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<br />GLENWOOD SPRINGS FIS <br /> <br />greater uncertainty in debris flow analysis on alluvial fans than the more typical clear <br />water river hydraulic analysis. In addition, the current Flood Insurance Program does not <br />recognize the unique hazard which debris flows pose. As a result, the community of <br />Glenwood Springs and the State of Colorado were reluctant to adopt the alluvial fan <br />floodplains developed for this study. As a result FEMA requested they be dropped from <br />the current effort. The USACE was instructed to: (a) remove the alluvial fan flood plains, <br />and associated zoning, (b) add the old HEC-2 model cross sections from the Roaring Fork <br />River, from the previous study, into the current model to ensure the river profiles for the <br />studies matched, and (c) revise the encroachment model to eliminate negative surcharges <br />in the floodway. <br /> <br />B. Hydrology <br />The Colorado and Roaring Fork River hydrology was based on a hydrology study <br />performed by the Corps of Engineers in 1977. The peak flows for the Roaring Fork and <br />Colorado Rivers upstream and downstream of the confluence were derived from flow <br />frequency relationships. The Roaring Fork River peak discharges for various return <br />periods are presented in Table 3.1 along with the concurrent flow in the Colorado River. <br />The Colorado River peak discharges for various return periods are presented in Table 3.2 <br />along with the concurrent flow in the Roaring Fork River. Concurrent flow is the <br />expected flow from a source other than the one discharging a given return frequency. <br /> <br />Table 3.1: Peak Flow on Roaring Fork River with Concurrent Flows on the Colorado River. <br /> <br /> Return Period <br />Location JOyr 50yr 100yr 500yr <br /> (cfs) (cfs) (cfs) (cfs) <br />Roaring Fork @ G. Springs (Peak) 13,500 19,000 21,200 28,000 <br />Colorado Abv. Roaring Fork (Cone) 17,000 23,000 25,000 31,000 <br />Colorado Blw. Roaring Fork 30,500 42,000 46,200 59,000 <br /> <br /> <br />Table 3.2: Peak Flow on the Colorado River with Concurrent Flows on the Roaring Fork Riyer. <br /> <br /> Return Period <br />Location 10yr 50yr 100yr 500yr <br /> (cfs) (cfs) (cfs) (cfs) <br />Colorado Abv. Roaring Fork (Peak) 21,500 29,000 32,500 41,000 <br />Roaring Fork @G. Springs (Cone) JO,300 13,600 15,000 18,000 <br />Colorado Blw. Roaring Fork 31,800 42,600 47,500 59,000 <br /> 2 <br />