infiltration basins require routine maintenance to: remove accumulated materials such as sediment, trash or debris from the forebay and the basin; maintain vegetation health if the BMP includes vegetation; and maintain integrity of side slopes, inlets, energy dissipators, and outlets. A summary table of standard inspection and maintenance indicators is provided within this Fact Sheet.

Non-Standard Maintenance or BMP Failure. If any of the following scenarios are observed, the BMP is not performing as intended to protect downstream waterways from pollution and/or erosion.

Corrective maintenance, increased inspection and maintenance, BMP replacement, or a different BMP type will be required.
• The BMP is not drained between storm events. Surface ponding longer than approximately 24 hours following a storm event may be detrimental to vegetation health, and surface or subsurface ponding longer than approximately 96 hours following a storm event poses a risk of vector (mosquito) breeding. Poor drainage can result from clogging of the underlying native soils, or clogging of covers applied at the basin surface such as topsoil, mulch, or rock layer. 2026-08-19 Item #12D Page 344 of 816
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The specific cause of the drainage issue must be determined and corrected. For surface-level basins (i.e., not underground infiltration galleries), surface cover materials can be removed and replaced, and/or native soils can be scarified or tilled to help reestablish infiltration. If it is determined that the underlying native soils have been compacted or do not have the infiltration capacity expected, or if the infiltration surface area is not accessible (e.g., an underground infiltration gallery) the [City Engineer] shall be contacted prior to any additional repairs or reconstruction.
• Sediment, trash, or debris accumulation has filled the forebay or other pretreatment device within one month, or if no forebay or other pretreatment device is present, has filled greater than 25% of the surface ponding volume within one maintenance cycle. This means the load from the tributary drainage area is too high, reducing BMP function or clogging the BMP.

This would require adding a forebay or other pretreatment measures within the tributary area draining to the BMP to intercept the materials if no pretreatment component is present, or increased maintenance frequency for an existing forebay or other pretreatment device.

Pretreatment components, especially for sediment, will extend the life of the infiltration basin. • Erosion due to concentrated storm water runoff flow that is not readily corrected by adding erosion control blankets, adding stone at flow entry points, or minor re-grading to restore proper drainage according to the original plan. If the issue is not corrected by restoring the BMP to the original plan and grade, the [City Engineer] shall be contacted prior to any additional repairs or reconstruction.

Other Special Considerations. If the infiltration basin is vegetated: Vegetated structural BMPs that are constructed in the vicinity of, or connected to, an existing jurisdictional water or wetland could inadvertently result in creation of expanded waters or wetlands. As such, vegetated structural BMPs have the potential to come under the jurisdiction of the United States Army Corps of Engineers, SDRWQCB, California Department of Fish and Wildlife, or the United States Fish and Wildlife Service. This could result in the need for specific resource agency permits and costly mitigation to perform maintenance of the structural BMP. Along with proper placement of a structural BMP, routine maintenance is key to preventing this scenario. 2026-08-19 Item #12D Page 345 of 816
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Summary of Standard Inspection and Maintenance
The property owner is responsible to ensure inspection, operation and maintenance of permanent BMPs on their property unless responsibility has been formally transferred to an agency, community facilities district, homeowners association, property owners association, or other special district. Maintenance frequencies listed in this table are average/typical frequencies. Actual maintenance needs are site-specific, and maintenance may be required more frequently. Maintenance must be performed whenever needed, based on maintenance indicators presented in this table.

The BMP owner is responsible for conducting regular inspections to see when maintenance is needed based on the maintenance indicators.

Threshold/Indicator Maintenance Action Typical Maintenance Frequency Accumulation of sediment, litter, or debris
in forebay and/or basin
Remove and properly dispose of accumulated materials, (without damage to vegetation when applicable).
• Inspect monthly. If the forebay is 25%
full* or more in one month, increase inspection frequency to monthly plus after every 0.1-inch or larger storm event. •Remove any accumulated materials found
within the infiltration area at each
inspection.
•When the BMP includes a forebay,
materials must be removed from the forebay when the forebay is 25% full*, or if accumulation within the forebay blocks flow to the infiltration area. 2026-08-19 Item #12D Page 346 of 816
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Threshold/Indicator Maintenance Action Typical Maintenance Frequency Obstructed inlet or outlet structure Clear blockage. • Inspect monthly and after every 0.5-inch or larger storm event.
•Remove any accumulated materials found
at each inspection.

Poor vegetation establishment (when the BMP includes vegetated surface by design) Re-seed, re-plant, or re-establish vegetation per original plans. • Inspect monthly.
• Maintain when needed.

Dead or diseased vegetation (when the BMP includes vegetated surface by design) Remove dead or diseased vegetation, reseed, re-plant, or re-establish vegetation per original plans. • Inspect monthly.
• Maintain when needed.

Overgrown vegetation (when the BMP includes vegetated surface by design) Mow or trim as appropriate. • Inspect monthly.
• Maintain when needed.

Erosion due to concentrated irrigation flow Repair/re-seed/re-plant eroded areas and adjust the irrigation system.
• Inspect monthly.
• Maintain when needed.

Erosion due to concentrated storm water runoff flow
Repair/re-seed/re-plant eroded areas, and make appropriate corrective measures such as adding erosion control blankets, adding stone at flow entry points, or minor regrading to restore proper drainage according to the original plan. If the issue is not corrected by restoring the BMP to the original plan and grade, the [City Engineer] shall be contacted prior to any additional repairs or reconstruction. • Inspect after every 0.5-inch or larger storm
event. If erosion due to storm water flow has been observed, increase inspection frequency to after every 0.1-inch or larger storm event. • Maintain when needed. If the issue is not
corrected by restoring the BMP to the original plan and grade, the [City Engineer] shall be contacted prior to any additional repairs or reconstruction. 2026-08-19 Item #12D Page 347 of 816
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Threshold/Indicator Maintenance Action Typical Maintenance Frequency Standing water in infiltration basin without subsurface infiltration gallery for longer than 24-96 hours following a storm event Make appropriate corrective measures such as adjusting irrigation system, removing obstructions of debris or invasive vegetation, or removing/replacing clogged or compacted surface treatments and/or scarifying or tilling native soils. Always remove deposited sediments before scarification, and use a hand-guided rotary tiller. If it is determined that the underlying native soils have been compacted or do not have the infiltration capacity expected, the [City Engineer] shall be contacted prior to any additional repairs or reconstruction. • Inspect monthly and after every 0.5-inch
or larger storm event. If standing water is observed, increase inspection frequency to after every 0.1-inch or larger storm event. • Maintain when needed.

Standing water in subsurface infiltration gallery for longer than 24-96 hours following a storm event This condition requires investigation of why
infiltration is not occurring. If feasible, corrective action shall be taken to restore infiltration (e.g., flush fine sediment or remove and replace clogged soils). BMP may require retrofit if infiltration cannot be restored. The [City Engineer] shall be contacted prior to any repairs or reconstruction. • Inspect monthly and after every 0.5-inch
or larger storm event. If standing water is observed, increase inspection frequency to after every 0.1-inch or larger storm event. • Maintain when needed.
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Threshold/Indicator Maintenance Action Typical Maintenance Frequency Presence of mosquitos/larvae For images of egg rafts, larva, pupa, and adult mosquitos, see http://www.mosquito.org/biology If mosquitos/larvae are observed: first, immediately remove any standing water by dispersing to nearby landscaping; second, make corrective measures as applicable to restore BMP drainage to prevent standing water. For subsurface infiltration galleries,
ensure access covers are tight fitting, with gaps or holes no greater than 1/16 inch, and/or install barriers such as inserts or screens that prevent mosquito access to the subsurface storage. If mosquitos persist following corrective measures to remove standing water, or if the BMP design does not meet the 96-hour drawdown criteria because the underlying native soils have been compacted or do not have the infiltration capacity expected, the [City Engineer] shall be contacted to determine a solution. A different BMP type, or a Vector Management Plan prepared with concurrence from the County of San Diego Department of Environmental Health, may be required. • Inspect monthly and after every 0.5-inch
or larger storm event. If mosquitos are observed, increase inspection frequency to after every 0.1-inch or larger storm event. • Maintain when needed
Damage to structural components such as
weirs, inlet or outlet structures
Repair or replace as applicable. • Inspect annually.
• Maintain when needed.
“25% full” is defined as ¼ of the depth from the design bottom elevation to the crest of the outflow structure (e.g., if the height to the outflow opening is 12 inches from the bottom elevation, then the materials must be removed when there is 3 inches of accumulation – this should be marked on the outflow structure). 2026-08-19 Item #12D Page 349 of 816
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E.15 INF-2 Bioretention
Photo Credit: Ventura County Technical Guidance Document Description
Bioretention (bioretention without underdrain) facilities are vegetated surface water systems that filter water through vegetation and soil, or engineered media prior to infiltrating into native soils. These facilities are designed to infiltrate the full DCV. Bioretention facilities are commonly incorporated into the site within parking lot landscaping, along roadsides, and in open spaces. They can be constructed inground or partially aboveground, such as planter boxes with open bottoms (no impermeable liner at the bottom) to allow infiltration. Treatment is achieved through filtration, sedimentation, sorption, infiltration, biochemical processes and plant uptake.

Typical bioretention without underdrain components include:
• Inflow distribution mechanisms (e.g, perimeter flow spreader or filter strips) • Energy dissipation mechanism for concentrated inflows (e.g., splash blocks or riprap) • Shallow surface ponding for captured flows
• Side slope and basin bottom vegetation selected based on expected climate and ponding depth • Non-floating mulch layer (optional)
• Media layer (planting mix or engineered media) capable of supporting vegetation growth • Filter course layer consisting of aggregate to prevent the migration of fines into uncompacted native soils or the optional aggregate storage layer
• Optional aggregate storage layer for additional infiltration storage • Uncompacted native soils at the bottom of the facility • Overflow structure
MS4 Permit Category
Retention
Manual Category
Infiltration
Applicable Performance
Standard
Pollutant Control
Flow Control
Primary Benefits
Volume Reduction
Treatment
Peak Flow Attenuation
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Design Adaptations for Project Goals
Full infiltration BMP for stormwater pollutant control. Bioretention can be used as a pollutant control BMP designed to infiltrate runoff from direct rainfall as well as runoff from adjacent tributary areas. Bioretention facilities must be designed with an infiltration storage volume (a function of the ponding, media and aggregate storage volumes) equal to the full DCV and able to meet drawdown time limitations.

Integrated stormwater flow control and pollutant control configuration. Bioretention facilities can be designed to provide flow rate and duration control. This may be accomplished by providing greater infiltration storage with increased surface ponding and/or aggregate storage volume for stormwater flow control
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Typical plan and section view of a Bioretention BMP
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Design Criteria and Considerations
Bioretention must meet the following design criteria. Deviations from the below criteria may be approved at the discretion of the City Engineer if it is determined to be appropriate:

Siting and Design Intent/Rationale
□
Placement observes geotechnical
recommendations regarding potential
hazards (e.g., slope stability, landslides,
liquefaction zones) and setbacks (e.g.,
slopes, foundations, utilities).

Must not negatively impact existing
site geotechnical concerns.
□
Selection and design of BMP is based on
infiltration feasibility criteria and
appropriate design infiltration rate
presented in Appendix C and D.

Must operate as a full infiltration
design and must be supported by
drainage area and in-situ infiltration
rate feasibility findings.
□
Contributing tributary area is ≤ 5 acres (≤
1 acre preferred).

Bigger BMPs require additional design
features for proper performance.

Contributing tributary area greater
than 5 acres may be allowed at the
discretion of the City Engineer if the
following conditions are met: 1)
incorporate design features (e.g. flow
spreaders) to minimizing short
circuiting of flows in the BMP and 2)
incorporate additional design features
requested by the City Engineer for
proper performance of the regional
BMP.
□
Finish grade of the facility is ≤ 2%. In long
bioretention facilities where the potential
for internal erosion and channelization
exists, the use of check dams is required.

Flatter surfaces reduce erosion and
channelization within the facility.

Internal check dams reduce velocity
and dissipate energy.

Surface Ponding
□
Surface ponding is limited to a 24-hour
drawdown time.
24-hour drawdown time is
recommended for plant health.
□
Surface ponding depth is ≥ 6 and ≤ 12
inches.

Surface ponding capacity lowers
subsurface storage requirements.

Deep surface ponding raises safety
concerns.

Surface ponding depth greater than 12
inches (for additional pollutant
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Siting and Design Intent/Rationale
control or surface outlet structures or
flow-control orifices) may be allowed
at the discretion of the City Engineer
if the following conditions are met: 1)
surface ponding depth drawdown
time is less than 24 hours; and 2)
safety issues and fencing requirements
are considered (typically ponding
greater than 18” will require a fence
and/or flatter side slopes) and 3)
potential for elevated clogging risk is
considered
□ A minimum of 2 inches of freeboard is
provided.

Freeboard provides room for head
over overflow structures and
minimizes risk of uncontrolled
surface discharge.
□ Side slopes are stabilized with vegetation
and are ≥ 3H: 1V.

Gentler side slopes are safer, less
prone to erosion, able to establish
vegetation more quickly and easier
to maintain.

Vegetation
□
Plantings are suitable for the climate and
expected ponding depth. A plant list to
aid in selection can be found in
Appendix E.20.

Plants suited to the climate and
ponding depth are more likely to
survive.
□ An irrigation system with a connection
to water supply is provided as needed.

Seasonal irrigation might be needed
to keep plants healthy.

Mulch (Optional or Mandatory – Dependent on jurisdiction) □
A minimum of 3 inches of well-aged,
shredded hardwood mulch that has been
stockpiled or stored for at least 12
months is provided. Mulch must be nonfloating to avoid clogging of overflow structure.

Mulch will suppress weeds and
maintain moisture for plant growth.

Aging mulch kills pathogens and
weed seeds and allows beneficial
microbes to multiply.

Media Layer
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Siting and Design Intent/Rationale
□
Media maintains a minimum filtration
rate of 5 in/hr over lifetime of facility. A
minimum initial filtration rate of 10
in/hr is recommended.

A high filtration rate through the soil
mix minimizes clogging potential
and allows flows to quickly enter the
aggregate storage layer, thereby
minimizing bypass.
□
Media is a minimum 18 inches deep,
meeting specifications outlined in
Appendix F.3:
.

A deep media layer provides
additional filtration and supports
plants with deeper roots.

Standard specifications shall be
followed.
□
Alternatively, for proprietary designs and
custom media mixes not meeting the
media specifications, the media meets
the pollutant treatment performance
criteria in Section F.1.

Greater surface area to tributary
area ratios decrease loading rates
per square foot and therefore
increase longevity.

Adjusted runoff factor is to account
for site design BMPs implemented
upstream of the BMP (such as rain
barrels, impervious area dispersion,
etc.). Refer to Appendix B.2
guidance.

Filter Course Layer (Optional)
□
A filter course is used to prevent migration
of fines through layers of the facility. Filter
fabric is not used.

Migration of media can cause
clogging of the aggregate storage
layer void spaces or subgrade.

Filter fabric is more likely to clog.
□ Filter course is washed and free of fines. Washing aggregate will help eliminate fines that could clog the
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Siting and Design Intent/Rationale
facility and impede infiltration.
□
Filter course calculations assessing
suitability for particle migration prevention
have been completed.

Gradation relationship between
layers can evaluate factors (e.g.,
bridging, permeability, and
uniformity) to determine if
particle sizing is appropriate or if
an intermediate layer is needed.

Aggregate Storage Layer (Optional)
□
Class 2 Permeable per Caltrans
specification 68-1.025 is recommended for
the storage layer. Washed, open-graded
crushed rock may be used, however a 4-6
inch washed pea gravel filter course layer at
the top of the crushed rock is required.

Washing aggregate will help
eliminate fines that could clog the
aggregate storage layer void
spaces or subgrade.
□
Maximum aggregate storage layer depth is
determined based on the infiltration
storage volume that will infiltrate within a
36-hour drawdown time.

A maximum drawdown time to
facilitate provision of adequate
stormwater storage for the next
storm event.

Inflow and Overflow Structures
□
Inflow and overflow structures are
accessible for inspection and maintenance.

Overflow structures must be connected to
downstream storm drain system or
appropriate discharge point.

Maintenance will prevent clogging
and ensure proper operation of
the flow control structures.
□
Inflow velocities are limited to 3 ft/s or
less or use energy dissipation methods (e.g.,
riprap, level spreader) for concentrated
inflows.

High inflow velocities can cause
erosion, scour and/or channeling.
□
Curb cut inlets are at least 12 inches wide,
have a 4-6 inch reveal (drop) and an apron
and energy dissipation as needed.

Inlets must not restrict flow and
apron prevents blockage from
vegetation as it grows in. Energy
dissipation prevents erosion.
□
Overflow is safely conveyed to a
downstream storm drain system or
discharge point. Size overflow structure to
pass 100-year peak flow for on-line basins
and water quality peak flow for off-line
basins.

Planning for overflow lessens the
risk of property damage due to
flooding.

Conceptual Design and Sizing Approach for Stormwater Pollutant Control Only To design bioretention for stormwater pollutant control only (no flow control required), the following steps should be taken:
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1. Verify that siting and design criteria have been met, including placement and basin area requirements, maximum side and finish grade slope, and the recommended media surface area tributary ratio.

2. Calculate the DCV per Appendix B based on expected site design runoff for tributary areas.

3. Use the sizing worksheet to determine if full infiltration of the DCV is achievable based on the available infiltration storage volume calculated from the bioretention without underdrain footprint area, effective depths for surface ponding, media and aggregate storage layers, and in-situ soil design infiltration rate for a maximum 36-hour drawdown time for the aggregate storage layer, with surface ponding no greater than a maximum 24-hour drawdown. The drawdown time can be estimated by dividing the average depth of the basin by the design infiltration rate of the underlying soil. Appendix D provides guidance on evaluating a site’s infiltration rate. A generic sizing worksheet is provided in Appendix B.4.

4. Where the DCV cannot be fully infiltrated based on the site or bioretention constraints, an underdrain can be added to the design (use biofiltration with partial retention factsheet).

Conceptual Design and Sizing Approach when Stormwater Flow Control is Applicable Control of flow rates and/or durations will typically require significant surface ponding and/or aggregate storage volumes, and therefore the following steps should be taken prior to determination of stormwater pollutant control design. Pre-development and allowable post-project flow rates and durations shall be determined as discussed in Chapter 6 of the manual.

1. Verify that siting and design criteria have been met, including placement requirements, maximum side and finish grade slopes, and the recommended media surface area tributary area ratio. Design for flow control can be achieved using various design configurations.

2. Iteratively determine the facility footprint area, surface ponding and/or aggregate storage layer depth required to provide infiltration storage to reduce flow rates and durations to allowable limits while adhering to the maximum drawdown times for surface ponding and aggregate storage. Flow rates and durations can be controlled using flow splitters that route the appropriate inflow amounts to the bioretention facility and bypass excess flows to the downstream storm drain system or discharge point.

3. If bioretention without underdrain facility cannot fully provide the flow rate and duration control required by the MS4 permit, an upstream or downstream structure with appropriate storage volume such as an underground vault can be used to provide additional control.

4. After bioretention without underdrain BMPs have been designed to meet flow control requirements, calculations must be completed to verify if stormwater pollutant control requirements to treat the DCV have been met.
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Maintenance Overview
Normal Expected Maintenance. Bioretention requires routine maintenance to: remove accumulated materials such as sediment, trash or debris; maintain vegetation health; maintain infiltration capacity of the media layer; replenish mulch; and maintain integrity of side slopes, inlets, energy dissipators, and outlets. A summary table of standard inspection and maintenance indicators is provided within this Fact Sheet.

Non-Standard Maintenance or BMP Failure. If any of the following scenarios are observed, the BMP is not performing as intended to protect downstream waterways from pollution and/or erosion.

Corrective maintenance, increased inspection and maintenance, BMP replacement, or a different BMP type will be required.
• The BMP is not drained between storm events. Surface ponding longer than approximately 24 hours following a storm event may be detrimental to vegetation health, and surface ponding longer than approximately 96 hours following a storm event poses a risk of vector (mosquito) breeding. Poor drainage can result from clogging of the media layer, filter course, aggregate storage layer, underlying native soils, or outlet structure. The specific cause of the drainage issue must be determined and corrected. If it is determined that the underlying native soils have been compacted or do not have the infiltration capacity expected, the [City Engineer] shall be contacted prior to any additional repairs or reconstruction. • Sediment, trash, or debris accumulation greater than 25% of the surface ponding volume within one month. This means the load from the tributary drainage area is too high, reducing BMP function or clogging the BMP. This would require pretreatment measures within the tributary area draining to the BMP to intercept the materials. Pretreatment components, especially for sediment, will extend the life of components that are more expensive to replace such as media, filter course, and aggregate layers.
• Erosion due to concentrated storm water runoff flow that is not readily corrected by adding erosion control blankets, adding stone at flow entry points, or minor re-grading to restore proper drainage according to the original plan. If the issue is not corrected by restoring the BMP to the original plan and grade, the [City Engineer] shall be contacted prior to any additional repairs or reconstruction.

Other Special Considerations. Bioretention is a vegetated structural BMP. Vegetated structural BMPs that are constructed in the vicinity of, or connected to, an existing jurisdictional water or wetland could inadvertently result in creation of expanded waters or wetlands. As such, vegetated structural BMPs have the potential to come under the jurisdiction of the United States Army Corps of Engineers, SDRWQCB, California Department of Fish and Wildlife, or the United States Fish and Wildlife Service. This could result in the need for specific resource agency permits and costly mitigation to perform maintenance of the structural BMP. Along with proper placement of a structural BMP, routine maintenance is key to preventing this scenario. 2026-08-19 Item #12D Page 358 of 816
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Summary of Standard Inspection and Maintenance
The property owner is responsible to ensure inspection, operation and maintenance of permanent BMPs on their property unless responsibility has been formally transferred to an agency, community facilities district, homeowners association, property owners association, or other special district. Maintenance frequencies listed in this table are average/typical frequencies. Actual maintenance needs are site-specific, and maintenance may be required more frequently. Maintenance must be performed whenever needed, based on maintenance indicators presented in this table.

The BMP owner is responsible for conducting regular inspections to see when maintenance is needed based on the maintenance indicators.

Threshold/Indicator Maintenance Action Typical Maintenance Frequency Accumulation of sediment, litter,
or debris
Remove and properly dispose of accumulated materials, without damage to the vegetation or compaction of the media layer. • Inspect monthly. If the BMP is 25% full*
or more in one month, increase inspection frequency to monthly plus after every 0.1-inch or larger storm event. •Remove any accumulated materials found
at each inspection.

Obstructed inlet or outlet structure
Clear blockage. • Inspect monthly and after every 0.5-inch or larger storm event.
•Remove any accumulated materials found
at each inspection.

Damage to structural components such as weirs, inlet or outlet structures Repair or replace as applicable. • Inspect annually.
• Maintain when needed.
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Threshold/Indicator Maintenance Action Typical Maintenance Frequency Poor vegetation establishment Re-seed, re-plant, or re-establish vegetation per original plans.
• Inspect monthly.
• Maintain when needed.

Dead or diseased vegetation Remove dead or diseased vegetation, re-seed, replant, or re-establish vegetation per original plans. • Inspect monthly. • Maintain when needed. Overgrown vegetation Mow or trim as appropriate. • Inspect monthly. • Maintain when needed.
2/3 of mulch has decomposed, or mulch has been removed
Remove decomposed fraction and top off with fresh mulch to a total depth of 3 inches. • Inspect monthly.
•Replenish mulch annually, or more
frequently when needed based on inspection.

Erosion due to concentrated irrigation flow
Repair/re-seed/re-plant eroded areas and adjust the irrigation system. • Inspect monthly.
• Maintain when needed.

Erosion due to concentrated storm water runoff flow
Repair/re-seed/re-plant eroded areas, and make appropriate corrective measures such as adding erosion control blankets, adding stone at flow entry points, or minor re-grading to restore proper drainage according to the original plan. If the issue is not corrected by restoring the BMP to the original plan and grade, the [City Engineer] shall be contacted prior to any additional repairs or reconstruction. • Inspect after every 0.5-inch or larger storm
event. If erosion due to storm water flow has been observed, increase inspection frequency to after every 0.1-inch or larger storm event. • Maintain when needed. If the issue is not
corrected by restoring the BMP to the original plan and grade, the [City Engineer] shall be contacted prior to any additional repairs or reconstruction. 2026-08-19 Item #12D Page 360 of 816
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Threshold/Indicator Maintenance Action Typical Maintenance Frequency Standing water in BMP for longer than 24 hours following a storm event Surface ponding longer than approximately 24 hours following a storm event may be detrimental to vegetation health Make appropriate corrective measures such as adjusting irrigation system, removing obstructions of debris or invasive vegetation, or repairing/replacing clogged or compacted soils. If it is determined that the underlying native soils have been compacted or do not have the infiltration capacity expected, the [City Engineer] shall be contacted prior to any additional repairs or reconstruction. • Inspect monthly and after every 0.5-inch
or larger storm event. If standing water is observed, increase inspection frequency to after every 0.1-inch or larger storm event. • Maintain when needed.

A different BMP type, or a Vector Management Plan
prepared with concurrence from the County of San Diego Department of Environmental Health, may be required. • Inspect monthly and after every 0.5-inch
or larger storm event. If mosquitos are observed, increase inspection frequency to after every 0.1-inch or larger storm event. • Maintain when needed.
“25% full” is defined as ¼ of the depth from the design bottom elevation to the crest of the outflow structure (e.g., if the height to the outflow opening is 12 inches from the bottom elevation, then the materials must be removed when there is 3 inches of accumulation – this should be marked on the outflow structure). 2026-08-19 Item #12D Page 361 of 816 Appendix E: BMP Design Fact Sheets
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E.16 INF-3 Permeable Pavement (Pollutant
Control)
Location: Kellogg Park, San Diego, California
Description
Permeable pavement is pavement that allows for percolation through void spaces in the pavement surface into subsurface layers. The subsurface layers are designed to provide storage of stormwater runoff so that outflows, primarily via infiltration into subgrade soils or release to the downstream conveyance system, can be at controlled rates. Varying levels of stormwater treatment and flow control can be provided depending on the size of the permeable pavement system relative to its drainage area, the underlying infiltration rates, and the configuration of outflow controls. Pollutant control permeable pavement is designed to receive runoff from a larger tributary area than site design permeable pavement (see SD-6B). Pollutant control is provided via infiltration, filtration, sorption, sedimentation, and biodegradation processes.

Typical permeable pavement components include, from top to bottom:
• Permeable surface layer
• Bedding layer for permeable surface
• Aggregate storage layer with optional underdrain(s)
• Optional final filter course layer over uncompacted existing subgrade.

MS4 Permit Category
Retention
Flow-thru Treatment
Control
Manual Category
Infiltration
Flow-thru Treatment
Control
Applicable Performance
Standard
Pollutant Control
Flow Control

Primary Benefits
Volume Reduction
Peak Flow Attenuation
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Typical plan and Section view of a Permeable Pavement BMP Subcategories of permeable pavement include modular paver units or paver blocks, pervious concrete, porous asphalt, and turf pavers. These subcategory variations differ in the material used for the 2026-08-19 Item #12D Page 363 of 816
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permeable surface layer but have similar functions and characteristics below this layer.

Design Adaptations for Project Goals
Site design BMP to reduce impervious area and DCV. See site design option SD-6B.

Full infiltration BMP for stormwater pollutant control. Permeable pavement without an underdrain and without impermeable liners can be used as a pollutant control BMP, designed to infiltrate runoff from direct rainfall as well as runoff from adjacent areas that are tributary to the pavement. The system must be designed with an infiltration storage volume (a function of the aggregate storage volume) equal to the full DCV and able to meet drawdown time limitations.

Partial infiltration BMP with flow-thru treatment for stormwater pollutant control. Permeable pavement can be designed so that a portion of the DCV is infiltrated by providing an underdrain with infiltration storage below the underdrain invert. The infiltration storage depth should be determined by the volume that can be reliably infiltrated within drawdown time limitations. Water discharged through the underdrain is considered flow-thru treatment and is not considered biofiltration treatment. Storage provided above the underdrain invert is included in the flow-thru treatment volume.

Flow-thru treatment BMP for stormwater pollutant control. The system may be lined and/or installed over impermeable native soils with an underdrain provided at the bottom to carry away filtered runoff. Water quality treatment is provided via unit treatment processes other than infiltration.

This configuration is considered to provide flow-thru treatment, not biofiltration treatment.

Significant aggregate storage provided above the underdrain invert can provide detention storage, which can be controlled via inclusion of an orifice in an outlet structure at the downstream end of the underdrain. PDPs have the option to add saturated storage to the flow-thru configuration in order to reduce the DCV that the BMP is required to treat. Saturated storage can be added to this design by including an upturned elbow installed at the downstream end of the underdrain or via an internal weir structure designed to maintain a specific water level elevation. The DCV can be reduced by the amount of saturated storage provided.

Integrated stormwater flow control and pollutant control configuration. With any of the above configurations, the system can be designed to provide flow rate and duration control. This may include having a deeper aggregate storage layer that allows for significant detention storage above the underdrain, which can be further controlled via inclusion of an outlet structure at the downstream end of the underdrain.
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Recommended Siting Criteria
Siting Criteria Intent/Rationale
□
Placement observes geotechnical
recommendations regarding potential hazards
(e.g., slope stability, landslides, liquefaction
zones) and setbacks (e.g., slopes, foundations,
utilities).

Must not negatively impact existing site
geotechnical concerns.
□
Selection must be based on infiltration
feasibility criteria.

Full or partial infiltration designs must be
supported by drainage area feasibility
findings.
□
Permeable pavement is not placed in an area
with significant overhanging trees or other
vegetation.

Leaves and organic debris can clog the
pavement surface.
□
Minimum depth to groundwater and bedrock
≥ 10 ft.

A minimum separation facilitates
infiltration and lessens the risk of
negative groundwater impacts.
□
Contributing tributary area includes effective
sediment source control and/or pretreatment
measures such as raised curbed or grass filter
strips.

Sediment can clog the pavement surface.
□
Direct discharges to permeable pavement are
only from downspouts carrying “clean” roof
runoff that are equipped with filters to remove
gross solids.

Roof runoff typically carries less
sediment than runoff from other
impervious surfaces and is less likely to
clog the pavement surface.

Recommended BMP Component Dimensions
BMP Component Dimension Intent/Rationale
Bedding Layer 1-2 inches (typical)
Bedding (e.g., sand, aggregate)
provided to stabilize and level the
surface.

Aggregate Storage ≥ 6 inches A minimum depth of aggregate provides structural stability for
expected pavement loads.

Underdrain Diameter ≥ 6 inches Smaller diameter underdrains are prone to clogging.
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Design Criteria and Considerations
Permeable pavements must meet the following design criteria. Deviations from the below criteria may be approved at the discretion of the [City Engineer] if it is determined to be appropriate:

Design Criteria Intent/Rationale
□
An impermeable liner or other hydraulic
restriction layer is included if site constraints
indicate that infiltration should not be allowed.

Lining prevents storm water from
impacting groundwater and/or sensitive
environmental or geotechnical features.

Incidental infiltration, when allowable,
can aid in pollutant removal and
groundwater recharge.
□
For pollutant control permeable pavement, the
ratio of the total drainage area (including the
permeable pavement) to the permeable
pavement should not exceed 4:1.

Higher ratios increase the potential for
clogging but may be acceptable for
relatively clean tributary areas.
□
Finish grade of the permeable pavement has a
slope ≤ 5%.

Flatter surfaces facilitate increased runoff
capture.

Permeable Surface Layer
□
Permeable surface layer type is appropriately
chosen based on pavement use and expected
vehicular loading.

Pavement may wear more quickly if not
durable for expected loads or
frequencies.
□
Permeable surface layer type is appropriate for
expected pedestrian traffic.

Expected demographic and accessibility
needs (e.g., adults, children, seniors,
runners, high-heeled shoes, wheelchairs,
strollers, bikes) requires selection of
appropriate surface layer type that will
not impede pedestrian needs.

Bedding Layer for Permeable Surface
□
Bedding thickness and material is appropriate
for the chosen permeable surface layer type.

Porous asphalt requires a 2- to 4-inch
layer of asphalt and a 1- to 2-inch layer
of choker course (single-sized crushed
aggregate, one-half inch) to stabilize the
surface.

Pervious concrete also requires an
aggregate course of clean gravel or
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Design Criteria Intent/Rationale
crushed stone with a minimum amount
of fines.

Permeable Interlocking Concrete Paver
requires 1 or 2 inches of sand or No. 8
aggregate to allow for leveling of the
paver blocks.

Similar to Permeable Interlocking
Concrete Paver, plastic grid systems also
require a 1- to 2-inch bedding course of
either gravel or sand.

For Permeable Interlocking Concrete
Paver and plastic grid systems, if sand is
used, a geotextile should be used
between the sand course and the
reservoir media to prevent the sand from
migrating into the stone media.
□
Aggregate used for bedding layer is washed
prior to placement.

Washing aggregate will help eliminate
fines that could clog the permeable
pavement system aggregate storage layer
void spaces or underdrain.

Media Layer (Optional) –used between bedding layer and aggregate storage layer to provide pollutant treatment control
□
The pollutant removal performance of the
media layer is documented by the applicant.

Media used for BMP design should be
shown via research or testing to be
appropriate for expected pollutants of
concern and flow rates.
□
A filter course is provided to separate the
media layer from the aggregate storage layer.

Migration of media can cause clogging of
the aggregate storage layer void spaces or
underdrain.
□
If a filter course is used, calculations assessing
suitability for particle migration prevention
have been completed.

Gradation relationship between layers
can evaluate factors (e.g., bridging,
permeability, and uniformity) to
determine if particle sizing is appropriate
or if an intermediate layer is needed.
□
Consult permeable pavement manufacturer to
verify that media layer provides required
structural support.

Media must not compromise the
structural integrity or intended uses of
the permeable pavement surface.
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Design Criteria Intent/Rationale
Aggregate Storage Layer
□
Aggregate used for the aggregate storage layer
is washed and free of fines.

Washing aggregate will help eliminate
fines that could clog aggregate storage
layer void spaces or underdrain.
□
Minimum layer depth is 6 inches and for
infiltration designs, the maximum depth is
determined based on the infiltration storage
volume that will infiltrate within a 36-hour
drawdown time.

A minimum depth of aggregate provides
structural stability for expected pavement
loads.

Underdrain and Outflow Structures
□
Underdrains and outflow structures, if used,
are accessible for inspection and maintenance.

Maintenance will improve the
performance and extend the life of the
permeable pavement system.
□
Underdrain outlet elevation should be a
minimum of 3 inches above the bottom
elevation of the aggregate storage layer.

A minimal separation from subgrade or
the liner lessens the risk of fines entering
the underdrain and can improve
hydraulic performance by allowing
perforations to remain unblocked.
□ Minimum underdrain diameter is 6 inches. Smaller diameter underdrains are prone to clogging.
□
Underdrains are made of slotted, PVC pipe
conforming to ASTM D 3034 or equivalent or
corrugated, HDPE pipe conforming to
AASHTO 252M or equivalent.

Slotted underdrains provide greater
intake capacity, clog resistant drainage,
and reduced entrance velocity into the
pipe, thereby reducing the chances of
solids migration.

Filter Course (Optional)
□ Filter course is washed and free of fines.

Washing aggregate will help eliminate
fines that could clog subgrade and
impede infiltration.

Conceptual Design and Sizing Approach for Site Design

1. Determine the areas where permeable pavement can be used in the site design to replace traditional pavement to reduce the impervious area and DCV. These permeable pavement areas can be credited toward reducing runoff generated through representation in stormwater calculations as pervious, not impervious, areas but are not credited for stormwater pollutant control. These permeable pavement areas should be designed as selfretaining with the appropriate tributary area ratio identified in the design criteria. 2026-08-19 Item #12D Page 368 of 816
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2. Calculate the DCV per Appendix B, taking into account reduced runoff from self-retaining permeable pavement areas.

Conceptual Design and Sizing Approach for Stormwater Pollutant Control Only To design permeable pavement for stormwater pollutant control only (no flow control required), the following steps should be taken:

1. Verify that siting and design criteria have been met, including placement requirements, maximum finish grade slope, and the recommended tributary area ratio for non-selfretaining permeable pavement. If infiltration is infeasible, the permeable pavement can be designed as flow-thru treatment per the sizing worksheet. If infiltration is feasible, calculations should follow the remaining design steps.

2. Calculate the DCV per Appendix B based on expected site design runoff for tributary areas.

3. Use the sizing worksheet to determine if full or partial infiltration of the DCV is achievable based on the available infiltration storage volume calculated from the permeable pavement footprint, aggregate storage layer depth, and in-situ soil design infiltration rate for a maximum 36-hour drawdown time. The applicant has an option to use a different drawdown time up to 96 hours if the volume of the facility is adjusted using the percent capture method in Appendix B.4.2.

4. Where the DCV cannot be fully infiltrated based on the site or permeable pavement constraints, an underdrain must be incorporated above the infiltration storage to carry away runoff that exceeds the infiltration storage capacity.

5. The remaining DCV to be treated should be calculated for use in sizing downstream BMP(s).

Conceptual Design and Sizing Approach when Stormwater Flow Control is Applicable Control of flow rates and/or durations will typically require significant aggregate storage volumes, and therefore the following steps should be taken prior to determination of stormwater pollutant control design. Pre-development and allowable post-project flow rates and durations should be determined as discussed in Chapter 6 of the manual.

1. Verify that siting and design criteria have been met, including placement requirements, maximum finish grade slope, and the recommended tributary area ratio for non-selfretaining permeable pavement. Design for flow control can be achieving using various design configurations, but a flow-thru treatment design will typically require a greater aggregate storage layer volume than designs which allow for full or partial infiltration of the DCV.

2. Iteratively determine the area and aggregate storage layer depth required to provide 2026-08-19 Item #12D Page 369 of 816
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infiltration and/or detention storage to reduce flow rates and durations to allowable limits. Flow rates and durations can be controlled from detention storage by altering outlet structure orifice size(s) and/or water control levels. MultJ-level orifices can be used within an outlet structure to control the full range of flows.

3. If the permeable pavement system cannot fully provide the flow rate and duration control required by this manual, a downstream structure with sufficient storage volume such as an underground vault can be used to provide remaining controls.

4. After permeable pavement has been designed to meet flow control requirements, calculations must be completed to verify if stormwater pollutant control requirements to treat the DCV have been met.

5. Maintenance Overview

6. Normal Expected Maintenance. Routine maintenance of permeable pavement includes:
removal of materials such as trash and debris accumulated on the paving surface; vacuuming of the paving surface to prevent clogging; and flushing paving and subsurface gravel to remove fine sediment. If the BMP includes underdrains and/or an outflow control structure, check and clear these features.

7. Non-Standard Maintenance or BMP Failure. If the permeable pavement area is not drained between storm events, or if runoff sheet flows across the permeable pavement area and flows off the permeable pavement area during storm events, the BMP is not performing as intended to protect downstream waterways from pollution and/or erosion. During storm events up to the 85th percentile storm event (approximately 0.5 to 1 inch of rainfall in San Diego County), runoff should not flow off the permeable pavement area. The permeable pavement area is expected to have adequate hydraulic conductivity and storage such that rainfall landing on the permeable pavement and runoff from the surrounding drainage area will go directly into the pavement without ponding or overflow (in properly designed systems, the surrounding drainage area is not more than half as large as the permeable pavement area). Following the storm event, there should be no standing water (puddles) on the permeable pavement area.

8. If storm water is flowing off the permeable pavement during a storm event, or if there is standing water on the permeable pavement surface following a storm event, this is an indicator of clogging somewhere within the system. Poor drainage can result from clogging of the permeable surface layer, any of the subsurface components, or the subgrade soils.

The specific cause of the drainage issue must be determined and corrected. Surface or subsurface ponding longer than approximately 96 hours following a storm event poses a risk of vector (mosquito) breeding. Corrective maintenance, increased inspection and maintenance, BMP replacement, or a different BMP type will be required. If poor drainage persists after flushing of the paving, subsurface gravel, and/or underdrain(s) when 2026-08-19 Item #12D Page 370 of 816
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applicable, or if it is determined that the underlying soils do not have the infiltration capacity expected, the [City Engineer] shall be contacted prior to any additional repairs or reconstruction.

9. Other Special Considerations. The runoff storage and infiltration surface area in this BMP are not readily accessible because they are subsurface. This means that clogging and poor drainage are not easily corrected. If the tributary area draining to the BMP includes unpaved areas, the sediment load from the tributary drainage area can be too high, reducing BMP function or clogging the BMP. All unpaved areas within the tributary drainage area should be stabilized with vegetation. Other pretreatment components to prevent transport of sediment to the paving surface, such as grass buffer strips, will extend the life of the subsurface components and infiltration surface. Along with proper stabilization measures and pretreatment within the tributary area, routine maintenance, including preventive vacuum/regenerative air street sweeping, is key to preventing clogging. 2026-08-19 Item #12D Page 371 of 816
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Summary of Standard Inspection and Maintenance
The property owner is responsible to ensure inspection, operation and maintenance of permanent BMPs on their property unless responsibility has been formally transferred to an agency, community facilities district, homeowners association, property owners association, or other special district. Maintenance frequencies listed in this table are average/typical frequencies. Actual maintenance needs are site-specific, and maintenance may be required more frequently. Maintenance must be performed whenever needed, based on maintenance indicators presented in this table.

The BMP owner is responsible for conducting regular inspections to see when maintenance is needed based on the maintenance indicators.

Accumulation of sediment, litter, or debris
on permeable pavement surface
Remove and properly dispose of accumulated materials. Inspect tributary area for exposed soil or other sources of sediment and apply stabilization measures to sediment source areas. Apply source control measures as applicable to sources of litter or debris. • Inspect monthly and after every 0.5-inch
or larger storm event.
•Remove any accumulated materials found
at each inspection.
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Threshold/Indicator Maintenance Action Typical Maintenance Frequency Weeds growing on/through the permeable
pavement surface
Remove weeds and add features as necessary to prevent weed intrusion. Use non-chemical methods (e.g., instead of pesticides, control weeds using mechanical removal, physical barriers, and/or physical changes in the surrounding area adjacent to pavement that will preclude weed intrusion into the pavement). • Inspect monthly.
•Remove any weeds found at each
inspection.

Standing water in permeable paving area or subsurface infiltration gallery for longer than 24-96 hours following a storm event This condition requires investigation of why
infiltration is not occurring. If feasible, corrective action shall be taken to restore infiltration (e.g., pavement should be swept with a vacuum power or regenerative air
street sweeper to restore infiltration rates, clear underdrains if underdrains are present). BMP may require retrofit if infiltration cannot be restored. The [City Engineer] shall be contacted prior to any repairs or reconstruction. • Inspect monthly and after every 0.5-inch
or larger storm event. If standing water is observed, increase inspection frequency to after every 0.1-inch or larger storm event. • Maintain when needed.
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Threshold/Indicator Maintenance Action Typical Maintenance Frequency Presence of mosquitos/larvae For images of egg rafts, larva, pupa, and adult mosquitos, see http://www.mosquito.org/biology If mosquitos/larvae are observed: first, immediately remove any standing water by dispersing to nearby landscaping; second, make corrective measures as applicable to restore BMP drainage to prevent standing water.

If mosquitos persist following corrective measures to remove standing water, or if the BMP design does not meet the 96-hour drawdown criteria because the underlying native soils have been compacted or do not have the infiltration capacity expected, the [City Engineer] shall be contacted to determine a solution. A different BMP type, or a Vector Management Plan prepared with concurrence from the County of San Diego Department of Environmental Health, may be required. • Inspect monthly and after every 0.5-inch or
larger storm event. If mosquitos are observed, increase inspection frequency to after every 0.1-inch or larger storm event. • Maintain when needed.

Obstructed underdrain or outlet structure (when the BMP includes outflow control structure for runoff released from subsurface storage via underdrain(s)) Clear blockage. • Inspect if standing water is observed for longer than 24-96 hours following a storm event.
• Maintain when needed.

Damage to structural components of subsurface infiltration gallery such as weirs or outlet structures Repair or replace as applicable. • Inspect annually.
• Maintain when needed.
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Threshold/Indicator Maintenance Action Typical Maintenance Frequency Damage to permeable paving surface (e.g., cracks, settlement, misaligned paver blocks, void spaces between paver blocks need fill
materials replenished)
Repair or replace damaged surface as appropriate.
• Inspect annually.
• Maintain when needed.
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E.17 INF-4 Dry Wells
Source: Cal/EPA Fact Sheet on Dry Wells
Description
A dry well typically consists of a gravity-fed pit, deeper than it is long or wide, lined with perforated casing and often backfilled with gravel or stone. Dry wells penetrate layers of poorly infiltrating soils, such as clays, allowing infiltration into deeper permeable layers. Dry wells reduce storm water runoff while increasing groundwater recharge. Dry wells require pretreatment. Pretreatment effectiveness is contingent upon proper maintenance pretreatment devices.

Criteria for Use of a Dry Well as an Infiltration BMP
A dry well may be acceptable as an “infiltration BMP” if it meets ALL the following criteria:
• The BMP meets the minimum geotechnical and groundwater investigation requirements listed in Appendix C; and
• The BMP is evaluated by approved infiltration rate assessment methods presented in Appendix D; and
• Implements an appropriate pretreatment BMP (refer to Appendix B.6.2 for selection); and • Dry wells serving lots other than single-family homes are registered with the US EPA (additional information and registration forms can be found at: https://www.epa.gov/uic).

MS4 Permit Category
Retention
BMP Manual Category
Infiltration
Applicable Performance
Standard
Pollutant Control
Flow Control
Primary Benefits
Volume Reduction
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E.18 PR-1 Biofiltration with Partial Retention
Location: 805 and Bonita Road, Chula vista, CA.

Description
Biofiltration with partial retention (partial infiltration and biofiltration) facilities are vegetated surface water systems that filter water through vegetation, and soil or engineered media prior to infiltrating into native soils, discharge via underdrain, or overflow to the downstream conveyance system. Where feasible, these BMPs have an elevated underdrain discharge point that creates storage capacity in the aggregate storage layer. Biofiltration with partial retention facilities are commonly incorporated into the site within parking lot landscaping, along roadsides, and in open spaces. They can be constructed in ground or partially aboveground, such as planter boxes with open bottoms to allow infiltration.

Treatment is achieved through filtration, sedimentation, sorption, infiltration, biochemical processes and plant uptake.

Typical biofiltration with partial retention components include:
• Inflow distribution mechanisms (e.g, perimeter flow spreader or filter strips) • Energy dissipation mechanism for concentrated inflows (e.g., splash blocks or riprap) • Shallow surface ponding for captured flows
• Side Slope and basin bottom vegetation selected based on climate and ponding depth • Non-floating mulch layer (Optional)
• Media layer (planting mix or engineered media) capable of supporting vegetation growth • Filter course layer consisting of aggregate to prevent the migration of fines into uncompacted native soils or the optional aggregate storage layer • Aggregate storage layer with underdrain(s)
• Uncompacted native soils at the bottom of the facility • Overflow structure
MS4 Permit Category
NA
Manual Category
Partial Retention
Applicable Performance
Standard
Pollutant Control
Flow Control
Primary Benefits
Volume Reduction
Treatment
Peak Flow Attenuation
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Typical plan and Section view of a Biofiltration with Partial Retention BMP 2026-08-19 Item #12D Page 378 of 816
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Design Adaptations for Project Goals
Partial infiltration BMP with biofiltration treatment for stormwater pollutant control.

