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21 completed as part of the failure investigation supports the eyewitness account that the dams overtopped during the storm event. Failure of the dams was primarily attributed to backward headcutting erosion during the overtopping process. Variations in elevation along the crests of both dams concentrated the overtopping flows in low spots, thereby accelerating the headcutting process. The duration and depth of overtopping sustained prior to failure is unknown. During the flood, several attempts were made to remove debris from the spillways at the LEV Dam. Emerald Valley Dams – Lessons Learned and Mitigation Measures x Non-uniform dam crest elevations promote concentrated flow in low spots during overtopping, and thus increase erosion due to headcutting. 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 Irregularities (depressions, oversteepened areas, animal burrows, etc.) and obstructions (pipes, debris, and large vegetation) along the upstream and/or downstream slopes generate concentrated and turbulent flows that increase erosion potential and headcutting. To minimize erosion and mitigate failure, it is ideal to have a uniform downstream slope. Locally steepened sections of embankments should be graded uniformly to provide a consistent dam cross section along the entire length of the embankment. x Multiple pipeline penetrations through the embankment with no filter zones result in backward erosion piping or failure due to pipe deterioration and defects. Debris within these pipes also prevents flood water from discharging, leading to overtopping of the dam. To mitigate failure, outlet works should be maintained so that they operate as intended. If trashracks are associated with these structures, they should be kept free of debris and possibly cleaned as flood events are approaching and/or occurring. x Vegetation and debris in the spillway prevent flood water from discharging fast enough to prevent overtopping of the dam. To limit dam overtopping and mitigate failure, the spillway should be maintained and kept free of vegetation and debris so that it can operate as designed. An open channel overflow spillway has less potential for clogging when compared to the “piped” spillways used at the LEV Dam. x Modifications and repairs that are not performed satisfactorily may contribute to dam failure. Modifications and repairs to existing small dams should be designed by engineers familiar with the state-of-practice for dam engineering and reviewed by appropriate regulatory agencies. Dams near Big Elk Meadows The Big Elk Meadows complex consists of five dams in series, including (listed from upstream to downstream): Sunset Lake Dam, Rainbow Lake Dam, Willow Lake Dam, Mirror Lake Dam, and Meadow Lake Dam. The dams are situated on the West Fork of the Little Thompson River within the Big Elk Meadows Subdivision, located approximately 5 miles upstream of US 36 along Larimer County Road 47. Sunset Lake Dam is located in northwestern Boulder County, whereas the other four dams are located in southwestern Larimer County. An aerial image of the five dams is shown in Photo 4. Photo 4: Aerial image of Big Elk Meadows dams. Meadow Lake Dam is classified as jurisdictional, low hazard. All other dams are classified as non- jurisdictional, low hazard. Since Meadow Lake Dam was classified as jurisdictional, it received regular inspections with the most recent inspection completed in 2008. Limited information was available for the other dams. Sunset Lake Dam Rainbow Lake Dam Willow Lake Dam Meadow Lake Dam Mirror Lake Dam