Stormwater Report 1

PROPOSED CONDITIONS The proposed site has been modeled as one drainage area totaling 4.68 acres. (See Fig.2 Proposed Drainage Area Map.) The point of analysis for this drainage area is the Type Double “B” Inlet at the rear of the site, which connects to the municipal storm sewer system within Deerfield Drive. DRAINAGE AREA 1 (DA-1) Drainage Area 1 consists of 4.68 acres of both pervious and impervious surfaces. The impervious area, 3.32 acres, consists of the existing and proposed parking lot and associated improvements. The pervious area, 0.27 acres, is comprised of landscaped and lawn areas that are in good condition. Two times of concentration have been calculated for this drainage area, one based on the current adjusted rainfall total and one for the future projected rainfall total. The times of concentration are 12.6 and 11.7 minutes, respectively. CURVE NUMBERS The CN values used have been taken from the TR-55 Manual “Urban Hydrology for Small Watersheds,” Table 2-2a and Table 2-2c, based on land cover and hydrologic soil groups. Table 4 – Curve Number (Proposed Conditions) Description Hydrologic Group Curve Number Landscaped Islands D 80 (Open Space-Good Conditions) Impervious D 98 PEAK FLOWS Using the information described above, the peak flows during the 2-, 10-, and 100-year storm events where calculated. Table 4 below summarizes the peak flows during each storm event. (See attached Hydraflow Output for Proposed Conditions Hydrographs, CN Values, and Time of Concentration Calculations.) As shown in this table, the proposed runoff rates and volumes are reduced from the existing to proposed conditions. Since this regulation requires that we show reductions at each time interval, we have included Table 7 in the appendix of this report which shows the minute-by-minute flows leaving the site for each storm event and compares existing and proposed conditions. Table 5 – Overall Peak Runoff Rates – Current Adjusted Rainfall Existing Site Proposed Site Proposed Site Existing Site Storm Event Peak Runoff Runoff Volume Peak Runoff Runoff Volume 2 Year 9.339 cfs 44,031 cf 9.179 cfs 43,147 cf 10 Year 15.35 cfs 73,038 cf 15.19 cfs 71,995 cf 100 Year 27.41 cfs 132,192 cf 27.27 cfs 130,994 cf 6