Agenda - Environmental Policy Board - 10/15/2018

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Table 17.4 shows a comparison of discharge from the site for the existing and proposed developed scenarios for the 100-year events. For existing conditions, a maximum peak flow of 28.3 cfs under the Highway 10 culvert occurs during the 100-year, 10-day snowmelt event. Peak flow discharge for proposed development conditions is slightly less (25.3 cfs) than existing conditions, excluding infiltration and assuming that no bio- retention facilities are i11l,u1pu1ated into the individual blocks. The existing peak flow rate (12.4 cfs) is exceeded for the 100-year, 24-hour rainfall event under proposed developed conditions (25.1 cfs), but has been significantly reduced from the peak rate of 132.6 cfs at the culvert crossing the RR tracks. Since infiltration is excluded, the numbers presented under proposed conditions are conservative. Slight reductions in peak flow rate discharge at the outlet and significant reductions in volume discharge could be achieved with the incorporation of properly designed and maintained infiltration basins. Volume discharge reductions of 30 to 40 percent could be achieved assuming a moderate rate of infiltration during the 100-year, 24-hour rainfall event and some infiltration during the last 15 days of the 30-day runoff simulation of the 100-year snowmelt event. Table 17.4. Comparison of peak flows and volumes discharged from site for 100-year events Model Existing conditions Proposed Proposed w/ some infiltration occurring 100-year 24-hour rainfall (5.9 inches precipitation) Peak flow Volume discharge (cfs) discharge (ac-ft) 12.4 25.1 24.7 54.9 150.3 92.1 100-year 10-day snowmelt (7.2 inches runoff) Peak flow Volume discharge (cfs) discharge (ac-ft) 28.3 25.3 24.8 334.7 520.5 356.2 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. 17-10