Biofiltration with partial retention can be designed so that a portion of the DCV is infiltrated by providing infiltration storage below the underdrain invert. The infiltration storage depth should be determined by the volume that can be reliably infiltrated within drawdown time limitations. Water discharged through the underdrain is considered biofiltration treatment. Storage provided above the underdrain within surface ponding, media, and aggregate storage is included in the biofiltration treatment volume.

Integrated stormwater flow control and pollutant control configuration. The system can be designed to provide flow rate and duration control by primarily providing increased surface ponding and/or having a deeper aggregate storage layer. This will allow for significant detention storage, which can be controlled via inclusion of an orifice in an outlet structure at the downstream end of the underdrain.

Must operate as a partial infiltration design and
must be supported by drainage area and in-situ
infiltration rate feasibility findings.
□
Contributing tributary area shall be ≤ 5
acres (≤ 1 acre preferred).

Bigger BMPs require additional design features
for proper performance.

Contributing tributary area greater than 5 acres
may be allowed at the discretion of the [City
Engineer} if the following conditions are met:
1) incorporate design features (e.g. flow
spreaders) to minimizing short circuiting of
flows in the BMP and 2) incorporate
additional design features requested by the
[City Engineer] for proper performance of the
regional BMP.
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Siting Criteria Intent/Rationale
□ Finish grade of the facility is ≤ 2%. Flatter surfaces reduce erosion and channelization within the facility.

Recommended BMP Component Dimensions
BMP Component Dimension Intent/Rationale
Freeboard ≥ 2 inches
Freeboard provides room for head
over overflow structures and
minimizes risk of uncontrolled
surface discharge.

Surface Ponding ≥ 6 and ≤ 12 inches
Surface ponding capacity lowers
subsurface storage requirements.

Deep surface ponding raises safety
concerns.

Surface ponding depth greater than
12 inches (for additional pollutant
control or surface outlet structures
or flow-control orifices) may be
allowed at the discretion of the
[City Engineer] if the following
conditions are met: 1) surface
ponding depth drawdown time is
less than 24 hours; and 2) safety
issues and fencing requirements are
considered (typically ponding
greater than 18” will require a fence
and/or flatter side slopes) and 3)
potential for elevated clogging risk
is considered.

Ponding Area Side Slopes 3H:1V or shallower
Gentler side slopes are safer, less
prone to erosion, able to establish
vegetation more quickly and easier
to maintain.

Mulch ≥ 3 inches
Mulch will suppress weeds and
maintain moisture for plant growth.

Aging mulch kills pathogens and
weed seeds and allows the
beneficial microbes to multiply.
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BMP Component Dimension Intent/Rationale
Media Layer ≥ 18 inches
A deep media layer provides
additional filtration and supports
plants with deeper roots.

Standard specifications shall be
followed.

For non-standard or proprietary
designs, compliance with Appendix
F.1 ensures that adequate treatment
performance will be provided.

Underdrain Diameter ≥ 6 inches Smaller diameter underdrains are prone to clogging.

Cleanout Diameter ≥ 6 inches Properly spaced cleanouts will facilitate underdrain maintenance.

Design Criteria and Considerations
Biofiltration with partial retention must meet the following design criteria and considerations.

Deviations from the below criteria may be approved at the discretion of the [City Engineer] if it is determined to be appropriate:

Design Criteria Intent/Rationale
Surface Ponding
□
Surface ponding is limited to a 24-hour
drawdown time.

Vegetation
□
Plantings are suitable for the climate and
expected ponding depth. A plant list to aid in
selection can be found in Appendix E.26
Plants suited to the climate and ponding
depth are more likely to survive.
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Design Criteria Intent/Rationale
□
An irrigation system with a connection to
water supply should be provided as needed.

Seasonal irrigation might be needed to
keep plants healthy.

Mulch (Optional or Mandatory – Dependent on jurisdiction) □
A minimum of 3 inches of well-aged, shredded
hardwood mulch that has been stockpiled or
stored for at least 12 months is provided.

Mulch must be non-floating to avoid clogging
of overflow structure.

Mulch will suppress weeds and maintain
moisture for plant growth. Aging mulch
kills pathogens and weed seeds and
allows the beneficial microbes to
multiply.

Media Layer
□
Media maintains a minimum filtration rate of 5
in/hr over lifetime of facility. An initial
filtration rate of 8 to 12 in/hr is recommended
to allow for clogging over time; the initial
filtration rate should not exceed 12 inches per
hour.

A filtration rate of at least 5 inches per
hour allows soil to drain between events,
and allows flows to relatively quickly
enter the aggregate storage layer, thereby
minimizing bypass. The initial rate
should be higher than long term target
rate to account for clogging over time.

However an excessively high initial rate
can have a negative impact on treatment
performance, therefore an upper limit is
needed.
□
Media is a minimum 18 inches deep, meeting
either of these two media specifications:

Section F.3 Bioretention Soil Media (BSM) or
specific jurisdictional guidance.

Alternatively, for proprietary designs and
custom media mixes not meeting the media
specifications, the media meets the pollutant
treatment performance criteria in Section F.1.

A deep media layer provides additional
filtration and supports plants with deeper
roots.

Standard specifications shall be followed.

For non-standard or proprietary designs,
compliance with Appendix F.1 ensures
that adequate treatment performance will
be provided.
□
Media surface area is 3% of contributing area
times adjusted runoff factor or greater. Unless
demonstrated that the BMP surface area can
be smaller than 3%.

Greater surface area to tributary area
ratios: a) maximizes volume retention as
required by the MS4 Permit and
b) decrease loading rates per square foot
and therefore increase longevity.

Adjusted runoff factor is to account for
site design BMPs implemented upstream
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Design Criteria Intent/Rationale
of the BMP (such as rain barrels,
impervious area dispersion, etc.). Refer
to Appendix B.2 guidance.
□
Where receiving waters are impaired or have a
TMDL for nutrients, the system is designed
with nutrient sensitive media design (see fact
sheet BF-2).

Potential for pollutant export is partly a
function of media composition; media
design must minimize potential for
export of nutrients, particularly where
receiving waters are impaired for
nutrients.

Filter Course Layer
□
A filter course is used to prevent migration of
fines through layers of the facility. Filter fabric
is not used.

Migration of media can cause clogging of
the aggregate storage layer void spaces or
subgrade. Filter fabric is more likely to
clog.
□ Filter course is washed and free of fines. Washing aggregate will help eliminate fines that could clog the facility
□
Filter course calculations assessing suitability
for particle migration prevention have been
completed.

Gradation relationship between layers
can evaluate factors (e.g., bridging,
permeability, and uniformity) to
determine if particle sizing is appropriate
or if an intermediate layer is needed.

Aggregate Storage Layer
□
Class 2 Permeable per Caltrans specification
68-1.025 is recommended for the storage layer.

Washed, open-graded crushed rock may be
used, however a 4-6 inch washed pea gravel
filter course layer at the top of the crushed
rock is required.

Washing aggregate will help eliminate
fines that could clog the aggregate
storage layer void spaces or subgrade.
□
Maximum aggregate storage layer depth below
the underdrain invert is determined based on
the infiltration storage volume that will
infiltrate within a 36-hour drawdown time.

A maximum drawdown time is needed
for vector control and to facilitate
providing storm water storage for the
next storm event.

Inflow, Underdrain, and Outflow Structures
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Design Criteria Intent/Rationale
□
Inflow, underdrains and outflow structures are
accessible for inspection and maintenance.

Maintenance will prevent clogging and
ensure proper operation of the flow
control structures.
□
Inflow velocities are limited to 3 ft/s or less or
use energy dissipation methods. (e.g., riprap,
level spreader) for concentrated inflows.

High inflow velocities can cause erosion,
scour and/or channeling.
□
Curb cut inlets are at least 12 inches wide, have
a 4-6 inch reveal (drop) and an apron and
energy dissipation as needed.

Inlets must not restrict flow and apron
prevents blockage from vegetation as it
grows in. Energy dissipation prevents
erosion.
□
Underdrain outlet elevation should be a
minimum of 3 inches above the bottom
elevation of the aggregate storage layer.

Slotted underdrains provide greater
intake capacity, clog resistant drainage,
and reduced entrance velocity into the
pipe, thereby reducing the chances of
solids migration.
□
An underdrain cleanout with a minimum 6-
inch diameter and lockable cap is placed every
250 to 300 feet as required based on
underdrain length.

Properly spaced cleanouts will facilitate
underdrain maintenance.
□
Overflow is safely conveyed to a downstream
storm drain system or discharge point. Size
overflow structure to pass 100-year peak flow
for on-line infiltration basins and water quality
peak flow for off-line basins.

Planning for overflow lessens the risk of
property damage due to flooding.

Nutrient Sensitive Media Design
To design biofiltration with partial retention with underdrain for stormwater pollutant control only (no flow control required), the following steps should be taken:
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Conceptual Design and Sizing Approach for Stormwater Pollutant Control Only To design biofiltration with partial retention and an underdrain for stormwater pollutant control only (no flow control required), the following steps should be taken:

1. Verify that siting and design criteria have been met, including placement requirements, contributing tributary area, maximum side and finish grade slopes, and the recommended media surface area tributary ratio.

2. Calculate the DCV per Appendix B based on expected site design runoff for tributary areas.

3. Generalized sizing procedure is presented in Appendix B.5. The surface ponding should be verified to have a maximum 24-hour drawdown time.

Conceptual Design and Sizing Approach when Stormwater Flow Control is Applicable Control of flow rates and/or durations will typically require significant surface ponding and/or aggregate storage volumes, and therefore the following steps should be taken prior to determination of stormwater pollutant control design. Pre-development and allowable post-project flow rates and durations should be determined as discussed in Chapter 6 of the manual.

2. Iteratively determine the facility footprint area, surface ponding and/or aggregate storage layer depth required to provide detention and/or infiltration storage to reduce flow rates and durations to allowable limits. Flow rates and durations can be controlled from detention storage by altering outlet structure orifice size(s) and/or water control levels.

Multi

3. -level orifices can be used within an outlet structure to control the full range of flows.

4. If biofiltration with partial retention cannot fully provide the flow rate and duration control required by this manual, an upstream or downstream structure with significant storage volume such as an underground vault can be used to provide remaining controls.

5. After biofiltration with partial retention has been designed to meet flow control requirements, calculations must be completed to verify if stormwater pollutant control requirements to treat the DCV have been met.

Maintenance Overview
Normal Expected Maintenance. Biofiltration with partial retention requires routine maintenance 2026-08-19 Item #12D Page 385 of 816
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to: remove accumulated materials such as sediment, trash or debris; maintain vegetation health; maintain infiltration capacity of the media layer; replenish mulch; and maintain integrity of side slopes, inlets, energy dissipators, and outlets. A summary table of standard inspection and maintenance indicators is provided within this Fact Sheet.

Non-Standard Maintenance or BMP Failure. If any of the following scenarios are observed, the BMP is not performing as intended to protect downstream waterways from pollution and/or erosion.

Corrective maintenance, increased inspection and maintenance, BMP replacement, or a different BMP type will be required.
• The BMP is not drained between storm events. Surface ponding longer than approximately 24 hours following a storm event may be detrimental to vegetation health, and surface ponding longer than approximately 96 hours following a storm event poses a risk of vector (mosquito) breeding. Poor drainage can result from clogging of the media layer, filter course, aggregate storage layer, underdrain, or outlet structure. The specific cause of the drainage issue must be determined and corrected.
• Sediment, trash, or debris accumulation greater than 25% of the surface ponding volume within one month. This means the load from the tributary drainage area is too high, reducing BMP function or clogging the BMP. This would require pretreatment measures within the tributary area draining to the BMP to intercept the materials. Pretreatment components, especially for sediment, will extend the life of components that are more expensive to replace such as media, filter course, and aggregate layers.
• Erosion due to concentrated storm water runoff flow that is not readily corrected by adding erosion control blankets, adding stone at flow entry points, or minor re-grading to restore proper drainage according to the original plan. If the issue is not corrected by restoring the BMP to the original plan and grade, the City Engineer shall be contacted prior to any additional repairs or reconstruction.

Other Special Considerations. Biofiltration with partial retention is a vegetated structural BMP.

Vegetated structural BMPs that are constructed in the vicinity of, or connected to, an existing jurisdictional water or wetland could inadvertently result in creation of expanded waters or wetlands.

As such, vegetated structural BMPs have the potential to come under the jurisdiction of the United States Army Corps of Engineers, SDRWQCB, California Department of Fish and Wildlife, or the United States Fish and Wildlife Service. This could result in the need for specific resource agency permits and costly mitigation to perform maintenance of the structural BMP. Along with proper placement of a structural BMP, routine maintenance is key to preventing this scenario. 2026-08-19 Item #12D Page 386 of 816
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Summary of Standard Inspection and Maintenance
The property owner is responsible to ensure inspection, operation and maintenance of permanent BMPs on their property unless responsibility has been formally transferred to an agency, community facilities district, homeowners association, property owners association, or other special district. Maintenance frequencies listed in this table are average/typical frequencies. Actual maintenance needs are site-specific, and maintenance may be required more frequently. Maintenance must be performed whenever needed, based on maintenance indicators presented in this table.

The BMP owner is responsible for conducting regular inspections to see when maintenance is needed based on the maintenance indicators.

Threshold/Indicator Maintenance Action Typical Maintenance Frequency Accumulation of sediment, litter, or debris Remove and properly dispose of accumulated materials, without damage to the vegetation or compaction of the media layer. • Inspect monthly. If the BMP is 25% full*
or more in one month, increase inspection frequency to monthly plus after every 0.1-inch or larger storm event. •Remove any accumulated materials found
at each inspection.

Obstructed inlet or outlet structure Clear blockage. • Inspect monthly and after every 0.5-inch or larger storm event.
•Remove any accumulated materials found
at each inspection.

Damage to structural components such as
weirs, inlet or outlet structures
Repair or replace as applicable. • Inspect annually.
• Maintain when needed.

Poor vegetation establishment Re-seed, re-plant, or re-establish vegetation per original plans.
• Inspect monthly.
• Maintain when needed.
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Threshold/Indicator Maintenance Action Typical Maintenance Frequency Dead or diseased vegetation Remove dead or diseased vegetation, reseed, re-plant, or re-establish vegetation per original plans.
• Inspect monthly.
• Maintain when needed.

Overgrown vegetation Mow or trim as appropriate. • Inspect monthly. • Maintain when needed.
2/3 of mulch has decomposed, or mulch has been removed
Remove decomposed fraction and top off
with fresh mulch to a total depth of 3
inches.
• Inspect monthly.
•Replenish mulch annually, or more
frequently when needed based on inspection.

Erosion due to concentrated irrigation flow Repair/re-seed/re-plant eroded areas and adjust the irrigation system.
• Inspect monthly.
• Maintain when needed.

Standing water in BMP for longer than 24 hours following a storm event Surface ponding longer than approximately 24 hours following a storm event may be detrimental to vegetation health Make appropriate corrective measures such as adjusting irrigation system, removing obstructions of debris or invasive vegetation, clearing underdrains, or
repairing/replacing clogged or compacted soils.
• Inspect monthly and after every 0.5-inch
or larger storm event. If standing water is observed, increase inspection frequency to after every 0.1-inch or larger storm event. • Maintain when needed.
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Threshold/Indicator Maintenance Action Typical Maintenance Frequency Presence of mosquitos/larvae For images of egg rafts, larva, pupa, and adult mosquitos, see http://www.mosquito.org/biology If mosquitos/larvae are observed: first, immediately remove any standing water by dispersing to nearby landscaping; second, make corrective measures as applicable to restore BMP drainage to prevent standing water.

If mosquitos persist following corrective measures to remove standing water, or if the BMP design does not meet the 96-hour drawdown criteria due to release rates controlled by an orifice installed on the underdrain, the [City Engineer] shall be contacted to determine a solution. A different BMP type, or a Vector Management Plan prepared with concurrence from the County of San Diego Department of Environmental Health, may be required. • Inspect monthly and after every 0.5-inch
or larger storm event. If mosquitos are observed, increase inspection frequency to after every 0.1-inch or larger storm event. • Maintain when needed.

Underdrain clogged Clear blockage. Inspect if standing water is observed for longer than 24-96 hours following a storm event. Maintain when needed. 2026-08-19 Item #12D Page 389 of 816
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E.19 BF-1 Biofiltration
Location: 43rd Street and Logan Avenue, San Diego, California Description
Biofiltration (Bioretention with underdrain) facilities are vegetated surface water systems that filter water through vegetation, and soil or engineered media prior to discharge via underdrain or overflow to the downstream conveyance system. Bioretention with underdrain facilities are commonly incorporated into the site within parking lot landscaping, along roadsides, and in open spaces. Because these types of facilities have limited or no infiltration, they are typically designed to provide enough hydraulic head to move flows through the underdrain connection to the storm drain system.

Treatment is achieved through filtration, sedimentation, sorption, biochemical processes and plant uptake.

Typical bioretention with underdrain components include:
• Inflow distribution mechanisms (e.g, perimeter flow spreader or filter strips) • Energy dissipation mechanism for concentrated inflows (e.g., splash blocks or riprap) • Shallow surface ponding for captured flows
• Side slope and basin bottom vegetation selected based on expected climate and ponding depth
• Non-floating mulch layer (Optional)
• Media layer (planting mix or engineered media) capable of supporting vegetation growth • Filter course layer consisting of aggregate to prevent the migration of fines into uncompacted native soils or the aggregate storage layer • Aggregate storage layer with underdrain(s)
• Impermeable liner or uncompacted native soils at the bottom of the facility • Overflow structure
MS4 Permit Category
Biofiltration
Manual Category
Biofiltration
Applicable Performance
Standard
Pollutant Control
Flow Control
Primary Benefits
Treatment
Volume Reduction (Incidental)
Peak Flow Attenuation (Optional)
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Typical plan and Section view of a Biofiltration BMP
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Design Adaptations for Project Goals
Biofiltration Treatment BMP for stormwater pollutant control. The system is lined or un-lined to provide incidental infiltration, and an underdrain is provided at the bottom to carry away filtered runoff. This configuration is considered to provide biofiltration treatment via flow through the media layer. Storage provided above the underdrain within surface ponding, media, and aggregate storage is considered included in the biofiltration treatment volume. Saturated storage within the aggregate storage layer can be added to this design by raising the underdrain above the bottom of the aggregate storage layer or via an internal weir structure designed to maintain a specific water level elevation.

Integrated stormwater flow control and pollutant control configuration. The system can be designed to provide flow rate and duration control by primarily providing increased surface ponding and/or having a deeper aggregate storage layer above the underdrain. This will allow for significant detention storage, which can be controlled via inclusion of an outlet structure at the downstream end of the underdrain.

Recommended Siting Criteria
Siting Criteria Intent/Rationale
□
Placement observes geotechnical
recommendations regarding potential hazards
(e.g., slope stability, landslides, liquefaction
zones) and setbacks (e.g., slopes, foundations,
utilities).

Must not negatively impact existing site
geotechnical concerns.
□
An impermeable liner or other hydraulic
restriction layer is included if site constraints
indicate that infiltration or lateral flows should
not be allowed.

Lining prevents storm water from
impacting groundwater and/or sensitive
environmental or geotechnical features.

Incidental infiltration, when allowable,
can aid in pollutant removal and
groundwater recharge.
□
Contributing tributary area shall be ≤ 5 acres
(≤ 1 acre preferred).

Bigger BMPs require additional design
features for proper performance.

Contributing tributary area greater than 5
acres may be allowed at the discretion of
the [City Engineer} if the following
conditions are met: 1) incorporate design
features (e.g. flow spreaders) to
minimizing short circuiting of flows in
the BMP and 2) incorporate additional
design features requested by the [City
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Siting Criteria Intent/Rationale
Engineer] for proper performance of the
regional BMP.
□ Finish grade of the facility is ≤ 2%.

Flatter surfaces reduce erosion and
channelization within the facility.
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Recommended BMP Component Dimensions
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BMP Component Dimension Intent/Rationale
Freeboard ≥ 2 inches
Freeboard provides room for head
over overflow structures and
minimizes risk of uncontrolled
surface discharge.

Surface Ponding ≥ 6 and ≤ 12 inches
Surface ponding capacity lowers
subsurface storage requirements.

Deep surface ponding raises safety
concerns.

Surface ponding depth greater than
12 inches (for additional pollutant
control or surface outlet structures
or flow-control orifices) may be
allowed at the discretion of the [City
Engineer] if the following
conditions are met: 1) surface
ponding depth drawdown time is
less than 24 hours; and 2) safety
issues and fencing requirements are
considered (typically ponding
greater than 18” will require a fence
and/or flatter side slopes) and 3)
potential for elevated clogging risk
is considered.

BMP Component Dimension Intent/Rationale
Ponding Area Side Slopes 3H:1V or shallower
Gentler side slopes are safer, less
prone to erosion, able to establish
vegetation more quickly and easier
to maintain.
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Mulch ≥ 3 inches
Mulch will suppress weeds and
maintain moisture for plant growth.

Aging mulch kills pathogens and
weed seeds and allows the beneficial
microbes to multiply.

Media Layer ≥ 18 inches
A deep media layer provides
additional filtration and supports
plants with deeper roots. Standard
specifications shall be followed. For
non-standard or proprietary
designs, compliance with F.1
ensures that adequate treatment
performance will be provided.

Underdrain Diameter ≥ 6 inches Smaller diameter underdrains are prone to clogging.

Cleanout Diameter ≥ 6 inches Properly spaced cleanouts will facilitate underdrain maintenance.

Design Criteria and Considerations
Bioretention with underdrain must meet the following design criteria. Deviations from the below criteria may be approved at the discretion of the [City Engineer] if it is determined to be appropriate:

Design Criteria Intent/Rationale
Surface Ponding
□
Surface ponding is limited to a 24-hour
drawdown time.

Surface ponding limited to 24 hours for
plant health. Surface ponding drawdown
time greater than 24-hours but less than
96 hours may be allowed at the
discretion of the [City Engineer] if
certified by a landscape architect or
agronomist.
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Design Criteria Intent/Rationale
Vegetation
□
Plantings are suitable for the climate and
expected ponding depth. A plant list to aid in
selection can be found in Appendix E.26.

Plants suited to the climate and ponding
depth are more likely to survive.
□
An irrigation system with a connection to
water supply should be provided as needed.

Seasonal irrigation might be needed to
keep plants healthy.

Mulch (Optional or Mandatory – Dependent on jurisdiction) □
A minimum of 3 inches of well-aged, shredded
hardwood mulch that has been stockpiled or
stored for at least 12 months is provided.

Mulch will suppress weeds and maintain
moisture for plant growth. Aging mulch
kills pathogens and weed seeds and
allows the beneficial microbes to
multiply.

A filtration rate of at least 5 inches per
hour allows soil to drain between events.

The initial rate should be higher than
long term target rate to account for
clogging over time. However an
excessively high initial rate can have a
negative impact on treatment
performance, therefore an upper limit is
needed.
□
Media is a minimum 18 inches deep, meeting
either of these two media specifications:

Section F.3 Bioretention Soil Media (BSM) or
specific jurisdictional guidance
Alternatively, for proprietary designs and
custom media mixes not meeting the media
specifications, the media meets the pollutant
treatment performance criteria in Section F.1.

A deep media layer provides additional
filtration and supports plants with deeper
roots.

Standard specifications shall be followed.

For non-standard or proprietary designs,
compliance with F.1 ensures that
adequate treatment performance will be
provided.
□ Media surface area is 3% of contributing area
times adjusted runoff factor or greater. Unless
Greater surface area to tributary area
ratios: a) maximizes volume retention as
required by the MS4 Permit and b)
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Design Criteria Intent/Rationale
demonstrated that the BMP surface area can
be smaller than 3%.
decrease loading rates per square foot
and therefore increase longevity.

Adjusted runoff factor is to account for
site design BMPs implemented upstream
of the BMP (such as rain barrels,
impervious area dispersion, etc.). Refer
to Appendix B.2 guidance.
□
Where receiving waters are impaired or have a
TMDL for nutrients, the system is designed
with nutrient sensitive media design (see fact
sheet BF-2).

Potential for pollutant export is partly a
function of media composition; media
design must minimize potential for
export of nutrients, particularly where
receiving waters are impaired for
nutrients.

Filter Course Layer
□
A filter course is used to prevent migration of
fines through layers of the facility. Filter fabric
is not used.

Migration of media can cause clogging of
the aggregate storage layer void spaces or
subgrade. Filter fabric is more likely to
clog.
□ Filter course is washed and free of fines.

Washing aggregate will help eliminate
fines that could clog the facility and
impede infiltration.
□
Filter course calculations assessing suitability
for particle migration prevention have been
completed.

Gradation relationship between layers
can evaluate factors (e.g., bridging,
permeability, and uniformity) to
determine if particle sizing is appropriate
or if an intermediate layer is needed.

Aggregate Storage Layer
□
Class 2 Permeable per Caltrans specification
68-1.025 is recommended for the storage layer.

Washed, open-graded crushed rock may be
used, however a 4-6 inch washed pea gravel
filter course layer at the top of the crushed
rock is required.

Washing aggregate will help eliminate
fines that could clog the aggregate
storage layer void spaces or subgrade.
□ The depth of aggregate provided (12-inch
typical) and storage layer configuration is
Proper storage layer configuration and
underdrain placement will minimize
facility drawdown time.
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Design Criteria Intent/Rationale
adequate for providing conveyance for
underdrain flows to the outlet structure.

Inflow, Underdrain, and Outflow Structures
□
Inflow, underdrains and outflow structures are
accessible for inspection and maintenance.

Maintenance will prevent clogging and
ensure proper operation of the flow
control structures.
□
Inflow velocities are limited to 3 ft/s or less or
use energy dissipation methods. (e.g., riprap,
level spreader) for concentrated inflows.

High inflow velocities can cause erosion,
scour and/or channeling.
□
Curb cut inlets are at least 12 inches wide, have
a 4-6 inch reveal (drop) and an apron and
energy dissipation as needed.

Properly spaced cleanouts will facilitate
underdrain maintenance.
□
Overflow is safely conveyed to a downstream
storm drain system or discharge point Size
overflow structure to pass 100-year peak flow
for on-line infiltration basins and water quality
peak flow for off-line basins.

Planning for overflow lessens the risk of
property damage due to flooding.

Conceptual Design and Sizing Approach for Stormwater Pollutant Control Only To design bioretention with underdrain for stormwater pollutant control only (no flow control 2026-08-19 Item #12D Page 399 of 816
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required), the following steps should be taken:

2. Calculate the DCV per Appendix B based on expected site design runoff for tributary areas.

3. Use the sizing worksheet presented in Appendix B.5 to size biofiltration BMPs.

Conceptual Design and Sizing Approach when Stormwater Flow Control is Applicable Control of flow rates and/or durations will typically require significant surface ponding and/or aggregate storage volumes, and therefore the following steps should be taken prior to determination of stormwater pollutant control design. Pre-development and allowable post-project flow rates and durations should be determined as discussed in Chapter 6 of the manual.

2. Iteratively determine the facility footprint area, surface ponding and/or aggregate storage layer depth required to provide detention storage to reduce flow rates and durations to allowable limits. Flow rates and durations can be controlled from detention storage by altering outlet structure orifice size(s) and/or water control levels.

MultJ-level orifices can be used within an outlet structure to control the full range of flows.

3. If bioretention with underdrain cannot fully provide the flow rate and duration control required by this manual, an upstream or downstream structure with significant storage volume such as an underground vault can be used to provide remaining controls.

4. After bioretention with underdrain has been designed to meet flow control requirements, calculations must be completed to verify if stormwater pollutant control requirements to treat the DCV have been met.

Maintenance Overview
Normal Expected Maintenance. Biofiltration requires routine maintenance to: remove accumulated materials such as sediment, trash or debris; maintain vegetation health; maintain infiltration capacity of the media layer; replenish mulch; and maintain integrity of side slopes, inlets, energy dissipators, and outlets. A summary table of standard inspection and maintenance indicators is provided within this Fact Sheet.

Non-Standard Maintenance or BMP Failure. If any of the following scenarios are observed, the 2026-08-19 Item #12D Page 400 of 816
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BMP is not performing as intended to protect downstream waterways from pollution and/or erosion.

Corrective maintenance, increased inspection and maintenance, BMP replacement, or a different BMP type will be required.
• The BMP is not drained between storm events. Surface ponding longer than approximately 24 hours following a storm event may be detrimental to vegetation health, and surface ponding longer than approximately 96 hours following a storm event poses a risk of vector (mosquito) breeding. Poor drainage can result from clogging of the media layer, filter course, aggregate storage layer, underdrain, or outlet structure. The specific cause of the drainage issue must be determined and corrected.
• Sediment, trash, or debris accumulation greater than 25% of the surface ponding volume within one month. This means the load from the tributary drainage area is too high, reducing BMP function or clogging the BMP. This would require pretreatment measures within the tributary area draining to the BMP to intercept the materials. Pretreatment components, especially for sediment, will extend the life of components that are more expensive to replace such as media, filter course, and aggregate layers.
• Erosion due to concentrated storm water runoff flow that is not readily corrected by adding erosion control blankets, adding stone at flow entry points, or minor re-grading to restore proper drainage according to the original plan. If the issue is not corrected by restoring the BMP to the original plan and grade, the [City Engineer] shall be contacted prior to any additional repairs or reconstruction.

Other Special Considerations. Biofiltration is a vegetated structural BMP. Vegetated structural BMPs that are constructed in the vicinity of, or connected to, an existing jurisdictional water or wetland could inadvertently result in creation of expanded waters or wetlands. As such, vegetated structural BMPs have the potential to come under the jurisdiction of the United States Army Corps of Engineers, SDRWQCB, California Department of Fish and Wildlife, or the United States Fish and Wildlife Service. This could result in the need for specific resource agency permits and costly mitigation to perform maintenance of the structural BMP. Along with proper placement of a structural BMP, routine maintenance is key to preventing this scenario. 2026-08-19 Item #12D Page 401 of 816
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Summary of Standard Inspection and Maintenance
The property owner is responsible to ensure inspection, operation and maintenance of permanent BMPs on their property unless responsibility has been formally transferred to an agency, community facilities district, homeowners association, property owners association, or other special district.

Maintenance frequencies listed in this table are average/typical frequencies. Actual maintenance needs are site-specific, and maintenance may be required more frequently. Maintenance must be performed whenever needed, based on maintenance indicators presented in this table.

The BMP owner is responsible for conducting regular inspections to see when maintenance is needed based on the maintenance indicators.

Threshold/Indicator Maintenance Action Typical Maintenance Frequency Accumulation of sediment, litter, or debris Remove and properly dispose of accumulated materials, without damage to the vegetation or compaction of the media layer. • Inspect monthly. If the BMP is 25% full*
or more in one month, increase inspection frequency to monthly plus after every 0.1-inch or larger storm event. •Remove any accumulated materials found
at each inspection.

Obstructed inlet or outlet structure Clear blockage. • Inspect monthly and after every 0.5-inch or larger storm event.
•Remove any accumulated materials found
at each inspection.

Damage to structural components such as
weirs, inlet or outlet structures
Repair or replace as applicable • Inspect annually.
• Maintain when needed.

Poor vegetation establishment Re-seed, re-plant, or re-establish vegetation per original plans.
• Inspect monthly.
• Maintain when needed.

Dead or diseased vegetation Remove dead or diseased vegetation, reseed, re-plant, or re-establish vegetation per original plans.
• Inspect monthly.
• Maintain when needed.
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Threshold/Indicator Maintenance Action Typical Maintenance Frequency Overgrown vegetation Mow or trim as appropriate. • Inspect monthly. • Maintain when needed.
2/3 of mulch has decomposed, or mulch has been removed
Remove decomposed fraction and top off
with fresh mulch to a total depth of 3
inches.
• Inspect monthly.
•Replenish mulch annually, or more
frequently when needed based on inspection.

Erosion due to concentrated irrigation flow Repair/re-seed/re-plant eroded areas and adjust the irrigation system.
• Inspect monthly.
• Maintain when needed.

Standing water in BMP for longer than 24 hours following a storm event Surface ponding longer than approximately 24 hours following a storm event may be detrimental to vegetation health Make appropriate corrective measures such as adjusting irrigation system, removing obstructions of debris or invasive vegetation, clearing underdrains, or
repairing/replacing clogged or compacted soils.
• Inspect monthly and after every 0.5-inch
or larger storm event. If standing water is observed, increase inspection frequency to after every 0.1-inch or larger storm event. • Maintain when needed.
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Threshold/Indicator Maintenance Action Typical Maintenance Frequency Presence of mosquitos/larvae For images of egg rafts, larva, pupa, and adult mosquitos, see http://www.mosquito.org/biology If mosquitos/larvae are observed: first, immediately remove any standing water by dispersing to nearby landscaping; second, make corrective measures as applicable to restore BMP drainage to prevent standing water.

Underdrain clogged Clear blockage. Inspect if standing water is observed for longer than 24-96 hours following a storm event. Maintain when needed. “25% full” is defined as ¼ of the depth from the design bottom elevation to the crest of the outflow structure (e.g., if the height to the outflow opening is 12 inches from the bottom elevation, then the materials must be removed when there is 3 inches of accumulation – this should be marked on the outflow structure).2026-08-19 Item #12D Page 404 of 816 Appendix E: BMP Design Fact Sheets
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E.20 BF-2 Nutrient Sensitive Media Design
Some studies of bioretention with underdrains have observed export of nutrients, particularly inorganic nitrogen (nitrate and nitrite) and dissolved phosphorus. This has been observed to be a short-lived phenomenon in some studies or a long term issue in some studies. The composition of the soil media, including the chemistry of individual elements is believed to be an important factor in the potential for nutrient export. Organic amendments, often compost, have been identified as the most likely source of nutrient export. The quality and stability of organic amendments can vary widely.

The biofiltration media specifications contained in the County of San Diego Low Impact Development Handbook: Appendix G -Bioretention Soil Specification (June 2014, unless superseded by more recent edition) and the City of San Diego Low Impact Development Design Manual (page B-18) (July 2011, unless superseded by more recent edition) were developed with consideration of the potential for nutrient export. These specifications include criteria for individual component characteristics and quality in order to control the overall quality of the blended mixes. As of the publication of this manual, the June 2014 County of San Diego specifications provide more detail regarding mix design and quality control.

The City and County specifications noted above were developed for general purposes to meet permeability and treatment goals. In cases where the BMP discharges to receiving waters with nutrient impairments or nutrient TMDLs, the biofiltration media should be designed with the specific goal of minimizing the potential for export of nutrients from the media. Therefore, in addition to adhering to the City or County media specifications, the following guidelines should be followed:

1. Select plant palette to minimize plant nutrient needs A landscape architect or agronomist should be consulted to select a plant palette that minimizes nutrient needs. Utilizing plants with low nutrient needs results in less need to enrich the biofiltration soil mix. If nutrient quantity is then tailored to plants with lower nutrient needs, these plants will generally have less competition from weeds, which typically need higher nutrient content. The following practices are recommended to minimize nutrient needs of the plant palette:
• Utilize native, drought-tolerant plants and grasses where possible. Native plants generally have a broader tolerance for nutrient content, and can be longer lived in leaner/lower nutrient soils.
• Start plants from smaller starts or seed. Younger plants are generally more tolerant of lower nutrient levels and tend to help develop soil structure as they grow. Given the lower cost of smaller plants, the project should be able to accept a plant mortality rate that is somewhat higher than starting from larger plants and providing high organic content.

2. Minimize excess nutrients in media mix
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Once the low-nutrient plant palette is established (item 1), the landscape architect and/or agronomist should be consulted to assist in the design of a biofiltration media to balance the interests of plant establishment, water retention capacity (irrigation demand), and the potential for nutrient export. The following guidelines should be followed:
• The mix should not exceed the nutrient needs of plants. In conventional landscape design, the nutrient needs of plants are often exceeded intentionally in order to provide a factor of safety for plant survival. This practice must be avoided in biofiltration media as excess nutrients will increase the chance of export. The mix designer should keep in mind that nutrients can be added later (through mulching, tilling of amendments into the surface), but it is not possible to remove nutrients, once added.
• The actual nutrient content and organic content of the selected organic amendment source should be determined when specifying mix proportions.

Nutrient content (i.e., C:N ratio; plant extractable nutrients) and organic content (i.e, % organic material) are relatively inexpensive to measure via standard agronomic methods and can provide important information about mix design. If mix design relies on approximate assumption about nutrient/organic content and this is not confirmed with testing (or the results of prior representative testing), it is possible that the mix could contain much more nutrient than intended.
• Nutrients are better retained in soils with higher cation exchange capacity.

Cation exchange capacity can be increased through selection of organic material with naturally high cation exchange capacity, such as peat or coconut coir pith, and/or selection of inorganic material with high cation exchange capacity such as some sands or engineered minerals (e.g., low P-index sands, zeolites, rhyolites, etc). Including higher cation exchange capacity materials would tend to reduce the net export of nutrients. Natural silty materials also provide cation exchange capacity; however potential impacts to permeability need to be considered. • Focus on soil structure as well as nutrient content. Soil structure is loosely defined as the ability of the soil to conduct and store water and nutrients as well as the degree of aeration of the soil. Soil structure can be more important than nutrient content in plant survival and biologic health of the system. If a good soil structure can be created with very low amounts of organic amendment, plants survivability should still be provided. While soil structure generally develops with time, biofiltration media can be designed to promote earlier development of soil structure. Soil structure is enhanced by the use of amendments with high humus content (as found in well-aged organic material). In addition, soil structure can be enhanced through the use of organic material with a distribution of particle sizes (i.e., a more heterogeneous mix). • Consider alternatives to compost. Compost, by nature, is a material that is continually evolving and decaying. It can be challenging to determine whether tests 2026-08-19 Item #12D Page 406 of 816
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previously done on a given compost stock are still representative. It can also be challenging to determine how the properties of the compost will change once placed in the media bed. More stable materials such as aged coco coir pith, peat, biochar, shredded bark, and/or other amendments should be considered.

With these considerations, it is anticipated that less than 10 percent organic amendment by volume could be used, while still balancing plant survivability and water retention. If compost is used, designers should strongly consider utilizing less than 10 percent by volume.

3. Design with partial retention and/or internal water storage An internal water storage zone, as described in Fact Sheet PR-1 is believed to improve retention of nutrients. For lined systems, an internal water storage zone worked by providing a zone that fluctuates between aerobic and anaerobic conditions, resulting in nitrification/denitrification. In soils that will allow infiltration, a partial retention design (PR-1) allows significant volume reduction and can also promote nitrification/denitrification.

Acknowledgment: This fact sheet has been adapted from the Orange County Technical Guidance Document (May 2011). It was originally developed based on input from: Deborah Deets, City of Los Angeles Bureau of Sanitation, Drew Ready, Center for Watershed Health, Rick Fisher, ASLA, City of Los Angeles Bureau of Engineering, Dr. Garn Wallace, Wallace Laboratories, Glen Dake, GDML, and Jason Schmidt, Tree People. The guidance provided herein does not reflect the individual opinions of any individual listed above and should not be cited or otherwise attributed to those listed. 2026-08-19 Item #12D Page 407 of 816
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E.21 BF-3 Proprietary Biofiltration Systems
The purpose of this fact sheet is to help explain the potential role of proprietary BMPs in meeting biofiltration requirements, when full retention of the DCV is not feasible. The fact sheet does not describe design criteria like the other fact sheets in this appendix because this information varies by BMP product model.

Criteria for Use of a Proprietary BMP as a Biofiltration BMP A proprietary BMP may be acceptable as a “biofiltration BMP” under the following conditions:

1. The BMP meets the minimum design criteria listed in Appendix F, including the pollutant treatment performance standard in Appendix F.1;

2. The BMP is designed and maintained in a manner consistent with its performance certifications (See explanation in Appendix F.2); and

3. The BMP is acceptable at the discretion of the City Engineer. The City Engineer has no obligation to accept any proprietary biofiltration BMP.

Guidance for Sizing a Proprietary BMP as a Biofiltration BMP Proprietary biofiltration BMPs must meet the same sizing guidance as non-proprietary BMPs. Sizing is typically based on capturing and treating 1.50 times the DCV not reliably retained. Guidance for sizing biofiltration BMPs to comply with requirements of this manual is provided in Appendix F.2.

Maintenance Overview
Refer to manufacturer for maintenance information.
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E.22 FT-1 Vegetated Swales
Location: Eastlake Business Center, Chula Vista, California; Photo Credit: Eric Mosolgo Description
Vegetated swales are shallow, open channels that are designed to remove stormwater pollutants by physically straining/filtering runoff through vegetation in the channel. Swales can be used in place of traditional curbs and gutters and are well-suited for use in linear transportation corridors to provide both conveyance and treatment via filtration. An effectively designed vegetated swale achieves uniform sheet flow through densely vegetated areas. When soil conditions allow, infiltration and volume reduction are enhanced by adding a gravel drainage layer underneath the swale. Vegetated swales with a subsurface media layer can provide enhanced infiltration, water retention, and pollutantremoval capabilities. Pollutant removal effectiveness can also be maximized by increasing the hydraulic residence time of water in swale using weirs or check dams.

Typical vegetated swale components include:
• Inflow distribution mechanisms (e.g., flow spreader)
• Surface flow
• Vegetated surface layer
• Check dams (if required)
• Optional aggregate storage layer with underdrain(s)
MS4 Permit Category
Flow-thru Treatment Control
Manual Category
Flow-thru Treatment Control
Applicable Performance
Standard
Pollutant Control
Primary Benefits
Treatment
Volume Reduction (Incidental)
Peak Flow Attenuation
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Typical plan and Section view of a Vegetated Swale BMP
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Design Adaptations for Project Goals
Site design BMP to reduce runoff volumes and storm peaks. Swales without underdrains are an alternative to lined channels and pipes and can provide volume reduction through infiltration. Swales can also reduce the peak runoff discharge rate by increasing the time of concentration of the site and decreasing runoff volumes and velocities.

Flow-thru treatment BMP for stormwater pollutant control. The system is lined or un-lined to provide incidental infiltration with an underdrain and designed to provide pollutant removal through settling and filtration in the channel vegetation (usually grasses). This configuration is considered to provide flow-thru treatment via horizontal surface flow through the swale. Sizing for flow-thru treatment control is based on the surface flow rate through the swale that meets water quality treatment performance objectives.

Design Criteria and Considerations
Vegetated swales must meet the following design criteria. Deviations from the below criteria may be approved at the discretion of the City Engineer if it is determined to be appropriate:

Siting and Design Intent/Rationale
□
Placement observes geotechnical
recommendations regarding potential
hazards (e.g., slope stability, landslides, and
liquefaction zones) and setbacks (e.g.,
slopes, foundations, utilities).

Must not negatively impact existing site
geotechnical concerns.
□
An impermeable liner or other hydraulic
restriction layer is included if site
constraints indicate that infiltration or
lateral flows should not be allowed.

Lining prevents stormwater from
impacting groundwater and/or sensitive
environmental or geotechnical features.

Incidental infiltration, when allowable, can
aid in pollutant removal and groundwater
recharge.
□
Contributing tributary area ≤ 2 acres. Higher ratios increase the potential for clogging but may be acceptable for
relatively clean tributary areas.
□
Longitudinal slope is ≥ 1.5% and ≤ 6%. Flatter swales facilitate increased water quality treatment while minimum slopes
prevent ponding.
□
For site design goal, in-situ soil infiltration
rate ≥ 0.5 in/hr (if < 0.5 in/hr, an
underdrain is required and design goal is for
pollutant control only).

Well-drained soils provide volume
reduction and treatment. An underdrain
should only be provided when soil
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Siting and Design Intent/Rationale
infiltration rates are low or per geotechnical
or groundwater concerns.

Surface Flow
□
Maximum flow depth is ≤ 6 inches or ≤ 2/3
the vegetation length, whichever is greater.

Ideally, flow depth will be ≥ 2 inches below
shortest plant species.

Flow depth must fall within the height
range of the vegetation for effective water
quality treatment via filtering.
□
A minimum of 1 foot of freeboard is
provided.

Freeboard minimizes risk of uncontrolled
surface discharge.
□
Cross sectional shape is trapezoidal or
parabolic with side slopes ≥ 3H:1V.

Gentler side slopes are safer, less prone to
erosion, able to establish vegetation more
quickly and easier to maintain.
□
Bottom width is ≥ 2 feet and ≤ 8 feet. A minimum of 2 feet minimizes erosion. A maximum of 8 feet prevents channel
braiding.
□
Minimum hydraulic residence time ≥ 10
minutes.

Longer hydraulic residence time increases
pollutant removal.
□
Swale is designed to safely convey the 10-yr
storm event unless a flow splitter is
included to allow only the water quality
event.

Planning for larger storm events lessens the
risk of property damage due to flooding.
□
Flow velocity is ≤ 1 ft/s for water quality
event. Flow velocity for 10-yr storm event
is ≤ 3 ft/s.

Lower flow velocities provide increased
pollutant removal via filtration and
minimize erosion.

Vegetated Surface Layer (amendment with media is Optional) □
Soil is amended with 2 inches of media
mixed into the top 6 inches of in-situ soils,
as needed, to promote plant growth
(optional). For enhanced pollutant control,
2 feet of media can be used in place of insitu soils. Media meets either of these two media specifications:

City of San Diego Low Impact City of San
Diego Storm Water Standards Appendix F,
February 2016);
Or County of San Diego Low Impact
Development Handbook, June 2014:

Appendix G -Bioretention Soil
Specification.

Amended soils aid in plant establishment
and growth. Media replacement for in-situ
soils can improve water quality treatment
and site design volume reduction.
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Siting and Design Intent/Rationale
□
Vegetation is appropriately selected lowgrowing, erosion-resistant plant species that effectively bind the soil, thrive under sitespecific climatic conditions and require little or no irrigation.

Plants suited to the climate and expected
flow conditions are more likely to survive.

Check Dams
□
Check dams are provided at 50-foot
increments for slopes ≥ 2.5%.

Check dams prevent erosion and increase
the hydraulic residence time by lowering
flow velocities and providing ponding
opportunities.

Filter Course Layer (For Underdrain Design)
□
A filter course is used to prevent migration
of fines through layers of the facility. Filter
fabric is not used.

Migration of media can cause clogging of
the aggregate storage layer void spaces or
subgrade. Filter fabric is more likely to clog.
□ Filter course is washed and free of fines.

Washing aggregate will help eliminate fines
that could clog the facility and impede
infiltration.
□
Filter course calculations assessing
suitability for particle migration prevention
have been completed.

Gradation relationship between layers can
evaluate factors (e.g., bridging, permeability,
and uniformity) to determine if particle
sizing is appropriate or if an intermediate
layer is needed.

