12000000053 Highland No. 2 Dams Inundation Report

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extend from the dam crest (gage height 36.67) to surveyed low spots near the downstream toes of each dam (gage height 10.95 for North Dam, gage height 24.48 for South Dam), 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 for North Dam, gage height 12.78 for South Dam), 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 are summarized below in Table 3 and Table 4 for the North and South Dams, respectively. Table 3. Highland Reservoir No. 2 North Dam Failure Comparison of HEC-HMS Piping Simulation Results Empirical Method Time to Peak Peak Outflow (minutes) (cfs) GROUND-LEVEL BREACH Froehlich 82 20,400 Von Thun and Gillette 80 19,700 USBR 13 16,200 BREACH INTO OUTLET CHANNEL Froehlich 64 19,100 Von Thun and Gillette 24 21,300 USBR 10 21,800 Table 4. Highland Reservoir No.2 South Dam Failure Comparison of HEC-HMS Piping Simulation Results Empirical Method Time to Peak Peak Outflow (minutes) (cfs) GROUND-LEVEL BREACH Froehlich 113 4.000 • Von Thun and Gillette 157 3,300 USBR 6" 1,200 BREACH INTO HIGHLAND DITCH Froehlich 86 9,500 Von Thun and Gillette 42 10,600 USBR 11 11.400 *Time of failure predicted in empirical equation was less than 0.1 hours. Failure time was rounded up to 0.1 hours(6 min). Of the six methods that were analyzed for the North Dam, the USBR method with a failure into the outlet channel resulted in the greatest predicted peak outflow of 21,800 cfs with a time to peak of 10 minutes. For the South Dam, the USBR method with a breach into Highland Ditch resulted in the greatest predicted peak outflow of 11,400 cfs with a time to peak of 11 minutes. These two hydrographs were 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 5 and Table 6 for the North and South Dams, respectively. The HEC- HMS output is attached in Appendix C and Appendix D for the North and South Dams, respectively. The full HEC-HMS models are included on the Project CD. HIGHLAND NO.2 NORTH AND SOUTH DAMS - 3 - June 15,2012 INUNDATION MAPPING REPORT