Agenda packet — Economic Development Committee, August 4, 2025 (part 3 of 3)

Several times during first year following installation.

During subsequent seasons at beginning and end of
wet season. Additional monitoring after periods of
heavy stormwater runoff is desirable.

If drawdown period is observed to have increased
significantly over the design drawdown period,
clean the drop inlet, wash the top 18 inches of
gravel, and clean the bore. This maintenance
activity may be reduced by minimizing upstream
erosion.

As required
Remove litter and debris from Infiltration Well area. As required Maintain pretreatment control measures in
accordance to its respective “Long-Term
Maintenance” section.

As required
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Treatment Control Measure Fact Sheet L-11: Dry Well
Development Guidance Manual E-77 May 2025
Description
[Note: Dry wells cannot be used in City maintained
areas]
A Dry Well is a bored, drilled, or driven shaft or
hole whose depth is greater than its width. A Dry
Well is designed specifically for alleviation of
flooding and the disposal of stormwater runoff. Dry
Wells design and function are similar to infiltration
trenches in that they are designed to temporarily
store and infiltrate stormwater runoff, primarily
from rooftops and other impervious areas with low
pollutant loadings. A Dry Well may either be a small
excavated pit filled with aggregate or a prefabricated storage chamber or pipe segment.

Dry Wells can be used to reduce the increased volume of stormwater runoff caused by building roofs. While generally not a significant source of stormwater runoff pollution, roofs are one of the most important sources of new or increased stormwater runoff volume from land development sites. Dry Wells can be used to indirectly enhance water quality by reducing the amount of stormwater runoff volume to be treated by other downstream treatment control measures.

A Dry Well cross-section schematic is presented in Figure L-11.1.

Advantages
• Can be placed below ground.
• Reduces peak stormwater runoff flows during small storm events.

Disadvantages
• Cannot be used in City maintained areas.
• Not appropriate for areas with low permeability soils (Type C or D soils) or high groundwater. • Cannot receive untreated stormwater runoff, except from rooftops. • Rehabilitation of failed Dry Wells requires complete reconstruction. • Not appropriate for industrial sites or locations with contaminated soils or where spills may occur because of the potential threat to groundwater contamination.

Planning and Site Considerations
• Integrate Dry Wells into open space buffers and other landscape areas when possible. • Plan for appropriate setback distances presented in Table L-11.1. • Pretreatment using Grassy Channels, Vegetated Buffer Strips, Vegetated Swales, Grassy Swales, and/or Vegetated Filter Strip is required to protect Dry Wells from high sediment loads.
• Install an observation well to check water levels, drawdown time, and evidence of clogging.

Photo Credit: Canale Landscaping
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Treatment Control Measure Fact Sheet L-11: Dry Well
Development Guidance Manual E-78 May 2025
Figure L-11.1. Dry Well Schematic
One example is a subsurface retention/infiltration system installation under a parking lot similar to the one shown below.

Photo Credit: Contech Engineered Solutions, LLC
www.contechs.com
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Treatment Control Measure Fact Sheet L-11: Dry Well
Development Guidance Manual E-79 May 2025
Design Criteria
Principal design criteria for Dry Wells are listed in Table L-11.1.

Table L-11.1. Dry Well Design Criteria
Design Parameter Design Criteria Notes
Tributary drainage area ≤ 10,000 ft2
Design volume WQV
Drawdown time for WQV 12 hr (maximum)
Soil permeability range 0.6-2.0 in/hr Saturated vertical permeability Groundwater separation 10 ft (minimum)
Between Dry Well bottom and
historical seasonally high
groundwater table
Trench surcharge depth (Dmax) 10 ft (maximum)
Setbacks
100 ft From wells, tanks, fields, springs
20 ft Down slope from foundation
100 ft Up slope from foundation
Filter media diameter 1.5-3 in Gravel or prefabricated media Observation well size 4-6 in Perforated PVC pipe with
removable cap
Pretreatment vegetated buffer
strip length 10 ft (minimum) Length in flow direction
Pretreatment vegetated buffer
strip slope 5% (maximum) Slope in flow direction
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Treatment Control Measure Fact Sheet L-11: Dry Well
Development Guidance Manual E-80 May 2025
Design Procedure
Step 1 – Calculate WQV
Calculate the effective tributary drainage area and WQV based on a 12-hour drawdown period.

Step 2 – Calculate Minimum Surface Area of Dry Well Bottom (Amin) Where
WQV = water quality volume (ft3);
tmax = maximum drawdown time = 12 hours;
I = site infiltration rate (soil permeability, in/hr); and s = safety factor
In the formula above, the safety factor accounts for possible inaccuracies in the infiltration rate measurement. A larger safety factor should be used when the infiltration rate is less certain. Safety factors typically range between two (2) and ten (10) and should be determined by a qualified geotechnical engineer or geologist based on field measurements of saturated vertical permeability at the proposed site.

Step 3 – Calculate Design Depth of Water Surcharge in Dry Well (Dmax) Where
WQV = water quality volume (ft3);
P = Porosity of Dry Well material (use 0.30). Note: Use of manufactured percolation tank modules can provide greater porosity than gravel.

Amin = Minimum surface area for Dry Well bottom required (ft2).

Note: Dmax should not exceed ten (10) feet. If necessary, increase Amin to keep Dmax ≤ 10 ft.

Step 4 – Design Observation Well
Provide a vertical section of perforated PVC pipe, four (4) to six (6) inches in diameter, installed flush with the top of the Dry Well on a footplate and with a locking, removable cap. The observation well is needed to monitor the infiltration rate in the Dry Well and is useful for marking the location of the Dry Well.
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Treatment Control Measure Fact Sheet L-11: Dry Well
Development Guidance Manual E-81 May 2025
Step 5 – Design Diversion Structure
Provide bypass or overflow of stormwater runoff volumes in excess of the WQV by means of a screened overflow pipe connected to the downstream storm drain system or grated overflow outlet.

Volume Retention Calculation
Dry wells may be used to achieve both the volume retention and treatment requirements. A Dry Well provides volume retention for the entire WQV calculated in Step 1 of the Design Procedure.

Construction Considerations
• If possible, stabilize the entire tributary area to the Dry Well before construction begins. If this is not possible, divert flow around the Dry Well site to protect it from sediment loads during construction.
• Once construction is complete, stabilize the entire tributary area to the Dry Well before allowing stormwater runoff to enter the Dry Well.
• Store excavated material at least ten (10) feet from the Dry Well to avoid backsliding and cave-ins.
• Install geotextile fabric on sides, bottom, and one foot below the surface of the Dry Well (see Table 8-8). Provide generous overlap at all seams. • Minimize compaction of the subgrade to the maximum extent practicable during construction to prevent loss of infiltrative capacity.
• Clean, washed gravel should be placed in the excavated Dry Well in lifts and lightly compacted with a plate compactor. Use of unwashed gravel can result in clogging.

Long-Term Maintenance
The City requires execution of a maintenance agreement with the property owner prior to final acceptance of a private development project, which includes treatment control measures such as Dry Wells. Such agreements typically include requirements such as those outlined in Table L11.2. The property owner or property owner’s designee is responsible for compliance with the agreement. The maintenance agreement must provide the City with complete access to the treatment control measure and its immediate vicinity at any time. #6..
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Treatment Control Measure Fact Sheet L-11: Dry Well
Development Guidance Manual E-82 May 2025
Table L-11.2. Inspection and Maintenance Requirements for Dry Wells Activity Schedule
Monitor infiltration rate in well after storm events by recording the drop in water depth versus time using a
calibrated rod or staff gauge.

Several times during first year following
installation. During subsequent seasons at
beginning and end of wet season. Additional
monitoring after periods of heavy stormwater
runoff is desirable.

Clean Dry Well when loss of infiltrative capacity is
observed. If infiltration rate is observed to have
decreased significantly over the design rate, remove
sediment and/or gravel from the Dry Well, and
replace with washed gravel if necessary. Clogging is
most likely occurring at the top Dry Well. This
maintenance activity is expensive, and the need for it
can be minimized through prevention of upstream
erosion.

As required
Remove litter and debris from Dry Well area. As required Inspect Dry Well to identify potential problems such
as standing water, trash, and debris, and sediment
accumulation.

At beginning and end of wet season. Additional
inspections after periods of heavy stormwater
runoff are desirable.

Maintain pretreatment control measures in
accordance to its respective “Long-Term
Maintenance” section.

As required
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APPENDIX F
Conventional Treatment Control Measure Fact
Sheets (C1 – C6)
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Treatment Control Measure Fact Sheet C-1: Constructed Wetland Development Guidance Manual F-1 May 2025
Description
A Constructed Wetland is a singlestage treatment system consisting of a forebay and a permanent pool with
aquatic plants. The function of a
Constructed Wetland is similar to a
Wet Pond in that influent stormwater
runoff flow mixes with a permanent
pool as it enters the basin. The
surcharge volume above the
permanent pool is slowly released
over a specified period (24 hours for
water quality volume (WQV)).

Constructed Wetlands require a
longer release period for the
surcharge volume than Wet Ponds
because the depth and volume of the
permanent pool for Constructed Wetlands are less than for Wet Ponds. A base flow is required to maintain the permanent water pool. Constructed Wetlands also differ from Wet Ponds in terms of the extensive presence of aquatic plants. Plants provide energy dissipation and enhance pollutant removal by sedimentation and biological uptake. A conceptual layout of a Constructed Wetland is shown in Figure C-1.1.

Constructed Wetlands differ from natural wetlands in that they are man-made and are designed to enhance stormwater quality. Diversion of stormwater directly to natural wetlands is not recommended because natural wetlands need to be protected from adverse effects of development. This is especially important because natural wetlands provide stormwater and flood control on a regional scale. Natural wetlands can be incorporated into the Constructed Wetlands system, but such action requires the approval of federal and state regulators.

Constructed Wetlands are generally not allowed to be used to mitigate the loss of natural wetlands.

Advantages
• Constructed Wetlands can provide substantial wildlife habitat and passive recreation. • Due to the presence of the permanent wet pool, Constructed Wetlands can provide significant water quality improvement for many constituents, including dissolved nutrients. • Widespread application with sufficient capture volume can provide significant control of channel erosion and enlargement caused by changes to stormwater flow.

Limitations
• Constructed Wetlands must have a continuous base flow to maintain aquatic plants. • There may be some aesthetic concerns about a facility that looks swampy.

Constructed Wetland. Source: Constructed wetlands (NRCS 656).

AgBMPs.(n.d.). Retrieved October 7, 2021, from
https://agbmps.osu.edu/bmp/constructed-wetlands-nrcs-656. #6..
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Treatment Control Measure Fact Sheet C-1: Constructed Wetland Development Guidance Manual F-2 May 2025
• There are public safety concerns when wetlands are constructed where there is public access.
• Mosquito and midge breeding is likely to occur in Constructed Wetlands. • Constructed Wetlands cannot be placed on steep, unstable slopes and require a relatively large footprint.

Planning and Siting Considerations
• Appropriate land uses include large residential developments and commercial, institutional, and industrial areas where the incorporation of green space and a Constructed Wetland into the landscape is desirable and feasible.
• It can be used effectively in combination with upstream treatment controls, such as vegetated buffer strips and vegetated swales.
• Requires relatively large areas (typically four to six percent of the tributary area) and are usually larger than Wet Ponds because the average depth is less. • Most appropriate for sites with low-permeability soils (Type C and D) that will support aquatic plant growth.
• Infiltration through a wetland bottom cannot be relied upon because the bottom is either covered by soils of low permeability or because the groundwater is higher than the wetland bottom.
• Wetland bottom channels require a near-zero slope.
• A base flow of water is required to maintain aquatic conditions. #6..
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Treatment Control Measure Fact Sheet C-1: Constructed Wetland Development Guidance Manual F-3 May 2025
Plan View
Section View
Figure C-1.1 Conceptual Layout of Constructed Wetland. Source: Adapted from Urban Storm Drain Criteria Manual Vol 3. – Best Management Practices, Urban Drainage and Flood Control District. (1999, November).
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Treatment Control Measure Fact Sheet C-1: Constructed Wetland Development Guidance Manual F-4 May 2025
Design Criteria
Design criteria for Constructed Wetlands are listed in Table C-1.1. A Design Data Summary Sheet is provided at the end of this fact sheet.

Table C-1.1. Constructed Wetland Design Criteria
Design Parameter Design
Criteria Notes
Design volume WQV See standard calculation fact sheet
Maximum drawdown time for WQV 24 hours Based on WQV
Minimum permanent pool volume 75% Percentage of WQV
Inlet/outlet erosion control – Provide energy dissipaters to reduce velocity Forebay
a. Volume
b. Area
c. Depth
5-10%
5-10%
2-4 ft
Percentage of WQV
Percentage of permanent pool surface area
Open-water Zone
a. Area (including forebay)
b. Depth
10-40%
2-4 ft
Percentage of permanent pool surface area
Wetland Zone
a. Area
b. Depth
50-70%

0.5-1 ft
Percentage of permanent pool surface area
30 to 50% should be 0.5 ft deep
Outlet Zone
a. Area
b. Depth
5-10%
3 ft
Percentage of permanent pool surface area
Minimum
Surcharge depth above permanent pool 2 ft Maximum
Basin length to width ratio 2:1 Minimum (larger preferred) Basin freeboard 1 ft Minimum
Wetland zone bottom slope 10% Maximum
Embankment side slope (H:V)
≥ 4:1
≥ 3:1
Inside
Outside (without retaining walls)
Maintenance access ramp slope (H:V) 10:1 or flatter
Maintenance access ramp width 16 ft Minimum – paved with concrete or permeable pavers
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Treatment Control Measure Fact Sheet C-1: Constructed Wetland Development Guidance Manual F-5 May 2025
Design Procedure
Step 1 – Calculate Water Quality Volume (WQV)
Using the Fact Sheet T-0, determine the tributary drainage area and WQV for 24-hour drawdown.

Step 2 – Determine Basin Minimum Volume for Permanent Pool (VPP) The volume of the permanent wetland pool (Vpp) shall be not less than 75% of the WQV.

Vpp ≥ 0.75 x WQV
Where
WQV = water quality volume (ft3).

Step 3 – Determine Basin Depths and Surface Areas
Distribution of the Constructed Wetland area is needed to achieve desired biodiversity.

Distribute component areas as follows:

Components Percent of Permanent
Pool Surface Area Design Water Depth
Forebay 5-10% 2 to 4 feet
Open-water zone 10-40% 2 to 4 feet
Wetland zones with
emergent vegetation 50-70%
6 to 12 inches
(30% to 50% of this area
should be 6 inches deep
with bottom slope ≤ 10%)
Outlet zone 5-10% 3 feet (minimum)
Estimate average depth of permanent pool (Davg) including all zones.

Estimate the water surface area of the permanent pool (App) based on actual Vpp.

App = Vpp / Davg
Where
Vpp = volume of permanent pool (ft3); and
Davg = average depth of permanent pool (ft).

Estimate water surface elevation of the permanent pool (WS Elevpp) based on site elevations.
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Treatment Control Measure Fact Sheet C-1: Constructed Wetland Development Guidance Manual F-6 May 2025
Step 4 – Determine Surcharge Depth of WQV above Permanent Pool and Maximum Water Surface Elevation
The surcharge depth of the WQV above the permanent pool’s water surface (DWQV) should be ≤ 2.0 feet.
a. Estimate WQV surcharge depth (DWQV) based on App.

DWQV = WQV/App
Where
WQV = water quality volume (ft3); and
App = surface area of permanent pool (ft2).
b. If DWQV > 2.0 feet, adjust value of Vpp and/or Davg to increase App and yield DWQV ≤

2.0. The water surface of the basin will be greater than App when the WQV is added to the permanent pool.
c. Estimate maximum water surface area (AWQV) with WQV based on basin shape. d. Recalculate Final DWQV based on AWQV and App. Note: Vpp and/or Davg can be adjusted to yield Final DWQV ≤ 2.0 feet.

Final DWQV = WQV/((AWQV + App)/2)
Where
WQV = water quality volume (ft3);
AWQV = maximum water surface area (ft2); and
App = surface area of permanent pool (ft2).
e. Calculate maximum water surface elevation in the basin with WQV.

WS ElevWQV = WS Elevpp + Final DWQV
Where
WS Elevpp = water surface elevation of permanent pool (ft); and Final DWQV = final surcharge depth of WQV (ft).

Step 5 – Determine Inflow Requirement (Qinflow)
A net inflow of water must be available through a perennial base flow or supplemental water source. Use the following equation and parameters to estimate the quantity of monthly inflow required at various times of the year. The maximum monthly requirement will govern the design requirement.

Qinflow = QE-P + Qseepage + QET
Where
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Treatment Control Measure Fact Sheet C-1: Constructed Wetland Development Guidance Manual F-7 May 2025
QE-P = Loss due to evaporation minus the gain due to precipitation (acre-ft/mo.) Qseepage = Loss or gain due to seepage to groundwater (acre-ft/mo.) QET = Loss due to evapotranspiration (additional loss through plant area above water surface not including the water surface) (acre-ft/mo.) Step 6 – Design Basin Forebay
The forebay provides a location for the sedimentation of larger particles and has a solid bottom surface to facilitate the mechanical removal of accumulated sediment. The forebay is part of the permanent pool and has a water surface area comprising 5 to 10% of the permanent pool water surface area and a volume comprising 5 to 10% of the WQV. The depth of the permanent pool in the forebay should be 2 to 4 feet. Provide forebay inlet with an energy dissipation structure and/or erosion protection. Trash screens at the inlet are recommended to reduce the dispersion of large trash articles throughout the basin.

Step 7 – Design Outlet Works
Provide outlet works that limit the maximum water surface elevation to WS ElevWQV. The Outlet Works are to be designed to release the WQV over a 24-hour period. Protect the outlet from clogging with a trash rack and a skimmer shield that extends below the outlet and above the maximum WQV depth. A single orifice outlet control is depicted in Figure C1.2. a. For single orifice outlet control or a single row of orifices at the permanent pool elevation (WS Elevpp) use the orifice equation based on the WQV (ft3) and depth of water above orifice centerline D (ft) to determine orifice area (in2):

Orifice Equation

Q = C× A × 2gD
Where Q = Flow rate
C = Orifice coefficient (use 0.61)
A = Area of orifice (in2)
g = Acceleration due to gravity (32.2 ft/sec2)
D = Depth of water above orifice centerline (DWQV in ft) The equation can be solved for A based on the WQV and design drawdown time (t) using the following conversion of the orifice equation
A = 𝑊𝑊𝑊𝑊𝑊𝑊

60.19 𝑥𝑥 𝐷𝐷0.5 𝑥𝑥 𝑡𝑡

Where
WQV = water quality volume (ft3);
D = depth of water above orifice centerline (DWQV); and #6..
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Treatment Control Measure Fact Sheet C-1: Constructed Wetland Development Guidance Manual F-8 May 2025
t = drawdown period (hrs) = 24 hrs
b. For perforated pipe outlets or vertical plates with multiple orifices, use the following equation to determine required area per row of perforations, based on the WQV (acre-ft) and depth of water above centerline of the bottom perforation D (ft).

Area/row (in2) = WQV/K24
Where
WQV = water quality volume (acre-ft);
K24 = 0.012D2 + 0.14D – 0.06 (from Denver UDFCD, 1999); and D = depth of water above orifice centerline (DWQV).

Select the appropriate perforation diameter and number of perforations per row (columns) with the objective of minimizing the number of columns and using a maximum perforation diameter of 2 inches. Rows are spaced at 4 inches on center from the bottom perforation.

Thus, there will be 3 rows for each foot of depth plus the top row. The number of rows (nr) may be determined as follows:
nr = 1 + (D x 3)
Where
D = depth of water above orifice center line.

Calculate total outlet area by multiplying the area per row by the number of rows.

Total Orifice Area = Ar x nr
Where
Ar = required area per row of perforations; and
nr = number of rows.

Step 8 – Design Basin Shape
Whenever possible, shape the basin with a gradual expansion from the inlet and a gradual contraction toward the outlet. The length-to-width ratio should be between 2:1 to 4:1, with 3:1 recommended. Internal baffling with berms or modification of inlet and outlet points may be necessary to achieve this ratio.

Step 9 – Design Basin Side Slopes
Side slopes should be stable and sufficiently gentle to limit rill erosion and facilitate maintenance. Internal side slopes should be no steeper than 4:1; external side slopes should be no steeper than 3:1.

Step 10 – Design Maintenance Access
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Treatment Control Measure Fact Sheet C-1: Constructed Wetland Development Guidance Manual F-9 May 2025
Provide all-weather access for maintenance vehicles to the bottom and outlet works. The maximum grades of access ramps should be 10 percent, and the minimum width should be 16 feet. Ramps should be paved with concrete.

Step 11 – Design Security Fencing
Provide aesthetic security fencing around the basin to protect habitat except when specifically waived by the City. The fencing design shall be approved by the City.

Step 12 – Select Vegetation
Select vegetation from the list of approved plants – Appendix L. Select wetland vegetation appropriate for planting in the wetland bottom. A qualified wetland specialist should be consulted regarding the selection and establishment of plants. The shallow littoral bench should have a 4- to 6-inch layer of organic topsoil. Berms and side-sloping areas should be planted with native or irrigated turf grasses. The selection of plant species for a Constructed Wetland shall take into consideration the water fluctuation likely to occur in the wetland. The permanent pool water level should be controlled as necessary to allow the establishment of wetland plants and raised to the final operating level after plants are established.

Volume Retention Calculation
No volume retention credit is provided for a Constructed Wetland system because it does not fully retain the WQV. However, a Constructed Wetland may be used to meet treatment control requirements.

Construction Considerations
• An impermeable liner may be required to maintain permanent pool levels in areas with porous soils.
• Install seepage collars on outlet piping to prevent seepage through embankments.

Maintenance Requirements
The City requires the execution of a maintenance agreement with the property owner prior to final acceptance of a private development project, which includes treatment controls such as Constructed Wetland. Such agreements will typically include requirements such as those outlined in Table C-1.2. The property owner or his/her designee is responsible for compliance with the agreement. Sample agreements are presented in Appendix I. The maintenance agreement and plan will provide the City designee with complete access to the Device and its immediate vicinity at any time.
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Treatment Control Measure Fact Sheet C-1: Constructed Wetland Development Guidance Manual F-10 May 2025
Table C-1.2. Inspection and Maintenance Requirements for Constructed Wetlands Activity Schedule
Remove litter and debris As required
Inspect basin to identify potential problems such as
trash and debris accumulation, damage from
burrowing animals, and sediment accumulation.

At beginning and end of the wet season. Additional
inspections after periods of heavy runoff are
desirable.

If permitted by the Department of Fish and Wildlife
or other agency regulations, stock basin with
mosquito fish to enhance natural mosquito and
midge control.

As required
Harvest vegetation for vector control and to
maintain open water surface area. Annually or more frequently if required Remove sediment when accumulation reaches 10
percent of original design depth or if resuspension is
observed. (Note: Sediment removal may not be
required in the main pool area for as long as 20
years.)
Clean in early spring so vegetation damaged during
cleaning has time to reestablish.

Clean forebay to minimize frequency of main basin
cleaning. As required

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Treatment Control Measure Fact Sheet C-2: Extended Detention Basin
Development Guidance Manual F-11 May 2025
Description
Extended Detention Basins are permanent
basins formed by excavation and/or
construction of embankments to temporarily
detain the WQV of stormwater runoff to allow
for the sedimentation of particulates to occur
before the runoff is discharged. An Extended
Detention Basin serves to reduce peak
stormwater runoff flow rates, as well as provide
treatment of stormwater runoff. Extended
Detention Basins are typically dry between
storms, although a shallow pool, one to three
feet deep, can be included in the design for
aesthetic purposes and to promote biological
uptake and conversion of pollutants. A bottom
outlet provides a controlled slow release of the
detained runoff over a specified time period. Extended Detention Basins can also be used to provide flood control by including additional storage capacity. The basic elements of an Extended Detention Basin are shown in Figure C-2.1. This configuration is most appropriate for large sites.

Advantages
• May be designed to provide other benefits such as recreation (such as playfields), wildlife habitat, and open space. Safety issues must be addressed. • Relatively easy and inexpensive to build and operate due to its simple design. • Useful in retrofit situations where their low hydraulic head requirements allow them to be sited within the constraints of the existing storm drain system. It can be designed into flood control basins or sometimes retrofitted into existing flood control basins. • It can be designed to meet trash control requirements.

Limitations
• Discharge from Extended Detention Basins may pose a risk to cold-water receiving waters because water retained in the permanent pool is typically heated over time. • Although wet Extended Detention Basins can increase property values, dry Extended Detention Basins can adversely affect the value of nearby properties due to the adverse aesthetics of dry, bare areas and inlet and outlet structures. Appropriate vegetation selection and maintenance can help to mitigate these adverse effects.

Detention Basins. Source: Detention basin
retrofitting. Detention Basin Retrofitting | City of Elgin, Illinois - Official Website. (n.d.). Retrieved October 7, 2021, from https://www.cityofelgin.org/1275/DetentionBasin-Retrofitting.

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Treatment Control Measure Fact Sheet C-2: Extended Detention Basin
Development Guidance Manual F-12 May 2025
Planning and Siting Considerations
• If constructed early in the land development cycle, Extended Detention Basins can serve as sediment trap during construction within the tributary area. • Surface basins are typical, but underground vaults may be appropriate in a small commercial development.
• Tributary Drainage Area: Small to medium-sized regional tributary areas with available open space and drainage areas greater than about five (5) acres; • Land area requirements: Approximately 0.5 to 2 percent of the tributary development area. • Soil Type: Can be used with almost all soils and geology, with minor adjustments for regions with rapidly percolating soils. In these areas, impermeable liners can be installed to prevent groundwater contamination. The base of the basin should not intersect the groundwater table because a permanently wet bottom can become a vector breeding ground. #6..
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Treatment Control Measure Fact Sheet C-2: Extended Detention Basin
Development Guidance Manual F-13 May 2025
Plan View
Section View
Figure C-2.1. Extended Detention Basin Conceptual Layout. Source: Adapted from Urban Storm Drain Criteria Manual Vol 3. – Best Management Practices, Urban Drainage and Flood Control District. (1999, November).

Design Criteria
Principal design criteria for Extended Detention Basins are listed in Table C-2.1. Extended Detention Basins may also serve as a flood control detention basin under the City’s Storm (see Figures 6-17 and 6-18)
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Treatment Control Measure Fact Sheet C-2: Extended Detention Basin
Development Guidance Manual F-14 May 2025
Drainage System Engineer Design Standards. Such dual-purpose basins must also conform to design criteria for detention basins set forth in Chapter 4 of the City’s Standard Specifications.

A Design Data Summary Sheet is provided at the end of this fact sheet.