Aggregate Storage Layer (For Underdrain Design)
□
The depth of aggregate provided (12-inch
typical) and storage layer configuration is
adequate for providing conveyance for
underdrain flows to the outlet structure.

Proper storage layer configuration and
underdrain placement will minimize facility
drawdown time.
□
Aggregate used for the aggregate storage
layer is washed and free of fines.

Washing aggregate will help eliminate fines
that could clog aggregate storage layer void
spaces or underdrain.

Inflow and Underdrain Structures
□
Inflow and underdrains are accessible for
inspection and maintenance.

Maintenance will prevent clogging and
ensure proper operation of the flow control
structures.
□
Underdrain outlet elevation should be a
minimum of 3 inches above the bottom
elevation of the aggregate storage layer.

A minimal separation from subgrade or the
liner lessens the risk of fines entering the
underdrain and can improve hydraulic
performance by allowing perforations to
remain unblocked.
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Siting and Design Intent/Rationale
□ Minimum underdrain diameter is 6 inches. Smaller diameter underdrains are prone to clogging.
□
Underdrains are made of slotted, PVC pipe
conforming to ASTM D 3034 or equivalent
or corrugated, HDPE pipe conforming to
AASHTO 252M or equivalent.

Slotted underdrains provide greater intake
capacity, clog resistant drainage, and
reduced entrance velocity into the pipe,
thereby reducing the chances of solids
migration.
□
An underdrain cleanout with a minimum 6-
inch diameter and lockable cap is placed
every 250 to 300 feet as required based on
underdrain length.

Properly spaced cleanouts will facilitate
underdrain maintenance.

Conceptual Design and Sizing Approach for Site Design

1. Determine the areas where vegetated swales can be used in the site design to replace traditional curb and gutter facilities and provide volume reduction through infiltration.

Conceptual Design and Sizing Approach for Storm Water Pollutant Control Only To design vegetated swales for storm water pollutant control only, the following steps should be taken:

1. Verify that siting and design criteria have been met, including bottom width and longitudinal and side slope requirements.

2. Calculate the design flow rate per Appendix B based on expected site design runoff for tributary areas.

3. Use the sizing worksheet to determine flow-thru treatment sizing of the vegetated swale and if flow velocity, flow depth, and hydraulic residence time meet required criteria. Swale configuration should be adjusted as necessary to meet design requirements.

Maintenance Overview
Normal Expected Maintenance. Vegetated swales require routine maintenance to: remove accumulated materials such as sediment, trash, and debris; maintain vegetation health; and maintain integrity of side slopes, channel bottom, inlets, energy dissipaters, weirs or check dams, and outlets to ensure runoff will be conveyed as uniform flow throughout the swale (i.e., flow will spread uniformly across the width of the swale as it is conveyed from upstream to downstream).

Non-Standard Maintenance or BMP Failure. If any of the following scenarios are observed, the BMP is not performing as intended to protect downstream waterways from pollution and/or erosion.

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type will be required.
• The BMP is not drained between storm events. Surface ponding longer than approximately 24 hours following a storm event may be detrimental to vegetation health, and surface ponding longer than approximately 96 hours following a storm event poses a risk of vector (mosquito) breeding. Poor drainage can result from deposited materials or overgrowth of vegetation within the swale blocking drainage conveyance or blocking an outlet structure, or localized erosion issues that cause channelization and prevent uniform flow throughout the swale. The specific cause of the drainage issue must be determined and corrected. If the issue is not corrected by restoring the BMP to the original plan and grade, the City reviewer shall be contacted prior to any additional repairs or reconstruction. • Sediment, trash, or debris accumulation blocking drainage becomes a chronic issue observed at every inspection. This means the load from the tributary drainage area is too high, reducing BMP function or clogging the BMP. This would require pretreatment measures within the tributary area draining to the BMP to intercept the materials. • Erosion due to concentrated storm water runoff flow that is not readily corrected by adding erosion control blankets, adding stone at flow entry points, or minor re-grading to restore proper drainage according to the original plan. If the issue is not corrected by restoring the BMP to the original plan and grade, the City reviewer shall be contacted prior to any additional repairs or reconstruction.
.
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Summary of Standard Inspection and Maintenance
The property owner is responsible to ensure inspection, operation and maintenance of permanent BMPs on their property unless responsibility has been formally transferred to an agency, community facilities district, homeowners association, property owners association, or other special district.

The BMP owner is responsible for conducting regular inspections to see when maintenance is needed based on the maintenance indicators.

Threshold/Indicator Maintenance Action Inspection and Maintenance Frequency Accumulation of sediment, litter, or
debris
Remove and properly dispose of accumulated materials, without damage to vegetation. • Inspect monthly. If accumulated materials
are observed blocking drainage, increase inspection frequency to monthly plus after every 0.1-inch or larger storm event. •Remove any accumulated materials found
at each inspection.

Obstructed inlet or outlet structure Clear blockage. • Inspect monthly and after every 0.5-inch or larger storm event.
•Remove any accumulated materials found
at each inspection.

Damage to structural components such as
weirs, inlet or outlet structures
Repair or replace as applicable. • Inspect annually.
• Maintain when needed.

Poor vegetation establishment Re-seed, re-plant, or re-establish vegetation per original plans.
• Inspect monthly.
• Maintain when needed.
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Threshold/Indicator Maintenance Action Inspection and Maintenance Frequency Dead or diseased vegetation Remove dead or diseased vegetation, re-seed, replant, or re-establish vegetation per original plans.
• Inspect monthly.
• Maintain when needed.

Overgrown vegetation Mow or trim as appropriate. • Inspect monthly. • Maintain when needed.

Erosion due to concentrated irrigation flow
Repair/re-seed/re-plant eroded areas and adjust the irrigation system. • Inspect monthly.
• Maintain when needed.

Standing water in BMP following a storm event
Make appropriate corrective measures such as adjusting irrigation system, removing obstructions of debris or invasive vegetation, loosening or replacing top soil to allow for better infiltration, or minor re-grading for proper drainage. If the issue is not corrected by restoring the BMP to the original plan and grade, the City reviewer shall be contacted prior to any additional repairs or reconstruction. • Inspect monthly and after every 0.5-inch or
larger storm event. If standing water is observed, increase inspection frequency to after every 0.1-inch or larger storm event. • Maintain when needed.
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Threshold/Indicator Maintenance Action Inspection and Maintenance Frequency Presence of mosquitos/larvae For images of egg rafts, larva, pupa, and adult mosquitos, see http://www.mosquito.org/biology If mosquitos/larvae are observed: first, immediately remove any standing water by dispersing to nearby landscaping; second, make corrective measures as applicable to restore BMP drainage to prevent standing water. If mosquitos persist following corrective measures to remove standing water, the City reviewer shall be contacted to determine a solution. A different BMP type, or a Vector Management Plan prepared with concurrence from the County of San Diego Department of Environmental Health, may be required. • Inspect monthly and after every 0.5-inch or
larger storm event. If mosquitos are observed, increase inspection frequency to after every 0.1-inch or larger storm event. • Maintain when needed.
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E.23 FT-2 Media Filters
Photo Credit: Contech Stormwater Solutions
Description
Media filters are manufactured devices that consist of a series of modular filters packed with engineered media that can be contained in a catch basin, manhole, or vault that provide treatment through filtration and sedimentation. The manhole or vault may be divided into multiple chambers where the first chamber acts as a presettling basin for removal of coarse sediment while the next chamber acts as the filter bay and houses the filter cartridges. A variety of media types are available from various manufacturers that can target pollutants of concern via primarily filtration, sorption, ion exchange, and precipitation. Specific products must be selected to meet the flow-thru BMP selection requirements described in Appendix J.5. Treatment effectiveness is contingent upon proper maintenance of filter units.

Typical media filter components include:
• Vault for flow storage and media housing
• Inlet and outlet
• Media filters
MS4 Permit Category
Flow-thru Treatment Control
Manual Category
Flow-thru Treatment Control
Applicable Performance
Standard
Pollutant Control
Flow Control
Primary Benefits
Treatment
Peak Flow Attenuation (Optional)
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Design Adaptations for Project Goals
Flow-thru treatment BMP for storm water pollutant control. Water quality treatment is provided through filtration. This configuration is considered to provide flow-thru treatment, not biofiltration treatment. Storage provided within the vault restricted by an outlet is considered detention storage and is included in calculations for the flow-thru treatment volume.

Integrated storm water flow control and pollutant control configuration. Media filters can also be designed for flow rate and duration control via additional detention storage. The vault storage can be designed to accommodate higher volumes than the storm water pollutant control volume and can utilize multi-stage outlets to mitigate both the duration and rate of flows within a prescribed range.

Design Criteria and Considerations
Media filters must meet the following design criteria. Deviations from the below criteria may be approved at the discretion of the City Engineer if it is determined to be appropriate:

Siting and Design Intent/Rationale
□
Placement observes geotechnical
recommendations regarding potential
hazards (e.g., slope stability, landslides,
and liquefaction zones) and setbacks
(e.g., slopes, foundations, utilities).

Must not negatively impact existing site
geotechnical concerns.
□
Recommended for tributary areas with
limited available surface area or where
surface BMPs would restrict uses.

Maintenance needs may be more labor intensive
for media filters than surface BMPs. Lack of
surface visibility creates additional risk that
maintenance needs may not be completed in a
timely manner.
□
Vault storage drawdown time ≤96
hours. Provides vector control.
□
Vault storage drawdown time ≤36 hours
if the vault is used for equalization of
flows for pollutant treatment.

Provides required capacity to treat back to back
storms. Exception to the 36 hour drawdown
criteria is allowed if additional vault storage is
provided using the curves in Appendix B.4.2.

Inflow and Outflow Structures
□
Inflow and outflow structures are
accessible by required equipment (e.g.,
vactor truck) for inspection and
maintenance.

Maintenance will prevent clogging and ensure
proper operation of the flow control structures.
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Conceptual Design and Sizing Approach for Storm Water Pollutant Control Only To design a media filter for storm water pollutant control only (no flow control required), the following steps should be taken

1. Verify that the selected BMP complies with BMP selection requirements in Appendix B.6.

2. Verify that placement and tributary area requirements have been met.

3. Calculate the required DCV and/or flow rate per Appendix B.6.3 based on expected site design runoff for tributary areas.

4. Media filter can be designed either for DCV or flow rate. To estimate the drawdown time, divide the vault storage by the treatment rate of media filters.

Conceptual Design and Sizing Approach when Storm Water Flow Control is Applicable Control of flow rates and/or durations will typically require significant vault storage volume, and therefore the following steps should be taken prior to determination of storm water pollutant control design. Pre-development and allowable post-project flow rates and durations should be determined as discussed in Chapter 6 of the manual.

1. Verify that placement and tributary area requirements have been met.

2. Iteratively determine the vault storage volume required to provide detention storage to reduce flow rates and durations to allowable limits. Flow rates and durations can be controlled from detention storage by altering outlet structure orifice size(s) and/or water control levels. Multilevel orifices can be used within an outlet structure to control the full range of flows to MS4.

3. If a media filter cannot fully provide the flow rate and duration control required by this manual, an upstream or downstream structure with appropriate storage volume such as an underground vault can be used to provide remaining controls.

4. After the media filter has been designed to meet flow control requirements, calculations must be completed to verify if storm water pollutant control requirements to treat the DCV have been met.

5. Verify that the vault drawdown time is 96 hours or less. To estimate the drawdown time:
a. Divide the vault volume by the filter surface area.
b. Divide the result (a) by the design filter rate.
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Maintenance Overview Normal Expected Maintenance. Media filters require routine maintenance to: remove accumulated materials such as sediment, trash, and debris; replace filter cartridges; and maintain integrity of any internal components such as weirs and piping. A summary table of standard inspection and maintenance indicators is provided within this Fact Sheet. Non-Standard Maintenance or BMP Failure. The normal expected maintenance described above ensures the BMP functionality. Lapses in the normal expected maintenance can lead to clogging of the BMP and potentially blocking the storm drain system. If clogging is observed, the BMP is not performing as intended to protect downstream waterways from pollution and/or erosion. In addition, clogged BMPs can lead to flooding, standing water and mosquito breeding habitat. Maintenance is critical to ensure the flood protection capacity of the storm drain system is not compromised. If proper routine maintenance is not performed, corrective maintenance and increased inspection and maintenance will be required. For persistent clogging or presence of mosquitos, contact the City reviewer to determine a permanent solution. For example, adding pretreatment measures within the tributary area draining to the BMP to intercept sediment, trash, and debris. Pretreatment components, especially for sediment, will extend the life of the filter media. For mosquitos, a Vector Management Plan, prepared with concurrence from the County of San Diego Department of Environmental Health, may be required.

Other Special Considerations. Media filters are proprietary systems that include proprietary media that must be replaced as part of normal expected maintenance. They are typically installed underground and may require entry into the underground vault to perform the maintenance. The BMP owner is responsible to hire a maintenance operator qualified to service the units. The maintenance operator must obtain the appropriate filter media and/or any parts that need to be replaced. If maintenance conditions require maintenance personnel to enter the underground structure, the maintenance personnel must be trained and certified in confined space entry. To find a qualified maintenance operator, the BMP owner shall contact the manufacturer of the proprietary BMP.

The design of media filters includes consideration of the specific pollutants expected from the area tributary to the media filter and the specific pollutants of concern for the downstream waterways. Therefore, it is expected that the filter media selected during design of the project will not be substituted. If a need arises to substitute a different filter configuration or filter media, the City reviewer shall be contacted prior to any changes. 2026-08-19 Item #12D Page 424 of 816
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Summary of Standard Inspection and Maintenance
The property owner is responsible to ensure inspection, operation and maintenance of permanent BMPs on their property unless responsibility has been formally transferred to an agency, community facilities district, homeowners association, property owners association, or other special district. Maintenance frequencies listed in this table are average/typical frequencies. Actual maintenance needs are site-specific, and maintenance may be required more frequently. Maintenance must be performed whenever needed, based on maintenance indicators presented in this table.

The BMP owner is responsible for conducting regular inspections to see when maintenance is needed based on the maintenance indicators.

Threshold/Indicator Maintenance Action Inspection and Maintenance Frequency Accumulation of sediment, litter, or debris. The threshold for removal of materials depends on the specific type of proprietary filter and configuration and shall be based on the manufacturer’s recommendation. In any case, materials must be removed if accumulation blocks flow through the BMP.

Remove and properly dispose of accumulated materials.
• Inspect monthly.
•Remove materials annually (minimum), or
more frequently when BMP reaches manufacturer’s threshold for removal of materials in less than one year, or if accumulation blocks outlet. 2026-08-19 Item #12D Page 425 of 816
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Threshold/Indicator Maintenance Action Inspection and Maintenance Frequency Spent or clogged filter media.

The threshold for changing media depends
on the specific type of proprietary media and shall be based on the manufacturer’s recommendation. In any case, media must be replaced if flow cannot pass through the media or passes through at less than the design capacity.

Remove and properly dispose filter media, and replace with fresh media. • Inspect condition of media annually or
more frequently if recommended by manufacturer.
• Inspect BMP drainage monthly and after
every 0.5-inch or larger storm event. If standing water has been observed, increase inspection frequency to after every 0.1-inch or larger storm event. • Maintain when needed based on
manufacturer’s threshold/indicator for the specific media, or if standing water in the BMP indicates flow cannot pass through the media.

Any other recommendations pursuant to the
proprietary filter manufacturer’s maintenance guide.

Any other actions pursuant to the
proprietary filter manufacturer’s maintenance guide.
•As recommended by the proprietary filter
manufacturer’s maintenance guide
Obstructed inlet or outlet structure Clear blockage. • Inspect monthly and after every 0.5-inch or larger storm event.
•Remove any accumulated materials found
at each inspection.
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Threshold/Indicator Maintenance Action Inspection and Maintenance Frequency Presence of mosquitos/larvae For images of egg rafts, larva, pupa, and adult mosquitos, see http://www.mosquito.org/biology If mosquitos/larvae are observed: first, immediately remove and properly dispose any standing water; second, remove any accumulated materials that obstruct flow through the BMP to restore BMP drainage to prevent standing water. Ensure access covers are tight fitting, with gaps or holes no greater than 1/16 inch, and/or install barriers such as inserts or screens that prevent mosquito access to the subsurface storage. If the BMP includes a permanent sump, contact the City reviewer to determine a permanent solution. A different BMP type, or a Vector Management Plan prepared with concurrence from the County of San Diego Department of Environmental Health, may be required. • Inspect monthly and after every 0.5-inch
or larger storm event. If mosquitos are observed, increase inspection frequency to after every 0.1-inch or larger storm event. • Maintain when needed.

Damage to structural components of the filtration system such as weirs, underdrains, inlet or outlet structures Repair or replace as applicable. • Inspect annually.
• Maintain when needed.
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E.24 FT-3 Sand Filters
Photo Credit: City of San Diego LID Manual
Description
Sand filters operate by filtering storm water through a constructed sand bed with an underdrain system. Runoff enters the filter and spreads over the surface. Sand filter beds can be enclosed within concrete structures or within earthen containment. As flows increase, water backs up on the surface of the filter where it is held until it can percolate through the sand. The treatment pathway is downward (vertical) through the media to an underdrain system that is connected to the downstream storm drain system. As storm water passes through the sand, pollutants are trapped on the surface of the filter, in the small pore spaces between sand grains or are adsorbed to the sand surface. The high filtration rates of sand filters, which allow a large runoff volume to pass through the media in a short amount of time, can provide efficient treatment for storm water runoff.

Typical sand filter components include:
• Forebay for pretreatment/energy dissipation
• Surface ponding for captured flows
• Sand filter bed
• Aggregate storage layer with underdrain(s)
• Overflow structure
MS4 Permit Category
Flow-thru Treatment Control
Manual Category
Flow-thru Treatment Control
Applicable Performance
Standard
Pollutant Control
Flow Control
Primary Benefits
Treatment
Volume Reduction (Incidental)
Peak Flow Attenuation (Optional)
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Typical plan and Section view of a Sand Filter BMP
Design Adaptations for Project Goals
Flow-thru treatment BMP for stormwater pollutant control. The system is lined or un-lined to 2026-08-19 Item #12D Page 429 of 816
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provide incidental infiltration, and an underdrain is provided at the bottom to carry away filtered runoff. This configuration is considered to provide flow-thru treatment via vertical flow through the sand filter bed. Storage provided above the underdrain within surface ponding, the sand filter bed, and aggregate storage is considered included in the flow-thru treatment volume. Saturated storage within the aggregate storage layer can be added to this design by including an upturned elbow installed at the downstream end of the underdrain or via an internal weir structure designed to maintain a specific water level elevation.

Integrated stormwater flow control and pollutant control configuration. The system can be designed to provide flow rate and duration control by primarily providing increased surface ponding and/or having a deeper aggregate storage layer above the underdrain. This will allow for significant detention storage, which can be controlled via inclusion of an outlet structure at the downstream end of the underdrain.

Design Criteria and Considerations
Sand filters must meet the following design criteria. Deviations from the below criteria may be approved at the discretion of the City Engineer if it is determined to be appropriate:

Lining prevents storm water from impacting
groundwater and/or sensitive environmental
or geotechnical features. Incidental
infiltration, when allowable, can aid in
pollutant removal and groundwater recharge.
□ Contributing tributary area (≤ 5 acres).

Bigger BMPs require additional design
features for proper performance.

Contributing tributary area greater than 5
acres may be allowed at the discretion of the
[City Engineer} if the following conditions
are met: 1) incorporate design features (e.g.
flow spreaders) to minimizing short circuiting
of flows in the BMP and 2)incorporate
additional design features requested by the
City Engineer for proper performance of the
regional BMP.
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Siting and Design Intent/Rationale
□ Finish grade of facility is < 6%. Flatter surfaces reduce erosion and channelization within the facility.
□ Earthen side slopes are ≥ 3H:1V.

Gentler side slopes are safer, less prone to
erosion, able to establish vegetation more
quickly and easier to maintain.
□
Surface ponding is limited to a 36-hour
drawdown time.

Provides required capacity to treat back to
back storms. Exception to the 36 hour
drawdown criteria is allowed if additional
surface storage is provided using the curves in
Appendix B.4.2.
□
Surface ponding is limited to a 96-hour
drawdown time.

Prolonged surface ponding can create a
vector hazard.
□
Maximum ponding depth does not exceed 3
feet.

Surface ponding capacity lowers subsurface
storage requirements and results in lower cost
facilities. Deep surface ponding raises safety
concerns.
□
Sand filter bed consists of clean washed
concrete or masonry sand (passing ¼ inch
sieve) or sand similar to the ASTM C33
gradation.

Washing sand will help eliminate fines that
could clog the void spaces of the aggregate
storage layer.
□
Sand filter bed permeability is at least 1
in/hr.

A high filtration rate through the media
allows flows to quickly enter the aggregate
storage layer, thereby minimizing bypass.
□
Sand filter bed depth is at least 18 inches
deep.

Different pollutants are removed in various
zones of the media using several mechanisms.

Some pollutants bound to sediment, such as
metals, are typically removed within 18 inches
of the media.
□
Aggregate storage should be washed, bankrun gravel.

Proper storage layer configuration and
underdrain placement will minimize facility
drawdown time.
□
Inflow, underdrains and outflow structures
are accessible for inspection and
maintenance.

Maintenance will prevent clogging and ensure
proper operation of the flow control
structures.
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Siting and Design Intent/Rationale
□
Inflow must be non-erosive sheet flow (≤ 3
ft/s) unless an energy-dissipation device,
flow diversion/splitter or forebay is
installed.

Concentrated flow and/or excessive volumes
can cause erosion in a sand filter and can be
detrimental to the treatment capacity of the
system.
□
Underdrain outlet elevation should be a
minimum of 3 inches above the bottom
elevation of the aggregate storage layer.

A minimal separation from subgrade or the
liner lessens the risk of fines entering the
underdrain and can improve hydraulic
performance by allowing perforations to
remain unblocked.
□ Minimum underdrain diameter is 6 inches. Smaller diameter underdrains are prone to clogging.
□
Underdrains should be made of slotted,
PVC pipe conforming to ASTM D 3034 or
equivalent or corrugated, HDPE pipe
conforming to AASHTO 252M or
equivalent.

Slotted underdrains provide greater intake
capacity, clog resistant drainage, and reduced
entrance velocity into the pipe, thereby
reducing the chances of solids migration.
□
Overflow is safely conveyed to a
downstream storm drain system or discharge
point.

Planning for overflow lessens the risk of
property damage due to flooding.

Conceptual Design and Sizing Approach for Storm Water Pollutant Control Only To design a sand filter for storm water pollutant control only (no flow control required), the following steps should be taken:

1. Verify that siting and design criteria have been met, including placement requirements, contributing tributary area, and maximum finish grade slope.

2. Calculate the required DCV and/or flow rate per Appendix J.5.3 based on expected site design runoff for tributary areas.

3. Sand filter can be designed either for DCV or flow rate. To estimate the drawdown time, divide the average ponding depth by the permeability of the filter sand.

Conceptual Design and Sizing Approach when Storm Water Flow Control is Applicable Control of flow rates and/or durations will typically require significant surface ponding and/or aggregate storage volumes, and therefore the following steps should be taken prior to determination of storm water pollutant control design. Pre-development and allowable post-project flow rates and durations should be determined as discussed in Chapter 6 of the Manual.

1. Verify that siting and design criteria have been met, including placement requirements, contributing tributary area, and maximum finish grade slope. 2026-08-19 Item #12D Page 432 of 816
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3. If a sand filter cannot fully provide the flow rate and duration control required by the MS4 permit, an upstream or downstream structure with appropriate storage volume such as an underground vault can be used to provide remaining controls.

4. After the sand filter has been designed to meet flow control requirements, calculations must be completed to verify if storm water pollutant control requirements to treat the DCV have been met.

Maintenance Overview
Normal Expected Maintenance. Sand filters require routine maintenance to: remove accumulated materials such as sediment, trash, and debris from the forebay; and clear the underdrain(s). To ensure runoff is passed through the sand bed, sand at the top of the sand bed (approximately 2 inches, or more if necessary) must be removed and replaced to restore flow when the drain time exceeds 24-96 hours. A summary table of standard inspection and maintenance indicators is provided within this Fact Sheet.

Non-Standard Maintenance or BMP Failure. The normal expected maintenance described above ensures the BMP functionality. Lapses in the normal expected maintenance can lead to clogging of the BMP and runoff bypassing the filter. If clogging is observed, the BMP is not performing as intended to protect downstream waterways from pollution and/or erosion. In addition, clogged BMPs can lead to flooding, standing water and mosquito breeding habitat. Corrective maintenance and increased inspection and maintenance will be required. For persistent clogging or presence of mosquitos, contact the City reviewer to determine a permanent solution. For example, adding pretreatment measures within the tributary area draining to the BMP to intercept sediment, trash, and debris. Pretreatment components, especially for sediment, will extend the life of the sand bed. For mosquitos, a Vector Management Plan, prepared with concurrence from the County of San Diego Department of Environmental Health, may be required.
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Summary of Standard Inspection and Maintenance
The property owner is responsible to ensure inspection, operation and maintenance of permanent BMPs on their property unless responsibility has been formally transferred to an agency, community facilities district, homeowners association, property owners association, or other special district. Maintenance frequencies listed in this table are average/typical frequencies. Actual maintenance needs are site-specific, and maintenance may be required more frequently. Maintenance must be performed whenever needed, based on maintenance indicators presented in this table.

The BMP owner is responsible for conducting regular inspections to see when maintenance is needed based on the maintenance indicators.

Threshold/Indicator Maintenance Action Inspection and Maintenance Frequency Accumulation of sediment, litter, or
debris in forebay and/or filter bed
Remove and properly dispose of accumulated materials.
• Inspect monthly. If the forebay is 25%
full* or more in one month, increase inspection frequency to monthly plus after every 0.1-inch or larger storm event. •Remove any accumulated materials found
within the filter bed at each inspection.
•When the BMP includes a forebay,
materials must be removed from the forebay when the forebay is 25% full*, or if accumulation within the forebay blocks flow to the filter bed.

Standing water in BMP for longer than 24-96 hours following a storm event Make appropriate corrective measures to restore drainage such as removing obstructions of debris from the forebay, clearing underdrains or repairing/replacing clogged sand bed. • Inspect monthly and after every 0.5-inch
or larger storm event. If standing water is observed, increase inspection frequency to after every 0.1-inch or larger storm event. • Maintain when needed.
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Threshold/Indicator Maintenance Action Inspection and Maintenance Frequency Clogged sand bed
This is indicated when the drain time of
the surface of the sand bed exceeds 24-96 hours.

Remove and properly dispose sand from the top of the sand bed (approximately 2 inches of sand, or as much as needed to restore flow). Restore sand depth to the design depth. • Inspect monthly and after every 0.5-inch
or larger storm event. If standing water is observed, increase inspection frequency to after every 0.1-inch or larger storm event. • Maintain when needed.

Obstructed inlet or outlet structure Clear blockage. • Inspect monthly and after every 0.5-inch or larger storm event.
•Remove any accumulated materials found
at each inspection.

Presence of mosquitos/larvae For images of egg rafts, larva, pupa, and adult mosquitos, see http://www.mosquito.org/biology If mosquitos/larvae are observed: first, immediately remove and properly dispose any standing water by dispersing to nearby landscaping; second, make corrective measures as applicable to restore BMP drainage to prevent standing water. If mosquitos persist following corrective measures to remove standing water, the City reviewer shall be contacted to determine a solution. A different BMP type, or a Vector Management Plan prepared with concurrence from the County of San Diego Department of Environmental Health, may be required. • Inspect monthly and after every 0.5-inch
or larger storm event. If mosquitos are observed, increase inspection frequency to after every 0.1-inch or larger storm event. • Maintain when needed
Damage to structural components of the BMP such as weirs, underdrains, inlet or outlet structures Repair or replace as applicable. • Inspect annually.
• Maintain when needed.
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E.25 FT-4 Dry Extended Detention Basin
Location: Rolling Hills Ranch, Chula Vista, California; Photo Credit: Eric Mosolgo
Description
Dry extended detention basins are basins that have been designed to detain stormwater for an extended period to allow sedimentation and typically drain completely between storm events. A portion of the dissolved pollutant load may also be removed by filtration, uptake by vegetation, and/or through infiltration. The slopes, bottom, and forebay of dry extended detention basins are typically vegetated. Considerable stormwater volume reduction can occur in dry extended detention basins when they are located in permeable soils and are not lined with an impermeable barrier. dry extended detention basins are generally appropriate for developments of ten acres or larger, and have the potential for multiple uses including parks, playing fields, tennis courts, open space, and overflow parking lots. They can also be used to provide flow control by modifying the outlet control structure and providing additional detention storage.

Typical dry extended detention basins components include:
• Forebay for pretreatment
• Surface ponding for captured flows
• Vegetation selected based on basin use, climate, and ponding depth • Low flow channel, outlet, and overflow device
• Impermeable liner or uncompacted native soils at the bottom of the facility MS4 Permit Category
Flow-thru Treatment Control
Manual Category
Flow-thru Treatment Control
Applicable Performance
Standard
Pollutant Control
Flow Control
Primary Benefits
Treatment
Volume Reduction (Incidental)
Peak Flow Attenuation
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Typical plan and Section view of a Dry Extended Detention Basin BMP Design Adaptations for Project Goals
Flow-thru treatment BMP for stormwater pollutant control. The system is lined or un-lined to 2026-08-19 Item #12D Page 437 of 816
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provide incidental infiltration and designed to detain stormwater to allow particulates and associated pollutants to settle out. This configuration is considered to provide flow-thru treatment, not biofiltration treatment. Storage provided as surface ponding above a restricted outlet invert is considered detention storage and is included in calculations for the flow-thru treatment volume.

Integrated stormwater flow control and pollutant control configuration. Dry extended detention basins can also be designed for flow control. The surface ponding can be designed to accommodate higher volumes than the stormwater pollutant control volume and can utilize multi- stage outlets to mitigate both the duration and rate of flows within a prescribed range.

Design Criteria and Considerations
Dry extended detention basins must meet the following design criteria. Deviations from the below criteria may be approved at the discretion of the City Engineer if it is determined to be appropriate:

Lining prevents storm water from
impacting groundwater and/or sensitive
environmental or geotechnical features.

Incidental infiltration, when allowable, can
aid in pollutant removal and groundwater
recharge.
□
Contributing tributary area is large (typically
≥ 10 acres).

Dry extended detention basins require
significant space and are more cost-effective
for treating larger drainage areas.
□ Longitudinal basin bottom slope is 0 - 2%. Flatter slopes promote ponding and settling of particles.
□
Basin length to width ratio is
≥ 2:1 (L:W).

A larger length to width ratio provides a
longer flow path to promote settling.
□
Forebay is included that encompasses 20 -
30% of the basin volume.

A forebay to trap sediment can decrease
frequency of required maintenance.
□ Side slopes are ≥ 3H:1V.

Gentler side slopes are safer, less prone to
erosion, able to establish vegetation more
quickly and easier to maintain.
□
Surface ponding drawdown time is between
24 and 96 hours.

Minimum drawdown time of 24 hours
allows for adequate settling time and
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Siting and Design Intent/Rationale
maximizes pollutant removal. Maximum
drawdown time of 96 hours provides vector
control.
□
Minimum freeboard provided is ≥1 foot for
offline facilities and ≥2 feet for online
facilities.

Freeboard provides room for head over
overflow structures and minimizes risk of
uncontrolled surface discharge.
□
Inflow and outflow structures are accessible
by required equipment (e.g., vactor truck) for
inspection and maintenance.

Maintenance will prevent clogging and
ensure proper operation of the flow control
structures.
□
A low flow channel or trench with a ≥ 2%
slope is provided. A gravel infiltration trench
is provided where infiltration is allowable.

Aids in draining or infiltrating dry weather
flows.
□
Overflow is safely conveyed to a
downstream storm drain system or discharge
point. Size overflow structure to pass 100-
year peak flow.

Planning for overflow lessens the risk of
property damage due to flooding.
□
The maximum rate at which runoff is
discharged is set below the erosive threshold
for the site.

Extended low flows can have erosive
effects.

Conceptual Design and Sizing Approach for Storm Water Pollutant Control Only To design dry extended detention basins for stormwater pollutant control only (no flow control required), the following steps should be taken:

1. Verify that siting and criteria have been met, including placement requirements, contributing tributary area, forebay volume, and maximum slopes for basin sides and bottom.

2. Calculate the DCV per Appendix B based on expected site design runoff for tributary areas.

3. Use the sizing worksheet to determine flow-thru treatment sizing of the surface ponding of the dry extended detention basin, which includes calculations for a maximum 96-hour drawdown time.

Conceptual Design and Sizing Approach when Stormwater Flow Control is Applicable Control of flow rates and/or durations will typically require significant surface ponding volume, and therefore the following steps should be taken prior to determination of storm water pollutant control design. Pre-development and allowable post-project flow rates and durations should be determined as discussed in Chapter 6 of the manual.

1. Verify that siting and criteria have been met, including placement requirements, tributary area, and maximum slopes for basin sides and bottom.

2. Iteratively determine the surface ponding required to provide detention storage to reduce flow 2026-08-19 Item #12D Page 439 of 816
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rates and durations to allowable limits. Flow rates and durations can be controlled from detention storage by altering outlet structure orifice size(s) and/or water control levels. Multilevel orifices can be used within an outlet structure to control the full range of flows.

3. If a dry extended detention basin cannot fully provide the flow rate and duration control required by this manual, an upstream or downstream structure with appropriate storage volume such as an additional basin or underground vault can be used to provide remaining controls.

4. After the dry extended detention basin has been designed to meet flow control requirements, calculations must be completed to verify if storm water pollutant control requirements to treat the DCV have been met.

Maintenance Overview
Normal Expected Maintenance. Dry extended detention basins require routine maintenance to:
remove accumulated materials such as sediment, trash or debris; maintain vegetation health; and maintain integrity of side slopes, inlets, energy dissipators, and outlets. A summary table of standard inspection and maintenance indicators is provided within this Fact Sheet.

Non-Standard Maintenance or BMP Failure. If any of the following scenarios are observed, the BMP is not performing as intended to protect downstream waterways from pollution and/or erosion.

Corrective maintenance, increased inspection and maintenance, BMP replacement, or a different BMP type will be required.
• The BMP is not drained between storm events. Surface ponding longer than approximately 24 hours following a storm event may be detrimental to vegetation health, and surface or underground ponding longer than approximately 96 hours following a storm event poses a risk of vector (mosquito) breeding. Poor drainage can result from clogging of underlying native soils and/or the outlet structure. The specific cause of the drainage issue must be determined and corrected. If it is determined that the drainage of the basin relies on infiltration and the underlying native soils have been compacted or do not have the infiltration capacity expected, the City reviewer shall be contacted prior to any additional repairs or reconstruction. • Sediment, trash, or debris accumulation greater than 25% of the surface ponding volume within one month. This means the load from the tributary drainage area is too high, reducing BMP function or clogging the BMP. This would require pretreatment measures within the tributary area draining to the BMP to intercept the materials. • Erosion due to concentrated storm water runoff flow that is not readily corrected by adding erosion control blankets, adding stone at flow entry points, or minor re-grading to restore proper drainage according to the original plan. If the issue is not corrected by restoring the BMP to the original plan and grade, the City reviewer shall be contacted prior to any additional repairs or reconstruction.

Other Special Considerations. Some above-ground dry extended detention basins are vegetated structural BMPs. Vegetated structural BMPs that are constructed in the vicinity of, or connected to, an existing jurisdictional water or wetland could inadvertently result in creation of expanded waters or 2026-08-19 Item #12D Page 440 of 816
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wetlands. As such, vegetated structural BMPs have the potential to come under the jurisdiction of the United States Army Corps of Engineers, SDRWQCB, California Department of Fish and Wildlife, or the United States Fish and Wildlife Service. This could result in the need for specific resource agency permits and costly mitigation to perform maintenance of the structural BMP. Along with proper placement of a structural BMP, routine maintenance is key to preventing this scenario.

Underground dry extended detention basins are typically designed to be cleaned from above-ground using a vactor. If maintenance conditions require maintenance personnel to enter the underground structure, the maintenance personnel must be trained and certified in confined space entry. 2026-08-19 Item #12D Page 441 of 816
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Summary of Standard Inspection and Maintenance
The property owner is responsible to ensure inspection, operation and maintenance of permanent BMPs on their property unless responsibility has been formally transferred to an agency, community facilities district, homeowners association, property owners association, or other special district. Maintenance frequencies listed in this table are average/typical frequencies. Actual maintenance needs are site-specific, and maintenance may be required more frequently. Maintenance must be performed whenever needed, based on maintenance indicators presented in this table.

The BMP owner is responsible for conducting regular inspections to see when maintenance is needed based on the maintenance indicators.

During the first year of operation of a structural BMP, inspection is recommended at least once prior to August 31 and then monthly from September through May. Inspection during a storm event is also recommended. After the initial period of frequent inspections, the minimum inspection and maintenance frequency can be determined based on the results of the first year inspections. “25% full” is defined as ¼ of the depth from the design bottom elevation to the crest of the outflow structure (e.g., if the height to the outflow opening is 12 inches from the bottom elevation, then the materials must be removed when there is 3 inches of accumulation – this should be marked on the outflow structure).

Threshold/Indicator Maintenance Action Inspection and Maintenance Frequency Accumulation of sediment, litter, or debris in
forebay and/or basin
Remove and properly dispose of accumulated materials, (without damage to vegetation when applicable).
• Inspect monthly. If the forebay is 25% full*
or more in one month, increase inspection frequency to monthly plus after every 0.1-inch or larger storm event. •Remove any accumulated materials found
within the basin area at each inspection.
•When the BMP includes a forebay,
materials must be removed from the forebay when the forebay is 25% full*, or if accumulation within the forebay blocks flow to the basin.

Obstructed inlet or outlet structure Clear blockage. • Inspect monthly and after every 0.5-inch or larger storm event.
•Remove any accumulated materials found
at each inspection.
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Appendix E: BMP Design Fact Sheets
E-171 2023
Threshold/Indicator Maintenance Action Inspection and Maintenance Frequency Poor vegetation establishment (when the BMP includes vegetated surface by design) Re-seed, re-plant, or re-establish vegetation per original plans. • Inspect monthly.
• Maintain when needed.

Dead or diseased vegetation (when the BMP includes vegetated surface by design) Remove dead or diseased vegetation, re-seed, re-plant, or re-establish vegetation per original plans. • Inspect monthly.
• Maintain when needed.

Overgrown vegetation (when the BMP includes vegetated surface by design) Mow or trim as appropriate. • Inspect monthly.
• Maintain when needed.

Erosion due to concentrated irrigation flow Repair/re-seed/re-plant eroded areas and adjust the irrigation system.
• Inspect monthly.
• Maintain when needed.

Erosion due to concentrated storm water runoff flow
Repair/re-seed/re-plant eroded areas, and make appropriate corrective measures such as adding erosion control blankets, adding stone at flow entry points, or minor regrading to restore proper drainage according to the original plan. If the issue is not corrected by restoring the BMP to the original plan and grade, the City reviewer shall be contacted prior to any additional repairs or reconstruction. • Inspect after every 0.5-inch or larger storm
event. If erosion due to storm water flow has been observed, increase inspection frequency to after every 0.1-inch or larger storm event. • Maintain when needed. If the issue is not
corrected by restoring the BMP to the original plan and grade, the City reviewer shall be contacted prior to any additional repairs or reconstruction. 2026-08-19 Item #12D Page 443 of 816
Appendix E: BMP Design Fact Sheets
E-172 2023
Threshold/Indicator Maintenance Action Inspection and Maintenance Frequency Standing water in above-ground BMP for longer than 24-96 hours following a storm event Make appropriate corrective measures such as adjusting irrigation system, removing obstructions of debris or invasive vegetation, or removing/replacing clogged or compacted surface treatments and/or scarifying or tilling native soils. Always remove deposited sediments before scarification, and use a hand-guided rotary tiller. If it is determined that the drainage of the basin relies on infiltration and the underlying native soils have been compacted or do not have the infiltration capacity expected, the City reviewer shall be contacted prior to any additional repairs or reconstruction. • Inspect monthly and after every 0.5-inch or
larger storm event. If standing water is observed, increase inspection frequency to after every 0.1-inch or larger storm event. • Maintain when needed.

Standing water in underground BMP for longer than 24-96 hours following a storm event Make appropriate corrective measures such as removing obstructions at the outlet, clearing underdrains, or flushing fine sediment from aggregate layer when applicable. If it is determined that the drainage of the basin relies on infiltration and the underlying native soils have been compacted or do not have the infiltration capacity expected, the City reviewer shall be contacted prior to any additional repairs or reconstruction. • Inspect monthly and after every 0.5-inch or
larger storm event. If standing water is observed, increase inspection frequency to after every 0.1-inch or larger storm event. • Maintain when needed.
2026-08-19 Item #12D Page 444 of 816
Appendix E: BMP Design Fact Sheets
E-173 2023
Threshold/Indicator Maintenance Action Inspection and Maintenance Frequency Presence of mosquitos/larvae For images of egg rafts, larva, pupa, and adult mosquitos, see http://www.mosquito.org/biology If mosquitos/larvae are observed: first, immediately remove and properly dispose any standing water; second, make corrective measures as applicable to restore BMP drainage to prevent standing water. For underground detention basins, ensure access covers are tight fitting, with gaps or holes no greater than 1/16 inch, and/or install barriers such as inserts or screens that prevent mosquito access to the subsurface storage. If mosquitos persist following corrective measures to remove standing water, or if the BMP design does not meet the 96-hour drawdown criteria due to release rates controlled by an orifice installed on the underdrain, the City reviewer shall be contacted to determine a solution. A different BMP type, or a Vector Management Plan prepared with concurrence from the County of San Diego Department of Environmental Health, may be required. • Inspect monthly and after every 0.5-inch or
larger storm event. If mosquitos are observed, increase inspection frequency to after every 0.1-inch or larger storm event. • Maintain when needed
Damage to structural components such as
weirs, inlet or outlet structures
Repair or replace as applicable. • Inspect annually.
• Maintain when needed.
2026-08-19 Item #12D Page 445 of 816
Appendix E: BMP Design Fact Sheets
E-174 2023
E.26 FT-5 Proprietary Flow-Thru Treatment
Control BMPs
The purpose of this fact sheet is to help explain the potential role of proprietary BMPs in meeting flow thru treatment control BMP requirements. The fact sheet does not describe design criteria like the other fact sheets in this appendix because this information varies by BMP product model.

Criteria for Use of a Proprietary BMP as a Flow-Thru Treatment Control BMP A proprietary BMP may be acceptable as a “flow-thru treatment control BMP” under the following conditions:

1. The BMP is selected and sized consistent with the method and criteria described in Appendix B.6;

2. The BMP is designed and maintained in a manner consistent with its performance certifications (See explanation in Appendix B.6); and

3. The BMP is acceptable at the discretion of the City Engineer. The City Engineer has no obligation to accept any proprietary flow-thru treatment control BMP.

Criteria for Use of a Proprietary BMP as a Flow-Thru Treatment Control BMP Proprietary flow-thru BMPs must meet the same sizing guidance as other flow-thru treatment control BMPs. Guidance for sizing flow-thru BMPs to comply with requirements of this manual is provided in Appendix B.6.

Maintenance Overview
Refer to manufacturer for maintenance information.
2026-08-19 Item #12D Page 446 of 816
Appendix E: BMP Design Fact Sheets
E-175 2023 2023
E.27
PL Plant List
Plant Name Irrigation Requirements Preferred Location in Basin Applicable Bioretention Sections (Un-Lined Facilities) Applicability to Flow-Through Planter?
(Lined Facility)
Latin Name Common Name
Temporary
Irrigation during
Plant
Establishment
Period
Permanent
Irrigation (Drip
/ Spray)(1) Basin Bottom
Basin Side
Slopes
Section A
Treatment-Only
Bioretention in
Hydrologic Soil Group
A or B Soils
Section B
Treatment-Only
Bioretention in
Hydrologic Soil
Group C or D soils
Section C
Treatment Plus Flow
Control
Bioretention in
Hydrologic Soil
Group A or B Soils
Section D
Treatment Plus
Flow Control
Bioretention in
Hydrologic Soil
Group C or D Soils
NO
Applicable to Unlined Facilities
Only
(Bioretention
Only)
YES
Can Use in Lined or
Un-Lined Facility
(Flow-Through
Planter OR
Bioretention)
TREES(2)
Alnus rhombifolia White Alder X X X X X X X X
Platanus racemosa California Sycamore X X X X X X X X
Salix lasiolepsis Arroyo Willow X X X X X X X
Salix lucida Lance-Leaf Willow X X X X X X X
Sambucus mexicana Blue Elderberry X X X X X X X

SHRUBS / GROUNDCOVER
Achillea millefolium Yarrow X X X X X
Agrostis palens Thingrass X X X X X X X
Anemopsis californica Yerba Manza X X X X X X X
Baccharis douglasii Marsh Baccahris X X X X X X X X
Carex praegracillis California Field Sedge X X X X X X X X Carex spissa San Diego Sedge X X X X X X X X
Carex subfusca Rusty Sedge X X X X X X X X X
Distichlis spicata Salt Grass X X X X X X X X
Eleocharis
macrostachya
Pale Spike Rush X X X X X X X X
Festuca rubra Red Fescue X X X X X X X
Festuca californica California Fescue X X X X X X
Iva hayesiana Hayes Iva X X X X X
Juncus Mexicana Mexican Rush X X X X X X X X X
Jucus patens California Gray Rush X X X X X X X X X
Leymus condensatus
‘Canyon Prince’
Canyon Prince Wild Rye X X X X X X X X X
Mahonia nevinii Nevin’s Barberry X X X X X X X
Muhlenburgia rigens Deergrass X X X X X X X X X
Mimulus cardinalis Scarlet Monkeyflower X X X X X X
Ribes speciosum Fushia Flowering Goose. X X X X X
Rosa californica California Wild Rose X X X X X X
Scirpus cenuus Low Bullrush X X X X X X X X
Sisyrinchium bellum Blue-eyed Grass X X X X X

1. All plants will benefit from some supplemental irrigation during hot dry summer months, particularly those on basin side slopes and further inland. 2. All trees should be planted a min. of 10’ away from any drain pipes or structures. 2026-08-19 Item #12D Page 447 of 816

C I T Y O F E N C I N I T A S B M P D E S I G N M A N U A L F-1 2023
2023
Appendix
F
Appendix F Biofiltration Standard
and Checklist
2026-08-19 Item #12D Page 448 of 816
Appendix F: Biofiltration Standard and Checklist
F-2 2023
Introduction
The MS4 Permit and this manual define a specific category of stormwater pollutant treatment BMPs called “biofiltration BMPs.” The MS4 Permit (Section E.3.c.1) states:

Biofiltration BMPs must be designed to have an appropriate hydraulic loading rate to maximize stormwater retention and pollutant removal, as well as to prevent erosion, scour, and channeling within the BMP, and must be sized to:
a) Treat 1.5 times the DCV not reliably retained onsite, OR b) Treat the DCV not reliably retained onsite with a flow-thru design that has a total volume, including pore spaces and pre-filter detention volume, sized to hold at least 0.75 times the portion of the DCV not reliably retained onsite.

