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Updated Storm Water Management Plan (SWMP) <br />City of Ramsey, Minnesota <br />either high-density residential developments or commerciallindustrial <br />developments. <br />This report recommends modifying the current development ordinances to <br />encourage infiltration and soil ripping of mass grading to compensate for lost <br />infiltration conditions as well as requiring oversized retention ponding to mitigate <br />and compensate for increases in runoff. Innovative solutions to the storm water <br />runoff increases associated with the increase in impervious surface will be <br />investigated and encouraged when deemed appropriate. Potential solutions <br />include pervious pavements, rain gardens, infiltration basins and low impact <br />development among others. <br />2. An integral part of this SWMP is updating the comprehensive storm water runoff <br />modeling performed in 2008. The 2008 modeling effort captured the existing <br />conditions throughout the entire city. This modeling includes: <br />a. Mapping out the small watersheds draining to general collection points <br />such as low points in roadways and intersections, wetlands or ponds. <br />b. Estimating the runoff from the 2, 10 and 100 -year rainfall events. <br />c. Routing the runoff through the existing system. <br />The existing system may be a pipe network, a pond, a wetland or a waterway. <br />The modeling predicts the high water levels and flows associated with each <br />rainfall event modeled. <br />This modeling provides a baseline for comparison purposes as new developments <br />change the drainage pattern. With this modeling information, City staff can <br />readily review the cumulative impacts of large developments for effects on the <br />baseline conditions across the entire watershed. <br />Storm and Sanitary Analysis (SSA) software was used in the comprehensive <br />modeling. This software is based on the industry standard EPASWMM process <br />and the St. Vennant equations. The model can be used to input actual rainfall <br />events from rain gauges and can model the transport of pollutants through the <br />system. This will be very useful in evaluating the BMP measures to address <br />future TMDLs. <br />3. Where the cumulative effect of regulated development is potential flooding, the <br />recommended practice is the construction of retention ponds or detention basins, <br />including infiltration, as a requirement of further development of the outlying <br />growth areas. It is further recommended that the post construction peak outflows <br />from new developments be limited to no more than the existing peak flow for the <br />2-, 10-, and 100 -year storms. The Third Generation Lower Rum River Watershed <br />Management Plan requires infiltration of the first 1" of runoff from new <br />impervious surfaces. This will better mitigate the cumulative effects of increased <br />impervious surfacing and increased runoff volume from new developments. <br />Section II <br />February 20, 2015March 6, 2015 Page 5 <br />