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EXECUTIVE SUMMARY <br />Historic rainfall along Colorado's Front Range from September 11- 18, 2013, caused devastating <br />flooding to developed areas along rivers and drainages. One of the heavily impacted drainages <br />was the Little Thompson River Drainage. As a result of the historic rainfall event and subsequent <br />flooding on the Little Thompson River, a series of five dams that serve as water supply for the <br />community of Big Elk Meadows were overtopped by the swollen river and failed. <br />The Colorado Division of Water Resources, Office of the State Engineer regulates all dams in <br />Colorado through the Dam Safety Branch. Dams are regulated, in part, by placing dams in <br />categories of hazard classification and size. The hazard classification is based on the consequences <br />of a sudden dam failure on life and property downstream. Hazard classifications of High, <br />Significant, Low and No Public Hazard are defined in the Rules and Regulations for Dam Safety and <br />Dam Construction (hereafter, the Rules). The size of a dam refers to the height of the dam, the <br />volume of water stored and the area of the reservoir impounded. The Rules define the size <br />categories of Non - Jurisdictional and Jurisdictional; large, small and minor. Of the failed Big Elk <br />Meadows dams, one was classified as a Jurisdictional, minor -sized Low hazard dam. The other <br />four failed dams were classified as Non - Jurisdictional sized Low hazard dams. <br />In the days and weeks following the September 2013 storm, members of the Dam Safety Branch <br />conducted field reconnaissance and site survey and data collection activities within the Big Elk <br />Meadows subdivision. During these site visits data were collected to confirm the dams had failed <br />and no longer posed any threat, and also to gather forensic data for future study of the dam <br />failures. Subsequent to those initial site visits, additional fieldwork was performed to gather <br />stream cross - section data at locations upstream and downstream of the dams to enable peak flow <br />(discharge) estimates. The collected field work, published flood studies, eyewitness interviews, <br />photographs and videos were assembled for a focused study to answer Dam Safety Branch <br />questions and in response to interests in the downstream community. Gage- adjusted radar rainfall <br />data focused on the study area from the September 8 to 18, 2013 storm was purchased. The <br />collected data were used in an HEC -HMS rainfall- runoff model for a study basin (including <br />subbasins) that extended from the headwaters of the Little Thompson downstream approximately <br />13.2 miles to the vicinity of the X Bar 7 Ranch communities. The modeling was performed to allow <br />estimates of peak runoff produced only by the rainfall by assuming the dams did not fail. A second <br />round of modeling was then performed to estimate the peak runoff from rainfall and include the <br />dam failure timing as reported by eyewitnesses. The purpose of this two -phase modeling effort <br />was to determine if the dam failures caused incremental damage on the downstream <br />environment; damage beyond that caused by the rainfall flooding. <br />The methods and procedures followed in this analysis meet the current industry standard and <br />used the best available data. Results of the field work and modeling were consistent and indicate <br />that the Big Elk Meadows dams failed from overtopping. <br />Report of the September 2013 Little Thompson River Flooding COLORADO <br />and Big Elk Meadows Dam Failures, June 2014 (Revised, Dec 2014) Division of Water Resources A&V <br />DAM SAFETY BRANCH <br />