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Vadnais Lakes Watershed Management Organization, Anoka County, Minnesota Depattinent of Natural Resources, Minnesota Department of Transportation, Minnesota Pollution Control Agency, U.S. Army Corps of Engineers, State of Minnesota Stormwater Manual or other regulatory agencies. g. The City of Lino Lakes may develop a Comprehensive Stormwater Management Plan (CSMP) as an alternative way to meet the stormwater requirements of the local watershed agencies for development within a defined area and a common resource of concem (ROC). The CSMP will be submitted to the applicable watershed district for review and must comply with the requirements of the watershed district. 3) Stormwater management plan modeling requirements. a. A hydrograph method or computer program based on Natural Resources Conservation Service Technical Release #20 (TR -20) and subsequent guidance must be used to analyze stormwater runoff for the design or analysis of flows and water levels within and off the project site. Composite curve numbers shall not include directly connected impervious surfaces. b. In determining curve numbers to model runoff in the post -development condition, the hydrologic soil group (HSG) of areas within construction limits is to be shifted down one classification (or one-half classification for HSG A) to account for the impacts of grading on soil structure unless the project specifications incorporate soil amendments in accordance with Rice Creek Watershed District Soil Amendment guidelines. c. The following curve numbers (Table 1) shall be utilized for modeling of those site areas not covered by impervious surface: Table 1. Curve Numbers for Use with Pervious Areas Hydrologic Soil Group Existing Runoff Curve Number* Post -Construction Runoff Curve Number** Undisturbed Land Disturbed Land A 39 39 49 B 61 61 74 C 74 74 80 D 80 80 80 * Curve numbers from USDA-NRCS, Technical Release 55 ** Rice Creek Watershed District 2013 Rules d. The analysis of flood levels, storage volumes, and discharge rates for waterbodies and stormwater management basins must include the NOAA Atlas 14 values, as amended, for the 2 - year, 10 -year and 100 -year return period, 24-hour rainfall events and the 10 -day snowmelt event, in order to identify the critical duration flood event. The ten-day snowmelt event is simulated by a 7.2 -inch, ten-day spring runoff event during which it is assumed the ground is frozen solid and no infiltration occurs (CN set to 100 for all areas). The City Engineer may require analysis of draft 06-15 ECS Ordinance.docx 17/29