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The discussion below for each of these parameters describes some of the details of the calibration <br />process. Associated tables provide the final parameters utilized in the modeling. Appendices D <br />and E provide more detail on the analyses and references utilized in development of these <br />parameters. <br />10.8 Unit Hydrographs <br />Unit hydrographs were developed for each of the drainage basins delineated. The Dam Safety <br />Branch has adopted use of the United States Bureau of Reclamation (USBR) synthetic unit <br />hydrograph procedure outlined in Cudworth (1989) as part of the process of transforming rainfall <br />to runoff. Input parameters were estimated assuming normal antecedent conditions using <br />guidance from the Hydrologic Basin Response Parameter Estimation Guidelines, (HBRPEG, 2008). <br />Physical parameters for each subbasin were estimated from GIS mapping and known <br />characteristics of the basins. Selection of the Rocky Mountain Thunderstorm dimensionless unit <br />hydrograph was based on the type of storm and rainfall depths that occurred in September 2013 <br />and the location of the basins. <br />Guidance for estimation of initial values for the lumped resistance parameter Kn was taken from <br />HBRPEG, 2008. For the storm and basins analyzed in this study, a typical mid -range value for K„ of <br />0.25 was used. During the subsequent calibration phase of the modeling the value of Kn was <br />reduced to 0.20. A summary of the physical basin parameters and initial selection of the unit <br />hydrograph values is presented below in Table 4. <br />Table 4 - Model Unit Hydrograph Parameters <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 23 of 48 - - <br />■ 9go rm:1ovilU41:1 <br />Lower <br />LT <br />Sunset <br />Rainbow <br />Willow <br />Mirror <br />Meadow <br />Muggins <br />North <br />Rowell <br />Variable <br />West <br />Below <br />Dam <br />Dam <br />Dam <br />Dam <br />Dam <br />Gulch <br />Fork <br />Gulch <br />Fork <br />PWS <br />Area, A <br />14.92 <br />0.41 <br />0.21 <br />0.07 <br />4.28 <br />17.79 <br />5.46 <br />7.55 <br />27.85 <br />4.00 <br />(sq. miles) <br />L (miles) <br />5.19 <br />1.54 <br />0.64 <br />0.27 <br />4.52 <br />8.72 <br />5.51 <br />7.96 <br />11.78 <br />3.64 <br />Lca (miles) <br />2.57 <br />0.88 <br />0.37 <br />0.25 <br />2.23 <br />4.44 <br />2.35 <br />4.95 <br />4.14 <br />0.51 <br />Slope, 5 <br />(ft /mile) <br />345.7 <br />669.5 <br />826.5 <br />1340.7 <br />375.2 <br />304.2 <br />470.1 <br />266.2 <br />247 <br />440 <br />Kn <br />0.2 <br />0.2 <br />0.2 <br />0.2 <br />0.2 <br />0.2 <br />0.2 <br />0.2 <br />0.2 <br />0.2 <br />Unit <br />Duration, <br />30 <br />15 <br />10 <br />5 <br />30 <br />60 <br />30 <br />60 <br />60 <br />15 <br />D (min) <br />Selected <br />Timestep <br />10 <br />10 <br />5 <br />2 <br />10 <br />20 <br />10 <br />20 <br />30 <br />10 <br />(min) <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 23 of 48 - - <br />■ 9go rm:1ovilU41:1 <br />