5.2. SR 08-08-2011 - Laserfiche WebLink

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' 17. Water Quality -Surface Water Runoff a. Compare the quantity and quality of site runoff before and after the project. Describe permanent controls to manage or treat runoff. Describe any ' storm water pollution prevention plans. ' b. Identify routes and receiving water bodies for runoff from the site; include major downstream water bodies as well as the immediate receiving waters. Estimate impact runoff on the quality of receiving waters. Existing Conditions The study area currently consists mainly of small grain straight rows, small wooded and grassed areas and a few small wetlands. Existing impervious areas include TH 10, which runs through the middle of the study area and some small farm and residential structures on the southwest corner of the study area. Approximately 15 acres of the study area is within subwatershed ER02 and drains to the Rum River. The remaining area is within subwatershed ER01 which drains to the Mississippi River (see Figure 17-1). The portions of the Mississippi River and the Rum River which are downstream of these subwatersheds are classified as Outstanding Resource Value Waters (ORVW). Both rivers are listed as impaired by the Minnesota Pollution Control Agency for mercury (Mississippi and Rum Rivers) and PCB's in fish tissue (Mississippi River). About 13 acres of the study area consists of Group B/D rated soils (mainly in the wetland areas) and the remaining area contains Group A rated soils. These soils ratings are based on hydrologic soils classifications with A soils having high infiltration rates even when thoroughly wetted. The infiltration rates range from 0.8 to 1.63 inches per hour. These soils consist chiefly of deep, well drained to excessively drained sands and gravel. Group A soils have a high rate of water transmission, therefore resulting in a low runoff potential. Group B soils have moderate infiltration rates ranging from 0.3 to 0.6 inches per hour when thoroughly wetted. Group B soils consist of deep moderately well to well drained soils with moderately fine to moderately coarse textures. Group D soils have very slow infiltration rates ranging from 0 to 0.2 inches per hour when thoroughly wetted. Group D soils are typically clay soils with high swelling potential, soils with high permanent water table, soils with a clay layer at or near the surface, or shallow soils over nearly impervious material. Proposed Conditions Due to the conceptual nature of the development scenarios, the amount of impervious surfaces in each land use was assumed based on the values presented in the City's Nondegradation Review (November 2007) and characteristics of existing developments in the City. Scenario 1 represents the development conditions with the least amount of impervious surfaces. Much of the agricultural and open space land uses are preserved under this scenario with the majority of the remaining area being developed to 75% impervious. Scenarios 2 and 3 largely consist of commercial/retail land use with 75% impervious and 95% respectively. Scenario 3 represents the most extreme development scenario with respect to impervious City of Elk River Draft Alternative Urban Areawide Review July 2011 Page 39 of 84