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It must be stressed that this portion of Ramsey does not have a natural outlet to the <br />Mississippi River. The preliminary drainage system described within this AUAR <br />assumes an outlet that takes advantage of publicly -owned, County land that extends from <br />Highway 10 to the Mississippi River south of the RTC site. Preliminary discussions with <br />the County on the use of this land have occurred, but resolution has not been agreed <br />upon. Advantages of using this land extend beyond the RTC site, to the entire sub - <br />watershed, including the eventual upgrade of Highway 10, which will also need a River <br />outlet. If permission cannot be obtained from the County to use its land, an outlet option <br />will need to be pursued to the southeast, along Highway 10. The small amount of water <br />that now leaves the site, runs southeast along the highway, but infiltrates within a short <br />distance. <br />Mitigation Approach <br />The quantity and quality approach laid -out earlier in this section describes an approach <br />driven by the need to comply with runoff control rules of the LRRWMO and water <br />quality requiieiiieiiis that are described in the next section. The runoff management <br />system proposed in Figure 17.2 is done in a preliminary sense to allow the City and the <br />developer to proceed with some knowledge of what design specifics will be needed. The <br />City will assure that the developer(s) will design and build the final drainage and runoff <br />management system within this overall framework, in compliance with the mandates of <br />the LRRWMO. <br />Implementation of BMPs in Preliminary Design <br />A system of BMPs can be initially proposed to meet the needs identified in the Item 12 <br />and 17 discussions. The first aspect of this design is the handling of the large-scale (100- <br />year) event. Figures 17.2 (a-c) illustrate the major management practice features that are <br />proposed to store and treat runoff in the central drainage corridor. This system is <br />designed to provide volume/peak reduction storage for the runoff, as well as water quality <br />treatment. This approach starts with large detention storage in ponds located on -site in <br />sub -watersheds 7 and 8, followed by similarly sized detention in ponds located in <br />subwatersheds 25 and 26 south of the active development site. This storage is <br />supplemented with additional smaller -scale storage in sub -watersheds 5, 11, 13, 21 and <br />22. Tables 17.1-17.5 describe the water quantity reductions in these ponds, and Tables <br />17.6 and 17.7 describe the water quality improvements for TP. <br />The ponding system provides both permanent pool storage for water quality treatment <br />and temporary flood storage above the permanent pool. The ponds in the central <br />drainage corridor are supplemented by two areas for additional storage of pre-treated <br />runoff. These two areas in sub -watersheds 8 and 19 will allow for water levels to raise <br />and take advantage of storage available. This water will be slowly drained by a <br />controlled outlet, but infiltration will also occur. Figure 17.3 is a schematic cross-section <br />of this approach. Keeping these areas dry except during high flows allows for their use <br />as open space, recreation areas for essentially all of the year, with the exception of that <br />time when they are needed to absorb flow. They then serve to dampen peaks, reduce <br />17-14 <br />