1515 Fourth Street Napa, CA 94559
www.rsacivil.com
707.252.3301 v 707.252.4966 f
STORMWATER CONTROL PLAN
FOR A REGULATED PROJECT
CAPITOLA SUBDIVISION
808 CAPITOLA DRIVE
NAPA, CALIFORNIA 94559
Prepared for:
Mercatus Partners
Attn: Christopher Russell, Managing Director
620 Newport Center Dr. Suite 1100
Newport Beach, CA 92660
Project #4123011.0
March 20, 2025
ATTACHMENT 6
CAPITOLA SUBDIVISION
STORMWATER CONTROL PLAN
Table of Contents
I.
Project Data
.......................................................................................................................................... 1 II.
Setting
................................................................................................................................................... 1 II.A. Project Location and Description .............................................................................................. 1 II.B. Existing Site Features and Conditions ....................................................................................... 2 II.C. Opportunities and Constraints for Stormwater Control ........................................................... 3 III.
Low Impact Development Design Strategies
....................................................................................... 3 III.A. Optimization of Site Layout....................................................................................................... 3 III.B. Use of Permeable Pavements ................................................................................................... 3 III.C. Dispersal of Runoff to Pervious Areas ....................................................................................... 3 III.D. Stormwater Control Measures ................................................................................................. 3 IV.
Documentation of Drainage Design
................................................................................................... 5 IV.A. Descriptions of Each Drainage Management Areas .................................................................. 5 IV.A.1. Drainage Management Areas ………………………………………………………………………………..5 IV.A.2. Drainage Management Area Descriptions ……………………………………………………………. 6 IV.B. Tabulation and Sizing Calculations ............................................................................................ 5 V.
Source Control Measures
..................................................................................................................... 6 V.A. Site activities and potential sources of pollutants .................................................................... 6 V.B. Potential Pollutant Sources and Source Control Measures …………………………………………..……. 6 VI.
Stormwater Facility Maintenance
....................................................................................................... 7 VI.A. Ownership and Responsibility for Maintenance in Perpetuity ................................................. 7 VI.B. Summary of Maintenance Requirements for Each Stormwater Facility .................................. 7 VII.
Construction Checklist
........................................................................................................................ 8 VIII.
Certification
........................................................................................................................................ 8 ATTACHMENT 6
CAPITOLA SUBDIVISION
STORMWATER CONTROL PLAN
TABLES
Table 1. Project Data Form
Table 2. Drainage Management Areas
Table 3. Potential Pollutant Sources and Source Control Measures Table 4. Construction Plan E.12 Checklist
FIGURES
Figure 1. Vicinity Map
Figure 2. Existing Site Conditions
Figure 3. Bioretention Cross Section
ATTACHMENTS
1. Soil Classification (4 Sheets)
2. Stormwater Control Plan (Sheet TM6)
3. Provision E.12 Sizing Calculator Spreadsheet
ATTACHMENT 6
CAPITOLA SUBDIVISION
STORMWATER CONTROL PLAN
I.
Project Data
Table 1.
Project Data Form
Project Name/Number Capitola Subdivision / 4123011.0
Application Submittal Date
Project Location 808 Capitola Drive
Napa, California 94559
APN: 046-020-020
Project Phase No. Not Applicable
Project Type and Description Construction of an 8-lot single family residential subdivision including streets, driveways, utilities and bioretention/detention facility.
Total Project Site Area 1.6± acres
Total New and Replaced Impervious Surface
Area
45,520 sq. ft. (including Capitola Drive new frontage
improvements)
Total Pre-Project Impervious Surface Area 10,977 sq. ft.
Total Post-Project Impervious Surface Area 45,520 sq. ft. (including Capitola Drive new frontage improvements)
II.
Setting
II.A.
Project Location and Description
This project involves the demolition of an existing residential house and driveway. The site will be developed into an 8-lot single-family residential subdivision with an extension of Capitola Drive and a new private street. The project location is shown in Figure 1 below.
