EBL&S Development Station Park Green Preliminary Stormwater Quality Management Strategy REV 1

Transcription

1 Development Preliminary Stormwater Quality Management Strategy REV 1

2 Preliminary Stormwater Quality Management Strategy February 2009 This report takes into account the particular instructions and requirements of our client. It is not intended for and should not be relied upon by any third party and no Arup North America Ltd responsibility is undertaken to any third 901 Market Street, Suite 260, party San Francisco, CA Tel Fax Job number

3 Document Verification Page 1 of 1 Job title Document title Job number File reference Document ref Revision Date Filename 0001Stormwater Quality Management.doc Draft 1 11/29/07 Description First draft Prepared by Checked by Approved by Name Nathan Will Grant McInnes John Eddy Signature Rev 1 Filename Description This document examines the proposed stormwater quality management with respect to an alternative program that was proposed in late 2008, and will go to public hearing in Prepared by Checked by Approved by Name Manish Dalia Nathan Will/Grant McInnes John Eddy Signature Filename Description Prepared by Checked by Approved by Name Signature Filename Description Prepared by Checked by Approved by Name Signature Issue Document Verification with Document

4 Contents Page 1 Introduction 1 2 Design Considerations Site Specific Constraints Permeable Pavements Biofiltration Planters and Rain Gardens Filterra Catch Basins Mechanical Devices Summary of BMP effectiveness 9 3 Sizing Calculations Site Run-off Coefficients Rainfall Intensity Flow-Based Design Criteria 11 4 Proposed Stormwater Quality Management System The Plan Alternative Conclusions 18 Tables Table 1: BMP Pollutant Removal Effectiveness Table 2: Water Quality Runoff Coefficients Figures Figure 1: Proposed Site Plan The Plan Figure 2: Proposed Site Plan Alternative 2 Figure 3: Potential Pollutants Figure 4: Porous Unit Paver Installation Figure 5: Rain Garden Cross Section Figure 6: Schematic of Filterra Unit Figure 7: Hydrodynamic Separation Products Figure 8: StormFilter Media Cartridge Figure 9: Catch Basin Inserts Figure 10: Stormwater BMP Overview in Streetscapes and Open Spaces The Plan Figure 11: Stormwater BMP Overview in Streetscapes and Open Spaces Alternative 2 Appendices

5 Appendix A Sizing Calculations Appendix B Design Curves

6 1 Introduction As proposed by EBL&S Development, will transform 12 acres of under-utilized retail acreage into a new neighborhood with housing, open space, retail, and commercial uses. Located at the northwest corner of the intersection of Concar Drive and South Delaware Street and adjacent to the Hayward Station Caltrain Station in San Mateo, CA, infrastructure will be a model of sustainable design. Refer to Figures 1 and 2 for plans of the proposed development for the two program alternatives. Figure 1 depicts the preferred alternative The Plan, while Figure 2 depicts Alternative 2. Figure 1: Proposed Site Plan The Plan \\S-FNAS01\JOBSIF\131178\4 INTERNAL PROJECT DATA\4-05 REPORTS & NARRATIVES\STORMWATER\ALTERNATIVE 2 FEBRUARY 2009 STUDY\0003SPG- PRELIMINARY STORMWATER QUALITY MANAGEMENT STRATEGY_REV1 FEBRUARY 2009.DOC Page1 24 November 2007

7 Figure 2: Proposed Site Plan Alternative 2 The ability of the on-site stormwater systems to cleanse stormwater before it is discharged into the City of San Mateo s system is vital for meeting regulatory requirements and to providing a highly sustainable design. The purpose of this report is to summarize the preliminary concept for the stormwater quality infrastructure design elements in the streets and public open spaces within the development for The Plan and Alternative 2. The report recommends a variety of Best Management Practices (BMPs) that will meet the standards of National Pollution Discharge Elimination System (NPDES) and the San Mateo Countywide C.3 Stormwater Technical Guidance Manual (March 2007). As required in the Design Guidelines, all parcel developers are responsible for integrating stormwater treatment BMPs that meet NPDES requirements before they may discharge into the storm drainage system. Design, reporting, and approval of the parcel-specific stormwater quality systems will be undertaken at the time of the parcel-specific Site Planning Architectural Review. Stormwater quality within the streetscapes will be managed using a combination of the following BMPs: Permeable Pavers Pavement systems that allow water to pass through to the pavement s underlying roadbed. These will reduce peak runoff that must be treated \\S-FNAS01\JOBSIF\131178\4 INTERNAL PROJECT DATA\4-05 REPORTS & NARRATIVES\STORMWATER\ALTERNATIVE 2 FEBRUARY 2009 STUDY\0003SPG- PRELIMINARY STORMWATER QUALITY MANAGEMENT STRATEGY_REV1 FEBRUARY 2009.DOC Page2 24 November 2007

