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The analysis provides timing of the peak discharges of the rainfall flooding, volume of the flows, <br />and timing and contributions of the dam failures on the flooding. The modeling estimated the <br />rainfall flood peak discharge that occurred at the X Bar 7 Ranch cross - section at approximately <br />5:30 am on September 12, 2013, at a rate of 15,997 cubic feet per second (cfs). The dam failures <br />increased flows in the river at the X Bar 7 Ranch cross section by a maximum of approximately <br />1,295 cfs and raised the water surface there by approximately 0.8 feet. The analysis showed the <br />dam failures occurred at approximately two hours (1), seven hours (2), eight hours (1) and 20 <br />hours (1) after the rainfall- induced peak discharge and that flows were generally receding at the <br />time of the failures. The maximum discharge resulting from flooding combined with the dam <br />failures were less than the previous flood -only peak discharge and the water surface did not <br />exceed what had been previously seen. <br />The sheer volume and erosive power of the river during and after the peak discharges lead to the <br />damages observed. Results of this study show Big Elk Meadows dam failure discharges did not <br />cause incremental damages in the downstream channel. <br />Possible causes of the surges and extensive damage experienced during September 12 and 13, <br />2013 and not satisfactorily captured in the modeling were discussed briefly. A more thorough <br />investigation of the entire reaches of the channels of all tributaries of the Little Thompson River <br />above the X Bar 7 Ranch community might reveal potential clues for conclusive determination of <br />the causes of those surges and damages witnessed. Additional research might include analysis of <br />streambank saturation and erosion, channel migration, and debris dam formation and subsequent <br />failure. <br />The September 2013 event is not the first extreme precipitation event to occur on the Little <br />Thompson River drainage. Rainfall data was acquired for the 1969 "Big Elk Meadows" storm, an <br />event of similar magnitude that occurred May 3 -9, 1969. The 1969 rainfall and resulting runoff <br />were modeled using the same procedures and model utilized for the September 2013 event. The <br />model results show a peak discharge of 3,800 cfs at the X Bar 7 Ranch cross section which is <br />consistent with a reported 3,900 cfs at the nearest Division of Water Resources stream gage <br />located approximately 6.4 miles downstream. <br />The difference in stream flooding response to similar magnitudes (depths) of rain between the <br />1969 and 2013 storms lies in the duration of the rainfall. The majority of the 2013 rainfall fell over <br />a period of 12 -18 hours whereas the 1969 rainfall fell steadily over a period of three days. The less <br />intense, longer duration storm allowed more infiltration and slower runoff of the rainfall to the <br />river channels, resulting in a lower peak discharge and less damaging flows. <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 />