A project applicant must be able to affirmatively demonstrate that a given BMP is designed and sized in a manner consistent with this definition to be considered as a “biofiltration BMP” as part of a compliant stormwater management plan. Retention is defined in the MS4 Permit as evapotranspiration, infiltration, and harvest and use of stormwater vs. discharge to a surface water system.

Contents and Intended Uses
This appendix contains a checklist of the key underlying criteria that must be met for a BMP to be considered a biofiltration BMP. The purpose of this checklist is to facilitate consistent review and approval of biofiltration BMPs that meet the “biofiltration standard” defined by the MS4 Permit.

This checklist includes specific design criteria that are essential to defining a system as a biofiltration BMP; however it does not present a complete design basis. This checklist was used to develop BMP Fact Sheets for PR-1 biofiltration with partial retention and BF-1 biofiltration, which do present a complete design basis. Therefore, biofiltration BMPs that substantially meet all aspects of the Fact sheets PR-1 or BF-1 should be able to complete this checklist without additional documentation beyond what would already be required for a project submittal. 2026-08-19 Item #12D Page 449 of 816
Appendix F: Biofiltration Standard and Checklist
F-3 2023
Other biofiltration BMP designs3
(including both non-proprietary and proprietary designs) may also meet the underlying MS4 Permit requirements to be considered biofiltration BMPs. These BMPs may be classified as biofiltration BMPs if they (1) meet the minimum design criteria listed in this appendix, including the pollutant treatment performance standard in Appendix F.1, (2) are designed and maintained in a manner consistent with their performance certifications (See explanation in Appendix F.2), if applicable, and (3) are acceptable at the discretion of the City Engineer. The applicant may be required to provide additional studies and/or required to meet additional design criteria beyond the scope of this document in order to demonstrate that these criteria are met.

Organization
The checklist in this appendix is organized into the seven (7) main objectives associated with biofiltration BMP design. It describes the associated minimum criteria that must be met in order to qualify a biofiltration BMP as meeting the biofiltration standard. The seven main objectives are listed below. Specific design criteria and associated manual references associated with each of these objectives is provided in the checklist in the following section.

1. Biofiltration BMPs shall be allowed only as described in the BMP selection process in this manual (i.e., retention feasibility hierarchy).

2. Biofiltration BMPs must be sized using acceptable sizing methods described in this manual.

3. Biofiltration BMPs must be sited and designed to achieve maximum feasible infiltration and evapotranspiration.

4. Biofiltration BMPs must be designed with a hydraulic loading rate to maximize pollutant retention, preserve pollutant control/sequestration processes, and minimize potential for pollutant washout.

5. Biofiltration BMPs must be designed to promote appropriate biological activity to support and maintain treatment processes.

6. Biofiltration BMPs must be designed to prevent erosion, scour, and channeling within the BMP.

7. Biofiltration BMP must include operations and maintenance design features and planning considerations to provide for continued effectiveness of pollutant and flow control functions.
3 Defined as biofiltration designs that do not conform to the specific design criteria described in Fact Sheets PR-1 or BF-1. This category includes proprietary BMPs that are sold by a vendor as well as non-proprietary BMPs that are designed and constructed of primarily of more elementary construction materials 2026-08-19 Item #12D Page 450 of 816
Appendix F: Biofiltration Standard and Checklist
F-4 2023
Biofiltration Criteria Checklist
The applicant shall provide documentation of compliance with each criterion in this checklist as part of the project submittal. The right column of this checklist identifies the submittal information that is recommended to document compliance with each criterion. Biofiltration BMPs that substantially meet all aspects of Fact Sheets PR-1 or BF-1 should still use this checklist; however additional documentation (beyond what is already required for project submittal) should not be required.

1. Biofiltration BMPs shall be allowed to be used only as described in the BMP selection process based on a documented feasibility analysis.

Intent: This manual defines a specific prioritization of pollutant treatment BMPs, where BMPs that retain water (retained includes evapotranspired, infiltrated, and/or harvested and used) must be used before considering BMPs that have a biofiltered discharge to the MS4 or surface waters. Use of a biofiltration BMP in a manner in conflict with this prioritization (i.e., without a feasibility analysis justifying its use) is not permitted, regardless of the adequacy of the sizing and design of the system.
□
The project applicant has demonstrated
that it is not technically feasible to retain
the full DCV onsite.

Document feasibility analysis and findings
in SWQMP per Appendix C.

2. Biofiltration BMPs must be sized using acceptable sizing methods.

Intent: The MS4 Permit and this manual defines specific sizing methods that must be used to size biofiltration BMPs. Sizing of biofiltration BMPs is a fundamental factor in the amount of stormwater that can be treated and also influences volume and pollutant retention processes.
□
The project applicant has demonstrated
that biofiltration BMPs are sized to meet
one of the biofiltration sizing options
available (Appendix B).

Submit sizing worksheets (Appendix B.5)
or other equivalent documentation with
the SWQMP.

3. Biofiltration BMPs must be sited and designed to achieve maximum feasible infiltration and evapotranspiration.

Intent: Various decisions about BMP placement and design influence how much water is retained via infiltration and evapotranspiration. The MS4 Permit requires that biofiltration BMPs achieve maximum feasible retention (evapotranspiration and infiltration) of stormwater volume.
2026-08-19 Item #12D Page 451 of 816
Appendix F: Biofiltration Standard and Checklist
F-5 2023
□
The biofiltration BMP is sited to allow for
maximum infiltration of runoff volume
based on the feasibility factors considered
in site planning efforts. It is also designed
to maximize evapotranspiration through
the use of amended media and plants.

Document site planning and feasibility
analyses in project submittal per Section

5.4. □
The biofiltration BMP meets the annual
retention target specified in Appendix B.

Included documentation that annual
retention target is met.

4. Biofiltration BMPs must be designed with a hydraulic loading rate to maximize pollutant retention, preserve pollutant control processes, and minimize potential for pollutant washout.

Intent: Various decisions about biofiltration BMP design influence the degree to which pollutants are retained. The MS4 Permit requires that biofiltration BMPs achieve maximum feasible retention of storm water pollutants.
□
Media selected for the biofiltration BMP
meets minimum quality and material
specifications, including the maximum
allowable design filtration rate and
minimum thickness of media.

OR
Provide documentation that media meets
the specifications.
2026-08-19 Item #12D Page 452 of 816
Appendix F: Biofiltration Standard and Checklist
F-6 2023
□
Alternatively, for proprietary designs and
custom media mixes not meeting the
media specifications, field scale testing
data are provided to demonstrate that
proposed media meets the pollutant
treatment performance criteria in Section
F.1 below.

Provide documentation of performance
information as described in Section F.1.
□
To the extent practicable, filtration rates
are outlet controlled (e.g., via an
underdrain and orifice/weir) instead of
controlled by the infiltration rate of the
media.

Include outlet control in designs or
provide documentation of why outlet
control is not practicable.
□
The water surface drains to at least 12
inches below the media surface within 24
hours from the end of storm event flow to
preserve plant health and promote healthy
soil structure.

Include calculations to demonstrate that
drawdown rate is adequate.
□ If nutrients are a pollutant of concern,
design of the biofiltration BMP follows
nutrient-sensitive design criteria.

Follow specifications for nutrient sensitive
design in Fact Sheet BF-2. Or provide
alternative documentation that nutrient
treatment is addressed and potential for
nutrient release is minimized.
□
Media gradation calculations or geotextile
selection calculations demonstrate that
migration of media between layers will be
prevented and permeability will be
preserved.

Follow specification for choking layer or
geotextile in Fact Sheet PR-1 or BF-1. Or
include calculations to demonstrate that
choking layer is appropriately specified.

5. Biofiltration BMPs must be designed to promote appropriate biological activity to support and maintain treatment processes.

Intent: Biological processes are an important element of biofiltration performance and longevity.
□
Plants have been selected to be tolerant of
project climate, design ponding depths
and the treatment media composition.

Provide documentation justifying plant
selection. Refer to the plant list in
Appendix E.26.
□
Plants have been selected to minimize
irrigation requirements.

Provide documentation describing
irrigation requirements for establishment
and long term operation.
2026-08-19 Item #12D Page 453 of 816
Appendix F: Biofiltration Standard and Checklist
F-7 2023
□
Plant location and growth will not impede
expected long-term media filtration rates
and will enhance long term infiltration
rates to the extent possible.

Provide documentation justifying plant
selection. Refer to the plant list in
Appendix E.26.

6. Biofiltration BMPs must be designed with a hydraulic loading rate to prevent erosion, scour, and channeling within the BMP.

Intent: Erosion, scour, and/or channeling can disrupt treatment processes and reduce biofiltration effectiveness.
□
Scour protection has been provided for
both sheet flow and pipe inflows to the
BMP, where needed.

Provide documentation of scour
protection as described in Fact Sheets PR1 or BF-1 or approved equivalent. □ Where scour protection has not been
provided, flows into and within the BMP
are kept to non-erosive velocities.

Provide documentation of design checks
for erosive velocities as described in Fact
Sheets PR-1 or BF-1 or approved
equivalent.
□
For proprietary BMPs, the BMP is used in
a manner consistent with manufacturer
guidelines and conditions of its third-party
certification4
(i.e., maximum tributary area,
maximum inflow velocities, etc., as
applicable).

Provide copy of manufacturer
recommendations and conditions of thirdparty certification.

7. Biofiltration BMP must include operations and maintenance design features and planning considerations for continued effectiveness of pollutant and flow control functions.

Intent: Biofiltration BMPs require regular maintenance in order provide ongoing function as intended. Additionally, it is not possible to foresee and avoid potential issues as part of design; therefore plans must be in place to correct issues if they arise. 4 Certifications or verifications issued by the Washington Technology Acceptance Protocol-Ecology program and the New Jersey Corporation for Advanced Technology programs are typically accompanied by a set of guidelines regarding appropriate design and maintenance conditions that would be consistent with the certification/verification 2026-08-19 Item #12D Page 454 of 816
Appendix F: Biofiltration Standard and Checklist
F-8 2023
□
The biofiltration BMP O&M plan
describes specific inspection activities,
regular/periodic maintenance activities
and specific corrective actions relating to
scour, erosion, channeling, media
clogging, vegetation health, and inflow
and outflow structures.

Include O&M plan with project submittal
as described in Chapter 7.
□
Adequate site area and features have been
provided for BMP inspection and
maintenance access.

Illustrate maintenance access routes,
setbacks, maintenance features as needed
on project water quality plans.
□
For proprietary biofiltration BMPs, the
BMP maintenance plan is consistent with
manufacturer guidelines and conditions of
its third-party certification (i.e.,
maintenance activities, frequencies).

Provide copy of manufacturer
recommendations and conditions of thirdparty certification. 2026-08-19 Item #12D Page 455 of 816
Appendix F: Biofiltration Standard and Checklist
F-9 2023
F.1Pollutant Treatment Performance Standard
Standard biofiltration BMPs that are designed following the criteria in Fact Sheets PR-1 and BF-1 are presumed to the meet the pollutant treatment performance standard associated with biofiltration BMPs. This presumption is based on the MS4 Permit Fact Sheet which cites analyses of standard biofiltration BMPs conducted in the Ventura County Technical Guidance Manual (July 2011).

For BMPs that do not meet the biofiltration media specification and/or the range of acceptable media filtration rates described in Fact Sheet, PR-1 and BF-1, additional documentation must be provided to demonstrate that adequate pollutant treatment performance is provided to be considered a biofiltration BMP. Project applicants have three options for documenting compliance:

1. Project applicants may provide documentation to substantiate that the minor modifications to the design is expected to provide equal or better pollutant removal performance for the project pollutants of concern than would be provided by a biofiltration design that complies with the criteria in Fact Sheets PR-1 and BF-1. Minor modifications are design elements that deviate only slightly from standard design criteria and are expected to either not impact performance or to improve performance compared to standard biofiltration designs. The reviewing agency has the discretion to accept or reject this documentation and/or request additional documentation to substantiate equivalent or better performance to BF-1 or PR-1, as applicable. Examples of minor deviations include:
• Different particle size distribution of aggregate, with documentation that system filtration rate will meet specifications.
• Alternative source of organic components, with documentation of material suitability and stability from appropriate testing agency. • Specialized amendments to provide additional treatment mechanisms, and which have negligible potential to upset other treatment mechanisms or otherwise deteriorate performances.

2. For proprietary BMPs, project applicants may provide evidence that the BMP has been certified for use as part of the Washington State Technology Assessment Protocol-Ecology certification program and meets each of the following requirements:
a. The applicant must demonstrate (using the checklist in this Appendix) that the BMP meets all other conditions to be considered as a biofiltration BMP. For example, a cartridge media filter or hydrodynamic separator would not meet biofiltration BMP design criteria regardless of Technology Acceptance Protocol-Ecology certification because they do not support effective biological processes. 2026-08-19 Item #12D Page 456 of 816
Appendix F: Biofiltration Standard and Checklist
F-10 2023
b. The applicant must select BMPs that have an active Technology Acceptance ProtocolEcology certification, with General Use Level Designation for the appropriate project pollutants of concern as identified in Table F.1-1. The list of certified technologies is updated as new technologies are approved (link below). Technologies with Pilot Use Level Designation and Conditional Use Level Designations are not acceptable. Refer to: http://www.ecy.wa.gov/programs/wq/stormwater/newtech/technologies.html. c. The applicant must demonstrate that BMP is being used in a manner consistent with all conditions of the Technology Acceptance Protocol-Ecology certification while meeting the flow rate or volume design criteria that is required for biofiltration BMPs under this manual. Conditions of Technology Acceptance Protocol-Ecology certification are available by clicking on the technology name at the website listed in bullet b. Additional discussion about sizing of proprietary biofiltration BMPs to comply with applicable sizing standards is provided below in Section F.2.

3. For BMPs that do not fall into options 1 or 2 above, the City Engineer may allow the applicant to submit alternative third-party documentation that the pollutant treatment performance of the system is consistent with the performance levels associated with the necessary Technology Acceptance Protocol-Ecology certifications. Table F.1-1 describes the required levels of certification and Table F.l-2 describes the pollutant treatment performance levels associated with each level of certification. Acceptance of this approach is at the sole discretion of the City Engineer. If Technology Acceptance Protocol-Ecology certifications are not available, preference shall be given to:
a. Verified third-party, field-scale testing performance under the Technology Acceptance Reciprocity Partnership Tier II Protocol. This protocol is no longer operated, however this is considered to be a valid protocol and historic verifications are considered to be representative provided that product models being proposed are consistent with those that were tested. Technology Acceptance Reciprocity Partnership verifications were conducted under New Jersey Corporation for Advance Testing and are archived at the website linked below. Note that Technology Acceptance Reciprocity Partnership verifications must be matched to pollutant treatment standards in Table F.1-2 then matched to an equivalent Technology Acceptance Protocol-Ecology certification in Table F.1-1.
b. Verified third-party, field-scale testing performance under the New Jersey Corporation for Advance Testing protocol. Note that New Jersey Corporation for Advance Testing verifications must be matched to pollutant treatment standards in Table F.1-2 then matched to an equivalent Technology Acceptance Protocol-Ecology certification in Table F.1-1.
2026-08-19 Item #12D Page 457 of 816
Appendix F: Biofiltration Standard and Checklist
F-11 2023
A list of field-scale verified technologies under Technology Acceptance Reciprocity Partnership Tier II and New Jersey Corporation for Advance Testing can be accessed at:
http://www.njcat.org/verification-process/technology-verification-database.html (refer to field verified technologies only).

Table F.1-1: Required Technology Acceptance Protocol-Ecology Certifications for Polltuants of Concern for Biofiltration Performance Standard
Project Pollutant of Concern Required Technology Acceptance ProtocolEcology Certification for Biofiltration Performance Standard
Trash Basic Treatment, Phosphorus Treatment, Enhanced
Treatment
Sediments Basic Treatment, Phosphorus Treatment, Enhanced Treatment
Oil and Grease Basic Treatment, Phosphorus Treatment, Enhanced Treatment
Nutrients Phosphorus Treatment1
Metals Enhanced Treatment
Pesticides Basic Treatment (including filtration)2 Phosphorus Treatment, Enhanced Treatment
Organics Basic Treatment (including filtration)2 Phosphorus Treatment, Enhanced Treatment
Bacteria and Viruses Basic Treatment (including bacteria removal processes)3 , Phosphorus Treatment, Enhanced
Treatment
1 – There is no Technology Acceptance Protocol-Ecology equivalent for nitrogen compounds; however systems that are designed to retain phosphorus (as well as meet basic treatment designation), generally also provide treatment of nitrogen compounds. Where nitrogen is a pollutant of concern, relative performance of available certified systems for nitrogen removal should be considered in BMP selection.
2 – Pesticides, organics, and oxygen demanding substances are typically addressed by particle filtration consistent with the level of treatment required to achieve Basic treatment certification; if a system with Basic treatment certification does not provide filtration, it is not acceptable for pesticides, organics or oxygen demanding substances. 3 – There is no Technology Acceptance Protocol-Ecology equivalent for pathogens (viruses and bacteria), and testing data are limited because of typical sample hold times. Systems with Technology Acceptance Protocol-Ecology Basic Treatment must be include one or more significant bacteria removal process such as media filtration, physical sorption, predation, reduced redox conditions, and/or solar inactivation. Where design options are available to enhance pathogen removal (i.e., pathogen-specific media mix offered by vendor), this design variation should be used. 2026-08-19 Item #12D Page 458 of 816
Appendix F: Biofiltration Standard and Checklist
F-12 2023
Table F.1-2: Performance Standards for Technology Acceptance Protocol-Ecology Certification Performance Goal Influent Range Criteria
Basic Treatment 20 – 100 mg/L TSS Effluent goal ≤ 20 mg/L TSS 100 – 200 mg/L TSS ≥ 80% TSS removal
>200 mg/L TSS > 80% TSS removal
Enhanced
(Dissolved Metals)
Treatment
Dissolved copper 0.005 – 0.02
mg/L Must meet basic treatment goal
and better than basic treatment
currently defined as >30%
dissolved copper
removal
Dissolved zinc 0.02 – 0.3 mg/L
Must meet basic treatment goal
and better than basic treatment
currently defined as >60%
dissolved zinc
removal
Phosphorous
Treatment
Total phosphorous 0.1 – 0.5
mg/L Must meet basic treatment goal and
exhibit ≥50% total phosphorous
removal
Oil Treatment Total petroleum hydrocarbon >
10 mg/L No ongoing or recurring visible
sheen in effluent
Daily average effluent Total
petroleum hydrocarbon
concentration < 10 mg/L Maximum
effluent Total petroleum
hydrocarbon concentration for a 15
mg/L for a discrete (grab) sample
Pretreatment 50 – 100 mg/L TSS ≤ 50 mg/L TSS
≥ 200 mg/L TSS ≥ 50% TSS removal
2026-08-19 Item #12D Page 459 of 816
Appendix F: Biofiltration Standard and Checklist
F-13 2023
F.2 Guidance on Sizing and Design of Approved
Equivalent Proprietary Biofiltration Systems
This section explains the general process for design and sizing of approved equifvalent proprietary biofiltration systems. This section assumes that the BMPs have been selected based on the criteria in Section F.1.

F.2.1 Guidance on Design per Conditions of
Certification/Verification
The biofiltration standard and checklist in this appendix requires that “the BMP is used in a manner consistent with manufacturer guidelines and conditions of its third-party certification.” Practically, what this means is that the BMP is used in the same way in which it was tested and certified. For example, it is not acceptable for a BMP of a given size to be certified/verified with a 100 gallon per minute treatment rate and be applied at a 150 gallon per minute treatment rate in a design.

Certifications or verifications issued by the Washington Technology Acceptance Protocol-Ecology program and the Technology Acceptance Reciprocity Partnership or New Jersey Corporation for Advance Testing programs are typically accompanied by a set of guidelines regarding appropriate design and maintenance conditions that would be consistent with the certification/verification. It is common for these approvals to specify the specific model of BMP, design capacity for given unit sizes, type of media that is the basis for approval, and/or other parameter. The applicant must demonstrate conclusively that the proposed application of the BMP is consistent with these criteria.

For alternate non-proprietary systems that do not have a Technology Acceptance ProtocolEcology/Technology Acceptance Reciprocity Partnership / New Jersey Corporation for Advance Testing certification (but which still must provide quantitative data per Appendix F.1), it must be demonstrate that the configuration and design proposed for the project is reasonably consistent with the configuration and design under which the BMP was tested to demonstrate compliance with Appendix F.1.

F.2.2 Sizing of Flow-Based Biofiltration BMP
This sizing method is only available when the BMP meets the pollutant treatment performance standard in Appendix F.1.

Approved equivilanet proprietary biofiltration systems are typically designed as a flow-based BMPs (i.e., a constant treatment capacity with negligible storage volume). Proprietary biofiltration is only acceptable if the sizing criteria in this Appendix and the retention performance standard identified in Appendix B.5 are satisfied. The applicable sizing method for biofiltration for this type of BMP is to treat 1.5 times the flow based DCV.. The applicable sizing method for biofiltration is therefore 2026-08-19 Item #12D Page 460 of 816
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reduced to: Treat 1.5 times the DCV.

The following steps should be followed to demonstrate that the system is sized to treat 1.5 times the DCV.

1. Calculate the flow rate required to meet the pollutant treatment performance standard without scaling for the 1.5 factor. Options include either:
a. Calculate the runoff flow rate from a 0.2 inch per hour uniform intensity precipitation event (See methodology below), or
b. Conduct a continuous simulation analysis to compute the size required to capture and treat 80 percent of average annual runoff; for small catchments, 5-minute precipitation data should be used to account for short time of concentration. Nearest rain gage with 5-minute precipitation data is allowed for this analysis.

2. Multiply the flow rate from Step 1 by 1.5 to compute the design flow rate for the biofiltration system.

3. Based on the conditions of certification/verification (discussed above), establish the design capacity, as a flow rate, of a given sized unit.

4. Demonstrates that an appropriate unit size and number of units is provided to provide a flow rate that meets the required flow rate from Step 2.

5. Provide supplemental retention BMPs that will satisfy retention requirements in Appendix B.2.

6. Demonstrate that the BMP design is expected to last 10 years before major maintenance is anticipated per guidance in Appendix B.5.5
Flow-based treatment control BMPs shall be sized to filter or treat the maximum flow rate of runoff produced from a rainfall intensity of 0.2 inch of rainfall per hour,. The following hydrologic method shall be used to calculate the flow rate to be filtered or treated:
[ = × L ×
Where:

Q = Design flow rate in cubic feet per second
C = Runoff factor, area-weighted estimate using Table B.1-1. i = Rainfall intensity of 0.2 in/hr.

A = Tributary area (acres) which includes the total area draining to the BMP, including any offsite or onsite areas that comingle with project runoff and drain to the BMP. Refer to Section

3.3.3 for additional guidance. Street projects consult Section 1.4.3. 2026-08-19 Item #12D Page 461 of 816
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Worksheet F.2-1: Flow-Thru Design Flows
Flow-Based Design Flows Worksheet B.6-1
1 Biofiltration Adjustment Factor (Step 2) AF= 1.5 unitless 2 Design rainfall intensity i= 0.20 in/hr
3 Area tributary to BMP (s) A= acres
4 Area-weighted runoff factor (estimate using
Appendix B.2) C= unitless
5 Calculate Flow Rate = AF x (C x i x A) Q= cfs
1) Approved equivalent proprietary systems, if used, shall provide certified treatment capacity equal to or greater than the calculated flow rate in Line 5; certified treatment capacity per unit shall be consistent with third party certifications.
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F.3Bioretention Soil Media (BSM)
SECTION 803 - BIORETENTION SOIL MEDIA COMPOSITION, TESTING, AND

INSTALLATION
803-1 GENERAL.

Bioretention Soil Media (BSM) is intended to filter storm water and support plant growth while minimizing the leaching of potential pollutants. This specification includes requirements that apply to BSM used in stormwater treatment BMPs, including bioretention and biofiltration. 803-2 BLENDED BSM CRITERIA AND TESTING REQUIREMENTS
803-2.1 General. Blended BSM shall consist of 60% to 80% by volume sand, up to 20% by volume topsoil, and up to 20% by volume compost. Sand, Topsoil, and Compost used in BSM shall conform to requirements listed in Sections 803-3, 803-4, and 803-5, respectively. For bioretention/biofiltration with outlet-controlled designs, it is likely that topsoil will need to be omitted or reduced to achieve permeability targets.

Alternative mix components and proportions may be utilized, provided that the whole blended mix conforms to whole BSM criteria, detailed in Section 803-2.3 through 803-2.5. Alternative mix designs may include alternative proportions and/or alternative organic amendments. Alternative mixes are subject to approval by the reviewing jurisdiction. Alternative mixes that use an alternative organic component (rather than compost) may be necessary when BMPs are installed in areas with nitrogen or phosphorus impaired receiving waters in order to meet more stringent BSM quality requirements as detailed in Section 803-2.5.
803-2.2 Testing and Submittals. At least 30 days prior to ordering materials, the Contractor shall submit the following to the local jurisdiction reviewer (upon request): source/supplier of BSM, location of source/supplier, a physical sample of the BSM, whole BSM test results from a third party independent laboratory, test results for individual component materials as required, and description of proposed methods and schedule for mixing, delivery, and placement of BSM. The test results shall be no older than 120 days and shall accurately represent the materials and feed stocks that are currently available from the supplier.

Test results shall demonstrate conformance to agronomic suitability and hydraulic suitability criteria listed in Sections 803-2.3 and 803-2.4, respectively. BSM for use in BMPs in areas with water quality impairments in receiving waters shall also comply with applicable Chemical Suitability criteria in Section 803-2.5. No delivery, placement, or planting of BSM shall begin until test results confirm the suitability of the BSM. The Contractor shall submit a written request for approval which shall be 2026-08-19 Item #12D Page 463 of 816
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accompanied by written analysis results from a written report of a testing agency. The testing agency must be registered by the State for agronomic soil evaluation laboratory test fees shall be paid for by the Contractor.
803-2.3 Agronomic Suitability. The BSM shall conform to the requirements herein to support plant growth. BSM which requires amending to comply with the below specifications shall be uniformly blended and tested in its blended state prior to testing and delivery. a) pH range shall be between 6.0-8.5.
b) Salinity shall be between 0.5 and 3.0 millimho/cm (as measure by electrical conductivity) c) Sodium absorption ratio (SAR) shall be less than 5.0 d) Chloride shall be less than 800 ppm.
e) Cation exchange capacity shall be greater than 10 meq/100 g. f) Organic matter shall be between 2 and 5%.
g) Carbon:Nitrogen ratio shall be between 12 and 40 (15 to 40 preferred).

Textural class fraction shall adhere to limits in Table 803-2.1, as determined by ASTM Method D422 or an approved alternative method:

TABLE 803-2.3
Textural Class (ASTM
D422)
Size Range Mass Fraction
(percent)
Gravel Larger than 2 mm 0 to 25 of total sample
Clay Smaller than 0.005 mm 0 to 5 of non-gravel
fraction
Test results shall show the following information:
a) Date of testing
b) Project name, contractor name, and source of materials and supplier name c) Copies of all testing reports including, at a minimum, analytical results sufficient to confirm compliance with all requirements listed in this section. 803-2.4 Hydraulic Suitability. BSM shall meet the have appropriate hydraulic properties for filtering stormwater. The BSM shall conform to the requirements herein to support plant growth. BSM which 2026-08-19 Item #12D Page 464 of 816
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requires amending, shall be uniformly blended and tested in its blended state prior to testing and delivery.
803-2.4.1 Testing. The saturated hydraulic conductivity of the whole BSM shall be measured according to the method detailed in the measurement of hydraulic conductivity (USDA Handbook 60, method 34b), commonly available as part of standard agronomic soil evaluation, or ASTM D24234 Permeability of Granular Soils (at approximately 85% relative compaction Standard Proctor, ASTM D698). BSM shall conform to hydraulic criteria associated with the BMP design configuration that best applies to the facility where the BSM will be installed (Section 803-2.4.2 or 803-2.4.3). 803-2.4.2 Systems with Unrestricted Underdrain System (i.e., media control). For systems with underdrains that are not restricted, the BSM shall meet the minimum and maximum measured hydraulic conductivity found in Table 803-2.4 to ensure adequate flow rate through the BMP and longevity of the system but reduce excessive velocities through the media. In all cases, an upturned elbow system on the underdrain, measuring 9 to 12 inches above the invert of the underdrain, should be used to control velocities in the underdrain pipe and reduce potential for solid migration through the system.
803-2.4.3 Systems with Restricted Underdrain System (i.e., outlet control). For systems in which the flow rate of water through the media is controlled via an outlet control device (e.g., orifice or valve) affixed to the outlet of the underdrain system, the hydraulic conductivity of the media should meet the requirements in Table 803-2.4 and the outlet control device should control the flow rate to between 5 and 12 inches per hour. This configuration reduces the sensitivity of system performance to the hydraulic conductivity, compaction, and clogging of the material, reduces the likelihood of preferential flow through media, and allows more precise design and control of system flow rates. For these reasons, outlet control should be considered the preferred design option over unrestricted underdrain systems.
803-2.4.4 Systems without Underdrains. For systems without underdrains, the BSM shall have a hydraulic conductivity of at least 5 inches per hour, or at least 2 times higher than the design infiltration rate of the underlying soil, whichever is greater.

Table 803-2.4.

Hydraulic Conductivity Requirements
Underdrain System Minimum (in/hr) Maximum (in/hr)
Unrestricted (media control) 8 24
Restricted (outlet control)
Preferred Design Option. 20 80
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803-2.5 Chemical Suitability for Areas Draining to Impaired Receiving Waters. 803-2.5.1 General. The chemical suitability criteria listed in this section do not apply to systems without underdrains, unless groundwater is impaired or susceptible to nutrient contamination. Limits for a given parameter only apply if that parameter is associated with a water quality impairment, priority water quality condition, and/or TMDL in the receiving water. Limits may be waived at the discretion of the reviewing jurisdiction if it is determined by the jurisdiction that it is unreasonable to meet the specification using locally-available materials (available within 100 miles). 803-2.5.2 Testing. Potential for pollutant leaching shall be assessed using either the Saturated Media Extract Method (aka, Saturation Extract) that is commonly performed by agronomic laboratories or the Synthetic Precipitation Leaching Procedure (SPLP) (EPA SW-846, Method 1312). If the saturation extract method is used, samples may be rinsed with up to five pore volumes before collecting extract for analysis.
803-2.5.3 BSM Limits in Areas Draining to Impaired Receiving Waters. The limits in this section are in terms of the concentration of a parameter in water that has been contacted with the BSM.

Table 803-2.5.3
Applicable Pollutant(s) Saturation Extract or SPLP Criteria Phosphorus* < 1 mg/L
Zinc < 1 mg/L
Copper < 0.04 mg/L
Lead < 0.025 mg/L
Arsenic < 0.02 mg/L
Cadmium < 0.01 mg/L
Mercury < 0.01 mg/L
Selenium < 0.01 mg/L
803-2.5.4 Alternative BSM for Reduced Phosphorus Leaching. In areas with impaired receiving waters, alternative BSM should be considered, especially if receiving waters are phosphorus impaired.

BSM with 20% compost may result in phosphorus leaching and soluble phosphorus test results in excess of the 1 mg/L limit presented in Table 803-2.5.3 Alternative organic amendments, such as coco coir pith and/or composted wood products, in place of compost should be considered in these areas.

Sand and soil components with higher levels of iron and aluminum should also be considered to limit the solubility of phosphorus.
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803-5.5.5. Nitrogen Impaired Receiving Waters. In areas with a downstream water quality impairment or TMDL for nitrogen, a combination of BSM composition and BMP design shall be used to reduce the potential for nitrate leaching from BMPs. • BSM: The C:N ratio of BSM shall be between 15 and 40 to reduce the potential for nitrate leaching.
• BMP design: BMPs shall be designed to either enhance infiltration into underlying soils or with internal water storage to promote reduction of nitrogen:
o If a BMP is installed with a liner, the BMP must include an internal saturated zone, consisting of at least an 18-inch thick layer of gravel, to enhance denitrification. o If a BMP does not include a liner, it must be installed with a retention zone below the underdrain discharge elevation, consisting of at least an 18-inch thick layer of gravel, to enhance infiltration into underlying soils.
803-3 SAND FOR BSM.
803-3.1 General. Sand used in BSM should preferably be washed prior to delivery. If sand is not washed it must still meet sieve analysis requirements in Table 1. 803-3.2 Gradation Limits. A sieve analysis shall be performed in accordance with California Test 202, ASTM D 422, or approved equivalent method to demonstrate compliance with the gradation limits shown in Table 803-3.2. Fines passing the No. 200 sieve shall be non-plastic. 2026-08-19 Item #12D Page 467 of 816
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TABLE 803-3.2
Percentage Passing Sieve (by weight)
Sieve Size (ASTM D422) Minimum Maximum
3/8 inch 100 100
#4 90 100
#8 70 100
#16 40 95
#30 15 70
#40 5 55
#100 0 15
#200 0 5
803-4 TOPSOIL FOR BSM.
803-4.1 General. Topsoil shall be free of hazardous materials and shall be consistent with a common definition of topsoil. Decomposed granite and derivatives of decomposed granite are not considered to be topsoil for the purpose of this specification.
803-4.2 Textural Class. Topsoil shall be classified as a sandy loam or a loamy sand according to the US Department of Agriculture soil classification system. In addition, a textural class analysis shall be performed in accordance with ASTM D422, or an approved alternative method to demonstrate compliance with the gradation limits in Table 803-4.2.

Table 803-4.2
Textural Class (ASTM
D422)
Size Range Mass Fraction
(percent)
Gravel Larger than 2 mm 0 to 25 of total sample
Clay Smaller than 0.005 mm 0 to 15 of non-gravel
fraction
803-5 COMPOST FOR BSM.
803-5.1 General. Compost shall be produced at a facility inspected and regulated by the local enforcement agency for CalRecycle. Compost should also preferably be certified by the U.S.

Composting Council’s Seal of Testing Assurance Program (USCC STA) or an approved equivalent program. Compost shall not be produced from biosolids feedstock. 803-5.1.1 Gradation Limits. A sieve analysis shall be performed in accordance with ASTM D 422, 2026-08-19 Item #12D Page 468 of 816
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or approved equivalent method to demonstrate compliance with the gradation limits show in Table 803-5.1.1.

Table 803-5.1.1
Sieve Size (ASTM D422) Percent Passing Sieve (by weight) 1/2” 97 to 100
2 mm 40 to 90
803-5.1.2 Material Content. Organic Material Content shall be 35% to 100% by dry weight and moisture shall be 25% to 60% wet weight basis. Physical contaminants (manmade inert materials) shall not exceed 1% by dry weight.
803-5.2 Compost Testing. Compost shall meet the following requirements as demonstrated through standard agronomic testing methods:
a) Carbon to nitrogen (C:N) ratio. C:N shall be between 15:1 and 40:1, preferably above 20:1 to reduce the potential for nitrogen leaching/washout. b) pH. pH shall be between 6.0 and 8.5.
c) Soluble Salt Concentration. Soluble Salt Concentration shall be less than 10 dS/m. (Method TMECC 4.10-A, USDA and U.S. Composting Council). d) Stability. Carbon Dioxide evolution rate shall be less than 3.0 mg CO2-C per g compost organic matter (OM) per day or less than 6 mg CO2-C per g compost carbon per day, whichever unit is reported. (Method TMECC 5.08-B, USDA and U.S. Composting Council). Alternatively a Solvita rating of 5.5 or higher is acceptable. 803-5.2.1 Pathogens and Pollutant Limits. Select pathogens shall pass US EPA Class A standard, 40 CFR Section 503.32(a). Trace Metals shall pass US EPA Class A standard, 40 CFR Section 503.13, Table 1 for Ceiling Concentrations.
803-6 DELIVERY, STORAGE, HANDLING, AND PAYMENT
803-6.1 General. BSM shall be thoroughly mixed prior to delivery using mechanical mixing methods such as a drum mixer. The Contractor shall protect soils and mixes from absorbing excess water and from erosion at all times.
803-6.1.1 Delivery. The Contractor shall not deliver or place soils in wet or muddy conditions. 2026-08-19 Item #12D Page 469 of 816
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803-6.1.2 Storage. The Contractor shall not store materials unprotected during large rainfall events (>0.25 inches). If water is introduced into the material while it is stockpiled, the Contractor shall allow the material to drain to the acceptance of the reviewing jurisdiction before placement. 803-6.1.3 Handling and Placement. BSM shall be lightly compacted and placed in loose lifts approximately 12 inches (300 mm) to ensure reasonable settlement without excessive compaction.

Compaction within the BSM area should not exceed 75 to 85% standard proctor within the BSM.

Machinery shall not be used in the bioretention facility to place the BSM. A conveyor or spray system shall be used for media placement in large facilities. Low ground pressure equipment may be authorized for large facilities at the discretion of the reviewing jurisdiction. Placement methods and BSM quantities shall account for approximately 10% loss of volume due to settling. Planting methods and timing shall account for settling of media without exposing plant root systems. 803-6.1.4 Hydraulic Suitability. The reviewing jurisdiction may request up to three double ring infiltrometer tests (ASTM D3385) or approved alternative tests to confirm that the placed material meets applicable hydraulic suitability criteria. In the event that the infiltration rate of placed material does not meet applicable criteria, the reviewing jurisdiction may require replacement and/or decompaction of materials. 803-6.2 Quality Control and Acceptance
803-6.2.1 General. Close adherence to the material quality controls herein are necessary in order to support healthy vegetation, minimize pollutant leaching, and assure sufficient permeability to infiltrate/filter runoff during the life of the facility. Amendments may be included to adjust agronomic properties. Acceptance of the material will be based on test results certified to be representative. Test results shall be conducted no more than 120 days prior to delivery of the blended BSM to the project site. For projects installing more than 100 cubic yards of BSM, batch-specific tests of the blended mix shall be provided to the reviewing jurisdiction for every 100 cubic yards of BSM along with a site plan showing the placement locations of each BSM batch within the facility. 803-6.5 Measurement and Payment. Quantities of mixed BSM will be measured as shown in the Bid. The volumetric quantity of mixed BSM to be paid for shall be the volume of BSM placed within the limits of the dimensions shown on the Plans.
803-7 AGGREGATE MATERIALS FOR BIORETENTION AND BIOFILTRATION

DRAINAGE LAYERS
803-7.1 General. This section provides material specifications for drainage layers below BSM in bioretention BMPs. This consists of a two-layer filter course placed below the BSM and above an open-graded aggregate stone reservoir.
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803-7.2 Rock and Sand Materials for Drainage Layers
803-7.2.1 General. All sand and stone products used in BSM drainage layers shall be clean and thoroughly washed.
803-7.2.2 Filter Course. Graded aggregate choker material is installed as a filter course to separate BSM from the drainage rock reservoir layer. The purpose of this layer is to limit migration of sand or other fines from the BSM. The filter course consists of two layers of choking material increasing in particle size. The top layer (closets to the BSM) of the filter course shall be constructed of thoroughly washed ASTMC33 Choker Sand as detailed in Table 200-1.5.5. The bottom layer of the filter course shall be constructed of thoroughly washed ASTM No. 8 aggregate material conforming to gradation limits contained in Table 200-1.2.1.
803-7.2.3 Open-Graded Aggregate Stone. Open-graded aggregate material is installed below filter course layers to provide additional storm water storage capacity and contain the underdrain pipe(s).

This layer shall be constructed of thoroughly washed AASHTO No. 57 open graded aggregate material conforming to gradation limits contained in Table 200-1.2.1. 803-7.3 Layer Thicknesses and Construction.
803-7.3.1 General. Aggregate shall be deposited on underlying layers at a uniform quantity per linear foot (meter), which quantity will provide the required compacted thickness within the tolerances specified herein without resorting to spotting, picking up, or otherwise shifting the aggregate material. 803-7.3.2 Filter Course Layers. Each of the two filter course layers (top layer of ASTM C33 Choker Sand and bottom layer of ASTM No. 8) shall be installed to a thickness of 3 inches (75 mm). Both layers shall be spread in single layers. Marker stakes should be used to ensure uniform lift thickness. 803-7.3.3 Aggregate Drainage and Storage Layer. The thickness of the aggregate drainage and storage layer (AASHTO No. 57) will depend on site specific design and shall be detailed in contract documents.
803-7.3.4 Spreading. Drainage layers shall be as delivered as uniform mixtures and each layer shall be spread in one operation. Segregation within each aggregate layer shall be avoided and the layers shall be free from pockets of coarse or fine material.
803-7.3.5 Compacting. Filter course material and aggregate storage material shall be lightly compacted to approximately 80% standard proctor without the use of vibratory compaction. 803-7.4 Measurement and Payment. Quantities of graded aggregate choker material and opengraded aggregate storage material will be measured as shown in the Bid. The volumetric quantities of graded aggregate choker stone material and open-graded storage material shall be those placed within 2026-08-19 Item #12D Page 471 of 816
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the limits of the dimensions shown on the Plans. The weight of material to be paid for will be determined by deducting (from the weight of material delivered to the Work) the weight of water in the material (at the time of weighing) in excess of 1% more than the optimum moisture content. No payment will be made for the weight of water deducted as provided in this subsection. 803-8 SUMMARY
Summary of BSM specification requirements for the City of San Diego and County of San Diego included in Table 803-8.

Table 803-8
Component Requirement
BSM Material Composition Sand: 60-80% by volume
Topsoil: 0-20% by volume
Compost: 20% by volume
Alternative Blends Acceptable? Yes, but they must meet performance-based specifications.

Sand Type Washed sand conforming to particle size
distribution
Topsoil Type Sandy loam or loamy sand with clay < 15% and gravel < 25%
Compost Type From a CalRecycle permitted facility. Biosolids derived materials are not acceptable
BSM Permeability 8-24 inches/hour for BMPs without outlet control; 15-80 inches/hour for BMPs with outlet control;
testing is required to demonstrate.

Agronomic Suitability
Requirements
Limits for salts and potential toxins. C:N ration
between 12 and 40.

Water Quality Related Limits? Requirements related to specific pollutants when water quality of receiving waters is impaired for
those pollutants.
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C I T Y O F E N C I N I T A S B M P D E S I G N M A N U A L G-1 2023
Appendix
I
Appendix G Guidance for
Continuous Simulation and
Hydromodification Management
Sizing Factors
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G.1 Guidance for Continuous Simulation
Hydrologic Modeling for Hydromodification
Management Studies in San Diego County
Region 9
G.1.1 Introduction
Continuous simulation hydrologic modeling is used to demonstrate compliance with the performance standards for hydromodification management in San Diego. There are several available hydrologic models that can perform continuous simulation analyses. Each has different methods and parameters for determining the amount of rainfall that becomes runoff, and for representing the hydraulic operations of certain structural BMPs such as biofiltration with partial retention or biofiltration. This Appendix is intended to:
• Identify acceptable models for continuous simulation hydrologic analyses for hydromodification management;
• Provide guidance for selecting climatology input to the models; • Provide standards for rainfall loss parameters to be used in the models; • Provide standards for defining physical characteristics of LID components; and • Provide guidance for demonstrating compliance with performance standards for hydromodification management.

This Appendix is not a user's manual for any of the acceptable models, nor a comprehensive manual for preparing a hydrologic model. This Appendix provides guidance for selecting model input parameters for the specific purpose of hydromodification management studies. The model preparer must be familiar with the user's manual for the selected software to determine how the parameters are entered to the model.

G.1.2 Software for Continuous Simulation Hydrologic
Modeling
The following software models may be used for hydromodification management studies in San Diego:
• HSPF – Hydrologic Simulation Program-FORTRAN, distributed by USEPA, public domain. • SDHM – San Diego Hydrology Model, distributed by Clear Creek Solutions, Inc. This is an HSPF-based model with a proprietary interface that has been customized for use in San Diego for hydromodification management studies.
• SWMM – Storm Water Management Model, distributed by USEPA, public domain. 2026-08-19 Item #12D Page 474 of 816
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Third-party and proprietary software, such as XPSWMM or PCSWMM, may be used for hydromodification management studies in San Diego, provided that:
• Input and output data from the software can interface with public domain software such as SWMM. In other words, input files from the third party software should have sufficient functionality to allow export to public domain software for independent validation. • The software's hydromodification control processes are substantiated.

G.1.3 Climatology Parameters
G.1.3.1 Rainfall
In all software applications for preparation of hydromodification management studies in San Diego, rainfall data must be selected from approved data sets that have been prepared for this purpose. As part of the development of the March 2011 Final HMP, long-term hourly rainfall records were prepared for public use. The rainfall record files are provided on the Project Clean Water website. The rainfall station map is provided in the March 2011 Final HMP and is included in this Appendix as Figure G.1-1.
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Figure G.1-1: Rainfall Station Map
Project applicants preparing continuous simulation models shall select the most appropriate rainfall data set from the rainfall record files provided on the Project Clean Water website. For a given project location, the following factors should be considered in the selection of the appropriate rainfall data set:
• In most cases, the rainfall data set in closest proximity to the project site will be the appropriate choice (refer to the rainfall station map).
• In some cases, the rainfall data set in closest proximity to the project site may not be the most applicable data set. Such a scenario could involve a data set with an elevation significantly different from the project site. In addition to a simple elevation comparison, the project proponent may also consult with the San Diego County’s average annual precipitation isopluvial map, which is provided in the San Diego County Hydrology Manual (2003). Review of this map could provide an initial estimate as to whether the project site is in a similar rainfall zone as compared to the rainfall stations. Generally, precipitation totals in San Diego County increase with increasing elevation.
• Where possible, rainfall data sets should be chosen so that the data set and the project location are both located in the same topographic zone (coastal, foothill, mountain) and major watershed unit (Upper San Luis Rey, Lower San Luis Rey, Upper San Diego River, Lower San Diego River, etc.).

For SDHM users, the approved rainfall data sets are pre-loaded into the software package. SDHM users may select the appropriate rainfall gauge within the SDHM program. HSPF or SWMM users shall download the appropriate rainfall record from the Project Clean Water website and load it into the software program.

Both the pre-development and post-project model simulation period shall encompass the entire rainfall record provided in the approved rainfall data set. Scaling the rainfall data is not permitted.

G.1.3.2 Potential Evapotranspiration
Project applicants preparing continuous simulation models shall select a data set from the sources described below to represent potential evapotranspiration.

For HSPF users, this parameter may be entered as an hourly time series. The hourly time series that was used to develop the BMP Sizing Calculator parameters is provided on the project clean water website and may be used for hydromodification management studies in San Diego. For SDHM users, the hourly evaporation data set is pre-loaded into the program. HSPF users may download the evaporation record from the Project Clean Water website and load it into the software program. 2026-08-19 Item #12D Page 476 of 816
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For HSPF or SWMM users, this parameter may be entered as monthly values in inches per month or inches per day. Monthly values may be obtained from the California Irrigation Management Information System "Reference Evapotranspiration Zones" brochure and map (herein "CIMIS ETo Zone Map"), prepared by California Department of Water Resources, dated January 2012. The CIMIS ETo Zone Map is available from www.cimis.gov, and is provided in this Appendix as Figure G.1-2.

