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2.2 Dam Breach Simulation in HEC-HMS <br /> The U.S. Army Corps of Engineers' (USACE)hydrologic modeling computer program HEC-HMS <br /> was used to simulate a clear-day piping failure of Highland No. 1 Dam using the breach parameters <br /> estimated by each of the empirical equations. As a conservative measure, the simulation which <br /> produced the greatest peak outflow was selected for use in the downstream hydraulic modeling and <br /> inundation mapping. <br /> For each HEC-HMS breach simulation the water surface was set at the spillway crest at the onset of <br /> the breach, and all water above the breach invert was assumed to drain. The assumption was made <br /> that the breach would extend from the dam crest (gage height 16.09) to a surveyed low spot near the <br /> downstream toe of the dam (gage height 10.28), with breach side slopes of 0.7:1 (horizontal to <br /> vertical). As recommended in the Guidelines for Dam Breach Analysis the piping failure was set to <br /> start at the midpoint of the final breach height. A storage-elevation table from the Division of Water <br /> Resources was used to model the reservoir storage. A copy of the storage-elevation table is attached <br /> in Appendix B. The time to peak and the peak outflow for each of the HEC-HMS simulations is <br /> summarized below in Table 2. <br /> Table 2. Highland Reservoir No. 1 Dam Failure <br /> Comparison of HEC-HMS Piping Simulation Results <br /> Empirical Method Time to Peak Peak Outflow <br /> (minutes) (cfs) <br /> Froehlich 125 850 <br /> Von Thun and Gillette 128 440 <br /> USBR 6* 150 <br /> *Time of failure predicted in empirical equation was less than 0.1 hours. <br /> Failure time was rounded up to 0.1 hours(6 min). <br /> Of the three methods that were analyzed, the Froehlich method resulted in the greatest predicted <br /> peak outflow of 850 cfs with a time to peak of 125 minutes. This hydrograph was routed <br /> downstream using HEC-HMS through representative reaches using the Muskingum-Cunge method. <br /> The reach descriptions,routing times, and predicted flow rates are summarized below in Table 3. <br /> The HEC-HMS output is attached in Appendix C, and the full HEC-HMS model is included on the <br /> Project CD. <br /> Table 3. Highland Reservoir No. 1 Dam Failure <br /> Summary of HEC-HMS Results Downstream of Highland No. 1 Dam <br /> Location Estimated Time from Start Estimated Peak <br /> of Breach Development Flow Rate(cfs) <br /> Peak outflow at breach 2 hr 5 min 850 <br /> 1-25 2 hr 19 min 840 <br /> Thomas Reservoir 2 hr 38 min 840 <br /> HIGHLAND No. 1 DAM - 2 - June 15,2012 <br /> INUNDATION MAPPING REPORT <br />