8 and will begin the cleansing process because the underlying roadbed provides filtration before the stormwater is conveyed to raingardens. These pavers will be the upper end of a treatment train where they are proposed in C Street, surrounding the central park. Rain Gardens These planted areas capture stormwater and treat runoff through natural sedimentation, infiltration, and biological uptake in a natural setting with soft edge conditions. Several rain gardens are proposed in the neighborhood open spaces. Biofiltration Planters These planters will treat runoff through biological uptake, natural sedimentation, and infiltration practices within a confined planter box. Biofiltration planters will be used throughout the streetscapes. Filterra Catch Basins In areas where space does not permit biofiltration planters, Filterra catch basins, or equivalent, will be implemented. Using a combination of unique soil filter media and natural absorption through plant species, large amounts of runoff can be treated in a compact area. Mechanical Devices Where biological devices are not feasible because of space and grade constraints, mechanical devices are proposed. There are numerous mechanical devices on the market, including media filters and hydrodynamic separators that are compact and capable of treating large volumes of water. Currently, mechanical devices are planned for the emergency vehicle access easement at the west property line. \\S-FNAS01\JOBSIF\131178\4 INTERNAL PROJECT DATA\4-05 REPORTS & NARRATIVES\STORMWATER\ALTERNATIVE 2 FEBRUARY 2009 STUDY\0003SPG- PRELIMINARY STORMWATER QUALITY MANAGEMENT STRATEGY_REV1 FEBRUARY 2009.DOC Page3 24 November 2007

9 2 Design Considerations Stormwater runoff collects impurities and pollutants, while passing over roofs, parking lots, landscapes and gutters. Without treatment, these pollutants enter our watercourses and ultimately, the ocean. The California BMP Handbook, excerpted in Figure 3, describes the types of pollutants that can be generated from various land uses. Figure 3: Potential Pollutants (Source California Stormwater BMP Handbook: New Development and Redevelopment; California Stormwater Quality Association, January 2003) The following sections outline the site constraints and describe the BMPs appropriate for along with design recommendations set forth by the San Mateo Countywide C.3 Stormwater Technical Guidance Manual for each of the proposed BMP types considered. 2.1 Site Specific Constraints The existing soil underlying contains significant amounts of clay, including Bay Mud, which creates an impervious layer across the site. Use of permeable pavers and infiltration systems will require a sub-drainage system to channel the water away and prevent ponding above the impervious layer. Further challenging water infiltration and rain garden design is the presence of a high groundwater table. The groundwater table is typically between 8 and 12 feet below finished grade, although there are areas where the ground water is within the depth of typical infiltration basins and rain gardens (3-4 feet). Where the water table is at or close to the basins and gardens, the installation of an impermeable barrier or membrane is recommended to separate the groundwater from the potentially polluted stormwater. Encouraged by good-neighborhood planning and to reduce the heat island effect, the majority of on-site parking is located in the building basements. The urban nature of the development precludes the use of swales for cleaning and conveying storm run-off, (i.e., \\S-FNAS01\JOBSIF\131178\4 INTERNAL PROJECT DATA\4-05 REPORTS & NARRATIVES\STORMWATER\ALTERNATIVE 2 FEBRUARY 2009 STUDY\0003SPG- PRELIMINARY STORMWATER QUALITY MANAGEMENT STRATEGY_REV1 FEBRUARY 2009.DOC Page4 24 November 2007

10 the conflict between linear, surface drainage facilities and pedestrian-oriented streetscape). 2.2 Permeable Pavements Permeable pavement slows and cleanses the flows as the storm water passes through the pavement and underlying soils to the sub-drain system. will use permeable pavers for surfacing C Street, which encircles the park for both program alternatives. The following are recommendations for designing the permeable pavement and base: 1) The pavement section is capable of supporting appropriate traffic loads. 2) Base material is composed of open-graded, crushed stone with no fines, to allow for subsurface water storage and drainage 3) Minimum infiltration rates in the base material of 5 in/hr 4) Installation of sub-grade collection system (slotted or perforated pipes) to direct the stormwater to rain gardens or biofiltration planters 5) Installation of a watertight barrier below aggregate base if the water table is within 10 feet of surface, as directed by the Bay Area Water Quality Board recommendations. Figure 4: Porous Unit Paver Installation Permeable pavements are only effective at removing Total Suspended Solids (TSS) and heavy metals, therefore this should not be considered a stand-alone BMP. Sub-drains from porous pavements shall flow to rain gardens in the open space. The design shall recognize that the use of porous pavement will reduce the peak flows of the required water quality storm events, and allow for reductions in the rain garden treatment area. \\S-FNAS01\JOBSIF\131178\4 INTERNAL PROJECT DATA\4-05 REPORTS & NARRATIVES\STORMWATER\ALTERNATIVE 2 FEBRUARY 2009 STUDY\0003SPG- PRELIMINARY STORMWATER QUALITY MANAGEMENT STRATEGY_REV1 FEBRUARY 2009.DOC Page5 24 November 2007

11 2.3 Biofiltration Planters and Rain Gardens Flow-through streetscape planters and rain gardens are proposed for the development, both of which have similar design criteria. The following are the design considerations for the implementation of these BMPs: 1) Minimum 18 of loamy sand for main infiltration medium with an infiltration rate of 5 in/hr 2) A required minimum 6 of ponding above mulch layer. This depth can be increased to 12, which reduces the needed size of biofiltration planters or rain gardens 3) Stormwater is delivered to the planters via storm drains, curb cuts, or overland flow. After treatment, storm drains will deliver water to existing networks on South Delaware Street, Garvey Way and Concar Drive. 4) The biofiltration planters shall be coupled with traditional catch basins downstream of the treatment unit. The catch basins will act as overflow basins during rainfall events in excess of the water quality event. 5) Rain gardens planted with trees will require a 40% reduction in effective area to allow for planting requirements of larger trees along C and B Streets. Figure 5: Rain Garden Cross Section 2.4 Filterra Catch Basins An alternative to traditional biofiltration basins is a pre-cast product (Filterra by Americast 1 ) that can also be integrated into the planter strip, while maintaining a raised curb. This effectively acts as a dual catchbasin with the upstream unit acting as a biofiltration cell for the water quality design flow and the downstream unit serving the larger rain event flows. The following are recommendations for the installation of the Filterra units: 1 \\S-FNAS01\JOBSIF\131178\4 INTERNAL PROJECT DATA\4-05 REPORTS & NARRATIVES\STORMWATER\ALTERNATIVE 2 FEBRUARY 2009 STUDY\0003SPG- PRELIMINARY STORMWATER QUALITY MANAGEMENT STRATEGY_REV1 FEBRUARY 2009.DOC Page6 24 November 2007