Table C-2.1. Extended Detention Basin Design Criteria
Design Parameter Design Criteria Notes
Design volume WQV 80% annual capture; 48-hr drawdown
Drawdown time for WQV 48 hr (minimum)
Outlet controls or pumping stations
must be designed to withdraw WQV
over a minimum period of 48 hours
Drawdown time for 50% WQV 12 hr (minimum) Time before release of 50% WQV Basin design volume 120% Percentage of WQV. Provide 20% sediment storage volume.

Inlet/outlet erosion control – Provide energy dissipaters to reduce velocity
Forebay
a. Volume
b. Drain time
5-10%
< 45 min
Percentage of WQV
Low-flow channel
a. Depth
b. Flow capacity
9 in.
200% Percentage of forebay outlet release
capacity
Upper stage
a. Bottom slope
b. Depth
c. Width
2%
2 ft
30 ft
Minimum
Minimum
Length to width ratio 2:1 Minimum (larger preferred)
Bottom stage
a. Volume
b. Depth
5-20%

1.5-3 ft
Percentage of WQV
Deeper than Upper Stage
Freeboard 1 ft Minimum
Embankment side slope (H:V) ≥ 4:1
≥ 3:1
Inside
Outside (without retaining walls)
Maintenance access ramp slope
(H:V) 10:1 or flatter
Maintenance access ramp width 16 ft Minimum – approach paved with asphalt concrete
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Treatment Control Measure Fact Sheet C-2: Extended Detention Basin
Development Guidance Manual F-15 May 2025
Design Procedure (for Trash Control see below)
Step 1 – Calculate Water Quality Volume (WQV)
Using Fact Sheet T-0, determine the tributary drainage area and WQV for 48-hour drawdown.

Step 2 – Determine Minimum Basin Storage Design Volume (VBS) The volume of the basin (VBS) shall be not less than 120% of the WQV. The additional 20 percent provides an allowance for sediment accumulation.

VBS = 1.2 x WQV
Where
WQV = water quality volume (ft3).

Step 3 – Design Outlet Works
The outlet works are to be designed to release the WQV over a 48-hour period, with no more than 50% released in 12 hours. Protect the outlet from clogging with a trash rack and a skimmer shield that extends below the outlet and above the maximum WQV depth. Refer to Figures C-2.2 and C-2.3 for schematics pertaining to structure geometry, grates, trash racks, and screens; outlet type: perforated riser pipe or orifice plate. a. For single orifice outlet control or a single row of orifices at the basin bottom surface elevation (Figure C-2.3), use the orifice equation based on the WQV (ft3) and depth of water above orifice centerline D (ft) to determine orifice area (in2):

Orifice Equation

Q = C× A × 2gD
Where
Q = Flow rate
C = Orifice coefficient (use 0.61)
A = Area of orifice (in2)
g = Acceleration due to gravity (32.2 ft/sec2)
D = Depth of water above orifice centerline (DWQV in ft) The equation can be solved for A based on the WQV and design drawdown time (t) using the following conversion of the orifice equation A = 𝑊𝑊𝑊𝑊𝑊𝑊

60.19 𝑥𝑥 𝐷𝐷0.5 𝑥𝑥 𝑡𝑡
Where
WQV = water quality volume (ft3);
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Treatment Control Measure Fact Sheet C-2: Extended Detention Basin
Development Guidance Manual F-16 May 2025
D = depth of water above orifice centerline (DWQV, ft); and t = drawdown period (hrs) = 48 hrs
b. For perforated pipe outlets or vertical plates with multiple orifices (Figure C-2.2), use the following equation to determine the required area per row of perforations (Ar) based on the WQV (acre-ft) and depth of water above the centerline of the bottom perforation D (ft).

Ar (in2) = WQV/K48
Where
WQV = water quality volume (acre-ft); and
K48 = 0.013D2 + 0.22D – 0.10
Select appropriate perforation diameter and number of perforations per row (columns) with the objective of minimizing the number of columns and using a maximum perforation diameter of 2 inches. Rows are spaced at 4 inches in the center from the bottom perforation. Thus, there will be 3 rows for each foot of depth plus the top row. The number of rows (nr) may be determined as follows:
nr = 1 + (D x 3)
Where
D = depth of water above orifice center line (ft).

Calculate the total outlet area by multiplying the area per row by the number of rows.

Total Orifice Area = Ar x nr
Where:

Ar = required area per row of perforations; and
nr = number of rows.

Step 4 – Provide Trash Rack/Gravel Pack
A trash rack or gravel pack around perforated risers shall be provided to protect outlet orifices from clogging. Trash racks are better suited for use with perforated vertical plates for outlet control and allow easier access to outlet orifices for purposes of inspection and cleaning. The trash rack shall be sized to prevent clogging of the primary water quality outlet without restricting the hydraulic capacity of the outlet control orifices.

Step 5 – Design Basin Shape
Whenever possible, shape the basin with a gradual expansion from the inlet and a gradual contraction from the middle toward the outlet. The length-to-width ratio should be a minimum of 2:1. Internal baffling with berms or modification of inlet and outlet points may be necessary to achieve this ratio.
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Treatment Control Measure Fact Sheet C-2: Extended Detention Basin
Development Guidance Manual F-17 May 2025
Step 6 – Two-Stage Design
A two-stage design, including a pool that often fills with frequently occurring runoff, minimizes standing water and sediment deposition in the remainder of the basin. a. Upper Stage: The upper stage should be a minimum of 2 feet deep, with the bottom sloped at 2 percent toward the low flow channel. The minimum width of the upper stage should be 30 feet.
b. Bottom Stage: The active storage basin of the bottom stage should be 1.5 to 3 feet deeper than the upper stage and store 5 to 20 percent of the WQV. A micro-pool below the active storage volume of the bottom stage, if provided, should be one-half the depth of the top stage or two (2) feet, whichever is greater.

Step 7 – Design Pond Forebay
The forebay provides a location for the sedimentation of larger particles and has a solid bottom surface to facilitate the mechanical removal of accumulated sediment. The forebay has a volume comprising 5 to 10% of the WQV. Provide the forebay inlet with an energy dissipation structure and/or erosion protection. A berm should separate the forebay from the upper stage of the basin. The outlet pipe from the forebay to the low-flow channel should be sized to drain the forebay volume in 45 minutes. The outlet pipe entrance should be offset from the forebay inlet to prevent short-circuiting.

Step 8 – Low-Flow Channel
The low-flow channel conveys flow from the forebay to the bottom stage. Erosion protection should be provided where the low-flow channel enters the bottom stage. The lining of the low-flow channel with concrete is recommended. The depth of the channel should be at least 9 inches. The flow capacity of the channel should be twice the release capacity of the forebay outlet.

Step 9 – Select Vegetation
Select vegetation from the list of approved plants – Appendix L. Bottom vegetation provides erosion protection and sediment entrapment. Basin bottoms, berms, and side slopes may be planted with native grasses or with irrigated turf.

Step 10 – Design Embankment Side Slopes
Design embankments to conform to State of California Division of Safety of Dams requirements if the basin dimensions cause it to fall under that agency’s jurisdiction.

Interior slopes should be no steeper than 4:1 and exterior slopes no steeper than 3:1.

Flatter slopes are preferable.

Step 11 – Inlet/Outlet Design
Basin inlet and outlet points should be provided with an energy dissipation structure and/or erosion protection.

Step 12 – Design Maintenance Access
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Treatment Control Measure Fact Sheet C-2: Extended Detention Basin
Development Guidance Manual F-18 May 2025
Provide all-weather access for maintenance vehicles to the bottom and outlet works. The maximum grades of access ramps should be 10 percent and the minimum width should be 16 feet. Ramps should be paved with concrete.

Step 13 – Provide Bypass
Provide for bypass or overflow of runoff volumes in excess of the WQV. Spillway and overflow structures should be designed in accordance with the applicable standards of the City.

Step 14 – Geotextile Fabric
Non-woven geotextile fabric used in conjunction with gravel packs around perforated risers shall conform to the specifications listed in Table C-2.2.

Table C-2.2. Non-woven Geotextile Fabric Specifications Property Test Reference Minimum
Specification
Grab Strength ASTM D4632 90 lbs
Elongation at peak load ASTM D4632 50%
Puncture Strength ASTM D3787 45 lbs
Permittivity ASTM D4491 0.7 sec-1
Burst Strength ASTM D3786 180 psi
Toughness % Elongation x Grab Strength 5,500 lbs
Ultraviolet Resistance (Percent strength
retained at 500 Weatherometer hours)
ASTM D4355 70%
Adapted from SSPWC, 1997.

Step 15 – Design Security Fencing
Provide aesthetic security fencing around the Extended Detention Basin to protect habitat except when specifically waived by the City. Fencing design shall adhere to the City of Modesto Standard Specifications and be approved by the City.

Design Procedure – Trash Control5
Extended Detention Basins are also known as dry ponds, holding ponds, retarding basins, or dry detention basins. These are typically topographical depressions and may include an underground system of pipes, chambers, concrete vaults, or similar void structures designed to store water. Detention Basins incorporate filtration through media or infiltration to underlying soils. Detention Basins also include wet retention basins designed to contain water on its surface year-round.
5 Based on the State Water Resources Control Board Categorically Certified Multi-Benefit Trash Treatment Systems – Detention Basin information sheet or any subsequent revisions.
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Treatment Control Measure Fact Sheet C-2: Extended Detention Basin
Development Guidance Manual F-19 May 2025
Extended Detention Basins must be designed, constructed, and maintained in accordance with the following five (5) requirements:

1. Trap trash particles that are 5 millimeters or greater at any time during a storm event for the following:
a. The peak flow rate generated by the region specific one-year, one-hour storm event from the applicable sub-drainage area (0.31 inches/hour); or b. The peak flow rate of the corresponding storm drain (if the Basin is designed to treat flows from the corresponding storm drain that is designed for less than the peak flow rate generated from a one-year, one-hour storm event).

2. Extended Detention Basins may include either or both of the following to trap particles for either flow described above in section 1.a or 1.b:
a. A screen at the system’s inlet, overflow, or bypass outlet; or b. An up-gradient structure designed to bypass flows exceeding the flows as described in section 1.a or 1.b6

3. The peak flow rates references in section 1.a shall be calculated using one of the following methods:
a. For small drainage areas (generally less than 50 acres) - the Rational Equation Method, which is expressed as Q = CiA, where:

Q = design flow rate (cubic feet per second),
C = runoff coefficient (dimensionless),
i = design rainfall intensity (0.31 inches/hour), and
A = subdrainage area (acres).
b. For large drainage areas (generally 50 acres or more) – Other accepted hydrologic mathematical methods that more accurately calculate peak flow rates from large drainage areas, provided a registered California-licensed professional engineer documents the calculations within the design plans.

4. For Extended Detention Basins that incorporate groundwater recharge capacity into the sizing of the Basin for the purpose of requirements related to the peak flow rates in item 1, above, the percolation rate below the Basin must either be measured directly or estimated employing conservative hydrogeologic assumptions.

5. A registered California licensed Professional Engineer shall stamp and sign Bioretention System design plans as required by California Business & Professions Code section 6700, et seq.
6 Upon approval by the appropriate Regional Water Quality Control Board Executive Officer, a 5 millimeter screen and/or upgradient structure may not be required if the Basin is designed for flood control from flows generated by very large storm events. #6..
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Treatment Control Measure Fact Sheet C-2: Extended Detention Basin
Development Guidance Manual F-20 May 2025
Vector breeding considerations must also be addressed due to the potential nuisance and human health effects.

Trash controls shall be installed and maintained by the property owner for all storm drains, catch basins, or inlets that are located within the boundaries of the parcel and that are operated and maintained by the property owner. Trash controls shall only be installed in storm drains, catch basins, or inlets within the Public RoW if the City has agreed to enter into a maintenance agreement with the property owner (Appendix I). #6..
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Treatment Control Measure Fact Sheet C-2: Extended Detention Basin
Development Guidance Manual F-21 May 2025
Figure C-2.2. Perforated Pipe Outlet Structure. Source: Adapted from Urban Storm Drain Criteria Manual Vol 3. – Best Management Practices, Urban Drainage and Flood Control District. (1999, November).
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Treatment Control Measure Fact Sheet C-2: Extended Detention Basin
Development Guidance Manual F-22 May 2025
Figure C-2.3. Orifice Plate Outlet Configuration. Source: Adapted from Urban Storm Drain Criteria Manual Vol 3. – Best Management Practices, Urban Drainage and Flood Control District. (1999, November).
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Treatment Control Measure Fact Sheet C-2: Extended Detention Basin
Development Guidance Manual F-23 May 2025
Volume Retention Calculation
No volume retention credit is provided for an Extended Detention Basin because it does not fully retain the WQV. However, an Extended Detention Basin may be used to meet treatment control requirements.

Construction Considerations
• Install seepage collars on outlet piping to prevent seepage through embankments. • Clearly mark areas to be used for Extended Detention Basins before site work begins to avoid soil disturbance and compaction during construction.

Maintenance Requirements
The City requires the execution of a maintenance agreement with the property owner prior to the final acceptance of a private development project, which includes treatment controls such as Extended Detention Basins. Such agreements will typically include requirements such as those outlined in Table C-2.3. The property owner or his/her designee is responsible for compliance with the agreement. The maintenance agreement and plan will provide the City designee with complete access to the device and its immediate vicinity at any time.

Maintenance of the device is the sole responsibility of the owner.

Trash Considerations
Because regular maintenance is required to maintain adequate trash capture capacity and to ensure that captured trash does not migrate offsite, the project applicant shall establish a maintenance schedule based on:
a. The maintenance frequency as required in the applicable State or Regional Water Board stormwater permit; and,
b. Site-specific factors including the design trash capture capacity of the Extended Detention Basin, local storm frequency, and characterization of trash and vegetation accumulation in the corresponding sub-drainage area.
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Treatment Control Measure Fact Sheet C-2: Extended Detention Basin
Development Guidance Manual F-24 May 2025
Table C-2.3. Inspection and Maintenance Requirements for Extended Detention Basins Activity Schedule
Remove litter and debris from the banks and basin bottom. As required Inspect Extended Detention Basin for the following items: clogging of outlet; differential settlement; cracking; erosion; leakage; tree growth on the embankment; the condition of riprap in the inlet, outlet, and pilot channels; sediment accumulation in the basin; trash and debris accumulation; damage from burrowing animals; and the health and density of grass turf on the basin side slopes and floor.

Correct observed problems as necessary.

At beginning and end of the
wet season. Additional
inspections after periods of
heavy runoff are desirable.

If permitted by the Department of Fish and Wildlife or other agency regulations, stock basin with mosquito fish to enhance natural mosquito and midge control.

As required
Remove sediment when accumulation reaches 25 percent of original design depth or if resuspension is observed. (Note: Sediment removal may not be required in the main basin for as long as 20 years.)
Clean in early spring so
vegetation damaged during
cleaning has time to
reestablish.

Clean forebay to minimize frequency of main basin cleaning. As required Trim vegetation and inspect monthly to prevent the establishment of woody vegetation and for aesthetic and vector reasons.

At beginning and end of the
wet season.

Control mosquitoes. As necessary
For trash control, regular maintenance is required to maintain adequate trash capture capacity and ensure that trapped trash does not migrate offsite. The owner should establish a maintenance schedule based on site-specific factors, including the size of the detention BMP, storm frequency, and estimated or measured trash loading area. #6..
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Treatment Control Measure Fact Sheet C-3: Wet Pond
Development Guidance Manual F-25 May 2025
Description
Wet Ponds are open earthen basins that feature a
permanent pool of water that is displaced by
stormwater runoff, in part or in total, during storm
runoff events. Like Extended Detention Basins, Wet
Ponds are designed to temporarily retain the WQV of
stormwater runoff and slowly release this volume
over a specified period (12 hours). Wet Ponds differ
from Extended Detention Basins in that the influent
stormwater runoff mixes with and displaces the
permanent pool as it enters the basin. The design
drawdown time for Wet Ponds (12 hours) is shorter
than for Extended Detention Basins (48 hours),
because enhanced treatment is provided in the
permanent pool. Wet Ponds differ from Constructed
Wetlands in having a greater average depth. A dryweather base flow is required to maintain a permanent pool. The primary removal mechanism is settling, but pollutant removal, particularly nutrients, also occur through biological activity in the pond. The basic elements of a Wet Pond are shown in Figure C-3.1.

Advantages
• Wet Ponds can be designed to provide other benefits such as recreation, wildlife habitat, and open space.
• Wet Ponds are often viewed as a public amenity when integrated into a park or open-space setting.
• The permanent pool can provide significant water quality improvement across a relatively broad spectrum of constituents including dissolved nutrients. • Can serve essentially any size tributary area.

Limitations
• Public safety must be considered with respect to access and use. • Potential for mosquito and midge breeding exists due to permanent water pool. • Discharge from Wet Ponds may pose a risk to cold-water receiving waters because water retained in the permanent pool is typically heated over time. • Base flow or supplemental water is required if water level is to be maintained, although ponds may be allowed to dry out during the dry season if non-stormwater flows are negligible.
• Algae growth may be a potential issue if the permanent water pool is maintained during the summer dry season.
• Ponds require a relatively large footprint.

Wet Pond - Charles River Watershed Association
Source: Low Impact Best Management Practice
(BMP) Information Sheet. www.charlesriver.org
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Treatment Control Measure Fact Sheet C-3: Wet Pond
Development Guidance Manual F-26 May 2025
• Depending on volume and depth, pond designs may require approval from the State Division of Safety of Dams.

Planning and Siting Considerations
• Wet Ponds are appropriate for use in the following settings:
o Where there is a need to achieve a reasonably high level of dissolved contaminant removal and/or sediment capture;
o Where base flow rates or other channel flow sources are relatively consistent yearround; or o In residential settings where aesthetic and wildlife habitat benefits can be appreciated and maintenance activities are likely to be consistently undertaken. • Wet Ponds are not suitable for:
o Dense urban areas;
o Sites with steep, unstable slopes; or
o Areas with long dry periods and high evaporation rates without a perennial groundwater base flow or supplemental water supply to maintain the permanent pool. • Tributary drainage areas are typically small to medium-sized regional areas greater than approximately 10 acres with available open space.
• Land area requirements are approximately two to three percent of the tributary development area.
• Most appropriate for sites with low-permeability soils (Types C or D).

Design Criteria
The principal design criteria for Wet Ponds are listed in Table C-3.1. #6..
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Treatment Control Measure Fact Sheet C-3: Wet Pond
Development Guidance Manual F-27 May 2025
Table C-3.1. Wet Pond Design Criteria
Design Parameter Design Criteria Notes
Design volume WQV 80% annual capture; and 12-hr drawdown Maximum drawdown time for
WQV 12 h Based on WQV
Minimum permanent pool
volume 100-150% Percentage of WQV
Inlet/outlet erosion control – Provide energy dissipaters to reduce velocity Forebay
a. Volume
b. Drain time
c. Depth
5-10%
< 45 min
2 to 4 ft
Percentage of WQV
Littoral Zone
a. Area
b. Depth
25-40%
6-18 in Percentage of permanent pool surface area
Deeper Zone
a. Area (including forebay)
b. Depth
c. Maximum depth
55-65%
4-8 ft
12 ft
Percentage of permanent pool surface area
Average depth
Pond length to width ratio 2:1 Minimum (larger preferred) Bottom width 30 ft Minimum
Pond freeboard 1 ft Minimum
Embankment side slope (H:V)
≥ 4:1
≥ 3:1
Inside
Outside (without retaining walls)
Maintenance access ramp slope
(H:V) 10:1 or flatter
Maintenance access ramp width 16 ft Minimum – approach paved with asphalt concrete
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Treatment Control Measure Fact Sheet C-3: Wet Pond
Development Guidance Manual F-28 May 2025
Plan View
Section View
Figure C-3.1. Conceptual Layout of Wet Pond. Source: Adapted from Urban Storm Drain Criteria Manual Vol 3. – Best Management Practices, Urban Drainage and Flood Control District. (1999, November).
(see Figures 6-17 and 6-18)
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Treatment Control Fact Sheet C-3: Wet Pond
Development Guidance Manual F-29 May 2025
Design Procedure
Step 1 – Calculate Water Quality Design Volume (WQV)
Using the Calculation of Stormwater Quality Design Flow and Volume Fact Sheet, determine the tributary drainage area and WQV for 12-hour drawdown.

Step 2 – Determine Minimum Volume for Permanent Pool (Vpp) The volume of the permanent pool (Vpp) shall be not less than 100% and up to 150% of the WQV.

Vpp = 1.0 to 1.5 x WQV
Where
WQV = water quality volume (ft3).

Step 3 – Determine Depth Zones
Distribution of the permanent pool area is needed to achieve the desired biodiversity. In addition to the forebay, two depth zones are required (Figure C-3.2). The Littoral Zone provides for aquatic plant growth along the perimeter of the pool. The Deeper Zone covers the remaining pond area and promotes sedimentation and nutrient uptake by phytoplankton. Distribute component areas as follows:

Components Percent of Permanent
Pool Surface Area Design Water Depth
Forebay 5-10% 2 to 4 feet
Littoral Zone 25-40% 6 to 18 inches
Deeper Zone 55-65% 4 to 8 feet average; 12 foot
max
Estimate average depth of permanent pool (Davg) including all zones.

Estimate the water surface area of the permanent pool (App) based on actual Vpp.

App = Vpp / Davg
Where
Vpp = minimum volume of permanent pool (ft3); and
Davg = average depth of permanent pool (ft).

Estimate water surface elevation of the permanent pool (WS Elevpp) based on site elevations.

Step 4 – Determine Inflow Requirement (Qinflow)
A net inflow of water must be available through a perennial base flow or supplemental water source. Use the following equation and parameters to estimate the quantity of #6..
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Treatment Control Fact Sheet C-3: Wet Pond
Development Guidance Manual F-30 May 2025
monthly inflow required at various times of the year. The maximum monthly requirement will govern the design requirement.

Qinflow = QE-P + Qseepage + QET
Where
QE-P = Loss due to evaporation minus the gain due to precipitation (acre-ft/mo.) Qseepage = Loss or gain due to seepage to groundwater (acre-ft/mo.) QET = Loss due to evapotranspiration (additional loss through plant area above water surface not including the water surface) (acre-ft/mo.)
Step 5 – Design Pond Forebay
The forebay provides a location for the sedimentation of larger particles and has a solid bottom surface to facilitate the mechanical removal of accumulated sediment. The forebay is part of the permanent pool and has a volume comprising 5 to 10% of the WQV. The depth of the permanent pool in the forebay should be 2 to 4 feet. Provide the forebay inlet with an energy dissipation structure and/or erosion protection. A berm consisting of rock and topsoil mixture should be part of the littoral bench to create the forebay and have a minimum top width of 8 feet and side slopes no steeper than 4:1. Trash screens at the inlet are recommended to reduce the dispersion of large trash articles throughout the basin.

Step 6 – Design Outlet Works
The outlet works are to be designed to release the WQV over a 12-hour period. Protect the outlet from clogging with a trash rack and a skimmer shield that extends below the outlet and above the maximum WQV depth. An outlet works for a Wet Pond, is depicted in Figure C-3.3.
a. For single orifice outlet control or a single row of orifices at the permanent pool elevation (WS Elevpp) use the orifice equation based on the WQV (ft3) and depth of water above orifice centerline D (ft) to determine orifice area (in2):

Orifice Equation

Q = C× A × 2gD
Where
Q = Flow rate
C = Orifice coefficient (use 0.61)
A = Area of orifice
g = Acceleration due to gravity (32.2 ft/sec2)
D = Depth of water above orifice centerline (DWQV)
The equation can be solved for A based on the WQV and design drawdown time (t) using the following conversion of the orifice equation:
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Treatment Control Fact Sheet C-3: Wet Pond
Development Guidance Manual F-31 May 2025
A= 𝑊𝑊𝑊𝑊𝑊𝑊

60.19 𝑥𝑥 𝐷𝐷0.5 𝑥𝑥 𝑡𝑡
Where
t = drawdown period (hrs) = 12 hrs
b. For perforated pipe outlets or vertical plates with multiple orifices use the following equation to determine required area per row of perforations, based on the WQV (acreft) and depth of water above centerline of the bottom perforation D (ft).

Ar (in2) = WQV/K12
Where
WQV = water quality volume (acre-ft); and
K12 = 0.008D2 + 0.056D – 0.012
Select the appropriate perforation diameter and number of perforations per row (columns) with the objective of minimizing the number of columns and using a maximum perforation diameter of 2 inches. Rows are spaced at 4 inches in center from the bottom perforation.

Thus, there will be three rows for each foot of depth plus the top row. The number of rows (nr) may be determined as follows:
nr = 1 + (D x 3)
Calculate the total outlet area by multiplying the area per row by the number of rows.

Total Orifice Area = Ar x nr
Step 7 – Design Basin Shape
Whenever possible, shape the basin with a gradual expansion from the inlet and a gradual contraction toward the outlet. The length-to-width ratio should be between 2:1 to 4:1, with 3:1 recommended. Internal baffling with berms or modification of inlet and outlet points may be necessary to achieve this ratio.

Step 8 – Design Embankment Side Slopes
Side slopes should be stable and sufficiently gentle to limit rill erosion and to facilitate maintenance. Interior slopes should be no steeper than 4:1 and exterior slopes no steeper than 3:1. Flatter slopes are preferable.

Side slopes above the permanent pool should be no steeper than 4:1, preferably 5:1 or flatter.

The littoral zone should be very flat (40:1 or flatter), with the depth ranging from 6 inches near the shore and extending to no more than 12 inches at the furthest point from the shore.

The side slope below the littoral zone shall be 3:1 or flatter. #6..
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Treatment Control Fact Sheet C-3: Wet Pond
Development Guidance Manual F-32 May 2025
Step 9 – Inlet/Outlet Design
Basin inlet and outlet points should be provided with an energy dissipation structure and/or erosion protection.

Step 10 – Design Maintenance Access
Provide all-weather access for maintenance vehicles to the bottom and outlet works. The maximum grades of access ramps should be 10 percent and minimum width should be 16 feet. Ramps should be paved with concrete.

Step 11 – Provide Bypass
Provide for bypass or overflow of runoff volumes in excess of the WQV. Spillway and overflow structures should be designed in accordance with the applicable standards of the City.

Step 12 – Provide Underdrains
Provide underdrain trenches near the edge of the pond. The trenches should be no less than 12 inches wide, filled with ASTM C-33 sand to within 2 feet of the pond’s permanent pool water surface, and with an underdrain pipe connected through a valve to the outlet.

These underdrains will permit the drying out of the pond when it has to be “mucked out” to restore the volume lost due to sediment deposition.

