APPENDIX C: STORMWATER CONTROL PLAN

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1 APPENDIX C: STORMWATER CONTROL PLAN

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3 STORM WATER MANAGEMENT PLAN FOR PLEASANT HILL, CALIFORNIA Prepared for Compliance with the Clean Water Program (C.3) and the San Francisco Bay Regional Water Quality Control Board Order No. R Printing date: March 16, 2015 Prepared for: DeNova Homes 1500 Willow Pass Court Concord, CA Prepared by: Meridian Associates, Inc Civic Court Suite 360 Concord, CA Job No

4 TABLE OF CONTENTS SECTION 1 PROJECT SETTING Project Location and Description... 1 Vicinity Map Site Features and Conditions Pre- & Post-Development Runoff Rates Stormwater Conveyance Systems... 1 SECTION 2 STORMWATER TREATMENT BMPs Location, Description and Maintenance of BMPs Splash Blocks Yard Swales/Private Drains Landcaped Swales/Slopes Other Landscaping Bubble-Up Drain Rock Private Inlet Bioretention Basins Sizing Calculations Additional Low Impact Development (LID) Measures... 6 SECTION 3 SOURCE CONTROL MEASURES Description of Site Activities and Potential Sources of Pollutants... 7 Table 3-1 Potential Pollutant Sources and Controls... 7 SECTION 4 SUMMARY OF PERMITING AND CODE COMPLIANCE ISSUES... 8 SECTION 5 BMP MAINTENANCE REQUIREMENTS Ownership and Responsibility for Maintenance in Perpetuity Commitment to Execute and Necessary Agreements Statement Accepting Responsibility for Operation and Maintenance of BMP s Until that Responsibility is Formally Transferred... 9 SECTION 6 CONSTRUCTION PLAN CHECKLIST Table 6-1 Construction Plan Checklist SECTION 7 CERTIFICATION APPENDIX A Storm Water Management Plan Figure 1

5 SECTION 1.0 PROJECT SETTING 1.1 Project Location and Description Subdivison 9403 Mercury at Taylor is a subdivision of Assessor s Parcel Numbers , - 010, -012, -014 & -016 located in Pleasant Hill, California. The project consists of 18 single-family homes and associated improvements on approximately 9.8 acres. VICINITY MAP PLEASANT HILL, CA NOT TO SCALE 1.2 Site Features and Conditions The site is vegetated open space with grasses and trees. There are residences to the west, south and southeast, Taylor Boulevard to the north, and the Contra Costa Canal to the east. Site topography is hilly, ranging from a high elevation of 215 in the southeast portion, to a low elevation of 97.8 in the northeast corner. The site drains in all directions, with the bulk of the runoff headed towards Taylor Boulevard and Mercury Way. Grading for the project will create 18 relatively flat pads and two bioretention basins, along with streets, parallel parking and cul-de-sacs. An Emergency Vehicle Access (EVA) roadway is provided for the property to the south, and a sanitary sewer maintenance Page 1

