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Elk River Subwatershed ER04W: stormwater Retrofit Assessment Page 2 <br />Executive Summary <br />Subwatershed ER04 as delineated in Elk River's MS4 permit Non-degradation plan was selected <br />by the City for retrofit assessment. Modeling results for ER04 as reported in the Non- <br />degradation plan showed relatively high phosphorus and total suspended solids loading to the <br />Mississippi River, a priority surface water resource. <br />For practical reasons the City and SWCD decided to narrow the scope of this assessment. The <br />subwatershed was subdivided to the areas east and west of U.S. Hwy 169. These two areas are <br />not hydraulically connected with the exception of periods of extremely high precipitation. The <br />subwatershed west of U.S. Hwy 169, ER04W, is the focus of this assessment. <br />Based on existing stormwater system maps and City staff knowledge of the system, ER04W was <br />divided into 12 catchments. The 12 catchments were subdivided into a total of 30 sub- <br />catchments based on significant differences in land use or drainage to existing stormwater <br />treatment BMPs. During desktop review and field reconnaissance we identified additional un- <br />mapped stormwater system pipes and re-drew some flow boundaries. After re-evaluating the <br />data, we determined that for modeling, the 30 sub-catchments should be re-grouped into 8 <br />catchments based on common outflow points. When present, an existing treatment basin was <br />used as the common outflow point. For this report, "catchment" will refer to one of these 8 <br />catchments. <br />Five sub-catchments were not modeled or assessed for treatment options for the following <br />reasons. Two drained to land locked wetlands that were not connected to the stormwater <br />system (7B and 8A). One consists of low density development and steep topography not well <br />suited to installing BMPs (1J). One consisted of a high percentage of open space and modeling <br />showed that it exports a relatively small amount of phosphorus and TSS (2B). One industrial <br />area has been tentatively identified for redevelopment (5B). <br />Catchments ER04W6, ER04W7 were modeled but not assessed for treatment options because <br />modeling showed that the existing treatment ponds and wetlands were capable of effectively <br />treating runoff from these catchments. <br />List of Catchments and Sub-catchments <br />Catchment Sub-catchments <br />ER04W1 1A, 1B, 1C, 1D, 1E, 1F, 1G, 1H, 11, 1J <br />ER04W2 2A, 2B <br />ER04W3 3A, 3B, 3C, 3D, 3E <br />ER04W4 4A, 4B, 4C <br />ER04W5 5A, 5B <br />ER04W6 6A, 6B, 6C, 6D, 6E <br />ER04W7 7A, 7B <br />ER04W8 8A <br />