Step 13 – Select Vegetation
Select vegetation from the list of approved plants – Appendix L. Bottom vegetation provides erosion protection and sediment entrapment. Berms and side slopes may be planted with native grasses or with irrigated turf. The shallow littoral bench should have a 4- to 6-inch-thick organic topsoil layer and be vegetated with aquatic species.

Step 14 – Design Security Fencing
Provide aesthetic security fencing around the Wet Pond to protect habitat except when specifically waived by the City. Fencing design shall adhere to the City of Modesto Standard Specifications and be approved by the City.

Volume Retention Calculation
No volume retention credit is provided for a Wet Pond because it does not fully retain the WQV. However, Wet Ponds may be used to meet treatment control requirements.

Construction Considerations
• An impermeable liner may be required to prevent infiltration and maintain permanent pool levels in areas with porous soils.
• Install seepage collars on outlet piping to prevent seepage through embankments.

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Treatment Control Fact Sheet C-3: Wet Pond
Development Guidance Manual F-33 May 2025
Figure C-3.2. Depth Zones for Wet Pond. Source: Adapted from Urban Storm Drain Criteria Manual Vol

3. – Best Management Practices, Urban Drainage and Flood Control District. (1999, November) Figure C-3.3. Outlet Works for Wet Pond. Source: Adapted from Urban Storm Drain Criteria Manual Vol

3. – Best Management Practices, Urban Drainage and Flood Control District. (1999, November).

Maintenance Requirements
The City requires the execution of a maintenance agreement with the property owner prior to the final acceptance of a private development project, which includes treatment controls such as Extended Detention Basins. Such agreements will typically include requirements such as those outlined in Table C-3.2. The property owner or his/her designee is responsible for compliance with the agreement. The maintenance agreement and plan will provide the City #6..
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Treatment Control Fact Sheet C-3: Wet Pond
Development Guidance Manual F-34 May 2025
designee with complete access to the device and its immediate vicinity at any time.

Maintenance of the device is the sole responsibility of the owner.

Table C-3.2. Inspection and Maintenance Requirements for Wet Ponds Activity Schedule
Remove litter and debris from the banks and pond bottom. As required Inspect Wet Pond for the following items: clogging of outlet; differential settlement; cracking; erosion; leakage; tree growth on the embankment; the condition of riprap in the inlet, outlet, and pilot channels; sediment accumulation in the basin; trash and debris accumulation; damage from burrowing animals; and the health and density of grass turf on the basin side slopes and floor. Correct observed problems as necessary.

At beginning and end of the
wet season. Additional
inspections after periods of
heavy runoff are desirable.

If permitted by the Department of Fish and Wildlife or other agency regulations, stock basin with mosquito fish to enhance natural mosquito and midge control.

As required
Harvest vegetation for vector control and to maintain effective permanent pool volume.

Annually or more frequently
if required
Remove sediment when accumulation reaches 25 percent of original design depth or if resuspension is observed. (Note: Sediment removal may not be required in the main pool area for as long as 20 years.) Clean in early spring so
vegetation damaged during
cleaning has time to
reestablish.

Clean forebay to minimize frequency of permanent pool cleaning. As required #6..
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Treatment Control Measure Fact Sheet C-4: Proprietary Treatment Control Development Guidance Manual F-35 May 2025
This Development Guidance Manual provides guidance for the selection and design of some of the common on-site stormwater treatment control measures for development projects. The standard treatment control measures included in this Manual are non-proprietary (public domain) designs that have been reviewed and determined to be generally acceptable. As such, the plan check review and approval process will typically be routine for development projects that have selected one or more of these control measures.

However, the City recognizes that these non-proprietary treatment control measures may not be appropriate for all projects due to physical site constraints. The City will allow the use of Proprietary Control Devices if the following criteria are met:
• The VRR has been met through the use of VRMs (Section 5); • The project applicant has demonstrated to the satisfaction of the City that the use of nonproprietary treatment controls is infeasible or impractical.

The most commonly encountered classes of proprietary stormwater quality control measures include hydrodynamic separation, catch basin insert technologies, cartridge filter-type controls, and proprietary biotreatment devices. It should be noted that vault systems should be avoided since they are typically equipped with high-flow rate media filters and are thus, less effective at pollutant removal and more challenging to maintain compared to the landscape-based LID type treatment controls. Therefore, vault systems should be considered only as a last resort, after all other viable alternatives have been thoroughly evaluated.

Proprietary Control Devices that are approved by the City for general use are listed in Appendix N along with the sizing criteria and criteria used for approval (Note: Appendix N does not include devices approved for trash control. For a list of certified and/or agency-approved devices contact the City). This list will be updated periodically when additional devices are added to the approved list.

In general, any Proprietary Control Device must be designed to treat the WQV or the water quality flow (WQF)7. Procedures to calculate the WQV and WQF are provided in Fact Sheet T-0.

Stormwater runoff in excess of the WQV or WQF may be diverted around or through the treatment device. Any proposed device must include all maintenance, operation, and construction requirements, as indicated in Appendix E and as recommended by the manufacturer.

The City encourages the development of innovative Proprietary Control Devices and may consider an alternative Proprietary Control Device that is not in Appendix N, on a ‘pilot basis.’ In order for a pilot project to be considered, the manufacturer and property owner must commit to participating in and funding a monitoring program to verify the device’s performance. Site designers should anticipate additional review time and contact the City early in the process to request consideration of pilot installation projects. If unsuccessful, the property owner will be required to install stormwater control measures as needed to meet the standards outlined in this Manual.
7 Use of alternative sizing criteria is allowed for certain devices as indicated in Appendix N. #6..
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Treatment Control Measure Fact Sheet C-4: Proprietary Treatment Control Development Guidance Manual F-36 May 2025
Description
A Trash Capture Device is a type of treatment
control that either (a) removes pollutants
and/or solids from stormwater runoff or (b)
captures, infiltrates, and/or reuses stormwater
runoff. A Trash Capture Device can include Full
Capture Systems (FCS) or Low Impact
Development (LID) controls.

For the purposes of this fact sheet and the use
of these controls within the City, C-5 Trash
Capture Devices must be FCS (see below) [note:
multi-benefit, LID, or public domain projects are
defined in Factsheets L-1, L-4, L-5, and C-2].

A Trash Capture Device that is an FCS is a
treatment control, or series of treatment
controls, including but not limited to, a multibenefit project (e.g., a bioretention facility that meets volume retention and trash control requirements, see L-1) or an LID control measure that traps all particles that are 5 mm or greater, and has a design treatment capacity that is either:
a) of not less than the peak flow rate, Q, resulting from a one-year, one-hour storm in the subdrainage area, or
b) appropriately sized to and designed to carry at least the same flows as the corresponding storm drain.

Trash Capture Devices are typically proprietary-based controls that meet the design and maintenance criteria of the Statewide Trash Amendments.4 Only those devices that have been approved by the State Water Resources Control Board may be implemented to meet the Statewide Trash Amendment requirements and within the City.

For a current list of certified and/or agency-approved devices, contact the City (depending on the location of the project).

Advantages
• Prevents trash from being transported into and through the storm drain system (for trash particles >5 mm) into the receiving waters.
• A wide range of proprietary devices provides options and flexibility to allow for site-specific conditions.
• Devices may be sized and custom-fit to existing stormwater infrastructure. 4 https://www.waterboards.ca.gov/water_issues/programs/trash_control/documentation.html Image 6-16. Diagram of a Hydrodynamic Trash Separator.

Source: Environmental Protection Agency. (n.d.). Trash
Capture Technologies. EPA. Retrieved October 7, 2021, from https://www.epa.gov/trash-free-waters/trash-capturetechnologies. #6..
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Treatment Control Measure Fact Sheet C-4: Proprietary Treatment Control Development Guidance Manual F-37 May 2025
• Some devices may be selected or configured to provide additional benefits, such as metal sorption or sediment capture.

Limitations
• Existing storm drain infrastructure may limit the types, size, and/or treatment capacity of some Trash Capture Devices.
• Optimal device performance requires routine maintenance consistent with the manufacturer's specifications.
• Some devices have not been widely used or tested in the field, which could increase the risk for flooding or infrastructure issues.

Planning and Siting Considerations
• Land area and size of the upstream catchment area draining to the Trash Capture Device. • Dimensions and/or condition of the catch basin/vault where the device will be installed. • The degree to which the proposed catch basin is connected to other catch basins. • Other requirements such as permitting, construction, and utility clearance (especially for retrofits).
• Cost of construction and long-term maintenance.

Design Criteria5
Principal design criteria for Trash Capture Devices are defined by the manufacturer. Refer to the list of State Water Resources Control Board certified devices that can be installed and consult with the applicable manufacturer. In order for the Trash Capture Device to meet the requirements of the Statewide Trash Amendments, the device must be designed, constructed, and maintained in accordance with the following four (4) requirements:

2. Trash Capture Devices may include either or both of the following to trap particles for either flow described above in section 1.a or 1.b:
a. A screen at the system’s inlet, overflow, or bypass outlet; or 5 Based on the State Water Resources Control Board Categorically Certified Multi-Benefit Trash Treatment Systems – List of Certified Trash Full Capture Systems Available to the Public
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Treatment Control Measure Fact Sheet C-4: Proprietary Treatment Control Development Guidance Manual F-38 May 2025
b. An up-gradient structure designed to bypass flows exceeding the flows as described in section 1.a or 1.b6.

4. A registered California licensed Professional Engineer shall stamp and sign the design plans as required by California Business & Professions Code section 6700, et seq.

Vector breeding considerations must also be addressed due to the potential nuisance and human health effects. Prior to installation of any Trash Capture Device, the local mosquito vector control district should be contacted to ensure the installation conforms to the local district’s visual inspection, treatment, and vector breeding minimizing guidelines.

Design Procedure
While the design procedure for a given project may vary, a general process for the design of Trash Capture Devices has been outlined below.

Step 1 – Evaluate maps and site location.

Evaluate the site location for stormwater infrastructure connectivity (i.e., storm drain lines, inlets, outfalls, and catchments) and identify the potential locations for the installation of a Trash Capture Device.
6 Upon approval by the appropriate Regional Water Quality Control Board Executive Officer, a 5 millimeter screen and/or upgradient structure may not be required if the Basin is designed for flood control from flows generated by very large storm events. #6..
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Treatment Control Measure Fact Sheet C-4: Proprietary Treatment Control Development Guidance Manual F-39 May 2025
Step 2 – Delineate catchment drainage areas.

Conduct a GIS Desktop analysis or field visit to define the boundary of each stormwater catchment associated with a potential Trash Capture Device.

Step 3 – Identify appropriate Trash Capture Devices for catchment drainage areas.

Obtain the list of certified and/or agency-approved Trash Capture Devices from the City and identify the number and type of devices needed to treat the catchment drainage areas.

Step 4 – Conduct field evaluations/surveys to further identify site constraints and design criteria required by the manufacturer of the selected Trash Capture Device.

Using design criteria and specifications for the selected Trash Capture Devices, conduct a field evaluation to identify any potential site constraints and to obtain the design criteria specified.

Construction Considerations
• If a site is deemed to be technically infeasible for installation, the relocation of a Trash Capture Device upstream may be able to account for site-specific constraints. • Trash Capture Devices shall be installed within the boundary of the project site location and not within the public right-of-way unless a prior agreement with the City has been reached.

Long-Term Maintenance
The City requires the execution of a maintenance agreement with the property owner prior to the final acceptance of a private development project, which includes treatment controls such as Trash Capture Devices. Such agreements will typically include requirements such as those outlined in Table C-5.1; however, the requirements are expected to be specific to each device and based on the manufacturer’s recommendations.

The property owner or his/her designee is responsible for compliance with the agreement. The maintenance agreement and plan will provide the City designee with complete access to the device and its immediate vicinity at any time. Maintenance of the device is the sole responsibility of the owner.
#6..
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Treatment Control Measure Fact Sheet C-4: Proprietary Treatment Control Development Guidance Manual F-40 May 2025
Table C-5.1. Example Inspection and Maintenance Requirements for Trash Capture Devices Activity Schedule
Cleaning devices A minimum of once per seasonal cycle. Additional inspections after periods of heavy runoff are desirable.

Cleaning of inlet filters
A minimum of three times per seasonal cycle.

Additional inspections after periods of heavy runoff are desirable.

Inspection and cleaning (applicable to all devices)
Follow manufacturers’ recommendations to ensure
devices operate at desired effectiveness. Make
necessary repairs in a timely manner.
#6..
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Treatment Control Measures Fact Sheet C-6: Media Filter Development Guidance Manual F-41 May 2025
Description
A Media Filter is a two-stage treatment
system including a pretreatment settling
basin and a filter bed containing sand or
other absorptive filtering media. The filter
bed is supported by a gravel base with an
underdrain system. As stormwater runoff
flows into the system, large particles settle
out in the first basin and finer particles and
other pollutants are removed in the
filtration bed. Three variations of public
domain Media Filters are presented –
above-ground Austin Sand Filter, DC
Underground Sand Filter, and the linear or
perimeter (Delaware) Sand Filter.

A typical configuration for an Austin sand filter is presented in Figure C-6.1. Principal components of the unit include a sedimentation basin and a filter bed. The sedimentation basin is designed to hold the entire WQV and to release that volume to the filter bed over the design drawdown time of 48 hours. Large sediment is removed through settling in the sedimentation basin. Fine particles and other pollutants are removed in the filtration basin as stormwater runoff passes through the filter media. Stormwater runoff volumes in excess of the WQV are bypassed around the unit.

A typical District of Columbia (DC) Underground Sand Filter, which was developed by the DC Environmental Regulation Administration, is presented in Figure C-6.2. The DC Underground Sand Filter is in a structural shell with three chambers. The shell may consist of precast or cast-in-place concrete. The plunge pool in the first chamber and the throat of the chamber, which are hydraulically connected by an underwater rectangular opening, absorb energy, and provide pretreatment, trapping grit and floating organic material such as oil, grease, and leaves. The second chamber contains a typical sand filter with a subsurface drainage system consisting of a perforated PVC pipe in a stone bed. The third chamber, or clearwell, collects the flow from the underdrain pipes, and overflow pipes when installed, and directs treated stormwater runoff to the storm drain system.

A typical Delaware sand filter is presented in Figure C-6.3. The system consists of two parallel concrete trenches, sedimentation, and filter, divided by a close-spaced wall. Stormwater runoff enters the sedimentation trench and causes the sedimentation pool to rise and overflow into the filter trench through weir notches at the top of the dividing wall. The weirs allow stormwater runoff to enter the filter bed as sheet flow to prevent scouring the sand. The permanent pool in the sedimentation trench is dead storage, which inhibits resuspension of particles deposited from earlier storm events and prevents heavier sediment from being washed into the filtration trench.

Floatable materials can reach the filter media through the surface overflow. Stormwater runoff entering the filtration trench is treated with media filtration and removed from the filtration unit through an underdrain pipe where it can be discharged to the storm drain system. Stormwater runoff volumes larger in excess of the WQV are bypassed around the filtration unit.

Media Filter
Photo Credit: Sacramento Stormwater Quality Partnership #6..
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Treatment Control Measures Fact Sheet C-6: Media Filter Development Guidance Manual F-42 May 2025
Figure C-6.1. Austin Sand Filter Schematic
#6..
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Treatment Control Measures Fact Sheet C-6: Media Filter Development Guidance Manual F-43 May 2025
Figure C-6.2. DC Underground Sand Filter Schematic
Figure C-6.3. Delaware Sand Filter Schematic
#6..
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Treatment Control Measures Fact Sheet C-6: Media Filter Development Guidance Manual F-44 May 2025
Advantages
• Provides effective treatment through settling and filtering while requiring relatively small space.
• Can be placed below ground.
• Suitable for use when water is not available for irrigation or base flow. • Suited for most site conditions. The presence of permeable soils is not a requirement. • Reduces peak stormwater runoff flows during small storm events.

Disadvantages
• Potential for clogging of media. Pretreatment or upstream treatment control measures to remove large sediment may be required to minimize or prevent media clogging. • Significant head loss through treatment units may limit use on flat surfaces. • May be more expensive to construct than many other types of treatment control measures.

Planning and Site Considerations
• Media sand filters are generally suited for sites where there is no base flow, and the sediment load is relatively low.
• Media sand filters are well suited for drier areas and/or urban areas because they do not require vegetation and require less surface space than many other treatment control measures.
• Selection of a media sand filter type depends on the size of the drainage area and the facility location. For large watersheds (i.e., up to 50 acres), an Austin Sand Filter is recommended. For small catchments requiring underground facilities (i.e., up to 1.5 acres), a DC Underground Sand Filter is recommended. Delaware Sand Filters are especially suitable for paved sites and industrial sites (up to 5 acres) because they can be situated to accept sheet flow from adjacent pavement.
• Because the filter media is imported sand or engineered adsorptive material, Media Filters are suited for most soil conditions, and the presence of permeable soils is not a requirement. • Approximately four (4) feet of hydraulic head is required to achieve design flow through the Austin and DC Underground Sand Filters. Delaware Sand Filters can operate with as little as two (2) feet of hydraulic head.
• For underground Media Filters, the load-carrying capacity of the filter structure must be considered if it is located under parking lots, driveways, roadways, and certain sidewalks. • Potential for mosquitoes due to standing water will be greatly reduced or eliminated if the Media Filter is properly designed, constructed, and operated to maintain its infiltration capacity.

Design Criteria
Principal design criteria for Austin Sand Filters are presented in Table C-6.1.

Principal design criteria for DC Underground Sand Filters are presented in Table C-6.2.

Principal design criteria for Delaware Sand Filters are presented in Table C-6.3. #6..
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Treatment Control Measures Fact Sheet C-6: Media Filter Development Guidance Manual F-45 May 2025
Table C-6.1. Austin Sand Filter Design Criteria
Design Parameter Design Criteria Notes
Sedimentation Basin
Tributary drainage area
≤ 50 acres
Design volume WQV
Sedimentation basin drawdown
time
40 hr (maximum) Based on WQV
Basin water depth 3-10 ft
Length to width ratio 2:1 (minimum)
Freeboard 1 ft Above maximum water surface
elevation
Inlet velocity
3 ft/sec
(maximum)
Provide inlet energy dissipater as
required to limit inlet velocity
Filtration Basin
Storage volume above filter bed 20% (minimum) Based on WQV Storage depth above filter bed 3 ft (minimum)
Gravel depth over sand filter 2 in (minimum)
Sand depth in filter bed 18 in (minimum)
Permeability coefficient for sand
filter

3.5 ft/day
Sand filter surface slope 3.5 ft/day
Sand filter surface slope 0%
Grave cover over underdrain 2 in (minimum)
Sand size (diameter) 0.02-0.04 in
Underdrain gravel size
(diameter)

0.5-2 in
Inside diameter of underdrain 6 in (minimum)
Underdrain pipe type PVC Schedule 40 or heavier
Underdrain slope 1% (minimum)
Underdrain perforation diameter 3/8 in (minimum)
Perforations per row 6 (minimum)
Space between perforation rows 6 in (minimum)
Gravel bed depth 16 in (minimum)
#6..
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Treatment Control Measures Fact Sheet C-6: Media Filter Development Guidance Manual F-46 May 2025
Table C-6.2. DC Underground Sand Filter Design Criteria Design Parameter Design Criteria Notes
Tributary drainage area ≤ 1.5 acres
Design volume WQV
Drawdown time 48 hr (maximum) Based on WQV
Gravel depth over filter media 2 in (minimum)
Sand filter depth 18 in (minimum)
Depth of cover gravel over
underdrain pipe
2 in (minimum)
Filter coefficient
2 ft/day
Volume of WQV in sediment
chamber
20% (minimum)
Underdrain slope
1% (minimum)
Diameter of upper level gravel
cover
1 in (maximum)
Clearwell length 3 ft (minimum)
Filter sand sizing – ASTM C 33 concrete sand
Gravel diameter size in underdrain 0.5-2 in (minimum)
Underdrain pipe size 6 in (minimum)
Schedule 40 reinforced PVC pipe
Perforation diameter in drainage
pipe
3/8 in (minimum)
Spacing between perforations
6 in (maximum)
Spacing between underdrain pipes 27 in (maximum) Center to center #6..
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Treatment Control Measures Fact Sheet C-6: Media Filter Development Guidance Manual F-47 May 2025
Table C-6.3. Delaware Sand Filter Design Criteria
Design Parameter Design Criteria Notes
Tributary drainage area ≤ 5 acres
Design volume WQV
Drawdown time 48 hr Based on WQV
Weir height between
sedimentation chamber and sand
filter
2 in
Gravel depth over sand 2 in (minimum)
Sand depth 18 in (minimum)
Gravel underdrain depth 16 in (minimum
Filter coefficient 2 ft/day
Top layer/underdrain gravel size,
diameter

0.5-2 in
Sand size - ASTM C33 concrete sand
Top layer slope 0%
Underdrain or bottom of filter
slope 0.5% (minimum)
Underdrain pipe size, diameter 6 in (minimum) PVC Schedule 40 Perforation size, diameter 3/8 in (minimum)
Number of holes per row 6 (minimum)
Spacing between rows 6 in (minimum)
Weephole diameter 3 in (minimum
Spacing between weepholes 9 in (minimum) Center to center Sedimentation chamber and sand
filter width
18-30 in
#6..
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Treatment Control Measures Fact Sheet C-6: Media Filter Development Guidance Manual F-48 May 2025
Design Procedures
The design procedures for an Austin sand filter system are listed below:

Step 1 – Calculate WQV
Using Fact Sheet C-2, calculate the effective tributary drainage area and WQV.

Step 2 – Determine Sedimentation Basin Volume (Vsb)
The sedimentation basin volume must be greater than or equal to the WQV.

Where
WQV = water quality volume (ft3)
Step 3 – Determine Sedimentation Basin Depth (dsb)
The allowable depth of water in the sedimentation basin (dsb) is limited by the available hydraulic head at the project site, which is based on the difference in elevation between the sedimentation basin inlet and filter bed outlet. The design sedimentation basin depth should be between three (3) and ten (10) feet. Select a design depth in the allowable range that yields the required Vsb given any footprint area constraints for the project site.

Step 4 – Determine Sedimentation Basin Area (Asb)
Where
Vsb = sedimentation basin volume (ft3); and
dsb = sedimentation basin depth (ft)
Step 5 – Determine Sedimentation Basin Shape
Determine overall length (Lsb) and width (Wsb) dimensions to yield the Asb for any given footprint constraints for the project site.

Where
Lsb = sedimentation basin length (ft); and
wsb = sedimentation basin width (ft).

The length-to-width ratio should be a minimum of 2:1. Internal baffling may be necessary to achieve this ratio and to mitigate short-circuiting and/or dead storage problems. #6..
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Treatment Control Measures Fact Sheet C-6: Media Filter Development Guidance Manual F-49 May 2025
If the sedimentation basin is not rectangular, shape the basin with a gradual expansion from the inlet and a gradual contraction toward the outlet. The sedimentation basin design should maximize the distance from where the heavier sediment is deposited near the inlet to where the outlet structure is located. This configuration will improve basin performance and reduce maintenance requirements.

Step 6 – Determine Sedimentation Basin Inlet/Outlet Design The sedimentation basin inlet and outlet points should be provided with energy dissipation structures and/or erosion protection. Energy dissipation devices may be necessary to reduce inlet velocities that exceed three (3) feet per second.

An outlet works must be provided that is designed to release the WQV to the filtration basin over a 40-hour period. See Fact Sheet C-3 Extended Detention Basin for outlet works design procedures.

Step 7 – Determine Sedimentation Basin Outlet Trash Rack Design A trash rack or gravel pack around perforated risers shall be provided to protect outlet orifices from clogging. Trash racks are better suited for use with perforated vertical plates for outlet control and allow easier access to outlet orifices for the purpose of inspection and cleaning.

Trash racks shall be sized to prevent clogging of the primary water quality outlet without restricting the hydraulic capacity of the outlet control orifices.

Step 8 – Determine Sediment Trap Design (Optional)
A sediment trap is a storage area that captures and removes sediment from the basin flow regime. In doing so, the sediment trap inhibits resuspension of solids during subsequent stormwater runoff events and improves long-term removal efficiency. The trap also maintains adequate volume to hold the WQV that would otherwise be partially lost due to sediment storage. Sediment traps may reduce maintenance requirements by reducing the frequency of sediment removal. It is recommended that the sediment trap volume be equal to ten (10) percent of the sedimentation basin volume. All water collected in the sediment trap shall drain within 40 hours. The drain pipe invert should be above the filtration basin sand bed surface. The minimum piping grading to the filtration basin should be 1/4 inch per foot, or two (2) percent slope. Access for cleaning the sediment trap drain system is necessary.

Step 9 – Determine Sedimentation Basin Liner Design
If the sedimentation basin is an earthen structure, and an impermeable liner is required to protect groundwater quality, the impermeable liner shall provide a maximum permeability of 1 x 10-6 cm/sec (ASTM Method D-2434). If an impermeable liner is not required, then a geotextile fabric liner, which meets the specifications listed in Table C-6.4, shall be installed unless the sedimentation basin is excavated to bedrock. #6..
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Treatment Control Measures Fact Sheet C-6: Media Filter Development Guidance Manual F-50 May 2025
Step 10 – Determine Minimum Filtration Basin Storage Volume (Vfbs) The filtration basin storage capacity above the filter media surface should be greater than or equal to 20 percent of the WQV. This capacity is necessary to account for backwater effects resulting from partially clogged filter media.

Where
WQV = water quality volume (ft3)
Step 11 – Determine Filter Bed Surface Area (Avbs)
Surface area is the primary design parameter for the filter bed and is a function of sand permeability, filter bed depth, hydraulic head, and filtration rate. The filter bed area should be the larger of the minimum area required for storage (Aff) and the minimum area required for flow (Afbs).

Determine minimum filter surface area required for storage (Afbs) Where
Vfbs = Storage volume above filter bed (ft3); and
dfbs = Depth of storage above filter bed (ft), 3 ft minimum Determine minimum filter surface area required for flow (Aff).

Where
WQV = water quality volume (ft3
);
df = filter bed depth (ft);
k = sand filter permeability coefficient (ft/hr) = 0.146 ft/hr; dfbs = depth of storage above filter bed (ft); and
tf = drawdown time for filter (hr) = 40 hr.

Use the larger of Afbs or Aff as the design value for filter bed area. #6..
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Treatment Control Measures Fact Sheet C-6: Media Filter Development Guidance Manual F-51 May 2025
Step 12 – Design Filtration Basin Inlet Structure
The inlet structure should spread flow uniformly across the filter media surface. Flow spreaders, weirs, or multiple orifice openings are recommended.

Step 13 – Design Filter Bed
The filter (sand) bed may be either of the two configurations described below. Note that sand bed depths are final, consolidated depths. Consolidation effects must be taken into account.

