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ERC CC&V Poverty Gulch Chicago Tunnel <br /> December 2016 <br /> spillway crest down an additional 0.4 feet. We also recommend that a riprap grade control structure be <br /> added at the north or downstream end of the outlet from the spillway at the location where flows from <br /> the spillway enter Poverty Gulch. This grade control structure will help ensure that a headcut doesn't <br /> occur. <br /> Prior to additional work being performed on the spillway and construction of the grade control structure, <br /> ERC believes the interim risk to the facility is very low. While we believe this additional work should be <br /> completed when practical,the following items minimize current risk: <br /> • The sediment pond is sized to contain the full 100-year inflow, so it is likely that in the event of <br /> the design storm,the spillway would not be activated. <br /> • The peak 100-year inflow into the sediment pond is approximately 7 cfs. With this flow the <br /> spillway would have a flow depth of about 0.6 feet. The spillway can pass approximately 17 cfs, <br /> or more than 200 percent of the 100-year storm without overtopping. <br /> • Outflow velocities for the spillway channel were calculated to be approximately 2.4 ft/s. The <br /> installed 6-inch riprap will minimize any erosion in this area. <br /> References <br /> Steffens and Associates, Inc. CCV Poverty Gulch SEDCAD Results for 5-, 10- and 100-year Storms, August <br /> 9, 2016. <br /> Steffens and Associates, Inc. CCV Poverty Gulch SEDCAD Maps, Drawing Numbers CCVAS11-6 and <br /> CCVSAII-7,August 2016a. <br /> 4 <br />