12000000053 Highland N. 3 Dam Inundation Report

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to calculate failure time, but the average breach width was fixed at 29 feet. A comparison of the breach parameters calculated by each of the empirical equations for both breach scenarios is shown below in Table 1. Table 1. Highland Reservoir No. 3 Dam Failure Comparison of Dam Breach Parameters Empirical Method Average Breach Time of Failure Width (feet) (minutes) GROUND-LEVEL BREACH Froehlich 80 169 Von Thun and Gillette 36 83 USBR 20 4 BREACH INTO OUTLET CHANNEL Froehlich 29 95 Von Thun and Gillette 29 26 USBR 29 6 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 Highland No. 3 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 spillway crest at the onset of the breach, and all water above the breach invert was assumed to drain. For the ground-level failure alternative the assumption was made that the breach would extend from the dam crest (gage height 16.64) to a surveyed low spot near the downstream toe of the dam (gage height 9.55), with breach side slopes of 0.7:1 (horizontal to vertical). For the outlet structure failure alternative the breach was assumed to extend to the channel invert (gage height 0), with breach side slopes of 1:1. As recommended in the Guidelines for Dam Breach Analysis, the piping failure was set to start at the midpoint of the final breach height. A storage-elevation table from a 1951 survey by Guy S. Newkirk was used to model the reservoir storage. A copy of the storage-elevation 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. HIGHLAND No.3 DAM -2 - June 15,2012 INUNDATION MAPPING REPORT