12000000065 Fuchs Dam Inundation Report

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Table 1. Fuchs Reservoir Dam Failure Comparison of Dam Breach Parameters Empirical Method Average Breach Width Time of Failure (ft) (min) Froehlich 54 26 MLM-WA 41 34 2.2 Dam Breach Simulation in HEC-HMS The U.S. Army Corps of Engineers' (USACE)hydrologic modeling computer program HEC-HMS was used to simulate a clear-day piping failure of Fuchs Dam using the breach parameters estimated by each of the empirical equations. As a conservative measure, the simulation which produced the greatest peak outflow was selected for use in the downstream hydraulic modeling and inundation mapping. For each HEC-HMS breach simulation the water surface was set at the service spillway crest (elevation 202.2 feet) at the onset of the breach, and the reservoir was assumed to drain completely. The assumption was made that the breach would extend from the dam crest (elevation 207.0 feet)to the elevation of the upstream invert of the outlet pipe (elevation 184.4 feet). The breach side slopes were set at 0.7:1 (horizontal to vertical) for the Froehlich (2008) simulation, and 1:1 for the MLM- WA (2007) simulation. As recommended in the Guidelines for Dam Breach Analysis,the piping failure was set to start at the midpoint of the final breach height, and both linear and sine wave progressions were tested. A capacity table provided by the SEO dated September 21, 1955 was used to model the reservoir storage. A copy of the capacity table is attached in Appendix B. The time to peak and the peak outflow for each of the HEC-HMS simulations is summarized below in Table 2. Table 2. Fuchs Reservoir Dam Failure Comparison of HEC-HMS Piping Simulation Results Linear Progression Sine Wave Progression Empirical Method Time to Peak Peak(cfs) Time to Peak Peak(cfs) Outflow(min) Outflow(min) Froehlich 26 6,600 21 6,200 MLM-WA 34 4,800 28 4,400 Of the methods that were analyzed,the Froehlich method with a linear breach progression resulted in the greatest predicted peak outflow of 6,600 cfs with a time to peak of 26 minutes. This hydrograph was routed downstream using HEC-HMS through representative reaches using the Muskingum-Cunge method. The reach descriptions, routing times, and predicted flow rates are summarized below in Table 3. The HEC-HMS output is attached in Appendix C, and the full HEC-HMS model is included on the Project CD. The elevations shown in this section of the report are relative to the vertical datum used in the 1985 Hydro-Triad drawings and the 2010 Davis Engineering survey. FUCHS DAM - 2 - June 15,2012 INUNDATION MAPPING REPORT