12 1) Unit shall be coupled with a catch basin, which captures flows larger than water quality events 2) Units shall be integrated into landscape strips and arranged to capture flows across the throat opening 3) Unit shall have an impermeable base, and shall be concrete if base is located within 1 foot of water table 4) The treated stormwater pipe shall be connected directly to the downstream catch basin Figure 6: Schematic of Filterra Unit 2.5 Mechanical Devices Mechanical devices will be used in locations where biofiltration techniques cannot be efficiently integrated into the site design. Although it is not a preferred BMP, the type of mechanical device selected will depend on a variety of site characteristics. The proposed mechanical devices have not been selected yet. Their applicability for will be determined nearer to construction to capitalize on the latest treatment technologies available. The following design considerations will apply regardless of the mechanical device used: 1) Size device per manufacturer s recommendations using site-specific precipitation data, surface area, and contributing area runoff coefficient 2) Potential pollutant characteristics of proposed watershed shall be estimated to determine the type of device selected The following mechanical devices are currently being considered: Hydrodynamic Separators Downstream Defender and Continuous Deflective Separation (CDS) systems are compact hydrodynamic separators that remove finer \\S-FNAS01\JOBSIF\131178\4 INTERNAL PROJECT DATA\4-05 REPORTS & NARRATIVES\STORMWATER\ALTERNATIVE 2 FEBRUARY 2009 STUDY\0003SPG- PRELIMINARY STORMWATER QUALITY MANAGEMENT STRATEGY_REV1 FEBRUARY 2009.DOC Page7 24 November 2007

13 sediment, particles, free oil, and debris from urban runoff. The design allows for easy inspection and unobstructed maintenance access. This system uses a combination of swirl-concentrator and flow-control technologies to maximize treatment. Figure 7: Hydrodynamic Separation Products Media Filters Utilize variable flow controls, media-filled cartridges, and a storage sump to capture and retain a broad spectrum of pollutants. Multiple cartridge heights allow the benefits of a filtration system within the available pipe characteristics. Filter media can be customized to treat site-specific pollutants. StormFilter is an example of a media filter BMP. Figure 8: StormFilter Media Cartridge Catch Basin Inserts These devices trap hydrocarbons and other contaminants such as metals, sand, silt, trash, and debris from stormwater runoff. These are easy to install in new and existing drop and curb inlets. Quick and easy maintenance is facilitated by Media-Pak cartridges. Triton Catch Basin inserts and Abtech s Smart Sponge / Ultra urban filter are examples of these devices. Figure 9: Catch Basin Inserts \\S-FNAS01\JOBSIF\131178\4 INTERNAL PROJECT DATA\4-05 REPORTS & NARRATIVES\STORMWATER\ALTERNATIVE 2 FEBRUARY 2009 STUDY\0003SPG- PRELIMINARY STORMWATER QUALITY MANAGEMENT STRATEGY_REV1 FEBRUARY 2009.DOC Page8 24 November 2007

14 2.6 Summary of BMP effectiveness Table 1 summarizes the typical pollutant removal effectiveness that can be expected by implementation of the BMPs within as discussed above. The rankings of the BMPs should be viewed as indicative only. Table 1: BMP Pollutant Removal Effectiveness Pollutant Removal Effectiveness Pollutant Porous Pavers (1) Bioretention (Planters and Rain Gardens) Mechanical seperators (2) Sediment High High High Nutrients Medium Medium Low Trash N/A High High Metals High High High Bacteria N/A High Medium Oil and Grease N/A High High Organics N/A High Medium (1) Removal effectiveness data taken from the Porous Pavement BMP fact sheet on: (2) Removal effectiveness is dependant on the particular device used. Ranking should be viewed as indicative only. \\S-FNAS01\JOBSIF\131178\4 INTERNAL PROJECT DATA\4-05 REPORTS & NARRATIVES\STORMWATER\ALTERNATIVE 2 FEBRUARY 2009 STUDY\0003SPG- PRELIMINARY STORMWATER QUALITY MANAGEMENT STRATEGY_REV1 FEBRUARY 2009.DOC Page9 24 November 2007

