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I I I I I I I I I I I I I I I I I I , I Per FEMA's designation of a Iimited,detaiI study only the 100-year flood profile and the IOO-year floodplain boundaries need to be analyzed. The Town of Gypsum at the downstream end of the study reach and the Town of Eagle at the upstream end are incorporated areas which were not included in this Task Order. However, the limited detail study of this reach will include the 100-year water surface profile through both towns, A USGS stream gaging station is located just below the Town of Gypsum. Background Information The current floor hazard information for communities along the Roaring Fork and Eagle Rivers is based on hydrologic studies completed 15 to 20 years ago. Therefore, it was decided in the T&C meeting to review previous hydrologic studies, conduct frequency analyses using the additional hydrologic data now available, and revise the peak discharges in the specific study reaches, if deemed necessary. The adopted peak discharges will be used to perform the hydraulic analyses of both stream reaches. New aerial mapping and cross section data provided by the Communities will be used in the hydraulic analyses to reflect physical changes that have occurred in the study areas as a result of new construction. This hydrologic study involves review of previous studies and a new statistical evaluation of peak flow data for the two USGS gaging stations on the main stem Roaring Fork River (Aspen and GIenwood Springs) and one station on the Eagle River below Gypsum. These stream gages are the ones that are most pertinent to the reaches covered under this study. The study includes a recommendation about whether current flood frequency studies are adequate and what discharge values should be used in the hydraulic studies for the specific reaches included in this Task Order. II. PREVIOUS HYDROLOGIC STUDIES Roaring Fork River The Sacramento District ofthe U.S. Army Corps of Engineers (COE) performed a study that was documented in "Internal Memorandum, Basalt, Colorado, Hydrology", August 1976. The COE study has been widely used in the Roaring Fork Basin and was used in the "Floodplain Information Report, Roaring Fork and Fryingpan Rivers, Eagle County, Colorado" prepared by Wright- McLaughlin Engineers in February 1978. These same peak discharges were used by the Department of Transportation for design of Colorado State Highway 82 bridges near Basalt. The "Eagle County Unincorporated Areas Flood Insurance Study" (January 1979) indicates that Gingery and Associates performed hydrologic and hydraulic analyses for that report. The streamflow data were again separated into snowmelt and rainfall events. A total of IS stream gages located in Eagle County were analyzed. The length of streamflow records varied from 6 to 43 years. They found that snowmelt peaks dominated for larger basins and cloudbursts produce the peaks in smaller basins, generally under 20 square miles. They used results form COE studies for reaches near Basalt. 2