2002-12-09 CC Packet - Laserfiche WebLink

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DISCHARGE FLOWRATES To meet the watershed district's discharge flowrate requirements the project will route the proposed development into Ponds I (IP) and 2 (2P). Pond I discharge will flow into Pond 2 and then Pond 2 will discharge to Peltier Ulke. The proposed development must not exceed the pre-developed I-year and I DO-year rainfall events. The flows for Ponds I and 2 begin at the pond's NWL line. The proposed development includes area not draining to either Pond I or 2. That area comprises of backyards and existing undeveloped space. The table below summarizes both pre-developed and developed conditions. . Table 2 - Discharge Rates for 1 year and 100 year Storms Storm Event Item Pre--developed Proposed (pond 2 + area not draining to ponds) I-vrflow(cfs) 0.93 0.88 lOO-vr flow (cfs) 28.0 19.2 The developed condition does not exceed the existing condition for both the I-year and 100- year events. For reference, please see Appendix B and C. INFILTRATION AND PERVIOUS AREA CALCULATION The required infiltration as determined by the worksheet is 0.6 acre-feet. The area provided on site for infiltration is 0.6 acre-feet with a surface area of 0.4 acres (See Grading Plan). This is based on an impervious area (on-site) of2.48 acres and a SCS Soil Type "B" soils. See Attachment D for reference. The pervious area was determined by taking the total on-site area and subtracting out the areas that eannot effectively hold runoff for extended periods of time. The types of area and the amounts are listed in the table below. Table 3 - Pervious Area Calculation Total Site Area 15.44 acres Impervious Area - 2.48 acres (includes bldg pads, streets and driveways) Wetland Area - 4.56 acres Pond Area (surface area at NWL) - 0.53 acres Effective Pervious Area 7.87 acres