07/25/2007 Env Bd Packet - Laserfiche WebLink

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Table 1: Curve Numbers Typically Used to Calculate Stormwater Runoff Under Pre - Development Conditions Hydrologic Sol/ Group A B C 0 Runoff Curve Number for Pasture in Good Condition 39 61 74 80 Runoff Curve Number for Open Space in Good Condition 39 61 74 80 Runoff Curve Number for Woods -Grass in Good Condition 32 58 72 79 The revised rules would define the "pre- settlement" condition as a prairie community at the time preceding the European settlement of Minnesota, as defined by the Marschner map (USDA, 1974). The Marschner map is used extensively in Minnesota as a reliable reference on pre - settlement land condition. Specific curve numbers to be used in modeling pre - settlement runoff are included in the rule and are as shown in Table 2: Table 2: Curve Numbers to be used to Calculate Stormwater Runoff from Upland Areas Under Pre - Settlement Conditions Hydrologic Soil Group A B C D Runoff Curve Number for Prairie Communities 30 58 71 78 The pre - settlement curve numbers proposed by the RCWD are slightly lower than those identified for the pre - development condition, meaning that the revised rule will result in lower maximum allowable peak flows. For permit applicants, the extent of the burden resulting from this change will depend on the site's soil types, pre - development condition, and proposed final land uses. Comments are solicited on the impact of this modeling change on the feasibility and cost of stormwater management. One impact of increased peak flow rates is disruption of a receiving waterbody's natural pattern of water elevation fluctuation, which can destabilize vegetation, promote erosion and injure highly valued fringe habitat. In addition to a peak flow criterion, the proposed rule would protect against harmful elevation changes (bounce) or increases in inundation directly by adopting the limits on increases in bounce and inundation period recommended by the Minnesota Stormwater Advisory Group in its 10