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23 Photo 8: Willow Lake Dam post-failure looking downstream near left abutment. Photo 9: Mirror Lake Dam post-failure looking downstream from left abutment. Photo 10: Meadow Lake Dam post-failure looking downstream from within reservoir. At the time of failure of Willow Lake Dam, Rainbow Lake Dam, Mirror Lake Dam, and Meadow Lake Dam, flood flows were receding, but the dams were still being overtopped. Overtopping durations generally ranged from 7 to 13 hours, with estimated depths up to about 3 feet. The table below summarizes the approximate overtopping durations and depths for each of the dams. The increases in flows due to the dam failure discharges never approached the peak discharges that had already occurred. Table 1: Summary of Overtopping Duration and Depth Dam Overtopping Duration (hours) Surveyed Overtopping Depth1 (ft) Sunset Lake Dam 7 1.11 Rainbow Lake Dam 12 2.65 (3) Willow Lake Dam 12 0.24 Mirror Lake Dam Unknown 3.42 (2) Meadow Lake Dam 13 1.90 Note: (1) Value in parenthesis is maximum depth reported by eyewitnesses. The September 2013 storm was not the first extreme flood event on the Little Thompson River watershed. The “Big Elk Meadows Storm” occurred in May of 1969, establishing the highest recorded peak discharge on the Little Thompson River. There were no reports of dam failures during the 1969 storm. While both the 1969 and 2013 storms had similar magnitudes of rainfall, the damaging flows of the 2013 storm are attributed to the duration of the rainfall. In the 1969 storm, rain fell steadily over a 3-day period, whereas in the 2013 storm the majority of rain fell during a 12- to 18-hour period. The Little Thompson River could not sustain the rapid rate of rainfall runoff due to the more intense, shorter duration 2013 storm, thus resulting in a higher rainfall-induced peak discharge and more damaging flows. Big Elk Meadows Dams – Lessons Learned and Mitigation Measures x Concentrated flows develop at low spots along a dam crest during overtopping, which increases erosion. Both Willow Lake Dam and Meadow Lake dam failed near the left abutment, where their crest elevations were lower. To minimize overtopping erosion and mitigate dam failure, areas where the dam crest is uneven or falls below the design freeboard should be filled and repaired. x Well maintained, low hazard embankment dams with uniform crest elevations and good vegetation cover (Photo 6) can survive overtopping durations in excess of 7 hours with depths up to 3 feet. Dam breach (i.e., dam failure) does not always occur immediately following dam overtopping. It is commonly assumed that dam overtopping is