Figure 1
. Vicinity Map
ATTACHMENT 6
CAPITOLA SUBDIVISION
STORMWATER CONTROL PLAN
The proposed use is consistent with the current zoning and General Plan land use. The project will include the construction of 8 residential houses, an extension of Capitola Drive, a new private road, and installation of new utilities along with a stormwater basin for bioretention/detention.
Refer to Attachment 2 for the overall scope of the project.
II.B.
Existing Site Features and Conditions
The project site is regular in shape, with elevations varying from approximately 18 at Capitola Drive to 37 at the house and rear landscaped area to approximately 46 at the northeast corner.
The site is currently used for a residential house that fronts Capitola Drive. The site is bounded by residential houses on all sides. See Figure 2 below for existing site conditions.
Figure 2
. Existing Site Conditions
Mapping by the U.S. Conservation Service has classified soil within this project area as Coombs gravelly loam (123) which is of the Hydraulic Soil Group C. Refer to Attachment 1 for Soils Map. The whole site drains to a blue-line creek located at the lower western portion. Stormwater is ultimately conveyed to the Napa River.
ATTACHMENT 6
CAPITOLA SUBDIVISION
STORMWATER CONTROL PLAN
II.C.
Opportunities and Constraints for Stormwater Control
A stormwater treatment facility has been integrated into the planning, design, construction, operation, and maintenance of the proposed development. The following potential opportunities and constraints were considered in determining the best stormwater control design for this development.
Opportunities for this site are the availability of landscaped areas in the yards. A bioretention facility will be installed to treat stormwater runoff prior to discharge from the site. Runoff will be conveyed to the bioretention facility from roof downspouts and surface flows from the streets. Once in the bioretention basin, runoff will be treated via infiltration together with the pollutant retention capabilities of the plants in the facilities. Treated water will be detained by a side opening in the overflow structure such that the post-development flow discharge will be maintained at, or below, pre-development levels. Ultimately runoff will outfall to the Napa River. See Attachment 2 for the location of the bioretention facility.
Constraints for this site includes the parcel shape and topography.
III.
Low Impact Development Design Strategies
III.A.
Optimization of Site Layout
1. Limitation of development envelope
The development of the houses will occur within the building setback lines per Section
17.08.030 of the City of Napa Municipal Code.
2. Preservation of natural drainage features
Natural drainage consists of sheet flow over the ground surface that concentrates in man- made surface drainage elements such as ditches, gutters, and onsite storm drain pipes. See constraints on Section II.C above.
3. Setbacks from creeks, wetlands, and riparian habitats - Not applicable.
4. Minimization of imperviousness
Landscaping will be used in the front and rear yards. Impervious areas will be minimized to the maximum extent practicable.
5. Use of drainage as a design element
A bioretention facility is incorporated into the aesthetic landscape design of the site. Grading and storm drain locations have been designed to direct runoff to the bioretention facility.
III.B.
Use of Permeable Pavements
Permeable pavements are not in the scope of this project.
III.C.
Dispersal of Runoff to Pervious Areas
Stormwater runoff will be directed to landscaped areas.
III.D.
Stormwater Control Measures
Runoff from the project site, including roof and paved areas, will be routed to one bioretention facility (see Attachment 2). The facility is designed and will be constructed to the criteria in the BASMAA Post-Construction Manual (January 2019), including the following features (see Figure 3):
ATTACHMENT 6
CAPITOLA SUBDIVISION
STORMWATER CONTROL PLAN
Figure 3
. Bioretention Cross Section
Each layer built flat, level, and to elevations specified in the plans:
o
Bottom of Gravel Layer (BGL)
o
Top of Gravel Layer (TGL)
o
Top of Soil Layer (TSL)
o
Overflow Grate
o
Facility Rim
12 inches of Class 2 permeable, Caltrans specification 68-2.02F (3).
18 inches sand/compost mix meeting BASMAA specifications.
4-inch diameter PVC SDR 35 perforated pipe underdrain, installed with the invert at the top of the Class 2 permeable layer with holes facing down, and connected to the overflow structure at that same elevation.
6-inch-deep reservoir between top of soil elevation and overflow side opening.