15 3 Sizing Calculations Streetscape runoff will be treated using biofiltration planters, Filterra catch basins, and mechanical devices prior to being discharged to the City of San Mateo drainage systems in Concar Drive, South Delaware Street, and Garvey Way. This section outlines the calculations used to size the water quality BMPs within. 3.1 Site Run-off Coefficients The runoff coefficient required for water quality treatment is less than that required for storm conveyance because water quality events are small, frequent events in which saturated soil conditions are not likely. Storm conveyance design requires larger runoff coefficients for flood control. The following C-values are outlined by the Bay Area Stormwater Management Agencies Association (BASMAA) for use in sizing stormwater treatment devices. Table 2: Water Quality Runoff Coefficients Water Quality Runoff Coefficients for Surface s Type of Surface Runoff Coefficient (C-Value) Concrete/ Pavement 0.80 Pervious Pavers 0.55 Grass/ Landscaping 0.10 Where a watershed has more than one surface, a composite runoff coefficient was calculated. The runoff coefficient calculations are located in Appendix A Rainfall Intensity There are two recommended approaches for sizing stormwater quality systems. One is the flow-based approach; the other is volume-based. In both cases, relatively small size storms are addressed as these make up the majority of storm events. The San Mateo Countywide C.3 Technical Guidance Manual identifies different sizing criteria based on project location. The site is located in Zone 6 (refer to figure 1 Appendix E in C.3 Manual). The following sizing events result: 1) Flow-Based Design Criteria: The water flow is defined as two (2) times the 85% hourly rainfall intensity of the contributing area s runoff. The resulting rainfall intensity (i wq ) is defined as the water quality intensity 0.2 in/hr. The Intensity Duration Frequency (IDF) curve for the San Francisco Airport Station in San Mateo can be seen in Appendix B1. 2) Volume-Based Design Criteria: The water volume required to catch 80% or more of runoff over the contributing surface area. Using a runoff coefficient of 1.0, the resulting unit basin storage volume at 80% is 0.75 inches. The runoff capture curve for the San Francisco Airport Station is in Appendix B2. Volume-based design criteria are used in sizing devices that rely on infiltration to the subgrade to the sub-surface collection system.. \\S-FNAS01\JOBSIF\131178\4 INTERNAL PROJECT DATA\4-05 REPORTS & NARRATIVES\STORMWATER\ALTERNATIVE 2 FEBRUARY 2009 STUDY\0003SPG- PRELIMINARY STORMWATER QUALITY MANAGEMENT STRATEGY_REV1 FEBRUARY 2009.DOC Page10 24 November 2007

16 3.3 Flow-Based Design Criteria Flow-based design criteria are used for the sizing of the biofiltration planters and are calculated using the rational method: Q wq = C * i wq * A Q wq = water quality design flow (cfs) C = water quality runoff coefficient i wq = design water quality intensity (in/hr); 0.2 in/hr for San Mateo A = watershed area (acres) The required size of the filtration unit can be calculated using a flow balance principle that states that the contributing water quality flow must be equal to the infiltration flow capacity of the planter. Q wq = Q bf Q bf = A bf * V bf A bf = Q wq / V bf Q bf = Flow capacity of Biofiltration unit (cfs) A bf = Required Area of Biofiltration (square feet) V bf = Biofiltration infiltration rate (in/hr) Sections describe alterations required for each flow-based BMP proposed for the site Biofiltration Planters Sizing of the biofiltration planters is dependent on the available width and specific constraints of the site. Assumptions for sizing the biofiltration planters are as follows: a) Biofiltration planters vary in width from 3.5 to 4 feet b) Allow for an 18-inch setback when a planter is located adjacent to parking c) V bf for the infiltration media is assumed to be 5 in/hr d) A bf shall be a minimum of 4% of impervious area of watershed The calculations for the watersheds anticipated to be treated using biofiltration planters can be found in Appendix A Raingardens Raingarden sizing is also dependent on the available width and constraints within the open spaces. Assumptions in sizing the raingardens are as follows: a) Raingardens vary in width between 20 and 30 feet b) V bf for the infiltration media is assumed to be 5 in/hr c) A bf shall be a minimum of 4% of impervious area of watershed d) A bf shall be a maximum 40% of overall raingarden area to account for large plantings and trees \\S-FNAS01\JOBSIF\131178\4 INTERNAL PROJECT DATA\4-05 REPORTS & NARRATIVES\STORMWATER\ALTERNATIVE 2 FEBRUARY 2009 STUDY\0003SPG- PRELIMINARY STORMWATER QUALITY MANAGEMENT STRATEGY_REV1 FEBRUARY 2009.DOC Page11 24 November 2007

17 The calculations for the watersheds anticipated to be treated in the raingardens can be found in Appendix A Filterra Catch Basins Given the high infiltration capacity, V bf, of the Filterra catch basins, these require less space than biofiltration planters. The units are especially applicable where available space is limited. A flow-based design was used to size the Filterra units.. Assumptions in sizing the Filterra catch basins are as follows: a) V bf for the infiltration media is assumed to be 8.33 in/hr b) 4 x6.5 catch basin unit can treat up to 0.35 acres c) 4 x8 catch basin unit is capable of treating up to 0.44 acres The calculations for the watersheds anticipated to be treated using Fliterra catch basins can be found in Appendix A Mechanical Devices Given that specific devices have not been selected for the site, the standard flow-based treatment rates are calculated for the characteristics of the contributing watersheds. The calculations for the anticipated design flows from the watersheds identified for mechanical devices can be found in Appendix A5. \\S-FNAS01\JOBSIF\131178\4 INTERNAL PROJECT DATA\4-05 REPORTS & NARRATIVES\STORMWATER\ALTERNATIVE 2 FEBRUARY 2009 STUDY\0003SPG- PRELIMINARY STORMWATER QUALITY MANAGEMENT STRATEGY_REV1 FEBRUARY 2009.DOC Page12 24 November 2007

