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The high - gradient mountain streams being modeled in the reach sections described above are <br />most suited for the Kinematic Wave method in HEC -HMS. A summary of the hydrologic routing <br />parameters selected for the base model is shown in Table 6. Note that channel length, slope, and <br />shape are fixed parameters, but Manning's n roughness was adjusted during the calibration phase. <br />A value of 0.05 was selected for the final modeling. <br />Table 6 — Model Channel Routing Parameters <br />Kinematic Wave <br />Variable <br />Channel Reach <br />West Fork <br />Little <br />Thompson <br />LT Above <br />PWS <br />LT Below <br />PMS <br />X Bar 7 <br />Length (ft) <br />21278 <br />8184 <br />27826 <br />13094 <br />Length (mi) <br />4.03 <br />1.55 <br />5.27 <br />2.48 <br />Slope (ft /ft) <br />0.03 <br />0.03 <br />0.03 <br />0.012 <br />Manning's n <br />0.05 <br />0.05 <br />0.05 <br />0.05 <br />Subreaches <br />2 <br />2 <br />2 <br />2 <br />Shape <br />Trapezoidal <br />Trapezoidal <br />Trapezoidal <br />Trapezoidal <br />Bottom Width (ft) <br />45 <br />45 <br />45 <br />50 <br />Side Slope (xH:1V) <br />2 <br />2 <br />2 <br />3 <br />10.11 Overtopping Failure Breach Parameters <br />The critical elements of an overtopping breach analysis are the breach bottom width, breach <br />shape, and time of failure. Time of failure is also described as Breach Development time. <br />Determining the value of these parameters typically requires comparing similar case histories or <br />using empirical methods published in literature. The ultimate breach size and shape (side slopes) <br />were measured directly during the September 24 and 25, 2013 field surveys. The survey <br />information is contained in Appendix B and shows the configuration of the breaches <br />approximately 12 days after the dam breaching. To accurately model peak breach discharges, <br />breach side slopes and bottom widths determined from that final embankment condition, <br />immediately after the reservoir has been drained, are ideally utilized. <br />Initial model runs used the bottom widths and side slopes surveyed during the September 24 -25 <br />site visits. Subsequent review of failure videos for Meadow Dam revealed that the actual breach <br />width of that dam immediately after failure was relatively narrow. It was felt that the much wider <br />widths measured in the field were the result of additional erosion of those embankments as the <br />remainder of the storm flows passed through them continuously for 12 days after breaching. The <br />final breach bottom widths were therefore adjusted based on that knowledge and validated using <br />Dam Safety Branch spreadsheets for determining dam breach parameters using the methods of <br />MacDonald Langrange- Monopolis and Froelich (DSB, 2010). <br />Determination of the breach development time was also challenging. Breach development time is <br />the total time for the breach to form and the reservoir to drain through that breach. The time <br />starts when the erosion of the embankment progresses to the point of meeting the upstream edge <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 25 of 48 - <br />■ 9go rm:1OvilU41:1 <br />