Concrete drop inlet overflow structure, with side opening and rim grate set to specified elevations, connected to the on-site storm drain system.
ATTACHMENT 6
CAPITOLA SUBDIVISION
STORMWATER CONTROL PLAN
Plantings selected for water conservation.
Irrigation system on a separate zone, with drip emitters and “smart” irrigation controllers.
Sign identifying the facility as a stormwater treatment facility.
Storm drain cleanout (SDCO) should be a minimum of 8 inches above the top of soil and can be a plug or grate.
30 mil liner at sides.
Area on the site that does not drain to a bioretention facility (see Attachment 2 for reference):
DMA 2 – The west half street area of Capitola Drive along lot 1 and Bioretention Facility, totaling 6,242 square feet. Grading in this area must conform with existing street elevations.
IV.
Documentation of Drainage Design
IV.A. Descriptions of Each Drainage Management Areas IV.A.1. Drainage Management Areas
Table 2
. Drainage Management Areas (DMAs) as shown on Attachment 2.
DMA
Name
DMA
perv
(Pervious Area,
square feet)
DMA
imp
(Impervious Area,
square feet)
Pervious
Pavers Area
(square feet)
Total Area
(square feet)
Bioretention
Facility Name
1 22,645 39,846
--
64,179
B
RF
#1
2 1,144 5,098
--
6,242
UNTREATED
IV.A.2. Drainage Management Area Descriptions
DMA 1
: Totaling 64,179 square feet, this DMA consists of Lots 1 to 8 as well as new Street A and the east side of Capitola Drive. Runoff from the roofs will drain out from downspouts to splash boxes that flows towards the street via landscape areas then along the street gutter toward a curb opening inlet that outfalls to BRF #1. This bioretention facility has a total treatment area of 1,688 square feet.
DMA 2
: Totaling 6,242 square feet, this DMA consists of the west side of Capitola Drive. Run-off from this area will remain untreated due to grading constraints and lack of available treatment area.
IV.B.
Tabulation and Sizing Calculations
Refer to Attachment 3 for Provision E.12 Sizing Calculator Spreadsheet.
ATTACHMENT 6
CAPITOLA SUBDIVISION
STORMWATER CONTROL PLAN
V.
Source Control Measures
V.A. Site activities and potential sources of pollutants On-site activities that could potentially produce stormwater pollutants are listed in Table 3 below:
V.B. Potential Pollutant Sources and Source Control Measures The site activities and potential sources of pollutants for the Capitola Subdivision project are listed in Table 3, below.
Table 3
. Potential Pollutant Sources and Source Control Measures Potential Sources of Runoff Pollutants Permanent Source Control BMPs Operational Source Control BMPs A. On-site storm drain inlets
(unauthorized non-stormwater
discharges and accidental spills or
leaks)
□
Mark all inlets with the words “No
Dumping! Flows to River” or
similar.
□
Maintain and periodically repaint or
replace inlet markings.
□
Provide stormwater pollution
prevention information to new site
owners, lessees, or operators.
□
See applicable operational BMPs in
Fact Sheet SC-74, “Drainage System
Maintenance.”
D
1 . Need for future indoor & structural
pest control
□
Building design shall incorporate
features that discourage entry of
pests.
□
Provide Integrated Pest
Management information to
owners, lessees, and operators.
D
2 . Landscape / outdoor pesticide use /
building and grounds maintenance
Final landscape plans will accomplish
all of the following:
□
Preserve existing native trees,
shrubs, and ground cover to the
maximum extent possible.
□
Minimize irrigation and runoff, to
promote surface infiltration
where appropriate, and to
minimize the use of fertilizers and
pesticides that can contribute to
stormwater pollution.
□
Where landscaped areas are used
to retain or detain stormwater,
specify plants that are tolerant of
saturated soil conditions.
□
Use pest-resistant plants,
especially adjacent to hardscape.
□
To insure successful
establishment, select plants
appropriate to site soils, slopes,
climate, sun, wind, rain, land use,
air movement, ecological
consistency, and plant
interactions.
□
Maintain landscaping using
minimum or no pesticides.