6 road is provided along Taylor Boulevard. New building pads will range in elevation from 126 to 173. With the exception of the slope required to provide the EVA, there is no grading above elevation 175, preserving the existing ridge line in the central portion of the site. A geotechnical investigation for the project was prepared by Stevens, Ferrone & Bailey, Engineering Company, Inc., Project No dated January 9, The investigation indicates that the site is underlain with sandstone material in the western 2/3 of the site, and claystone exists in the eastern 1/3 of the site. The assessment concluded that the site was suitable for residential subdivision construction, and recommended procedures for site grading, foundation design, retaining walls, roadway construction and soil drainage. Hydrologic Soil Group A (high infiltration rate) was estimated for the majority of the site, while the northeast portion (approximately encompassing proposed lots 9-18 was estimated as Hydologic Soil Group D (low infiltration rate). The project does not rely on infiltration rates of the native soils for the treatment of runoff, as the proposed basins will incorporate their own infiltration media. Minor infiltration in proposed landscaped areas, while significant in some open space areas, is not included as a factor in BMP sizing. 1.3 Pre- & Post-Development Peak Runoff Rates The existing site is 100% vegetated (9.8 acres). The proposed improvements will introduce 2.25 acres of impervious area (23% of the site), 4.33 acres of graded slopes and landscape areas (44.2% of the site), and preserve 3.22 acres of existing vegetated area (32.8% of the site). 1.4 Storm water Conveyance Systems A series of coordinated Best Management Practices (BMPs) will be used to remove pollutants, slow runoff, and release it to the downstream storm drain system. The Storm Water Management Plan is shown in Appendix A, Figure 1. Storm water control is focused on filtering runoff at the source of potential pollutants, beginning on each lot. Runoff from roofs is collected in downspouts. Downspout runoff will be handled using several methods: 1) Released to a splash block and drainage swale, and/or 2) connected to a private drain system. Drainage swales and private drain systems will typically lead to a front yard inlet at the back of curb, where runoff will be allowed to settle in the inlet. Runoff will be released to the street (via a bubble-up) and flow to a bioretention basin. The basin located south of Lot 4 will accept runoff from Lots 1 through 5 and the west half of Lot 6. The basin between lots 10 and 11 will accept runoff from Lots 7 through 18 and the east half of Lot 6. All street and driveway runoff will flow to a bioretention basin for treatment, except for the small portion of street connecting to Mercury Way. All runoff from newly graded slopes will be directed to a bioretention basin, except the slope behind lots Page 2

7 4 through 15, which will be landscaped, with runoff entering the existing roadside swale along Taylor Boulevard. SECTION 2 STORMWATER TREATMENT BMP S 2.1 Location, Description and Maintenance of BMP s The project BMP locations are shown on the Storm Water Management Plan, (Appendix A, Figure 1). Project BMPs are as follows: No. BMP Location(s) Landscaping 1) Splash blocks Located at downspouts not connected to private drains 2) Yard swales/private drains All lot runoff is directed to a yard drain, and released to the street via a bubble-up and/or curb sleeve 3) Landscaped swales/slopes From lot swales with gravity flow (no private drains), and flow over the slope behind lots 4 through 15. 4) Other landscaping Relatively flat areas adjacent to the project streets, along with modifications to the roadside swale along Taylor Boulevard. Drainage features 5) Bubble-up At the back of curb on lots, allowing sediments to settle before runoff is released to the street 6) Drain rock Bubble ups may havea drain rock component to allow for infiltration in low volume rain events. 7) Private inlet At storm drain angle points/junctions 8) Bioretention basin To collect runoff for infiltration and detention Splash blocks Splash blocks provide a disconnect between the roof downspouts and yard drain systems, allowing runoff to flow through landscaped areas for filtration. Where positive flow cannot be provided in a swale, downspouts may be connected to a private drain system. The original installation will be by the developer as part of the project landscaping. Maintenance will be by the homeowner to ensure the splash block is functioning properly and directing runoff away from the building foundation and towards the front yard private inlet(s) Yard Swales / Private Drains Yard swales are used in conjunction with splash blocks to allow additional runoff filtration through landscaped areas prior to entering the front yard inlet. Private drains may be used when positive flow Page 3