18 4 Proposed Stormwater Quality Management System To efficiently treat all the streetscape and open-space run-off, a variety of postconstruction BMPs have been selected that are integrated into the site design. Each BMP will treat the required water quality runoff prior to connection with the drainage system (refer to calculation tables in Appendix A for both program Alternatives). Run-off generated within the parcel blocks is assumed to be treated by BMPs within each parcel block, the design of which will be confirmed during the Site Planning and Architectural Review (SPAR) process. 4.1 The Plan The streetscapes have been divided into watersheds according to street profiles and slopes for The Plan (refer to Figure 10). Descriptions of each watershed are included in the Conceptual Hydrology Report Dated February The proposed watersheds for The Plan are treated as follows: Basins A, B, C and a majority of D do not have provisions for water quality measures. These basins are composed of City of San Mateo right-of-way and are not part of the proposed project. South Delaware Street is to be narrowed (by the City of San Mateo) per the Rail Corridor plan, and water quality improvements should be made at that time. The portion of Basin D that will require onsite treatment is the area between blocks MU-2 and MU-3. A small mechanical device is proposed to treat the runoff prior to entry to the storm system in Concar Drive. Watersheds E and F are located on A Street where multiple biofiltration planters are proposed on both sides of the streets (see Appendix A). These planters will be integrated into the streetscape between the proposed trees and streetlights and upstream of the overflow catchbasins. \\S-FNAS01\JOBSIF\131178\4 INTERNAL PROJECT DATA\4-05 REPORTS & NARRATIVES\STORMWATER\ALTERNATIVE 2 FEBRUARY 2009 STUDY\0003SPG- PRELIMINARY STORMWATER QUALITY MANAGEMENT STRATEGY_REV1 FEBRUARY 2009.DOC Page13 24 November 2007

19 Figure 10: Stormwater BMP Overview in Streetscapes and Open Spaces The Plan Watershed G is located in B Street and will flow via multiple curb cuts or slotted drains to the proposed rain garden in OS-10. Overflows will be directed to a catch basin at the intersection of Charles Way and South Delaware Street. Watershed H is also located in B Street and multiple flow-through biofiiltration planters are proposed between the trees in the landscape area. The planters will be upstream of a catch basin at the intersection with Charles Way. Watershed I consists predominantly of the central park and the surrounding C Street Loop where the majority of the sidewalks and streets will be permeable pavers. These permeable pavers will reduce total suspended solids and heavy metals, while reducing the volume of runoff. The rain garden along the north side of the park treats the remainder of the run-off. The streets and park will be graded to direct flows to the rain garden, with overflows directed to a catch basin located near the intersection with B street. Even with a reduction in the effective treatment area, this basin has excess capacity and could be utilized by some of the surrounding buildings, depending on the project phasing. \\S-FNAS01\JOBSIF\131178\4 INTERNAL PROJECT DATA\4-05 REPORTS & NARRATIVES\STORMWATER\ALTERNATIVE 2 FEBRUARY 2009 STUDY\0003SPG- PRELIMINARY STORMWATER QUALITY MANAGEMENT STRATEGY_REV1 FEBRUARY 2009.DOC Page14 24 November 2007

20 Watersheds J, K, and L are located along the western property line shared with the Joint Powers Board. An emergency vehicle access easement and vehicular access to parcels are required at this boundary; therefore, there is limited space for biological treatment BMPs. One large, or several small, mechanical devices are currently proposed to treat the runoff flows in this location. The type of device is not yet determined, but numerous options are available to meet the regulations. Watersheds M and N will use a flow-through BMPs similar to watersheds E, F, and H. Multiple biofiltration cells will be located between the trees and light posts upstream of catchbasins. Watersheds O, P, Q, and R are located within streets adjacent to townhomes. These watersheds complicate water quality treatment with driveway entrances for each unit that result in small landscape pockets. Filterra units are proposed because of the small footprints required and their ability to be efficiently integrated into the streetscape landscaping. Watershed P can be treated using either a Filterra catchbasin or biofiltration planters. 4.2 Alternative 2 The streetscapes have been divided into watersheds according to street profiles and slopes with respect to the Alternative 2 development (refer to Error! Reference source not found.). Descriptions of each watershed are included in the Conceptual Hydrology Report, dated February The proposed watersheds for Alternative 2 are treated as follows. Basins A, B, C and a majority of D do not have provisions for water quality measures. These basins are composed of City of San Mateo right-of-way and are not part of the proposed project. South Delaware Street is to be narrowed (by the City of San Mateo) per the Rail Corridor plan, and water quality improvements should be made at that time. The portion of Basin D that will require onsite treatment is the area between blocks MU-2 and MU-3. A small mechanical device is proposed to treat the runoff prior to entry to the storm system in Concar Drive. Watersheds E and F are located on A Street where multiple biofiltration planters are proposed on both sides of the streets (see Appendix A). These planters will be integrated into the streetscape between the proposed trees and streetlights, and upstream of the overflow catchbasins. \\S-FNAS01\JOBSIF\131178\4 INTERNAL PROJECT DATA\4-05 REPORTS & NARRATIVES\STORMWATER\ALTERNATIVE 2 FEBRUARY 2009 STUDY\0003SPG- PRELIMINARY STORMWATER QUALITY MANAGEMENT STRATEGY_REV1 FEBRUARY 2009.DOC Page15 24 November 2007