Determine the appropriate reference evapotranspiration zone for the project from the CIMIS ETo Zone Map. The monthly average reference evapotranspiration values are provided below in Table G.1-1.

Figure G.1-2: California Irrigation Management Information System "Reference Evapotranspiration Zones"
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Table G.1-1: Monthly Average Reference Evapotranspiration by ETo Zone (inches/month and inches/day) for use in SWMM Models for Hydromodification Management Studies in San Diego County CIMIS Zones 1, 4, 6, 9, and 16 (See CIMIS ETo Zone Map) January February March April May June July August September October November December Zone in/month in/month in/month in/month in/month in/month in/month in/month in/month in/month in/month in/month 1 0.93 1.4 2.48 3.3 4.03 4.5 4.65 4.03 3.3 2.48 1.2 0.62 4 1.86 2.24 3.41 4.5 5.27 5.7 5.89 5.58 4.5 3.41 2.4 1.86 6 1.86 2.24 3.41 4.8 5.58 6.3 6.51 6.2 4.8 3.72 2.4 1.86 9 2.17 2.8 4.03 5.1 5.89 6.6 7.44 6.82 5.7 4.03 2.7 1.86 16 1.55 2.52 4.03 5.7 7.75 8.7 9.3 8.37 6.3 4.34 2.4 1.55 January February March April May June July August September October November December Days 31 28 31 30 31 30 31 31 30 31 30 31
Zone in/day in/day in/day in/day in/day in/day in/day in/day in/day in/day in/day in/day 1 0.030 0.050 0.080 0.110 0.130 0.150 0.150 0.130 0.110 0.080 0.040 0.020 4 0.060 0.080 0.110 0.150 0.170 0.190 0.190 0.180 0.150 0.110 0.080 0.060 6 0.060 0.080 0.110 0.160 0.180 0.210 0.210 0.200 0.160 0.120 0.080 0.060 9 0.070 0.100 0.130 0.170 0.190 0.220 0.240 0.220 0.190 0.130 0.090 0.060 16 0.050 0.090 0.130 0.190 0.250 0.290 0.300 0.270 0.210 0.140 0.080 0.050 2026-08-19 Item #12D Page 478 of 816
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G.1.4 Land Characteristics and Loss Parameters
In all software applications for preparation of hydromodification management studies in San Diego, rainfall loss parameters must be consistent with this Appendix unless the preparer can provide documentation to substantiate use of other parameters, subject to local jurisdiction approval. HSPF and SWMM use different processes and different sets of parameters. SDHM is based on HSPF, therefore parameters for SDHM and HSPF are presented together in Section G.1.4.1. Parameters that have been pre-loaded into SDHM may be used for other HSPF hydromodification management studies outside of SDHM. Parameters for SWMM are presented separately in Section G.1.4.2.

G.1.4.1 Rainfall Loss Parameters for HSPF and SDHM
Rainfall losses in HSPF are characterized by PERLND/PWATER parameters and IMPLND parameters, which describe processes occurring when rainfall lands on pervious lands and impervious lands, respectively. "BASINS Technical Notice 6, Estimating Hydrology and Hydraulic Parameters for HSPF," prepared by the USEPA, dated July 2000, provides details regarding these parameters and summary tables of possible ranges of these parameters. Table G.1-2, excerpted from the abovementioned document, presents the ranges of these parameters.

For HSPF studies for hydromodification management in San Diego, PERLND/PWATER parameters and IMPLND parameters shall fall within the "possible" range provided in EPA Technical Note 6. To select specific parameters, HSPF users may use the parameters established for development of the San Diego BMP Sizing Calculator, and/or the parameters that have been established for SDHM. Parameters for the San Diego BMP Sizing Calculator and SDHM are based on research conducted specifically for HSPF modeling in San Diego.

Documentation of parameters selected for the San Diego BMP Sizing Calculator is presented in the document titled, San Diego BMP Sizing Calculator Methodology, prepared by Brown and Caldwell, dated January 2012 (herein "BMP Sizing Calculator Methodology"). The PERLND/PWATER parameters selected for development of the San Diego BMP Sizing Calculator represent a single composite pervious land cover that is representative of most pre-development conditions for sites that would commonly be managed by the BMP Sizing Calculator. The parameters shown below in Table G.1-3 are excerpted from the BMP Sizing Calculator Methodology. 2026-08-19 Item #12D Page 479 of 816
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Table G.1-2: HSPF PERLND/PWATER and IMPLND Parameters from EPA Technical Note 6 Range of Values
Name Definition Units Typical Possible Function of ... Comment Min Max Min Max
PWAT – PARM2FOREST Fraction forest cover none 0.0 0.50 0.0 0.95 Forest cover Only impact when SNOW is activeLZSN Lower Zone Nominal Soil Moisture Storage inches 3.0 8.0 2.0 15.0 Soils, climate CalibrationINFILT Index to Infiltration Capacity in/hr 0.01 0.25 0.001 0.50 Soils, land use Calibration, divides surface and subsurface flowLSUR Length of overland flow feet 200 500 100 700 Topography Estimate from high resolution topo maps or GISSLSUR Slope of overland flow plane ft/ft 0.01 0.15 0.001 0.30 Topography Estimate from high resolution topo maps or GIS KVARY Variable groundwater recession 1/inches 0.0 3.0 0.0 5.0 Baseflow recession variation Used when recession rate varies with GW levels AGWRC Base groundwater recession none 0.92 0.99 0.85 0.999 Baseflow recession Calibration PWAT – PARM3PETMAX Temp below which ET is reduced deg. F 35.0 45.0 32.0 48.0 Climate, vegetation Reduces ET near freezing, when SNOW is activePETMIN Temp below which ET is set to zero deg. F 30.0 35.0 30.0 40.0 Climate, vegetation Reduces ET near freezing, when SNOW is activeINFEXP Exponent in infiltration equation none 2.0 2.0 1.0 3.0 Soils variability Usually default to 2.0INFILD Ratio of max/mean infiltration capacities none 2.0 2.0 1.0 3.0 Soils variability Usually default to 2.0DEEPFR Fraction of GW inflow to deep recharge none 0.0 0.20 0.0 0.50 Geology, GW recharge Accounts for subsurface lossesBASETP Fraction of remaining ET from baseflow none 0.0 0.05 0.0 0.20 Riparian vegetation Direct ET from riparian vegetation AGWETP Fraction of remaining ET from active GW none 0.0 0.05 0.0 0.20 Marsh/wetlands extent Direct ET from shallow GW PWAT – PARM4
CEPSC Interception storage capacity inches 0.03 0.20 0.01 0.40 Vegetation type/density, land use Monthly values usually used UZSN Upper zone nominal soil moisture storage inches 0.10 1.0 0.05 2.0 Surface soil conditions, land use Accounts for near surface retention NSUR Manning's n (roughness) for overland flow none 0.15 0.35 0.05 0.50 Surface conditions, residue, etc. Monthly values often used for croplands INTFW Interflow inflow parameter none 1.0 3.0 1.0 10.0 Soils, topography, land use Calibration, based on hydrograph separationIRC Interflow recession parameter none 0.5 0.70 0.30 0.85 Soils, topography, land use Often start with a value of 0.7, and then adjust LZETP Lower zone ET parameter none 0.2 0.70 0.1 0.9 Vegetation type/density, root depth Calibration IWAT – PARM2
LSUR Length of overland flow feet 50 150 50 250 Topography, drainage system Estimate from maps, GIS, or field survey SLSUR Slope of overland flow plane ft/ft 0.01 0.05 0.001 0.15 Topography, drainage Estimate from maps, GIS, or field survey NSUR Manning's n (roughness) for overland flow none 0.03 0.10 0.01 0.15 Impervious surface conditions Typical range is 0.05 to 0.10 for roads/parking lots RETSC Retention storage capacity inches 0.03 0.10 0.01 0.30 Impervious surface conditions Typical range is 0.03 to 0.10 for roads/parking lots IWAT – PARM3 (PETMAX and PETMIN, same values as shown for PWAT – PARM3) 2026-08-19 Item #12D Page 480 of 816
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Table G.1-3: HSPF PERLND/PWATER Parameters from BMP Sizing Calculator Methodology Hydrologic Soil
Group
A
Hydrologic Soil
Group
B
Hydrologic Soil
Group
C
Hydrologic Soil
Group
D
Slope 5% 10% 15% 5% 10% 15% 5% 10% 15% 5% 10% 15%
PWAT_PAR
M2 Units
FOREST None 0 0 0 0 0 0 0 0 0 0 0 0
LZSN inches 5.2 4.8 4.5 5.0 4.7 4.4 4.8 4.5 4.2 4.8 4.5 4.2 INFILT in/hr 0.090 0.070 0.045 0.070 0.055 0.040 0.050 0.040 0.032 0.040 0.030 0.020 LSUR Feet 200 200 200 200 200 200 200 200 200 200 200 200 SLSUR ft/ft 0.05 0.1 0.15 0.05 0.1 0.15 0.05 0.1 0.15 0.05 0.1 0.15 KVARY 1/inche
s
3 3 3 3 3 3 3 3 3 3 3 3
AGWRC None 0.92 0.92 0.92 0.92 0.92 0.92 0.92 0.92 0.92 0.92 0.92 0.92 PWAT_PAR
M3
PETMAX (F) F 35 35 35 35 35 35 35 35 35 35 35 35
PETMIN (F) F 30 30 30 30 30 30 30 30 30 30 30 30
INFEXP None 2 2 2 2 2 2 2 2 2 2 2 2
INFILD None 2 2 2 2 2 2 2 2 2 2 2 2
DEEPFR None 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 BASETP None 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 AGEWTP None 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 PWAT_PAR
M4
CEPSC inches 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.08 UZSN inches 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 NSUR None 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 INTFW None 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 IRC None 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 LZETP None 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 2026-08-19 Item #12D Page 481 of 816
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Parameters within SDHM are documented in "San Diego Hydrology Model User Manual," prepared by Clear Creek Solutions, Inc. (as of the development of the Manual, the current version of the SDHM User Manual is dated January 2012). Parameters established for SDHM represent "grass" (non-turf grasslands), "dirt," "gravel," and "urban" cover. The documented PERLND and IMPLND parameters for the various land covers and soil types have been pre-loaded into SDHM. SDHM users shall use the parameters that have been pre-loaded into the program without modification unless the preparer can provide documentation to substantiate use of other parameters.

G.1.4.2 Rainfall Loss Parameters for SWMM
In SWMM, rainfall loss parameters (parameters that describe processes occurring when rainfall lands on pervious lands and impervious lands) are entered in the "subcatchment" module. In addition to specifying parameters, the SWMM user must also select an infiltration model.

The SWMM Manual provides details regarding the subcatchment parameters and summary tables of possible ranges of these parameters. For SWMM studies for hydromodification management in San Diego, subcatchment parameters shall fall within the range provided in the SWMM Manual. Some of the parameters depend on the selection of the infiltration model. For consistency across the San Diego region, SWMM users shall use the Green-Ampt infiltration model for hydromodification management studies. Table G.1-4 presents SWMM subcatchment parameters for use in hydromodification management studies in the San Diego region.
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Table G.1-4: Subcatchment Parameters for SWMM Studies for Hydromodification Management in San Diego
SWMM
Parameter
Name
Unit Range Use in San Diego
Name
X-Coordinate
Y-Coordinate
Description
Tag
Rain Gauge
Outlet
N/A N/A – project-specific Project-specific
Area acres (ac) Project-specific Project-specific
Width feet (ft) Project-specific Project-specific
% Slope percent (%) Project-specific Project-specific
% Imperv percent (%) Project-specific Project-specific
N-imperv -- 0.011 – 0.024 presented
in Table A.6 of SWMM
Manual
default use 0.012 for smooth
concrete, otherwise provide
documentation of other surface
consistent with Table A.6 of SWMM
Manual
N-Perv -- 0.05 – 0.80 presented in
Table A.6 of SWMM
Manual
default use 0.15 for short prairie
grass, otherwise provide
documentation of other surface
consistent with Table A.6 of SWMM
Manual
Dstore-Imperv inches 0.05 – 0.10 inches
presented in Table A.5
of SWMM Manual

0.05 Dstore-Perv inches 0.10 – 0.30 inches
presented in Table A.5
of SWMM Manual

0.10 %ZeroImperv percent (%) 0% – 100% 25%
Subarea
routing
-- OUTLET

IMPERVIOUS

PERVIOUS
Project-specific, typically OUTLET
Percent
Routed
% 0% – 100% Project-specific, typically 100%
Infiltration Method HORTON
GREEN_AMPT
CURVE_NUMBER
GREEN_AMPT
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SWMM
Parameter
Name
Unit Range Use in San Diego
Suction Head
(Green-Ampt)
Inches 1.93 – 12.60 presented
in Table A.2 of SWMM
Manual
Hydrologic Soil Group A: 1.5
Hydrologic Soil Group B: 3.0
Hydrologic Soil Group C: 6.0
Hydrologic Soil Group D: 9.0
Conductivity
(Green-Ampt)
Inches per
hour

0.01 – 4.74 presented in
Table A.2 of SWMM
Manual by soil texture
class

0.00 – ≥0.45 presented
in Table A.3 of SWMM
Manual by hydrologic
soil group
Hydrologic Soil Group A: 0.3
Hydrologic Soil Group B: 0.2
Hydrologic Soil Group C: 0.1
Hydrologic Soil Group D: 0.025
Note: reduce conductivity by 25% in
the post-project condition when
native soils will be compacted.

Conductivity may also be reduced by
25% in the pre-development
condition model for redevelopment
areas that are currently concrete or
asphalt but must be modeled
according to their underlying soil
characteristics. For fill soils in postproject condition, see Section G.1.4.3.

Initial Deficit
(Green-Ampt)
The difference between
soil porosity and initial
moisture content.

Based on the values
provided in Table A.2
of SWMM Manual, the
range for completely
dry soil would be 0.097
to 0.375
Hydrologic Soil Group A: 0.33
Hydrologic Soil Group B: 0.32
Hydrologic Soil Group C: 0.31
Hydrologic Soil Group D: 0.30
Note: in long-term continuous
simulation, this value is not
important as the soil will reach
equilibrium after a few storm events
regardless of the initial moisture
content specified.

Groundwater yes/no yes/no NO
LID Controls Project Specific
Snow Pack
Land Uses
Initial Buildup
Curb Length
Not applicable to hydromodification
management studies
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A schematic of the basic SWMM setup for hydromodification management studies is shown below, with the LID module is shown as a feature within the hydrology computational block. Surface water hydrology is distinguished from groundwater, however the groundwater module is not typically used in hydromodification management studies.

The rainfall and climatology input time series data are used to generate surface runoff which in turn is hydraulically routed through the collection system and storage/treatment facilities. The figure includes the following terms in the water balance equation:
• P = Precipitation
• E/T = Evaporation / Transpiration
• I/S = Infiltration / Seepage
• Q = Runoff
Evapotranspiration was previously addressed above; the remainder of this section discusses the other hydrologic losses and parameters.

Soil and Infiltration Parameters
Of the infiltration options available in SWMM, the Green-Ampt equation can best handle variable water content conditions in the shallow soil layers beneath the ground surface, which is critical for long-term continuous simulation of surface water hydrology. The Green-Ampt parameters suggested in Table G.1-4 are referenced according to hydrologic soil group. Green-Ampt parameters can also be determined by relating infiltration parameters to soil texture properties, as identified by in-situ geotechnical analysis results or published County soil survey information. Infiltration parameters 2026-08-19 Item #12D Page 485 of 816
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include:
• Capillary Tension (Suction Head): a measure of how tightly water is held within the soil pore space;
• Saturated Hydraulic Conductivity: a measure of how quickly the water can be drained vertically; and
• Initial Moisture Deficit: a measure of the initial soilwater deficit, also known as porosity (i.e., the volumetric fraction of water within the soil pore space under initially dry conditions).

Note that when SWMM is used without the Groundwater module, there is no distinction between the upper and lower zone soil moisture storage as in HSPF/SDHM. The LID module does however distinguish several layers/zones within each facility, and these are described below.

Overland Flow Parameters
Overland flow parameters describe the slope and length characteristics of shallow surface runoff.

These are determined by identifying representative overland flow paths for each subcatchment using available digital topographic data for pre-development conditions and the proposed grading plan for post-project conditions. Overland flow path lengths and slopes are measured directly from the available information. Generally, overland flow paths should be less than 1,000 feet in length, otherwise channelized flow is likely present and should be modeled hydraulically. Overland flow path widths are determined based on the subcatchment area divided by the corresponding flow path length for each subcatchment.

Although Surface Storage is not depicted in SWMM schematic, it is a component of the water balance equation and includes excess runoff that is held in both hydrologic depression storage and hydraulic storage units.

LID Module
There are two approaches for representing LID facilities in SWMM:
• Modeling Approach No. 1: Place LID controls within the appropriate subcatchment and then adjust parameters accordingly to reflect untreated areas within the parent subcatchment; and
• Modeling Approach No. 2: Create a new subcatchment for each LID control, allowing “runon” from the treated portion of the parent subcatchment. 2026-08-19 Item #12D Page 486 of 816
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Modeling Approach No.1 schematic is presented below. As described above, a portion of the impervious subarea from a given subcatchment can be routed onto the pervious area for infiltration (see arrow denoting subarea routing fraction). When the LID module of SWMM is used, the portion of the impervious area that is captured and treated by an LID facility is specified (see arrow denoting LID area fraction). The remaining impervious area, if any, is routed directly to the outlet.

Modeling Approach No. 1 (LID within Parent Subcatchment) The first approach is the easiest of the two for representing LID facilities in SWMM, as it allows a mix of controls to be placed within an existing subcatchment and each facility can capture and treat a different portion of the runoff generated from the parent subcatchment (i.e., outside of the LID footprint). A drawback of this approach is that it will not appropriately represent LID facilities in series (i.e., where the outflow from one LID control becomes the inflow to another LID control). No adjustments to the parent subcatchment hydrology parameters are needed if the cumulative LID area is small in comparison to the subcatchment area. However when the cumulative LID area is significant (e.g., greater than 10% of the subcatchment), at a minimum, the imperviousness and overland flow width values will need to be adjusted to compensate for the parent subcatchment area that was replaced with the cumulative LID footprint area.

Modeling Approach No.2 schematic is presented below. In this approach the LID facility is assigned to a new subcatchment and runoff from upstream subcatchments can be directed to this new subcatchment (i.e., “run-on”). In this way, LID controls can be modeled in series. Adjustments to the imperviousness and overland flow width values in the parent subcatchment will need to be made. For typical development or redevelopment sites that are evaluated in hydromodification management studies, LID capture areas often comprise a large portion of the parent subcatchments, and therefore Impervious Area
Pervious Area
Hydrologic Outlet
(Hydraulic Inlet)
Subarea
routing
fraction
LID area
fraction
LID Overflow
+ Drain flow LID
Area
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this is the preferred approach.

Modeling Approach No. 2 (LID in New Subcatchment)
More details on the use and application of LID controls are provided in the SWMM Manual and program help file. Suggested parameter values for use with hydromodification management studies in San Diego are provided in Appendix G.1.5.

G.1.4.3 Pervious Area Rainfall Loss Parameters in Post-Project Condition (HSPF, SDHM, and SWMM)
The following guidance applies to HSPF, SDHM, and SWMM. When modeling pervious areas in the post-project condition, fill soils shall be modeled as hydrologic soil group Type D soils, or the project applicant may provide an actual expected infiltration rate for the fill soil based on testing (must be approved by the City Engineer for use in the model). Where landscaped areas on fill soils will be re-tilled and/or amended in the post-project condition, the landscaped areas may be modeled as Type C soils. Areas to be re-tilled and/or amended in the post-project condition must be shown on the project plans. For undisturbed pervious areas (i.e., native soils, no fill), use the actual hydrologic soil group, the same as in the pre-development condition.

G.1.5 Modeling Structural BMPs (Ponds and LID Features) There are many ways to model structural BMPs. There are standard modules for several pond or LID elements included in SDHM and SWMM. Users may also set up project-specific stage-storagedischarge relationships representing structural BMPs. Regardless of the modeling method, certain Impervious Area
Pervious Area
Hydrologic Outlet
(Hydraulic Inlet)
Subarea
routing
fraction
LID
Area
LID Overflow
+ Drain flow
“Run-on”
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characteristics of the structural BMP, including infiltration of water from the bottom of the structural BMP into native soils, porosity of bioretention soils and/or gravel sublayers, and other programspecific parameters must be consistent with those presented below, unless the preparer can provide documentation to substantiate use of other parameters, subject to local jurisdiction approval. The geometry of structural BMPs is project-specific and shall match the project plans.

G.1.5.1 Infiltration into Native Soils Below Structural BMPs Infiltration into native soils below structural BMPs may be modeled as a constant outflow rate equal to the project site-specific design infiltration rate (Worksheet D.5-1) multiplied by the area of the infiltrating surface (and converted to cubic feet per second). This infiltration rate is not the same as an infiltration parameter used in the calculation of rainfall losses, such as the HSPF INFILT parameter or the Green-Ampt conductivity parameter in the SWMM subcatchment module. It must be sitespecific and must be determined based on the methods presented in Appendix D of this manual.

For preliminary analysis when site-specific geotechnical investigation has not been completed, project applicants proposing infiltration into native soils as part of the structural BMP design shall prepare a sensitivity analysis to determine a potential range for the structural BMP size based on a range of potential infiltration rates. As shown in Appendices C and D of this manual, many factors influence the ability to infiltrate storm water. Therefore even when soils types A and B are present, which are generally expected to infiltrate storm water, the possibility that a very low infiltration rate could be determined at design level must be considered. The range of potential infiltration rates for preliminary analysis is shown below in Table G.1-5.

Table G.1-5: Range of Potential Infiltration Rates to be Studied for Sensitivity Analysis when Native Infiltration is Proposed but Site-Specific Geotechnical Investigation has not been Completed Hydrologic Soil Group at
Location of Proposed
Structural BMP
Low Infiltration Rate for
Preliminary Study
(inches/hour)
High Infiltration Rate for
Preliminary Study
(inches/hour)
A 0.02 2.4
B 0.02 0.52
C 0 0.08
D 0 0.02
The infiltration rates shown above are for preliminary investigation only. Final design of a structural BMP must be based on the project site-specific design infiltration rate (Worksheet D.5-1).

G.1.5.2 Structural BMPs That Do Not Include Sub-Layers (Ponds) To model a pond, basin, or other depressed area that does not include processing runoff through 2026-08-19 Item #12D Page 489 of 816
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sublayers of amended soil and/or gravel, create a stage storage discharge relationship for the pond, and supply the information to the model according to the program requirements. For HSPF users, the stage-storage-discharge relationship is provided in FTABLES. SDHM users may use the TRAPEZOIDAL POND element for a trapezoidal pond or IRREGULAR POND element to request the program to create the stage-storage-discharge relationship, use the SSD TABLE element to supply a user-created stage-storage-discharge relationship, or use other available modules such as TANK or VAULT. For SWMM users, the stage-storage relationship is supplied in the storage unit module, and the stage-discharge relationship may be represented by various other modules such as the orifice, weir, or outlet modules. Stage-storage and stage-discharge curves for structural BMPs must be fully documented in the project-specific HMP report and must be consistent with the structural BMP(s) shown on project plans.

For user-created stage-discharge relationships, refer to local drainage manual criteria for equations representing hydraulic behavior of outlet structures. Users relying on the software to develop the stage-discharge relationship may use the equations built into the program. This manual does not recommend that all program modules calculating stage-discharge relationships must be uniform because the flows to be controlled for hydromodification management are low flows, calculated differently from the single-storm event peak flows studied for flood control purposes, and hydromodification management performance standards do not represent any performance standard for flood control drainage design. Note that for design of emergency outlet structures, and any calculations related to single-storm event routing for flood control drainage design, stage-discharge calculations must be consistent with the local drainage design requirements. This may require separate calculations for stage-discharge relationship pursuant to local manuals. The HMP flow rates shall not be used for flood control calculations.

G.1.5.3 Structural BMPs That Include Sub-Layers (Bioretention and Other LID) G.1.5.3.1 Characteristics of Bioretention Soil Media
The bioretention soil media used in bioretention, biofiltration with partial retention, and biofiltration structural BMPs is a sandy loam. The following parameters presented in Table G.1-6 are characteristics of a sandy loam for use in continuous simulation models.

Table G.1-6: Characteristics of Sandy Loam to Represent Bioretention Soil Media in Continuous Simulation for Hydromodification Management Studies in San Diego Soil Texture Porosity Field Capacity Wilting Point Conductivity Suction Head
Sandy Loam 0.4 0.2 0.1 5 inches/hour 1.5 inches
• Porosity is the volume of pore space (voids) relative to the total volume of soil (as a fraction). 2026-08-19 Item #12D Page 490 of 816
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• Field Capacity is the volume of pore water relative to total volume after the soil has been allowed to drain fully (as a fraction). Below this level, vertical drainage of water through the soil layer does not occur.
• Wilting point is the volume of pore water relative to total volume for a well dried soil where only bound water remains (as a fraction). The moisture content of the soil cannot fall below this limit.
• Conductivity is the hydraulic conductivity for the fully saturated soil (in/hr or mm/hr). • Suction head is the average value of soil capillary suction along the wetting front (inches or mm).

Figures G.1-3 and G.1-4, from http://www.stevenswater.com/articles/irrigationscheduling.aspx, illustrate unsaturated soil and soil saturation, field capacity, and wilting point. 2026-08-19 Item #12D Page 491 of 816
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Figure G.1-3: Unsaturated Soil Composition
Unsaturated soil is composed of solid particles, organic material and pores. The pore space will contain air and water.

Figure G.1-4: Soil saturation, field capacity, and wilting point 2026-08-19 Item #12D Page 492 of 816
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G.1.5.3.2 Characteristics of Gravel
For the purpose of hydromodification management studies, it may be assumed that water moves freely through gravel, not limited by hydraulic properties of the gravel. For the purpose of calculating available volume, use porosity of 0.4, or void ratio of 0.67. Porosity is equal to void ratio divided by (1 + void ratio).

G.1.5.3.3 Additional Guidance for SDHM Users
The module titled "bioretention/rain garden element" may be used to represent bioretention or biofiltration BMPs. SDHM users using the available "bioretention/rain garden element" shall customize the soil media characteristics to use the parameters from Table G.1-6 above, and select "gravel" for gravel sublayers. All other input variables are project-specific. "Native infiltration" refers to infiltration from the bottom of the structural BMP into the native soil. This variable is projectspecific, see Section G.1.5.1.

G.1.5.3.4 Additional Guidance for SWMM Users
The latest version of SWMM (version 5.1.012) includes the following eight types of LID controls:
• Bio-Retention Cell: surface storage facility with vegetation in an bioretention soil mixture placed above a gravel drainage bed.
• Rain Garden: same setup as bio-retention cell, but without an underlying gravel bed. • Green Roof: bio-retention cell with shallow surface storage and soil layers, underlain by a drainage mat that conveys excess percolated rainfall to the regular roof drainage system. • Infiltration Trench: drainage swale or narrow storage basin filled with gravel or other porous media designed to capture and infiltrate runoff to the native soil below. • Permeable Pavement: continuous pavement systems with porous concrete, asphalt mix, or paver blocks above a sand or gravel drainage bed with gravel storage layer below. • Rain Barrel: container (cistern) to collect roof runoff for later use (e.g., landscape irrigation) or release.
• Rooftop Disconnection: to simulate redirection of downspout discharge onto pervious landscaped areas and lawns instead of directly into storm drains. • Vegetative Swale: grassed conveyance channel (drainage ditch or swale) with vegetation designed to slow down runoff to allow more time for infiltration into the native soil below.

The "bio-retention cell" LID control may be used to represent bioretention or biofiltration BMPs.

For bio-retention cells, a number of LID process layers have been defined in SWMM and these are described below. Table G.1-7 provides parameters required for the standard "bio-retention cell" available in SWMM. The parameters are entered in the LID Control Editor. 2026-08-19 Item #12D Page 493 of 816
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Table G.1-7: Parameters for SWMM "Bio-Retention Cell" Module for Hydromodification Management Studies in San Diego
SWMM Parameter
Name Unit Use in San Diego
Surface
Berm Height
also known as Storage
Depth
inches Project-specific
Vegetative Volume
Fraction
also known as Vegetative
Cover Fraction
--- 0
Surface Roughness --- 0 (this parameter is not applicable to bio-retention cell) Surface Slope --- 0 (this parameter is not applicable to bio-retention cell) Soil
Thickness inches project-specific
Porosity --- 0.40
Field Capacity --- 0.2
Wilting Point --- 0.1
Conductivity Inches/hour 5
Conductivity Slope --- 5
Suction Head inches 1.5
Storage
Thickness
also known as Height inches Project-specific
Void Ratio --- 0.67
Seepage Rate
also known as
Conductivity
Inches/hour
Conductivity from the storage layer refers to infiltration from the bottom of the structural BMP into the native soil.

This variable is project-specific, see Section G.5.1.

Use 0 if the bio-retention cell includes an impermeable liner
Clogging Factor --- 0
Underdrain
Flow Coefficient
Also known as Drain
Coefficient
--- Project-specific
Flow Exponent
Also known as Drain
Exponent
--- Project-specific, typically 0.5
Offset Height
Also known as Drain
Offset Height
Inches Project-specific
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Surface Layer
This process layer receives direct rainfall (and run-on from upstream subcatchments) and the resultant storm water is available for ponding, infiltration, evapotranspiration, or overflow to the outlet. The following parameters are used:
• Berm Height: This value is the maximum depth that water can pond above the ground surface before overflow occurs. In some cases, this volume may overlap with the hydraulic representation of existing surface storage or another proposed BMP facility. In any case, the user must avoid double-counting the physical storage volume. • Vegetation Volume Fraction: This represents the surface storage volume that is occupied by the stems and leaves of vegetation within the bio-retention cell.

Soil Layer
This process layer is typically composed of an amended soil or compost mix. Water that infiltrates into this component is stored in the soil void space and is available for evapotranspiration via plant roots or can percolate into the storage layer below. The following parameters are used:
• Thickness: This parameter represents the depth of the amended soil layer. • Porosity: Ratio of pore space volume to soil volume.
• Field Capacity: Pore water volume ratio after the soil has been drained. • Wilting Point: Pore water volume ratio after the soil has been dried. • Conductivity: This represents the saturated hydraulic conductivity. • Conductivity Slope: Rate at which conductivity decreases with decreasing soil moisture content.
• Suction Head: This represents the capillary tension of water in the soil.

Porosity, conductivity and suction head values as a function of soil texture were included in Table G.1-5. The flow of water through partially saturated soil is less than under fully saturated conditions.

The SWMM program accounts for this reduced hydraulic conductivity to predict the rate at which infiltrated water moves through a layer of unsaturated soil when modeling groundwater or LID controls. The conductivity slope is a dimensionless curve-fitting parameter that relates the partially saturated hydraulic conductivity to the soil moisture content.

Storage Layer
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This process layer is typically composed of porous granular media such as crushed stone or gravel.

Water that percolates into this component is stored in the void space and is available for infiltration into the native soil, or collected by an underdrain and discharged to the outlet. The following parameters are used:
• Thickness: This parameter represents the depth of the stone base. • Void Ratio: Volume of void space relative to volume of solids. Note, by definition, Porosity = Void Ratio ÷ (1 + Void Ratio).
• Seepage Rate: Filtration rate from the granular media into the native soil below. A value of zero should be used if the facility has an impermeable bottom (e.g., concrete) or is underlain by an impermeable liner.
• Clogging Factor: This value is determined by the total volume of treated runoff to completely clog the bottom of the layer divided by the void volume of the layer.

Drain Layer
This process layer is used to characterize the discharge rate of an underdrain system to the outlet. The following parameters are used:

Flow Coefficient: This value (coupled with the flow exponent described below) characterizes the rate of discharge to the outlet as a function of the height of water stored in the bio-retention cell. The coefficient can be determined by the following equation:
where,
cg is the orifice discharge coefficient, typically 0.60-0.65 for thin walled plates and higher for thicker walls;
ALID is the cumulative footprint area (ft2) of all LID controls; D is the underdrain orifice diameter (in); and
g is the gravitational constant (32.2 ft/s2).
• Flow Exponent: A value of 0.5 should be used to represent flow through an orifice.

Offset Height: This represents the height of the underdrain above the bottom of the storage layer in the bio-retention cell.
68 605
2 gD
A
cC
LID
g






















=
π
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G.1.6 Flow Frequency and Duration
The continuous simulation model will generate an hourly flow record as its output. This hourly flow record must then be processed to determine pre-development and post-project flow rates and durations. Compliance with hydromodification management requirements of this manual is achieved when results for flow frequency and duration meet the performance standards. The performance standards are as follows (also presented in Chapter 6 of this manual):

1. For flow rates ranging from 10 percent, 30 percent or 50 percent of the pre-development 2- year runoff event (0.1Q2, 0.3Q2, or 0.5Q2) to the pre-development 10-year runoff event (Q10), the post-project discharge rates and durations shall not deviate above the predevelopment rates and durations by more than 10 percent over and more than 10 percent of the length of the flow duration curve. The specific lower flow threshold will depend on the erosion susceptibility of the receiving stream for the project site (see Section 6.3.4).

To demonstrate that a flow control facility meets the hydromodification management performance standard, first pre-development Q2 and Q10 must be identified, then a flow duration summary must be generated and compared for pre-development and post-project conditions between the appropriate fraction of Q2 to Q10. The range from a fraction of Q2 to Q10 represents the range of geomorphically significant flows for hydromodification management in San Diego. The upper bound of the range of flows to control is pre-development Q10 for all projects. The lower bound of the range of flows to control, or "lower flow threshold" is a fraction of pre-development Q2 that is based on the erosion susceptibility of the stream and depends on the specific natural system (stream) that a project will discharge to. Tools have been developed in the March 2011 Final HMP for assessing the erosion susceptibility of the stream (see Section 6.3.4). Simply multiply the pre-development Q2 by the appropriate fraction (e.g., 0.1Q2) to determine the lower flow threshold.

The following guidelines shall be used for determining flow rates and durations.

G.1.6.1 Determining Flow Rates
In the context of hydromodification management in San Diego, Q2 and Q10 refer to flow rates determined based on either continuous simulation hydrologic modeling or an approved regression equation. Either method may be applied, provided that the same methodology is be applied to determination of both Q2 and Q10 (i.e. cannot mix and match methods at a POC) and be consistent across all POCs for the project (i.e. cannot mix and match methods between multiple POCs).

G.1.6.1.1 Determining Flow Rates from Regression Equation The following approved regression equation may be used to determine pre-development Q2 and Q10:
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[
= 3.60 ×
E.'c
× 4E.cde
[E = 6.56 × E.cfe × 4.Ec
where:

Q2 = 2-year recurrence interval discharge in cubic feet per second Q10 = 10-year recurrence interval discharge in cubic feet per second A = Drainage area in square miles
P = Mean annual precipitation in inches (Refer to Table 6-1) TABLE G.1-8. Mean Annual Precipitation
Gauge Latitude Longitude Mean Annual Precipitation
(inches)
Oceanside 33.2105556 -117.353333 12.29
Encinitas 33.044567 -117.277213 10.73
Kearney Mesa 32.835118 -117.128456 11.43
Fashion Valley 32.7652778 -117.1758333 10.75
Bonita 32.6561111 -117.0341667 10.88
Poway 32.9522222 -117.0472222 13.08
Fallbrook AP 33.354669 -117.251279 16.18
Lake Wohlford 33.166423 -117.004955 16.63
Ramona 33.0480556 -116.8608333 16.57
Lake Henshaw 33.2386111 -116.7616667 21.58
Borrego 33.2211111 -116.3369444 4.00
Lindbergh 32.7337 -117.1767 10.75
Escondido 33.1197222 -117.095 14.67
Flinn Springs 32.847104 -116.857801 15.55
Lake Cuyamaca 32.9894 -116.5867 31.30
Lower Otay 32.6111 -116.9319 11.90
San Onofre 33.3513889 -117.5319444 11.13
San Vicente 32.912082 -116.926513 16.47
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Gauge Latitude Longitude Mean Annual Precipitation
(inches)
Santee 32.839016 -117.024857 13.15
G.1.6.1.2 Determining Flow Rates from Continuous Hourly Flow Output Flow rates for hydromodification management studies in San Diego must be based on partial duration series analysis of the continuous hourly flow output. Partial duration series frequency calculations consider multiple storm events in a given year. To construct the partial duration series:

1. Parse the continuous hourly flow data into discrete runoff events. The following separation criteria may be used for separation of flow events: a new discrete event is designated when the flow falls below an artificially low flow value based on a fraction of the contributing watershed area (e.g., 0.002 to 0.005 cfs/acre) for a time period of 24 hours. Project applicants may consider other separation criteria provided the separation interval is not more than 24 hours and the criteria is clearly described in the submittal document.

2. Rank the peak flows from each discrete flow event, and compute the return interval or plotting position for each event.

Readers who are unfamiliar with how to compute the partial-duration series should consult reference books or online resources for additional information. For example, Hydrology for Engineers, by Linsley et all, 1982, discusses partial-duration series on pages 373-374 and computing recurrence intervals or plotting positions on page 359. Handbook of Applied Hydrology, by Chow, 1964, contains a detailed discussion of flow frequency analysis, including Annual Exceedance, Partial-Duration and Extreme Value series methods, in Chapter 8. The US Geological Survey (USGS) has several hydrologic study reports available online that use partial duration series statistics (see http://water.usgs.gov/ and http://water.usgs.gov/osw/bulletin17b/AGU_Langbein_1949.pdf).

Pre-development Q2 and Q10 shall be determined from the partial duration analysis for the predevelopment hourly flow record. Pre-development Q10 is the upper threshold of flow rates to be controlled in the post-project condition. The lower flow threshold is a fraction of the pre-development Q2 determined based on the erosion susceptibility of the receiving stream. Simply multiply the predevelopment Q2 by the appropriate fraction (e.g., 0.1Q2) to determine the lower flow threshold.

To prepare the peak flow frequency curves, use the return interval on the x-axis and the flow rate on the y-axis. Compare the post-project peak flow frequency curve to the pre-development peak flow frequency curve to determine if it meets performance criteria for post-project peak flows (criteria number 2 presented under Section G.1.6).
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G.1.6.2 Determining Flow Durations from Continuous Hourly Flow Output Flow durations must also be summarized within the range of flows to control. Flow duration statistics provide a simple summary of how often a particular flow rate is exceeded. To prepare this summary:

1. Rank the entire hourly runoff time series output.

2. Extract the portion of the ranked hourly time series output from the lower flow threshold to the upper flow threshold – this is the portion of the record to be summarized.

3. Divide the applicable portion of the record into 100 equal flow bins (compute the difference between the upper flow threshold (cfs) and lower flow threshold (cfs) and divide this value by 99 to establish the flow bin size).

4. Count the number of hours of flow that fall into each flow bin.

Both pre-development and post-project flow duration summary must be based on the entire length of the flow record. Compare the post-project flow duration summary to the pre-development flow duration summary to determine if it meets performance criteria for post-project flow rates and durations (criteria presented under Section G.1.6).
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G.2 Sizing Factors for Hydromodification
Management BMPs
This section presents sizing factors for design of flow control structural BMPs based on the sizing factor method identified in Chapter 6.3.5.1. The sizing factors included here have been updated based on the requirements in the 2015 MS4 permit and are different than the sizing factors presented in previous manuals. These updated values replace the previous sizing factors which shall no longer be used for sizing of hydromodification flow control BMPs. A discussion of the rationale for the update is included below.

The sizing factors included in previous edition was re-printed from the "San Diego BMP Sizing Calculator Methodology," dated January 2012, prepared by Brown and Caldwell (herein "BMP Sizing Calculator Methodology"). These sizing factors were linked to the specific details and descriptions that were presented in the BMP Sizing Calculator Methodology, which included certain assumptions and limited options for modifications. The sizing factors were developed based on the 2007 MS4 Permit.

Some of the original sizing factors developed based on the 2007 MS4 Permit and presented in the BMP Sizing Calculator Methodology were not compatible with new requirements of the 2015 MS4 Permit, and therefore were not included in the February 2016 manual. Since publishing the 2016 Model Manual, the Copermittees have developed updated hydromodification factors that more accurately represent the BMP configurations specified in this model manual and account for the revised flow-duration performance standard of the 2015 MS4 Permit (110% exceedance allowance for entire flow-duration curve).

The updated sizing factors were generated using continuous simulation models in USEPA SWMM in accordance with the procedures, methodologies, and values presented in Appendix G.1. All sizing factors are in relation to the effective impervious area draining to the BMP.

The sizing factor method is intended for simple studies that do not include diversion, do not include significant offsite area draining through the project from upstream, and do not include offsite area downstream of the project area. Use of the sizing factors is limited to the specific structural BMPs described in this Appendix. When using the sizing factor methodology, the area fraction reported in the sizing tables represents the plan view area at the surface of the BMP before any ponding occurs.

The BMP footprint as defined by this methodology is depicted in Figure G.2-1. 2026-08-19 Item #12D Page 501 of 816
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Figure G.2-1: Representation of BMP Footprint for use of Sizing Factors Sizing factors are available for the following specific structural BMPs:
• Full infiltration condition:
o Infiltration: Sizing factors available for A, B, C, and D soils represent surface and/or below-ground structures (infiltration vaults).
• Partial infiltration condition:
o Biofiltration with partial retention: sizing factors available for A, B, C and D soils represent a bioretention area with bioretention soil media and gravel storage layer, with an underdrain, with gravel storage below the underdrain and a flow control orifice, with no impermeable liner
• No infiltration condition:
o Biofiltration: sizing factors available for A, B, C and D soils represent a bioretention area with bioretention soil media and gravel storage layer, with an underdrain and flow control orifice, without gravel storage below the underdrain, with an impermeable liner. (Formerly known as flow-through planter and/or biofiltration with impermeable liner)
• Other
o Cistern: sizing factors available for A, B, C, or D soils represent a vessel with a 2026-08-19 Item #12D Page 502 of 816
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flow control orifice outlet to meet the hydromodification management performance standard. For this BMP, the sizing factor result is a volume in cubic feet, not a surface footprint in square feet.

Sizing factors were created based on three rainfall basins: Lindbergh Field, Oceanside, and Lake Wohlford.

The following information is needed to use the sizing factors:
• In Encinitas, use the Oceanside rainfall basin data
• Hydrologic soil group at the project site (use available information pertaining to existing underlying soil type such as soil maps published by the Natural Resources Conservation Service)
• Pre-development and pre-project slope categories (flat = 0% – 5%, moderate = 5% – 10%, steep = >10%)
• Area tributary to the structural BMP
• Area weighted runoff factor (C) for the area draining to the BMP from Table G.2-1. Note:
runoff coefficients and adjustments presented in Appendices B.1 and B.2 are for pollutant control only and are not applicable for hydromodification management studies • Fraction of Q2 to control (see Chapter 6.3.4)5
When using the sizing factor method, Worksheet G.2-1 may be used to present the calculations of the required minimum areas and/or volumes of BMPs as applicable. Additionally, the “BMP Sizing Spreadsheet V3.0” available at projectcleanwater.org implements the sizing factor methodology. 5 All updated sizing factors refer to the “High Susceptibility” threshold value of 0.1Q2, where Q2 is determined using the Weibull Plotting position and results of the SWMMM Model runs for unit pervious catchments (refer to Table G.2-2)
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Figure G.2-2: Rainfall Basin Map
Table G.2-1: Runoff factors for surfaces draining to BMPs for Hydromodification Sizing Factors Method
Surface Runoff Factor
Roofs 1.0
Concrete 1.0
Pervious Concrete 0.10
Porous Asphalt 0.10
Grouted Unit Pavers 1.0
Solid Unit Pavers on granular base, min. 3/16 inch joint space 0.20 Crushed Aggregate 0.10
Turf block 0.10
Amended, mulched soils 0.10
Landscape 0.10
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Worksheet G.2-1: Sizing Factor Worksheet
Areas Draining to BMP Sizing Factors Minimum BMP Size
DMA
Name
Area
(sf) Soil Type
PreProject
Slope
Post-Project
Surface Type
Runoff
Factor (From
Table G.2-1)
Surface
Area
Surface
Volume Subsurface Volume Surface Area (sf) Volume (cf) Subsurface Volume (cf) Total DMA
Area
Minimum BMP Size*
Proposed BMP Size*
*Minimum BMP Size = Total of rows above. *Proposed BMP Size > Minimum BMP size.

Site Information
Project Name: Hydrologic Unit Project Applicant: Rain Gauge:

Jurisdiction: Total Project Area:

Assessor’s Parcel Number: Low Flow Threshold:

BMP Name: BMP Type:
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G.2.1 Unit Runoff Ratios and Low Flow Control Orifice
Design
G.2.1.1 Unit Runoff Ratios
Table G.2-2 presents unit runoff ratios for calculating pre-development Q2, to be used when applicable to determine the lower flow threshold for low flow control orifice sizing for biofiltration with partial retention, biofiltration, biofiltration with impermeable liner, or cistern BMPs. There is no low flow control orifice in the infiltration BMP or bioretention BMP. The unit runoff ratios are updated from the previously reported BMP Sizing Calculator methodology ratios to account for changes in modeling methodologies. Unit runoff ratios for "urban" and "impervious" cover categories were not transferred to this manual due to the requirement to control runoff to pre- development condition (see Chapter

6.3.3).

How to use the unit runoff ratios:

Obtain unit runoff ratio from Table G.2-2 based on the project's rainfall basin, hydrologic soil group, and pre-development slope (for redevelopment projects, pre-development slope may be considered if historic topographic information is available, otherwise use pre-project slope). Multiply the area tributary to the structural BMP (A, acres) by the unit runoff ratio (Q2, cfs/acre) to determine the predevelopment Q2 to determine the lower flow threshold, to use for low flow control orifice sizing.

