Stormwater Report 1

EXISTING CONDITIONS The existing site has been modeled as one drainage area totaling 4.68 acres. (See Fig.1 Existing 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. The drainage area includes all areas of disturbance, as well as any other areas that drain to the point of analysis. 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 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 13.6 and 12.7 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 2 – Curve Number (Existing 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 2 below summarizes the peak flows during each storm event. (See attached Hydraflow Output for Existing Conditions Hydrographs, CN Values, and Time of Concentration Calculations.) Table 3 – Existing Peak Runoff Rates Current Adjusted Rainfall Future Projected Rainfall Existing Site Existing Site Existing Site Existing Site Storm Event Peak Runoff Runoff Volume Peak Runoff Runoff Volume 2 Year 9.339 cfs 44,031 cf 11.28 cfs 53,349 cf 10 Year 15.35 cfs 73,038 cf 18.49 cfs 88,329 cf 100 Year 27.41 cfs 132,192 cf 34.83 cfs 169,002 cf 5