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<br />. <br /> <br />.) <br /> <br />. <br /> <br />DRAFT SOUTH PLA1TE RIVER TRIBUTARIES HYDROLOGY REPORT <br /> <br />FOUR MILE CREEK, HORSE CREEK, WEST CREEK, AND TROUT CREEK <br /> <br />2.0 Hydrologic Analysis <br /> <br />Streamflow gage data for the watersheds tributary to the South Platte River are not <br />available (USGS, 2004). Therefore, as part of the USGS assessment of the fire impacts <br />on the Hayman Area hydrology, pm-bUIll, post-bUIll, and long-term conditions were <br />evaluated using a rainfall-runoff watershed model. Based on this premise, the HEC-HMS <br />(version 2.2.2) computer model wal!i used in this study to complete the hydrologic <br />analysis for the four tributaries. <br /> <br />To be consistent with the hydrologic: methodology established by the USGS, the Soil <br />Conservation Services (SCS) hydrologic procedure within the HEC-HMS model was <br />selected for use in this analysis. Th(~ SCS dimensionless unit hydrograph was used to <br />compute the resultant flood hydro graphs for the tributaries. The SCS parameters of <br />runoff curve number (ReN), lag time, and initial abstraction were defined for each of the <br />individual sub-basins. <br /> <br />The USGS established a pre-burn local regional regression equation based on the analysis <br />of five stream gages near the Hayman Fire area. The USGS then used the regression <br />equation as the basis for calibrating their overall Hayman fire area HEC-HMS model for <br />the pre-bum, 100-year peak discharge.. The USGS then adjusted the calibrated model to <br />reflect the post-burn RCN values and lag times (USGS 2004). <br /> <br />The USGS provided the pre-bum and post-burn RCN values for the overall Hayman fire <br />area to Moser & Associates. Moser & Associates used the information to determine a <br />weighted composite RCN value speciHc to the individual sub-basins that were delineated <br />for the four tributary watersheds. Moser & Associates determined sub-basin RCN values <br />for post burn conditions. <br /> <br />In addition, for each individual sub-basin the SCS lag time and initial abstraction values <br />were calculated. The individual sub-basin SCS characteristics along with a drawing <br />depicting the sub-basin area, locations, and boundaries were then provided back to the <br />USGS for both the pre-burn and post-burn conditions. <br /> <br />The USGS used the information provided by Moser & Associates to determine the long- <br />term hydrologic characteristics of the individual tributary sub-basins as watershed <br />recovery and stabilizing hydrologic conditions are expected to develop over a period of <br />several years after a fire. <br /> <br />The USGS developed a method to determine the number of years over which recovery <br />from the fire would likely occur. In 1the South Platte River Basin, the USGS estimated <br />the long-tenn recovery to be six years. After a period of approximately six years, the <br />watershed recovery process results in only approximately a 5% reduction per year in the <br />10o-year flow values (USGS 2004). <br /> <br /> <br />Page 4 <br />