Configuration A – Sand Bed with Underdrain
The sand layer shall be a minimum depth of 18 inches and shall consist of 0.02-0.04 inch diameter sand. Below the sand is a layer of 0.5-2 inch diameter gravel that provides a minimum of two (2) inches of cover over the top of lateral underdrain pipes. #6..
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Treatment Control Measures Fact Sheet C-6: Media Filter Development Guidance Manual F-52 May 2025
No gravel is required under the lateral pipes. A layer of geotextile fabric meeting the specifications in Table C-6.4 must separate the sand and gravel and must be wrapped around the lateral pipes.

Drainage matting meeting the specifications in Table C-6.4 should be placed under the lateral pipes to provide adequate vertical and horizontal hydraulic conductivity to the later pipes.

In areas with high sediment load (total suspended solids concentrations greater than or equal to 200 mg/L0, the two- (2-) inch layer of stone on top of the sand filter should be underlain with Enkadrain 9120 filter fabric or equivalent meeting the specifications in Table C-6.4.

Configuration B – Sand Bed with Trench Underdrain (Figure C-6.4) The top layer shall be 12-18 inches of 0.02-0.04 inch diameter sand. Lateral pipes shall be placed in trenches with a covering of 0.5-2 inch diameter gravel and geotextile fabric. The lateral pipes shall be underlain by a layer of drainage matting (Table C-6.4).

In areas with high sediment load (total suspended solids concentrations greater than or equal to 200 mg/L, the two- (2-) inch layer of stone on top of the sand filter should be underlain with Enkadrain 9120 filter fabric or equivalent meeting the specifications in Table C-6.4.

Figure C-6.4. Austin Sand Filter Bed with Underdrain
#6..
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Treatment Control Measures Fact Sheet C-6: Media Filter Development Guidance Manual F-53 May 2025
Figure C-6.5. Austin Sand Filter Bed with Trench Underdrain Step 14 – Design Filtration Basin Underdrain Piping
Underdrain piping consists of the main collector pipe(s) and perforated lateral branch pipes.

The piping should be reinforced to withstand the weight of the overburden. Internal diameters of lateral branch pipes should be six (6) inches or greater and perforations should be 3/8 inch. Each row of perforations should contain at least six (6) holes, and the maximum spacing between rows of perforations should not exceed six (6) inches. All piping is to be Schedule 40 PVC or greater strength. The minimum grade of piping shall be 1/8 inch per foot, or one (1) percent slope (slopes down to one-half [0.5] percent are acceptable with prior approval). Access for cleaning all underdrain piping is needed.

Note: No drawdown time is associated with the sand filter, only the sedimentation basin.

Thus, it is not necessary to have a specific-design orifice for the filter bed outlet structure.

Step 15 – Design Filtration Basin Liner
If the filtration basin is an earthen structure and an impermeable liner is required to protect groundwater quality, the liner shall provide a maximum permeability of 1 x 10-6 cm/sec (ASTM Method D-2434). If an impermeable liner is not required, then a geotextile fabric liner shall be installed that meets the specifications listed in Table C-6.5 unless the basin has been excavated to bedrock.
#6..
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Treatment Control Measures Fact Sheet C-6: Media Filter Development Guidance Manual F-54 May 2025
Table C-6.4. Geotextile Fabric Specifications for Media Filters Parameter Test Method Specifications
Geotextile Fabric
Material Nonwoven geotextile fabric
Unit weight 8 oz/yd3 (minimum)
Filtration rate 0.08 in/sec (minimum)
Puncture strength ASTM D-751 (Modified) 125 lbs (minimum) Mullen burst strength ASTM D-751 400 lb/in
2 (minimum)
Tensile strength AST D-1682 300 lbs (minimum)
Equiv. opening size US Standard Sieve No. 80 (minimum)
Drainage Matting
Material Non-woven geotextile fabric
Unit weight 20 oz/yd3 (minimum)
Flow rate (fabric) 180 gpm/ft
2 (minimum)
Permeability ASTM D-2434 12.4 x 10-2
cm/sec
Grab strength ASTM D-1682
Dry: Lg 90/Wd 70
Wet: Lg 95/Wd 70

Puncture strength COE CW-02215 42 lbs (minimum)
Mullen burst strength ASTM D-1117 400 lb/in
2 (minimum)
Equiv. opening size ASTM D-1682 No. 100 (70-120)
Flow rate (drainage core) Drexel University 14 gpm/ft width Filter Fabric
Material Non-woven geotextile fabric
Unit weight 4.3 oz/yd3 (minimum)
Filtration rate 120 gpm/ft
2 (minimum)
Puncture strength ASTM D-751 (Modified) 60 lbs (minimum) Thickness 0.8 in (minimum)
#6..
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Treatment Control Measures Fact Sheet C-6: Media Filter Development Guidance Manual F-55 May 2025
The design procedures for a DC Underground Sand Filter system are listed below:

Step 1 – Determine Maximum Water Depth
Determine maximum allowable depth of water (2h) in the filtration basin considering elevation differences between inlet and outlet invert elevations. This height will establish weir height or elevation of inlet invert for bypass and orifices.

Step 2 – Determine Sand Filter Area (Afm)
Determine the minimum area of the DC Underground Sand Filter using the Austin Filter formula for partial sedimentation treatment.

Where
WQV = water quality volume (ft3);
df = filter sand bed depth (ft);
k = sand filter permeability coefficient (ft/hr) = 0.0833 ft/hr; h = one-half the maximum allowable water depth (2h) (ft); and tf = drawdown time for filter (hr) = 40 hr.

Step 3 – Determine Filter Width (wf)/Length (Lf)
Considering site constraints, select a filter width (Wf). Calculate the filter length (Lf) using the minimum area required (Afm).

Where
Afm = filter surface area based on flow (ft2
); and
Wf = filter bed width (ft).

Round the length and determine the adjusted area (Af).

Where
Wf = filter bed width (ft); and
Lf = filter bed length (ft).

Note: From this point forward, formulae assume a rectangular cross-section of the filter shell. #6..
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Treatment Control Measures Fact Sheet C-6: Media Filter Development Guidance Manual F-56 May 2025
Step 4 – Determine Storage Volume
Above filter voids (Vtf):

Where
Af = adjusted filter surface area (ft2); and
h = one-half the maximum allowable water depth (2h) (ft).

In filter voids (Vv), assuming 40% voids:

Where
Af = adjusted filter surface area (ft2
);
df = filter sand bed depth (ft); and
dg = filter gravel bed depth (ft).

Step 5 – Determine Flow-through Filter Volume during Filling (VQ) Where
k = 2 ft/day = 0.0833 ft/hr;
Af = adjusted filter surface area (ft2);
df = filter sand bed depth (ft);
dg = filter gravel bed depth (ft); and
tf = 1 hr (to fill voids).

Step 6 – Determine Net Volume to be Stored in Sediment Chamber awaiting Filtration (Vst) Where
WQV = water quality volume (ft3);
Vtf = storage volume above filter voids (ft3
);
Vv = storage volume in filter voids (ft3
); and
VQ = flow-through filter volume during filling (ft3).
#6..
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Treatment Control Measures Fact Sheet C-6: Media Filter Development Guidance Manual F-57 May 2025
Step 7 – Determine Minimum Permanent Pool Length (Lpm)
Where
Vst = net volume stored in sediment chamber (ft3
);
h = one-half the maximum allowable water depth (2h) (ft); and Wf = filter bed width (ft).

See Figure C-6.6 for dimensional relationships.

Step 8 – Determine Minimum Sediment Chamber Length (Ls) If Vst > 0.2 x WQV:

If Vst < 0.2 x WQV:

Where
Vst = net volume stored in sediment chamber (ft3);
h = one-half the maximum allowable water depth (2h) (ft); Wf = filter bed width (ft); and
WQV = water quality volume (ft3).

Note: It may be economical to adjust final dimensions to correspond with standard precast structures or to round off results to simplify measurements during construction. #6..
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Treatment Control Measures Fact Sheet C-6: Media Filter Development Guidance Manual F-58 May 2025
Ap Af Acw
Figure C-6.6. DC Underground Sand Filter Dimensional Relationships W f
Inflow
Pipe
Optional
Bypas s Pi pe
Access Door
60" Mini mum
Heads pace for
2h maintenance
d f Outfl ow
L s Col lector Pipe
Lp L f
d g
Lcw
Pipe
L t
Underdrai n @ minimum 0.5% slope
Where A p
A f
A cw
W f
Ls
L
p
L f
L
cw
L t
2h
d f
d
g
= Area of sediment chamber
= Area of s and filter
= Area of clearwell
= Width of filter
= Mini mum length of sediment chamber
= Final length of permanent pool
= Filter length
= Length of clearwell
= Total length, s um of L cw + Lp+ L f
= Maximum achievable ponding depth over filter
= s and bed depth
= gravel depth
#6..
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Treatment Control Measures Fact Sheet C-6: Media Filter Development Guidance Manual F-59 May 2025
Step 9 – Determine Final Permanent Pool Length (Lp)
If Lpm < (Ls + 2):

If Lpm > (Ls + 2):

Where
Lpm = minimum permanent pool length (ft); and
Ls = minimum sediment chamber length (ft).

Step 10 – Determine Clearwell Length (Lcw)
Set the clearwell length (Lcw) for adequate maintenance and/or access for monitoring flow rate and effluent water chemistry (minimum 3 feet).

Step 11 – Design Filter bed
Top Gravel Layer
The washed gravel at the top of the filter should be two (2) inches thick, and composed of

0.5-2-inch diameter stone. In areas with high sediment load (total suspended solids concentrations greater than or equal to 200 mg/L), the 2-inch layer of stone on top of the sand filter should be underlain with filter fabric meeting the specifications in Table C-6.4.

Sand Layer
The sand layer should be a minimum depth of 18 inches consisting of ASTM C33 concrete sand. A layer of geotextile fabric meeting the specifications in Table C-6.4 must separate the sand and gravel layer below.

Gravel Layer
The gravel layer surrounding the underdrain pipes should be at least 16 inches thick, and composed of 0.5-2-inch diameter stone that provides at least a two (2) inch cover over the tops of the underdrain pipes.

Step 12 – Design Underdrain Piping
Underdrain piping consists of the main collector pipe(s) and perforated lateral branch pipes.

The piping should be reinforced to withstand overburden weight. Internal diameters of lateral branch pipes should be six (6) inches or greater with perforations of 3/8 inch. Each row of perforations should contain at least six (6) holes, and the maximum spacing between rows of perforation should not exceed six (6) inches. All piping must be Schedule 40 PVC or greater strength. The minimum grade of piping shall be 1/8 inch per foot, or one (1) percent slope (slopes down to one-half [0.5] percent are acceptable with prior approval). Access for cleaning all underdrain pipes is needed.
#6..
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Treatment Control Measures Fact Sheet C-6: Media Filter Development Guidance Manual F-60 May 2025
Step 13 – Design Weepholes
In addition to an underdrain system, weepholes should be installed between the filter chamber and the clearwell to provide relief in case of pipe clogging. The weepholes should be three (3) inches in diameter with a minimum spacing of nine (9) inches center to center. The openings on the filter side of the dividing wall should be covered to the width of the trench with 12-inch high plastic hardware cloth of ¼-inch mesh or galvanized steel wire, minimum wire diameter 0.03-inch, No. 4 mesh hardware cloth anchored firmly to the dividing wall structure and folded a minimum of six (6) inches back under the bottom stone.

Step 14 – Design Dewatering Drain
A six (6) inch diameter DIP or PVC dewatering drain with a gate valve must be installed at the top of the stone/sand filter bed through the partition separating the filtration chamber from the clearwell chamber.

Step 15 – Design Bypass Pipe
Where a bypass pipe is needed, it shall be DIP or PVC with supports at a minimum of every 18 inches.

The design procedures for a Delaware sand filter system are listed below:

Step 1 – Determine Maximum Water Depth
Based on site constraints, determine the maximum ponding depth over the filter (2h). If an overflow device is built into the filter shell, size the overflow weir.

Step 2 – Determine Sand Filter/Sediment Chamber Surface Area (Asm) The filter shell must have the capacity to accept and store the WQV. The dimensions are sized to provide a filter area, which process the WQV in the desired drawdown time (48 hours). The areas of the sedimentation chamber and filter bed are typically set equal. The required areas are calculated as follows depending on the maximum depth of water above the filter bed:

If 2h < 2.67 ft:

If 2h > 2.67 ft:
#6..
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Treatment Control Measures Fact Sheet C-6: Media Filter Development Guidance Manual F-61 May 2025
Where
Afm = filter surface area based on flow (ft2
);
WQV = water quality volume (ft3);
h = one-half the maximum allowable water depth (2h) (ft); df = filter bed depth (ft);
k = sand filter permeability coefficient (ft/hr) = 0.0833 ft/hr; and tf = drawdown time for filter (hr) = 48 hr.

Step 3 – Determine Sediment Chamber and Filter Width
Site considerations usually dictate final dimensions of the facility. Sediment chambers and filter chambers are normally 18 to 30 inches wide. Use of standard grates requires a width of 26 inches.

Step 4 – Determine Sediment Chamber and Filter Length and Adjusted Area Where
Lf = filter length (ft);
Afm = filter surface area based on flow (ft2); and
Wf = filter width (ft).

Round the length upward as appropriate, and calculate the adjusted area.

Where
Af = adjusted filter surface area (ft2
);
Lf = filter length (ft); and
Wf = filter width (ft).

Step 5 – Determine Storage Volume in Filter Voids (Vv)
Assuming 40% voids:

Where
Af = adjusted filter surface area (ft2
);
df = filter sand bed depth (ft); and
dg = filter gravel bed depth (ft).
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Treatment Control Measures Fact Sheet C-6: Media Filter Development Guidance Manual F-62 May 2025
Step 6 – Determine Flow-through Filter during Filling (VQ) Where
k = 2 ft/day = 0.0833 ft/hr;
Af = adjusted filter surface area (ft2);
df = filter sand bed depth (ft);
dg = filter gravel bed depth (ft); and
tf = 1 hr (to fill voids).

Step 7 – Determine Net Volume to be Stored in Chambers awaiting Filtration (Vst) Where
WQV = water quality volume (ft3
);
Vv = storage volume in filter voids (ft3); and
VQ = flow-through filter volume during filling (ft
3 ).

Step 8 – Determine Available Storage in Chambers (Vsf)
Where
h = one-half the maximum allowable water depth (2h) (ft); Af = adjusted filter surface area (ft2
); and
As = adjusted sediment chamber area (ft2
).

If Vsf ≥ Vst, proceed with design.

If Vsf < Vst, adjust width and/or length, and repeat Steps 3-8.

Step 9 – Design Filter bed
Top Gravel Layer
The washed gravel at the top of the filter should be two (2) inches thick, and composed of 0.5- 2-inch diameter stone. In areas with high sediment load (total suspended solids concentrations greater than or equal to 200 mg/L), the 2-inch layer of stone on top of the sand filter should be underlain with filter fabric meeting the specifications in Table C-6.4. #6..
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Treatment Control Measures Fact Sheet C-6: Media Filter Development Guidance Manual F-63 May 2025
Sand Layer
The sand layer should be a minimum depth of 18 inches consisting of ASTM C33 concrete sand. A layer of geotextile fabric meeting the specifications in Table C-6.4 must separate the sand and gravel layer below.

Step 10 – Design Underdrain Piping
Underdrain piping consists of the main collector pipe(s) and perforated lateral branch pipes.

Note: No drawdown time is associated with the sand filter, only the sedimentation basin.

Thus, it is not necessary to have a specific-design orifice for the filter bed outlet structure.

Shallow rectangular drain tiles may be fabricated from such materials as fiberglass structural channels, saving several inches of filter depth. Drain tiles should be in two (2) foot lengths and spaced to provide gaps 1/8-inch less than the smallest gravel size on all four sides. Sections of tile may be cast in the dividing wall between the filter and the sedimentation chamber to provide shallow outfall orifices.

Step 11 – Design Weepholes
In addition to an underdrain system, weepholes should be installed between the filter chamber and the sedimentation chamber to provide relief in case of pipe clogging. The weepholes should be three (3) inches in diameter with a minimum spacing of nine (9) inches center to center. The openings on the filter side of the dividing wall should be covered to the width of the trench with 12-inch high plastic hardware cloth of ¼-inch mesh or galvanized steel wire, minimum wire diameter 0.03-inch, No. 4 mesh hardware cloth anchored firmly to the dividing wall structure and folded a minimum of six (6) inches back under the bottom stone.

Step 12 – Design Grates and Covers
Grates and cast steel covers are designed to take the same wheel loads as adjacent pavement.

Where possible, use standard-sized grates to reduce costs. Grates and covers should be supported by a galvanized steel perimeter frame.
#6..
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Treatment Control Measures Fact Sheet C-6: Media Filter Development Guidance Manual F-64 May 2025
Step 13 – Design Hood/Traps
In applications where trapping of hydrocarbons or other floating pollutants is required, large-storm overflow weirs should be equipped with a 10-gauge aluminum hood or commercially available catch basin trap. The hood or trap should extend a minimum of one (1) foot into the sedimentation chamber.

Step 14 – Design Dewatering Drain
A six (6) inch diameter dewatering drain with gate valve is to be installed at the top of the stone/sand filter bed through the partition separating the filter chamber from the sedimentation chamber.

Volume Retention Calculation
No volume retention credit is available for Media Filters because they do not fully retain the WQV. However, Media Filters may be used to meet treatment control requirements.

Construction Considerations
• Erosion and sediment control measures must be configured to prevent any inflow of stormwater runoff into the Media Filter during its construction. • The Media Filter must be adequately protected once constructed and not placed in service until all soil surfaces in the drainage watershed have been stabilized with vegetated cover. If stormwater runoff from the project site enters the Media Filter system prior to site revegetation, all contaminated materials must be removed and replaced with new, clean materials.
• The top of the Media Filter must be completely level. No grade is allowed. • The inverts of the notches, multiple orifices, or weirs dividing the sedimentation chamber from the filter chamber must be completely level. Otherwise, water will not arrive at the filter as sheet flow, and only the downgradient end of the filter will function. • Inflow grates or slotted curbs may conform to the grade of the completed pavement as long as the filters, notches, multiple orifices, and weirs connecting the sedimentation and filter chambers are completely level.
• If precast concrete lids are used, lifting rings or threaded sockets must be provided to allow easy removal with lifting equipment. Lifting equipment must be readily available to the facility operators.
• Where underdrains are used, the minimum slope of the pipe shall be one-half (0.5) percent.

Where only gravel-filtered water conveyance is provided, the filter floor must be sloped towards weepholes at a minimum slope of one-half (0.5) percent. #6..
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Treatment Control Measures Fact Sheet C-6: Media Filter Development Guidance Manual F-65 May 2025
Long-Term Maintenance
The City requires execution of a maintenance agreement with the property owner prior to final acceptance of a private development project, which includes treatment control measures such as Media Filters. Such agreements typically include requirements such as those outlined in Table C-6.5. The property owner or property owner’s designee is responsible for compliance with the agreement. The maintenance agreement must provide the City with complete access to the treatment control measure and its immediate vicinity at any time. Treatment control measure maintenance is the responsibility of the property owner. A sample maintenance agreement is presented in Appendix I.

Table C-6.5. Inspection and Maintenance Requirements for Media Filters Activity Schedule
Remove cover grates or precast lids on the
chambers and inspect to determine if the system is
functioning properly.

Quarterly during first year of operation
Inspect for standing water, sediment, trash, and
debris. Identify and correct observed problems. Semiannually after first year of operation Inspect facility after a large storm even to
determine if the facility is draining completely
within its design drawdown period.

At least once during the wet season
Prevent grass and vegetative waste from washing
into the filter.

As required
Remove trash and debris collected on the inlet
grates to maintain the inflow capacity of the filter.

At least weekly during the wet season or before
significant storm events
Remove top two (2) inches of sand and dispose of
sediment if the facility drain time exceeds its design
drawdown period. Restore media depth to 18
inches when overall media depth drops to 12
inches.

As required. Discoloration of the filter may be an
indication of clogging
Remove accumulated sediment in the
sedimentation basin.

Every ten (10) years or when sediment occupies ten
(10) percent of the basin volume, whichever is less
Dispose of sand, gravel, or filter cloth contaminated
with petroleum hydrocarbons in accordance with
applicable laws.

As required
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APPENDIX G

VOLUME RETENTION REQUIREMENT WORKSHEET
#6..
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Development Guidance Manual G-1 May 2025
APPENDIX G. VOLUME RETENTION REQUIREMENT

WORKSHEET
Note: If VRRREMAIN > 0, the volume retention requirement is not fully met. Meeting the volume retention requirement may be an iterative process. Designers should return to prior steps to explore alternative combinations of VRWs and LID-Based Volume Retention Control Measures. If the meeting of the full volume retention requirement is infeasible, a Volume Retention Requirement Waiver Application must be submitted.
*Apply reductions to VRR as appropriate for Significant Redevelopment in Section 5 Project: ________________________________________________________________________________ Detail: ________________________________________________________________________________ Design by: ____________________________________________________________________________ Date: ________________________________________________________________________________

1. Project Drainage Area Characteristics: Pre-project
a. Weighted Runoff Coefficient (CrPRE)
b. Total Drainage Area (APRE)
c. Pre-project Runoff Volume (VolPRE)*
VolPRE = (0.50/12) x APRE x CrPRE
CrPRE = ___________
APRE = ____________ft2
VolPRE= ___________ft3

2. Project Drainage Area Characteristics: Post-project
a. Weighted Runoff Coefficient (Cra)
b. Total Drainage Area (APOST)
c. Post-project Runoff Volume (VolPOST)
VolPOST = (0.50/12) x APOST x Cra
d. Volume Retention Requirement*
VRR = VolPOST – VolPRE
Cra = ____________
APOST = __________ft2
VolPOST= _________ft3
VRR = ___________ft3

3. Volume Retention Measures:

Total number of VRMs in project
Total Volume Retention Credits from VRMs (∑ VolVRM)
Total Tributary Impervious Area Reduction Credits for application to effective area calculation (∑ Areacredit)
Remaining Volume retention required from LID Treatment Controls (VRRTREAT) VRRTREAT = VRR – ∑ VolVRM
No. VRMs = _________
∑ VolVRM = __________ft3
∑ Areacredit = ________ft2
VRRTREAT = __________ft3

4. Volume Retention Credits
Total Volume retention Credits from LID Treatment Controls (∑ VolTREAT) Total Volume retention Provided (VRRPROVIDED)
VRRPROVIDED = ∑ VolVRM (line 3b) + ∑ VolTREAT (line 4a) Volume retention remaining (VRRREMAIN)
VRRREMAIN = VRR – VRRPROVIDED
∑ VolTREAT = _________ft3
VRRPROVIDED = ________ ft3
VRRREMAIN = ________ ft3
#6..
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APPENDIX H

VOLUME RETENTION REQUIREMENT WAIVER

APPLICATION
#6..
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Development Guidance Manual H-1 May 2025
APPENDIX H. VOLUME RETENTION REQUIREMENT WAIVER

APPLICATION
A waiver may be granted if the volume retention requirement (VRR) cannot be met due to site constraints, as described in Section 5.

Meeting the VRR is an iterative process. Designers should return to prior steps to explore alternative combinations of volume retention measures (VRMs) and low impact development (LID) Treatment Controls prior to submitting a waiver application request.

If the project cannot meet the full VRR, the project must still retain the volume to the maximum extent practicable and treat the full water quality volume (WQV)/water quality flow (WQF). In addition, projects that cannot fully meet the VRR must select Treatment Controls with a medium to high removal efficiency for the pollutant of concern (Tables 3-1 and 7-2).

The burden of proof is on the project applicant to demonstrate that it is technically infeasible to meet the VRR. Economic hardship is not an acceptable reason for noncompliance. In general, the City does not expect to grant waivers for the VRR.

The final determination will be made by City of Modesto Community and Economic Development Department. The City has the authority to reject a VRR Waiver request if VRM and/or LID Treatment Controls are considered feasible at the project site.

Consideration of a waiver request requires applicants to do the following:
 Reduce volume to the maximum extent practicable, even if the full VRR cannot be met.  Treat the full WQV/WQF.
 Consider all of the VRM and LID Treatment Controls. Applicants must show why certain VRM and/or LID Treatment Controls are not feasible at the development site.  Demonstrate that site conditions warrant technical infeasibility, as described in Section 5.  Submit this application with or prior to preliminary site plan submission.  Obtain the signature and stamp of the project engineer registered in California.  Submit the Volume Retention Design Worksheet (Appendix G) along with this application. #6..
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Development Guidance Manual H-2 May 2025
Volume Retention Requirement Waiver Application
Property Owner/Developer Information
Name of Business:

Contact Person:

Street:

City: State: Zip:

Phone Number:

Email Address:

Mailing Address:

City: State: Zip:___________
Plan Preparer Information
Name of Business:

Contact Person:

Street:

City: State: Zip:

Phone Number:

Email Address:

Mailing Address:

City:

State:_________Zip:____________
Project Name:_________________________________________________________________ Project Category (check all that apply – see Section 2 of the Stormwater Quality Control Criteria Plan for project definitions):

A) Priority
Land Use
Projects
High Density
Residential
Industrial Commercial Mixed Urban Public Transportation Station
B) Priority
Projects
Residential
Subdivision
Commercial/
Industrial
Automotive
Repair Shops
Retail
Gasoline Outlets
Restaurants
Parking Lots Significant
Redevelopment
Property Description (include location, size, land uses, etc.):
#6..
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Development Guidance Manual H-3 May 2025
Volume Retention Requirement ________ (Volume Retention Worksheet, line 2e) Volume Retention Provided ________ (Volume Retention Worksheet, line 4b) Volume Retention Remaining ________ (Volume Retention Worksheet, line 4c) Types of BMPs Implemented on Project Site (check all that apply) Describe Why a Volume Retention Requirement Waiver is Needed (include specifics on site conditions that warrant technical infeasibility & attach supporting documentation:

Describe Alternative Compliance Design
The portion of the volume that cannot be reliably retained on-site (Volume Retention Worksheet, line 4c) must be treated and temporarily held on-site and released into the City stormwater network once the storm event passed. Attach supporting documentation:

Type and Number of LID Treatments Proposed
Bioretention (L-1) Porous Pavement Filter (L-6)
Stormwater Planter (L-2) Vegetated (Dry) Swale (L-7))
Tree-well Filter (L-3) Grassy Swale (L-8)
Water Quality Infiltration
Basin (L-4)
Grassy Filter Strip (L-9)
Water Quality Infiltration
Trench (L-5)
Type and Number of Volume Retention Proposed
Rain Garden (V-1) Grassy Channel (V-4)
Rain Barrel/Cistern (V-2) Vegetated Buffer Strip (V-5)
Interception Trees (V-3)
#6..
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Development Guidance Manual H-4 May 2025

APPLICATION FEE PER CURRENT CITY OF MODESTO FEE STRUCTURE

CERTIFICATION
I hereby certify that the information provided in this Application is correct.