21 Figure 11: Stormwater BMP Overview in Streetscapes and Open Spaces Alternative 2 Watershed G is located in B Street and will flow via multiple curb cuts or slotted drains to the proposed rain garden in OS-10. Overflows will be directed to a catch basin at the intersection of Charles Way and S. Delaware Street. Watershed H is also located in B Street and multiple flow-through biofiiltration planters are proposed between the trees in the landscape area. The planters will be upstream of a catch basin at the intersection with Charles Way. Watershed I consists predominantly of the central park and the surrounding C Street Loop where the majority of the sidewalks and streets will be permeable pavers. These permeable pavers will reduce total suspended solids and heavy metals, and reduce the volume of runoff. The rain garden along the north side of the park treats the remainder of the run-off. The streets and park will be graded to direct flows to the rain garden, with overflows being directed to a catch basin located near the intersection with B Street. Even with a reduction in the effective treatment area, this basin has excess capacity, and could be utilized by some of the surrounding buildings, depending on the project phasing. Watersheds J and L are located along the western property line shared with the Joint Powers Board. An emergency vehicle access easement and vehicular access to parcels are required at this boundary; therefore, there is limited space for biological treatment BMPs. One large, or several small, mechanical devices are currently proposed to treat the runoff flows in this location. The type of device is not yet determined, but numerous options are available to meet the regulations. \\S-FNAS01\JOBSIF\131178\4 INTERNAL PROJECT DATA\4-05 REPORTS & NARRATIVES\STORMWATER\ALTERNATIVE 2 FEBRUARY 2009 STUDY\0003SPG- PRELIMINARY STORMWATER QUALITY MANAGEMENT STRATEGY_REV1 FEBRUARY 2009.DOC Page16 24 November 2007

22 Watershed K is composed of the southern end of the central park and drains to the western property line shared with the Joint Powers Board. There are plans for some retail buildings in this area, and similar to watersheds J and L, an emergency vehicle access easement and vehicular access to parcels are required at this boundary; therefore, there is limited space for biological treatment BMPs. A small mechanical device is currently proposed to treat the runoff flows in this location; however, this area could be combined with watersheds J and L and a larger mechanical device could be used. The type of device is not yet determined, but numerous options are available to meet the regulations. Watersheds M and N are constrained because the alleys are designed for fire access with minimal landscaped pockets with insufficient room for flow through BMPs. A small mechanical device is currently proposed to treat the runoff flows in this location. The type of device is not yet determined, but numerous options are available to meet the regulations. Watersheds O, P, Q, and R will use flow-through BMPs similar to watersheds E, F, and H. Multiple biofiltration cells will be located between the trees and light posts upstream of catchbasins. Watersheds S and T are located within streets adjacent to townhomes. These watersheds complicate water quality treatment with driveway entrances for each unit that result in small landscape pockets. Filterra units are proposed because of the small footprints required and their ability to be efficiently integrated into the streetscape landscaping. \\S-FNAS01\JOBSIF\131178\4 INTERNAL PROJECT DATA\4-05 REPORTS & NARRATIVES\STORMWATER\ALTERNATIVE 2 FEBRUARY 2009 STUDY\0003SPG- PRELIMINARY STORMWATER QUALITY MANAGEMENT STRATEGY_REV1 FEBRUARY 2009.DOC Page17 24 November 2007

23 5 Conclusions The water quality treatment system proposed for each program alternative for Station Park Green is a comprehensive approach with BMPs located near the source of rainfall rather than at the end of the line. This approach to water quality treatment allows for the integration of biological systems into the urban landscape while meeting or exceeding regulations. Through this integrated design, stormwater runoff within the Station Park Green Development will be adequately treated to meet the requirement of the San Mateo Countywide C.3 Stormwater Technical Guidance Manual (March 2007), California Stormwater BMP Handbook: New Development and Redevelopment; California Stormwater Quality Association (January 2003), and the NPDES permit. \\S-FNAS01\JOBSIF\131178\4 INTERNAL PROJECT DATA\4-05 REPORTS & NARRATIVES\STORMWATER\ALTERNATIVE 2 FEBRUARY 2009 STUDY\0003SPG- PRELIMINARY STORMWATER QUALITY MANAGEMENT STRATEGY_REV1 FEBRUARY 2009.DOC Page18 24 November 2007

24 Appendix A Sizing Calculations

25 A1 Composite C-Value The Plan The following is the Composite C-value Calculation used to size the Stormwater BMPs (refer to section 3.1) Contributing Area (Area in Acres) 1,2 Grass/ Porous Watershed OS Pavers Paved Total Area Grass/ OS 3 Composite C*A Values Porous Pavers Paved Composite Composite C-value (Acres) (Acres) (Acres) (Acres) C*A A B C D E F G H I J K L M N O P Q R S Total 7.05 Total Areas correlate to watershed areas defined in Conceptual Hydrology Study 2- Development parcels excluded from calculations 3- Source of C-value from San Mateo County C3 guidebook (2007) 0003SPG- PRELIMINARY STORMWATER QUALITY MANAGEMENT STRATEGY_REV1 FEBRUARY 2009.DOC Page 1 Draft 1 24 November 2007

26 A2 Composite C-Value Alternative 2 The following is the Composite C-value Calculation used to size the Stormwater BMPs (refer section 3.1) Contributing Area (Area in Acres) 1,2 Grass/ Porous Watershed OS Pavers Paved Total Area Grass/ OS 3 Composite C*A Values Porous Pavers Paved Composite Composite C-value (Acres) (Acres) (Acres) (Acres) C*A A B C D E F G H I J K L M N O P Q R S T Total 6.28 Total Areas correlate to watershed areas defined in Conceptual Hydrology Study 2- Development parcels excluded from calculations 3- Source of C-value from San Mateo County C3 guidebook (2007) 0003SPG- PRELIMINARY STORMWATER QUALITY MANAGEMENT STRATEGY_REV1 FEBRUARY 2009.DOC Page 2 Draft 1 24 November 2007

