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<br />(References 36, 37, and 38). The topographic mapping for the Eagle River was flown <br />over a 2-week period during the middle of October 1998 when the flow was <br />approximately 270 cfs to 300 cfs at a location downstream of Gypsum. This is a <br />relatively low flow for the Eagle River, and represents only 3.5 percent of the 100-year <br />flow of 8,030 cfs. The average depth of water in the channel during the mapping was <br />negligible. The model input channel geometry assumes negligible channel geometry <br />below the water surface at the time of the aerial mapping (Reference 36). <br /> <br />The topographic mapping for the. Colorado River was flown in October 1998 when the <br />flow was approximately 1,300 cfs to 1,400 cfs. Although this is.a relatively low flow for <br />the Colorado River, it represents approximately 5.8 percent of the 100-year flow. On <br />average, the depth of water in the channel during mapping was approximately 2 feet to <br />3 feet (Reference 36). <br /> <br />The 100- and 500-yearfloodplain boundaries are shown on the FIRM. On this map, the <br />100-year floodplain boundary corresponds to the boundary of the areas of special flood <br />hazards (Zones A and AE), and the 500-year floodplain boundary corresponds to the <br />boundary of areas of moderate flood hazards. In cases where the 100- and 500-year <br />floodplain boundaries are close together, only the 100-year floodplain boundary has been <br />shown. Small areas within the floodplain boundaries may lie above the flood elevations <br />but cannot be shown because of limitations of the map scale and/or lack of detailed <br />topographic data. <br /> <br />For the streams. studied by approximate methods, only the 100-year floodplain boundary <br />is shown on the FIRM. <br /> <br />Within the Town of Eagle, Eby Creek was studied by approximate methods and the <br />boundary of the 100-year flood was developed from normal depth calculations and <br />topographic maps at a scale of 1: 1 ,200, with a contour interval of 2 feet (Reference 39). <br /> <br />For the Town of Gypsum, approximate and shallow flooding boundaries were delineated <br />on these maps and on topographic maps at a scale of 1 :24,000 with a contour interval of <br />20 feet (Reference 33). <br /> <br />Within the Towns of Minturn and Red Cliff, for streams studied by approximate <br />methods, the boundary of the 100-year flood was.delineated using topographic maps at a <br />scale of 1 :2,400, with a contour interval of 5 feet (References 27 and 39). <br /> <br />For the Town of Vail, boundaries of the shallow flooding areas were delineated, using <br />field inspection and engineering judgment, on topographic maps at a scale of 1 :2,400 <br />with contour intervals of 2, 10, and 20 feet. (Reference 29). The Gore Creek floodplain <br />was manually redelineated based on Geographic Information 1 System data that were <br />supplied by the Town of Vail in November 2003. The supplied topographic coverage <br />had a contour interval of 2 feet. <br /> <br />Approximate 100-year floodplain boundaries in some portions of the study area were <br />taken directly from the Flood Hazard Boundary Map (FHBM) for Eagle County, <br />Colorado and other FHBMs for the incorporated communities within Eagle County, <br />where appropriate (Reference 22). <br /> <br />The SC determined that some areas shown on the FHBM are areas of minimal flooding; <br />therefore, they were not delineated on the maps. <br /> <br />28 <br />