HomeMy WebLinkAboutAppendix 4.09-1, Storm Drainage Technical StudyAppendix 4.9-1:
Storm Drainage Technical Study
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TECHNICAL MEMORANDUM
Date: April 20, 2021
Subject: Storm Drainage Technical Study
Southline Development Project
The following summarizes the storm drain analysis performed by BKF Engineers at the planning level for
the Southline Development Project. This has been prepared for review by the Cities of South San
Francisco and San Bruno, and for use in the environmental documents for the Southline Development
Project.
Regulatory Setting
South San Francisco is part of the Colma Creek watershed within the County of San Mateo’s Colma
Creek Flood Control Zone. The City of South San Francisco owns and maintains storm drainage
infrastructure within the public rights-of-way, which then discharge to County flood control facilities.
The State of California regulates water quality in the region through the San Francisco Bay Regional
Water Quality Control Board (RWQCB). The City of South San Francisco is a permittee of the Municipal
Regional NPDES Permit (MRP) administered by the RWQCB. As a permittee, the City is responsible for
requiring sections C.10 (trash capture) and C.3 (stormwater treatment) to be met by all qualifying
development projects within the City.
Existing Conditions
The Southline Project Site and off-site improvements within the City of South San Francisco are located
within the Southern Region Watershed, Subwatershed B, Area 45, as defined by the 2016 South San
Francisco Storm Drain Master Plan1. The off-site improvements within the City of San Bruno’s
jurisdiction are within the Lower Level Region Watershed, Subwatershed A. Onsite storm drainage
systems convey storm runoff from each parcel within the existing Project Site area to the adjacent
public roadways. The south portion of the Site area drains to a 42” RCP located in Tanforan Avenue. The
northwest portion of the Site drains to a 4’x4’ box culvert located in South Maple Avenue. Parcels
located on the east appear to drain north via the roadway gutter in both Dollar Avenue and South
Linden Avenue, as there is no public underground storm drain infrastructure in these two roadways. The
gutter flow in S. Linden drains to a 7’x5.5’ box culvert that crosses Linden Avenue north of the site.
There is one parcel on Dollar Avenue assumed to drain south to Tanforan. Refer to Exhibit A for the
existing site drainage map, and Table A for the 10-year and 25-year peak runoff flows to each drainage
basin.
The 2016 Storm Drain Master Plan identified no recommended facility improvements for infrastructure
serving the Project Site within Subwatershed B. However, the Master Plan did recommend
improvements within Tanforan Avenue to infrastructure serving the adjacent Lower Region watershed,
Subwatershed A. The recommended improvements call for the future installation of a parallel 66” pipe
in addition to the existing 42” RCP located within Tanforan Avenue. Additional downstream segments
are also identified for capacity improvements. These recommended improvements are due to
anticipated demands from General Plan build-out within the Lower Region Watershed, and are not the
result of the planned future demands from development of the Southline project, which is within the
1 2016 SSF Storm Drain Master Plan, Figures 1-3, 1-4, and Exhibit 2.
Southern Region Watershed. The project will coordinate with the City of South San Francisco to
preserve existing space within Tanforan Avenue for the possible installation of the 66” pipe as part of a
future capital improvement project.
Proposed Improvements
Onsite storm runoff will be captured and treated onsite using LID methods (landscaped bioretention
and biofiltration planters) per current NPDES C3 requirements. Treated runoff will discharge to the
public storm drain system through new onsite underground storm drain piping. The project will be
served by the existing public storm drain infrastructure within Tanforan Avenue and Maple Avenue, and
does not anticipate construction of new public storm drainage infrastructure within those roadways or
downstream. The project will be constructing a new public roadway (Southline Avenue) through the
site. This roadway will contain new public storm drain infrastructure, as well as public bioretention
planters, to capture, treat, and convey storm runoff from the new public roadway and adjacent sidewalk
areas. This new storm drain infrastructure will be connected to the existing infrastructure in Tanforan
Avenue through new public storm drain piping in Dollar Avenue. Proposed public storm drainage
improvements or public storm water treatment areas within the City of San Bruno will also be
connected to the existing public storm drainage system. All proposed off-site improvements will
continue to drain to the existing storm drain infrastructure within the same existing subwatersheds.
The Project proposes to maintain or reduce the pre-project flows to both the north and south drainage
basins. The Project also proposes to reduce the overall peak runoff from the site by 12%from the
existing condition by increasing the pervious area (landscaping) for the site. This balance of flow shed
and reduction of peak run-off will apply to both Phase 1 of the project as well as the complete specific
plan development in future phases. Refer to Exhibits B1 and B2 for the proposed site drainage map, and
Tables B1 and B2 for the peak flows and reductions to each drainage basin.
