12000000053 McIntosh 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 McIntosh 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 (elevation 5079.07) to a surveyed low spot near the downstream toe of the dam (elevation 5061.00),2 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. A storage-elevation table from the 1995 Hydrology Report for McIntosh Lake Dam by JR Engineering 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. Table 2. McIntosh Reservoir Dam Failure Comparison of HEC-HMS Piping Simulation Results Empirical Method Time to Peak Peak Outflow (minutes) (cfs) Froehlich 109 12,200 Von Thun and Gillette 102 10,900 USBR 8 5,800 Of the three methods that were analyzed, the Froehlich method resulted in the greatest predicted peak outflow of 12,200 cfs with a time to peak of 109 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. Table 3. McIntosh Reservoir Dam Failure Summary of HEC-HMS Results Downstream of McIntosh Dam Location Estimated Time from Start of Estimated Peak Flow Breach Development Rate(cfs) Peak outflow at breach 1 hr 49 min 12,200 Airport Rd. 2 hr 0 min 11,900 Hover St. 3 hr 17 min 10,900 Hwy 287(Main St.) 4 hr 29 min 10,100 Hwy 119 in Boulder Co. 5 hr 34 min 9,300 Hwy 119 in Weld Co. 9 hr 33 min 6,200 1-25 12 hr 57 min 4,800 2 Elevations are in NAVD 88. MCINTOSH DAM - 2 - June 15,2012 INUNDATION MAPPING REPORT