27 A3 Biofiltration Planters Sizing The Plan Refer to section for sizing calculation methodology Water -shed FLOW CALCULATIONS Runoff Coefficien t 2 Effecitve Imperviou s Area 3 Rainfall Intensity 4 Design Flow 5 Soil Infiltratio n Rate 6 BIOFILTRATION PLANTER SIZING Area Require d 7 Width Length Numbe r Planter Area Provide d Area 1 C A i wq Q wq V bf A bf (ft) (ft) (ea) (acres) (sf) (in/hr) (cf/hr) (in/hr) (sf) (sf) E F H M N P Areas correlate to watershed areas defined in Conceptual Hydrology Study 2- Composite runoff coefficient correlates to Appendix A1 for watershed 3- Effective Impervious Area = A * C/ 0.80 (c-value for pervious surface) 4- Water Quality rainfall intensity calculated per San Mateo C3 and California Stormwater BMP Handbook (refer section 3.2) 5- Q wq = C* i wq * A 6- Infiltration Rate assumed for typical biofiltration soil media 7- A bf = Q / V bf (Abf must be at least 4% of drainage area according to San Mateo C.3 stormwater guide) 0003SPG- PRELIMINARY STORMWATER QUALITY MANAGEMENT STRATEGY_REV1 FEBRUARY 2009.DOC Page 3 Draft 1 24 November 2007

28 A4 Biofiltration Planters Sizing Alternative 2 Refer to section for sizing calculation methodology Water -shed FLOW CALCULATIONS Runoff Coefficien t 2 Effecitve Imperviou s Area 3 Rainfall Intensity 4 Design Flow 5 Soil Infiltratio n Rate 6 BIOFILTRATION PLANTER SIZING Area Require d 7 Width Length Numbe r Planter Area Provide d Area 1 C A i wq Q wq V bf A bf (ft) (ft) (ea) (acres) (sf) (in/hr) (cf/hr) (in/hr) (sf) (sf) E F H O P Q R Areas correlate to watershed areas defined in Conceptual Hydrology Study 2- Composite runoff coefficient correlates to Appendix A1 for watershed 3- Effective Impervious Area = A * C/ 0.80 (c-value for pervious surface) 4- Water Quality rainfall intensity calculated per San Mateo C3 and California Stormwater BMP Handbook (refer section 3.2) 5- Q wq = C* i wq * A 6- Infiltration Rate assumed for typical biofiltration soil media 7- A bf = Q / V bf (Abf must be at least 4% of drainage area according to San Mateo C.3 stormwater guide) 0003SPG- PRELIMINARY STORMWATER QUALITY MANAGEMENT STRATEGY_REV1 FEBRUARY 2009.DOC Page 4 Draft 1 24 November 2007

29 A5 Raingarden Sizing The Plan Refer to Section for sizing calculation methodology Watershed FLOW CALCULATIONS Runoff Coefficient 2 Effecitve Impervious Area 3 Rainfall Intensity 4 Design Flow 5 Soil Infiltration Rate 6 RAINGARDEN SIZING Area Required 7 Rain Garden Area 8 Effective Area 9 Area 1 C A i wq Q wq V bf A bf (acres) (sf) (in/hr) (cf/hr) (ft/hr) (sf) (sf) (sf) G I Areas correlate to watershed areas defined in Conceptual Hydrology Study 2- Composite runoff coefficient correlates to Appendix A1 for watershed 3- Effective Impervious Area = A * C/ 0.80 (c-value for pervious surface) 4- Water Quality rainfall intensity calculated per San Mateo C3 and California Stormwater BMP Handbook (refer section 3.2) 5- Q wq = C* i wq * A 6- Infiltration Rate assumed for typical biofiltration soil media 7- A bf = Q / V bf (Abf must be at least 4% of drainage area according to San Mateo C.3 stormwater guide) 8- Raingarden area is approximated per Specific Plan 9- Effecitve Area is the area of entire rain garden suitable for filtration, 60% devoted to larger plantings 0003SPG- PRELIMINARY STORMWATER QUALITY MANAGEMENT STRATEGY_REV1 FEBRUARY 2009.DOC Page 5 Draft 1 24 November 2007

30 A6 Raingarden Sizing Alternative 2 Refer to Section for sizing calculation methodology Watershed FLOW CALCULATIONS Effecitve Impervious Area 3 Soil Infiltration Rate 6 RAINGARDEN SIZING Runoff Rainfall Design Area Coefficient 2 Intensity 4 Flow 5 Required 7 Rain Garden Effective C A i wq Q wq V bf A bf Area 8 Area 9 Area 1 (acres) (sf) (in/hr) (cf/hr) (ft/hr) (sf) (sf) (sf) G I Areas correlate to watershed areas defined in Conceptual Hydrology Study 2- Composite runoff coefficient correlates to Appendix A1 for watershed 3- Effective Impervious Area = A * C/ 0.80 (c-value for pervious surface) 4- Water Quality rainfall intensity calculated per San Mateo C3 and California Stormwater BMP Handbook (refer section 3.2) 5- Q wq = C* i wq * A 6- Infiltration Rate assumed for typical biofiltration soil media 7- A bf = Q / V bf (Abf must be at least 4% of drainage area according to San Mateo C.3 stormwater guide) 8- Raingarden area is approximated per Specific Plan 9- Effecitve Area is the area of entire rain garden suitable for filtration, 60% devoted to larger plantings 0003SPG- PRELIMINARY STORMWATER QUALITY MANAGEMENT STRATEGY_REV1 FEBRUARY 2009.DOC Page 6 Draft 1 24 November 2007

