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. . . . . . . . I. . . I. i. . . . i. . . . . . . . . I. . . I. . . . . . . . . I. . . . . I. . i. . . '. . . . I. . . T bl 4 P k tOO D" h Ii SI edUSGSG" S a e " ea -year lSC ar2e or e ect al!ln2 tatlons USGS tOO-year Station Station Discharge ID Name (cfs) 09065100 Cross Creek Near Minturn, Co. 859 09067000 Beaver Creek At Avon, Co. 285 09070500 Colorado River Near Dotsero, Co. 23,570 There are several reservoirs on the Colorado River which provide some incidental flood control. Because it is likely that spillway facilities have been designed to evacuate flood flows in excess of the 100-year discharge, it has been assumed that reservoir will not affect downstream 100-year flood discharge. In actuality, there will be some flood flow attenuation due to surcharge capacity in the reservoir and travel times from the reservoir inflow points to the spillway. Regardless, the gage records analyzed above implicitly consider reservoir storage in a general matter. Additional analysis of the reservoirs and basin would be needed to more accurately estimate peak discharges. Communities should regulate the approximate floodplains on a case-by-case basis. FEMA requires that a detailed floodplain analysis must be performed by a professional engineer and submitted by the developer or development proponent for development projects involving more 5 acres or more than 50 units (whichever is more restrictive). Hvdraulic Analvsis Hydraulic methods used for the approximate floodplain delineations shown on the Eagle County FIRM panels are not available or have not been documented by FEMA. For future studies, an approximate 100-year flood elevation at a particular location on the river may be obtained by using the loo-year peak flow value (as provided in this report) in conjunction with an acceptable hydraulic analysis. Hydraulic analyses for determining loo-year water surface elevations for approximate floodplains may be performed using FEMA's Quick-2 computer program, the Corps of Engineers' HEC-2 computer program, F1owmaster, or other acceptable hydraulic methods. Channel and floodplain cross-sections may be obtained from detailed topographic mapping or from field surveys. Cross-sections obtained from USGS quadrangle maps are not advised. Floodplain Mappinl! The existing FIRMs for Eagle County and Incorporated Areas depict areas subject to 100-year flooding based on approximate technical methods. The map does not show flood profiles, base flood elevations, or sufficient detail in order for county and municipal officials, engineers, developers, and land owners to make floodplain determinations with any certainty. The Colorado Water Conservation Board can be contacted to request technical assistance for approximate floodplain analyses as needed. 9