Table G.2-2: Unit Runoff Ratios for Sizing Factor Method Rain Gauge Soil Pre-Project
Slope Q2 (cfs/acre) Q10 (cfs/acre)
Lake Wohlford A Flat 0.256 0.518
Lake Wohlford A Moderate 0.275 0.528
Lake Wohlford A Steep 0.283 0.531
Lake Wohlford B Flat 0.371 0.624
Lake Wohlford B Moderate 0.389 0.631
Lake Wohlford B Steep 0.393 0.633
Lake Wohlford C Flat 0.490 0.729
Lake Wohlford C Moderate 0.495 0.733
Lake Wohlford C Steep 0.496 0.735
Lake Wohlford D Flat 0.548 0.784
Lake Wohlford D Moderate 0.554 0.788
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Rain Gauge Soil Pre-Project
Slope Q2 (cfs/acre) Q10 (cfs/acre)
Lake Wohlford D Steep 0.556 0.788
Oceanside A Flat 0.256 0.679
Oceanside A Moderate 0.277 0.694
Oceanside A Steep 0.285 0.700
Oceanside B Flat 0.377 0.875
Oceanside B Moderate 0.391 0.879
Oceanside B Steep 0.395 0.881
Oceanside C Flat 0.488 0.981
Oceanside C Moderate 0.497 0.985
Oceanside C Steep 0.499 0.986
Oceanside D Flat 0.571 0.998
Oceanside D Moderate 0.575 0.999
Oceanside D Steep 0.576 0.999
Lindbergh A Flat 0.057 0.384
Lindbergh A Moderate 0.073 0.399
Lindbergh A Steep 0.082 0.403
Lindbergh B Flat 0.199 0.496
Lindbergh B Moderate 0.220 0.509
Lindbergh B Steep 0.230 0.513
Lindbergh C Flat 0.335 0.601
Lindbergh C Moderate 0.349 0.610
Lindbergh C Steep 0.354 0.613
Lindbergh D Flat 0.429 0.751
Lindbergh D Moderate 0.437 0.753
Lindbergh D Steep 0.439 0.753
G.2.1.2 Flow Control Orifice Design
When used as hydromodification flow control BMPs, biofiltration with partial retention, biofiltration, 2026-08-19 Item #12D Page 507 of 816
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and cistern BMPs include a low flow control orifice to control the rate that flow is released from the underdrain or primary outlet. The sizing factors were developed using a standard process for sizing the low flow control orifice, therefore BMPs designed using the sizing factor method must size the low flow control orifice using the same basis. The low flow control orifice must be designed to release the lower flow threshold flow rate (fraction of pre-development Q2) when the water surface elevation in the BMP is equal to the crest elevation of the next outflow structure. To size the low flow control orifice, determine the head on the orifice measured from the bottom of the orifice to the minimum elevation of the next outflow structure of the BMP. The next outflow structure is typically the BMP overflow structure, except in some multJ-use BMPs (e.g., BMPs that are designed for flood control in addition to hydromodification management). In this application, the difference between the bottom of the orifice and the centroid of the orifice is small relative to the total head for the calculation and may be neglected in the calculation by measuring from the orifice invert. This calculation is automated in the “BMP Sizing Spreadsheet V3.0” posted on www.projectcleanwater.org.

Steps to size the low flow control orifice:
• Determine pre-development Q2 using the unit runoff ratios above. • Multiply pre-development Q2 by 0.1 to determine the low flow threshold flow rate. Note sizing factors are only available for streams with high susceptibility to erosion where the low flow threshold is 0.1Q2.
• Determine the head (H) on the orifice measured from the bottom of the orifice to the minimum elevation of the next outflow structure of the BMP. • Use the orifice equation (below) and solve for the maximum orifice area to release the lower flow threshold flow rate.
• Consider how the orifice will be created. Determine the constructible dimension(s) (e.g., a standard drill bit diameter) that will produce an orifice with an area equal to or less than the maximum orifice area. The final orifice area determined based on constructible dimensions shall not exceed the maximum orifice area.
[ = × × (64.4 H)
E.d
where:

Q = Flow rate in cubic feet per second
C = Orifice coefficient; in this application use C = 0.65 A = Area in square feet
H = Head in feet
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G.2.2 Sizing Factors for "Infiltration" BMP
Table G.2-3 presents sizing factors for calculating the required surface area (A) for an infiltration BMP. There is no underdrain and therefore no low flow orifice in the infiltration BMP. Sizing factors were developed for hydrologic soil groups A B, C, and D. This BMP is generally not applicable in hydrologic soil groups C and D, but applicants have the option if there are no geotechnical or water balance issues and the underlying design infiltration rate for the BMP is greater than 0.5 inches per hour. The infiltration BMP is surface ponding feature that allows infiltration into the native or amended soils of the BMP surface.
• Ponding layer: a nominal 36-inch ponding layer shall be included below the overflow elevation.
• Design infiltration rate: the design infiltration rate shall be greater than 0.5 inches per hour.
• Overflow structure: San Diego Regional Standard Drawing Type I Catch Basin (D-29). For the purposes of hydromodification flow control other type of overflow structures are allowed.

Infiltration Facility BMP Example Illustration
How to use the sizing factors for flow control BMP sizing:

Obtain sizing factors from Table G.2-3 based on the project's lower flow threshold fraction of Q2, hydrologic soil group, pre-project slope, and rain gauge (rainfall basin). Multiply the area tributary to the structural BMP (A, square feet) by the area weighted runoff factor (C, unitless) (see Table G.2-1) by the sizing factors to determine the required surface area (A, square feet) for the infiltration BMP.

The civil engineer shall provide the necessary surface area of the BMP on the plans.

Additional steps to use this BMP as a combined pollutant control and flow control BMP:
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The BMP sized using the sizing factors in Table G.2-3 meets both pollutant control and flow control requirements.

Table G.2-3: Sizing Factors for Hydromodification Flow Control Infiltration BMPs Designed Using Sizing Factor Method
Lower Flow
Threshold Soil Group Pre-ProjectSlope
Rain Gauge A

0.1Q2 A Flat Lindbergh 0.055

0.1Q2 A Moderate Lindbergh 0.055

0.1Q2 A Steep Lindbergh 0.055

0.1Q2 B Flat Lindbergh 0.045

0.1Q2 B Moderate Lindbergh 0.045

0.1Q2 B Steep Lindbergh 0.045

0.1Q2 C Flat Lindbergh 0.035

0.1Q2 C Moderate Lindbergh 0.035

0.1Q2 C Steep Lindbergh 0.035

0.1Q2 D Flat Lindbergh 0.030

0.1Q2 D Moderate Lindbergh 0.030

0.1Q2 D Steep Lindbergh 0.030

0.1Q2 A Flat Oceanside 0.060

0.1Q2 A Moderate Oceanside 0.060

0.1Q2 A Steep Oceanside 0.060

0.1Q2 B Flat Oceanside 0.050

0.1Q2 B Moderate Oceanside 0.050

0.1Q2 B Steep Oceanside 0.050

0.1Q2 C Flat Oceanside 0.050

0.1Q2 C Moderate Oceanside 0.050

0.1Q2 C Steep Oceanside 0.045

0.1Q2 D Flat Oceanside 0.035
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Lower Flow
Threshold Soil Group Pre-ProjectSlope
Rain Gauge A

0.1Q2 D Moderate Oceanside 0.035

0.1Q2 D Steep Oceanside 0.035

0.1Q2 A Flat L Wohlford 0.085

0.1Q2 A Moderate L Wohlford 0.085

0.1Q2 A Steep L Wohlford 0.085

0.1Q2 B Flat L Wohlford 0.070

0.1Q2 B Moderate L Wohlford 0.070

0.1Q2 B Steep L Wohlford 0.070

0.1Q2 C Flat L Wohlford 0.055

0.1Q2 C Moderate L Wohlford 0.055

0.1Q2 C Steep L Wohlford 0.055

0.1Q2 D Flat L Wohlford 0.040

0.1Q2 D Moderate L Wohlford 0.040

0.1Q2 D Steep L Wohlford 0.040
Q2 = 2-year pre-project flow rate based upon partial duration analysis of long-term hourly rainfall records A = Surface area sizing factor for flow control
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G.2.3 Sizing Factors for Biofiltration with Partial Retention Table G.2-4 presents sizing factors for calculating the required surface area (A) biofiltration with partial retention BMP. The bioretention BMP consists of four layers:
• Ponding layer: 12-inches active storage, [minimum] 2-inches of freeboard above overflow relief
• Media Layer: 18-inches of soil [bioretention soil media] • Filter Course: 6-inches
• Storage Layer: 18-inches of gravel at 40 percent porosity for A and B soils and 12-inches of gravel at 40 percent porosity for C and D soils. The underdrain offset for A and B soils shall be 18-inches, for C soils is shall be 6-inches and for D soils is shall be 3 inches. • Overflow Structure: San Diego Regional Standard Drawing Type I Catch Basin (D-29). For the purposes of hydromodification flow control other type of overflow structures are allowed.

This BMP does not include an impermeable layer at the bottom of the facility to prevent infiltration into underlying soils, regardless of hydrologic soil group. If a facility is to be lined, the designer must use the sizing factors for biofiltration (Refer to Appendix G.2.4).

Biofiltration with Partial Retention BMP Example Illustration How to use the sizing factors for flow control BMP Sizing:

Obtain sizing factors from Table G.2-4 based on the project's lower flow threshold fraction of Q2, hydrologic soil group, pre-project slope, and rain gauge (rainfall basin). Multiply the area tributary to the structural BMP (A, square feet) by the area weighted runoff factor (C, unitless) (see Table G.2-1) 2026-08-19 Item #12D Page 512 of 816
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by the sizing factors to determine the required surface area (A, square feet). Select a low flow control orifice for the underdrain that will discharge the lower flow threshold flow at the overflow riser elevation. Standard head (H) for this calculation (based on the standard detail) is 3.0 feet for A or B soils, 3.5 feet for C soils, or 3.75 feet for D soils. The civil engineer shall provide the necessary surface area of the BMP and the underdrain and orifice detail on the plans.

Additional steps to use this BMP as a combined pollutant control and flow control BMP:

The BMP sized using the sizing factors in Table G.2-4 meets both pollutant control and flow control requirements.

Table G.2-4: Sizing Factors for Hydromodification Flow Control Biofiltration with Partial Retention BMPs Designed Using Sizing Factor Method
Lower Flow
Threshold Soil Group
PreProject
Slope
Aggregate below low
orifice invert
(inches)
Rain Gauge A

0.1Q2 A Flat 18 Lindbergh 0.080

0.1Q2 A Moderate 18 Lindbergh 0.080

0.1Q2 A Steep 18 Lindbergh 0.080

0.1Q2 B Flat 18 Lindbergh 0.065

0.1Q2 B Moderate 18 Lindbergh 0.065

0.1Q2 B Steep 18 Lindbergh 0.060

0.1Q2 C Flat 6 Lindbergh 0.050

0.1Q2 C Moderate 6 Lindbergh 0.050

0.1Q2 C Steep 6 Lindbergh 0.050

0.1Q2 D Flat 3 Lindbergh 0.050

0.1Q2 D Moderate 3 Lindbergh 0.050

0.1Q2 D Steep 3 Lindbergh 0.050

0.1Q2 A Flat 18 Oceanside 0.080

0.1Q2 A Moderate 18 Oceanside 0.075

0.1Q2 A Steep 18 Oceanside 0.075

0.1Q2 B Flat 18 Oceanside 0.070
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Lower Flow
Threshold Soil Group
PreProject
Slope
Aggregate below low
orifice invert
(inches)
Rain Gauge A

0.1Q2 B Moderate 18 Oceanside 0.070

0.1Q2 B Steep 18 Oceanside 0.070

0.1Q2 C Flat 6 Oceanside 0.070

0.1Q2 C Moderate 6 Oceanside 0.070

0.1Q2 C Steep 6 Oceanside 0.070

0.1Q2 D Flat 3 Oceanside 0.070

0.1Q2 D Moderate 3 Oceanside 0.070

0.1Q2 D Steep 3 Oceanside 0.070

0.1Q2 A Flat 18 L Wohlford 0.110

0.1Q2 A Moderate 18 L Wohlford 0.110

0.1Q2 A Steep 18 L Wohlford 0.105

0.1Q2 B Flat 18 L Wohlford 0.090

0.1Q2 B Moderate 18 L Wohlford 0.085

0.1Q2 B Steep 18 L Wohlford 0.085

0.1Q2 C Flat 6 L Wohlford 0.065

0.1Q2 C Moderate 6 L Wohlford 0.065

0.1Q2 C Steep 6 L Wohlford 0.065

0.1Q2 D Flat 3 L Wohlford 0.060

0.1Q2 D Moderate 3 L Wohlford 0.060

0.1Q2 D Steep 3 L Wohlford 0.060
Q2 = 2-year pre-project flow rate based upon partial duration analysis of long-term hourly rainfall records A = Surface area sizing factor for flow control
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G.2.4 Sizing Factors for Biofiltration
Table G.2-5 presents sizing factors for calculating the required surface area (A) for a biofiltration BMP (formerly known as flow-through planter and/or biofiltration BMP with impermeable liner). The BMPs consist of four layers:
• Ponding layer: 12-inches active storage, [minimum] 2-inches of freeboard above overflow relief
• Media Layer: 18-inches of soil [bioretention soil media] • Filter Course: 6-inches
• Storage Layer: 12-inches of gravel at 40 percent porosity. The underdrain offset shall be 3-inches.
• Overflow Structure: San Diego Regional Standard Drawing Type I Catch Basin (D-29).

For the purposes of hydromodification flow control other type of overflow structures are allowed.

This BMP includes an impermeable liner to prevent infiltration into underlying soils.

Biofiltration BMP Example Illustration
How to use the sizing factors for flow control BMP Sizing:

Obtain sizing factors from Table G.2-5 based on the project's lower flow threshold fraction of Q2, hydrologic soil group, pre-project slope, and rain gauge (rainfall basin). Multiply the area tributary to the structural BMP (A, square feet) by the area weighted runoff factor (C, unitless) (see Table G.2-1) by the sizing factors to determine the required surface area (A, square feet). Select a low flow control orifice for the underdrain that will discharge the lower flow threshold flow at the overflow riser elevation. Standard head (H) for this calculation (based on the standard detail) is 3.75 feet for all soil groups. The civil engineer shall provide the necessary surface area of the BMP and the underdrain and 2026-08-19 Item #12D Page 515 of 816
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orifice detail on the plans.

Additional steps to use this BMP as a combined pollutant control and flow control BMP:

The BMP sized using the sizing factors in Table G.2-5 meets both pollutant control and flow control requirements except for surface drawdown requirements. Applicant must perform surface drawdown calculations and if needed develop a vector management plan (Refer to Section 6.3.7) or revise the BMP design to meet the drawdown requirements. If changes are made to the BMP design applicants must perform site specific continuous simulation modeling (Refer to Appendix G).

Table G.2-5: Sizing Factors for Hydromodification Flow Control Biofiltration BMPs Designed Using Sizing Factor Method
Lower Flow
Threshold
Soil Group Pre-Project Slope Rain Gauge A

0.1Q2 A Flat Lindbergh 0.320

0.1Q2 A Moderate Lindbergh 0.300

0.1Q2 A Steep Lindbergh 0.285

0.1Q2 B Flat Lindbergh 0.105

0.1Q2 B Moderate Lindbergh 0.100

0.1Q2 B Steep Lindbergh 0.095

0.1Q2 C Flat Lindbergh 0.055

0.1Q2 C Moderate Lindbergh 0.050

0.1Q2 C Steep Lindbergh 0.050

0.1Q2 D Flat Lindbergh 0.050

0.1Q2 D Moderate Lindbergh 0.050

0.1Q2 D Steep Lindbergh 0.050

0.1Q2 A Flat Oceanside 0.150

0.1Q2 A Moderate Oceanside 0.140

0.1Q2 A Steep Oceanside 0.135

0.1Q2 B Flat Oceanside 0.085

0.1Q2 B Moderate Oceanside 0.085

0.1Q2 B Steep Oceanside 0.085
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Lower Flow
Threshold
Soil Group Pre-Project Slope Rain Gauge A

0.1Q2 C Flat Oceanside 0.075

0.1Q2 C Moderate Oceanside 0.075

0.1Q2 C Steep Oceanside 0.075

0.1Q2 D Flat Oceanside 0.070

0.1Q2 D Moderate Oceanside 0.070

0.1Q2 D Steep Oceanside 0.070

0.1Q2 A Flat L Wohlford 0.285

0.1Q2 A Moderate L Wohlford 0.275

0.1Q2 A Steep L Wohlford 0.270

0.1Q2 B Flat L Wohlford 0.150

0.1Q2 B Moderate L Wohlford 0.145

0.1Q2 B Steep L Wohlford 0.145

0.1Q2 C Flat L Wohlford 0.070

0.1Q2 C Moderate L Wohlford 0.070

0.1Q2 C Steep L Wohlford 0.070

0.1Q2 D Flat L Wohlford 0.060

0.1Q2 D Moderate L Wohlford 0.060

0.1Q2 D Steep L Wohlford 0.060
Q2 = 2-year pre-project flow rate based upon partial duration analysis of long-term hourly rainfall records A = Surface area sizing factor for flow control
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G.2.5 Sizing Factors for "Cistern" BMP
Table G.2-6 presents sizing factors for calculating the required volume (V) for a cistern BMP. In this context, a "cistern" is a detention facility that stores runoff and releases it at a controlled rate. A cistern can be a component of a harvest and use system, however the sizing factor method will not account for any retention occurring in the system. The sizing factors were developed assuming runoff is released from the cistern. The sizing factors presented in this section are to meet the hydromodification management performance standard only. The cistern BMP is based on the following assumptions:
• Cistern configuration: The cistern is modeled as a 4-foot tall vessel. However, designers could use other configurations (different cistern heights), as long as the lower outlet orifice is sized to properly restrict outflows and the minimum required volume is provided. • Cistern upper outlet: The upper outlet from the cistern would consist of a weir or other flow control structure with the overflow invert set at an elevation of 7/8 of the water height associated with the required volume of the cistern – V. For the assumed 4-foot water depth in the cistern associated with the sizing factor analysis, the overflow invert is assumed to be located at an elevation of 3.5 feet above the bottom of the cistern. The overflow weir would be sized to pass the peak design flow based on the tributary drainage area.

How to use the sizing factors:

Obtain sizing factors from Table G.2-6 based on the project's lower flow threshold fraction of Q2, hydrologic soil group, pre-project slope, and rain gauge (rainfall basin). Multiply the area tributary to the structural BMP (A, square feet) by the area weighted runoff factor (C, unitless) (see Table G.2-1) by the sizing factors to determine the required volume (V, cubic feet). Select a low flow control orifice that will discharge the lower flow threshold flow at the overflow elevation (i.e. when there is 3.5 feet of head over the lower outlet orifice or adjusted head as appropriate if the cistern overflow elevation is not 3.5 feet tall). The civil engineer shall provide the necessary volume of the BMP and the lower outlet orifice detail on the plans.

Additional steps to use this BMP as a combined pollutant control and flow control BMP:

A cistern could be a component of a full retention, partial retention, or no retention BMP depending on how the outflow is disposed. However use of the sizing factor method for design of the cistern in a combined pollutant control and flow control system is not recommended. The sizing factor method for designing a cistern does not account for any retention or storage occurring in BMPs combined with the cistern (i.e., cistern sized using sizing factors may be larger than necessary because sizing factor method does not recognize volume losses occurring in other elements of a combined system).

Furthermore when the cistern is designed using the sizing factor method, the cistern outflow must be set to the low flow threshold flow for the drainage area, which may be inconsistent with requirements 2026-08-19 Item #12D Page 518 of 816
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for other elements of a combined system. To optimize a system in which a cistern provides temporary storage for runoff to be either used onsite (harvest and use), infiltrated, or biofiltered, project-specific continuous simulation modeling is recommended. Refer to Sections 5.6 and 6.3.6.

Table G.2-6: Sizing Factors for Hydromodification Flow Control Cistern BMPs Designed Using Sizing Factor Method
Lower Flow
Threshold Soil Group Pre-Project Slope Rain Gauge V

0.1Q2 A Flat Lindbergh 0.54

0.1Q2 A Moderate Lindbergh 0.51

0.1Q2 A Steep Lindbergh 0.49

0.1Q2 B Flat Lindbergh 0.19

0.1Q2 B Moderate Lindbergh 0.18

0.1Q2 B Steep Lindbergh 0.18

0.1Q2 C Flat Lindbergh 0.11

0.1Q2 C Moderate Lindbergh 0.11

0.1Q2 C Steep Lindbergh 0.11

0.1Q2 D Flat Lindbergh 0.09

0.1Q2 D Moderate Lindbergh 0.09

0.1Q2 D Steep Lindbergh 0.09

0.1Q2 A Flat Oceanside 0.26

0.1Q2 A Moderate Oceanside 0.25

0.1Q2 A Steep Oceanside 0.25

0.1Q2 B Flat Oceanside 0.16

0.1Q2 B Moderate Oceanside 0.16

0.1Q2 B Steep Oceanside 0.16

0.1Q2 C Flat Oceanside 0.14

0.1Q2 C Moderate Oceanside 0.14

0.1Q2 C Steep Oceanside 0.14

0.1Q2 D Flat Oceanside 0.12
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Lower Flow
Threshold Soil Group Pre-Project Slope Rain Gauge V

0.1Q2 D Moderate Oceanside 0.12

0.1Q2 D Steep Oceanside 0.12

0.1Q2 A Flat L Wohlford 0.53

0.1Q2 A Moderate L Wohlford 0.49

0.1Q2 A Steep L Wohlford 0.49

0.1Q2 B Flat L Wohlford 0.28

0.1Q2 B Moderate L Wohlford 0.28

0.1Q2 B Steep L Wohlford 0.28

0.1Q2 C Flat L Wohlford 0.14

0.1Q2 C Moderate L Wohlford 0.14

0.1Q2 C Steep L Wohlford 0.14

0.1Q2 D Flat L Wohlford 0.12

0.1Q2 D Moderate L Wohlford 0.12

0.1Q2 D Steep L Wohlford 0.12
Q2 = 2-year pre-project flow rate based upon partial duration analysis of long-term hourly rainfall records V = Cistern volume sizing
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C I T Y O F E N C I N I T A S B M P D E S I G N M A N U A L H-1 2023
Appendix
I
Appendix H Guidance for
Investigating Potential Critical
Coarse Sediment Yield Areas
The following guidance provides methodologies for protecting CCSYAs:

H.1. Step 1: Identify CCSYAs
H.2. Step 2: Avoidance of Onsite CCSYAs
H.3. Step 3: Bypass Onsite and Upstream CCSYAs
H.4. Step 4: Demonstrate No Net Impact
H.5. References
H.6. PCCSYAs: Regional WMAA Maps
H.7. Downstream System Sensitivity to Coarse Sediment
H.8. Calculation Methodology for Ep and Sp
H.9. Mitigation Measures Fact Sheets
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Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-2 2023
H.1 Step 1: Identify CCSYAs
A critical coarse sediment yield area (CCSYA) is an active or potential source of bed sediment to downstream channel reaches. When a Priority Development Project (PDP) is constructed, it has the potential to negatively impact characteristics of sediment supply and delivery which can lead to degradation of receiving waters. In order to prevent these impacts, PDP applicants must examine the tributary areas delineated in the project’s storm water management plans and identify sources of critical coarse sediment within the following locations:
• Onsite CCSYAs: CCSYAs identified within the project’s property boundary as indicated in the SWQMP. Refer to Section 1.3 for defining a project. • Upstream CCSYAs: CCSYAs identified within the drainage area draining through the project’s property boundary as indicated in the SWQMP. Refer to Section 1.3 for defining a project.

Applicants must first identify potential critical coarse sediment yield areas (PCCSYAs) using one of the methods presented in Appendix H.1.1. Once these PCCSYAs are identified, applicants may either accept the PCCSYA mapping as final, or may elect to further refine the results of the mapping through consideration of the refinement methods outlined in Appendix H.1.2. At the end of Step 1, applicants will have identified CCSYAs that must be avoided and bypassed by the project.

H.1.1 Identification Methods
Applicants must identify both onsite and upstream CCSYAs by referring to the Regional Watershed Management Area Analysis PCCSYA maps provided in Appendix H.6.. 2026-08-19 Item #12D Page 522 of 816
Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-3 2023
H.1.2 Refinement Options
After identifying PCCSYAs using one of the methods above, the applicant may either accept the PCCSYA mapping as final, or may elect to further refine the results of the mapping through consideration of one or more of the refinement methods outlined below.

H.1.2.1 Depositional Analysis
Areas identified as PCCSYAs may be removed from consideration if it is demonstrated that these sources are deposited into existing systems prior to reaching the first downstream unlined water of the state. Systems resulting in deposition may include existing natural sinks, existing structural BMPs, existing hardened MS4 systems, or other existing similar features that produce a peak velocity from the discrete 2-year, 24 hour runoff event of less than three feet per second in the system being analyzed. Applicants electing to perform depositional analysis to refine PCCSYA mapping must refer to the detailed guidance provided in Appendix H.7.1.

H.1.2.2 Threshold Channel Analysis
Areas identified as PCCSYAs may be removed from consideration if they discharge to a “threshold channel” that does not exhibit characteristics associated with significant bed load movement during design flows. Applicants electing to perform threshold channel analysis to refine PCCSYA mapping must refer to the detailed guidance provided in Appendix H.7.2.

H.1.2.3 Coarse Sediment Source Area Verification
Areas identified as PCCSYAs may be removed from consideration if an applicant demonstrates that these areas actually consist of fine grained sediment. Applicants electing to perform coarse sediment source area verification to refine PCCSYA mapping must refer to the detailed guidance provided in Appendix H.7.3.

H.1.2.4 Verification of Geomorphic Landscape Units (GLUs) If an applicant has identified sources of critical coarse sediment via the alternate WMAA Method discussed in Appendix H.1.1, PCCSYAs mapping may be refined through verification of GLUs. If this method is used, applicants must refer to detailed guidance provided in Appendix H.6.1.

H.2 Step 2: Avoidance of Onsite CCSYAs
A key element of preserving the stability of receiving waters is to avoid changes in bed sediment supply by avoiding development on CCSYAs. Avoidance is best achieved through proper site design. The following are some potential strategies that should be considered while determining the site layout to avoid CCSYAs:
• The civil engineer shall designate onsite CCSYAs that are to be avoided (undisturbed) for the purpose of preserving coarse soil supply. When feasible, use and/or access restriction should be established for these areas.
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Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-4 2023
• Minimize new impervious footprint. Refer to SD-3 in Chapter 4 for guidance on minimizing impervious footprint.

If onsite CCSYAs are not avoided per the metrics defined below, the applicant must demonstrate no net impact to the receiving water using guidance in Appendix H.4.

H.2.1 Avoidance Metrics
If the applicant has identified onsite CCSYAs using the WMAA Mapping Method, encroachments of up to 5% into the onsite CCSYAs may be permitted. Refer to Appendix H.6.3 for supporting rationale for 5% encroachment.

H.3 Step 3: Bypass Onsite and Upstream
CCSYAs
Another key element of preserving the stability of receiving waters is to maintain current bed sediment supply characteristics through effective bypass of onsite and upstream sediment sources. Upstream bed sediment sources may include overland flow from CCSYAs and/or concentrated channel flows.

Applicants must ensure both onsite and upstream sources of bed sediment are effectively bypassed through their project. If onsite and/or upstream CCSYAs are not effectively bypassed per the criteria below, applicant must demonstrate no net impact to the receiving water per the guidance presented in Appendix H.4.

H.3.1 Bypass CCSYAs from Hillslopes
Both onsite and upstream hillslopes mapped as CCSYAs must be effectively bypassed through and/or around the proposed project site.
• Proposed hardened drainage systems (e.g. storm drains, drainage ditches) that convey the bed sediment from the hillslopes to the downstream waters of the state should maintain a peak velocity from the discrete 2-year, 24-hour runoff event greater than three feet per second. o When drainage ditches are proposed for bypass, this velocity may be achieved by designing to the minimum dimensions listed in the San Diego Regional Standard drawing D-75.
o When an 18” concrete storm drain is proposed for bypass, this velocity may typically be achieved by maintaining a storm drain slope of ≥0.5%. In instances where 2 year, 24-hour peak flow rates associated with the storm drain are less than 1.1 cfs, applicants may refer to the table below for minimum slopes needed to maintain three feet per second. Applicants may interpolate the values from the table below, or may elect to perform more detailed cleansing velocity calculations presented in Appendix H.7.1. 2026-08-19 Item #12D Page 524 of 816
Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-5 2023
2-Year, 24-Hour
Peak Flow (cfs)
Minimum Slope for 18”
Concrete Storm Drain
<0.25 n/a, this PCCSYA is considered de-minimis

0.25 2.0%

0.50 1.0%

1.10 0.5%
• Storm water runoff that contains the bed sediment from CCSYAs must not be routed through detention basins or other facilities with restricted outlets that will trap sediment. Bypass systems shall be designed as necessary so that the bed material is conveyed to the downstream receiving water. Structural BMPs (including most flow-thru BMPs) are likely to trap sediment. • For scenarios where a BMP must be constructed to treat offsite drainage area and there are CCSYAs outside of the project footprint, it may be feasible to achieve mitigation by construction of an outlet structure that can convey the bed load to the downstream receiving water and clear water through a bypass structure to a BMP. • Proposed crossings (culverts, driveways, etc.) should not impede the transport of upstream critical coarse sediment. Crossings should be designed to avoid headwater conditions that would result in the trapping/settling of sediment.

H.3.2 Bypass CCSYAs from Channels
Projects that effectively avoid and bypass CCSYAs mapped in Step 1 of this guidance are not required to take specific action to ensure bypass of channel flows. This guidance does not set forth channel bypass criteria for this scenario because it recognizes that existing regulator mechanisms (such as 401 certifications, site design requirements, etc) are generally sufficient to preserve the sediment transport functions of onsite channels.

However, projects that do not effectively avoid and bypass the CCSYAs mapped in Step 1, will be required to specifically account for bypass of channel flows as part of the demonstration of no net impact outlined in Appendix H.4.

H.3.3 De Minimis Upstream CCSYA
Applicants have an option to exclude de minimis upstream CCSYAs. De minimis upstream CCSYAs consist of coarse hillslope areas that are not significant contributors of bed sediment yield due to their small size, and are considered by the owner and County as not practicable to bypass to the downstream waters of the state. In limited scenarios where all of the criteria below are satisfied, de minimis upstream CCSYAs may be omitted from consideration.
• De minimis upstream CCSYA is not disturbed through the proposed project activities. • De minimis upstream CCSYA is not part of an upstream drainage contributing more than 0.31 total acres to the project site.
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Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-6 2023
• Multiple de minimis upstream CCSYAs cannot be adjacent to each other and hydraulically connected.
• The SWQMP must document the reason why each de minimis upstream CCSYA could not be bypassed to the downstream waters of the state.

The 0.31-acre (13,500 square feet) de minimis threshold was established using 0.25 cfs as the cut off peak flow for the 2-year, 24-hour event, rational method equation and the following assumptions:
• C = 0.225 (average runoff coefficient (C) for soil type A and B); • Average 6-hour, 2-year storm depth = 1.5 inches;
• Time of concentration = 6 minutes; and
• 2-year peak intensity = 3.51 in/hr. (based on procedures from the County Hydrology Manual).

The strategies for sediment bypass do not mitigate for the reduction of CCSYA that have been replaced by development onsite but can only mitigate scenarios where development hinders movement of bed sediment through the project footprint. When preservation of existing channels and/or implementation of sediment bypass measures is not feasible and/or not implemented, the applicant must demonstrate no net impact to the receiving water via the guidance presented in Appendix H.4.

H.4 Step 4: Demonstrate No Net Impact
When impacts to CCSYAs cannot be avoided or effectively bypassed, the applicant must demonstrate that their project generates no net impact to the receiving water per the performance metrics identified herein.
• Appendix H.4.1 provides background on the state of the current science for predicting hydromodification impacts due to reductions in sediment supply; • Appendix H.4.2 defines the management standard that will be the basis for evaluating whether “no net impact to the receiving water” is achieved; • Appendix H.4.3 identifies the type of mitigation measures (i.e., additional flow control, stream rehabilitation, and applicant proposed mitigation measures) that can be used to meet the management standard;
• Appendix H.8 provides the methodology for calculation of Erosion Potential (Ep) and Sediment Supply Potential (Sp); and
• Appendix H.9 provides fact sheets for implementation of the mitigation measures.

H.4.1 Background
Channel form, by definition, is composed of bed and bank material as well as channel geometry (in plan, cross-section, and profile); however, the dominant forces typically controlling channel form are 2026-08-19 Item #12D Page 526 of 816
Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-7 2023
discharge and sediment supply (notably bed material) since a stream’s most basic function is to convey water and sediment (Knighton, 1998). The interaction between form and function is qualitatively described through Lane’s relationship:

Equation H.4-1: Lane’s Relationship
[ × g ∝ [ × X
Where:

Qs = Sediment discharge
D = Particle diameter or size of sediment
Qw = Streamflow
S = Stream slope
Lane’s relationship qualitatively states that the sediment load (size and volume of sediment), which is the first half of the relationship, is proportional to the stream power (volume of runoff and slope) which is represented by the second half of the relationship. The sediment discharge (Qs) in the relationship is the coarser part of sediment load, referred to as the “bed sediment”, since this is the part of the load which largely molds the bed formation (Lane, 1955). Lane’s relationship (Equation H.4-1) cannot be used for quantitative calculations since the proportionality is not necessarily linear.

For a stream at equilibrium, Lane’s relationship states that if one of the variables changes and the other variables do not change proportionately, then the stream channel is no longer in equilibrium.

Sediment load and stream power can change considerably during and following new development, leading to changes in the equilibrium state of the receiving channel. • Typically, sediment load increases during the construction period, due to the additional exposure of bare soil during the grading and construction process, and before landscaping vegetation has stabilized the soil. This is regulated through the construction-phase BMP requirements established by the Construction General Permit and/or the MS4 Permit. • Following the construction period, sediment load typically decreases to below predevelopment levels, as less sediment is available from areas that have been paved or stabilized by landscape vegetation. When this decrease is not regulated, the bed sediment supplied to the stream (first half of the relationship) is reduced and the sediment transport capacity (stream power) is increased due to increased flows and durations resulting from the addition of impervious areas (second half of the relationship). This may result in degradation of the stream system as illustrated in Figure H.4-1.
2026-08-19 Item #12D Page 527 of 816
Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-8 2023
Stream in equilibrium Post-construction condition with no flow control and/or sediment supply regulations
Schematics credit: SCCWRP
Figure H.4-1: Illustration of Lane’s Relationship
Lane’s relationship is useful for making qualitative predictions concerning channel impacts due to changes in runoff and/or sediment loads from the watershed. Although this qualitative assessment is useful for understanding how the watershed responds to development, quantitative predictions are valuable for determining the magnitude of response and they can inform the identification of locations where the greatest management attention should be invested.

Lane’s relationship can be supplemented by the use of quantitative predictions which allow the evaluation of the stream under changing conditions. Quantitative predictions will include bed sediment supply calculations for the first half of the Lane relationship, and bed sediment transport capacity calculations for the second half of the Lane relationship. Imbalances between the bed sediment supply rate and transport capacity determines the rate of sediment deposition or erosion in the channel and the associated channel change (Wilcock et al., 2009).

The common practice is to use the Erosion Potential (Ep) metric to evaluate the changes in sediment transport capacity and the Sediment Supply Potential (Sp) metric to evaluate the changes in bed sediment supply for susceptible receiving channels of concern. In regards to Ep metric, • SCCWRP Technical Report 667 (SCCWRP, 2012) states:
“The underlying premise of the erosion potential approach advances the concept of flow duration control by addressing in-stream processes related to sediment transport. An erosion potential calculation combines flow parameters with stream geometry to assess long term 2026-08-19 Item #12D Page 528 of 816
Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-9 2023
(decadal) changes in the sediment transport capacity. The cumulative distribution of shear stress, specific stream power and sediment transport capacity across the entire range of relevant flows can be calculated and expressed using an erosion potential metric, Ep.” • SCCWRP Technical Report 753 (SCCWRP, 2013) states the following based on review of field measurements from 61 sites in Southern California:
“Results indicate that channel enlargement is highly dependent on the ratio of post- to preurban sediment-transport capacity over cumulative duration simulations of 25 years (load ratio, a.k.a. erosion potential), which explained nearly 60% of the variance.” For the purposes of implementing mitigation measures within the MS4-permitted region of the County of San Diego: this manual defines Ep as the ratio of post-project/pre-development (natural) long-term transport capacity or work; and Sp as the ratio of post-project/pre-project (existing) longterm bed sediment supply. Guidance for calculating Ep and Sp are provided in Appendix H.8.

H.4.2 Management Standard
This guidance defines a sediment supply management standard through which no net impact to receiving water can be quantitatively indicated. This management standard is demonstrated through the Net Impact Index (NII), a dimensionless index that must be used by the applicant to evaluate if there is, or is not, a net impact to the receiving water. NII is defined in this manual as the ratio of Ep to Sp. Mitigation measures shall be designed to meet the NII management standard shown in Equation H.4-2 to achieve no net impact to the receiving water. The NII management standard is based on Lane’s relationship (Ep is directly proportional to Sp) and an allowance of 10% (based on Appendix H.4.2.1). This represents the most appropriate current understanding of how to quantitatively account for sediment supply changes without replacing bed sediment sources (Palhegyi and Rathfelder, 2007 and Parra, 2015).
2026-08-19 Item #12D Page 529 of 816
Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-10 2023
Equation H.4-2: Net Impact Index
i(( =
1j
Xj ≤ 1.1
Where:

NII = Net Impact Index
Ep = Erosion Potential
Sp = Sediment Supply Potential
If NII ≤ 1.1, then the project produces no net impact to the receiving water in terms of coarse sediment yield, and no further analysis is required. If NII > 1.1, then the project generates an impact on the receiving water and the project is required to implement mitigation measures defined in Appendix H.4.3 such that the NII is reduced to a compliant value (NII ≤ 1.1).

H.4.2.1 Allowance to the NII Management Standard
This manual establishes the NII defined in Appendix H.4.2 as the management standard for coarse sediment supply. The 10% allowance to the management standard is supported by the following research studies or projects:
• The authors of the USACE report for channel design (USACE, 2001) state that, “achieving an optimum Capacity-Supply Ratio, within 10 percent of unity, should ensure dynamic stability while allowing the river itself to recover some of the fluvial detail that cannot be engineered.” • The authors of SCCWRP Technical Report 605 (SCCWRP, 2010), “anticipate that changes of less than 10% in either driver [discharge or sediment flux] are unlikely to instigate, on their own, significant channel changes. This value is a conservative estimate of the year-to-year variability in either discharge or sediment flux that can be accommodated by a channel system in a state of dynamic equilibrium.”
• Sediment transport and supply measurements and calculations are inherently inexact.

Discrepancies of up to 10% should not be a source of concern (PCR et al., 2002).

H.4.3 Types of Mitigation Measures
The following text discusses mitigation measures that may be used by the applicant to meet the NII management standard defined in Appendix H.4.2. These include:
• Additional Flow Control;
• Stream Rehabilitation; and
• Applicant Proposed Mitigation Measures
Appendix H.9 provides additional guidance for implementation of these mitigation measures. 2026-08-19 Item #12D Page 530 of 816
Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-11 2023
H.4.3.1 Additional Flow Control
One option for managing bed sediment supply reductions is to provide additional detention and retention of site runoff to compensate for the reduction of bed sediment supply. This measure requires increasing flow attenuation by adding storage volume in structural BMPs. This management option accounts for changes in hydrology, channel geometry, and bed/bank material, but not sediment supply. For example, if there is a 30% reduction in bed-load due to proposed
urbanization, then the sediment supply potential (Sp) equals

0.7. Assuming the appropriate range is +10%,
hydromodification controls can be sized and situated such that the post-project effective in-stream work is lowered to less than 77% of the baseline pre-development condition.

Structural BMPs designed for hydromodification control
utilize the following two basic principles:
• Detain runoff and release it in a controlled way that either mimics pre-development in-stream sediment
transport capacity, mimics flow durations, or reduces
flow durations to account for a reduction in bed
sediment supply.
• Manage excess runoff volumes through one or more of the following pathways: (1) infiltration; (2) evapotranspiration; (3) storage and use; (4) discharge at a rate below the critical low flowrate; or (5) discharge downstream to a receiving water that is not susceptible to hydromodification impacts.

If desired, structural BMPs can be designed to support flood control and LID objectives in addition to hydromodification control. To the maximum extent possible, structural BMPs should be designed to receive flows from developed areas only. This facilitates design optimization as well as avoiding intercepting coarse sediments from open spaces that should ideally be passed through to the stream channel.

A fact sheet for additional flow control is provided in Appendix H.9.1.

H.4.3.2 Stream Rehabilitation
Hydromodification control can be achieved by stream rehabilitation projects including: drop structures, grade control structures, bed and bank reinforcement, increased channel sinuosity or meandering, increased channel width, and flow diversion. The objective of these in-stream controls, or stream restoration measures, is to reduce or maintain the overall Erosion Potential (Ep) of the receiving channel by modifying its hydraulic properties and/or bed/bank material resistance without fully replacing sediment supply or controlling increases in runoff. Stream rehabilitation is only an 2026-08-19 Item #12D Page 531 of 816
Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-12 2023
option where the receiving channel of concern is already impacted by erosive flows and shows evidence of excessive sediment, erosion, deposition, or is a hardened channel.

Stream rehabilitation projects are subject to the permitting requirements of the resource agencies.

Stream rehabilitation projects may require the following permits:
• California Department of Fish and Wildlife – 1602 Streambed Alteration Agreement. • US Fish and Wildlife Service – Authorization under the Endangered Species Act. • US Army Corps of Engineers – Clean Water Act Section 404 Permit. • Regional Water Quality Control Board – Clean Water Act Section 401 Water Quality Certification.
• Local Grading Permit
A fact sheet for stream rehabilitation is provided in Appendix H.9.2.

H.4.3.3 Applicant Proposed Mitigation Measures
The applicant may propose a mitigation measure not identified in this manual if it will achieve no net impact to the receiving water. Additional analysis may be requested by the City prior to approval of the mitigation measure to substantiate the finding of no net impact to the receiving water.

H.5 References
American Society of Civil Engineers and Water Environment Federation. 1992. Design and Construction of Urban Stormwater Management Systems. ASCE Manual No. 77, WEF Manual No. 20, Alexandria, VA.

ASCE, 1970. Design and construction of sanitary and storm sewers, ASCE Manual of Engineering Practice No. 37, WPCF Manual of Practice No. 9. Prepared by the Joint Committee of the American Society of Civil Engineers and the Water Pollution Control Federation Chow, V.T. 1959. Open-Channel Hydraulics. McGraw-Hill Book Company.

Fischenich, C. 2001. “Stability Thresholds for Stream Restoration Materials.” EMRRP Technical Notes Collection (ERDC TN-EMRRP-SR-29), U.S. Army Engineer Research and Development Center, Vicksburg, MS.

Hawley, R.J., and Bledsoe, B.P. 2011. “How do flow peaks and durations change in suburbanizing semJ-arid watersheds? A southern California Study,” Journal of Hydrology, Elsevier, Vol 405, pp 69-82.

Knighton D., 1998. Fluvial Forms and Processes: A New Perspective. Arnold Publishers.

Lane, E.W. 1955. The Importance of Fluvial Morphology in Hydraulic Engineering. American Society of Civil Engineers. Proceedings, Vol. 81, Paper No. 745. 2026-08-19 Item #12D Page 532 of 816
Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-13 2023
Los Angeles Regional Water Quality Control Board (LARWQCB). 2012. Los Angeles County MS4 Permit (Order No. R4-2012-0175).

National Resource Conservation Service (NRCS), 2007. National Engineering Handbook Part 654 - Stream Restoration Design.

Palhegyi, G.E. and Rathfelder, K. 2007. “Applying the Erosion Potential Methodology to Natural Channel Design Procedures in Southern California”. Presented at CASQA, Sept 2007, Orange County, California.

Parra, L.A., 2015. Addressing Critical Coarse Sediment Impact by Defining Dimensionless Indexes of Coarse Sediment Production, Transport and Net Impact.

Parra, L.A., 2016. Link between Flow Duration Curve and Erosion Work – Expanded.

PCR Services Corporation, Philip Williams and Associates, and Balance Hydrologics, Inc. 2002.

Baseline Geomorphic and Hydrologic Conditions Report, Ranch Mission Viejo: Portions of the San Juan and Western San Mateo Watersheds, prepared for Rancho Mission Viejo.

Renard, K.G., G.R. Foster, G.A. Weesies, D.K. McCool and D.C. Yoder, 1997. Predicting Soil Erosion by Water. A guide to conservation planning with Revised Universal Soil Loss Equation (RUSLE). U.S. Department of Agriculture, Agriculture Handbook No. 703.

San Diego, County of. 2011. Final Hydromodification Management Plan.

Santa Clara Valley Urban Runoff Pollution Prevention Program (SCVURPPP). 2005.

Hydromodification Management Plan. April 21, 2005.

Shields, F.D. 1996. Hydraulic and Hydrologic Stability In: A Brooks and FD Shields (Eds.), River Channel Restoration, 23-74.

Shields, F.D., A. Brooks and J. Haltiner. 1999. Geomorphological Approaches to Incised Stream Channel Restoration in the United States and Europe. In: SE Darby and A Simon (Eds.) Incised River Channels.

Southern California Coastal Water Research Project (SCCWRP), 2008. Stream Channel Classification and Mapping Systems: Implications for Assessing Susceptibility to Hydromodification Effects in Southern California, Technical Report 562.

Southern California Coastal Water Research Project (SCCWRP), 2005. Effect of Increases in Peak Flows and Impervious on the Morphology of Southern California Streams, Technical Report 450.

Southern California Coastal Water Research Project (SCCWRP), 2010. Hydromodification Screening Tools: GIS-based catchment analyses of potential changes in runoff and sediment discharge.

Technical Report 605.

Southern California Coastal Water Research Project (SCCWRP), 2010a. Hydromodification Screening Tools: Field Manual for Assessing Channel Susceptibility. Technical Report 606. 2026-08-19 Item #12D Page 533 of 816
Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-14 2023
Southern California Coastal Water Research Project (SCCWRP), 2012. Hydromodification Assessment and Management in California. Technical Report 667.

Southern California Coastal Water Research Project (SCCWRP), 2013. Modeling and Managing Hydromodification Effects: summary of Available Tools and Decision-Making Approach.

Technical Report 753.

USACE, 2001. Channel Restoration Design for Meandering Rivers. US Army Corps of Engineers, Final Report, ERDC/CHL CR-01-1. Prepared by Soar, P.J., and Thorne, C.R.

USDA, 1974. Present Sediment Yield Rates for Western United States. U. S. Department of Agriculture Soil Conservation Service.

Wilcock, P., J. Pitlick, and Y. Cui. 2009. Sediment Transport Primer Estimating Bed-Material Transport in Gravel-bed Rivers. USDA General Technical Report, RMRS-GTR-226.

H.5.1 Terms of Reference
The guidance described in Appendix H of this manual was developed by Geosyntec Consultants (Geosyntec) on behalf of the County of San Diego and the City of San Diego. Appendix H was specifically developed to provide PDP applicants guidance to meet the MS4 Permit Provision E.3.c.(2)(b) within the MS4-permitted region within the San Diego County. This guidance is not intended to be used for purposes, other than to meet this MS4 Permit requirement.

The guidance was developed with input from a Technical Advisory Committee (TAC) members through a series of meetings conducted in January 2016. The TAC input resulted in a streamlined guidance enhanced to provide applicants with simplified methods to determine impacts to coarse sediment delivery based on complex scientific principles. TAC participants included:

GLUs characterize the magnitude of sediment production from areas through three factors judged to exert the greatest influence on the variability on sediment-production rates: geology types, hillslope gradient, and land cover. The Regional WMAA document and the GIS layers for the map can be found on the Project Clean Water website at the following address:
http://www.projectcleanwater.org/index.php?option=com_content&view=article&id=248&Itemid =219
The regional-level mapping is based on the following sources:

Dataset Source Year Description
Elevation USGS 2013 1/3rd Arc Second (~10 meter cells) digital elevation model for San Diego County
Land Cover SanGIS 2013 Ecology-Vegetation layer for San Diego County downloaded from SanGIS
Geology
Kennedy, M.P.,
and Tan, S.S. 2002
Geologic Map of the Oceanside 30’x60’ Quadrangle,
California, California Geological Survey, Regional
Geologic Map No. 2, 1:100,000 scale.