□
See applicable operational BMPs in
Fact Sheet SC-41, “Building and
Grounds Maintenance.”
□
Provide IPM information to new
owners, lessees and operators.
O. Miscellaneous drain or wash water or
other sources
Boiler drain lines
□
Boiler drain lines shall be directly
or indirectly connected to the
sanitary sewer system and may
not discharge to the storm drain.
If architectural copper is used,
implement the following BMPs for
management of rinse water during
installation:
ATTACHMENT 6
CAPITOLA SUBDIVISION
STORMWATER CONTROL PLAN
Potential Sources of Runoff Pollutants Permanent Source Control BMPs Operational Source Control BMPs
Condensate drain lines
Rooftop equipment
Drainage sumps
Roofing, gutters, and trim
Other sources
□
Condensate drain lines may
discharge to landscaped areas if
the flow is small enough that
runoff will not occur. Condensate
drain lines may not discharge to
the storm drain system.
□
Rooftop equipment with
potential to produce pollutants
shall be roofed and/or have
secondary containment.
□
Any drainage sumps on-site shall
feature a sediment sump to
reduce the quantity of sediment
in pumped water.
□
If possible, purchase copper
materials that have been pre-
patinated at the factory.
□
If patination is done on-site, prevent
rinse water from entering storm
drains by discharging to landscaping
or by collecting in a tank and hauling
off-site.
□
Consider coating the copper
materials with an impervious
coating that prevents further
corrosion and runoff.
□
Implement the following BMPs
during routine maintenance:
□
Prevent rinse water from entering
storm drains by discharging to
landscaping or by collecting in a
tank and hauling off-site.
VI.
Stormwater Facility Maintenance
VI.A.
Ownership and Responsibility for Maintenance in Perpetuity The applicant accepts responsibility for interim operation and maintenance of stormwater treatment facilities until such time as this responsibility is formally transferred to a subsequent owner.
An Operations & Maintenance Plan will be prepared for the project. The owner shall execute a Post- Construction BMP Maintenance Agreement with the City of Napa prior to completion of construction.
VI.B.
Summary of Maintenance Requirements for Each Stormwater Facility The bioretention/detention facility will be maintained on the following schedule at a minimum. Details of maintenance responsibility and procedures will be included in an Operation and Maintenance Plan to be submitted for approval prior to the completion of construction.
At no time will synthetic pesticides or fertilizers be applied, nor will any soil amendments, other than aged compost mulch or sand/compost mix, be introduced.
Daily:
The facility will be examined for visible trash during regular policing of the site, including the soil surface and trash capture screen, and trash will be removed. Debris and sediment should be removed to ensure the bioretention facility maintains its treatment area and volume capacity.
After Significant Rain Events:
A significant rain event is one that produces approximately a half-inch or more rainfall in a 24-hour period. Within 24 hours after each such event, the following will be conducted:
The surface of the facilities will be observed to confirm there is no excessive ponding. All facilities are designed to pond up to a 6” reservoir for stormwater treatment. Larger rainfall events may exceed this reservoir and overflow into the drop inlet.
Inlets will be inspected, and any accumulations of trash or debris will be removed.
ATTACHMENT 6
CAPITOLA SUBDIVISION
STORMWATER CONTROL PLAN
The surface of the mulch layer will be inspected for movement of material. Mulch will be replaced and raked smooth if needed.
Prior to the Start of the Rainy Season:
In September of each year, the facility will be inspected to confirm there are no accumulation of debris that would block flow, and that growth and spread of plantings does not block inlets or the movement of runoff across the surface of the facilities. The metering structure and openings shall be inspected, and any accumulation of debris or sediment shall be removed.
Annual Landscape Maintenance:
In December – February of each year, vegetation will be cut back as needed, debris removed, and plants and mulch replaced as needed. The concrete work will be inspected for damage. The elevation of the top of soil and mulch layer will be confirmed to be consistent with the 6-inch reservoir depth.
VII.