8 to the front yard inlet cannot be provided. Accumulated runoff in the front yard inlet will rise to the level of the bubble-up grate and flow to the street. Installation of private drains will be by the developer as part of the project landscaping. Maintenance will be by the homeowner to ensure the system is functioning properly and to remove debris or obstructions Landscaped swales/slopes Landscaped swales and landscaped slopes will be used to filter sediment and deter erosion on the graded slopes and graded swales of the project. To be effective, landscape materials must be able to filter sediments from stormwater runoff. The surface components of these areas will primarily consist of clumped/clustered vegetation consisting of grasses, groundcovers, shrubs and other plants, along with landscape materials such as river-rock, cobbles and decorative borders. The original landscape installation by the Developer, per the project Landscape Plans. Landscape area maintenance outside private lots will be the responsibility of the Homeowners Association (HOA). The HOA will maintain/repair any landscaped areas that have degraded to the point where filtration of sediments is no longer occurring, or is considered no longer effective. Maintenance will consist of replanting, trimming, debris removal, and repairs to any related irrigation system Other Landscaping Some relatively flat areas of the site will be landscaped by the Developer per the project Landscape Plans. Landscaping on these areas is considered part of the Homeowners Association maintenance area. These landscape areas are not essential to the proper function of the Storm Water Management Plan, but will be designed, installed and maintained by the HOA. There may be cases where landscaping on private lots creates a risk of erosion, in which case, the HOA will have the authority to intervene and remedy the risk situation. The existing roadside swale along Taylor Boulevard will be modified for treatment mitigation. Improvements will include, but are not limited to re-grading, vegetation, check dams, subdrains, infiltration material and/or drain rock, and a shallow settlement area at the exiting inlet near the northeast corner of the site. The settlement area, if installed, will be no deeper than 3, and is intended to capture any sediment that would otherwise enter the existing inlet. The mitigation does not include the existing Taylor Boulevard pavement area. The BMPs installed are for mitigation of the newly graded slope and sewer maintenance road only Bubble-Up Bubble-ups are a drain inlet located adjacent to the street. Runoff entering the inlet is allowed to settle for sediment removal, infiltrate into drain rock (if drain rock is used) and, in some cases, evaporate. When the inlet fills in a storm, runoff rises to the grate of the inlet and is released to the street. Roof downspouts and area drains that connect to the bubble-up are the responsibility of the homeowner. Maintenance of the bubble-up will consist of removal of debris and accumulated sediments, either by flushing, vacuuming, or replacement of the drain rock. Normal maintenance is Page 4

9 expected to be performed by the homeowner, but the HOA has the right to perform maintenance if the integrity of the overall system is threatened Drain Rock Bubble-ups/drain inlets may have a base comprised of drain rock (1 cubic foot, minimum) for filtration and capture of sediment in low-flow conditions. When runoff fills the base rock area, it rises to the grate and is released to the street swale. Maintenance is described above in section See Appendix A, Figure Private Inlet Private Inlets will be installed on lots that cannot achieve direct surface flow to the back of curb inlet. Other private inlets may be installed at angle points or junctions in the pipe alignment, but are not intended to receive significant surface runoff. The private inlet system should always terminate at a back of curb private inlet (bubble-up). Each homeowner will maintain the private drain line that lies on their lot. The Homeowner s Association has the right to perform maintenance should the homeowner neglect to do so, or if a significant repair is required Bioretention basins A bioretention basin is a soil and plant-based filtration and retention area intended to remove pollutants. The surface vegetation is underlain by an 18 thick layer of bio-treatment soil mix (BSM), capable of achieving a minimum infiltration (percolation) rate of 5 per hour. Beneath the BSM layer is a 12 thick layer of class II aggregate and a subdrain. The original installation of the bioretention swales will be by the Developer, per the project Landscape Plans. The bioretention swale will be maintained by the HOA, to ensure effective filtering of the runoff. Effective filtering is intended to mean that the bioretention basin as installed by the Developer has not been modified or neglected such that the vegetation area is decreased, and the BSM and subdrains are functioning as intended. The HOAn will maintain/repair any bioretention basin that has been neglected, improperly moved or modified, or is considered no longer effective. Maintenance will consist of replanting, trimming, debris removal, and repairs to any related piping or overflow risers and grates.. Typical sections of the bioretention swales are shown in Appendix A, Figure Sizing Calculations The 4% method was used to determine the required areas (and depths) for the bioretention basins, which combines 4% of impervious area contributing to a basin with 1% of the pervious (landscaped) area contributing to the basin. When the required area is determined, the basin is designed to provide a minimum 6 depth of available ponding, all of which must be underlain by BSM. Each Drainage Management Area (DMA) is tabulated on the Stormwater Management Plan (Appendix A, Figure 1) Page 5