Application Prepared by:

Print Name and Firm Signature of Project Engineer in Above-Named Firm Professional Title Affix Professional Registration Stamp of AboveNamed Person with Signature and Expiration Date #6..
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APPENDIX I

MAINTENANCE AGREEMENTS AND FORMS
This appendix includes the following maintenance agreements and forms:

I-1: Stormwater Treatment Device Access and Maintenance Agreement I-2: Stormwater Quality Control Plan Owner’s Certification Statement A template agreement is provided in this Appendix, however it is recommended that the project proponent use the most recent template agreement available on the City website https://www.modestogov.com/1263/Forms
#6..
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Development Guidance Manual I1-1 May 2025
APPENDIX I-1: STORMWATER TREATMENT DEVICE ACCESS AND

MAINTENANCE AGREEMENT TEMPLATE
#6..
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Development Guidance Manual I2-1 May 2025
APPENDIX I-2: SWQCP OWNER’S CERTIFICATION STATEMENT
OWNER’S CERTIFICATION

STORMWATER QUALITY CONTROL PLAN
for
(PROJECT NAME)
This Project Stormwater Quality Control Plan (Plan) was prepared for (Project Owner / Developer) by (Name of Preparing Firm/Individual) . This Plan is intended to comply with all requirements specified in the City of Modesto Development Standards for new development and redevelopment projects.

The undersigned understands that stormwater pollution control measures are enforceable requirements under the Development Standards. The undersigned, while owning the property on which such control measures are to be implemented, is responsible for the implementation of the provisions of this Plan and for the maintenance of all structural stormwater pollution control measures and agrees to ensure that the conditions on the project site conform to the requirements specified in the Development Standards.

Once the undersigned transfers its interest in the project property, its successors-ininterest shall bear the aforementioned responsibility to maintain structural stormwater pollution control measures and to implement and amend this Plan.

Name of Owner ___________________________________________________ Address of Owner__________________________________________________ _________________________________________________________________ Phone Number of Owner____________________________________________ Signature
Print Name
Title
Date
#6..
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- 1 -
RECORDING REQUESTED BY:

City of Modesto
PLEASE RETURN TO / MAIL TO:

City of Modesto City Clerk
P.O. Box 642
Modesto, CA 95353
SPACE ABOVE THIS LINE FOR RECORDER’S USE
ACCESS AND MAINTENANCE AGREEMENT FOR:
(CHECK ALL THAT APPLY)
❏ VOLUME RETENTION MEASURE (V1-V5)
❏ TREATMENT CONTROL MEASURE (L1-L11 and C1-C6)
❏ TRASH FULL CAPTURE SYSTEM / MULTI-BENEFIT PROJECT
OWNER:

PROPERTY ADDRESS:

APN:

THIS AGREEMENT is made and entered into in Modesto, California, this day of , , by and between ,
hereinafter referred to as “Owner” and the CITY OF MODESTO, a municipal corporation, located in the County of Stanislaus, State of California hereinafter referred to as “CITY”; WHEREAS, the Owner owns real property (“Property”) in the City of Modesto, County of Stanislaus, State of California, more specifically described in Exhibit “A” and depicted in Exhibit “B”, each of which exhibits is attached hereto and incorporated herein by this reference; WHEREAS, at the time of initial approval of development project known as within the Property described herein,
the City of Modesto City Council Resolution No. or
Planning Commission Resolution No. or
Planning Department Certificate of Approval No. required the project to employ on-site volume retention measures to reduce or contain all storm water runoff on site and/or Treatment Control Measures and/or Trash Full Capture System/MultiBenefit Project(s) to minimize pollutants in urban runoff.

VOLUME RETENTION MEASURE - TREATMENT CONTROL MEASURE – TRASH FULL

CAPTURE SYSTEM
WHEREAS, the Owner has chosen to install a
, hereinafter referred to as “Device(s),
as the on-site control measure(s) to minimize pollutants in urban runoff; WHEREAS, said Device(s) has/have been installed in accordance with plans and specifications accepted by the City;
#6..
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WHEREAS, said Device(s), with installation on private property and draining only private property, is a private facility with all maintenance or replacement, therefore, the sole responsibility of the Owner in accordance with the terms of this Agreement; WHEREAS, the Owner is aware that periodic and continuous maintenance consistent with the Device requirements/protocols, including, but not necessarily limited to, filter material replacement and sediment removal, is required to assure peak performance of Device(s) and that, furthermore, such maintenance activity will require compliance with all Local, State, or Federal laws and regulations, including those pertaining to confined space and waste disposal methods, in effect at the time such maintenance occurs; NOW THEREFORE, it is mutually stipulated and agreed as follows:

1. The recitals set forth above, the exhibits set forth in the recitals, the introductory paragraph preceding the recitals, and all defined terms set forth in both, are hereby incorporated into this Agreement as set forth herein in full.

2. Owner hereby provides the City or City’s designee complete access, of any duration, to the Device(s) and its immediate vicinity at any time, upon reasonable notice, or in the event of emergency, as determined by City’s Utilities Director no advance notice, for the purpose of inspection, sampling, testing of the Device(s), and in case of emergency, to undertake all necessary repairs or other preventative measures at Owner’s expense as provided in paragraph 4 below. City shall make every effort at all times to minimize or avoid interference with Owner’s use of the property.

3. Owner shall use its best efforts diligently to maintain the Device(s) in a manner assuring peak performance at all times. All reasonable precautions shall be exercised by Owner and Owner’s representative or contractor in the removal and extraction of material(s) from the Device(s) and the ultimate disposal of the material(s) in a manner consistent with all relevant laws and regulations in effect at the time. As may be requested in writing from time to time by the City, the Owner shall provide the City with documentation identifying the material(s) removed, the quantity, and disposal destination.

4. In the event Owner, or its successors or assigns, fails to accomplish the necessary maintenance contemplated by this Agreement, within five (5) days of being given written notice by the City, the City is hereby authorized to cause any maintenance necessary to be done and charge the entire cost and expense thereof to the Owner or Owner’s successors or assigns, including administrative costs, attorneys fees and interest thereon at the maximum rate authorized by the Civil Code from the date of the notice of expense until paid in full.

5. The City may require the owner to post security in form and for a time period satisfactory to the City of guarantee the performance of the obligations state herein. Should the Owner fail to perform the obligations under the Agreement, the City may, in the case of a cash bond, act for the Owner using the proceeds from it, or in the case of a surety bond, require the sureties to perform the obligations of the Agreement. As an additional remedy, the Director may withdraw any previous stormwater related approval with respect to the property on which the Device(s) has/have been installed until such time as Owner repays to City its reasonable cost incurred in accordance with paragraph 3 above. #6..
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- 3 -

6. This Agreement shall be recorded in the Office of the Recorder of Stanislaus County, California at the expense of the City and shall constitute notice to all successors and assigns of the title to said Property of the obligation herein set forth, as also a lien in such amount as will fully reimburse the City, including interest as herein above set forth, subject to foreclosure in event of default in payment.

7. In event of legal action occasioned by any default or action of the Owner, or its successors or assigns, then the Owner and its successors or assigns agree(s) to pay all costs incurred by the City in enforcing the terms of this Agreement, including reasonable attorney’s fees and costs, and that the same shall become part of the lien against said Property.

8. It is the intent of the parties hereto that burdens and benefits herein undertaken shall constitute covenants that run with said Property and constitute a lien there against.

9. The obligations herein undertaken shall be binding upon the heirs, successors, executors, administrators and assigns of the parties hereto. The term “Owner” shall include not only the present Owner, but also its heirs, successors, executors, administrators, and assigns.

Owner shall notify any successor to title of all or part of the Property about the existence of this Agreement. Owner shall provide such notice prior to such successor obtaining an interest in all or part of the Property. Owner shall provide a copy of such notice to the City at the same time such notice is provided to the successor.

10. Time is of the essence in the performance of this Agreement.

11. Any notice to a party required or called for in this Agreement shall be served in person, or by deposit in the U.S. Mail, first class postage prepaid, to the address set forth below. Notice(s) shall be deemed effective upon receipt, or seventy-two (72) hours after deposit in the U.S.

Mail, whichever is earlier. A party may change a notice address only by providing written notice thereof to the other party.

IF TO CITY: IF TO OWNER:

City of Modesto
Utilities Department
P.O. Box 642
Modesto, CA 95353
#6..
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- 4 -
IN WITNESS THEREOF, the parties hereto have affixed their signatures as of the date first written above.

CITY OF MODESTO, a Municipal
Corporation
By:

JOSEPH P. LOPEZ
City Manager
ATTEST:

By: OWNER: DIANE NAYARES-PEREZ, CMC, City Clerk
By:
(SEAL) (Signature)
(Print Name & Title)
By: (Signature)
(Print Name & Title)

*Corporations – signature of two (2) officers required
or one (1) officer plus corporate seal.

Partnership – signature of a partner required
Sole Proprietorship – signature of proprietor required
LLC – signature of proprietor or partner required
Approved as to Form:

By:

JOSE M. SANCHEZ
City Attorney

Approved as to sufficiency:

By:

Toby Wells,
City Engineer
#6..
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- 5 -
Exhibit A
Legal Description of Property
#6..
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- 6 -
Exhibit B
Property Map
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APPENDIX J
STORMWATER QUALITY CONTROL PLAN and

MAINTENANCE PLAN SUBMITTAL GUIDANCE
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APPENDIX J. SWQCP AND MAINTENANCE PLAN

SUBMITTAL GUIDANCE
This appendix includes the following templates and forms:

J-1: Stormwater Quality Control Plan Guidance
J-2: Stormwater Quality Control Plan [Template]
J-3: Stormwater Maintenance Plan Guidance
J-4: Stormwater Maintenance Plan [Template]
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Development Guidance Manual J1-1 May 2025
APPENDIX J-1: STORMWATER QUALITY CONTROL PLAN GUIDANCE
This appendix identifies the information and format that must be submitted to the City as a part of the Project Stormwater Quality Control Plan (SWQCP). The SWQCP must be submitted to and approved by the City of Modesto (City) stormwater staff prior to issuance of building or use permits for the project.

The full Guidance Manual for Development Stormwater Quality Control Measures is available for download at https://www.modestogov.com/1515/Storm-Water.

I. Cover Page
The cover page must include the following information:
 Project name, address, and Assessor’s Parcel Number (APN)  Property owner/developer’s name and contact information  Brief property description (should be consistent with Section III)  Plan preparer’s name and contact information
 Dates submitted, revised (first and subsequent submittals, as needed)  Signed and stamped by a licensed California Engineer
II. Owner’s Certification Statement
The owner’s certification statement requires the owner to legally accept responsibility for inspection and maintenance of the identified stormwater control measures. Required information includes:
• Project name
• Property owner’s name, contact information, and signature III. Project Description
The project description provides general information about the project type, description of the project, and site information. The following information must be included:

1. Project Category – type of development, type of project, City zoning, and associated pollutant categories of concern

2. Project Information – location, project and drainage area, receiving water(s), topography, depth to groundwater, etc.

3. Project Narrative – narrative description about the proposed project including the size, location, land uses, expected pollutant-generating activities, and any other pertinent information

4. Site Maps
a. Provide a vicinity map showing the location of the project relative to principal landmarks.
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Development Guidance Manual J1-2 May 2025
b. Provide a site map showing boundaries of the site, acreage and drainage patterns/contour lines. Show each discharge location from the site and any drainage flowing onto the site.
c. Identify locations of existing and proposed storm drain facilities, private sanitary sewer systems and grade-breaks for purposes of pollution prevention. d. With legend, show locations of expected sources of pollution generation (outdoor work and storage areas, heavy traffic areas, delivery areas, trash enclosures, fueling areas, industrial clarifiers, wash-racks, etc). Identify any areas having contaminated soil or where toxins are stored or have been stored/disposed of in the past. e. Delineates and describes the drainage management areas (DMAs) with arrows showing the flow direction.
f. With legend, indicate types and locations of structural stormwater control measures that will be built to permanently control stormwater pollution.

IV. Stormwater Control Measures
Identify the types of control measures provided for the project site.

1. Summary Matrix – identify each type of control measure provided.

2. Site Design Control Measures
a. Describe each Site Design Control Measure that will be utilized and how each conforms to design criteria.
b. If a Site Design Control Measure is not applicable to the project, provide a statement of justification describing why the control measure is not applicable. c. If implementation of an applicable Site Design Control Measure is not feasible due to project site conditions, provide a statement of justification describing why implementation is not feasible.

3. Source Control Measures
a. Describe each Source Control Measure that will be utilized and how each conforms to the design criteria. (Note: Source Control S-1 is required for all projects)

4. Volume Retention Measures
a. Describe each Volume Retention Measure (VRM) that will be utilized. b. Provide the summary sheet in Appendix G to track conformance with the Volume Reduction Requirement (VRR). This sheet must show:
a. Pre- and post-project stormwater runoff volume
b. Applicable volume retention credits
c. Volume retention requirement
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Development Guidance Manual J1-3 May 2025
c. If implementation of VRMs is not feasible and/or the VRR cannot be met, use the Waiver Application in Appendix H to justify why implementation is not feasible. All VRMs must be taken into consideration.

5. Treatment Control Measures
a. Describe each Treatment Control Measure to be provided. b. If the VRR was not fully met through the use of VRMs, LID-based Treatment Controls must be used. The summary sheet in Appendix G should be used to continue tracking conformance with the VRR. If implementation of VRMs is not feasible and/or the VRR cannot be met, use the Waiver Application in Appendix H to justify why implementation is not feasible. All VRMs must be taken into consideration. c. Summarize design data for Treatment Control Measures on appropriate design procedure forms. Provide detailed supporting calculations for design data values in a clear and organized manner.

V. Maintenance Plan and Responsibility
The Project SWQCP must include information on the responsible party(ies) that will operate, inspect, and maintain stormwater control measures. The owner/developer must sign a certified, legally-binding maintenance agreement [Appendix I].

1. Provide a summary of the control measures to be provided and responsible parties responsible for maintenance of each control.

2. Provide complete contact information for each identified responsible party.

3. Indicate any anticipated transfer of responsibility due to future transfer of ownership or annexation.

4. Provide a statement that a detailed Maintenance Plan will be prepared in accordance with SWQCCP requirements.
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Development Guidance Manual J2-1 May 2025
APPENDIX J-2: STORMWATER QUALITY CONTROL PLAN TEMPLATE
The following worksheets and sections are required for a SWQCP submittal.

Section I. Cover Page
Stormwater Quality Control Plan
Project Name:

Street:

City: Modesto State: California Zip Code:

Assessor’s Parcel Number (APN):

Property Owner/Developer Information:

Name of Business:

Contact Person:

Street:

City: State: Zip:

Phone Number:

Email Address:

Plan Preparer Information:

Name of Business:

Contact Person:

Street:

City: State: Zip:

Phone Number:

Email Address:

Property Description (location, size, land uses, etc.)
Submittal Information
Date of First Submittal
Date(s) Revised
The cover page must be signed and stamped by a licensed California Engineer #6..
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Development Guidance Manual J2-2 May 2025
Section 2. Owner’s Certification
Stormwater Quality Control Plan
For
Project Name:__________________________________________________________________________ This Project Stormwater Quality Control Plan (Plan) was prepared for ________________________ by _____________________________. This Plan is intended to comply with all requirements specified in the City of Modesto Development Guidance Manual for new development and redevelopment projects.

The undersigned understands that stormwater pollution control measures are enforceable requirements under the Development Guidance Manual. The undersigned, while owning the property on which such control measures are to be implemented, is responsible for the implementation of the provisions of this Plan and for the maintenance of all structural stormwater pollution control measures and agrees to ensure that the conditions on the project site conform to the requirements specified in the Development Guidance Manual.

Once the undersigned transfers its interest in the project property, its successors-in-interest shall bear the aforementioned responsibility to maintain structural stormwater pollution control measures and to implement and amend this Plan.

Name of Owner:

Address of Owner: _____________________________________________________________________ Phone Number of Owner: _______________________________________________________________ Email Address of Owner:

Signature:

Print Name:

Title:

Date:
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Development Guidance Manual J2-3 May 2025
Section 3. Project Description

1. Development
(select one – Development Guidance Manual Section 2.1)
- New Development - New Development Infill - Significant Redevelopment

2. Project Category
(select all that apply – Development Guidance Manual Section 2.1) Priority Land Use
- High Density Residential
- Industrial
- Commercial
- Mixed Urban
- Public Transportation Station
Priority Project
- Residential Subdivision
- Commercial/Industrial
- Automotive Repair Shops
- Retail Gasoline Outlets
- Restaurants
- Parking Lots
- Significant Redevelopment
City Zoning:

3. PLU and Priority Project Categories and Associated Pollutants of Concern (select all that apply – Development Guidance Manual Table 3-1) Project Category
Pollutant Category of Concern
SedimentNutrientsMetalsTrashOxygen DemandToxic OrganicsBacteria Residential X X X1 X X X
Commercial/Industrial (any size)
(including auto repair, RGO, restaurants) X X X X X X X Parking Lots (≥ 5,000 SF or 25 spaces) X X X X X
Mixed Urban X X X X X X X
Public Transportation Stations X
X = Pollutant assumed to be present in stormwater runoff from project area unless applicant demonstrates otherwise. 1 – Metals are a pollutant category of concern for the street and road infrastructure portion of the residential development.

If applicable, identify other pollutants of concern specific to this project site not mentioned above. #6..
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Development Guidance Manual J2-4 May 2025

2. Project Information
General Project Site Information Table 1
Latitude: Longitude: Elevation:

Total Project Area (AT)(acres) Total Drainage Area (acres) Total Existing (Pre) Impervious Area
(acres)
Total Post-Project Impervious
Area (acres)
Project Location
Receiving Water(s)
General Project Site Information Table 2
Post-Project Land Use Type(s)
Topography
Hydraulic Head
Depth to Ground Water
Type of Soil _________________________________________________________________________ #6..
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Development Guidance Manual J2-5 May 2025

3. Project Narrative
Provide a narrative description of the proposed project, including the size, location, land uses found in the drainage area, expected pollutant-generating activities, and any other pertinent information. #6..
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Development Guidance Manual J2-6 May 2025

4. Site Maps
Provide the following types of maps and information:
• Vicinity map showing the location of the project relative to principal landmarks. • Site map showing boundaries of the site, acreage and drainage patterns/contour lines. Show each discharge location from the site and any drainage flowing onto the site.
• Identify locations of existing and proposed storm drain facilities, private sanitary sewer systems and grade-breaks for purposes of pollution prevention.
• With legend, show locations of expected sources of pollution generation (outdoor work and storage areas, heavy traffic areas, delivery areas, trash enclosures, fueling areas, industrial clarifiers, wash-racks, etc). Identify any areas that have contaminated soil or where toxins are stored or have been stored/disposed of in the past. • Delineates and describes the site Drainage Management Areas (DMAs) with arrows showing the direction of flow on the site. • With legend, indicate types and locations of structural stormwater control measures that will be built to permanently control stormwater pollution.
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Development Guidance Manual J2-7 May 2025
Section 4. Stormwater Control Measures

1. Summary Matrix of Control Measures
* Can be designed to meet trash control requirements; ** Provide justification if the control measure is not applicable to the project.

Applicable Not
Applicable Quantity
Site Design
Control
Measures
(G-1) Conserve Natural Areas**
(G-2) Protect Slopes and Channels**
(G-3) Minimize Soil Compaction**
(G-4) Minimize Impervious Areas**
Source Control Measures
(S-1) Storm Drain Message and Signage**
(S-2) Outdoor Material Storage Area
(S-3) Outdoor Trash Storage & Waste Handling**
(S-4) Outdoor Loading/Unloading Dock Area
(S-5) Outdoor Repair/Maintenance Area
(S-6) Outdoor Vehicle/ Equipment Wash Area
(S-7) Fuel and Maintenance Area
Volume
Retention
Measures
(V-1) Rain Garden
(V-2) Rain Barrel/Cistern
(V-3) Interception Trees
(V-4) Grassy Channel
(V-5) Vegetated Buffer Strip
LID-Based Treatment Control Measures
(L-1) Bioretention*
(L-2) Stormwater Planter
(L-3) Tree-well Filter
(L-4) Water Quality Infiltration Basin*
(L-5) Water Quality Infiltration Trench*
(L-6) Porous Pavement Filter
(L-7) Vegetated (Dry) Swale
(L-8) Grassy Swale
(L-9) Grassy Filter Strip
(L-10) Infiltration Well
(L-11) Dry Well
Conventional Treatment
Control Measures
(C-1) Constructed Wetland
(C-2) Extended Detention Basin*
(C-3) Wet Pond
(C-4) Proprietary Treatment Control*
(C-5) Trash Capture Device
(C-6) Media Filter
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Development Guidance Manual J2-8 May 2025

2. Site Design Controls
a) Describe each Site Design Control Measure that will be utilized and how each conforms to the design criteria.
b) If a Site Design Control Measure is not applicable to the project, provide a statement of justification describing why the control measure is not applicable. c) If implementation of an applicable Site Design Control Measure is not feasible due to project site conditions, provide a statement of justification describing why implementation is not feasible.

Add space as needed
G-1 Conserve Natural Areas
G-2 Protect Slopes and Channels
G-3 Minimize Soil Compaction
G-4 Minimize Impervious Area
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Development Guidance Manual J2-9 May 2025

3. Source Control Measures
Describe each Source Control Measure that will be utilized and how each conforms to the design criteria (Note: Source Control S-1 is required for all projects). Add space as needed.

Description of Source Controls Provided
S-1 Storm Drain Message and Signage
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Development Guidance Manual J2-10 May 2025

4. Volume Retention Measures
a) Describe each Volume Retention Measure (VRM) that will be utilized. b) Provide the summary sheet in Appendix G to track conformance with the Volume Reduction Requirement (VRR). This sheet must show:
a. Pre- and post-project stormwater runoff volume
b. Applicable volume retention credits
c. Volume retention requirement
c) If implementation of VRMs is not feasible and/or the VRR cannot be met, use the Waiver Application in Appendix H to justify why implementation is not feasible. All VRMs must be taken into consideration.

Add space as needed.
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Development Guidance Manual J2-11 May 2025
Description of Volume Retention Measures
#6..
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Development Guidance Manual J2-12 May 2025

5. Treatment Control Measures
a) Describe each Treatment Control Measure to be provided. b) If the VRR was not fully met through the use of VRMs, LID-based Treatment Controls must be used. The summary sheet in Appendix G should be used to continue tracking conformance with the VRR. If implementation of VRMs is not feasible and/or the VRR cannot be met, use the Waiver Application in Appendix H to justify why implementation is not feasible. All VRMs must be taken into consideration.
c) Summarize design data for Treatment Control Measures on appropriate design procedure forms.

Provide detailed supporting calculations for design data values in a clear and organized manner.

Add space as needed.

Description of Treatment Control Measures
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Development Guidance Manual J2-13 May 2025
Section 5. Maintenance Plan and Responsibility
Provide a summary of the control measures to be implemented and responsible parties responsible for maintenance of each control measure.

Control Measures Implemented
Responsible Party Contact Information
Contact Person:

Street:

City: State: Zip Code:

Phone Number:

Email Address:

Indicate any anticipated transfer of responsibility due to future transfer of ownership or annexation. #6..
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Development Guidance Manual J3-1 May 2025
APPENDIX J-3: STORMWATER MAINTENANCE PLAN GUIDANCE
This appendix identifies the information that shall be included in a maintenance plan. Refer to the Fact Sheets for individual control measure and device-specific maintenance requirements.

I. Cover Page
 Project Name, address, and Assessor’s Parcel Number (APN)  Property Owner/Developer’s name and contact information  Operations and Maintenance name and contact information II. Baseline Descriptions
 List responsible parties for operation and maintenance of the stormwater control measures on site and associated contact information.
 Identify the intended method of providing financing for operation, inspection, routine maintenance, and upkeep of stormwater control measures.  Provide a description of the types of O&M activities that will take place at this site.  List all permanent stormwater control measures (see SWQCP Section 4).

III. Routine Maintenance
Provide the following for each stormwater control measure:
 List and describe all maintenance and waste disposal activities that will be performed and distinguish between the maintenance appropriate for a 2-year establishment period and expected long-term maintenance.

For example, maintenance requirements for vegetation in a constructed wetland may be more intensive during the first few years until the vegetation is established. The postestablishment maintenance plan shall address maintenance needs (e.g. pruning, irrigation, weeding) for a larger, more stable system.

Include maintenance performance procedures for facility components that require relatively unique maintenance knowledge, such as specific plant removal/replacement, landscape features, or constructed wetland maintenance. These procedures shall provide enough detail for a person unfamiliar with maintenance to perform the activity or identify the specific skills or knowledge necessary to perform and document the maintenance.  Identification of the equipment and materials required to perform the maintenance.  Parties responsible for the O&M plan and number of years for which records will be retained.
 As appropriate, list all housekeeping procedures for prohibiting illicit discharges or potential illicit discharges to the storm drain. Identify housekeeping BMPs that reduce maintenance of treatment control measures.
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Development Guidance Manual J3-2 May 2025
IV. Training
Identify persons to be trained and when the training occurs. Training shall include:
a. Good housekeeping procedures defined in the plan.
b. Proper maintenance of all pollution mitigation devices. c. Identification and cleanup procedures for spills and overflows. d. Large-scale spill or hazardous material response.
e. Safety concerns when maintaining devices and cleaning spills.

V. Inspections
Create and maintain on site, a log for inspector names, dates, and stormwater control measure devices to be inspected and maintained. Provide a checklist for each inspection and maintenance category (see Example Maintenance Log). Maintenance activities may include, but not be limited to the following:
 Perform annual testing of any mechanical or electrical devices prior to wet weather.  Report any significant changes in stormwater control measures to the site management. As appropriate, assure mechanical devices are working properly and/or landscaped BMP plantings are irrigated and nurtured to promote thick growth.  Note any significant maintenance requirements due to spills or unexpected discharges.  As appropriate, perform maintenance and replacement as scheduled and as needed in a timely manner to assure stormwater control measures are performing as designed and approved.
 Assure unauthorized low-flow discharges from the property do not by-pass stormwater control measures.
 Perform an annual assessment of each pollution generation operation and its associated stormwater control measures to determine if any part of the pollution reduction train can be improved.
 For trash controls, regular maintenance is required to maintain adequate trash capture capacity and ensure that trapped trash does not migrate offsite. The owner should establish a maintenance schedule based on site-specific factors, including the size of the detention BMP, storm frequency, and estimated or measured trash loading area.