Kennedy, M.P.,
and Tan, S.S. 2008
Geologic Map of the San Diego 30’x60’ Quadrangle,
California, California Geological Survey, Regional
Geologic Map No. 3, 1:100,000 scale.

Todd, V.R. 2004
Preliminary Geologic Map of the El Cajon 30’x60’
Quadrangle, Southern California, United States
Geological Survey, Southern California Areal
Mapping Project, Open File Report 2004-1361,
1:100,000 scale.

Jennings et al. 2010
“Geologic Map of California,” California Geological
Survey, Map No. 2 – Geologic Map of California,
1:750,000 scale
The regional data set is a function of the inherent data resolution of the macro-level data sets and may not conform to all site conditions, or does not reflect changes to particular areas that have occurred since the underlying data was developed. This means slopes, geology, or land cover at the project site can be mischaracterized in the regional data set. If an applicant feels the Regional WMAA analysis inaccurately mapped their project area, they may elect to perform a site-specific GLU analysis based on data collected from project-level investigations to refine the mapping as outlined below.

The following PCCSYAs may be removed from the mapping without performing the full GLU analysis described in Appendix H.6.1 a) areas under 10% slope, b) paved areas. 2026-08-19 Item #12D Page 535 of 816
Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-16 2023
The City of Encinitas has made the regional data set for Potential Critical Coarse Sediment Yield Areas available on the myEncinitas mapping application. A project applicant should first check the myEncinitas app to determine if potential critical coarse sediment yield areas may exist within the project drainage boundaries (i.e., within or draining through the project). To do this, use the following procedure:

1. Open the City website: www.cityofencinitas.org

2. Type in the address of the project and click “View Property Info”

3. Once in the mapping application, click on the arrow at the left, then click on “Legend”

4. Open the layer group called “Site Profiler Service” by clicking on the adjacent plus sign

5. Open the layer group called “Stormwater” by clicking on the adjacent plus sign

6. Turn on the “Potential Critical Coarse Sediment Yield Areas” layer by clicking in the box adjacent to the layer so that a check mark appears in the box. Also turn on the Stormwater and the Site Profiler Service layer groups in the same away

7. The Potential Critical Coarse Sediment Yield Areas layer will appear as the orange areas on the map.

Alternatively, a project applicant may use the map in Figure H-1 to view the location of Potential Critical Coarse Sediment Yield Areas throughout Encinitas.

Generally, if the map does not indicate potential critical coarse sediment yield areas may exist within the project drainage boundaries, no further analysis is necessary. However, the City Engineer has the discretion to require additional project-level investigation even when the WMAA map does not indicate the presence of potential critical coarse sediment yield areas within the project site.

If the project is shown to impact potential critical coarse sediment yield areas based on the WMAA map, or if the City Engineer requires, project-level GLU analysis can be performed (see Section 6.2.1).

Project-level GLU analysis will either confirm or invalidate the finding of the Regional WMAA maps.

For project-level GLU analysis, the civil engineer shall determine slopes, geology, and land cover categories existing at the project site, and intersect this data to determine GLUs existing at the project site. The data provided in H.1 will assist the civil engineer to characterize the site.

When it has been determined based on the GLU analysis that potential critical coarse sediment yield areas are present within the project boundary, and it has been determined that downstream systems require protection (see Section 6.2.2), additional analysis may be performed that may refine the extents of actual critical coarse sediment yield areas to be protected onsite (see Section 6.2.3). Procedures for additional analysis are provided in the following sections. 2026-08-19 Item #12D Page 536 of 816
Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-17 2023
Figure H-1: Potential Critical Coarse Sediment Yield Areas within the City of Encinitas 2026-08-19 Item #12D Page 537 of 816
Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-18 2023
H.6.1 Site-Specific GLU Analysis
In order to perform a site-specific GLU analysis the applicant must first delineate the project boundary and any areas draining through the project boundary. The applicant must then determine appropriate slopes, geology, and land cover categories for this area as identified below.

There are four slope categories in the GLU analysis. Category numbers shown (1 to 4) were assigned for the purpose of GIS processing.
• 0% to 10% (1)
• 10% to 20% (2)
• 20% to 40% (3)
• >40% (4)
There are seven geology categories in the GLU analysis:
• Coarse bedrock (CB)
• Coarse sedimentary impermeable (CSI)
• Coarse sedimentary permeable (CSP)
• Fine bedrock (FB)
• Fine sedimentary impermeable (FSI)
• Fine sedimentary permeable (FSP)
• Other (O)
There are six land cover categories in the GLU analysis:
• Agriculture/grass
• Forest
• Developed
• Scrub/shrub
• Other
• Unknown
Project site slopes shall be classified into the categories based on project-level topography. Project site geology may be determined from geologic maps (may be the same as regional-level information) or classified in the field by a qualified geologist. Table H.6-1 provides information to classify geologic map units into each geology category. Project site land cover shall be determined from aerial 2026-08-19 Item #12D Page 538 of 816
Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-19 2023
photography and/or field visit. For reference, Table H.6-2 provides information to classify land cover categories from the SanGIS Ecology-Vegetation data set into land cover categories. The civil engineer shall not rely on the SanGIS Ecology-Vegetation data set to identify actual land cover at the project site (for project-level investigation land cover must be confirmed by aerial photo or field visit).

Intersect the geologic categories, land cover categories, and slope categories within the project boundary to create GLUs. The GLUs listed in Table H.6-3 are considered to be PCCSYAs. Note the GLU nomenclature is presented in the following format: Geology – Land Cover – Slope Category (e.g., "CB-Agricultural/Grass-3" for a GLU consisting of coarse bedrock geology, agricultural/grass land cover, and 20% to 40% slope).

GLUs are created by intersecting the geologic categories, land cover categories, and slope categories.

This is a similar procedure to intersecting land uses with soil types to determine runoff coefficients or runoff curve numbers for hydrologic studies, but there are three categories to consider for the GLU analysis (slope, geology, and land cover), and the GLUs are not to be composited into a single GLU.

When GLUs have been created, determine whether any of the GLUs listed in Table H.6-3 are found within the project boundary. The GLUs listed in Table H.6-3 are considered to be PCCSYAs.

If none of the GLUs listed in Table H.6-3 are present within the project boundary and area draining through the project boundary, no measures for protection of critical coarse sediment yield areas are necessary. If one or more GLUs listed in Table H.6-3 are present within the project boundary, they shall be considered critical coarse sediment yield areas. Complete Worksheet H.6-1 to document verification of GLUs.

Table H.6-1: Geologic Grouping for Different Map Units
Map
Unit Map Name
Anticipated Grain
size of Weathered
Material
Bedrock or
Sedimentary
Impermeable
/ Permeable
Geology
Grouping
gr-m Jennings; CA Coarse Bedrock Impermeable CB
grMz Jennings; CA Coarse Bedrock Impermeable CB
Jcr El Cajon 30' x 60' Coarse Bedrock Impermeable CB
Jhc El Cajon 30' x 60' Coarse Bedrock Impermeable CB
Jsp El Cajon 30' x 60' Coarse Bedrock Impermeable CB
Ka El Cajon 30' x 60' Coarse Bedrock Impermeable CB
Kbm Oceanside 30' x
60' Coarse Bedrock Impermeable CB
Kbp Oceanside 30' x
60' Coarse Bedrock Impermeable CB
Kcc Oceanside 30' x
60' Coarse Bedrock Impermeable CB
Kcg Oceanside 30' x
60' Coarse Bedrock Impermeable CB
Kcm El Cajon 30' x 60' Coarse Bedrock Impermeable CB
Kcp El Cajon 30' x 60' Coarse Bedrock Impermeable CB
2026-08-19 Item #12D Page 539 of 816
Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-20 2023
Map
Unit Map Name
Anticipated Grain
size of Weathered
Material
Bedrock or
Sedimentary
Impermeable
/ Permeable
Geology
Grouping
Kd
San Diego &
Oceanside 30' x
60'
Coarse Bedrock Impermeable CB
Kdl Oceanside 30' x
60' Coarse Bedrock Impermeable CB
Kg Oceanside 30' x
60' Coarse Bedrock Impermeable CB
Kgbf Oceanside 30' x
60' Coarse Bedrock Impermeable CB
Kgd
San Diego &
Oceanside 30' x
60'
Coarse Bedrock Impermeable CB
Kgdf Oceanside 30' x
60' Coarse Bedrock Impermeable CB
Kgh San Diego 30' x
60' Coarse Bedrock Impermeable CB
Kgm El Cajon 30' x 60' Coarse Bedrock Impermeable CB
Kgm1 El Cajon 30' x 60' Coarse Bedrock Impermeable CB
Kgm2 El Cajon 30' x 60' Coarse Bedrock Impermeable CB
Kgm3 El Cajon 30' x 60' Coarse Bedrock Impermeable CB
Kgm4 El Cajon 30' x 60' Coarse Bedrock Impermeable CB
Kgp Oceanside 30' x
60' Coarse Bedrock Impermeable CB
Kgr El Cajon 30' x 60' Coarse Bedrock Impermeable CB
Kgu San Diego 30' x
60' Coarse Bedrock Impermeable CB
Khg Oceanside 30' x
60' Coarse Bedrock Impermeable CB
Ki Oceanside 30' x
60' Coarse Bedrock Impermeable CB
Kis Oceanside 30' x
60' Coarse Bedrock Impermeable CB
Kjd Oceanside 30' x
60' Coarse Bedrock Impermeable CB
KJem El Cajon 30' x 60' Coarse Bedrock Impermeable CB
KJld El Cajon 30' x 60' Coarse Bedrock Impermeable CB
Kjv El Cajon 30' x 60' Coarse Bedrock Impermeable CB
Klb El Cajon 30' x 60' Coarse Bedrock Impermeable CB
Klh Oceanside 30' x
60' Coarse Bedrock Impermeable CB
Klp El Cajon 30' x 60' Coarse Bedrock Impermeable CB
2026-08-19 Item #12D Page 540 of 816
Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-21 2023
Map
Unit Map Name
Anticipated Grain
size of Weathered
Material
Bedrock or
Sedimentary
Impermeable
/ Permeable
Geology
Grouping
Km Oceanside 30' x
60' Coarse Bedrock Impermeable CB
Kmg Oceanside 30' x
60' Coarse Bedrock Impermeable CB
Kmgp El Cajon 30' x 60' Coarse Bedrock Impermeable CB
Kmm Oceanside 30' x
60' Coarse Bedrock Impermeable CB
Kpa Oceanside 30' x
60' Coarse Bedrock Impermeable CB
Kpv El Cajon 30' x 60' Coarse Bedrock Impermeable CB
Kqbd Oceanside 30' x
60' Coarse Bedrock Impermeable CB
Kr Oceanside 30' x
60' Coarse Bedrock Impermeable CB
Krm Oceanside 30' x
60' Coarse Bedrock Impermeable CB
Krr Oceanside 30' x
60' Coarse Bedrock Impermeable CB
Kt
San Diego &
Oceanside 30' x
60'
Coarse Bedrock Impermeable CB
Ktr Oceanside 30' x
60' Coarse Bedrock Impermeable CB
Kvc Oceanside 30' x
60' Coarse Bedrock Impermeable CB
Kwm Oceanside 30' x
60' Coarse Bedrock Impermeable CB
Kwp Oceanside 30' x
60' Coarse Bedrock Impermeable CB
Kwsr Oceanside 30' x
60' Coarse Bedrock Impermeable CB
m Jennings; CA Coarse Bedrock Impermeable CB
Mzd Oceanside 30' x
60' Coarse Bedrock Impermeable CB
Mzg Oceanside 30' x
60' Coarse Bedrock Impermeable CB
Mzq Oceanside 30' x
60' Coarse Bedrock Impermeable CB
Mzs Oceanside 30' x
60' Coarse Bedrock Impermeable CB
sch Jennings; CA Coarse Bedrock Impermeable CB
2026-08-19 Item #12D Page 541 of 816
Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-22 2023
Map
Unit Map Name
Anticipated Grain
size of Weathered
Material
Bedrock or
Sedimentary
Impermeable
/ Permeable
Geology
Grouping
Kp
San Diego &
Oceanside 30' x
60'
Coarse Bedrock Impermeable CB
Ql El Cajon 30' x 60' Coarse Sedimentary Impermeable CSI QTf El Cajon 30' x 60' Coarse Sedimentary Impermeable CSI Ec Jennings; CA Coarse Sedimentary Impermeable CSI
K Jennings; CA Coarse Sedimentary Impermeable CSI
Kccg San Diego 30' x
60' Coarse Sedimentary Impermeable CSI
Kcs San Diego 30' x
60' Coarse Sedimentary Impermeable CSI
Kl
San Diego,
Oceanside & El
Cajon 30' x 60'
Coarse Sedimentary Impermeable CSI
Ku Jennings; CA Coarse Sedimentary Impermeable CSI
Qvof Oceanside 30' x
60' Coarse Sedimentary Impermeable CSI
Qvop8a San Diego 30' x
60' Coarse Sedimentary Impermeable CSI
Qvop9a San Diego 30' x
60' Coarse Sedimentary Impermeable CSI
Tmsc San Diego 30' x
60' Coarse Sedimentary Impermeable CSI
Tmss San Diego 30' x
60' Coarse Sedimentary Impermeable CSI
Tp San Diego & El
Cajon 30' x 60' Coarse Sedimentary Impermeable CSI
Tpm San Diego 30' x
60' Coarse Sedimentary Impermeable CSI
Tsc San Diego 30' x
60' Coarse Sedimentary Impermeable CSI
Tscu San Diego 30' x
60' Coarse Sedimentary Impermeable CSI
Tsd San Diego & El
Cajon 30' x 60' Coarse Sedimentary Impermeable CSI
Tsdcg San Diego 30' x
60' Coarse Sedimentary Impermeable CSI
Tsdss San Diego 30' x
60' Coarse Sedimentary Impermeable CSI
Tsm Oceanside 30' x
60' Coarse Sedimentary Impermeable CSI
2026-08-19 Item #12D Page 542 of 816
Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-23 2023
Map
Unit Map Name
Anticipated Grain
size of Weathered
Material
Bedrock or
Sedimentary
Impermeable
/ Permeable
Geology
Grouping
Tso Oceanside 30' x
60' Coarse Sedimentary Impermeable CSI
Tst
San Diego,
Oceanside & El
Cajon 30' x 60'
Coarse Sedimentary Impermeable CSI
Tt
San Diego &
Oceanside 30' x
60'
Coarse Sedimentary Impermeable CSI
Tta Oceanside 30' x
60' Coarse Sedimentary Impermeable CSI
Tmv
San Diego,
Oceanside & El
Cajon 30' x 60'
Coarse Sedimentary Impermeable CSI
Tsi Oceanside 30' x
60' Coarse Sedimentary Impermeable CSI
Qvoa
San Diego &
Oceanside 30' x
60'
Coarse Sedimentary Impermeable CSI
Qvoa11 Oceanside 30' x
60' Coarse Sedimentary Impermeable CSI
Qvoa12 Oceanside 30' x
60' Coarse Sedimentary Impermeable CSI
Qvoa13 Oceanside 30' x
60' Coarse Sedimentary Impermeable CSI
Qvoc Oceanside 30' x
60' Coarse Sedimentary Impermeable CSI
Qvop
San Diego &
Oceanside 30' x
60'
Coarse Sedimentary Impermeable CSI
Qvop1
San Diego &
Oceanside 30' x
60'
Coarse Sedimentary Impermeable CSI
Qvop10
San Diego &
Oceanside 30' x
60'
Coarse Sedimentary Impermeable CSI
Qvop10
a
San Diego 30' x
60' Coarse Sedimentary Impermeable CSI
Qvop11
San Diego &
Oceanside 30' x
60'
Coarse Sedimentary Impermeable CSI
2026-08-19 Item #12D Page 543 of 816
Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-24 2023
Map
Unit Map Name
Anticipated Grain
size of Weathered
Material
Bedrock or
Sedimentary
Impermeable
/ Permeable
Geology
Grouping
Qvop11
a
San Diego 30' x
60' Coarse Sedimentary Impermeable CSI
Qvop12
San Diego &
Oceanside 30' x
60'
Coarse Sedimentary Impermeable CSI
Qvop13
San Diego &
Oceanside 30' x
60'
Coarse Sedimentary Impermeable CSI
Qvop2
San Diego &
Oceanside 30' x
60'
Coarse Sedimentary Impermeable CSI
Qvop3
San Diego &
Oceanside 30' x
60'
Coarse Sedimentary Impermeable CSI
Qvop4
San Diego &
Oceanside 30' x
60'
Coarse Sedimentary Impermeable CSI
Qvop5
San Diego &
Oceanside 30' x
60'
Coarse Sedimentary Impermeable CSI
Qvop6 San Diego 30' x
60' Coarse Sedimentary Impermeable CSI
Qvop7
San Diego &
Oceanside 30' x
60'
Coarse Sedimentary Impermeable CSI
Qvop8
San Diego &
Oceanside 30' x
60'
Coarse Sedimentary Impermeable CSI
Qvop9 San Diego 30' x
60' Coarse Sedimentary Impermeable CSI
Tsa Oceanside 30' x
60' Coarse Sedimentary Impermeable CSI
Qof Oceanside 30' x
60' Coarse Sedimentary Permeable CSP
Qof1 Oceanside 30' x
60' Coarse Sedimentary Permeable CSP
Qof2 Oceanside 30' x
60' Coarse Sedimentary Permeable CSP
Q Jennings; CA Coarse Sedimentary Permeable CSP
Qa Oceanside 30' x
60' Coarse Sedimentary Permeable CSP
2026-08-19 Item #12D Page 544 of 816
Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-25 2023
Map
Unit Map Name
Anticipated Grain
size of Weathered
Material
Bedrock or
Sedimentary
Impermeable
/ Permeable
Geology
Grouping
Qd Oceanside 30' x
60' Coarse Sedimentary Permeable CSP
Qf Oceanside 30' x
60' Coarse Sedimentary Permeable CSP
Qmb
San Diego &
Oceanside 30' x
60'
Coarse Sedimentary Permeable CSP
Qop
San Diego &
Oceanside 30' x
60'
Coarse Sedimentary Permeable CSP
Qw
San Diego &
Oceanside 30' x
60'
Coarse Sedimentary Permeable CSP
Qyf Oceanside 30' x
60' Coarse Sedimentary Permeable CSP
Qt El Cajon 30' x 60' Coarse Sedimentary Permeable CSP
Qoa1-2 Oceanside 30' x
60' Coarse Sedimentary Permeable CSP
Qoa2-6 Oceanside 30' x
60' Coarse Sedimentary Permeable CSP
Qoa5 Oceanside 30' x
60' Coarse Sedimentary Permeable CSP
Qoa6 Oceanside 30' x
60' Coarse Sedimentary Permeable CSP
Qoa7 Oceanside 30' x
60' Coarse Sedimentary Permeable CSP
Qoc Oceanside 30' x
60' Coarse Sedimentary Permeable CSP
Qop1 Oceanside 30' x
60' Coarse Sedimentary Permeable CSP
Qc El Cajon 30' x 60' Coarse Sedimentary Permeable CSP
Qu El Cajon 30' x 60' Coarse Sedimentary Permeable CSP
Qoa
San Diego,
Oceanside & El
Cajon 30' x 60'
Coarse Sedimentary Permeable CSP
Qop2-4 San Diego 30' x
60' Coarse Sedimentary Permeable CSP
Qop3 Oceanside 30' x
60' Coarse Sedimentary Permeable CSP
Qop4 Oceanside 30' x
60' Coarse Sedimentary Permeable CSP
2026-08-19 Item #12D Page 545 of 816
Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-26 2023
Map
Unit Map Name
Anticipated Grain
size of Weathered
Material
Bedrock or
Sedimentary
Impermeable
/ Permeable
Geology
Grouping
Qop6
San Diego &
Oceanside 30' x
60'
Coarse Sedimentary Permeable CSP
Qop7
San Diego &
Oceanside 30' x
60'
Coarse Sedimentary Permeable CSP
Qya
San Diego,
Oceanside & El
Cajon 30' x 60'
Coarse Sedimentary Permeable CSP
Qyc
San Diego &
Oceanside 30' x
60'
Coarse Sedimentary Permeable CSP
Mzu
San Diego &
Oceanside 30' x
60'
Fine Bedrock Impermeable FB
gb Jennings; CA Fine Bedrock Impermeable FB
JTRm El Cajon 30' x 60' Fine Bedrock Impermeable FB
Kat Oceanside 30' x
60' Fine Bedrock Impermeable FB
Kc El Cajon 30' x 60' Fine Bedrock Impermeable FB
Kgb Oceanside 30' x
60' Fine Bedrock Impermeable FB
KJvs El Cajon 30' x 60' Fine Bedrock Impermeable FB
Kmv El Cajon 30' x 60' Fine Bedrock Impermeable FB
Ksp El Cajon 30' x 60' Fine Bedrock Impermeable FB
Kvsp Oceanside 30' x
60' Fine Bedrock Impermeable FB
Kwmt Oceanside 30' x
60' Fine Bedrock Impermeable FB
Qv Jennings; CA Fine Bedrock Impermeable FB
Tba San Diego 30' x
60' Fine Bedrock Impermeable FB
Tda Oceanside 30' x
60' Fine Bedrock Impermeable FB
Tv Oceanside 30' x
60' Fine Bedrock Impermeable FB
Tvsr Oceanside 30' x
60' Fine Bedrock Impermeable FB
Kgdfg Oceanside 30' x
60' Fine Bedrock Impermeable FB
2026-08-19 Item #12D Page 546 of 816
Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-27 2023
Map
Unit Map Name
Anticipated Grain
size of Weathered
Material
Bedrock or
Sedimentary
Impermeable
/ Permeable
Geology
Grouping
Ta San Diego 30' x
60' Fine Sedimentary Impermeable FSI
Tcs Oceanside 30' x
60' Fine Sedimentary Impermeable FSI
Td
San Diego &
Oceanside 30' x
60'
Fine Sedimentary Impermeable FSI
Td+Tf San Diego 30' x
60' Fine Sedimentary Impermeable FSI
Qls
San Diego,
Oceanside & El
Cajon 30' x 60'
Fine Sedimentary Impermeable FSI
Tm Oceanside 30' x
60' Fine Sedimentary Impermeable FSI
Tf
San Diego,
Oceanside & El
Cajon 30' x 60'
Fine Sedimentary Impermeable FSI
Tfr El Cajon 30' x 60' Fine Sedimentary Impermeable FSI To San Diego & El
Cajon 30' x 60' Fine Sedimentary Impermeable FSI
Qpe
San Diego &
Oceanside 30' x
60'
Fine Sedimentary Permeable FSP
Mexico San Diego 30' x
60' NA NA Permeable Other
Kuo San Diego 30' x
60' NA (Offshore) NA Permeable Other
Teo
San Diego &
Oceanside 30' x
60'
NA (Offshore) Sedimentary Permeable Other
Tmo Oceanside 30' x
60' NA (Offshore) Sedimentary Permeable Other
Qmo San Diego 30' x
60' NA (Offshore) Sedimentary Permeable Other
QTso San Diego 30' x
60' NA (Offshore) Sedimentary Permeable Other
af
San Diego &
Oceanside 30' x
60'
Variable,
dependent on
source material
Sedimentary Other
2026-08-19 Item #12D Page 547 of 816
Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-28 2023
Table H.6-2: Land Cover Grouping for SanGIS Ecology-Vegetation Data Set Id SanGIS Legend SanGIS Grouping Land Cover
Grouping
1 42000 Valley and Foothill Grassland Grasslands, Vernal Pools, Meadows, and Other
Herb Communities
Agricultural/Grass
2 42100 Native Grassland Agricultural/Grass
3 42110 Valley Needlegrass Grassland Agricultural/Grass 4 42120 Valley Sacaton Grassland Agricultural/Grass
5 42200 Non-Native Grassland
Grasslands, Vernal Pools,
Meadows, and Other
Herb Communities
Agricultural/Grass
6 42300 Wildflower Field Agriculture/Grass
7 42400 Foothill/Mountain Perennial
Grassland Agriculture/Grass
8 42470 Transmontane Dropseed
Grassland Agriculture/Grass
9 45000 Meadow and Seep Agriculture/Grass
10 45100 Montane Meadow Agriculture/Grass
11 45110 Wet Montane Meadow Agriculture/Grass
12 45120 Dry Montane Meadows Agriculture/Grass
13 45300 Alkali Meadows and Seeps Agriculture/Grass
14 45320 Alkali Seep Agriculture/Grass
15 45400 Freshwater Seep Agriculture/Grass
16 46000 Alkali Playa Community Agriculture/Grass
17 46100 Badlands/Mudhill Forbs Agriculture/Grass
18 Non-Native Grassland Agriculture/Grass
19 18000 General Agriculture
Non-Native Vegetation,
Developed Areas, or
Unvegetated Habitat
Agriculture/Grass
20 18100 Orchards and Vineyards Agriculture/Grass
21 18200 Intensive Agriculture Agriculture/Grass
22 18200 Intensive Agriculture - Dairies,
Nurseries, Chicken Ranches Agriculture/Grass
23 18300 Extensive Agriculture -
Field/Pasture, Row Crops Agriculture/Grass
24 18310 Field/Pasture Agriculture/Grass
25 18310 Pasture Agriculture/Grass
26 18320 Row Crops Agriculture/Grass
27 12000 Urban/Developed Developed
28 12000 Urban/Develpped Developed
29 81100 Mixed Evergreen Forest
Forest
Forest
30 81300 Oak Forest Forest
31 81310 Coast Live Oak Forest Forest
32 81320 Canyon Live Oak Forest Forest
33 81340 Black Oak Forest Forest
34 83140 Torrey Pine Forest Forest
35 83230 Southern Interior Cypress Forest Forest
36 84000 Lower Montane Coniferous Forest Forest
2026-08-19 Item #12D Page 548 of 816
Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-29 2023
Id SanGIS Legend SanGIS Grouping Land Cover
Grouping
37 84100 Coast Range, Klamath and
Peninsular Coniferous Forest Forest
38 84140 Coulter Pine Forest Forest Forest
39 84150 Bigcone Spruce (Bigcone Douglas
Fir)-Canyon Oak Forest Forest
40 84230 Sierran Mixed Coniferous Forest Forest
41 84500 Mixed
Oak/Coniferous/Bigcone/Coulter Forest
42 85100 Jeffrey Pine Forest Forest
43 11100 Eucalyptus Woodland
Non-Native Vegetation,
Developed Areas, or
Unvegetated Habitat
Forest
44 60000 RIPARIAN AND

BOTTOMLAND HABITAT
Riparian and Bottomland
Habitat
Forest
45 61000 Riparian Forests Forest
46 61300 Southern Riparian Forest Forest
47 61310 Southern Coast Live Oak Riparian
Forest Forest
48 61320 Southern Arroyo Willow Riparian
Forest Forest
49 61330 Southern Cottonwood-willow
Riparian Forest Forest
50 61510 White Alder Riparian Forest Forest
51 61810 Sonoran Cottonwood-willow
Riparian Forest Forest
52 61820 Mesquite Bosque Forest
53 62000 Riparian Woodlands Forest
54 62200 Desert Dry Wash Woodland Forest
55 62300 Desert Fan Palm Oasis Woodland Forest
56 62400 Southern Sycamore-alder Riparian
Woodland Forest
57 70000 WOODLAND
Woodland
Forest
58 71000 Cismontane Woodland Forest
59 71100 Oak Woodland Forest
60 71120 Black Oak Woodland Forest
61 71160 Coast Live Oak Woodland Forest
62 71161 Open Coast Live Oak Woodland Forest
63 71162 Dense Coast Live Oak Woodland Forest
64 71162 Dense Coast Love Oak Woodland Forest
65 71180 Engelmann Oak Woodland Woodland Forest
66 71181 Open Engelmann Oak Woodland Forest
67 71182 Dense Engelmann Oak Woodland Forest
2026-08-19 Item #12D Page 549 of 816
Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-30 2023
Id SanGIS Legend SanGIS Grouping Land Cover
Grouping
68 72300 Peninsular Pinon and Juniper
Woodlands Forest
69 72310 Peninsular Pinon Woodland Forest
70 72320 Peninsular Juniper Woodland and
Scrub Forest
71 75100 Elephant Tree Woodland Forest
72 77000 Mixed Oak Woodland Forest
73 78000 Undifferentiated Open Woodland Forest
74 79000 Undifferentiated Dense Woodland Forest
75 Engelmann Oak Woodland Forest
76 52120 Southern Coastal Salt Marsh
Bog and Marsh
Other
77 52300 Alkali Marsh Other
78 52310 Cismontane Alkali Marsh Other
79 52400 Freshwater Marsh Other
80 52410 Coastal and Valley Freshwater
Marsh Other
81 52420 Transmontane Freshwater Marsh Other
82 52440 Emergent Wetland Other
83 44000 Vernal Pool Grasslands, Vernal Pools,
Meadows, and Other
Herb Communities
Other
84 44320 San Diego Mesa Vernal Pool Other
85 44322 San Diego Mesa Claypan Vernal
Pool (southern mesas) Other
86 13100 Open Water
Non-Native Vegetation,
Developed Areas, or
Unvegetated Habitat
Other
87 13110 Marine Other
88 13111 Subtidal Other
89 13112 Intertidal Other
90 13121 Deep Bay Other
91 13122 Intermediate Bay Other
92 13123 Shallow Bay Other
93 13130 Estuarine Other
94 13131 Subtidal Other
95 13133 Brackishwater Other
96 13140 Freshwater
Non-Native Vegetation,
Developed Areas, or
Unvegetated Habitat
Other
97 13200 Non-Vegetated Channel,
Floodway, Lakeshore Fringe Other
98 13300 Saltpan/Mudflats Other
99 13400 Beach Other
100 21230 Southern Foredunes
Dune Community
Scrub/Shrub
101 22100 Active Desert Dunes Scrub/Shrub
102 22300 Stabilized and Partially-Stabilized
Desert Sand Field Scrub/Shrub
2026-08-19 Item #12D Page 550 of 816
Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-31 2023
Id SanGIS Legend SanGIS Grouping Land Cover
Grouping
103 24000 Stabilized Alkaline Dunes Scrub/Shrub
104 29000 ACACIA SCRUB Scrub/Shrub
105 63000 Riparian Scrubs
Riparian and Bottomland
Habitat
Scrub/Shrub
106 63300 Southern Riparian Scrub Scrub/Shrub
107 63310 Mule Fat Scrub Scrub/Shrub
108 63310 Mulefat Scrub Scrub/Shrub
109 63320 Southern Willow Scrub Scrub/Shrub
110 63321 Arundo donnax
Dominant/Southern Willow Scrub Scrub/Shrub
111 63330 Southern Riparian Scrub Scrub/Shrub
112 63400 Great Valley Scrub Scrub/Shrub
113 63410 Great Valley Willow Scrub Scrub/Shrub
114 63800 Colorado Riparian Scrub Scrub/Shrub
115 63810 Tamarisk Scrub Scrub/Shrub
116 63820 Arrowweed Scrub Scrub/Shrub
117 31200 Southern Coastal Bluff Scrub
Scrub and Chaparral
Scrub/Shrub
118 32000 Coastal Scrub Scrub/Shrub
119 32400 Maritime Succulent Scrub Scrub/Shrub
120 32500 Diegan Coastal Sage Scrub Scrub/Shrub
121 32510 Coastal form Scrub/Shrub
122 32520 Inland form (> 1,000 ft. elevation) Scrub/Shrub 123 32700 Riversidian Sage Scrub Scrub/Shrub
124 32710 Riversidian Upland Sage Scrub Scrub/Shrub
125 32720 Alluvial Fan Scrub Scrub/Shrub
126 33000 Sonoran Desert Scrub Scrub/Shrub
127 33100 Sonoran Creosote Bush Scrub Scrub/Shrub
128 33200 Sonoran Desert Mixed Scrub Scrub/Shrub
129 33210 Sonoran Mixed Woody Scrub Scrub/Shrub
130 33220 Sonoran Mixed Woody and
Succulent Scrub Scrub and Chaparral Scrub/Shrub
131 33230 Sonoran Wash Scrub Scrub/Shrub
132 33300 Colorado Desert Wash Scrub Scrub/Shrub
133 33600 Encelia Scrub Scrub/Shrub
134 34000 Mojavean Desert Scrub Scrub/Shrub
135 34300 Blackbush Scrub Scrub/Shrub
136 35000 Great Basin Scrub Scrub/Shrub
137 35200 Sagebrush Scrub Scrub/Shrub
138 35210 Big Sagebrush Scrub Scrub/Shrub
139 35210 Sagebrush Scrub Scrub/Shrub
140 36110 Desert Saltbush Scrub Scrub/Shrub
141 36120 Desert Sink Scrub Scrub/Shrub
142 37000 Chaparral Scrub/Shrub
2026-08-19 Item #12D Page 551 of 816
Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-32 2023
Id SanGIS Legend SanGIS Grouping Land Cover
Grouping
143 37120 Southern Mixed Chaparral Scrub/Shrub
144 37120 Southern Mixed Chapparal Scrub/Shrub
145 37121 Granitic Southern Mixed Chaparral Scrub/Shrub 146 37121 Southern Mixed Chaparral Scrub/Shrub
147 37122 Mafic Southern Mixed Chaparral Scrub/Shrub
148 37130 Northern Mixed Chaparral Scrub/Shrub
149 37131 Granitic Northern Mixed
Chaparral Scrub/Shrub
150 37132 Mafic Northern Mixed Chaparral Scrub/Shrub
151 37200 Chamise Chaparral Scrub/Shrub
152 37210 Granitic Chamise Chaparral Scrub/Shrub
153 37220 Mafic Chamise Chaparral Scrub/Shrub
154 37300 Red Shank Chaparral Scrub/Shrub
155 37400 SemJ-Desert Chaparral Scrub/Shrub
156 37500 Montane Chaparral Scrub/Shrub
157 37510 Mixed Montane Chaparral Scrub/Shrub
158 37520 Montane Manzanita Chaparral Scrub/Shrub
159 37530 Montane Ceanothus Chaparral Scrub/Shrub
160 37540 Montane Scrub Oak Chaparral Scrub/Shrub
161 37800 Upper Sonoran Ceanothus
Chaparral Scrub/Shrub
162 37830 Ceanothus crassifolius Chaparral Scrub/Shrub
163 37900 Scrub Oak Chaparral Scrub/Shrub
164 37A00 Interior Live Oak Chaparral Scrub/Shrub
165 37C30 Southern Maritime Chaparral Scrub and Chaparral Scrub/Shrub 166 37G00 Coastal Sage-Chaparral Scrub Scrub/Shrub
167 37K00 Flat-topped Buckwheat Scrub/Shrub
168 39000 Upper Sonoran Subshrub Scrub Scrub/Shrub
169 Diegan Coastal Sage Scrub Scrub/Shrub
170 Granitic Northern Mixed Chaparral Scrub/Shrub
171 Southern Mixed Chaparral Scrub/Shrub
172 11000 Non-Native Vegetation Non-Native Vegetation,
Developed Areas, or
Unvegetated Habitat
Unknown
173 11000 Non-Native VegetionVegetation Unknown
174 11200 Disturbed Wetland Unknown
175 11300 Disturbed Habitat Unknown
176 13000 Unvegetated Habitat Unknown
177 Disturbed Habitat Unknown
Table H.6-3: Potential Critical Coarse Sediment Yield Areas GLU Geology Land Cover Slope (%)
2026-08-19 Item #12D Page 552 of 816
Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-33 2023
CB-Agricultural/Grass3 Coarse Bedrock Agricultural/Grass 20% - 40% CB-Agricultural/Grass4 Coarse Bedrock Agricultural/Grass >40% CB-Forest-2 Coarse Bedrock Forest 10 – 20%
CB-Forest-3 Coarse Bedrock Forest 20% - 40%
CB-Forest-4 Coarse Bedrock Forest >40%
CB-Scrub/Shrub-4 Coarse Bedrock Scrub/Shrub >40%
CB-Unknown-4 Coarse Bedrock Unknown >40%
CSJAgricultural/Grass-2 Coarse Sedimentary
Impermeable Agricultural/Grass 10 – 20%
CSJAgricultural/Grass-3 Coarse Sedimentary
Impermeable Agricultural/Grass 20% - 40%
CSJAgricultural/Grass-4 Coarse Sedimentary
Impermeable Agricultural/Grass >40%
CSPAgricultural/Grass-4 Coarse Sedimentary Permeable Agricultural/Grass >40% CSP-Forest-3 Coarse Sedimentary Permeable Forest 20% - 40% CSP-Forest-4 Coarse Sedimentary Permeable Forest >40%
CSP-Scrub/Shrub-4 Coarse Sedimentary Permeable Scrub/Shrub >40% 2026-08-19 Item #12D Page 553 of 816
Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-34 2023
Worksheet H.6-1: Verification of GLUs
Verification of GLUs Worksheet H.6-1
Detailed project-level review of GLUs may be performed to verify the presence or absence of potential critical coarse sediment yield areas within the project site and/or upstream areas. Use this form to document the evaluation of slope, geology, and land cover combined to determine the site-specific GLUs. Complete all sections of this form.

Project Name:

Project Tracking Number / Permit Application Number:
1 What are the pre-project slopes? ϒ 0% to 10% (1)
ϒ 10% to 20% (2)
ϒ 20% to 40% (3)
ϒ >40% (4)
2 What is the underlying geology? Refer to
Appendix H.6 to classify geologic categories
into a geology grouping.

Note: site-specific geology may be determined
in the field by a qualified geologist.

ϒ Coarse bedrock (CB)
ϒ Coarse sedimentary impermeable (CSI)
ϒ Coarse sedimentary permeable (CSP)
ϒ Fine bedrock (FB)
ϒ Fine sedimentary impermeable (FSI)
ϒ Fine sedimentary permeable (FSP)
ϒ Other (O)
3 What is the pre-project land cover? Refer to
Appendix H.6 for land cover category
definitions.

Note: Land cover shall be determined from
aerial photography and/or field visit.

ϒ Agriculture/grass
ϒ Forest
ϒ Developed
ϒ Scrub/shrub
ϒ Other
ϒ Unknown
4 List the GLU(s) within the project site and/or
upstream areas.

Note the GLU nomenclature format is as
follows: Geology – Land Cover – Slope
Category (e.g. “CB-Agricultural/Grass-3” for a
GLU consisting of coarse bedrock geology,
agricultural/grass land cover, and 20% to 40%
slope).
2026-08-19 Item #12D Page 554 of 816
Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-35 2023
Worksheet H.6-1; Page 2 of 2
5 Photo(s)
Insert photos representative of the slopes, land cover, and geology. 6 Are any of the GLUs found within the project
boundary and/or upstream areas listed in Table
H.6-3?
ϒ Yes Go to 7
ϒ No Go to 8
7 End – Provide management measures for preservation of coarse sediment supply as described in this guidance document, or the project applicant may elect to determine whether downstream systems would be sensitive to reduction of coarse sediment yield from the project site and/or perform site-specific method for mapping critical coarse sediment yield areas. 8 End – Site-specific GLUs do not warrant preservation of coarse sediment supply, no measures for protection of critical coarse sediment yield areas onsite are necessary. Optional: use the note section below to provide justification for these findings. 9 Notes
2026-08-19 Item #12D Page 555 of 816
Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-36 2023
H.6.2 Assumptions for Regional WMAA PCCSYA Maps
This appendix summarizes the assumptions used while developing Regional WMAA PCCSYA maps that are not discussed in Appendix H.6.1:
• Critical coarse sediment would be generated from GLUs that are o composed of geologic units likely to generate coarse sediment (i.e. produces greater than 50% sand (0.074 mm; no. 200 sieve) by weight when weathered); and o have a potential for high relative sediment production (GLUs that produce soil loss greater than 8.4 tons/acre/year are assigned a high relative rating, this corresponds to 42% of the total coarse soil loss from the MS4-permitted region within the County of San Diego)
• Relative sediment production was assigned using RUSLE analysis of GLUs. It was assumed that this relative rating represents sediment production from sheet erosion, rill erosion, gullies and lower order channels, since these features are mostly on the hillslopes that are represented by the GLUs.
o While performing the RUSLE analysis to assign the relative ranking, C factor from the regional maps from USEPA was adjusted to 0 for developed land covers to account for management actions implemented on developed sites (e.g. impervious surfaces). • WMAA mapping does not account for sediment production from in-stream sediment supply (since these are mostly protected through other regulations) and sediment production from mass failures like landslides which are difficult to estimate on a regional scale without performing extensive field investigations.
• Regional WMAA map assumes that all receiving waters require coarse sediment and the map also does not account for potential existing impediments that may hinder delivery of coarse sediment to receiving waters.

For additional details refer to the Regional WMAA document on the Project Clean Water website at the following address:
http://www.projectcleanwater.org/index.php?option=com_content&view=article&id=248&Itemid =219
2026-08-19 Item #12D Page 556 of 816
Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-37 2023
H.6.3 Encroachment Allowance for Regional PCCSYA WMAA
Map
When an applicant uses the regional PCCSYA map from WMAA to define onsite CCSYAs an encroachment allowance of up to 5% is allowed.

The following provides the supporting rational for 5% encroachment:

Step 1. Sp has to be greater than 0.5, based on current understanding of risks to receiving waters arising from changes in sediment production (SCCWRP Technical Report 605, 2010).

Step 2. Estimated Sp (Equation H.8-11) = 0.7*SYRUSLE+0.3*SYNHD = 0.7*0.42 + 0.3*1 = 0.59 a. Based on RUSLE analysis conducted during Regional WMAA the GLUs mapped as PCCSYAs contribute 42% of the bed sediment yield (i.e. SYRUSLE = 0.42) b. Disturbance to NHDPlus channels are protected through 401 water quality certifications issued by the RWQCB, so it is assumed that SYNHD =1 Step 3. Dividing the Sp estimate from Step 2 by the required Sp in Step 1 provides the factor of safety that is currently implicit in the regional WMAA PCCSYA map = 0.59/0.5 =

1.18 or 18% factor of safety
Step 4. The remaining factor of safety after accounting for the proposed encroachment of 5% = 18% - 5% = 13%
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Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-38 2023
H.7 PCCSYAs: Refinement Options
If an applicant has identified onsite and/or upstream PCCSYAs and elects to perform additional optional analyses to refine the PCCSYA designation, the guidance presented below should be followed. Protection of critical coarse sediment yield areas is a necessary element of hydromodification management because coarse sediment supply is as much an issue for causing erosive conditions to receiving streams as are accelerated flows. However, not all downstream systems warrant preservation of coarse sediment supply nor all source areas need to be protected. The following guidance shall be used to refine PCCSYA designations:
• Depositional Analysis (Appendix H.7.1)
• Threshold Channel Analysis (Appendix H.7.2)
• Coarse Sediment Source Area Verification (Appendix H.7.3) H.7.1 Depositional Analysis
Areas identified as PCCSYAs may be removed from consideration if it is demonstrated that these sources are deposited into existing systems prior to reaching the first downstream unlined water of the state. Systems resulting in deposition may include existing natural sinks, existing structural BMPs, existing hardened MS4 systems, or other existing similar features. Applicants electing to perform depositional analysis to refine PCCSYA mapping must meet the following criteria to qualify for exemption from CCSYA designation:
• The existing hardened MS4 system that is being analyzed should be upstream of the first downstream unlined waters of the state; and
• The peak velocity from the discrete 2-year, 24-hour runoff event for the existing hardened MS4 system that is being analyzed is less than three feet per second.

The three feet per second criteria is consistent with the recommended minimum velocity for storm and sanitary sewers in ASCE Manual of Engineering Practice No. 37 (ASCE, 1970).

In limited scenarios, applicant may have the option to establish site specific minimum self-cleansing velocity using Equation H.7-1 or other appropriate equations instead of using the default three feet per second criteria. This site specific analysis must be documented in the SWQMP and the City has the discretion to request additional analysis prior to approving a site specific minimum self-cleansing velocity. If an applicant chooses to establish a site specific minimum self-cleansing velocity for refinement, then the applicant must design any new bypass hardened conveyance systems proposed by the project to meet the site specific criteria.
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Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-39 2023
Equation H.7-1: Minimum Self Cleansing Velocity
=

1.486
M

'
l
m)A/ − 1In

l
Where:

V = minimum self-cleansing velocity (ft/sec)
R = hydraulic radius (ft)
n = Manning’s roughness coefficient (unitless)
B = constant equal to 0.04 for clean granular particles (unitless) sg = specific gravity of sediment particle (unitless): Use 2.65 Dg = sediment particle diameter (inches): Use 0.20 in
H.7.2 Threshold Channel Analysis
A threshold channel is a stream channel in which channel boundary material has no significant movement during the design flow. If there is no movement of bed load in the stream channel, then it is not anticipated that reductions in sediment supply will be detrimental to stream stability because the channel bed consists of the parent material and not coarse sediment supplied from upstream. In such a situation, changes in sediment supply are not considered a geomorphic condition of concern.

SCCWRP Technical Report 562 (2008) states the following in regards to sand vs. gravel bed behavior/threshold vs. live-bed contrasts:
“Sand and gravel systems are quite varied in their transport of sediment and their sensitivity to sediment supply. On the former, sand-bed channels typically have live beds, which transport sediment continuously even at relatively low flows. Conversely, gravel/cobble-bed channels generally transport the bulk of their bed sediment load more episodically, requiring higher flow events for bed mobility (i.e., threshold behavior).”
“Sand-bed streams without vertical control are much more sensitive to perturbations in flow and sediment regimes than coarse-grain (gravel/cobble) threshold channels. This has clear implications in their respective management regarding hydromodification (i.e., sand systems being relatively more susceptible than coarser systems). This also has direct implications for the issue of sediment trapping by storm water practices in watersheds draining to sand-bed streams, as well as general loss of sediment supply following the conversion from undeveloped sparsely-vegetated to developed well-vegetated via irrigation.”
The following provides guidance for evaluating whether a stream channel is a threshold channel or not. This determination is important because while accounting for changes in bed sediment supply is appropriate for quantifying geomorphic impacts in non-threshold stream channels, it is not considered appropriate for threshold channels. The domain of analysis for this evaluation shall be the same as that used to evaluate susceptibility, per SCCWRP Technical Report 606, Field Manual for Assessing 2026-08-19 Item #12D Page 559 of 816
Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-40 2023
Channel Susceptibility (2010). This domain is defined by the following upstream and downstream boundaries:
• From the point of compliance proceed downstream until reaching one of the following:
o At least one reach downstream of the first grade-control point (preferably second downstream grade control location);
o Tidal backwater/lentic (still water) waterbody;
o Equal order tributary (Strahler 1952);
o A 2-fold increase in drainage area.

