06-08-26 Planning Commission Meeting Packet

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Stormwater Management Plan Inwood Commercial Carlson Engineering, Inc. Page 4 of 6 1.5 Rate Control Analysis All stormwater from the site drains to existing stormwater ponds 100 and 200. Based on the Inwood Stormwater Management Plan, Inwood Commercial areas were assumed to have 30 percent impervious area within the 1,000 Foot Shoreland District Boundary, and 75% of the area outside of the district boundary, which is indicated in the notes section on Figure 3 of the Inwood Stormwater Management Plan (See Appendix A). The majority of the Inwood Commercial site was slated to drain to Stormwater Pond 200, and a small southern portion to drain to Stormwater Pond 100. It will be up to the future lot development plans to correctly discharge the future impervious area to the right pond and not exceed the maximum allowable. Since the Inwood Commercial proposed impervious amount, which includes 4 ghost platted lots, is less than maximum allowable from each catchment area, proposed runoff rates will be less than the design runoff rates. Please see Figure 5 for an impervious exhibit. When the 5 individual lots develop, a percent impervious analysis should be completed to ensure that runoff rates are not increased to the ponds. 1.6 Water Quality and Volume Reduction Analysis Based on the Inwood Stormwater Management Plan, the ponds were designed to meet NURP standards as well as South Washington Watershed District phosphorus reduction requirements. Additionally, the infiltration basin was designed to provide volume reduction meeting the South Washington Watershed District and MPCA requirements for the entire site in a fully developed condition. Because the proposed site will have less impervious than the original assumptions used in the water quality and volume reductions calculations, the site will generate less runoff and require less water quality treatment and volume reduction than the approved master development plan. 1.7 Storm Sewer Design Proposed storm sewer will be used to collect stormwater from the site and to route it to existing pond 200. Lot 2, Block 2 will need utilize existing storm sewer stubs into the site from 5 th Street N to route stormwater south to existing Pond 100, behind Kwik Trip, as to not exceed the maximum allowable impervious to Pond 200. The rational method was used to determine anticipated flows to each catch basin and the pipes were sized using those flows along with Manning’s equation. All proposed storm sewer has been designed to accommodate a 10-year storm at a minimum. See Figure 4 for Storm Sewer Drainage Map and Appendix B for storm sewer design spreadsheet.