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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 Foothills 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 the reservoir was assumed to drain completely. The assumption was made that the <br /> breach would extend from the dam crest(elevation 5216.35) to a surveyed low spot near the <br /> downstream toe of the dam (elevation 5165.43),1 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 an October 2001 as- <br /> built plan set by Smith Geotechnical Engineering Consultants was used to model the reservoir <br /> storage. A copy of the storage-elevation table is attached in Appendix B. The time to peak and the <br /> peak outflow for each of the HEC-HMS simulations is summarized below in Table 2. <br /> Table 2. Foothills Reservoir Dam Failure <br /> Comparison of HEC-HMS Piping Simulation Results <br /> Empirical Method Time to Peak Peak Outflow <br /> (minutes) (cfs) <br /> Froehlich 49 72,300 <br /> Von Thun and Gillette 54 69,400 <br /> USBR 28 92,200 <br /> MLM-WA 96 52,300 <br /> Of the four methods that were analyzed, the USBR method resulted in the greatest predicted peak <br /> outflow of 92,200 cfs with a time to peak of 28 minutes. This hydrograph was routed downstream <br /> using HEC-HMS through representative reaches using the Muskingum-Cunge method. The reach <br /> descriptions,routing times, and predicted flow rates are summarized below in Table 3. The HEC- <br /> HMS output is attached in Appendix C, and the full HEC-HMS model is included on the Project <br /> CD. <br /> The surveyed dam crest and downstream toe elevations were found by cross referencing surveyed gage heights <br /> with the storage-elevation table,which included both gage heights and elevations.The vertical datum of these <br /> elevations is unknown. <br /> FOOTHILLS DAM - 2 - June 15,2012 <br /> INUNDATION MAPPING REPORT <br />