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Table 17.5. Comparison of peak flows and volumes discharged from site for 1-year and <br />10-year events <br />Model <br />1-year 24-hour rainfall <br />(2.3 inches precipitation) <br />Peak flow (cfs) <br />Volume (ac-ft) <br />10-year 24-hour rainfall <br />(4.1 inches precipitation) <br />Peak flow (cfs) <br />Volume (ac-ft) <br />Existing conditions 2.3 0.3 <br />Proposed 14.4 14.4 <br />F ,. vs, ,tea 7.8 4.0 <br />w/ infiltration occurring <br />7.1 0.7 <br />23.1 65.0 <br />22.6 35.4 <br />Smaller, more frequent rainfall events are critical for water quality. Achievement of <br />long-term year-round water quality benefits requires the ability to retain and treat smaller <br />storm events. To meet LRRWMO water quality requirements based on NURP design <br />criteria, the final design should provide for a dead storage volume of at least 38 ac-ft, <br />which is the volume required to accommodate the runoff volume from a 2.5-inch rainfall <br />event (excluding infiltration in basins). <br />Water Quality Modeling <br />Water Quality Under Developed Conditions. As shown in a previous section, the amount <br />of water leaving the site under current conditions is minimal. Consequently, the amount <br />of pollution associated with the runoff is equally minimal. However, this all changes <br />once development occurs. An increase in nonpoint pollution from this site will occur <br />from many new sources, including some or all of the following: <br />• Automobile, truck and bus traffic (oil, exhaust, vehicle decomposition); <br />• Lawn and landscaping chemicals (fertilizer and pesticide); <br />• Litter; <br />• Vegetative debris; <br />• Pet waste; <br />• Fueling spillage from the convenience stations; <br />• Increased sanding and salting; and <br />• New construction (erosion, debris). <br />The pollutant removal efficiencies of the proposed stormwater management practices <br />were assessed using the P8 Urban Catchment Model (Program for Predicting Polluting <br />Particle Passage through Pits, Puddles and Ponds, developed by William Walker). This <br />approach allowed for the evaluation of different runoff scenarios, as well as the <br />prediction of pollutant loads passing through the proposed development and eventually <br />into the Mississippi River. Model results presented are for a complete year with a long <br />term average precipitation depth (23.85 inches). This scenario is different than those <br />17-11 <br />