TABLE A: EXISTING PEAK RUNOFFSOUTHLINE PROJECT - STORM DRAIN STUDYEXISTING STORM DRAIN FLOW TABLE:ACI10Q10Q10SITE WATERSHEDCONNECTION LOCATIONTOTAL SITE AREA (AC)IMPERVIOUS AREA (AC)PERVIOUS AREA (AC)WEIGHTED RUNOFF COEFFICIENTTIME OF CONCENTRATION (MIN)RAIN INTENSITY (10-YEAR STORM) (IN/HR)RUNOFF (CFS)TOTAL RUNOFF (CFS)A11.9411.67 0.27 0.94101.9621.89B0.470.38 0.09 0.82 10 1.960.75C8.628.24 0.38 0.92 10 1.9615.57D5.514.56 0.94 0.84 10 1.969.05TOTAL:26.5324.85 1.68 47.2747.27ACI25Q25Q25SITE WATERSHEDCONNECTION LOCATIONTOTAL SITE AREA (AC)IMPERVIOUS AREA (AC)PERVIOUS AREA (AC)WEIGHTED RUNOFF COEFFICIENTTIME OF CONCENTRATION (MIN)RAIN INTENSITY (25-YEAR STORM) (IN/HR)RUNOFF (CFS)TOTAL RUNOFF (CFS)A11.9411.67 0.27 0.94102.3426.13B0.470.38 0.09 0.82 10 2.340.90C8.628.24 0.38 0.92 10 2.3418.59D5.514.56 0.94 0.84 10 2.3410.81TOTAL:26.5324.85 1.68 56.4356.43NOTES:1Runoff coefficients: C=0.95 for Impervious; C=0.3 for Pervious (landscape)2The rain intensity value is based on NOAA Atlas 14, Vol 6, Version 2.3Weighted Runoff Coefficient = [ (Impervious Area/Total Site Area) * 0.95 ] + [ (Pervious Area/Total Site Area) * 0.3 ]4Runoff, Q, is calculated using the Rational Method, Q=CIA.22.6424.62EXISTING STORM DRAIN TABLE 10-YEARSOUTHNORTHEXISTING STORM DRAIN TABLE 25-YEARSOUTH27.03NORTH29.401/7/2021BKF Engineers#20180624
TABLE B1: PROPOSED PEAK RUNOFF - SPECIFIC PLANSOUTHLINE PROJECT - STORM DRAIN STUDYFULL SPECIFIC PLAN SITE (TECH OFFICE SCENARIO)ACI10Q10PROPOSED WATERSHEDCONNECTION LOCATIONTOTAL SITE AREA (AC)IMPERVIOUS AREA (AC)PERVIOUS AREA (AC)WEIGHTED RUNOFF COEFFICIENTTc (MIN)RAINFALL 10-YR (IN/HR)PROPOSED Q10 (CFS)EXISTING Q10 (CFS)DELTA Q10 (CFS)A SOUTH13.3810.622.760.82101.9621.4022.64-1.24B NORTH13.169.893.270.79101.9620.3324.62-4.29TOTAL:26.5320.516.0241.7347.27-5.53ACI25Q25PROPOSED WATERSHEDCONNECTION LOCATIONTOTAL SITE AREA (AC)IMPERVIOUS AREA (AC)PERVIOUS AREA (AC)WEIGHTED RUNOFF COEFFICIENTTc (MIN)RAINFALL 25-YR (IN/HR)PROPOSED Q25 (CFS)EXISTING Q25 (CFS)DELTA Q25 (CFS)A SOUTH13.3810.622.760.82102.3425.5527.03-1.48B NORTH13.169.893.270.79102.3424.2729.40-5.13TOTAL:26.5320.516.0249.8356.43-6.60NOTES:1Runoff coefficients: C=0.95 for Impervious; C=0.3 for Pervious (landscape)2The rain intensity value is based on NOAA Atlas 14, Vol 6, Version 2.3Weighted Runoff Coefficient = [ (Impervious Area/Total Site Area) * 0.95 ] + [ (Pervious Area/Total Site Area) * 0.3 ]4Runoff, Q, is calculated using the Rational Method, Q=CIA.5PROPOSED STORM DRAIN TABLE (10-YEAR STORM)PROPOSED STORM DRAIN TABLE (25-YEAR STORM)Proposed site areas are based on the Tech Office scenario, which has less landscaped / pervious areas than the proposed Life Science scenario, and is therefore the worst-case scenario.4/20/2021BKF Engineers#20180624
TABLE B2: PROPOSED PEAK RUNOFF - PHASE 1SOUTHLINE PROJECT - STORM DRAIN STUDYPHASE 1 DEVELOPMENTACI10Q10PROPOSED WATERSHEDCONNECTION LOCATIONTOTAL SITE AREA (AC)IMPERVIOUS AREA (AC)PERVIOUS AREA (AC)WEIGHTED RUNOFF COEFFICIENTTc (MIN)RAINFALL 10-YR (IN/HR)PROPOSED Q10 (CFS)EXISTING Q10 (CFS)DELTA Q10 (CFS)A SOUTH14.0011.022.980.81101.9622.2722.64-0.37B NORTH12.5310.791.740.86101.9621.1124.62-3.51TOTAL:26.5321.814.7343.3847.27-3.88ACI25Q25PROPOSED WATERSHEDCONNECTION LOCATIONTOTAL SITE AREA (AC)IMPERVIOUS AREA (AC)PERVIOUS AREA (AC)WEIGHTED RUNOFF COEFFICIENTTc (MIN)RAINFALL 25-YR (IN/HR)PROPOSED Q25 (CFS)EXISTING Q25 (CFS)DELTA Q25 (CFS)A SOUTH14.0011.022.980.81102.3426.5927.03-0.44B NORTH12.5310.791.740.86102.3425.2029.40-4.20TOTAL:26.5321.814.7351.7956.43-4.64NOTES:1Runoff coefficients: C=0.95 for Impervious; C=0.3 for Pervious (landscape)2The rain intensity value is based on NOAA Atlas 14, Vol 6, Version 2.3Weighted Runoff Coefficient = [ (Impervious Area/Total Site Area) * 0.95 ] + [ (Pervious Area/Total Site Area) * 0.3 ]4Runoff, Q, is calculated using the Rational Method, Q=CIA.5PROPOSED STORM DRAIN TABLE (10-YEAR STORM)PROPOSED STORM DRAIN TABLE (25-YEAR STORM)This runoff analysis is based on the worst-case scenario whereby all existing impervious area remains in place within the future phase areas of the site after construction of Phase 1.4/20/2021BKF Engineers#20180624
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