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of the dam crest, and continues until the erosion progresses to the upstream toe of the dam <br />allowing the reservoir to fully drain through the resulting opening. Since the BEMA dams survived <br />extended periods of overtopping it was felt they failed relatively slowly. Initial model runs utilized <br />a breach development time of 1 hour. This resulted in lowering the peak discharges. Based on <br />review of the previously noted videos, the failure development time for all dam failures was <br />reduced to 30 minutes and utilized in the final modeling. The reduced breach development time <br />resulted in higher peak discharges. Table 7 presents a summary of the overtopping failure breach <br />parameters used in the modeling. <br />Table 7 — Model Overtopping Breach Parameters <br />HEC -HMS <br />SUNSET <br />RAINBOW <br />WILLOW <br />MIRROR <br />MEADOW <br />Variable <br />DAM <br />DAM <br />DAM <br />DAM <br />DAM <br />Top Elevation <br />7530.1 <br />7518.9 <br />7515.0 <br />7499.4 <br />7498.0 <br />(ft) <br />Bottom <br />7520.1 <br />7506.2 <br />7499.5 <br />7489.4 <br />7481.4 <br />Elevation (ft) <br />Bottom Width (ft) <br />25 <br />29 <br />18 <br />16 <br />20 <br />Avg Side Slope <br />(xH:1V) <br />1 <br />0.9 <br />0.8 <br />1 <br />1 <br />Development <br />0.5 <br />0.5 <br />0.5 <br />0.5 <br />0.5 <br />Time (HR) <br />Trigger Method <br />Time <br />Time <br />Time <br />Time <br />Time <br />Trigger Time <br />12Sep2013 <br />12Sep2013 <br />12Sep2013 <br />13Sep2013 <br />12Sep2013 <br />(DDMMMYYYY <br />HH:MM) (MDT) <br />07:41 <br />12:24 <br />12:02 <br />02:00 <br />13:40 <br />Progression <br />Sine Wave <br />Sine Wave <br />Sine Wave <br />Sine Wave <br />Sine Wave <br />Method <br />Triggering the initiation of failure can be accomplished by three methods in HEC -HMS: elevation, <br />duration at elevation, and a specific time. Through the investigation process we were able to <br />obtain eyewitness timelines of the failures, as have been previously described. Those trigger times <br />of failures were used in the modeling and are also shown in Table 7. <br />10.12 Model Calibration Cross Sections <br />As was previously discussed, numerous cross - sections were selected at various locations upstream <br />and downstream of the dams and on selected tributaries at places where indirect measurements <br />could be made and peak discharge estimated. This study also utilized information on indirect <br />measurements and discharge estimates provided by others as a means to check and validate the <br />results (NRCS, 2013). <br />Dam Safety Branch collected survey data were reduced and input into either HEC -RAS or <br />FlowMaster software to estimate peaks flows at the surveyed cross sections. Peak flow estimates <br />were determined assuming flow occurred at either critical depth or normal depth depending on <br />the location of the cross section. For large flood events and relatively long reaches of stream, flow <br />Report of the September 2013 Little Thompson River Flooding COLORADO <br />and Big Elk Meadows Dam Failures, June 2014 (Revised, Dec 2014) A&V Division of Water Resources <br />Page 26 of 48 - - <br />■ 9go rm:1OvilH41:1 <br />