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I I I I I I I I I I I I I I I I I I I I. I:\TRODUCTlOI' 1.1 Background In the sununer of 2002, severe drought conditions in the western United States contributed to an unusually large number of \\ildfires in Colorado. The Hayman Fire burned approximately 138.000 acres (216 sq. mi.) in the South Plane Ri\"cr Basin [USGS. 2004], or approximately 10 percent of the basin upstream of the contluence \\ith the North Fork South Platte River. near the community of South Plane. The fire was located in the southwestern comer of Douglas County. and included portions of Jefferson. Park. Teller and El Paso Counties. Due to both the temporary and relatively long-term increase in runoff from wildfire bum areas, the State of Colorado (through the Colorado Water Conservation Board) and Douglas County sought to prepare a new floodplain map for a portion of the South Platte River dO\\nstream of Cheesman Reservoir. upstream of the confluence of the North Fork South Platte River. A vicinity map of the current study area is provided in Figure 1.1. Specifically. the overall reach associated \\ith this study includes approximately 19.6 miles along the South Platte Ri"cr between the communities of South Platte and Wigwam. Within this 19,6.mile reach. hydraulic analyses were conducted for the foIlO\\ing: (a) a 0,9.mile reach in the vicinit:, of South Platte (at the contluence with the North Fork South Platte River); (b) a 13..t-mile reach from ~ighthawk to just below Wigwam: and (c) a 1.7-mile reach through the Wigwam area. A 3.6-mile reach of the river between South Platte and Nighthawk was not analyzed as part of this study. 1.2 Pre,'ious Studies The most pertinent previous study associated \\ith this area is the recently completed. "Analysis of Post-Fire Hydrologic Hazards for the 2002 Hayman. Coal Seam. and ~1issionary Ridge Wildfires. Colorado" [USGS, 200--l]. This report documents detailed hydrologic analyses associated ,\ith post~\\ildtire conditions for the Ha:man Burn area. as well as two other burn areas in Colorado. Of specific interest to the current study. the USGS report pro,'ides post- wildfire long-tenn (6-year) recovery peak tlows for the 100-year. 6-hour rainfall peak on the basins surrounding the South Platte Ri,'er \\ithin the areas affected by the Hayman \\ildfire in 2002, These long-tenn reco"ery 1 OO-year flows were used as the basis for the hydraulic analyses and floodplain mapping completed for the current study. Also of interest to the current study is the Douglas County Flood Insurance Study [FE~tA. 1996] which documents the delineation of a 1.5-mile reach of detailed 100-year COC\\CB03_South P]3n~_n:port do.; A\dm~... eo\sdThG bGhHR5., he.