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• <br />• <br />• <br />FINAL DRAFT 02 -23 -2011 <br />Curve Numbers for Use with Existing Condition Pervious Areas <br />Hydrologic Soil Group <br />Runoff Curve Number* <br />A <br />39 <br />B <br />61 <br />C <br />74 <br />D <br />80 <br />* Curve numbers from USDA -NRCS, Technical Release 55 <br />(8) Design Criteria <br />(a) Infiltration BMPs must be designed to provide: <br />1. Adequate pretreatment measures before discharge of runoff to the primary <br />infiltration area; <br />2. Drawdown within 48 -hours or 72 -hours from the end of a storm event, for surface <br />or sub - surface features, respectively. Soil infiltration rates shall be based on the appropriate HSG <br />classification and associated infiltration rates (see Table 1 in Appendix A). Infiltration area will <br />be limited to the horizontal areas subject to prolonged wetting; and <br />3. A minimum of three feet of separation from the seasonal high water table. <br />4. Design and placement in accordance with the Minnesota Department of Health <br />guidance called "Evaluating Proposed Stormwater Infiltration Projects in Vulnerable Wellhead <br />Protection Areas." <br />(b) Permanent sedimentation and water quality ponds must be designed to provide: <br />1. Water quality features consistent with NURP criteria and City wet pond criteria; <br />2. A permanent wet pool with dead storage at least equal to the runoff from a 2.5- <br />inch rainfall over the area tributary to the pond; and <br />3. An outlet structure capable of preventing migration of floating debris and oils for <br />at least the one -year storm. <br />(c) Detention basins must be designed to provide: <br />1. An outlet structure to control the 2 -year & 100 -year frequency events to existing <br />peak runoff sites; and <br />2. An identified overflow spillway sufficiently stabilized to convey flows greater <br />than the 100 -year critical storm event. <br />17 <br />