BRISBANE BAYLANDS INFRASTRUCTURE PLAN FEBRUARY 2011 APPENDIX O DRAFT
PRELIMINARY STORM DRAIN CALCULATIONS ASSOCIATED WITH BRISBANE BAYLANDS REDEVELOPMENT BRISBANE, CALIFORNIA Prepared by BKF Engineers BKF Job No.: 20070090-25 November 2010 Page 1 of 8
INTRODUCTION Universal Paragon Corporation is proposing improvements to approximately 550-acres of land to create a 12 million square foot world class mixed-use development. Located in Brisbane, California, the project site is bounded by the Sunnydale Avenue and Sunset Scavengers to the north, Brisbane Lagoon to the south, Bayshore Boulevard to the west and US Highway 101 to the east. The project area west of the railroad tracks was formerly a railyard operated by Southern Pacific Railroad while the area east of the railroad tracks is a former landfill currently being used for soil recycling operations. This analysis documents that the proposed on-site infrastructure is capable of supporting the on-site 25-year design storm event without ponding, that the proposed Central Drainage Channel (CDC) is capable of conveying the 100-year peak storm event water elevation, and that off-site flooding on Bayshore Boulevard for the 100-year storm event is significantly reduced. STORM DRAIN MODEL PARAMETERS 1. HYDROLOGIC DESIGN CRITERIA The Rational Method Equation is used to compute the peak rate of runoff from the site. The proposed model computes the peak rate of runoff for the 25-year storm with the 100-year storm event Hydraulic Grade Line (HGL) water elevation in the channel. The Rational Method Equation is: Q = CIA Q = peak discharge (cfs) C = runoff coefficient (dimensionless) I = rainfall intensity for an event whose duration is equal to the time of concentration for the watershed (inches/hour) A = drainage area (acres). 2. COMPUTER PROGRAM As described in the November 2003 City of Brisbane Storm Drainage Master Plan, (SDMP) by RBF Consulting, the computer program XP-SWMM is used to evaluate the performance of the proposed system. For the purposes of storm drainage analysis, the program is divided into a runoff and a hydraulics module. The runoff module is used to calculate the runoff hydrographs at system nodes, while the hydraulics module defines the physical infrastructure of the system and routes the runoff module through the system. Per the SDMP, tidal conditions have been included in developing the storm drain model. BKF has used the storm drain model and runoff module assumptions developed by RBF as a baseline for this model. 3. DRAINAGE AREAS Drainage areas were developed based on the conceptual grading plan prepared by BKF Engineers, dated November 2010. For reference, the conceptual grading plan is included as Exhibit 4 in this analysis. Page 2 of 8
4. PRECIPITATION INTENSITY DATA As described in the SDMP, precipitation distributions and design rainfall intensities were developed in accordance with the Return Period-Duration Specific Regional Equation described in the Santa Clara Valley Water District Hydrology Procedures handbook. Rainfall distributions were taken over a 24-hour period and developed for 2-yr, 10-yr, 25-yr, and 100-yr storm events. 5. RUNOFF COEFFICIENT The runoff coefficient is a measure of the amount of rainfall that runs off from the site at the storm peak. The Soil Conservation Service (SCS) curve number method is used to determine the portion of rainfall that develops in the storm water runoff. The SCS method combines land use classifications and soil types to develop a curve number (CN). Since the project site infiltrates at a slow rate due to the presence of bay mud deposits, on-site soils are classified as either Type C or D per the SDMP. A CN of 95-98 is used to classify impervious surfaces, whereas pervious surfaces are defined by a CN ranging from 60-84. 6. TIME OF CONCENTRATION As described in the SDMP, the SCS method is used to develop values for the time of concentration. The equation uses the longest runoff flow path, the slope of the watershed and the SCS curve number to develop the values for time of concentration. 7. TIDAL CONDITIONS Per the SDMP, recent studies has shown that high tidal levels can be expected during large storm events due to the presence of powerful off-shore winds and low atmospheric pressure. BKF has maintained the storm drain model tidal conditions developed by RBF, thus the model assumes a maximum stage tidal cycle elevation of 6.0-feet (NGVD 29). 8. HEADLOSS COEFFICIENT FOR JUNCTIONS Manhole losses (hl) are computed as hl=k V2/2g, where K is the standard head loss coefficient, V is the velocity leaving the junction structure and g is the gravitational constant. The head loss coefficient is based on the geometry of the junction structure and connecting pipes. The flow depths for the 25-year and 100- year flow rates are computed using the hydraulic grade line slope instead of the pipe slope. The hydraulic grade line slope is the slope between the pipe inlet water elevation (pipe invert elevation plus depth of flow) and the pipe outlet water elevation (pipe invert elevation plus downstream pipe flow depth plus manhole losses). 9. FREEBOARD The project freeboard criterion is based on the guidelines defined by the SDMP. These guidelines establish that projects design drainage facilities to provide a minimum of one-foot from the calculated 100-year peak stage where ponding may Page 3 of 8
