2016-04-26_PWETC_AgendaPacket - Laserfiche WebLink

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a) Find the ratio of existing runoff volumes from the parcel (Vp,,.ce, ) to the total subwatershed (V,,,,,,identified by the City): % contribution= Vp,,.ce,l Vtotat x 100 b) Find the proportionate share of the problem (identified by the City). Vreduction= % contribution X Vprobdem c) Derive the % decrease in existing site runoff required on the development site to mitigate the proportionate share by the following formula: %existing runoff volume reduction (Vparced - Vreduction)l Vparced X 100 d) Apply the above reduction to the 1-year, 2-year, 10-year and 100-year existing site runoff volumes. The proposed design runoff volumes cannot exceed these reduced runoff volumes. 4) Design Computations. a) Hydrologic Data Format: All hydrologic data shall be completed using NRCS methodology; i.e. HydroCAD or TR20/TR55, XP-SWMM or a comparable, City approved method. Hydraulic calculations will be accepted in the rational method format or in commonly used software packages such as FHWA HY-8, Eagle Point or XP-SWMM or a compatible, City approved method. These computations shall be submitted to the City upon request. b) Rainfalls: Rainfall amounts for hydrologic analysis shall be based on the precipitation frequency estimates of NOAA Atlas 14 for the 24-hour return period from 1 to 100 Years. City of Roseville analyses shall use the values in the following table. Rainfall Frequency Rainfall(Inches) 1-Year 24-Hour 2.5 2-Year 24-Hour 2.8 10-Year 24-Hour 4.2 100-Year 24-Hour 7.4 c) Infiltration-Prohibitive Sites: For projects not meeting the infiltration/volume control requirement as stated in Section 2(a), design engineers and developers shall determine the pollutant removal efficiency of the best management practices (BMPs) incorporated into the site plan using the available industry standard models, including P8 (using a standard NURP particle size distribution for the analysis), PondNET or a comparable model approved by the City. d) Wet-Detention Pond Storm Water Treatment: As an alternative to preparing a site-specific model, the development may provide a treatment volume (dead storage) of not less than two and one-half (2.5) inches multiplied by the runoff coefficient calculated over the contributing drainage area to the pond. For example, a one (1) acre impervious site with a runoff coefficient of 0.90 that drains to a common treatment pond would be required to provide a dead storage volume of 0.19 acre-feet or eight thousand two hundred (8,200) cubic feet. The Natural Resources Conservation Service Method may also be used upon City approval. e) Volume Reduction Calculation: The volume reduction (in cubic feet) provided by surface infiltration practices shall be computed using the following: Page 3 of 8 12/11/14