12000000063 Hyatt Dam Inundation Report

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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 Hyatt 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 the reservoir was assumed to drain completely. The assumption was made that the breach would extend from the dam crest (gage height 23.41) to the downstream toe (gage height 0.00), with breach side slopes of 0.7:1 (horizontal to vertical). 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 storage-elevation curve from the 1995 Hydrologic Analysis for Spillway Adequacy for Hyatt Lake by Gregory Hammer was used to model the reservoir storage. A copy of the storage-elevation curve 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. Hyatt Lake 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(mm) Froehlich 46 12,700 37 11,800 Von Thun and Gillette 53 11,100 43 10,200 USBR2 30 11,800 27 10,900 Of the methods that were analyzed,the Froehlich method with a linear breach progression resulted in the greatest predicted peak outflow of 12,700 cfs with a time to peak of 46 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. 2 During a phone conversation with Colorado Dam Safety Engineer Greg Hammer,it was decided that the 12 minute time of failure calculated by the USBR method was too rapid for a dam of this size and type.For this reason,the failure time was set to 30 minutes for the USBR simulation in HEC-HMS.A 12 minute failure time would have resulted in a peak outflow only slightly higher than the peak predicted by the Froehlich method. HYATT DAM - 2 - June 21,2012 INUNDATION MAPPING REPORT