03/28/2001 Env Bd Packet - Laserfiche WebLink

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POLAR I S *G R O U Pe 8200 HUMBOLT AVENUE SOUTH . SUITE 902. BLOOMINGTON. MN 55431 . 952.881.0878 . FAX: 952.881.0023 March 13, 2001 Mr. William A. T. McCully, P.E. Glenn Rehbein Companies 8651 Naples St. NE Blaine, MN 55449 Re: Spirit Hills Development, Lino Lakes, MN Dear William: Thanks for this opportunity to work with you. This letter presents Polaris Group's analysis of the hydrology of the proposed Spirit Hills Development. The analysis is based on our discussions with you, the grading plans we received from you. on March 8th, our discussions with the staff of Rice Creek Watershed District and a site visit. We have attached a general location map and a reduced version of the development plan (Figures 1 and 2). The District staff indicated the following primary issues to address in the development plans: • Rate Control - maintain discharge from the development to pre - development rates for 1 -year and 100 -year events. • Water Quality - provide dead storage for runoff from 2.5" storm. • Volume Control - provide infiltration basins to infiltrate runoff from roadways during a 0.34inch rain event. • Flood Level Control - maintain 1st floor/basement levels of homes 2 -feet above 100-year level in wetland/ponds, and 1 -foot above emergency overflow elevation. • Floodplain - compensate for floodplain fill (100 year level in Rice Lake 886.0) There are three primary watersheds on site .(See Figure 3). Most of the property is a soybean field that is now fallow.. A small portion to the north is wooded and approximately 6.4 acres of drainage comes from the existing residential area to the east. The two wetlands on site are also shown on the map. The soil investigation results show sandy soils typical of the Anoka sand plain. Type B soils were selected for the hydrologic analysis. The standard SCS methods of the HydroCAD model were used to model the runoff. Curve numbers were selected from the attached chart (Chart 1).- Model results are attached. Proposed changes to the existing watershed are shown on Figure 4. The proposed condition model assumes all fallow area is converted to 1/3 -acre residential area. Based on the curve •