10 The calculated treatment area for bioretention swales and bioretention basins is that area that is underlain by 18 of treatment soil (BSM) and achieves the required ponding depth. Check dams or other measures may be required in order to achieve the required ponding depth over the entire length/width of the treatment area. 2.3 Additional Low Impact Development (LID) Measures In accordance with section C.3.c of Order R , the project incorporates (at a minimum) the following additional LID measures: Tree planting. A minimum of 36 trees. Landscaping Soil Quality To meet the ideal bulk density for front yard landscaping Swimming Pool Water Pools should be drained into the sanitary sewer system Landscaping that minimizes irrigation and runoff Efficient irrigation systems Storm drain inlet stenciling No Dumping Drains to Bay Page 6

11 SECTION 3 SOURCE CONTROL MEASURES 3.1 Description of Site Activities and Potential Sources of Pollutants Subdivision 9403 Mercury at Taylor is a single-family residential subdivision. Potential sources of pollutants are generally limited to homeowner activities such as landscaping, vehicle washing, and common household pollutants. In general, any pollutants likely to be present will be treated by a combination of private lot landscaping, drain rock and bioretention basins as described in the previous section. Table 3-1 below, shows the potential pollutant sources and proposed controls. TABLE 3-1 POTENTIAL POLLUTANT SOURCES AND CONTROLS Potential Source of Pollutant Runoff Permanent BMP S Operational BMP s On-Site Storm Drain Inlets Need for Future Indoor & Structural Pest Control Landscaping/Outdoor Pesticide Use Vehicle Washing Driveway and Roadways Rooftops Storm drain inlets will be marked with the words No Dumping! Drains to Bay General building practices will be designed to minimize the potential for pest intrusion Landscaping will be designed in order to minimize irrigation and runoff requirements Vehicle washing will be performed off-site, in garages, or on the private streets; runoff, if any, will flow through the bio-retention basin prior to discharge to the storm drain system. Excess driveway runoff and roadway runoff will flow through a bioretention basin prior to discharge to the storm drain system Rooftop runoff discharges in two ways: 1) Onto splash blocks and into yard swales, landscaping and/or private drains. 2) Into a private drain system, which terminates at a back of curb inlet, and is released to the street via a bubbleup. 1) Inlet markings will be periodically maintained. 2) Stormwater pollution prevention information will be provided to new homeowners as part of a new homeowner package Integrated pest management information will be provided to new homeowners as part of a new homeowner package Homeowners will be provided with information regarding general stormwater pollution prevention information, including pesticide and fertilizer use As part of the new homeowner package, homeowners will be provided with information regarding potential pollutant sources, including vehicle washing Maintenance of bioretention basins will be conducted regularly in order to ensure proper functionality Splash blocks shall be inspected to ensure positive drainage away from the building foundation. Maintenance of swales and bioretention basins shall be performed regularly to ensure effective filtration. The HOA has the right to repair any defects that are impeding the effective operation of any swale, bioretention basin, landscaped area and/or private drain system. Page 7

12 SECTION 4 SUMMARY OF PERMIT AND CODE COMPLIANCE ISSUES The stormwater control BMP s, as described in this report, should not conflict with City of Pleasant Hill codes or other development requirements. All stormwater controls were designed in accordance with manufacturer s recommendations, Clean Water Program guidelines, California Stormwater Quality Association (CASQA) standards, and applicable City of Pleasant Hill standard drawings. It is the intention of this Storm Water Management Plan to conform with the descriptions and details outlined in the Contra Costa County C.3 Stormwater requirements Any conflicts, should they arise, will be resolved through the City s development review process or during subsequent permitting. Page 8

