2014-04-09_REVISION - M1977493 (2)

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Ray Lazuk March 19, 2014 Page 11 The two parallel CMP culverts near Station 118 +00, shown in Photograph 4, do not match the invert and slope of the channel. The size of these culverts is generally adequate. However, the invert of one of the culverts is about six inches above the channel, and the other culvert is at a negative slope. This causes water to back up in the channel at low flows and overtop the channel at peak flows. The 14 -inch HDPE culvert on the upper channel just before the confluence of the upper and lower channels at about Station 58 +00 has sufficient capacity to pass the 10 -year rainfall event but causes overtopping during the 10 -year rain -on -snow event. The existing 14 -inch HDPE culvert at this location is shown in Photograph 5. Lastly, the discharge end of the 24 -inch CMP culvert on the lower channel just before the confluence of the upper and lower channels at about Station 58 +00 is damaged. The existing 24 -inch CMP culvert is shown in Photograph 6. Diversion Structures The flood routing capacities shown in the figures assume that the overflow structures at Kokomo Creek and Searle Gulch function as intended. These structures were designed with overflow provisions that would convey some unimpacted surface runoff into the Climax industrial area during major storm events. This configuration is intended to protect the canal by preventing uncontrolled overtopping if runoff flows exceed the interceptor capacity. The flood routing abilities of the West Interceptor downstream of Kokomo Creek and Searle Gulch rely on the existing overflow structures. Photograph 7 and Photograph 8 show the existing configuration of the interceptor at these two locations. For this analysis, the gates into the interceptor were assumed to be fully open, and all runoff was routed through the interceptor until the headwater backed up to the crest of the respective overflow structure. However, if the operators foresee the potential for overtopping in the interceptor reach downstream of the diversion structure, these gates could be partially or fully closed to bypass the interceptor. These gates are part of the management system to prevent overtopping.