VI. Additional Information
Spill Plan
 Provide a spill plan and include spill plan documents as an appendix, if applicable.  Provide emergency notification procedures (phone and agency/persons to contact).  As appropriate for site, provide emergency containment and cleaning procedures. #6..
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Development Guidance Manual J3-3 May 2025
Facility Changes
Operational or facility changes which significantly affect the character or quantity of pollutants discharging into the stormwater control measures will require modifications to the Maintenance Plan and/or additional stormwater control measures.

Revisions to Pollution Mitigation Measures
If future correction or modification of pass stormwater control measures or procedures is required, the owner shall obtain approval from the governing stormwater agency prior to commencing any work. Corrective measures or modifications shall not cause discharges to bypass or otherwise impede existing stormwater control measures.

Monitoring & Reporting Program
The City may conduct a site inspection to evaluate compliance with the SWQCP Report and/or require a Monitoring & Reporting Program to assure the stormwater control measures approved for the site are performing according to design.

If required, the Maintenance Plan shall include performance testing and reporting protocols.

Site Map
 Provide a site map showing boundaries of the site, acreage and drainage patterns/contour lines. Show each discharge location from the site and any drainage flowing onto the site.

Distinguish between soft and hard surfaces on the map.
 Identify locations of existing and proposed storm drain facilities, private sanitary sewer systems and grade-breaks for purposes of pollution prevention.  With legend, show locations of expected sources of pollution generation (outdoor work and storage areas, heavy traffic areas, delivery areas, trash enclosures, fueling areas, industrial clarifiers, wash-racks, etc.). Identify any areas having contaminated soil or where toxins are stored or have been stored/disposed of in the past.
 With legend, indicate types and locations of stormwater control measures that will be built to permanently control stormwater pollution. Distinguish between pollution prevention, treatment, sewer diversion, and containment devices.
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Development Guidance Manual J4-1 May 2025
APPENDIX J-4: STORMWATER MAINTENANCE PLAN TEMPLATE
Stormwater Maintenance Plan – Cover Page
Project Name:

Street:

City: State: Zip Code:

Assessor’s Parcel Number (APN):

Property Owner/Developer Information:

Name: _______________________________
Title:_________________________________
Street: _______________________________
City:_________ State:______ Zip:_________
Phone Number:________________________
Email Address: ________________________
Operations & Maintenance Contact Person:

Name: _______________________________
Title:_________________________________
Street: _______________________________
City:________ State:______ Zip:__________
Phone Number:________________________
Email Address: ________________________
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Development Guidance Manual J4-2 May 2025
II. Baseline Descriptions
If applicable list persons responsible for operation and maintenance of the stormwater control measures on site not mentioned above. Include contact information for each person listed (phone number, emails, and address).

Identify the intended method of providing financing for operation, inspection, routine maintenance, and upkeep of stormwater control measures.

Provide a description of the types of O&M activities that will take place at this site List all permanent stormwater control measures (see SWQCP Section D).

Quantity Stormwater Control Measure
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Development Guidance Manual J4-3 May 2025
III. Routine Maintenance
List and describe all maintenance and waste disposal activities that will be performed and distinguish between the maintenance appropriate for a 2-year establishment period and expected long-term maintenance.

Routine Maintenance Activities for:

No. Maintenance Task Frequency of Task
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Development Guidance Manual J4-4 May 2025
Identification of the equipment and materials required to perform the maintenance.

Parties responsible for the O&M plan shall retain records for at least: Years.

List all housekeeping procedures for prohibiting illicit discharges or potential illicit discharges to the storm drain system. Identify housekeeping BMP’s that reduce maintenance of treatment control measures. #6..
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Development Guidance Manual J4-5 May 2025
IV. Training
Identify appropriate persons to be trained and when the training shall occur.

VI. Additional Information
Spill Plan
Provide a spill plan and include spill plan documents as an appendix, if applicable.

Downstream Receiving Body of Water:

1. Provide emergency notification procedures (phone number and agency/person(s) to contact).

2. As appropriate for site, provide emergency containment cleaning procedures.

Facility Changes
Operational or facility changes which significantly affect the character or quantity of pollutants discharging into the stormwater control measures will require modifications to the Maintenance Plan and/or additional stormwater control measures.

Revisions to Pollution Mitigation Measures
If future correction or modification of pass stormwater control measures or procedures is required, the owner shall obtain approval from the governing stormwater agency prior to commencing any work. Corrective measures or modifications shall not cause discharge to by-pass or otherwise impede existing stormwater control measures.

Monitoring and Reporting Program
The City may conduct a site inspection to evaluate compliance with the SWQCP Report and/or require a Monitoring & Reporting Program to assure the stormwater control measures approved for the site are performing according to design.

If required, the Maintenance Plan shall include performance testing and reporting protocols. #6..
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Development Guidance Manual J4-6 May 2025
Site Map
 Provide a site map showing boundaries of the site, acreage and drainage patterns/contour lines. Show each discharge location from the site and any drainage flowing onto the site. Distinguish between soft and hard surfaces on the map.  Identify locations of existing and proposed storm drain facilities, private sanitary sewer systems and grade-breaks for purposes of pollution prevention.
 With legend, show locations of expected sources of pollution generation (outdoor work and storage areas, heavy traffic areas, delivery areas, trash enclosures, fueling areas, industrial clarifiers, wash-racks, etc.). Identify any areas having contaminated soil or where toxins are stored or have been stored/disposed of in the past.  With legend, indicate types and locations of stormwater control measures that will be built to permanently control stormwater pollution. Distinguish between pollution prevention, treatment, sewer diversion, and containment devices. #6..
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Development Guidance Manual J4-7 May 2025
Example Maintenance Log
Facility Name:

Address:

BMP Type/ID:

Year: Maintenance Log No.

Date of
Inspection Inspectors Name Trash(Y/N)
Sediment
(Y/N)
Blocked
Inlet/Outlet
(Y/N)
Performed All Routine
Maintenance (Y/N) Comments / Notes

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APPENDIX K

HYDROLOGIC SOIL GROUPS
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Development Guidance Manual K-1 May 2025
APPENDIX K. HYDROLOGIC SOIL GROUPS
This appendix includes information on the Hydrologic Soil Groups in the City of Modesto to use in designing various stormwater control measures:

Relevance of Hydrologic Soil Groups Information
The hydrologic soil groups of a development area are pertinent to design of controls that involve infiltration and for identifying sites appropriate for detention basins. The predominant soil group will control the effectiveness of infiltration facilities or the suitability of an area for impounding water. Hydrologic soil group information should be used for preliminary siting studies only; actual design should be based on in-situ soil investigations and testing by a qualified engineer or geologist.

Table F-1. Typical Infiltration Rates
Soil Type Infiltration Rate (in/hr)
A 1.00 – 8.3
B 0.50 – 1.00
C 0.17 – 0.27
D 0.02 – 0.10
Infiltration rates shown represent the range covered by multiple sources such as ASCE.

Hydrologic Soil Groups
The hydrologic soil groups are classified by the United States Department of Agriculture (USDA) Natural Resources Conservation Service (NRCS), formerly the Soil Conservation Service. There are four hydrologic soil groups: A, B, C and D. Soils may be classified by two groups.

Soil Groups A and B
 Group A soils have a low runoff potential and high infiltration rate, as the soils typically are sands and gravel.
 Soil group B includes soils with moderate infiltration rates when completely wetted. Group B soils are sandy loam soils with moderately fine to moderately coarse textures.  Typically the best candidate soils for construction of infiltration facilities  Have high infiltration rates, unless the soils under consideration have been compacted during construction.

Soil Groups C and D
 Group C soils have slow infiltration rates when thoroughly wetted and these soils typically are silty-loam soils with an impeding layer or soils with moderately fine to fine texture.  Group D soils have a high runoff potential and very slow infiltration rate when thoroughly wetted. Group D soils include clay soils with high swelling potential, soils in a permanently high water table and shallow soils over nearly impervious material.  Usually more appropriate for detention basins.

This information is a general overview. For more specific information, consult the City of Modesto or contact the NRCS at (530) 662-3986.
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APPENDIX L

PLANTS SUITABLE FOR VEGETATIVE CONTROL

MEASURES
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Development Guidance Manual L-1 May 2025
APPENDIX L. PLANTS SUITABLE FOR VEGETATIVE

CONTROL MEASURES
Vegetation serves primarily to maintain soil porosity and prevent erosion. The effectiveness and aesthetic appeal of control measures are enhanced by selection of appropriate vegetative cover. Turf grass is preferred, and some other ground covers also may be appropriate. An important maintenance consideration in the selection of appropriate vegetation is whether irrigation is planned for the site.

Consult with City stormwater staff regarding the selection of appropriate vegetation and specifically with the Public Works Department - Trees and Forestry Services to identify which trees are appropriate for a given location and desired outcome/use [209-342-2253].

Table L-1 provides a sample list of appropriate vegetative covers. Additional suggested vegetative species are listed in Table L-2 and L-3.1
The tables are intended as guides in selecting vegetative covers. For specific species suitability and care information, refer to the sources listed for these tables.

Table L-1. Sample List of Appropriate Vegetative Covers Plant Name
Common (Latin)
Appropriate
Species Maintenance and Usage Notes*
Bermuda Grass
(Cynodon)
Santa Ana
hybrid
Common
Moderate maintenance. Dormant (brown) in winter.

Heat tolerant. Erosion control, swales.

Fescue (Festuca) Red fescue
(F. rubra)
Low to moderate maintenance. Tolerates some shade and poor soil. Lawns, swales, erosion control.
“Kentucky 31"
Tall Fescue
(F. elatior)
Low maintenance. Tolerate shade and compacted soils. Rapid germination. Lawns, swales, erosion control. Useful as overseed for Bermuda grass during dormant (winter) season.

Ryegrass (Lolium) Perennial
(L. perenne)
Moderate maintenance. Heat intolerant. Fast sprouting. Useful as overseed for Bermuda grass during dormant (winter) season.

Swales.

Annual
(L. multiflorum)
Annual (may live several seasons in mild climate). Moderate maintenance. Heat intolerant. Fast growing. Useful as overseed for winter-dormant species. Swales.
*Generally, these species will require supplemental irrigation.

Sources: ASCE, MWCG, Sunset
1 California Native Plant Society https://calscape.org/loc-37.9176,-121.171%20(san%20joaquin%20county)/cat-all-plants/ord-popular #6..
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Development Guidance Manual L-2 May 2025
Table L-2. Additional Sample List of Appropriate Vegetative Covers Plant Name Common (Latin) Appropriate Species Usage Notes Orchard grass (Dactylis) “Akaroa” or “Berber”
(D. glomerata)
Irrigated and Non-irrigated Sites
Wheatgrass (Agropyron) “Luna” or “Topar” pubescent
(A. intermedium trichophorum)
Irrigated and Non-irrigated Sites
Zorro Fescue (Vulpia) (V. myuros) Irrigated and Non-irrigated Sites Creeping wild Rye
(Leymus)
(L. triticoides) Nonirrigated Sites
Brome (Bromus) Blando
(B. mollis)
Nonirrigated Sites
California or “Cucamonga”
(B. carinatus)
Nonirrigated Sites
Sources: NRCS-FOTG. Manual of Standards for Erosion and Sediment Control Measures, Association of Bay Area Governments, 1995.
#6..
- 650 -
Development Guidance Manual L-3 May 2025
Botanical Name Common Name Height
Water Requirement Sun Requirement Deciduous/
Extremely Evergreen
Low
Very
Low Low Moderate - High Full Sun Part Shade Shade
Annual Herbs
Achyrachaena mollis Blow Wives 1 - 1.3 ft X X
Acmispon americanus American Bird's-foot
Trefoil
1 ft X X X X
Amaranthus blitoides Procumbent Pigweed
Amaranthus palmeri Palmer's Amaranth
Ammannia coccinea Valley Redstem 1.6 ft
Amsinckia douglasiana Douglas' Fiddleneck 0.7 - 3.9
ft
Amsinckia eastwoodiae Eastwood's Fiddleneck 0.7 - 3.9
ft
X
Amsinckia intermedia Common Fiddleneck 0.7 - 3.9
ft
X X
Amsinckia lycopsoides Bugloss-flowered
Fiddleneck

0.7 - 3 ft
Amsinckia menziesii Menzies' Fiddleneck 0.7 - 3.9
ft
X
Amsinckia tessellata Fiddleneck 0.7 - 3.9
ft
X
Amsinckia tessellata var.
tessellata
Desert Fiddleneck 0.7 - 3.9
ft
Ancistrocarphus filagineus Woolly Fishhooks
Astragalus didymocarpus Dwarf White Milkvetch 1 - 11.8
in
Astragalus gambelianus Gambel's Dwarf Milk
Vetch

0.8 -

11.8 in
X
Astragalus tener Alkali Milkvetch 0.8 -

11.8 in
Astragalus tener var. tener Alkali Milkvetch 1.6 -

11.8 in
Table L-3. Stanislaus County Native Plant List (obtained from the California Native Plant Society) #6..
- 651 -
Development Guidance Manual L-4 May 2025
Botanical Name Common Name Height
Water Requirement Sun Requirement Deciduous/
Extremely Evergreen
Low
Very
Low Low Moderate - High Full Sun Part Shade Shade
Annual Herbs (continued)
Athysanus pusillus Common Sandweed
Atriplex argentea Silver Saltweed 0.49 -

2.6 ft
X X
Atriplex argentea var.
expansa
Silverscale 0.49 -

2.6 ft
X
Atriplex cordulata Heartscale 0.33 -

1.6 ft
Atriplex coronata Crownscale 4 - 11.8
in
Atriplex coronata var.
coronata
Crownscale 4 - 11.8
in
Atriplex depressa Brittlescale 11.8 in
Atriplex minuscula Lesser Saltscale 1.3 ft
Atriplex persistens Vernal Pool Smallscale 9.8 in
Atriplex serenana Tractscale 1 - 3.3 ft X
Atriplex serenana var.
serenana
Bractscale 1 - 3.3 ft
Atriplex subtilis Subtle Orach 11.8
Bidens frondosa Sticktight 0.7 - 4 ft
Blennosperma nanum Common Stickyseed
Blennosperma nanum var.
nanum
Common Blennosperma
Blepharizonia laxa Glandular Big Tarweed
Bowlesia incana Hoary Bowlesia
Calandrinia menziesii Red Maids 1.3 ft X X
Callitriche marginata [a] Winged Water Starwort
Camissonia campestris Field Primrose 1.9 - 9.8
in
Table L-3. Stanislaus County Native Plant List (obtained from the California Native Plant Society) #6..
- 652 -
Development Guidance Manual L-5 May 2025
Botanical Name Common Name Height
Water Requirement Sun Requirement Deciduous/
Extremely Evergreen
Low
Very
Low Low Moderate - High Full Sun Part Shade Shade
Annual Herbs (continued)
Camissonia campestris ssp.
campestris
Mojave Suncup
Camissonia contorta Plains Evening Primrose 1.2 -

11.8 in
Castilleja attenuata Valley Tassels 1.6 ft
Castilleja exserta Purple Owl's Clover 1.5 ft X X X
Castilleja exserta ssp. exserta Pale Purple Owlclover
Centromadia fitchii Fitch Spikeweed 1.6 ft
Centromadia pungens Common Spikeweed 4 ft X
Centromadia pungens ssp.
pungens
Common Spikeweed
Chenopodium berlandieri Pitseed Goosefoot 0.33 -

9.8 ft
Clarkia modesta Waltham Creek Clarkia 1.6 ft
Clarkia purpurea Purple Clarkia 3.3 ft X X
Clarkia purpurea ssp.
purpurea
Winecup Clarkia 3.3 ft X X
Clarkia purpurea ssp.
quadrivulnera
Purple Clarkia 3.3 ft X
Clarkia speciosa Red Spotted Clarkia 2 ft X X X
Clarkia unguiculata Elegant Clarkia 3.3 ft X X X
Claytonia perfoliata Miner's Lettuce 1 - 1.3 ft X X X
Claytonia perfoliata ssp.
perfoliata
Miner's Lettuce 1.3 ft X X
Collinsia bartsiifolia White Blue-eyed Mary
Collinsia bartsiifolia var.
bartsiifolia
White Blue Eyed Mary 0.33 -

1.2 ft
Collinsia sparsiflora Spinster's Blue-eyed
Mary
Table L-3. Stanislaus County Native Plant List (obtained from the California Native Plant Society) #6..
- 653 -
Development Guidance Manual L-6 May 2025
Botanical Name Common Name Height
Water Requirement Sun Requirement Deciduous/
Extremely Evergreen
Low
Very
Low Low Moderate - High Full Sun Part Shade Shade
Annual Herbs (continued)
Collinsia sparsiflora var.
collina
Spinster's Blue Eyed Mary 1.9 - 7.9
in
Convolvulus simulans Small-flowered Morningglory
Croton Setiger Turkey Mullein 7.9 in X X X
Crassula aquatica [c] Water Pygmyweed
Crassula connata [c] Pygmy-weed 0.8 - 4 in
Cryptantha nevadensis Nevada Catseye 1.6 ft
Cucurbita foetidissima [a] Missouri Gourd 1 ft X X X
Cuscuta californica [d] Chaparral Dodder
Cuscuta campestris [d] Field Dodder
Cuscuta indecora [d] Bigseed Alfalfa Dodder
Cuscuta subinclusa [d] Canyon Dodder
Deinandra kelloggii Kellogg's Tarweed 3.3 - 4.9
ft
X
Descurainia pinnata Western Tansymustard
Downingia bella Hoover's Downingia
Downingia bicornuta Doublehorn Calicoflower 9.6 in
Downingia bicornuta var.
picta
Doublehorn Calicoflower
Downingia ornatissima Folded Calicoflower
Downingia ornatissima var.
ornatissima
Folded Calicoflower
Downingia pulchella Flatface Calicoflower
Eclipta prostrata False Daisy 3.3 ft X
Epilobium brachycarpum Panicled Willow Herb 0.7 - 7 ft X X Epilobium campestre Smooth Boisduvalia 0.33 -

1.8 ft
Table L-3. Stanislaus County Native Plant List (obtained from the California Native Plant Society) #6..
- 654 -
Development Guidance Manual L-7 May 2025
Botanical Name Common Name Height
Water Requirement Sun Requirement Deciduous/
Extremely Evergreen
Low
Very
Low Low Moderate - High Full Sun Part Shade Shade
Annual Herbs (continued)
Epilobium cleistogamum Selfing Willowherb 0.49 -

1.1 ft
Epilobium densiflorum Denseflower Willowherb 0.16 -

3.3 ft
Eriastrum pluriflorum Many Flowered Eriastrum 0.07 - 1
ft
X X
Eriogonum gracile Slender Woolly
Buckwheat
1 - 2 ft X X X D
Eriogonum nudum var.
pubiflorum
Fremont's Wild
Buckwheat
1 - 3.3 ft
Eriogonum roseum Wand Buckwheat 1.6 - 2.6
ft
X
Eryngium racemosum [a] Delta Button-celery
Erysimum capitatum [a] Sanddune Wallflower X
Erythranthe guttata [a] Seep Monkey Flower 2 - 5 ft X X X D Eschscholzia californica [a] California Poppy 0.16 - 2
ft

X X X D
Eschscholzia lobbii Fryingpans 1.9 - 6 in X X
Galium aparine Common Bedstrawm
Cleavers

10.8 in
Gilia capitata Blue Field Gilia 0.33 - 3
ft
X X
Gilia capitata ssp. staminea Bluehead Gilia
Gilia tricolor Tricolor Gilia 0.33 -

1.2 ft
X X
Gilia tricolor ssp. tricolor Bird's-eye Gilia
Gnaphalium palustre Western Marsh Cudweed 11.8 in
Gutierrezia californica [b] California Matchweed 1 - 2 ft X X D Helianthus annuus Sunflower 5 - 10 ft X X
Table L-3. Stanislaus County Native Plant List (obtained from the California Native Plant Society) #6..
- 655 -
Development Guidance Manual L-8 May 2025
Botanical Name Common Name Height
Water Requirement Sun Requirement Deciduous/
Extremely Evergreen
Low
Very
Low Low Moderate - High Full Sun Part Shade Shade
Annual Herbs (continued)
Hesperevax caulescens Hogwallow Starfish
Hesperomecon linearis Narrow-leaved Meconella 0.1 - 1.3 ft
Heterotheca sessiliflora [a] False Goldenaster 2 - 4 ft X X X Holocarpha obconica San Joaquin Tarweed 3.9 ft
Holocarpha virgata Pitgland Tarweed 0.7 - 4 ft X
Holocarpha virgata ssp.
virgata
Narrow Tarplant 4 ft X
Laennecia coulteri Coulter's Horseweed
Lasthenia californica California Goldfields 0.5 - 1.3
ft

X X X X
Lasthenia chrysantha Alkali-sink Goldfields 11 in
Lasthenia ferrisiae Ferris' Goldfields 1.3 ft
Lasthenia fremontii [a] Fremont's Goldfields 1.2 ft
Lasthenia glaberrima Smooth Goldfields 1.2 ft
Lasthenia gracilis Common Goldfields
Lasthenia microglossa Small-ray Goldfields 9.8 in
Lasthenia minor Coastal Goldfields 1.2 ft
Lasthenia platycarpha Alkali Goldfields 11.8 in
Layia chrysanthemoides Smooth Tidy Tips 1.7 ft X
Layia fremontii Fremont's Tidytips 1.3 ft
Lepidium acutidens [a] Alkali Pepperwort
Lepidium dictyotum Alkali Pepperweed
Lepidium latipes San Diego Pepperweed 4 in
Lepidium nitidum Peppergrass 1.3 ft X
Leptosiphon bicolor Bicolor Linanthus 0.8 - 6 in X X
Table L-3. Stanislaus County Native Plant List (obtained from the California Native Plant Society) #6..
- 656 -
Development Guidance Manual L-9 May 2025
Botanical Name Common Name Height
Water Requirement Sun Requirement Deciduous/
Extremely Evergreen
Low
Very
Low Low Moderate - High Full Sun Part Shade Shade
Annual Herbs (continued)
Lessingia nemaclada Slenderstem Lessingia 1.2 in
Lessingia pectinata
Lessingia pectinata var.
tenuipes
Limnanthes douglasii Common Meadowfoam 1.6 ft X X X X
Limnanthes douglasii ssp.
rosea
Douglas' Meadowfoam 1.6 ft X
Limosella acaulis Owyhee Mudwort
Lindernia dubia Yellowseed False
Pimpernel

10.7 -

11.8 in
Lupinus benthamii Spider Lupine 0.7 - 2.3
ft
X X
Lupinus bicolor [a] Miniature Lupine 0.26 -

1.3 ft
X X
Lupinus microcarpus Chick Lupine 0.33 -

2.6 ft
X X
Lupinus microcarpus var.
densiflorus
Dense Flowered
Platycarpos

0.33 -

2.6 ft
X X
Lupinus microcarpus var.
microcarpus
Valley Lupine 0.33 -

2.6 ft
X X
Lupinus succulentus Succulent Lupine 2 - 4 ft X X X
Matricaria discoidea Pineapple Weed
Mentzelia affinis Yellowcomet 1.9 in
Micropus californicus Q-tips
Micropus californicus var.
californicus
Cottontop
Microseris campestris San Joaquin Silverpuffs 1.6 ft
Microseris douglasii Douglas' Silverpuffs 1.9 in
Table L-3. Stanislaus County Native Plant List (obtained from the California Native Plant Society) #6..
- 657 -
Development Guidance Manual L-10 May 2025
Botanical Name Common Name Height
Water Requirement Sun Requirement Deciduous/
Extremely Evergreen
Low
Very
Low Low Moderate - High Full Sun Part Shade Shade
Annual Herbs (continued)
Microseris douglasii ssp.
douglasii
Mimulus latidens Broadtooth
Monkeyflower
4 - 10.7
in
Mimulus pilosus False Monkeyflower 0.07 -

1.2 ft
Mimulus tricolor Tricolor Monkeyflower 5.5 in
Monolopia lanceolata Common Monolopia 2.6 ft X X X
Monolopia major Cupped Monolopia 2.6 ft
Montia fontana Water Chickweed
Myosurus minimus Tiny Mousetail 2.4 - 4.8
in
Myosurus sessilis Vernal Pool Mousetail 4 in
Najas guadalupensis Southern Waternymph 1 ft
Navarretia leucocephala Whitehead
Pincushionplant

1.2 - 5.9
in
Navarretia pubescens Downy Pincushionplant 0.46 -

1.1 ft
X
Nemophila pedunculata Littlefoot Nemophila X X
Oenothera deltoides [a] Dune Primrose 0.33 -

3.3 ft
X X X
Oenothera deltoides ssp.

