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I I I I I I I I I I I I I I I I I I I Ill. HYDROLOGY With the exception of the O.9-mile reach of the South Platte River at the community of South Platte. the hydrologic information for this study was obtained from the report. "Analysis of Post-Fire Hydrologic Hazards for the 2002 Ha)man. Coal Seam and Missionary Ridge Wildfires, Colorado" [USGS. 2004]. The discharge profile used for the current hydraulic analysis for both the main detailed study reach and the approximate study reach \',:as prepared by correlating peak flows at specific nodes within the USGS hydrologic model to physical locations along the South Platte River. The dO\\TIstream limit of the area included in the USGS hydrologic model for the Hayman Bum area is located near the community of Nighthawk. Consequently. the USGS report did not provide any specitic hydrologic information for the South Platte River near the community of South Plane. which is located approximately 4 miles dO\\TIstream of Nighthawk at the confluence with the North Fork South Platte River.just upstream of Platte Canyon. The peak flows used for the hydraulic analysis of this lower reach were developed by analyzing historical gage data both on the South Platte River downstream of South Plane. and on the North Fork South Platte River which confluences \\ith the South Platte Ri,"er at the community of South Platte. This analysis assumed that the timing of the lOO-year (I-percent annual chance) peak discharges from the various sources in the study area would be nearly concurrent. thereby allowing for direct addition of the peak flows. Annual peak flows were obtained from the USGS for gaging station #06707000 (North Fork South Platte River at South Platte. CO) and gaging station #06707500 (South Platte River at South Platte. CO). Peak flow frequency analyses were conducted with data for the period of record at each of the two gages based on procedures outlined in Bulletin No. l7B [U.S, Water Resources Council. 1981 J using the USGS computer program PEAKFQ. The 100-year discharge for the North Fork was added directly to the IOO-year discharge determined by the USGS for the Hayman Burn area near Nighthawk. In order to estimate the 1 OO~year discharge from the drainage area tributaf}" to the South Platte Ri,'er between ;'\ighthawk and South Platte. the intervening drainage area was assumed to contribute runoff during the 1 DO-year e"ent at the unit runoff rate represented by the estimated 100-year discharge at the South Platte River gage. The resulting 100-year discharge for the portion of the South Plane Ri\"CT watershed dO\\TIstream of Nighthawk was added directly to the 100-year peak flows from both the North Fork and the Hayman Burn area. The resulting 100~ year discharge profile used in the hydraulic model is summarized in Table 3.1. Documentation of the hydrologic evaluation completed for this study is provided in Appendix B. COCWCB03_$QlIUt Plane_repondoc 6 A,dmso'\ eo'Slhi'G E'GhHR">. I,c.