Construction Plan E.12 Checklist
Table 4
. Construction Plan E.12 Checklist
Stormwater
Control Plan
Page #
Source Control or Treatment Control Measure
See Plan
1 Bioretention Facilit
y
SCP Site Plan
i
n Attachment
2 VIII.
Certification
The preliminary design of stormwater treatment facilities and other stormwater pollution control measures in this plan are in accordance with the current edition of the BASMAA Post-Construction Manual, dated January 2019.
_____________________________________
Preparer
Derek Dittman, PE
ATTACHMENT 6
CAPITOLA SUBDIVISION
STORMWATER CONTROL PLAN
ATTACHMENT 1
SOIL CLASSIFICATION
ATTACHMENT 6
Hydrologic Soil Group—Napa County, California
(Capitola Drive Mercatus)
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
8/25/2023
4238750
4238760
4238770
4238780
4238790
4238800
4238810
4238820
4238750
4238760
4238770
4238780
4238790
4238800
4238810
4238820
563570
563580
563590
563600
563610
563620
563630
563640
563650
563660
563670
563680
563690
563570
563580
563590
563600
563610
563620
563630
563640
563650
563660
563670
563680
563690
38° 17' 42'' N
122° 16' 23'' W
38° 17' 42'' N
122° 16' 17'' W
38° 17' 40'' N
122° 16' 23'' W
38° 17' 40'' N
122° 16' 17'' W
N
Map projection: Web Mercator Corner coordinates: WGS84 Edge tics: UTM Zone 10N WGS84 0 25 50 100 150
Feet
0 5 10 20 30
Meters
Map Scale: 1:602 if printed on A landscape (11" x 8.5") sheet.
Soil Map may not be valid at this scale.
ATTACHMENT 6
MAP LEGEND
MAP INFORMATION
Area of Interest (AOI)
Area of Interest (AOI)
Soils
Soil Rating Polygons
A
A/D
B
B/D
C
C/D
D
Not rated or not available
Soil Rating Lines
A
A/D
B
B/D
C
C/D
D
Not rated or not available
Soil Rating Points
A
A/D
B
B/D
C
C/D
D
Not rated or not available
Water Features
Streams and Canals
Transportation
Rails
Interstate Highways
US Routes
Major Roads
Local Roads
Background
Aerial Photography
The soil surveys that comprise your AOI were mapped at
1:24,000.
Warning: Soil Map may not be valid at this scale.
Enlargement of maps beyond the scale of mapping can cause misunderstanding of the detail of mapping and accuracy of soil line placement. The maps do not show the small areas of contrasting soils that could have been shown at a more detailed scale.
Please rely on the bar scale on each map sheet for map
measurements.
Source of Map:
Natural Resources Conservation Service
Web Soil Survey URL:
Coordinate System:
Web Mercator (EPSG:3857)
Maps from the Web Soil Survey are based on the Web Mercator projection, which preserves direction and shape but distorts distance and area. A projection that preserves area, such as the Albers equal-area conic projection, should be used if more accurate calculations of distance or area are required.
This product is generated from the USDA-NRCS certified data as of the version date(s) listed below.
Soil Survey Area:
Napa County, California
Survey Area Data:
Version 15, Sep 1, 2022
Soil map units are labeled (as space allows) for map scales 1:50,000 or larger.
Date(s) aerial images were photographed:
Mar 26, 2022—Apr
25, 2022
The orthophoto or other base map on which the soil lines were compiled and digitized probably differs from the background imagery displayed on these maps. As a result, some minor shifting of map unit boundaries may be evident.
Hydrologic Soil Group—Napa County, California
(Capitola Drive Mercatus)
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
8/25/2023
ATTACHMENT 6
Hydrologic Soil Group
Map unit symbol
Map unit name
Rating
Acres in AOI
Percent of AOI
123
Coombs gravelly loam,
2 to 5 percent slopes
C
1.6 100.0%
Totals for Area of Interest
1.6 100.0%
Description
Hydrologic soil groups are based on estimates of runoff potential. Soils are assigned to one of four groups according to the rate of water infiltration when the soils are not protected by vegetation, are thoroughly wet, and receive precipitation from long-duration storms.