Cognata [a]
Birdcage Evening
Primrose
Pectocarya penicillata Northern Pectocarya 0.8 - 9.8
in
Persicaria lapathifolia Willow Weed 0.2 - 2.6
ft
Phacelia ciliata Great Valley Phacelia 0.33 -

1.8 ft
X
Table L-3. Stanislaus County Native Plant List (obtained from the California Native Plant Society) #6..
- 658 -
Development Guidance Manual L-11 May 2025
Botanical Name Common Name Height
Water Requirement Sun Requirement Deciduous/
Extremely Evergreen
Low
Very
Low Low Moderate - High Full Sun Part Shade Shade
Annual Herbs (continued)
Phacelia ciliata var. ciliata Phacelia ciliata var. ciliata X X Phacelia distans Wild Heliotrope 0.16 -

2.6 ft
X X
Phacelia tanacetifolia Lacy Phacelia 2 - 4 ft X X
Pholistoma membranaceum White Fiesta Flower 0.16 - 3
ft
Plagiobothrys acanthocarpus Adobe Popcornflower 0.33 -

1.3 ft
Plagiobothrys bracteatus Bracted Popcornflower 0.33 -

1.3 ft
Plagiobothrys canescens Valley Popcornflower 0.33 - 2
ft
Plagiobothrys fulvus Field Popcornflower 2 ft
Plagiobothrys humistratus Low Popcornflower 0.33 -

1.3 ft
Plagiobothrys leptocladus Alkali Plagiobothrys 4 - 11.8 in
Plagiobothrys nothofulvus Rusty Popcornflower 0.7 - 2.3 ft
Plagiobothrys stipitatus Stalked Popcornflower 0.33 -

1.6 ft
Plagiobothrys stipitatus var.
micranthus
Stalked Popcornflower 0.33 -

1.6 ft
Plagiobothrys stipitatus var.
stipitatus
Showy Great Valley
Popcornflower

0.33 -

1.6 ft
Plagiobothrys tenellus Pacific Popcornflower 1.9 -

11.8 in
Plagiobothrys trachycarpus Rough-nutlet
Popcornflower

0.16 -

1.3 ft
Table L-3. Stanislaus County Native Plant List (obtained from the California Native Plant Society) #6..
- 659 -
Development Guidance Manual L-12 May 2025
Botanical Name Common Name Height
Water Requirement Sun Requirement Deciduous/
Extremely Evergreen
Low
Very
Low Low Moderate - High Full Sun Part Shade Shade
Annual Herbs (continued)
Plantago elongata Coast Plantain 7.1 in X
Platystemon californicus Creamcups 0.7 - 1 ft X X
Plectritis ciliosa Longspur Seablush 1.6 - 2.6
ft
Plectritis macrocera Longhorn Seablush 2 - 2.6 ft
Pluchea odorata [a] Marsh Fleabane 2 - 4 ft X X
Pluchea odorata var. odorata
[a]
Saltmarsh-fleabane 4.9 ft
Pogogyne douglasii Douglas' Pogogyne 0.8 - 1.5
ft
X
Pogogyne zizyphoroides Sacramento Beardstyle 1.9 - 6.2
in
Psilocarphus brevissimus Short Woollyheads 1.2 in
Psilocarphus brevissimus var.
brevissimus
Dwarf Woollyheads
Psilocarphus chilensis Round Woolly-marbles
Psilocarphus oregonus Oregon Woollyheads
Psilocarphus tenellus Slender Woolly-marbles
Ranunculus sceleratus Cursed Buttercup 1 - 2 ft
Rorippa curvisiliqua [a] Curvepod Yellowcress 1 ft
Rorippa palustris [a] Bog Yellowcress 3.3 ft
Rotala ramosior Lowland Rotala
Rigiopappus leptocladus Wireweed
Salvia carduacea Thistle Sage 0.33 -

3.3 ft
X X
Salvia columbariae Chia 0.33 -

1.6 ft
X X X
Table L-3. Stanislaus County Native Plant List (obtained from the California Native Plant Society) #6..
- 660 -
Development Guidance Manual L-13 May 2025
Botanical Name Common Name Height
Water Requirement Sun Requirement Deciduous/
Extremely Evergreen
Low
Very
Low Low Moderate - High Full Sun Part Shade Shade
Annual Herbs (continued)
Solanum americanum [b] American Black
Nightshade

2.6 - 4.9
ft
X X
Spergularia marina Saltmarsh Sand-spurrey
Stebbinsoseris heterocarpa Grassland Silverpuffs
Stephanomeria virgata Rod Wirelettuce 1.9 -

11.8 in
X
Stellaria nitens Shining Chickweed 9.8 in
Stephanomeria virgata ssp.
pleurocarpa
Wand Wirelettuce
Symphyotrichum subulatum Eastern Annual Saltmarsh
Aster
Thysanocarpus curvipes Hairy Lacepod 0.33 -

2.6 ft
X
Trichostema lanceolatum Vinegarweed 3.3 ft X
Trifolium albopurpureum Indian Clover 6 in X X
Trifolium ciliolatum Foothill Clover X X
Trifolium depauperatum Cowbag Clover E
Trifolium depauperatum var.
amplectens
Pale Sack Clover
Trifolium depauperatum var.
depauperatum
Dwarf Sack Clover
Trifolium depauperatum var.
truncatum
Truncate Sack Clover
Trifolium gracilentum Pinpoint Clover X X
Trifolium variegatum Whitetip Clover
Trifolium willdenovii Tomcat Clover X X
Triphysaria eriantha Johnny-tuck 1.2 ft
Triphysaria eriantha ssp.
eriantha
Butter'n Eggs
Table L-3. Stanislaus County Native Plant List (obtained from the California Native Plant Society) #6..
- 661 -
Development Guidance Manual L-14 May 2025
Botanical Name Common Name Height
Water Requirement Sun Requirement Deciduous/
Extremely Evergreen
Low
Very
Low Low Moderate - High Full Sun Part Shade Shade
Annual Herbs (continued)
Tropidocarpum gracile Dobie Pod 0.33 -

1.6 ft
Uropappus lindleyi Uropappus 2.3 ft X
Veronica peregrina Neckweed 11.8 in
Veronica peregrina ssp.
xalapensis
Purslane Speedwell
Wislizenia refracta Spectacle Fruit 0.16 -

6.6 ft
Wislizenia refracta ssp.
californica
Spectacle Fruit
Xanthium strumarium Cocklebur
Ferns
Azolla filiculoides Mosquito Fern 2.4 in X
Marsilea vestita Hairy Waterclover 1.1 ft X
Marsilea vestita ssp. vestita Hairy Waterclover 1.1 ft X Pilularia americana American Pillwort
Grasses
Agrostis exarata Spike Bentgrass 1 - 3.3 ft X X
Alopecurus carolinianus Carolina Foxtail 1.7 ft
Alopecurus saccatus Pacific Foxtail 1.5 ft
Bolboschoenus maritimus Alkali Bulrush 3 ft X X E
Bromus arizonicus Arizona Brome 1.3 - 3 ft X
Carex barbarae Valley Sedge 1.6 - 3.3
ft

X X X E
Cyperus eragrostis Tall Flatsedge 3 ft X X E
Cyperus erythrorhizos Red Rooted Cyperus 2.5 - 3.3
ft
X
Table L-3. Stanislaus County Native Plant List (obtained from the California Native Plant Society) #6..
- 662 -
Development Guidance Manual L-15 May 2025
Botanical Name Common Name Height
Water Requirement Sun Requirement Deciduous/
Extremely Evergreen
Low
Very
Low Low Moderate - High Full Sun Part Shade Shade
Grasses (continued)
Cyperus esculentus Chufa Sedge 8.4 in
Cyperus niger Black Flatsedge 1.6 ft
Deschampsia danthonioides Annual Hair Grass 1.3 - 2 ft
Distichlis spicata Saltgrass 1.1 - 1.6
ft
X X
Eleocharis acicularis Spike Rush 5.9 - 8.4
in
X X
Eleocharis macrostachya Common Spikerush 1.6 - 3.3
ft
X X E
Elymus multisetus Big Squirreltail 2 ft X X
Eragrostis hypnoides Creeping Love Grass 4 in
Eragrostis mexicana Mexican Lovegrass
Eragrostis mexicana ssp.
virescens
Chilean Love Grass
Eragrostis pectinacea Tufted Lovegrass 0.33 -

2.6 ft
Eragrostis pectinacea var.
pectinacea
Carolina Love Grass
Eriochloa acuminata Tapertip Cupgrass
Festuca microstachys Pacific Fescue 2.5 ft X X
Fimbristylis vahlii Vahl's Fimbry 1.2 in
Hordeum depressum Low Barley 1.8 ft X X
Juncus acuminatus Taper Tip Rush 1.5 - 3 ft X
Juncus balticus Baltic Rush 3 ft X X E
Juncus balticus ssp. ater Baltic Rush
Juncus bufonius Toad Rush 1 ft X X
Juncus bufonius var. bufonius Toad Rush X
Table L-3. Stanislaus County Native Plant List (obtained from the California Native Plant Society) #6..
- 663 -
Development Guidance Manual L-16 May 2025
Botanical Name Common Name Height
Water Requirement Sun Requirement Deciduous/
Extremely Evergreen
Low
Very
Low Low Moderate - High Full Sun Part Shade Shade
Grasses (continued)
Juncus bufonius var.
congestus
Clustered Toad Rush 1 ft X
Juncus effusus Soft Rush 4.9 - 6.6
ft
X X E
Juncus occidentalis Slender Juncus Rush 1 - 2 ft X X
Juncus bufonius var.
occidentalis
Western Toad Rush
Juncus oxymeris Pointed Rush 2 ft X X X
Juncus xiphioides Irisleaf Rush 1 - 3 ft X X E
Leersia oryzoides Rice Cutgrass 3.3 - 5 ft X
Leptochloa fusca Bearded Sprangletop
Leptochloa fusca ssp.
fascicularis
Bearded Sprangletop 3.3 ft
Melica californica California Melicgrass 1 - 4.3 ft X X X Muhlenbergia rigens Deergrass 4 - 5 ft X X E
Panicum capillare Witch Grass 3.3 ft
Paspalum distichum Knotgrass 0.9 - 2 ft X
Phalaris lemmonii Lemmon's Canary Grass 2.5 - 4.9
ft
Poa secunda One Sided Blue Grass 1.4 - 3.3
ft
X
Poa secunda ssp. secunda Pine Bluegrass 3.3 ft X
Puccinellia simplex California Alkaligrass 9.8 in
Schoenoplectus acutus Hardstem Bulrush 3 - 10 ft X X E
Schoenoplectus acutus var.
occidentalis
Tule 13 ft X
Stipa cernua Nodding Needle Grass 1.7 - 3.3
ft

X X X D
Table L-3. Stanislaus County Native Plant List (obtained from the California Native Plant Society) #6..
- 664 -
Development Guidance Manual L-17 May 2025
Botanical Name Common Name Height
Water Requirement Sun Requirement Deciduous/
Extremely Evergreen
Low
Very
Low Low Moderate - High Full Sun Part Shade Shade
Perennial Herbs
Alisma triviale Water Plantain 1 ft X X
Allium serra Jeweled Onion X X
Ambrosia psilostachya Western Ragweed 3.5 - 7 ft X X X D Artemisia douglasiana Douglas' Sagewort 8 ft X X X X X D Asclepias fascicularis Narrow Leaf Milkweed 1.7 - 3.3
ft

X X X D
Astragalus asymmetricus San Joaquin Milk Vetch 1.6 - 3.9 ft
X X
Astragalus douglasii Douglas' Milkvetch 0.7 - 3.3
ft
Atriplex fruticulosa Ballscale 1 - 1.6 ft
Baccharis glutinosa Saltmarsh Baccharis 3.3 - 7 ft X X
Bacopa eisenii Gila River Water Hyssop
Bidens laevis Bur Marigold 0.5 - 8 ft X X
Brodiaea elegans Harvest Brodiaea 1.6 ft X X X
Brodiaea elegans ssp.
elegans
Harvest Brodiaea 1.6 ft X X
Callitriche marginata [a] Winged Water Starwort
Castilleja affinis [a] Indian Paintbrush 2 ft X
Castilleja affinis ssp. Affinis
[a]
Coast Indian Paintbrush 2 ft X
Ceratophyllum demersum Hornwort 3.3 - 9.8
ft
Chenopodium californicum California Goosefoot 0.7 - 3.3 ft
Clematis ligusticifolia [h] Virgin's Bower 1 - 30 ft X X X D Cressa truxillensis Alkali Weed 9.8 in X X
Cucurbita foetidissima [a] Missouri Gourd 1 ft X X X
Table L-3. Stanislaus County Native Plant List (obtained from the California Native Plant Society) #6..
- 665 -
Development Guidance Manual L-18 May 2025
Botanical Name Common Name Height
Water Requirement Sun Requirement Deciduous/
Extremely Evergreen
Low
Very
Low Low Moderate - High Full Sun Part Shade Shade
Perennial Herbs (continued)
Datura wrightii Toluaca 1 - 5 ft X X X
Delphinium parryi San Bernardino Larkspur 2 - 4 ft X X
Delphinium parryi ssp. parryi Parry's Larkspur 0.5 - 3.5 ft
X X
Delphinium variegatum Royal Larkspur 1.6 - 2.8
ft
X X X
Delphinium variegatum ssp.
variegatum
Royal Larkspur
Dichelostemma capitatum
ssp. capitatum
Blue Dicks X X
Dipterostemon capitatus Blue Dicks 1.5 - 2 ft X X D
Elatine californica California Waterwort
Elodea canadensis Elodea X
Epilobium ciliatum Northern Willow Herb 1.6 - 6 ft X X X Epilobium ciliatum ssp.
ciliatum
Fringed Willowherb 6 ft X
Erigeron foliosus Leafy Fleabane 0.7 - 3.3
ft
X
Erigeron foliosus var. foliosus
[e]
Leafy Fleabane
Eryngium castrense Great Valley Coyotethistle

1.6 ft
Eryngium racemosum [a] Delta Button-celery
Eryngium vaseyi Coyotethistle
Erysimum capitatum [a] Sanddune Wallflower X
Erythranthe guttata [a] Seep Monkey Flower 2 - 5 ft X X X D Eschscholzia californica [a] California Poppy 0.16 - 2
ft

X X X D
Table L-3. Stanislaus County Native Plant List (obtained from the California Native Plant Society) #6..
- 666 -
Development Guidance Manual L-19 May 2025
Botanical Name Common Name Height
Water Requirement Sun Requirement Deciduous/
Extremely Evergreen
Low
Very
Low Low Moderate - High Full Sun Part Shade Shade
Perennial Herbs (continued)
Euthamia occidentalis Western Goldentop 3.5 - 7 ft X X
Frankenia salina Alkali Heath 1 ft X X D
Fritillaria agrestis Stinkbells 1.6 ft X
Galium trifidum Threepetal Bedstraw 0.8 - 1.6
ft
X
Glycyrrhiza lepidota Wild Licorice 1.3 - 4 ft X X
Grindelia camporum Great Valley Gumweed 2 - 6.6 ft X X D Gutierrezia californica [b] California Matchweed 1 - 2 ft X X D Helenium puberulum Sneezeweed 5 ft X X
Helianthus petiolaris [a] Prairie Sunflower
Heliotropium curassavicum Seaside Heliotrope 1.2 - 1.6
ft
X X
Heliotropium curassavicum
var. oculatum
Seaside Heliotrope 0.33 - 2
ft
Heterotheca grandiflora [a] Telegraph Weed 2.5 - 3.5
ft
X X D
Heterotheca sessiliflora [a] False Goldenaster 2 - 4 ft X X X Heterotheca sessiliflora ssp.
echioides
Bristly Goldenaster 4 ft X X
Hydrocotyle verticillata Whorled Marsh
Pennywort
X
Iva axillaris Poverty Weed
Lasthenia fremontii [a] Fremont's Goldfields 1.2 ft
Lathyrus jepsonii Jepson's Pea 6.6 ft X X X
Lathyrus jepsonii var.
californicus
California Pea 8.2 ft X X
Lemna gibba Thallus
Lepidium acutidens [a] Alkali Pepperwort
Table L-3. Stanislaus County Native Plant List (obtained from the California Native Plant Society) #6..
- 667 -
Development Guidance Manual L-20 May 2025
Botanical Name Common Name Height
Water Requirement Sun Requirement Deciduous/
Extremely Evergreen
Low
Very
Low Low Moderate - High Full Sun Part Shade Shade
Perennial Herbs (continued)
Lithophragma cymbalaria Mission Woodland Star 0.33 -

1.2 ft
Lomatium utriculatum Common Lomatium 1.6 ft X X
Lupinus bicolor [a] Miniature Lupine 0.26 -

1.3 ft
X X
Lupinus formosus Summer Lupine 0.7 - 2.6
ft
X X
Lycopus americanus American Water
Horehound

0.7 - 1.8
ft
X
Lomatium caruifolium Caraway Leaved
Lomatium

0.49 -

1.5 ft
X
Lomatium caruifolium var.
caruifolium
Alkali Desertparsley 1.5 ft X
Lupinus formosus var.
robustus
Summer Lupine 0.7 - 2.6
ft
Malvella leprosa Alkali Mallow 0.33 -

1.3 ft
X X
Nitrophila occidentalis Boraxweed 1 ft
Oenothera deltoides [a] Dune Primrose 0.33 -

3.3 ft
X X X
Oenothera deltoides ssp.

Cognata [a]
Birdcage Evening
Primrose
Oenothera elata Hooker's Evening
Primrose
5 ft X X X
Oenothera elata ssp.
hirsutissima
Hairy Evening Primrose X X
Persicaria amphibia Swamp Knotweed 7.9 - 1.2
in
X D
Persicaria hydropiperoides Water Pepper 1 - 3.3 ft X D
Table L-3. Stanislaus County Native Plant List (obtained from the California Native Plant Society) #6..
- 668 -
Development Guidance Manual L-21 May 2025
Botanical Name Common Name Height
Water Requirement Sun Requirement Deciduous/
Extremely Evergreen
Low
Very
Low Low Moderate - High Full Sun Part Shade Shade
Perennial Herbs (continued)
Persicaria punctata Water Smartweed 0.49 -

3.3 ft
X D
Phyla nodiflora Common Lippia 2.4 - 6 in X
Pluchea odorata [a] Marsh Fleabane 2 - 4 ft X X
Pluchea odorata var. odorata
[a]
Saltmarsh-fleabane 4.9 ft
Potamogeton nodosus Long-leaved Pondweed
Primula clevelandii Padre's Shootingstar 0.5 - 1 ft X X X D Pseudognaphalium
stramineum
Chilean Cudweed 2.3 ft X
Ranunculus californicus California Buttercup 0.6 - 2.3
ft

X X X X D
Ranunculus canus Sacramento Valley
Buttercup

0.36 -

2.1 ft
Rorippa curvisiliqua [a] Curvepod Yellowcress 1 ft
Rorippa palustris [a] Bog Yellowcress 3.3 ft
Sagittaria latifolia Wappato 0.7 - 4.9
ft
X
Sagittaria montevidensis
Sagittaria montevidensis ssp.
calycina
Sagittaria sanfordii Sanford's Arrowhead 4.3 ft
Sanicula bipinnata Poison Sanicle 2 ft X
Sesuvium verrucosum Western Sea-purslane
Solanum americanum [b] American Black
Nightshade

2.6 - 4.9
ft
X X
Solidago velutina ssp.
californica
California Goldenrod 1.5 - 5 ft X X X X E
Table L-3. Stanislaus County Native Plant List (obtained from the California Native Plant Society) #6..
- 669 -
Development Guidance Manual L-22 May 2025
Botanical Name Common Name Height
Water Requirement Sun Requirement Deciduous/
Extremely Evergreen
Low
Very
Low Low Moderate - High Full Sun Part Shade Shade
Perennial Herbs (continued)
Spergularia macrotheca Large Flowered Sand
Spurry
X
Spergularia macrotheca var.
leucantha
Stachys albens White Hedge Nettle 1.6 - 8 ft X X X E
Suaeda nigra Bush Seepweed 3 - 4.9 ft X X E
Symphyotrichum subulatum
var. parviflorum
Triteleia laxa Ithuriel's Spear 2 ft X X X X D
Typha domingensis Southern Cattail 13 ft X X
Urtica dioica Common Nettle
Urtica dioica ssp. holosericea Hoary Nettle
Utricularia macrorhiza Common Bladderwort X
Verbena bracteata Bracted Vervain
Shrubs
Allenrolfea occidentalis Iodine Bush 1 - 7 ft X
Baccharis pilularis Coyote Bush 1.5 - 10
ft

X X X X E
Baccharis salicifolia Mulefat 6 - 12 ft X X E/D
Cephalanthus occidentalis California Buttonbush 3 - 20 ft X X D Diplacus aurantiacus Bush Monkey Flower 3.9 - 5 ft X X X X E Ericameria linearifolia Linear Leaved
Goldenbush
5 ft X X
Erigeron foliosus var. foliosus
[e]
Leafy Fleabane
Eriogonum nudum Nude Buckwheat 0.5 - 1 ft X X X D
Frangula californica ssp.
tomentella
Hoary Coffeeberry 20 ft X X X
Table L-3. Stanislaus County Native Plant List (obtained from the California Native Plant Society) #6..
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Development Guidance Manual L-23 May 2025
Botanical Name Common Name Height
Water Requirement Sun Requirement Deciduous/
Extremely Evergreen
Low
Very
Low Low Moderate - High Full Sun Part Shade Shade
Shrubs (continued)
Gutierrezia californica [b] California Matchweed 1 - 2 ft X X D Isocoma acradenia Alkali Goldenbush 3.3 ft
Isocoma acradenia var.
bracteosa
Lupinus albifrons Silver Lupine 3.2 - 5 ft X X E
Lupinus albifrons var.
albifrons
Silver Lupine 3 - 5 ft X
Quercus wislizeni [f] Interior Live Oak 15 - 50 ft X X X X E Rosa californica California Wildrose 8 - 10 ft X X X X X D Rubus ursinus [g] Pacific Blackberry 2 - 6 ft X X X X D Salix exigua [f] Sandbar Willow 10 - 23 ft X X D
Salix exigua var. hindsiana [f] Sandbar Willow 16.4 ft X D Salix lasiandra var. lasiandra
[f]
Yellow Willow 33 - 53 ft X D
Salix lasiolepis [f] Arroyo Willow 7 - 35 ft X X D
Sambucus nigra [f] Black Elderberry 13.1 - 30
ft
X D
Sambucus nigra ssp.

Caerulea [f]
Blue Elderberry 20 - 30 ft X X X X D
Senecio flaccidus Threadleaf Ragwort 5 ft X X X
Senecio flaccidus var.
douglasii
Douglas' Groundsel 5 ft X
Solanum americanum [b] American Black
Nightshade

2.6 - 4.9
ft
X X
Solanum umbelliferum Bluewitch Nightshade 3.3 ft X X X D Toxicodendron diversilobum
[g]
Poisonoak 1.6 - 13
ft

X X X D
Vitis californica [g] California Grape 10 - 40 ft X X X X D Table L-3. Stanislaus County Native Plant List (obtained from the California Native Plant Society) #6..
- 671 -
Development Guidance Manual L-24 May 2025
Botanical Name Common Name Height
Water Requirement Sun Requirement Deciduous/
Extremely Evergreen
Low
Very
Low Low Moderate - High Full Sun Part Shade Shade
Succulents
Crassula aquatica [c] Water Pygmyweed
Crassula connata [c] Pygmy-weed 0.8 - 4 in
Leptosyne stillmanii [c] Stillman's Coreopsis 0.16 - 1
ft
X
Trees6 - Consult with Public Works Department – Trees and Forestry Services prior to the selection of trees Acer negundo Box Elder 35 - 66 ft X X X D
Aesculus californica California Buckeye 13.1 -

39.4 ft

X X X X D
Alnus rhombifolia White Alder 49.2 - 82
ft

X X X D
Fraxinus latifolia Oregon Ash 35 - 82 ft X X X X D
Juglans hindsii Northern California Black
Walnut
60 ft X X X
Populus fremontii Fremont Cottonwood 39.4 -
114.8 ft
X X D
Populus fremontii ssp.
fremontii
Fremont Cottonwood 66 ft X
Quercus douglasii Blue Oak 16 - 82 ft X X X D
Quercus lobata Valley Oak 60 - 100
ft
X X D
Sambucus nigra [f] Black Elderberry 13.1 - 30
ft
X D
Sambucus nigra ssp.

Caerulea [f]
Blue Elderberry 20 - 30 ft X X X X D
Salix exigua [f] Sandbar Willow 10 - 23 ft X X D
Salix exigua var. hindsiana [f] Sandbar Willow 16.4 ft X D 6 Consult with the City Trees and Forestry Services to identify which trees are appropriate for a given location and desired outcome/use [209-342-2253].

Table L-3. Stanislaus County Native Plant List (obtained from the California Native Plant Society) #6..
- 672 -
Development Guidance Manual L-25 May 2025
Botanical Name Common Name Height
Water Requirement Sun Requirement Deciduous/
Extremely Evergreen
Low
Very
Low Low Moderate - High Full Sun Part Shade Shade
Salix gooddingii Goodding's Black Willow 15 - 40 ft X X D Trees (continued)
Salix laevigata Red Willow 30 - 50 ft X X X D
Salix lasiandra [f] Shining Willow 3 - 30 ft X X D
Salix lasiolepis [f] Arroyo Willow 7 - 35 ft X X D
Quercus wislizeni [f] Interior Live Oak 15 - 50 ft X X X X E Vines
Clematis ligusticifolia [h] Virgin's Bower 1 - 30 ft X X X D Cuscuta californica [d] Chaparral Dodder
Cuscuta campestris [d] Field Dodder
Cuscuta subinclusa [d] Canyon Dodder
Rubus ursinus [g] Pacific Blackberry 2 - 6 ft X X X X D Toxicodendron diversilobum
[g]
Poisonoak 1.6 - 13
ft

X X X D
Vicia hassei [d] Slender Vetch 0.7 - 2.3
ft
Vitis californica [g] California Grape 10 - 40 ft X X X X D Reference: California Native Plant Society website https://calscape.org/ Notes:
[a] Annual herb, Perennial herb
[b] Annual herb, Perennial herb, Shrub
[c] Annual herb, Succulent
[d] Annual herb, Vine
[e] Perennial herb, Shrub
[f] Shrub, Tree
[g] Shrub, Vine
[h] Vine, Perennial herb
Table L-3. Stanislaus County Native Plant List (obtained from the California Native Plant Society) #6..
- 673 -

APPENDIX M

STANDARD CALCULATIONS FOR DIVERSION

STRUCTURE DESIGN
#6..
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Development Guidance Manual M-1 May 2025
APPENDIX M. STANDARD CALCULATIONS FOR DIVERSION

STRUCTURE DESIGN
Introduction
Stormwater runoff in excess of the water quality flow (WQF) or volume (WQV) is to be diverted around or through the treatment control measure. This appendix provides equations and design criteria necessary to design diversion structures to divert stormwater runoff in excess of the WQV or WQF around or through the treatment control measures.

Diversion Structure Design
Capture or isolation of the WQV is typically achieved by employing one of the following techniques:
 Divert the WQV into the treatment control measure from the on-site storm drain system using weirs or orifices at or upstream of the point of entrance to the treatment control measure.
 Bypassing flows in excess of the WQV within the treatment control measure using weirs and pipes for channel or pipe storm drain systems or routing excessive flows through a vegetated swale.

By employing diversion techniques, the WQF or WQV is treated and retained and stormwater runoff that exceeds the WQV or WQF is diverted or bypassed, untreated, directly to the downstream storm drain system or rockwells.

Equations and criteria to design a diversion structure are provided below. Alternative designs may be considered subject to approval.

All diversion structures are designed using the on-site storm design event. The drainage design storm is established by the governing agency and is not the same as the WQF or WQV. The drainage design storm is used to design the conveyance system, i.e. pipes, swales, etc. of the site without regard for treatment. The design engineer must ensure sufficient head room in the on-site system above the diversion to accommodate overflows.

Diverting Flows at the Inlet or Upstream of the Treatment Control Device Diverting flow at the inlet to the treatment control is the more common approach to divert excess runoff. Figure M-1 illustrates the more commonly used diversion structures. The height of the weir to divert the flow is determined as follows:

Treatment Control Measures Designed Based on the WQV

1. Determine the WQV (Section 6)

2. Utilizing design techniques provided in the treatment control measure fact sheets, determine the maximum height of the water level in the treatment control measure when the entire WQV is being stored.

