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(c) The following curve numbers (Table 1) shall be utilized for modeling of those site areas not <br />covered by impervious surface: <br /> <br /> Table 1. Curve numbers for use with pervious areas <br />Hydrologic Soil <br />Group <br />Existing Runoff <br />Curve Number * <br />Post-Construction Runoff Curve <br />Number** <br />Undisturbed Land Disturbed Land <br />A 39 39 49 <br />B 61 61 74 <br />C 74 74 80 <br />D 80 80 80 <br />* Curve numbers from USDA-NRCS, Technical Release 55 <br />** Rice Creek Watershed District 2013 Rules <br /> <br /> (d) The analysis of flood levels, storage volumes, and discharge rates for waterbodies and stormwater <br />management basins must include the NOAA Atlas 14 values, as amended, for the two-year, ten-year and <br />100-year return period, 24-hour rainfall events and the ten-day snowmelt event, in order to identify the <br />critical duration flood event. The ten-day snowmelt event is simulated by a 7.2 inch, ten-day spring runoff <br />event during which it is assumed the ground is frozen solid and no infiltration occurs (CN set to 100 for all <br />areas). The City Engineer may require analysis of additional precipitation durations to determine the critical <br />duration flood event. Analysis of the ten-day snowmelt event is not required for stormwater management <br />detention basins with a defined outlet elevation at or below the 100-year, 24-hour event elevation. <br /> (4) Water quality treatment. <br /> (a) Land development activities creating impervious surface shall address the use of better site design <br />(BSD) techniques as outlined in the better site design and low impact development sections of the <br />Minnesota Stormwater Manual (MPCA, 2014 and subsequent revisions). <br /> (b) The water quality treatment volume standard for all projects, except public linear projects, is <br />determined as follows (Table 2): <br /> <br /> Table 2. Required quality volume calculations for new or reconstructed <br />developments <br />BMP BMP Design <br />Variation Water Quality Treatment Volume Calculation [ft3] <br />Infiltration Infiltration <br />Feature Impervious surface [ft2] * 1.1 [in] /12 [in/ft] <br />Water Reuse Irrigation Impervious surface [ft2] * 1.1 [in] / 12 [in/ft] <br />Biofiltration Underdrain Impervious surface [ft2] * 1.1 [in] / (0.65 * 12 [in/ft]) <br />Filtration Sand or Rock <br />Filter Impervious surface [ft2] * 1.1 [in] / (0.50 * 12 [in/ft]) <br />Stormwater <br />Wetlands <br />Shallow <br />Wetland Impervious surface [ft2] * 1.1 [in] / (0.40 * 12 [in/ft])