The soils in the United States are assigned to four groups (A, B, C, and D) and three dual classes (A/D, B/D, and C/D). The groups are defined as follows:
Group A. Soils having a high infiltration rate (low runoff potential) when thoroughly wet. These consist mainly of deep, well drained to excessively drained sands or gravelly sands. These soils have a high rate of water transmission.
Group B. Soils having a moderate infiltration rate when thoroughly wet. These consist chiefly of moderately deep or deep, moderately well drained or well drained soils that have moderately fine texture to moderately coarse texture.
These soils have a moderate rate of water transmission.
Group C. Soils having a slow infiltration rate when thoroughly wet. These consist chiefly of soils having a layer that impedes the downward movement of water or soils of moderately fine texture or fine texture. These soils have a slow rate of water transmission.
Group D. Soils having a very slow infiltration rate (high runoff potential) when thoroughly wet. These consist chiefly of clays that have a high shrink-swell potential, soils that have a high water table, soils that have a claypan or clay layer at or near the surface, and soils that are shallow over nearly impervious material. These soils have a very slow rate of water transmission.
If a soil is assigned to a dual hydrologic group (A/D, B/D, or C/D), the first letter is for drained areas and the second is for undrained areas. Only the soils that in their natural condition are in group D are assigned to dual classes.
Rating Options
Aggregation Method:
Dominant Condition
Component Percent Cutoff:
None Specified
Hydrologic Soil Group—Napa County, California
Capitola Drive Mercatus
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
8/25/2023
ATTACHMENT 6
Tie-break Rule:
Higher
Hydrologic Soil Group—Napa County, California
Capitola Drive Mercatus
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
8/25/2023
ATTACHMENT 6
CAPITOLA SUBDIVISION
STORMWATER CONTROL PLAN
ATTACHMENT 2
STORMWATER CONTROL PLAN (SHEET TM6)
ATTACHMENT 6
ATTACHMENT 6
CAPITOLA SUBDIVISION
STORMWATER CONTROL PLAN
ATTACHMENT 3
PROVISION E.12 SIZING CALCULATOR SPREADSHEET
ATTACHMENT 6
Provision E.12 Sizing Calculator
See the instructions and the BASMAA Post-Construction Manual Step 1:
Enter Total Site
Area
Step 2:
List names
of all DMAs
and square
footage of
each
Step 4:
If the DMA is
"Drains to Self
Retaining" or
"Drains to
Bioretention"
enter runoff
factor from
Table 4-1
Step 6:
For "Drains to
Self-Retaining"
DMAs, enter
the
name of
receiving DMA
Step 5:
Slide
(move)
number
from this
column to
correct
column
(F or H-Q)
Total Site Area:
70,421
DMA Names
Square
Feet
Self-
Treating
Self-
Retaining Runoff Factor Untreated
Name of
Receiving
DMA BA #1
DMA-1
perv
22,645 0.1 2,265
DMA-1
imp
39,846 1 39,846
DMA-2
perv
1,144 0.1 1,144
DMA-2
imp
5,098 1 5098
Total DMAs
68,733
0 0
6,242
42,111
0 0
0 0
0 0
0 0.04
0.04 0.04
0.04 0.04
0.04 0.04
0.04 1,684
0 0
0 0
0 0
0 Total Facilities
1,688
1,688
0 0
0 0
0 0
0 DMAs + Facilities
70,421
OK
OK
OK
OK
OK
OK
OK
OK
OK
Step 8:
Iterate sizes of facility footprints and DMAs until all footprints are at least the minimum AND
DMAs + Facilities equals Total Site Area
Step 9:
Check to make sure Areas Draining to each Receiving Self-Retaining Area do not exceed maximum 2:1 ratio.
Step 10:
Check results on this spreadsheet are consistent with what is shown on the SCP Exhibit.
Footprint on Exhibit
Minimum Size
Sizing Factor
Step 3:
If DMA is "Self-
Treating" or "Self-
Retaining,"
copy
square footage to
appropriate column
Step 7:
Enter Facilty Footprints
BIORETENTION FACILITIES
ATTACHMENT 6