2014-04-09_REVISION - M1977493 (2)

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Ray Lazuk March 19, 2014 Page 12 The overflow structure at Kokomo Creek consists of a 48 -inch CMP crossing under the service road. The overflow structure at Searle Gulch consists of a 10- foot -wide weir that discharges into a box culvert crossing under the service road. The two 24 -inch slide gates that can be used to divert water into the Climax industrial area at the Searle Gulch diversion structure were assumed to be closed for this analysis. Table 3 provides a summary of the flow rate and percentage of the peak flow rate that would overflow at these structures for the storm events considered, assuming that the gates into the interceptor were fully open. Table 3. West Interceptor Overflow Structure Flow Rates. The West Interceptor after Searle Gulch consists of 46 -inch to 50- inch - diameter CMP until the end of the West Interceptor at Tucker Creek. The runoff entering this piped section during the 10 -year rainfall and rain -on -snow events exceeds the gravity -flow capacity of the pipe, causing the inlet at the Searle Gulch diversion structure to be submerged. The capacity of this piped section apparently has been exceeded before, as indicated by several overflow culverts that have been installed near existing interceptor inlets to route runoff over and across the interceptor pipe and into the industrial area. The locations of the interceptor inlets — two of which have been capped — and the overflow culverts are shown on Figure 7b. Any water that crosses the West Interceptor through these overflow culverts would discharge into and be fully contained in Mayflower TSF. 10 -Year Rain - Overflow 10 -Year Rainfall On -Snow Structure Event Event Kokomo Creek 21 cfs (21 %) 28 cfs (25 %) Searle Gulch 140 cfs (54 %) 154 cfs (56 %) The West Interceptor after Searle Gulch consists of 46 -inch to 50- inch - diameter CMP until the end of the West Interceptor at Tucker Creek. The runoff entering this piped section during the 10 -year rainfall and rain -on -snow events exceeds the gravity -flow capacity of the pipe, causing the inlet at the Searle Gulch diversion structure to be submerged. The capacity of this piped section apparently has been exceeded before, as indicated by several overflow culverts that have been installed near existing interceptor inlets to route runoff over and across the interceptor pipe and into the industrial area. The locations of the interceptor inlets — two of which have been capped — and the overflow culverts are shown on Figure 7b. Any water that crosses the West Interceptor through these overflow culverts would discharge into and be fully contained in Mayflower TSF.