31 A7 Filterra Unit Sizing The Plan Refer to Section for sizing calculation methodology Watershed FLOW CALCULATIONS FILTERRA SIZING Runoff Coefficient 2 Effecitve Impervious Area 3 Rainfall Intensity 4 Design Flow 5 Soil Infiltration Rate 6 Area Required 7 Width Length number Filterra C A i wq Q wq V bf A bf (ft) (ft) (ea) Area Area 1 (acres) (sf) (in/hr) (cf/hr) (ft/hr) (sf) (sf) O Q R Areas correlate to watershed areas defined in Conceptual Hydrology Study 2- Composite runoff coefficient correlates to Appendix A1 for watershed 3- Effective Impervious Area = A * C/ 0.80 (c-value for pervious surface) 4- Water Quality rainfall intensity calculated per San Mateo C3 and California Stormwater BMP Handbook (refer section 3.2) 5- Q wq = C* i wq * A 6- Infiltration Rate assumed for typical biofiltration soil media 7- A bf = Q / V bf (Abf minumum sized according to a 4.5' x 6' unit treating 0.35 Acres) 0003SPG- PRELIMINARY STORMWATER QUALITY MANAGEMENT STRATEGY_REV1 FEBRUARY 2009.DOC Page 7 Draft 1 24 November 2007

32 A8 Filterra Unit Sizing Alternative 2 Refer to Section for sizing calculation methodology FLOW CALCULATIONS Runoff Coefficient 2 Effecitve Impervious Area 3 Rainfall Intensity 4 Design Flow 5 Soil Infiltration Rate 6 FILTERRA SIZING Area Required 7 Width Length number Filterra Watershed Area 1 C A i wq Q wq V bf A bf (ft) (ft) (ea) Area (acres) (sf) (in/hr) (cf/hr) (ft/hr) (sf) (sf) S T Areas correlate to watershed areas defined in Conceptual Hydrology Study 2- Composite runoff coefficient correlates to Appendix A1 for watershed 3- Effective Impervious Area = A * C/ 0.80 (c-value for pervious surface) 4- Water Quality rainfall intensity calculated per San Mateo C3 and California Stormwater BMP Handbook (refer section 3.2) 5- Q wq = C* i wq * A 6- Infiltration Rate assumed for typical biofiltration soil media 7- A bf = Q / V bf (Abf minumum sized according to a 4.5' x 6' unit treating 0.35 Acres) 0003SPG- PRELIMINARY STORMWATER QUALITY MANAGEMENT STRATEGY_REV1 FEBRUARY 2009.DOC Page 8 Draft 1 24 November 2007

33 A9 Mechanical Devices Flow Calculations The Plan The calculated flows calculated below will inform the selection of future mechanical devices FLOW CALCULATIONS Runoff Coefficient 2 Effecitve Impervious Area 3 Rainfall Intensity 4 Design Flow 5 Area 1 C A i wq Q wq Water-shed (acres) (sf) (in/hr) (cf/hr) J K L Areas correlate to watershed areas defined in Conceptual Hydrology Study 2- Composite runoff coefficient correlates to Appendix A1 for watershed 3- Effective Impervious Area = A * C/ 0.80 (c-value for pervious surface) 4- Water Quality rainfall intensity calculated per San Mateo C3 and California Stormwater BMP Handbook (refer section 3.2) 5- Q wq = C* i wq * A 0003SPG- PRELIMINARY STORMWATER QUALITY MANAGEMENT STRATEGY_REV1 FEBRUARY 2009.DOC Page 9 Draft 1 24 November 2007

34 A10 Mechanical Devices Flow Calculations Alternative 2 The calculated flows calculated below will inform the selection of future mechanical devices FLOW CALCULATIONS Runoff Coefficient 2 Effecitve Impervious Area 3 Rainfall Intensity 4 Design Flow 5 Area 1 C A i wq Q wq Water-shed (acres) (sf) (in/hr) (cf/hr) J K L M N Areas correlate to watershed areas defined in Conceptual Hydrology Study 2- Composite runoff coefficient correlates to Appendix A1 for watershed 3- Effective Impervious Area = A * C/ 0.80 (c-value for pervious surface) 4- Water Quality rainfall intensity calculated per San Mateo C3 and California Stormwater BMP Handbook (refer section 3.2) 5- Q wq = C* i wq * A 0003SPG- PRELIMINARY STORMWATER QUALITY MANAGEMENT STRATEGY_REV1 FEBRUARY 2009.DOC Page 10 Draft 1 24 November 2007

35 Appendix B Design Curves

36 B1 IDF Curve 85 th Percentile Water Quality Intensity (i wq )= 2 * rainfall intensity (i wq )= 0.20 in/hr 0003SPG- PRELIMINARY STORMWATER QUALITY MANAGEMENT STRATEGY_REV1 FEBRUARY 2009.DOC Page A12 Draft 1 24 November 2007

37 B2 Runoff Capture Curve 0003SPG- PRELIMINARY STORMWATER QUALITY MANAGEMENT STRATEGY_REV1 FEBRUARY 2009.DOC Page A13 Draft 1 24 November 2007