Agenda - Council - 02/26/2019 - Laserfiche WebLink

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The primary benefit of incorporating infiltration BMPs into the site is achieved during small storm events. Table 17.5 compares site discharge for the 1-year and 10-year, 24- hour rainfall events considering site design that first excludes infiltration in the basins and then considers infiltration in the basins. For existing conditions, a peak flow of 2.3 cfs discharge at the Highway 10 culvert for the 1-year, 24-hour event is due to local drainage south of the railroad tracks only, as there is no flow leaving the site at the railroad tracks. As discussed earlier, the flow discharging from Highway 10 is small and does not reach the Mississippi River. For proposed development conditions excluding infiltration in the basins, peak discharge into the Mississippi River would be 14.4 cfs. By incorporating infiltration basins into the site, peak flow is reduced by 50 percent (7.8 cfs) and volume discharge is also significantly reduced.. The 10-year, 24-hour rainfall event results in a slight reduction in peak flow, but significant reduction in volume (50 percent) by incorporating the infiltration BMPs. Table 17.5. Comparison of peak flows and volumes discharged from site for 1-year and 10-year events 1-year 24-hour rainfall 10-year 24-hour rainfall Model (2.3 inches precipitation) (4.1 inches precipitation) Peak flow (cfs) Volume (ac-ft) Peak flow (cfs) Volume (ac-ft) Existing conditions 2.3 0.3 7.1 0.7 Proposed 14.4 14.4 23.1 65.0 Proposed 7.8 4.0 22.6 35.4 w/ infiltration occurring Proposed — mitigation plan with revisions 19.1 25.3 24.5 84.2 requested by County Smaller, more frequent rainfall events are critical for water quality. Achievement of long-term year-round water quality benefits requires the ability to retain and treat smaller storm events. To meet LRRWMO water quality requirements based on NURP design criteria, the final design should provide for a dead storage volume of at least 38 ac-ft, which is the volume required to accommodate the runoff volume from a 2.5-inch rainfall event (excluding infiltration in basins). Water Quality Modeling Water Quality Under Developed Conditions. As shown in a previous section, the amount of water leaving the site under current conditions is minimal. Consequently, the amount of pollution associated with the runoff is equally minimal. However, this all changes once development occurs. An increase in nonpoint pollution from this site will occur from many new sources, including some or all of the following: 17-11