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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 con'idor. TI'tis 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 dev. elopment 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 /> <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 />volume and enhance recharge. A similar feature is also proposed for sub-watershed 26. <br />This would be the last BMP in the chain of BMPs installed tlzroughout the site and south <br />of it before flows reaches the Mississippi River. Table 17.7 showed the dramatic water <br />quality improvement that this system could provide. Such an approach is mandated by <br />the Phase II discharge requirements (next section) and the MNRRA/Critical Area <br />guidelines (Item 27). <br /> <br />Figure 17.3 Schematic of Flood Storage/Infiltration Portion of Central Corridor. <br /> <br /> I <br /> I <br /> I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />Ii <br /> <br />Grass\parkland <br /> <br />(lined) <br /> <br />Flood storage <br /> <br />Trail <br /> <br />Figure not drawn to scale <br /> <br />44 <br /> <br />The runoff calculations in this section included determination of the 100-year event <br />runoff and a condition supplemented by additional on-site storage. Figures 17.2 (a-c) <br />showed the preliminary concept for the central drainage corridor. These figures illustrate <br /> <br />D-12- <br /> <br />! <br />I <br /> <br /> <br />