2014-04-09_REVISION - M1977493 (2)

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Ray Lazuk March 19, 2014 Page 9 for the 10 -year rainfall event with no base flow and for the 10 -year rainfall event with the 2 -year snowmelt base flow (i.e. rain -on -snow event). 5.0 RESULTS The estimated flood routing capacity of the interceptor system in terms of each interceptor's ability to route the 10 -year rainfall event and the 10 -year rain -on -snow event is illustrated in Figures 5, 6, and 7 and described in the following sections. The coefficients and other assumed parameters used in this analysis were based on expected site conditions during the respective events. Actual flooding conditions may vary from the information presented here depending on antecedent moisture conditions and snowmelt runoff conditions at the time of the storm. 5.1 Chalk Mountain Interceptor The majority of the Chalk Mountain Interceptor system has sufficient capacity to convey the 10- year rainfall event without overtopping. However, the 36 -inch CMP culvert crossing under the road to the Observatory near Station 95 +00 is expected to overtop by a few inches. This culvert is shown in Photograph 1. Water that overtops the Chalk Mountain Interceptor at this location would either flow down the interceptor access road and back into the interceptor or overflow the access road and be fully contained in Chalk Mountain Reservoir, which is within the Climax industrial area. Figure 5 shows the alignment of the Chalk Mountain Interceptor and highlights the reach with insufficient capacity to pass the storm events evaluated. 5.2 East Interceptor The model results indicate that the East Interceptor is capable of passing the 10 -year rainfall event and the 10 -year rain -on -snow event without overtopping. Figures 6a and 6b show the alignment of the East Interceptor, highlighting the entire interceptor as having sufficient capacity to pass the storm events evaluated.