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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 />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 throughout 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 />Figure 17.3 Schematic of Flood Storage/Infiltration Portion of Central Corridor. <br />17-15 <br />