07/08/2020 P&Z Packet - Laserfiche WebLink

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of the site drains to an existing private ditch/wetland area to the north of the site. A small portion at the south of the site drains through an existing culvert along Birch St (CSAH 34). The western portion of the site drains to the northwest to an existing private ditch. The surface water will flow through three new ponds. Pond 200 located in the middle of the site will pick up surface water from the south portion of the site and direct it towards Pond 100. Pond 100 will pick up surface water from the majority of the site and discharge to the west to an existing private ditch. Pond 400 will receive the remaining surface water and discharge to an existing ditch to the north. After the project is completed, the site will include approximately 8.8 acres of new impervious surfaces including the streets, sidewalks, paths, parking lots, and rooftops. Due to poor soils in the south portion of the site and a high groundwater table in the north portion of the site, infiltration is not feasible. The applicant used a HydroCAD model based on the Atlas 14, 24-hour storms to design the storm sewer conveyance system and ponds. The site is designed to reduce site runoff rates for the 2-, 10-, and 100-year rainfall events." Per the RCWD CAPROC dated April 8, 2020: Soil conditions vary over the sloping site, with HSG D sandy lean clays (CL) at the higher elevations, and HSG A/B poorly graded sands and sands with silt (SP/SP-SM) with sandy lean clays below. The HSG D soils in addition to the high water table below the HSG AB soils make infiltration infeasible for the site. Thus, stormwater ponds are acceptable to meet the water quality requirement. The pond sizing, and outlets and overflows are consistent with design criteria. 100% of the project area either flows to the ponds or meets design criteria. Additional TSS removal is not required. The applicant has met all the Water Quality requirements and the design criteria of RCWD Rule C: Stormwater Management Plans. Discharge Rate Comparison 2-year (cfs) 10-year (cfs) 100-year (cfs) Existing Proposed Existing Proposed Existing Proposed North to Private Ditch 3.5 2.3 5.1 3.9 6.9 5.7 West to Private Ditch 11.6 9-.2 6.8 34.1 2&7 25.7 82.8 74--7 73.9 South through Culvert under Birch Street 4.4 3.5 8.0 6.5 14.6 11.9 Totals* 19.2 1 421-.5-9.8 46.7 3-- 32.2 103.3 87.0 *Difference due to time of concentration Post -development stormwater discharge rates are less than the pre -development rates. N.