occur. Although the on-site portions of the project contain the 25-year design flow entirely within the proposed drainage channel area and piping system, the 100-year storm event overland release is provided through the public right-of-way with a minimum of 1-foot of freeboard to the proposed building finished floors. Throughout the majority of the site of the site, the provided freeboard exceeds the required minimum. EXISTING STORM DRAIN SYSTEM MODEL The results of the existing system model prepared by RBF as part of the SDMP are included in Appendix A with the Existing system model pipe and node layout is documented on Exhibit 1. Existing off-site conditions and flows that discharge through the site and used in the RBF model have been included in the proposed storm drain system model. PROPOSED STORM WATER DESIGN CRITERIA The proposed Baylands storm drainage collection system will be designed in compliance with the City of Brisbane requirements, the SDMP and supplemental project specific storm drainage reports. Existing on-site storm drainage pipes, structures, and pumps will be removed and/or replaced to allow for the installation of the proposed landfill clay cap and other measures required to complete landfill closure. Approximately 47-acres of the existing Beatty Avenue watershed have been re-routed to the Bayshore Drainage Area, resulting in a total Bayshore catchment area of 278-acres. A combination of pipes, culverts, and conveyance and catchment structures will direct the storm water runoff from the on-site streets and buildings, and the upper reach areas of the Bayshore Drainage Area to 10 outfalls discharging to the CDC. Storm Drain Pipe System Proposed storm drain pipes and structures are sized based on the calculated flows using Manning s equation. The pipes will be connected using fusion-welded High-Density Polyethylene (HDPE) pipe. Since refuse and bay mud soil conditions are susceptible to significant settlement, fusion-welded joints are designed to be flexible and are capable of withstanding tolerable levels of differential settlement. In combination with the rainfall intensity data for the project area, the rational equation is used to develop runoff volumes and flows. Storm Drain pipes and structures are then sized based on the calculated flows using Manning s equation. The preliminary line sizing and layout, shown on Exhibit 2, is based on the following criteria: Pipe Material: HDPE and Concrete Box Culverts Manning s Number: 0.013 Collection System Design Storm: 25-year in pipe system and 100-year overland release thru streets Channel Design Storm: 100-year storm HGL Tidal Cycle: Mean High Higher Water Level of 6.0 feet (SDMP) Page 4 of 8
Minimum Pipe Slope 0.20% after settlement 3-feet minimum cover over the top of the pipes and culverts where feasible Flexible connections at storm drain structures and buildings Minimizing pipe depths to prevent trenching into the proposed Clay Cap RESULTS On-site storm drainage collection facilities are sized to convey the peak flow rate from a 25-year storm event entirely within the piping system. The 25-year peak storm hydraulic grade line for the on-site piping system is below the rim elevations of the storm drain structures. Since overland release is through the public street rights-of-way, minor surcharging of the on-site infrastructure is acceptable and anticipated during the 100-year peak storm event. To accommodate the 100-year peak storm event, building finished floors will be set to provide a minimum of 1-foot of freeboard above the 100-year storm event HGL. In addition, the proposed central open drainage channel is sized to handle the 100-year storm event HGL for both the upper and lower reach tributary areas with a minimum of 3-feet of freeboard under the current tidal conditions. The 100-year design storm event water elevations are 6.0-feet at the Highway 101 outfall, 8.4-feet at the downstream side of the railroad culvert, and 8.8-feet at the beginning of the drainage channel near the Roundhouse. As recommended by the SDMP, the 100-year flooding depth on Bayshore Boulevard has been reduced by up to 1.6-feet. With the addition of approximately 267,300 cubic feet of storage within the CDC, the duration of 100-year peak storm event flooding at Bayshore has been reduced from 173 minutes to 82 minutes. The model results for the proposed system are included in Appendix B and storm drain model layout is shown on Exhibit 3. CONCLUSIONS As a result of the modeled preliminary improvements, the proposed infrastructure and CDC is capable of supporting the on-site site 25-year design storm drain runoff without ponding and reduces off-site flooding on Bayshore Boulevard for the 100-year storm event. Infrastructure sizing and layout will be finalized during the Civil Improvement Plan process. Page 5 of 8
EXHIBITS Page 6 of 8
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APPENDIX A Page 7 of 8
APPENDIX B Page 8 of 8