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small volume of Sunset Lake was quickly absorbed by the flood and had the least impact on on the <br />outflow hydrographs. <br />7,000 <br />6,000 <br />5,000 <br />4,000 <br />V <br />0 3,000 <br />2,000 <br />1,000 <br />0 <br />18:00 <br />OUTFLOW HYDROGRAPHS AT EACH DAM <br />INCLUDING DAM BREAHCES <br />Willow and Rainbow Failures <br />Sunset Failure <br />0000 0600 <br />11Sep2013 <br />Sunset Dam <br />— Rainbow Dam <br />— Willow Dam <br />— Mirror Dam <br />— Meadow Dam <br />Meadow Failure <br />Mirror Failure <br />1200 1800 0000 0600 1200 18:00 <br />12Sep2013 I 13Sep2013 <br />Figure 22 — Breach model outflow hydrographs at each dam including dam breaches (corresponds to model data <br />shown in Table 11. Plot of data is truncated to a 1.75 -day period to enhance resolution of the events.) <br />The hydrographs in Figure 22 showthat Willow, Rainbow and Meadow dams failed after more <br />than 12 hours of contiunuous overtopping and following a second period of intense rain. Figure 22 <br />shows the dam failures after a brief increase in stream flows just before noon on September 12. <br />Mirror Dam, the final dam in the series to fail, failed after another period of intense rain in the <br />early morning hours of September 13, as is shown by the third, smaller peak after midnight. The <br />model results show that although the peak runoff discharges from the drainage basins had already <br />occurred, the dams were still being overtopped at the time of failure, and flood flows were <br />receeding up to the time of failure. The model results show that inflows at the time of failure, <br />combined with the sudden increases in outflows due to the dams failing, did not produce outflows <br />reaching the magnitude of peak runoff discharges that had occurred priorto the the dams failing. <br />Figure 23 shows the hydrographs produced bythe dam breach model atthe calibration cross - <br />section locations previously described. The hydrograph plots showthat since the flows had <br />dropped well belowthe peaks at the time of the dam failures, the sudden increases in flows due to <br />the dam failure discharges did not approach the peak discharges that had already occurred. <br />Report of the September 2013 Little Thompson River Flooding COLORADO <br />and Big Elk Meadows Dam Failures, June 2014 (Revised, Dec 2014) a ntutsionof Water Resouxes <br />Page 32 of 48 - _ Ci�partrnclt of RUturaY Rr�a..r_ <br />DAM SAFETY BRANCH <br />