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 Page 15 Upper and Lower Emerald Valley Dam Failure Report <br /> January 23, 2015 <br /> <br /> <br />Figure 18: Above Keeton Only Unit Hydrograph <br />7.2.2 Subbasin Loss Rates <br />Subbasin loss rates were determined using the Initial and Uniform Loss Rate <br />method described in HBRPEG, 2008. While taken as a simplified alternative to <br />the Green and Ampt Loss Rate method normally used by the Dam Safety <br />Branch, the selected method still performs a rigorous computation for a <br />composite value of uniform loss (designated CNSTL or XKSAT in HBRPEG, 2008, <br />and ‘Constant Rate’ in HEC-HMS) and was judged as an acceptable method for <br />this investigation. <br /> <br />For the Initial and Uniform Loss Rate method, the value for CNSTL is taken as <br />the Green and Ampt composite value of hydraulic conductivity, XKSAT, <br />adjusted for vegetation cover for each subbasin. Therefore, soils information <br />for each subbasin delineated in this investigation was downloaded from NRCS <br />Soil Survey website to determine this variable. A summary of the initial loss <br />parameters for the base model is shown in Table 4. The initial loss rate <br />parameter was also determined to be sensitive and relatively small changes <br />resulted in significant changes in results during the calibration process. <br /> <br />Table 4 – Model Subbasin Loss Parameters <br />Variable Above Dams <br />SB_1 <br />Above Dams <br />SB_2 <br />Above Keeton <br />Only <br />Initial Loss (in) 0.6 0.6 0.6 <br />Uniform Rate <br />(in/hr) 0.6 0.6 1.0 <br />% Imperv. 2 2 0 <br /> <br /> <br /> <br /> <br />