13 SECTION 5 BMP MAINTENANCE REQUIREMENTS Upon completion of Subdivision 9403 Mercury at Taylor construction activity, BMP maintenance will be transferred to the Homeowner s Association (HOA). In general, future homeowners are responsible for their own lots. However, all BMPs on private lots are the responsibility of the homeowner, with the HOA reserving the right to maintain or repair any BMPs essential to the function of the pollutant and sediment removal system, such as drain rock, landscaping, swales, private drains, and bubble-ups. The HOA will maintain the bioretention basins. The City of Pleasant Hill reserves the right to inspect, maintain and repair any items deemed necessary. 5.1 Ownership and Responsibility for Maintenance in Perpetuity Commitment to Execute and Necessary Agreements The Developer will provide any necessary easements or rights of entry to the City or other agencies as required, and the HOA for access and inspection of storm water BMP s and to make provision of easements or rights of entry a condition of sale Statement Accepting Responsibility for Operation and Maintenance of BMP s Until that Responsibility is Formally Transferred The Developer agrees to maintain the BMPs on Subdivision 9403 Mercury at Taylor until one of the following occurs: 1) Acceptance of maintenance responsibility by the City of Pleasant Hill, or other applicable agency, including the filing of all required easements and establishment of a special district or other permanent funding mechanism; or 2) Transfer of responsibility to the HOA, which will be responsible for maintenance, execution of Codes, Covenants, and Responsibilities or other agreements that run with the land and will requires future owners to provide and pay for maintenance of stormwater BMP s, and execution of a Stormwater Management Facilities Operation and Maintenance Agreement and Right of Entry in the form provided by the City of Fremont, or other applicable agency Stormwater Facilities Operation and Maintenance Plan The Developer will submit, with the application for building permits, a draft Stormwater Facilities Operation and Maintenance Plan including detailed maintenance requirements and a maintenance schedule. Page 9

14 SECTION 6 CONSTRUCTION PLAN CHECKLIST The following table describes the stormwater control BMP s selected for Subdivision 9403 Mercury at Taylor. TABLE 6-1 CONSTRUCTION PLAN CHECKLIST Stormwater BMP Description Details Management Plan Section Splash Blocks (roof-runoff disconnect) None Section Yard swales/private drains. Yard swales Project Landscape Plans and/or private drains will discharge to a Project Plot Plans private back of curb inlet Private drains will discharge via a bubble up into the street Section Section Section Section Section Bioretention basins. Areas that filter runoff through vegetation, a BSM layer and a subdrain layer prior to release to a storm drain or the street. Other Landscaping. Landscaped areas on non-private lots that are not an essential element of stormwater control will be maintained by the HOA. Mitigation BMP maintenance for BMSs added to the existing roadside swale along Taylor Boulevard will become the responsibility of the City. Bubble-up. An inlet that retains runoff until it overflows the inlet grate. They are used at the back of curb on each lot where private drains discharge. Drain Rock. Sediment is trapped in the drain rock component during low-flow events. Higher volume events release the runoff to the street. Private Inlet. Runoff is collected from the downspouts and conveyed to a back of curb inlet. Runoff from roofs is never discharged to the public storm drain pipe without entering a bioretention basin. Appendix A, Figure 1 Project Landscape Plans Project Landscape Plans Appendix A, Figure 1 Appendix A, Figure 1 Project Landscape Plans Project Plot Plans Page 10

15 Stormwater Management Plan Section BMP Description Bioretention basin. A filtration pond that allows runoff to settle and infiltrate through a vegetated layer, BSM layer and subdrain layer prior to connection to a storm drain system. Details Appendix A, Figure 1 Project Landscape Plans SECTION 7 CERTIFICATION The selection, sizing, and preliminary design of treatment BMP s and other control measures in this plan are intended to meet the requirements of Regional Water Quality Control Board (RWQCB) Order R If there is a discrepancy between a requirement or detail as described herein, or in the project plans, and the requirements of the City of Pleasant Hillt or the RWQCB, the more stringent requirement shall apply, unless indicated differently in writing by the affected regulatory agency. The Developer/HOA, or its representative, may propose alternate and/or additional BMP for use on this site, subject to the review and approval of the HOA, the City of Pleasant Hill, and the RWQCB OWNERS CERTIFICATION per the requirements of the City, Post-Construction Storm Water Pollution Prevention Design Standards, or other applicable agencies and documents. I, the undersigned, certify that all land clearing, construction and development shall be done pursuant to the approved plan. DeNova Homes ADDENDUM Maintenance of SWMP BMPs will be maintained per the recorded Maintenance Agreement with the Subdivision 9403 Mercury at Taylor Homeowners Association. Page 11