Agenda - Environmental Policy Board - 10/15/2018

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17. Water Quality-Stormwater Runoff 17a. Compare the quantity and quality of site runoff before and after the project. Describe permanent controls to manage or treat runoff. Describe any stormwater pollution prevention plans. 17b. 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. For an AUAR the following guidance should be followed in addition to that in "EAW Guidelines": • it is expected that an AUAR will have a detailed analysis of stormwater issues • a map of the proposed stormwater manuguriwrct system and of the water bodies that will receive stormwater should be provided • the description of the stormwater systems would identify on -site and "regional" detention ponding and also indicate whether the various ponds will be new water bodies or converted existing ponds or wetlands. Where on -site ponds will be used but have not yet been designed, the discussion should indicate the design standards that will be followed. • if present in or adjoining the AUAR area, the following types of water bodies must be given special analyses o lakes: within the TC metro area a nutrient budget analysis must be prepared for any `priority lake" identified by the Metropolitan Council. Background Watershed Setting. The details of the surface water management system being proposed for the RTC site are best described by joining Items 17a and b into a single discussion. Figure 17.1 illustrates the entire watershed within which the RTC site lays. The watershed extends from north of Lake Itasca to the Mississippi River, covering an area of approximately 2,687 acres. The larger watershed can be sub -divided into a series of 31 sub -watersheds, which were shown in Figure 12.1 in Item 12. Each of these smaller units was characterized for water quantity and quality modeling under existing conditions, and was subsequently modeled for fully developed conditions as proposed under the City's 2001 Comprehensive Plan, as amended in 2002 (Item 5, Figure 5.4), and the preferred site development (Item 6, Figure 6.1). Drainage through the site. The principal drainage feature currently passing through the site, and evident in Figure 12.1, is a well- to poorly -defined swale that occurs from the northwest corner of the site to the middle of the site, whereupon it disappears. Historically, this swale appears to have been a more significant drainage feature, but 17-1