OR demonstrate sufficient flow attenuation through existing hydrologic modeling. • From the point of compliance proceed upstream for 20 channel top widths OR to the first grade control in good condition, whichever comes first.

Applicant must complete Worksheet H.7-1 to document selection of the domain of analysis. If the entire domain of analysis is classified as a threshold channel, then the PDP can be exempt from the MS4 Permit requirement for sediment supply. The following definitions from the Natural Resources Conservation Service’s (NRCS) National Engineering Handbook Part 654 - Stream Restoration Design (2007) are helpful in understanding what a threshold channel is. • Alluvial Channel: Streams and channels that have bed and banks formed of material transported by the stream. There is an exchange of material between the inflowing sediment load and the bed and banks of an alluvial channel (NRCS, 2007). • Threshold Channel: A channel in which channel boundary material has no significant movement during the design flow (NRCS, 2007).

The key factor for determining whether a channel is a threshold channel is the composition of its bed material. Larger bed sediment consisting primarily of cobbles and boulders are typically immobile, unless the channel is a large river with sufficient discharge to regularly transport such grain sizes as bed load. As a rule-of-thumb, channels with bed material that can withstand a 10-year peak discharge without incipient motion are considered threshold channels and not live-bed alluvial channels.

Threshold channel beds typically consist of cobbles, boulders, bedrock, or very dense vegetation (e.g., a thicket). Threshold channels also includes channels that have existing grade control structures that protect the stream channels from hydromodification impacts.

For a project to be exempt from coarse sediment supply requirements, the applicant must submit the following for approval by the City:
• Photographic documentation and grain size analysis used to determine the d50 of the bed material; and
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Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-41 2023
• Calculations that show that the receiving water of concern meets the specific stream power criteria defined below or a finding from a geomorphologist that the stream channel has existing grade control structures that protect the stream channel from hydromodification impacts.

Specific Stream Power
Specific (i.e., unit) stream power is the rate at which the energy of flowing water is expended on the bed and banks of a channel (refer to Equation H.7-2). SCCWRP studies have found that locating channels on a plot of Specific Stream Power at Q10 (as calculated by the Hawley et al. method optimized for Southern California watersheds – Figure H.7-2) versus median channel grain size is a good predictor of channel stability. The Q10 equation from SCCWRP TR 606 is presented as Equation H.7-3.

Equation H.7-2: Calculation of Specific Stream Power
XjopLqLp Xr.oUO 4,so. =
2,rUt Xr.oUO 4,so.
ℎUMMot uLgrℎ
=
v[X
s
Where:
v: Specific Weight of Water (9810 N/m3)
Q: Flow Rate (dominant discharge in many cases, m3
/sec)
S: Slope of Channel
w: Channel Width (meters)
Equation H.7-3: Calculation of Q10 using the Hawley et al. method Q10cfs = 18.2 * A0.87 * P0.77
Where:

Q10cfs: 10 year Flow Rate in cubic feet per second
A: Drainage Area in sq. miles
P: Mean Annual Precipitation in inches
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Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-42 2023
Figure H.7-1: Threshold of stream instability based on specific stream power and channel sediment diameter
Since the SCCWRP TR 606 Q10 (Equation H.7-3) does not explicitly consider watershed imperviousness, adjustment factors (AF) shown in Figure H.7-2 were developed using the following Equation H.7-4 for Q10 from SCCWRP TR 654 to account for imperviousness while estimating Q10.

Equation H.7-4: Calculation of Q10 using equation from SCCWRP TR 654 Q10 = e3.61 * A0.865 * DD0.804 * P224

0.778 * IMP0.096
Where:

Q10: 10 year Flow Rate
A: Drainage Area in sq. miles
DD: Drainage Density
P224: 2-Year 24-Hour Precipitation in inches
IMP: Watershed Imperviousness
Adjustment factors were developed as part of this methodology by changing the watershed imperviousness in Equation H.7-4 and keeping the remaining terms constant. Adjustment factor for imperviousness of 3.6% was set to 1; since it is the mean imperviousness of the dataset used to develop the stability curve in Figure H.7-1. Updated Q10 equation with adjustment factor is presented as Equation H.7-5 below:
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Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-43 2023
Equation H.7-5: Calculation of Q10 with Adjustment Factor for Watershed Imperviousness Q10cfs = AF * 18.2 * A0.87 * P0.77
Where:

Q10cfs: 10 year Flow Rate in cubic feet per second
AF: Adjustment Factor
A: Drainage Area in sq. miles
P: Mean Annual Precipitation in inches
Figure H.7-2: Adjustment factor to account for imperviousness while estimating Q10 Steps for evaluating the specific stream power criteria are presented below:
• Step 1: Calculate the specific stream power for the receiving water. Use Equation H.7-2, H.7- 5 and Figure H.7-2. Directly connected imperviousness shall be estimated using guidance provided in the Water Quality Equivalency guidance document. • Step 2: Determine the d50 of representative cross section within the domain of analysis. • Step 3: Use results from Step 1 and Step 2; and Figure H.7-1 to determine if the receiving water meets the specific stream power criteria. Receiving water shall be considered meeting the specific stream power criteria when the point plotted based on results from Step 1 and Step 2 is below the solid line in Figure H.7-1.
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Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-44 2023
H.7.3 Coarse Sediment Source Area Verification
When it has been determined that PCCSYAs are present, and it has been determined that downstream systems require protection, additional analysis may be performed that may refine the extents of actual CCSYAs to be protected onsite. The following analysis shall be performed to determine if the mapped PCCSYAs are a significant source of bed sediment supply to the receiving water, based on the coarse sediment proportion of the soil onsite
• Obtain a grain size distribution per ASTM D422 for the project’s PCCSYA that is being evaluated.
• Identify whether the source material is a coarse grained or fine grained soil. Coarse grained is defined as over 50% by weight coarse than no. 200 sieve (i.e., d50 > 0.074 mm). • By performing this analysis, the applicant can exclude PCCSYAs that are determined to be fine grained (i.e., d50 < 0.074 mm). Fine grained soils are not considered significant sources of bed sediment supply.
• Applicant shall include the following information in the SWQMP when this refinement option is performed:
o Map with locations on where the grain size distribution analysis was performed; o Photographic documentation; and
o Grain size distribution.
• Additional grain size distribution analysis may be requested at specific locations by the City prior to approval of this refinement.

Areas that are not expected to be a significant source of bed sediment supply (i.e. fine grained soils) to the receiving stream do not require protection and are not considered CCSYAs.

If it is determined that the PCCSYAs are producing sediment that is critical to receiving streams, or if the optional additional analysis presented above has not been performed, the project must provide management measures for protection of critical coarse sediment yield (refer to Appendix H.2, H.3 and H.4).
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Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-45 2023
Worksheet H.7-1: Domain of Analysis
Domain of Analysis Worksheet H.7-1
Use this form to document the domain of analysis
Project Name:

Project Tracking Number / Permit Application Number:

Part 1: Identify Domain of Analysis
Project Location (at proposed storm water discharge point) 1 Address:
2 Latitude (decimal degrees):
3 Longitude (decimal degrees):
4 Watershed:

Basis for determining downstream limit:

Channel length from discharge point
to downstream limit:

Basis for determining upstream limit:

Channel length from discharge point
to upstream limit:
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Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-46 2023
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Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-47 2023
Worksheet H.7-1; Page 2 of 2
Photo(s)
Map or aerial photo of site. Include channel alignment and tributaries, project discharge point, upstream and downstream limits of analysis, ID number and boundaries of geomorphic channel units, and any other features used to determine limits (e.g. exempt water body, grade control). 2026-08-19 Item #12D Page 567 of 816
Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-48 2023
H.8 Calculation Methodology for Ep and Sp
One method for quantifying hydromodification impacts to stream channels, which takes into account changes in the four factors in Lane’s relationship (i.e., hydrology, channel geometry, bed and bank material, and sediment supply), is to compare long-term changes in sediment transport capacity, or instream work, to bed sediment supply. For the purposes of demonstrating no net impact within the MS4-permitted region of the County of San Diego, Erosion Potential (Ep) is defined as the ratio of post-project/pre-development (natural) long-term transport capacity or work. To calculate Ep, the hydrology, channel geometry, and bed/bank material factors mentioned above need to be characterized for both land use scenarios. Sediment Supply Potential (Sp) is defined as the ratio of post-project/pre-project (existing) long-term bed sediment supply. While evaluating changes in discharge and sediment supply is done primarily as a desktop analysis, geomorphic field assessment is often necessary to characterize channel geometry and bed/bank material, and to ground truth assumptions for the desktop analyses. This appendix provides methodologies for the following:
• Calculation of Ep, and
• Calculation of Sp.

H.8.1 Calculation of Ep
Erosion Potential (Ep) is defined as the ratio of post-project/pre-development (natural) long-term transport capacity or work. To calculate Ep, the hydrology, channel geometry, and bed/bank material factors mentioned above need to be characterized for both land use scenarios. Traditionally, Ep is calculated based on a watershed-scale analysis (using future built out conditions) of the area tributary to a given receiving channel of concern at the point of compliance. However, watershed-scale continuous hydrologic modeling might not be feasible for small projects, with this understanding specific simplification steps for project-scale modeling are provided in this appendix. The applicant shall perform Ep calculations using one of the following methods, as applicable:
• Simplified Ep Method: Applicable when the default low flow threshold of 0.1Q2 is used and no changes to the receiving water are proposed. Refer to Appendix H.8.1.1. • Standard Ep Method: Applicable for all scenarios. Refer to Appendix H.8.1.2.

H.8.1.1 Simplified Ep Method
The simplified method is based on the relationships developed by Parra (2016) between the flow duration curve in the pre-development and post-project conditions and the standard simplified work equation. These relationships were developed using standard hydraulic equations and approximations that are applicable for channels of any lateral slope and the following geometrical cross sections: (a) wide rectangular sections; (b) relatively wide parabolic sections, and (c) triangular sections. The simplified Ep method is only applicable when the default low flow threshold of 0.1Q2 has been selected by the applicant for flow duration control and no changes to the receiving water geometry 2026-08-19 Item #12D Page 568 of 816
Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-49 2023
are proposed. Applicants shall follow Steps 1 through 3 to calculate Ep using the simplified methodology:

1. Perform continuous hydrologic simulation for the pre-development and post-project condition following guidelines in Appendix G. Generate flow bins and flow duration tables for the range of flows from 0.1Q2 to Q10.

2. Calculate the total work in the pre-development and the post-project condition using Equation H.8-1
Equation E.8-1: Total Work (Simplified)
u = w∆ry.
@
yz
A[
e{⁄
− (0.1[

)
e{⁄

I
.d
[
{
Where:

Wt = Total Work [dimensionless]
∆tj = Duration per flow bin
Q = Flow Rates estimated in STEP 1 [cfs] for a typical bin “j”. Usually, in Flow Duration Curve (FDC) analyses, the number of bins is 100, so j = 1 to n (with n= 100). However, the number of bins can be as small as 20 (n = 20).

Q2 = Pre-development 2-year peak flow [cfs]
m = exponent based on the function of the receiving channels geometry. • For narrow creek where the top width is 7 times or less the corresponding depth, m = 1/4.
• For intermediate creeks, where the top width is more than 7 times but less than 25 times the depth, m = 4/13.
• For wide creeks, where the top width is more than 25 times the depth, m = 2/5.

3. Ep is calculated by dividing the total work of the post-project condition by that of the predevelopment (natural) condition. Ep is expressed as:

Equation H.8-2: Ep (Simplified)
Ep = Wt,post / Wt,pre
Where:

Ep = Erosion Potential [unitless]
Wt,post = Total Work associated with the post-project condition [unitless] Wt,pre = Total Work associated with the pre-development condition [unitless] 2026-08-19 Item #12D Page 569 of 816
Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-50 2023
H.8.1.2 Standard Ep Method
While using the standard method, Ep calculation must be performed using the receiving water information from the point of compliance. Suggested steps for performing an Ep analysis are shown in the Figure H.8-1 below. This appendix describes each analysis step shown in Figure H.8-1, including the inputs and outputs of each step.

Figure H.8-1: Erosion Potential Flow Chart
STEP 1: CONTINUOUS HYDROLOGIC ANALYSIS
Hydrologic models are applied to simulate the hydrologic response of the watershed under predevelopment and post-project conditions for a continuous period of record. Modeling software appropriate for this type of simulation includes USEPA’s Storm Water Management Model (SWMM), Hydrological Simulation Program – Fortran (HSPF) developed by the USGS and USEPA, USACE’s Hydrologic Modeling System (HEC-HMS), and the San Diego Hydrology Model (SDHM) developed by Clear Creek Solutions, Inc. SDHM uses an HSPF computational engine, long-term precipitation data, and is a visually-oriented interactive tool for automated modeling and facility sizing.

Input parameters for these continuous simulations are hourly precipitation data for a long-term (>30 years) record, sub-catchment delineation, impervious cover, soil type, vegetative cover, terrain steepness, lag time or flow path length, and monthly evapotranspiration rate. The primary output is a 2026-08-19 Item #12D Page 570 of 816
Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-51 2023
simulated discharge record associated with the receiving channel of concern. Flow routing through drainage conveyances is necessary for continuous hydrologic analysis at the watershed scale. Appendix G provides guidance for developing continuous simulation models.

Traditionally, a hydrograph (Figure H.8-2) is the primary means for graphically comparing discharge records; however, a hydrograph is not ideal because long-term flow records span several decades.

Figure H.8-2: Example Hydrograph Comparison
Instead, a more effective means for comparing long-term continuous discharge records is to create a flow histogram, which differentiates the simulated flowrates into distinct “flow bins” so that the duration of flow for each bin can be tabulated. One method for establishing the distribution of flow bins is to increment the flow bins according to increments of flow stage using a hydraulic analysis, such as the normal depth equation. In this way, the hydraulic analysis step (Step 2) can be considered an input to the continuous hydrologic analysis step. While there is no established rule of thumb for how many flow bins are necessary, it is suggested that no less than 20 be used for an Ep analysis. An example of a flow histogram is provided on Figure H.8-3.

Figure H.8-3: Example Flow Duration Histogram
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Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-52 2023
Flow duration curves are another commonly used method for graphically interpreting long-term flow records. A flow duration curve is simply a plot of flowrate (y-axis) versus the cumulative duration, or percentage of time, that a flowrate is equaled or exceeded in the simulation record (x-axis). Figure H.8-4 provides an example flow duration curve comparison.

Figure H.8-4: Example Flow Duration Curve
Scaling Factor for Project-Scale Modeling
Project-scale flow rates derived from continuous hydrologic simulation can be scaled using the ratio of the pre-development 2-year peak discharge for the watershed and project catchment (i.e., Q2 watershed / Q2 project catchment) so that hydraulic and effective work calculations can be performed at the point of compliance with a larger tributary watershed. This scaling translates the runoff from the project catchment to its contribution to erosivity in the down gradient receiving channel, without the need for a complex watershed-scale continuous hydrologic model.

Applicant can estimate the scaling factor using Equation H.8-3. The scaling factor equation was developed using the 2-year peak flow rate empirical equation from Hawley and Bledsoe (2011) and removing the terms (average annual precipitation and imperviousness (pre-development condition as required by the MS4 Permit) that are constant.

Equation H.8-3: Scaling Factor
XpUtLM} #Upr,. = ~

Jy Ä
E.''c
Where:

Awatershed = total watershed drainage area at the point of compliance(mi2) Aproject= total project drainage area (mi2
)
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Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-53 2023
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Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-54 2023
STEP 2: HYDRAULIC ANALYSIS
Hydraulic parameters, such as stage, effective shear stress, and flow velocity, are computed for each designated flow bin using channel geometry and roughness data. Hydraulic calculations can be as simple as using the normal flow equation and obtaining results for the central channel or as complicated as using hydraulic models which account for backwater effects, such as HEC-RAS.

Using the formula for unit tractive force (Chow 1959), effective shear stress is expressed using equation H.8-4
Equation H.8-4: Effective Shear Stress
Å = v
X
Where:
τ = Effective Shear Stress [lb/ft2]
v = Unit Weight of Water [62.4 lb/ft3
]
R= Hydraulic Radius [ft]
S = Energy Gradient Assumed Equal to Longitudinal Slope [ft/ft].

Normal depth can be estimated using Manning’s equation (Equation H.8-5). Several sources provide lists of roughness coefficients for use in hydraulic analysis (Chow, 1959).

Equation H.8-5: Manning’s Equation
[ =

1.49 E.'cXE.d
M
,. =

1.49 E.'cXE.d
M
Where:

Q = Peak Flowrate [cfs]
V = Average Flow Velocity [ft/s]
A = Cross-Section Flow Area [ft2
]
R = Hydraulic Radius [ft] = A/P
P = Wetted Perimeter [ft]
S = Energy Gradient Assumed Equal to Longitudinal Slope [ft/ft] n = Manning Roughness [unit less]
Channel geometry inputs should be characterized by surveying cross sections and longitudinal profiles of the active channel at strategic locations. Methods of collecting topographic survey data 2026-08-19 Item #12D Page 574 of 816
Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-55 2023
can range from traditional survey techniques (auto level, cloth tape, and survey rod), to conducting a detailed ground based LiDAR survey.

STEP 3: WORK ANALYSIS
Hydraulic results for each flow bin along with the critical bed/bank material strength parameters are input into a work or sediment transport function in order to produce a work or transport rating curve.

An example of such a rating curve is provided on Figure H.8-3. The work equations can range from simplistic indices, material-specific sediment transport equations, or more complex functions based on site-calibrated sediment transport rating curves.
• Simplistic indices: An acceptable equation for effective work, as stated in the Los Angeles Regional MS4 Permit (LARWQCB, 2012) is expressed using equation H.8-6:

Equation H.8-6: Effective Work
u = (Å − Å
)
.d
Where:

W = Work [dimensionless];
τ = Effective Shear Stress [lb/ft2];
τc = Critical Shear Stress [lb/ft2];
V = Mid-Channel Flow Velocity [ft/s]
• Material-specific sediment transport equations: Material specific sediment transport equations are allowed to estimate the sediment transport capacity in the post-project and predevelopment condition. • Site-calibrated sediment transport curves: Applicants may have an option to use sitecalibrated sediment transport curves. In the future these may be available based on monitoring efforts being performed to support the County of San Diego’s Hydromodification Management Plan.

The critical shear stress to be used in equation H.8-6 must be estimated using one of the following:
• Shear stress corresponding to the critical flow rate or low flow threshold (Qc). Qc is the flowrate that results in incipient motion of bed or bank material, whichever is least resistant.

Qc is expressed as a fraction of the pre-development 2-year peak flow. The allowable low flow threshold Qc can be estimated as 10%, 30%, or 50% of the pre-development 2-year peak flow (0.1Q2, 0.3Q2, or 0.5Q2) depending on the receiving stream susceptibility to erosion, per SCCWRP Technical Report 606, Field Manual for Assessing Channel Susceptibility (SCCWRP, 2010). If a channel susceptibility assessment is not performed, then the conservative default is a Qc equal to 0.1Q2.
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Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-56 2023
• Bed and bank material can also be characterized through a geomorphic field assessment. For each stream location analyzed, a measure of critical shear stress can be obtained for the weakest bed or bank material prevalent in the channel. For non-cohesive material, a Wolman pebble count or sieve analysis can be used to obtain a grain size distribution, which can be converted to a critical shear stress using empirical relationships or published reference tables. For cohesive material, an in-situ jet test or reference tables are used. For banks reinforced with vegetation, reference tables are generally used. Appropriate references for critical shear stress values are provided in ASCE No.77 (1992) and Fischenich (2001). To account for the effects of vegetation density and channel irregularities, the applied shear stress can be partitioned into channel form and bed/bank roughness components. SCCWRP Technical Report 667 also has guidance for estimating critical shear stress.
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Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-57 2023
STEP 4: CUMULATIVE WORK ANALYSIS
Cumulative work is a measure of the long-term total work or sediment transport capacity performed at a creek location. It incorporates the distribution of both discharge magnitude and duration for the flow rates simulated. The cumulative work analysis must be performed up to the maximum geomorphically significant flow of Q10. To calculate cumulative work, first multiply the work (from STEP 3) and duration associated with each flow bin (from STEP 1). Then, the total work is obtained by summing the cumulative for all flow binds (Qc to Q10). This analysis can be expressed as:

Equation H.8-7: Cumulative Work
WÑ = wWÖ ∆tÖ
á
Öz
Where:

Wt = Total Work [dimensionless]
Wi = Work per flow bin [dimensionless]
∆t = Duration per flow bin [hours]
n = number of flow bins
The distribution of cumulative work, also referred to as a work curve (or work histogram), is helpful in understanding which flow rates are performing the most work on the channel of interest. An example work curve is provided in Figure H.8-5.
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Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-58 2023
Figure H.8-5: Example Work Curve
STEP 5: EROSION POTENTIAL ANALYSIS
Ep is calculated by simply dividing the total work of the post-project condition by that of the predevelopment (natural) condition. Ep is expressed as:

Equation H.8-8: Erosion Potential
Ep = Wt,post / Wt,pre
Where:

Ep = Erosion Potential [unitless]
Wt,post = Total Work associated with the post-project condition [unitless] Wt,pre = Total Work associated with the pre-development condition [unitless] As applicable, the applicant must use Worksheet H.8-1 and H.8-2 to document the Ep calculations for each point of compliance.
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Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-59 2023
Erosion Potential (Ep) Analysis Worksheet H.8-1
Background Information
1 Low Flow Threshold: results of SCCWRP channel
susceptibility analysis (Select 0.1*Q2 if analysis has not been performed).
 0.1*Q2
 0.3*Q2
 0.5*Q2
2 Selected Ep Method  Simplified Ep Method
 Standard Ep Method
2 Hydrologic Analysis: Select hydrologic analysis method.  Project-Scale
 Project-Scale and WatershedScale Continuous Simulation 4 Number of Points of Compliance (Copy and complete
worksheet for each Point of Compliance) unitless
Step 1: Hydrologic Analysis (not applicable for Simplified Ep Method) 5 Project-Scale Q2 (from continuous simulation) cfs
6 Project Area draining to the point of compliance sq. miles 7 Watershed Area draining to the point of compliance sq. miles 8 Scaling Factor for Flows (Line 7/Line 6)0.667 unitless 9 Low flow threshold (factor from Line 1 x Line 6) cfs
10 Watershed-Scale Q10 at Point of Compliance (from
continuous simulation or Project Q10 * Line 8) cfs
Hydrologic analysis results (Attach results of continuous simulation including:
full pre-development runoff time series at POC, full post-development runoff time series at POC, and flow duration histogram and/or cumulative flow duration curve for each POC).
 Yes
 No
Step 2: Hydraulic Analysis (not applicable for Simplified Ep Method) 11 Provide details about the cross-section (width, depth, slope, roughness, etc.) 2026-08-19 Item #12D Page 579 of 816
Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-60 2023
Erosion Potential (Ep) Analysis Worksheet H.8-1
Step 3: Work Analysis (not applicable for Simplified Ep Method) 12 Select work index, equation, or transport curve method for use in work analysis.
 Equation H.8-6
 Sediment Transport Equation
 Sediment Transport Curve
 Other: ______________
13 Describe/Justify selection in Line 12 above:
14 Calculate work done for each flow bin under the predevelopment and post-project condition using Worksheet H.8-2. Or similar documentation for
sediment transport modeling or transport curve analysis.  Yes
 No
Step 4: Cumulative Work Analysis
14 Cumulative pre-development work
(Equation H.8-1 for Simplified Ep Method)
(from Worksheet H.8.-2 for Standard Ep Method)

15 Cumulative post-project work
(Equation H.8-1 for Simplified Ep Method)
(from Worksheet H.8-2 for Standard Ep Method)

Step 5: Erosion Potential Analysis
16 Erosion Potential ( Line 15 / Line 14 ) unitless
2026-08-19 Item #12D Page 580 of 816
Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-61 2023
Work Calculations (Supplement to Worksheet H.8-1) Worksheet H.8-2 1 Channel Slope (ft/ft)
2 Channel Roughness (n) (unitless)
3 Low Flow Threshold (Line 9 from Worksheet H.8-1) cfs
4 Critical Shear Stress (lb/ft2)

A B C D E F G H I J K
Bin
Flow (cfs) Duration (hours)
Hydraulic
Radius (ft)
Average
Velocity
(ft/s)
Shear
Stress
(lb/ft2)
Work (unitless)
Lower
Limit
Upper
Limit Average Predevelopment
PostProject Predevelopment PostProject
1 2
3 4
5 6
7 8
9 10
11 12
13 14
15 16
17 18
19 20
n
Sum (Bins 1 to n) =
2026-08-19 Item #12D Page 581 of 816
Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-62 2023
Worksheet H.8-2 Key
A Number of flow bins, add additional rows as needed
B Lower limit for the corresponding flow bin
C Upper limit for the corresponding flow bin
D Average flow for the corresponding flow bin; [(B + C)/2] E Duration in hours for the corresponding flow bin in pre development condition F Duration in hours for the corresponding flow bin in post project condition G Hydraulic radius (in feet) associated with the average flow for the corresponding flow bin (from Manning’s equation and/or hydraulic analysis) H Average flow velocity (in fps) associated with the average flow for the corresponding flow bin (from Manning’s equation and/or hydraulic analysis) I Shear stress (lb/ft2
) associated with the average flow for the corresponding flow bin = γ * Hydraulic Radius*Slope = 62.4 * G * Line 1
J Pre-development work for associated flow bin
J = 0; If (I – Line 4) ≤ 0
J = E * (I – Line 4)1.5 * H; If (I – Line 4) > 0
K Post-project work for associated flow bin
K = 0; If (I – Line 4) ≤ 0
K = F * (I – Line 4)1.5 * H; If (I – Line 4) > 0
Note: If the receiving water dimensions are different in pre-development and post-project condition then Worksheet H.8-2 is not valid for work calculations. 2026-08-19 Item #12D Page 582 of 816
Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-63 2023
H.8.2 Calculation of Sp
While there are many categories of erosion processes (e.g., landslides, debris flows, gullies, tree throw, animal burrows, sheetwash erosion, wind erosion, dry ravel, bank erosion), in this evaluation processes will be simplified to sediment production from hillslopes and channels. Under ideal circumstances, the total bed sediment supply rate (tons/year) would be calculated for both the post-project built-out condition and pre-project condition using a watershed-scale Geomorphic Landscape Unit (GLU) and Geomorphic Channel Unit (GCU) approach which:
(1) identifies different sources of sediment supply based on categories of terrain slope, geology, land cover, and stream order;
(2) estimates the base erosion rate of those sources (GLUs and GCUs); (3) approximates the sediment delivery ratio (SDR) to the receiving channel; (4) evaluates the coarse bed-load fraction of the sources; and (5) integrates these considerations into a bed-load yield rate for both the existing condition and proposed built-out condition.

However, calculation of sediment yield rates for each GLU (tons/mi2 -yr) and GCU (tons/mJ-yr) using
the available science is inherently inexact and requires extensive field calibration. Additionally, performing the geospatial calculations necessary for such a comprehensive GLU and GCU analysis may not be straightforward for some project applicants. Since the objective is to determine the fraction of reduction in bed sediment supply in the post-project condition compared to the pre-project condition, but not to determine the bed sediment yield in physical units (tons/year/acre, for example) the following simplifications are allowed. These simplifications take into consideration the regional sediment yield map shown in Figure H.8.6.
2026-08-19 Item #12D Page 583 of 816
Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-64 2023
Figure H.8-6: Regional Sediment Yield Map
According to a regional sediment yield map of the Western US (USDA, 1974), hillslope processes (sheet and rill erosion) account for approximately 40% of the sediment yield in the San Diego County region, while channel processes (in-stream and gully erosion) account for approximately 60% of the sediment yield. Figure H.8-7 shows the different erosion processes. Provision E.3.a.(3)(a) of the MS4 Permit requires, “maintenance or restoration of natural storage reservoirs and drainage corridors (including topographic depressions, areas of permeable soils, natural swales, and ephemeral and intermittent streams)”, effectively making maintenance or restoration of channels and gullies within a project site a site design requirement.
2026-08-19 Item #12D Page 584 of 816
Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-65 2023
Figure H.8-7: Different Erosion Processes that Contribute Sediment Source: http://www.fairfaxcounty.gov/nvswcd/youyourland/soil.htm Sediment yield from hillslope processes (sheet and rill erosion) can be estimated using the Revised Universal Soil Loss Equation (RUSLE) and a sediment delivery ratio. For channel processes, the best available regional datasets are the USGS National Hydrography Dataset (NHD) and the NHDPlus dataset from USEPA and USGS (http://www.horizon-systems.com/nhdplus/). Both these datasets may not include the lowest order channels or gullies in the stream network, which can contribute a considerable amount of sediment produced from channel processes. Since the lower order channels and gullies originate and are mostly on the hillslopes, it is assumed for the Sp analysis that the sediment yield from lower order channels and gullies is proportional to the sediment yield from hill slopes.

Based on feedback received during the TAC meetings (Appendix H.5.1) the following distribution is proposed for the calculation of Sp:
• 70% of bed sediment yield ratio from RUSLE analysis (assumed to account for sediment yield from hillslope processes (sheet and rill erosion) and channels and gullies not part of the NHDPlus dataset); and
2026-08-19 Item #12D Page 585 of 816
Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-66 2023
• 30% of bed sediment yield ratio from channels in the NHDPlus dataset.

Note:
• If an applicant elects to map the waters of the state, the Sp distribution shall be revised to o 40% of bed sediment yield ratio from RUSLE analysis;
o 30% of bed sediment yield ratio from waters of the state that are not part of NHDPlus dataset; and
o 30% of bed sediment yield ratio from channels in the NHDPlus dataset.

SCALE OF ANALYSIS
The project applicant shall perform the Sp analysis at point (or points) where runoff leaves the project site6. The steps for performing an Sp analysis are shown in Figure H.8-8 and described below.

Figure H.8-8: Sediment Supply Potential Flow Chart
6 In limited scenarios, the County has the discretion allow for a watershed-scale Sp analysis to be performed at the point of compliance it the future built-out conditions of the watershed are used in the analysis. 2026-08-19 Item #12D Page 586 of 816
Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-67 2023
STEP 1: RUSLE ANALYSIS
RUSLE analysis is assumed to account for sediment yield from hillslope processes (sheet and rill erosion) and channels and gullies not part of the NHDPlus dataset. The change in bed sediment yield in the post-project condition compared to the pre-project condition using the RUSLE analysis must be estimated using equation H.8-9. This equation is a modified form of the standard RUSLE equation.

Only hillslopes that are anticipated to generate coarse sediment must be used in this analysis. Since Sp is a dimensionless index the terms that are relatively constant in the pre and post project condition, such as rainfall factor, have been removed.

Equation H.8-9: Sediment Yield (Hillslope)
Xà â0ä! =
4,/r − 4.,ãopr ∑å × ç × ZX × × 4é

4.o − 4.,ãopr ∑å × ç × ZX × é
Where:

A = Hillslope Area (acres)
K = Soil erodibility factor, this value can be obtained from regional K factor map from SWRCB or web soil survey or site-specific grain size analysis
LS = Slope length and steepness factor, this value can be obtained from the regional LS factor map from SWRCB or site-specific determination using look up tables based on slope and horizontal slope length from USDA Agriculture Handbook Number 703 (Renard et al., 1997) or other relevant sources
C= Cover management factor, use regional C factor map from USEPA or site-specific information; this is the reciprocal of the amount of surface cover on soil, whether it be vegetation, temporary mulch or other material. It is roughly the percentage of exposed soil, i.e., 95 percent cover yields a “C” value of 0.05. Use C=0 for areas where management actions are implemented (e.g. impervious areas)
P = Practice factor, only included in post-project condition. This term is added to account for sediment yield from engineered slopes. Practice factor of 0.25 shall be used for fill slopes and a practice factor of 0.50 shall be used for cut slopes. Use a practice factor of 1 for undisturbed areas.

The applicant may be allowed to receive credit for bed sediment yield from engineered slopes on the project perimeter directly discharging to conveyance systems if all of the following criteria are met:
• The engineered slopes consist of coarse bed material. This is confirmed by performing grain size distribution per ASTM D422 for the engineered slope and verifying that the d50 is greater than no. 200 sieve (0.074 mm).
• Cover factor in the post project condition shall not be greater than the cover factor used in the pre project condition for the same area.
• A maximum practice factor of 0.25 may applied to proposed fill slopes. A maximum practice factor of 0.50 may be applied to proposed cut slopes.
2026-08-19 Item #12D Page 587 of 816
Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-68 2023
• A statement from the geotechnical engineer is included in the SWQMP certifying that the engineered slope will be stable even after accounting for bed sediment generation and the anticipated soil loss during the planned lifetime of the engineered slope is acceptable.

Additional analysis and/or documentation may be requested by the City prior to approval of the credit for bed sediment yield from engineered slopes.

STEP 2: CHANNEL ANALYSIS
If an NHDPlus mapped channel exists within the project property boundary, applicants must consider the sediment production from this existing channel system. The change is bed sediment yield in the post-project condition compared to the pre-project condition from channels in the NHDPlus dataset must be estimated using equation H.8-10 (SYNHD). This equation is based on screening-level GIS calculations of stream length that will be contributing sediment in the post-project condition in the watershed tributary to the point of compliance.

Equation H.8-10: Sediment Yield (NHD)
Xàèê$ =
ZJ
Z

Where:

Lpost = Length of NHDplus streams in the watershed contributing to bed sediment supply in the post-project condition [miles]
Lpre = Length of NHDplus streams in the watershed contributing to bed sediment supply in the pre-project existing condition [miles]
STEP 3: SEDIMENT SUPPLY POTENTIAL ANALYSIS
Sediment Supply Potential (Sp) is defined as the ratio of post-project/pre-project (existing) long-term bed sediment supply. Sp must be calculated using equation H.8-11 presented below:

Equation H.8-11: Sediment Supply Potential
X = 0.7 × Xà â0ä! + 0.3 × Xàèê$
Where:

Sp = Sediment Supply Potential [unitless]
SYRUSLE = Change in bed sediment yield from hillslopes and lower order channels and gullies not part of NHDPlus dataset [unitless]
SYNHD = Change in bed sediment yield from channels in NHDPlus dataset [unitless] When estimating Sp the following additional conditions apply:
2026-08-19 Item #12D Page 588 of 816
Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-69 2023
• Projects that do not have onsite NHDPlus channels shall omit consideration of SYNHD and weighting factors depicted in Equation H.8-11. This simply results in Sp = SYRUSLE. • It must be assumed that the sediment yield from an area that drains to a structural BMP is zero. Consideration of sediment yield from an area draining to the structural BMP may be allowed if sediment bypass measures are implemented upstream of the structural BMP.

However, additional analysis may be requested by the City to substantiate the sediment yield estimates proposed by the applicant from implementing sediment bypass measures. • For scenarios where an upstream coarse sediment yield area drains through the project footprint and the project footprint cuts off conveyance of bed sediment generated upstream of the project footprint to the point of compliance, (e.g., via debris basins) the contribution from the upstream area shall be assumed to be zero.

As applicable, the applicant must use Worksheet H.8-3 to document the Sp calculations for each point of compliance.
2026-08-19 Item #12D Page 589 of 816
Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-70 2023
Sediment Supply Potential (Sp) Analysis Worksheet H.8-3 1 Scale of Analysis  Project Scale
 Watershed Scale (built-out condition)
Step 1: RUSLE Analysis
2 GLU
Pre-Project Post-Project
A K LS C A*K*LS*C A K LS C P A*K*LS*C*P
1 2
3 4
5 6
7 8
Add additional rows as needed
3 Sum Pre-Project Sum Post-Project
4 Xà â0ä!: ( Sum Post-Project/ Sum Pre-Project) (From Line 3) unitless Step 2: Channel Analysis: NHDPlus Channels
5 Lpre (from GIS analysis of pre-project existing condition) miles 6 Lpost (from GIS analysis of post-project condition) miles 7 Xàèê$: ( Line 6 / Line 5 ) unitless
Step 3: Sediment Supply Potential Analysis
8 RUSLE Analysis Bed Sediment Yield Ratio Calculated ( Line 4 ) unitless 9 Channel Bed Sediment Yield Ratio from NHDPlus dataset ( Line 7 )
unitless
10 Sediment Supply Potential Calculated using Equation H.8-11. ( 0.7 x Line 8 + 0.3 x Line 9)
unitless
2026-08-19 Item #12D Page 590 of 816
Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-71 2023
H.9 Mitigation Measures Fact Sheets
The following fact sheets were developed to assist the project applicants with designing mitigation measures:
• Additional flow control
• Stream Rehabilitation
H.9.1 Additional Flow Control
Description
Additional flow control refers to the modification
of post-development flow rates and durations
beyond the levels required by standard HMP
criteria (i.e. control of flow rates and durations
from Qc to Q10). Additional flow control can
mitigate the effect of decreased sediment delivery
by equivalently limiting sediment transport
capacity. BMPs providing additional flow control
are detention/retention type BMPs and will
typically be larger than those that meet HMP
criteria only. The performance standard for
additional flow control can be demonstrated
through the NII management standard.
2026-08-19 Item #12D Page 591 of 816
Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-72 2023
Management Standard and Sizing Approach
The management standard additional flow control BMPs need to meet to demonstrate that there is no net impact to the receiving waters is presented in the equation below:
i(( =
1j
Xj ≤ 1.1
Where:
1j: is the ratio of post-project/predevelopment sediment transport capacity
Xj: ratio of post-project/pre-project
(existing) long-term bed sediment
supply
Note: Redevelopment projects typically do not have critical coarse sediment yield areas onsite because management
actions have been implemented onsite (e.g. impervious areas, etc.). Refer to Appendix H.8 for methodologies to calculate Ep and Sp.

Project applicants must demonstrate that the NII management standard will be met under the postproject scenario through the following steps:

1. Calculate the Sp at the point of compliance using guidance in Appendix H.8.2.

2. Determine the Target Ep: EpTarget ≤ 1.1 * Sp

3. Calculate the pre-development sediment transport capacity or work (Ep denominator). Refer to Section 6.3.3 for definition of pre-development and refer to Appendix H.8.1 for guidance on calculating the sediment transport capacity or work.

4. Iteratively size additional flow control BMPs and calculate the post-project sediment transport capacity (Ep numerator) until the target Ep is reached.

5. Summarize the calculations performed to size the BMPs in the SWQMP.

In addition to the general approach outlined above, additional flow control BMPs must meet the design criteria presented in the Appendix E Fact Sheets. Deviations from these criteria may be approved at the discretion of the City if it is determined appropriate.

Design Adaption for Project Goals
NII management standard is met by additional flow control. Larger BMPs may be able to provide adequate additional flow control to meet the required performance standard. In this scenario no additional sediment BMPs are required.

For example, project that has an Sp = 0 (i.e. 100% of the bed sediment in the drainage area to the point of compliance is impacted by the project) can be mitigated by designing a BMP such that there is no discharge within the geomorphically significant flow range (i.e. Qc to Q10). 2026-08-19 Item #12D Page 592 of 816
Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-73 2023
NII management standard is not fully met by additional flow control. Additional flow control alone may not be able to entirely meet the NII management standard due to site, or other, constraints.

In scenarios where the target Ep cannot be met by additional flow control, additional BMPs that increase the supply of bed sediment or reduce the susceptibility of the receiving channel will be required.

Note: Additional flow control BMPs can be independent BMPs that provide flow control only or they can be integrated with storm water pollutant control BMPs.

Conceptual Design and Sizing Approach
The following steps detail an approach that can be used to appropriately size BMPs that provide additional flow control:

Step 1: Calculate the Sediment Supply Potential (Sp) based on pre- and post-project condition at the point of compliance.
• Refer to Appendix H.8.2 for methodology to calculate Sp. Applicant must document this analysis using Worksheet H.8-3.

Step 2: Determine the Target Ep based on the results of Step 1. • EpTarget ≤ 1.1 * Sp
Step 3: Perform continuous simulation modeling for pre-development condition. • Perform continuous simulation (refer to Appendix G) for the pre-development condition.
• Determine the flow durations for the pre-project scenario as described in Appendix G.1.6.2.

Step 4: Perform pre-development work analysis.
• Calculate the cumulative work performed by the range of geomorphically significant flows for the pre-development scenario, (refer to Step 3 and Step 4 in Appendix H.8.1 for calculation of work).

Step 5: Implement flow control BMPs and perform continuous simulation modeling for post-project scenario.
• Appropriately size pollutant control and hydromodification management BMPs according to the procedures presented in this manual.
• Perform continuous simulation (refer to Appendix G) for the post-project condition. • Determine the flow durations for the post-project scenario as described in Appendix G.1.6.2.
• Typically, BMPs sized to satisfy the flow duration control will provide for some level of Sp reduction and will ensure that the minimum design standards and sizing requirements are met.

Step 6: Perform post-project work analysis.
• Follow the steps presented in Step 4 to determine the post-project total work.

Step 7: Calculate Ep and determine if Target Ep has been met. 2026-08-19 Item #12D Page 593 of 816
Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-74 2023
• Divide the post-project total work by the pre-development total work and determine if the target Ep has been met.
• If the target Ep is met by the standard BMPs, document results and compliance with hydrologic and sediment supply performance standards.
• If the target Ep is not met, proceed to Step 8.

Step 8: Provide additional flow control storage and calculate Ep. • Following the procedures presented in the previous steps, iteratively calculate Ep for increasingly large BMPs until the target Ep is met.
• Document results and compliance with hydrologic and NII management standard.

As applicable, the applicant must use Worksheet H.8-1, Worksheet H.8-3 and Worksheet H.9-1 to document sizing of the additional flow control mitigation measure. 2026-08-19 Item #12D Page 594 of 816
Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-75 2023
Additional Flow Control Mitigation Measure Worksheet H.9-1 1 Sediment Supply Potential
(Line 10 of Worksheet H.8-3) unitless
2 Attached completed Worksheet H.8-3 and associated
documentation
 Yes
 No
3 Target Ep ≤ 1.1 * Line 1 unitless
4 Erosion Potential
(Line 16 of Worksheet H.8-1) unitless
5 Attached completed Worksheet H.8-1 and associated
documentation
 Yes
 No
6 Is Line 4 ≤ Line 3?
If Yes, NII management standard is met.

If No, increase the size of the BMP and recalculate Line 4.  Yes
 No
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Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-76 2023
H.9.2 Stream
Rehabilitation
Description
Hydromodification control can be achieved by
stream rehabilitation projects including: drop
structures, grade control structures, bed and bank
reinforcement, increased channel sinuosity or
meandering, increased channel width, and flow
diversion. The objective of these in-stream
controls, or stream restoration measures, is to reduce or maintain the overall Erosion Potential (Ep) of the receiving channel. Stream rehabilitation option is only available when the receiving channel of concern is already impacted by erosive flows and shows evidence of excessive sediment, erosion, deposition, or is a hardened channel.

Management Standard and Sizing Approach
The management standard stream rehabilitation projects need to meet to demonstrate that there is no net impact to the receiving waters is presented in the equation below:
i(( =
1j
Xj ≤ 1.1
Where:
1j: is the ratio of post-project/pre-development sediment transport capacity Xj: ratio of post-project/pre-project (existing) long-term bed sediment supply Note: Stream rehabilitation project reduce Ep by modifying the stream’s hydraulic properties and/or bed/bank material resistance without fully replacing sediment supply or controlling increases in runoff. Refer to Appendix H.8 for methodologies to calculate Ep and Sp.

Design Adaption for Project Goals
The following describes different types of stream rehabilitation projects that could be implemented to meet the NII management standard by reducing or maintaining the overall Ep:

Drop Structures: Drop structures are designed to reduce the average channel slope, thereby reducing the shear stresses generated by stream flows. These controls can be incorporated as natural looking rock structures with a step-pool design which allows drop energy to be dissipated into the pools while providing a reduced longitudinal slope between structures.

Grade Control Structures: Grade control structures are designed to maintain the existing channel slope while allowing for minor amounts of local scour. These control measures are often buried and entail a narrow trench across the width of the stream backfilled with concrete or similar material, as well as the creation of a “plunge pool” feature by placing boulders and vegetation on the downstream 2026-08-19 Item #12D Page 596 of 816
Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-77 2023
side of the sill. A grade control structure provides a reduced footprint and impact as compared to drop structures, which are designed to alter the channel slope.

Bed and Bank Reinforcement: Channel reinforcement serves to increase bed and bank resistance to instream erosion. A number of vegetated approaches are widely used. Such approaches include large woody debris, live crib walls, vegetated mechanically stabilized earth, live siltation, live brushlayering, willow posts and poles, live staking, live fascine, rootwad revetment, live brush mattresses, and vegetated reinforcement mats. These technologies provide erosion control that stabilizes bed and bank surfaces and allows for re-establishment of native plants, which serves to further increase channel stability.

Channel Sinuosity: Increasing channel sinuosity (meandering) can serve to reduce the channel slope, thereby reducing the shear stresses generated by stream flows. However, forcing a channel to be too sinuous is likely to lead to subsequent channel avulsion (cutting a new stream path) to a straighter course. Channel sinuosity needs to be supported by a geomorphic basis of design that shows the proposed form and gradient are appropriate for the valley slope, sediment, and water regime. This support may take the form of reference reaches in similar watersheds that have supported the proposed morphology over a significant period of time, or comparison between the proposed form and typical literature values.

Channel Widening: Increasing the width-to-depth ratio of a stream’s cross section is meant to spread flows out over a wider cross section with lower depths, thereby reducing shear stress for a given flow rate. This approach can be a useful management strategy in incised creeks to restore them to equilibrium conditions once vertical incision has ceased. As with sinuosity, it is important to develop a robust geomorphic basis of design that shows the increase in width-to-depth ratio to be sustainable.

Flow Diversion: Flow diversions can be designed to divert the excess flows caused by development to an hydromodification management exempt water body so that the shear stresses do no increase in the susceptible receiving water. When diversions are proposed to a water body exempt through watershed management area analysis, the applicant is required to provide a supporting analysis that the excess flows diverted to the exempt water body do not invalidate the exemption.

Design Considerations
Each stream rehabilitation project is to some degree unique because of differences in geomorphic process, morphology and previous watershed history. For this reason, this fact sheet does not provide a prescriptive ‘cookery book’ approach for rehabilitating streams, but instead provides guidelines and recommendations. Shields (1996) provides a helpful overview of the analytical steps involved in stream restoration and Shields et al. (1999) provides examples of approaches used to rehabilitate incised channels. Applicant will need to provide geomorphic and engineering information to support their proposed project approach. It is recommended that multiple lines of technical evidence be used by applicants to develop creek restoration plans based on the preponderance of evidence for design criteria such as channel width, depth, slope and planform. It is also important to understand that all creek rehabilitation projects must comply with relevant Federal, State and local regulations and 2026-08-19 Item #12D Page 597 of 816
Appendix H: Guidance for Investigation Potential Critical Coarse Sediment Yield Areas H-78 2023
permits. These will likely include obtaining permits from the RWQCB, USACE and California DF&W, and may involve additional permits or consultation with USDF&W and FEMA, as well as permits from the City. The proposed design shall also meet local drainage design guidelines for channel design.

Encinitas City CouncilAugust 19, 2026

Encinitas City Council
August 19, 2026