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10.9 Subbasin Loss Rates Subbasin loss rates were determined using the Initial and Uniform Loss Rate method described in HBRPEG, 2008. As a simplified alternative to the Green and Ampt Loss Rate method normally used by the Dam Safety Branch, the selected method still performs a rigorous computation for a composite value of uniform loss (designated CNSTL or XKSAT in HBRPEG, 2008, and `Constant Rate' in HEC -HMS) and was judged as an acceptable method for this investigation. For the Initial and Uniform Loss Rate method, the value for CNSTL is taken as the Green and Ampt composite value of hydraulic conductivity, XKSAT, adjusted for vegetation cover for each subbasin. Therefore, soils information for each subbasin delineated in this investigation was downloaded from NRCS Soil Survey website to determine this variable. The data were then processed to develop rainfall loss parameters using GIS and the Dam Safety Branch rainfall losses spreadsheet using procedures outlined in HBRPEG, 2008. Details of the analysis are provided in Appendix D. A summary of the initial loss parameters for the base model is shown in Table 5. The initial values for the Uniform Rate parameter was also determined to be sensitive and relatively small changes resulted in significant changes in results during the calibration process. The values of Uniform Rate utilized are shown in Table 5. A more thorough discussion of development of these parameters and changes to them are contained in Appendices D and E. Table 5 — Model Subbasin Loss Parameters 10.10 Hydrologic Channel Routing Due to the configuration of the five BEMA dams, each one downstream of the one before, no "reaches" exists between the reservoirs and routing between them was not required. Channel reaches are required and were utilized from the Meadow Dam basin to the HWY 36 element along the West Fork. On the main stem of the Little Thompson a routing reach is required between the HWY 36 junction element and the Pinewood Springs [PWS] junction element, between the PWS junction and the Confluence junction element, and then a final routing reach is required between the Confluence and X Bar 7 outflow elements. These routing reaches were of sufficient length to warrant channel routing to accurately portray the time lag of runoff hydrographs from the subbasins, from element to element. Report of the September 2013 Little Thompson River Flooding COLORADO and Big Elk Meadows Dam Failures, June 2014 (Revised, Dec 2014) A&V Division of Water Resources Page 24 of 48 - ■ 9go rm:1ovilU41:1 Lower LT Sunset Rainbow Willow Mirror Meadow Muggins North Rowell Variable West Below Dam Dam Dam Dam Dam Gulch Fork Gulch Fork PWS Initial Loss 0.4 0.4 0.4 0.4 0.4 0.3 0.3 0.3 0.3 0.3 (in) Uniform Rate 0.194 0.206 0.212 0.164 0.204 0.272 0.215 0.214 0.267 0.226 (in /hr) Imperv. 6 3 17 13 13 9 9 11 12 14 10.10 Hydrologic Channel Routing Due to the configuration of the five BEMA dams, each one downstream of the one before, no "reaches" exists between the reservoirs and routing between them was not required. Channel reaches are required and were utilized from the Meadow Dam basin to the HWY 36 element along the West Fork. On the main stem of the Little Thompson a routing reach is required between the HWY 36 junction element and the Pinewood Springs [PWS] junction element, between the PWS junction and the Confluence junction element, and then a final routing reach is required between the Confluence and X Bar 7 outflow elements. These routing reaches were of sufficient length to warrant channel routing to accurately portray the time lag of runoff hydrographs from the subbasins, from element to element. Report of the September 2013 Little Thompson River Flooding COLORADO and Big Elk Meadows Dam Failures, June 2014 (Revised, Dec 2014) A&V Division of Water Resources Page 24 of 48 - ■ 9go rm:1ovilU41:1