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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 />Grass\parkland <br />Trail <br />Baseflow channel <br />(lined) <br />Flood storage <br />Figure not drawn to scale <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 />the ponding system concept for storage during the large-scale event. Reference to Figure <br />6.6, however, shows that many additional smaller ponds exist on the site. Use of these <br />ponds and consideration of the infiltration that naturally occurs through the sandy soils <br />inherent to the site, yield a net reduction in flow leaving the site. Even further reduction <br />can be made during the design and construction phases with the iiieoipoiation of <br />additional BMPs. These features can also be used to filter inflow to the shallow <br />groundwater system and replace some of the recharge lost to increased urbanization. The <br />City can expect that volumes will be reduced if these features are incorporated in block <br />design runoff routing. The largest benefit would likely accrue from installation <br />associated with large parking lot surfaces. Further reductions can be explored during the <br />detailed design phase. <br />The final BMP element proposed for runoff control is the use of solids removal pre- <br />treatment at storm sewer outfalls. These installations can be any of a wide variety of <br />forebays or installed sumps/filters that remove particulates from stormwater prior to <br />discharge into any of the drainageways throughout the site. These will also reduce <br />overall pollutant removal and will be a major part of the city's Phase II list of available <br />BMPs. <br />17-15 <br />