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Table 17.4 shows a comparison of discharge from the site for the existing and proposed <br />developed scenarios for the 100-year events. For existing conditions, a maximum peak <br />flow of 28.3 cfs under the Highway 10 culvert occurs during the 100-year, 10-day <br />snowmelt event. Peak flow discharge for proposed development conditions is slightly <br />less (25.3 cfs) than existing conditions, excluding infiltration and assuming that no bio- <br />retention facilities are incorporated into the individual blocks. The existing peak flow <br />rate (12.4 cfs) is exceeded for the 100-year, 24-hour rainfall event under proposed <br />developed conditions (25.1 cfs), but has been significantly reduced from the peak rate of <br />132.6 cfs at the culvert crossing the RR tracks. Since infiltration is excluded, the <br />numbers presented under proposed conditions are conservative. Slight reductions in peak <br />flow rate discharge at the outlet and significant reductions in volume discharge could be <br />achieved with the incorporation of properly designed and maintained infiltration basins. <br />Volume discharge reductions of 30 to 40 percent could be achieved assuming a moderate <br />rate of infiltration during the 100-year, 24-hour rainfall event and some infiltration during <br />the last 15 days of the 30-day runoff simulation of the 100-year snowmelt event. <br />Since the original analysis was developed for the April 8, 2003 draft AUAR, additional <br />analysis has been performed of the County alternative outlet based on discussions with <br />the County. Refer to Appendix J for additional detail regarding results from the <br />additional analysis. The last rows in Tables 17.4 and 17.5 show the results of the changes <br />that occur when the infiltration area (26b in Figure 12.4) is reduced to exclude any <br />surface water storage within the Regional Park and when all expected County and State <br />roadway upgrades are included. All of the details associated with the selected drainage <br />outlet will be developed at the design stage. <br />Table 17.4. Comparison of peak flows and volumes discharged from site for 100-year <br />events <br />Model <br />100-year 24-hour <br />(5.9 inches precipitation) <br />Peak flow <br />discharge (cfs) <br />rainfall <br />Volume <br />discharge (ac-ft) <br />100-year 10-day <br />(7.2 inches <br />Peak flow <br />discharge (cfs) <br />snowmelt <br />runoff) <br />Volume <br />discharge (ac-ft) <br />Existing conditions <br />12.4 <br />54.9 <br />28.3 <br />334.7 <br />Proposed <br />25.1 <br />150.3 <br />25.3 <br />520.5 <br />Proposed w/ some <br />infiltration occurring <br />24.7 <br />92.1 <br />24.8 <br />356.2 <br />Proposed — mitigation <br />plan with revisions <br />requested by County <br />25.4 <br />176.9 <br />25.4 <br />556.6 <br />17-10 <br />