3. Set the height of the diversion weir to the maximum height of the water level. #6..
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Development Guidance Manual M-2 May 2025

4. Determine weir dimensions needed to divert peak flows of the drainage design storm using the following equation for a rectangular sharp-crested weir Qd = C xL x h1.5 eqn H-1
Where
Qd = Peak flow rate for drainage design storm, cfs
L = Effective length of weir, ft
C = Weir discharge coefficient
h = Depth of the flow above the crest of the weir, ft
The discharge coefficient “C” accounts for many factors, such as velocity of approach, in the weir equation. The height of the weir (H) and the height of the flow over the weir (h) are two characteristics of the sharp-crested weir that affect the value of C. Table M-1 can be used to approximate C for rectangular sharp-crested weirs without end contractions.

5. Provide sufficient head room in the treatment control to accommodate depth of flow over the weir.

Table M-1. Weir Discharge Coefficient (C) for Rectangular Sharp-crested Weirs Without End Contractions1 H/h
Head (h) over weir, ft

0.2 0.4 0.6 0.8 1.0 2.0 5.0

0.5 4.18 4.13 4.12 4.11 4.11 4.10 4.10

1.0 3.75 3.71 3.69 3.68 3.68 3.67 3.67

2.0 3.53 3.49 3.48 3.47 3.46 3.46 3.45

10.0 3.36 3.32 3.30 3.30 3.29 3.29 3.28
∞ 3.32 3.28 3.26 3.26 3.25 3.25 3.24

1. From Lindsay and Franzini (1979)
Treatment Control Measures Designed Based on the WQF

1. Establish the size of the on-site drainage system (pipe diameter or dimensions) based on the drainage design storm

2. Determine the WQF (Section 6)

3. Determine the depth of flow in the on-site drainage system when carrying the WQF using Manning’s equation (eqn H-2)
eqn H-2
Where
WQF = Water Quality Flow, cfs
n = Manning’s roughness coefficient
#6..
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Development Guidance Manual M-3 May 2025
A = Cross sectional area of drainage pipe or channel, ft2 R = Hydraulic radius, ft
S = Slope of pipe or channel, ft/ft

4. Using nomographs or computer programs, determine the depth of flow at WQF. Set the weir height at this depth.

5. Using Equation H-1, establish weir dimensions. Provide sufficient head room in treatment control to accommodate flows over the weir.

Bypassing Excess Flows within the Treatment Control Measure For certain site conditions, bypassing runoff in excess of the WQV must be achieved in the treatment control measure. When this occurs, the control measure must be designed to ensure the bypass system can be accommodated in the unit, i.e. sufficient depth, width and length to accommodate pipes, length of weirs, etc. The following discusses design considerations for the different treatment control measures.

Bypassing Flows through Infiltration and Sedimentation/Filtration Treatment Control Measures Weirs, orifices or pipes in treatment control measures are used to bypass runoff in excessive of the WQV and WQF. Design of these measures is similar to the approach described above under diverting flows at the inlet to the treatment control measure. Bypass for filtration devices occurs in the sedimentation chamber.

Weirs
Weirs are commonly used to bypass excess storm events. Determining the height of the weir is based on the maximum water elevation in a treatment control device when holding the entire WQV. To design the weir, use the procedures established under Diversion Structures for Treatment Control Measures Designed Using the WQV.

Orifices
Orifices can be considered in place of weirs or pipes. To avoid drawing floatables into the bypass, a hooded orifice (Figure M-2) should be designed using the equation H-3:

Qd = C xA x (2gh)0.5 eqn H-3
Where
Qd = Peak flow rate for drainage design storm, cfs
C = Orifice discharge coefficient, (use 0.6)
A = Area of orifice, ft2
H = Depth of the water above midpoint of orifice, ft
G = 32.2 ft/sec2.

Hoods should extend into one-third of the permanent pool depth or one-foot whichever is greater. Commercial catch basin traps can be used in lieu of a hood. #6..
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Development Guidance Manual M-4 May 2025
Determining the elevation of the orifice is based on determining the maximum water elevation in a treatment control device when holding the entire WQV. Use the procedures established under Diversion Structures for Treatment Control Measures Designed Using the WQV to establish the elevation of the mid-point of the orifice opening.

The size of the orifice is determined by using Equation H-3 for the orifice to bypass the peak flow of the on-site storm.

Ensure sufficient head room in the treatment unit to accommodate flows through orifice.

Pipes
Pipes can also be employed to bypass excess runoff. Determining the invert elevation of the bypass inlet is based on determining the maximum water elevation in a treatment control device when holding the entire WQV. To do this, use the procedures established under Diversion Structures for Treatment Control Measures Designed Using the WQV to design a diversion weir.

For filtration control measures, a hooded inlet using a 90° elbow should be considered at the inlet to the bypass pipe to prevent drawing floatables into the bypass (Figure M-2).

Hoods should extend into one-third of the permanent pool depth or one-foot whichever is greater. Commercial catch basin traps can be used in lieu of a hood.

For infiltration control measures (Figure M-3) bypass pipes are perforated and wrapped with filter fabric to avoid drawing sediment and small particles into the bypass pipe. Hoods are not necessary for these overflow pipes.

Bypass pipes are sized using the Manning’s equation (eqn H-4) using the peak flow of the drainage design storm and assuming they are flowing full. Under these conditions, Manning’s equation reduces to:
eqn H-4
Where
D = Diameter of pipe, ft
Qd = Peak flow rate for drainage design storm, cfs
n = Manning’s coefficient for pipe material
s = Slope of pipe, ft/ft (0.5% minimum required)
Provide sufficient head room in the treatment control to accommodate flows.

Routing Excess Runoff Through a Vegetated Swale
The depth of flow in a Vegetated Swale at WQF is determined using a roughness coefficient of

0.2. If additional flows beyond the WQF are to be directed to the vegetated swale, the roughness coefficient for these flows will be lower (approximately 0.03), because the flows exceeding the WQF do not flow through the swale and are only influenced by surface friction/roughness. Swales with distinctly different roughness coefficients can be designed using an equivalent roughness coefficient that is determined based on the roughness associated with #6..
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Development Guidance Manual M-5 May 2025
the wetted perimeters (P). For most on-site Vegetated Swale designs, there will be two different “n” values. An equivalent “ne” value can be determined using equation H-5:
eqn H-5
An iterative approach is used to develop an equivalent “ne”, that can be calculated with most computer hydraulic program applications:

1. Estimate an equivalent roughness coefficient (estimated “ne”);

2. Use the estimated roughness coefficient to determine the depth of flow using trial and error solution of Equation H-2 substituting the peak flow of the drainage design storm for the WQF;

3. Use the calculated depth to determine the wetted perimeter for the drainage system;

4. Use the wetted perimeter associated with each “n” for the drainage system and using Equation H-5 to calculate the equivalent roughness coefficient (calculated “ne”), and compare to the estimated “ne”; and

5. Continue the process until the calculated “ne” equals the estimated “ne”. This value is the equivalent roughness coefficient and used to design the Vegetated Swale according to recommendations provided in Fact Sheet L-7.

Note - This approach results in conservative n values. High flows in the swale may cause some vegetation to bend resulting in a lower n1 and lower equivalent “ne”.

AREA RUNOFF

CONTRIBUTING

STORM DRAIN SYSTEM

EXCESS RUNOFF TO

CONTROL MEASURE

TO TREATMENT

MAXIMUM DEPTH OF WQV

WEIR HEIGHT EQUAL TO

BEING RETAINED IN

CONTROL MEASURE

PIPE INTERCEPTOR ISOLATION/DIVERSION STRUCTURE

CONTROL MEASURE

TO TREATMENT

DIVERSION
WEIR

TO STORM DRAIN
SYSTEM

SURFACE CHANNEL DIVERSION STRUCTURE

TOP OF ISOLATION BAFFLE MUST

BE GREATER THAN MAX WATER

SURFACE ELEV OVER DIVERSION

WEIR FOR DESIGN STORM FLOW
Figure M-1. Common Diversion Structures at Inlets
North Tennessee Water Quality BMP Manual Design and Maintenance of Structural BMPs. (2008). (tech.). #6..
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Development Guidance Manual M-6 May 2025
Figure M-2. Illustration of Pipe Bypass in Infiltration Trench North Tennessee Water Quality BMP Manual Design and Maintenance of Structural BMPs. (2008). (tech).

Figure M-3. llustration of Pipe Bypass in a Filtration Device North Tennessee Water Quality BMP Manual Design and Maintenance of Structural BMPs. (2008). (tech.).

D.C. FILTER IN-LINE BY-PASS

ELEVATION OF INLET INVERT

FOR BYPASS PIPE EQUAL TO

MAXIMUM SURFACE ELEVATION

WHEN SQDV RETAINED IN UNIT

EXAMPLE OF HOODED ORIFICE

INTO FILTER

IN LIEU OF WEIR TO DRAW WQV

DRAINAGE SYSTEM
MIN. 0.5% SLOPE

BYPASS PIPE

HOODED INLET

TO STORM

PERFORATED PIPE
MINIMUM SLOPE 0.5%

WRAPPED IN FABRIC
FILTER (TABLE 5-16) BEING RETAINED IN UNIT

SURFACE ELEVATION WHEN SQDV

FORATED PIPE EQUAL TO MAXIMUM

HEIGHT OF INLET INVERT TO PER-
#6..
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APPENDIX N

APPROVED PROPRIETARY CONTROL MEASURES
#6..
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Development Guidance Manual N-1 May 2025
APPENDIX N. APPROVED PROPRIETARY CONTROL

MEASURES
This appendix lists proprietary stormwater treatment devices that have been approved by the City for general use in new development and significant redevelopment projects. As noted within the Guidance Manual (Section 5), projects must first show that the volume retention requirement (VRR) is met through the use of volume retention measures (VRMs) prior to the utilization of conventional treatment controls.

It should be noted that Appendix I does not include devices approved for trash control. For a list of certified and/or agency-approved devices contact the City. In addition, vector breeding considerations must be addressed when considering what types of trash treatment controls to implement due to the potential nuisance and human health effects. Those devices that have been reviewed and approved by Vector Control are identified within the State’s certified list of trash treatment control devices and should be confirmed with the City.

To provide a rational basis for approval of proprietary devices, the City has elected to recognize as approved for general and pilot use those proprietary devices that have been approved for general, conditional, or pilot use by other selected major stormwater programs that have established and are actively conducting a comprehensive testing protocol and approval process.

Currently, the City recognizes the lists of proprietary devices approved for general, conditional, and pilot use as well as trash capture systems from the following stormwater programs:
 Sacramento Stormwater Quality Partnership (website: http://www.sacstormwater.org/)  State of Washington Department of Ecology Stormwater Program (website:
http://www.ecy.wa.gov/programs/wq/stormwater/newtech/index.html)  State Water Board Certified Full Capture System List of Trash Treatment Control Devices (website:
www.waterboards.ca.gov/water_issues/programs/stormwater/trash_implementation.html) The City may recognize lists from other programs in the future and will update Appendix I accordingly.
#6..
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APPENDIX O

EXAMPLE CALCULATION
#6..
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Development Guidance Manual O-1 May 2025
APPENDIX O. EXAMPLE CALCULATION
This Appendix provides an example calculation to illustrate the application of the volume retention requirements (VRR), volume retention measures (VRMs), tributary impervious area credit and low impact development (LID)-based Treatment Control Measures. The calculation begins at Step 5, Apply VRMs. Real world development applications should also adhere to Steps 1 – 4 as described in Section 2.

SITE CONDITIONS
A commercial site design (Figure O-1), is used for the example calculation. This is a new development scenario and it is assumed that the pre-project conditions primarily consisted of disturbed soils with some undisturbed open space and no impervious cover elements.

STEP 5: APPLY Volume retention Measures
The sub steps for Step 5 as described in Section 5 are as follows:
 Calculate the VRR (post - pre)
 Select VRMs
 Determine volume retention
 Determine remaining VRR
 Determine tributary impervious area credits
Calculate Volume retention Requirement
Pre-Project Volume
Site Element Element
Area ft2
Fraction of
Total Area
Runoff
Coefficient
Weighted Runoff
Coefficient
(Fraction of Total
Area * Runoff
Coefficient)

0.51-inch Storm
Volume, ft3
(Total Project Area *
Total Weighted
Runoff Coefficient
*(0.51/12))
Disturbed soils 76,750 0.91 0.25 0.228
Undisturbed open space /trees 7,250 0.09 0.05 0.004
Total 84,000 0.233 831
#6..
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Development Guidance Manual O-2 May 2025
Post-Project Volume
Site Element Element
Area ft2
Fraction of
Total Area
Runoff
Coefficient
Weighted Runoff
Coefficient
(Fraction of Total
Area * Runoff
Coefficient)

0.51-inch Storm
Volume, ft3
(Total Project Area *
Total Weighted
Runoff Coefficient *
(0.51/12))
Permeable pavers 2,410 0.03 0.60 0.02
Roofs 15,200 0.18 0.95 0.17
Parking lot 22,961 0.27 0.95 0.26
Driveway 14,531 0.17 0.95 0.16
Plaza 4,755 0.06 0.95 0.05
Walkways 5,607 0.07 0.95 0.06
Amended soils 11,286 0.13 0.05 0.01
Undisturbed open
space /trees 7,250 0.09 0.05 0.00
Total 84,000 0.74 2,647
Pre-project
Volume 831
VRR (post – pre) 1,816
Select Volume retention Measures & Determine Volume retention Thirty-four evergreen, Interception Trees were selected as the VRMs for this site. Additional details on Interception Trees can be found in Fact Sheet V-3.

Parameter Unit Value
No. of trees ea 34
Avg. canopy diameter ft 20
Unit projected canopy area
(area of tree projected over impervious area)
ft2 314
Total canopy area
(unit projected canopy area * no. of trees) ft2 10,676
Percent interception % 40%
Volume retention
(0.51” * Total canopy area * Percent interception / 12 in/ft) ft3 181 Determine Remaining Volume retention Requirement
Remaining VRR (VRR – Interception Tree Volume retention):
1,816 ft3 – 181 ft3 = 1,635 ft3
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Development Guidance Manual O-3 May 2025
Determine Tributary Impervious Area Credits
Each VRM fact sheet describes how impervious area credits are calculated. See Fact Sheet V-3 for details on calculating the credits associated with Interception Trees.

Site Element Area of Canopy Coverage ft2
Impervious Area Credit ft2
(Area of Canopy Coverage *
Percent Interception)
Parking lot 5,018 2,007
Plazas 3,737 1,495
Walkways 1,921 768
Total 10,676 4,270
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Development Guidance Manual O-4 May 2025
Figure O-1. Example Commercial Site Design. Modified from LID Center. Source: Guillette, A. (n.d.). Anne Guillette, LEED Accredited Professional Low Impact Design Studio (formerly with the Low Impact Development Center). Low Impact Development: An Alternative Site Design Strategy. https://doi.org/2016
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Development Guidance Manual O-5 May 2025
STEP 6: APPLY LID TREATMENT CONTROLS
If VRR was not met through use of VRMs (Step 5), LID-based Treatment Control Measures must be used to further reduce volume. Treatment control measures must be designed to treat the WQF or WQV.

In this example calculation, the VRR was not fully met through the use of Interception Trees, as a result, the development will also apply the following LID-based Treatment Controls:
 Parking Lot Bioretention
 Vegetated Swale
 Bioretention Cell
 Tree-well Filter
Calculations for each of the LID-based Treatment Control Measures are provided in the tables below.

Parking Lot Bioretention Unit Value
Tributary area
(Area draining to parking lot bioretention)
ft2 22,961
Tributary impervious area credit
(see Tributary Impervious Area Credits in Step 5) ft2 2,007 Effective tributary area
(Tributary area – Tributary impervious area credit) ft2 20,954 WQV1
(Unit Basin Storage Volume2 * Effective tributary area * 1ft/12in) ft3 559 Bioretention area ft2 1,563
Depth of ponding zone ft 0.50
Depth of planting zone ft 1.50
Treatment volume provided1
(Bioretention area * Depth of ponding zone) ft3 782
Volume retention
(Depth of ponding zone * Bioretention area * 0.25) +
(Depth of planting zone * Bioretention area * 0.1)
ft3 430
1: WQV is treatment volume required; Treatment volume provided is the amount of treatment actually provided by parking lot bioretention
2: See Figure 6-1
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Development Guidance Manual O-6 May 2025
Vegetated Swale Unit Value
Tributary area
(Area draining to vegetated swale) ft2 5,188
Tributary impervious area credit
(see Tributary Impervious Area Credits in Step 5) ft2 - Effective tributary area
(Tributary area – Tributary impervious area credit) ft2 5,188 WQV1
(Unit Basin Storage Volume2 * Effective tributary area * 1ft/12in) ft3 138 Swale area ft2 798
Depth of ponding zone ft 0.67
Depth of planting zone ft 1.50
Treatment volume provided1
(Swale area * Depth of ponding zone) ft3 535
Volume retention
(Depth of ponding zone * Swale area * 0.25) +
(Depth of planting zone * Swale area * 0.1)
ft3 253
1: WQV is treatment volume required; Treatment volume provided is the amount of treatment actually provided by vegetated swale
2: See Figure 6-1
Tree-well Filters Unit Value
Tributary area
(Area draining to tree-well filter) ft2 4,755
Tributary impervious area credit
(see Tributary Impervious Area Credits in Step 5) ft2 1,495 Effective tributary area
(Tributary area – Tributary impervious area credit)
ft2 3,260
WQV1 (Unit Basin Storage Volume2 * Effective tributary area * 1ft/12in) ft3 87
No. of filters ea 12
Unit filter area ft2 16
Total filter area
(No. of filters * Unit filter area) ft2 192
Depth of ponding zone ft 0.75
Depth of planting zone ft 1.50
Treatment volume provided1
(Total filter area * Depth of ponding zone)
ft3 144
Volume retention
(Depth of ponding zone * Total filter area * 0.25) +
(Depth of planting zone * Total filter area * 0.1)
ft3 65
1: WQV is treatment volume required; Treatment volume provided is the amount of treatment actually provided by bioretention cell
2: See Figure 6-1
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Development Guidance Manual O-7 May 2025

SUMMARY OF VOLUME RETENTION
Control Measure Units Volume retention
Tree Interception ft3 181
Parking Lot Bioretention ft3 430
Vegetated Swale ft3 253
Bioretention Cell ft3 909
Tree-well Filters ft3 65
Total Volume retention ft3 1,838
VRR ft3 1,816
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APPENDIX P

REFERENCES
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Development Guidance Manual P-1 May 2025
APPENDIX P. REFERENCES
Basin Sizer, Version 1.45. (2007). Caltrans – Basin Sizer. https://svctenvims.dot.ca.gov/basinsizer/.

Bay, A. for the C. (2021, October). Reduce your stormwater. Pervious Pavers. https://www.stormwater.allianceforthebay.org/take-action/installations/pervious-pavers.

California Stormwater Quality Association (CASQA), 2021. Stormwater Best Management Practice Online Handbook: Development. Available through the CASQA website www.casqa.org
Caltrans. (n.d.). Erosion Control Toolbox: Sidewalk Stormwater Planter | Caltrans. https://dot.ca.gov/programs/design/lap-erosion-control-design/tool-1-lap-erosion-controltoolbox/tool-1kk-37-sidewalk-stormwater-planter.

Computing Stormwater Runoff Rates and Volumes. (2004). NJDEP New Jersey Department of Environmental Protection. https://www.njstormwater.org/bmp_manual2.htm.

Constructed Wetlands (NRCS 656) | AgBMPs. (n.d.). Agbmps.osu.edu. https://agbmps.osu.edu/bmp/constructed-wetlands-nrcs-656.

Contech Engineered Solutions (2025). http://www.contechs.com Cornell University Urban Horticulture Institute. (2007). Using Porous Asphalt and CU-Structural Soil®. http://www.hort.cornell.edu/uhi/outreach/pdfs/cu_porous_asphalt.pdf.

Costello, D.J., Greenfield, P.F., & Lee, P.L. (1991).

Detention Basin Retrofitting. (2023). City of Elgin, Illinois - Official Website. https://www.cityofelgin.org/1275/Detention-Basin-Retrofitting.

Ekhuemelo, David. (2016). Importance of Forest and Trees in Sustaining Water Supply and Rainfall. Nigeria Journal of Education, Health And Technology Research (Njehetr). NJEHETR – My WordPress Blog.
https://www.researchgate.net/publication/310458060_IMPORTANCE_OF_FOREST_AND_TR EES_IN_SUSTAINING_WATER_SUPPLY_AND_RAINFALL.

Ekka, S., & Hunt, B. (2020, February 10). Swale Terminology for Urban Stormwater Treatment | NC State Extension Publications. https://content.ces.ncsu.edu/swale-terminology-for-urbanstormwater-treatment.

Fairfax County Virginia. (n.d.). Control Heavy Runoff - Solving Drainage and Erosion Problems | Northern Virginia Soil and Water Conservation District. https://www.fairfaxcounty.gov/soilwater-conservation/drainage-problem-control-runoff.

Filter Strips/Grassed Riparian Buffers (NRCS 393 & 390) | AgBMPs. (n.d.). AgBMPs. https://agbmps.osu.edu/bmp/filter-stripsgrassed-riparian-buffers-nrcs-393-390.

Guillette, A. (n.d.). Anne Guillette, LEED Accredited Professional Low Impact Design Studio. Low Impact Development: An Alternative Site Design Strategy. #6..
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Development Guidance Manual P-2 May 2025
Haltiner, J. (2006). Philip Williams & Associates, Ltd. Hydromodification: An Introduction and Overview. https://www.slideserve.com/mary/hydromodification-an-introduction-andoverview-powerpoint-ppt-presentation.

Henao Casas, Jose & Walther, Marc & Kalwa, Fritz & Rausch, Randolf. (2019). Numerical and Analytical Assessment of Stormwater Infiltration via Vadose Zone Wells and Infiltration Trenches. 10.13140/RG.2.2.14098.07368
Hinman, C. (2007). A Guide for Design, Installation, and Maintenance – Rain Garden Handbook for Western Washington.
https://apps.ecology.wa.gov/publications/documents/1310027.pdf Hinman, C. (2012). Low Impact Development: Technical Guidance Manual for Puget Sound. https://docslib.org/doc/4059614/technical-guidance-manual-for-puget-sound Home. Low Impact Development. (2002, November). Low Impact Development. https://lowimpactdevelopment.org/.

IFAS Communications. (2025). Rain Barrels - Sarasota County - University of Florida, Institute of Food and Agricultural Sciences - UF/IFAS. http://sfyl.ifas.ufl.edu/sarasota/gardening-andlandscaping/horticulture-residential/florida-yards-and-neighborhoods/rain-barrels.

Low Impact Best Management Practice (BMP) Information Sheet. (n.d.). www.charlesriver.org Low Impact Development Handbook Stormwater Management Strategies. (2014, July). https://www.sandiegocounty.gov/content/dam/sdc/dpw/WATERSHED_PROTECTION_PROG RAM/watershedpdf/S-BMP/Filtration_O&M_Form.pdf.

MSD. (2016). Comply with Consent Decree | MSD. https://louisvillemsd.org/comply-consentdecree.

New Jersey Future Green Infrastructure Developers Guide. (2020, May). Bioretention System. https://developersguide.njfuture.org/bmp/bioretention-system/.

North Tennessee Water Quality BMP Manual. (2008). Design and Maintenance of Structural BMPs.
https://cms1files.revize.com/elizabethton/document_center/stormwater_program/Stormw ater_Management_Manual/WQ_BMP_Title_Page.pdf.

Oregon State University. (n.d.). Choose the right Rain Garden. https://extension.oregonstate.edu/node/119221/printable/print.

Pat Rector. (n.d.). Rutgers Cooperative Extension – Rutgers New Jersey Agricultural Experiment Station. https://njaes.rutgers.edu/.

Rain Gardens & Rebate Program – Flows to Bay. (n.d.).
https://www.flowstobay.org/preventing-stormwater-pollution/at-home/rain-gardens/.

Roadway Bioretention Areas. (2024). https://raleighnc.gov/SupportPages/roadwayBioretention-areas.

Roseen, R. M., Ballestero, T. P., & Houle, J. J. (2022). University of New Hampshire Stormwater Center 2009 Biannual Report. https://scholars.unh.edu/stormwater/76/ #6..
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Development Guidance Manual P-3 May 2025
Sacramento Stormwater Quality Partnership. (2018, July). Stormwater Quality Design Manual for the Sacramento Region. https://www.beriverfriendly.net/stormwater-quality-designmanual/ Schueler, T. (2007). Urban Stormwater Retrofit Practices (3rd ed., Vol. 1, p. 184). https://allaboutwatersheds.org/groups/RCWP/resources/Manual%2003%20- %20Urban%20Stormwater%20Retrofit%20Practices%20Manual%20Appendices.pdf.

Stormwater Discovery Tours. (2022, December). Ball Field Infiltration Basin - Stormwater Discovery Tours. https://stormwater.cob.org/tour-squalicum-creek-park/site-ball-fieldinfiltration-basin/ Stormwater Management Manual. [Eugene, Or.]: City of Eugene. (2008). 2-58--62. Print. http://ceppcontent.eugene1.net;7087/publishedcontent/publish/pw/stormwater/docs/ch2 e.pdf
Stringham, T. K., Snyder, K. A., Snyder, D. K., Lossing, S. S., Carr, C. A., & Stringham, B. J. (2018).

Rainfall Interception by Singleleaf Piñon and Utah Juniper: Implications for Stand-Level Effective Precipitation. Rangeland Ecology & Management, 71(3), 327–335. https://doi.org/10.1016/j.rama.2017.12.009.

Trash Capture Technologies. (2020, August 10). Trash Capture Technologies – US EPA. https://www.epa.gov/trash-free-waters/trash-capture-technologies.

Urban Storm Drain Criteria Manual Vol 3. – Best Management Practices, Urban Drainage and Flood Control District (UDFCD). (1999, rev. 2008). USDCM Volume 3 PDF | PDF | Stormwater | Clean Water Act
Infiltration Stormwater Planter Configuration. Urban Drainage and Flood Control District (UDFCD). (1999, rev. 2008). Drainage Criteria Manual (Vol 3). Denver, CO.

Urban Drainage and Flood Control District (UDFCD). (1999, rev. 2008). Urban Drainage Criteria Manual Vol 3 – Best Management Practices Stormwater Quality. Denver, CO.

U.S. Environmental Protection Agency (EPA). (2007). Reducing Stormwater Costs through Low Impact Development (LID) Strategies and Practices. EPA 841-F-07-006.

Vegetated Swale Specifications. (2021, October). City of Sandy, OR. https://www.ci.sandy.or.us/publicworks/page/vegetated-swale-specifications.

Water-Quality Swales: Low-impact development fact sheet. (n.d.). Low-impact development fact sheet. https://catalog.extension.oregonstate.edu/em9209/html. #6..
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Economic Development CommitteeAugust 4, 2025

Economic Development Committee
August 4, 2025