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Ray Lazuk <br />March 19, 2014 <br />Page 10 <br />5.3 West Interceptor <br />In its current configuration, the West Interceptor does not have adequate capacity to pass the <br />10 -year rainfall event without overtopping. Figures 7a and 7b show the alignment of the West <br />Interceptor and highlight the reaches with insufficient capacity to pass the storm events <br />evaluated. As a general observation, the slope of the West Interceptor is very flat, which <br />promotes slower velocities and deeper water depths. Water that overtops the West Interceptor <br />channel south of 3Dam would be fully contained in Tenmile TSF. Water that overtops the West <br />Interceptor channel between 3Dam and 5Dam would discharge directly into the Mayflower TSF <br />or into the lower reaches of Kokomo Creek and Searle Creek, which also discharge into <br />Mayflower TSF. <br />The upper end of the West Interceptor consists of two separate channels that combine at about <br />Station 58 +00. The stationing in Figures 7a and 7b follows the upper channel until the 36 -inch <br />culvert near Station 28 +00 that directs runoff to the lower channel, then follows the lower <br />channel to the confluence at Station 58 +00. This is the route that the majority of runoff will <br />follow during a storm event. The two sections that are not stationed are: 1) the lower channel <br />from the start of the interceptor system to the discharge from the 36 -inch culvert, and 2) the <br />upper channel from the 36 -inch culvert to the confluence of the upper and lower channels at <br />about Station 58 +00. These two small reaches collect minimal runoff and are expected to pass <br />the 10 -year rainfall and rain -on -snow events, except as noted below. <br />Three of the reaches shown to have insufficient capacity in Figures 7a and 7b correspond to <br />three restrictive culvert locations. There is also currently damage at a fourth culvert. These <br />culvert locations are described below. <br />The 1,890- foot -long 46- inch - diameter CMP section that begins at about Station 137 +00 and <br />discharges into Searle Gulch is severely damaged in several locations and has been partially <br />replaced with 42 -inch HDPE pipe, as shown in Photograph 2. The entrance to this pipe is <br />shown in Photograph 3. The overflow pipe indicated in Photograph 3 was likely installed after <br />previous occurrences of overtopping at this location. Regardless of the damage, the existing <br />pipe cannot pass the 10 -year rainfall event because of its very flat slope of 0.002 feet per foot. <br />