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18 <br />(4) Stormwater management plan modeling requirements. The City adopts by <br />reference RCWD Rule C4. Modeling for Stormwater Management Plans, as amended. <br />(b) A hydrograph method or computer program based on Natural Resources <br />Conservation Service Technical Release #20 (TR-20) and subsequent guidance must <br />be used to analyze stormwater runoff for the design or analysis of flows and water <br />levels within and off the project site. Composite curve numbers shall not include directly <br />connected impervious surfaces. <br />(c) In determining curve numbers to model runoff in the post-development <br />condition, the hydrologic soil group (HSG) of areas within construction limits is to be <br />shifted down one classification (or one-half classification for HSG A) to account for the <br />impacts of grading on soil structure unless the project specifications incorporate soil <br />amendments in accordance with Rice Creek Watershed District Soil Amendment <br />guidelines. <br />(d) The following curve numbers (Table 1) shall be utilized for modeling of those <br />site areas not covered by impervious surface: <br />Table 1. Curve numbers for use with pervious areas <br />Hydrologic <br />Soil Group <br /> <br />Existing <br />Runoff Curve <br />Number * <br /> <br />Post-Construction Runoff Curve <br />Number** <br />Undisturbed <br />Land <br />Disturbed <br />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 />The analysis of flood levels, storage volumes, and discharge rates for <br />waterbodies and stormwater management basins must include the NOAA Atlas 14 <br />values, as amended, for the two-year, ten-year and 100-year return period, 24-hour <br />rainfall events and the ten-day snowmelt event, in order to identify the critical duration <br />flood event. The ten-day snowmelt event is simulated by a 7.2 inch, ten-day spring <br />runoff event during which it is assumed the ground is frozen solid and no infiltration <br />occurs (CN set to 100 for all areas). The City Engineer may require analysis of <br />additional precipitation durations to determine the critical duration flood event. Analysis