Laserfiche WebLink
<br />the existing published flow values, the original flow values were <br />used' for this restudy. <br /> <br />In the U.S. Army Corps of Engineers Flood Hazard Information report <br />for the Uncompahgre River (Reference 5), frequency curves for <br />rainfall and snowmelt events at the Delta and Colona stream gages <br />were developed. The Delta gage is located approximately 20 miles <br />downstream of Olathe, and the Colana gage measures flow on the <br />Uncompahgre River approximately 20 miles upstream of Olathe. These <br />frequency curves We1:'e computed by statistical analyses of stream <br />gage data for 58 years of record at the Delta gage and 57 years of <br />record at the Colona gage. The independent frequency curves for <br />rainfall and snowmelt events were combined statistically to get all <br />event frequency curves at the gage locations. Since the two naIl <br />event" frequency curves were nearly identical at the two gage <br />locations, the peak discharges for the 10-, 50-, 100-, and 500-year <br />floods were computed to be constant between the gages. This <br />indicates that attenuation and stream diversions counterbalance <br />tributary inflow between the stations. <br /> <br />Peak discharge-drainage area relationships for the Gunnison and <br />Uncompahgre Rivers are shown in Table 1. <br /> <br />3.2 Hydraulic Analyses <br /> <br />Analyses of the hydraulic characteristics of flooding from the <br />sources studied were carried out to provide estimates of the <br />elevations of floods of the selected recurrence intervals. <br /> <br />Cross sections used in the backwater analyses for the Gunnison and <br />Uncompahgre Rivers were developed by the u.s. Army Corps of <br />Engineers (Reference 5). The below-water sections of all cross <br />sections were obtained by field measurement. All bridges, dams, <br />and culverts were field surveyed to obtain elevation data and <br />structural geometry. Several small private driveway bridges which <br />were not considered sturdy enough to withstand peak floods were <br />assumed to be washed out. The results obtained from the HEC-2 <br />computer program were adjusted in the vicini ty of bridges to more <br />accurately represent actual flooding conditions (Reference 10). <br /> <br />After the flood of 1984, the Gunnison River moved slightly (50 to <br />100 feet) to the north, toward the right bank. The channel and <br />overbanks at other cross sections appear similar to the 1976 <br />topography. The U.S. Highway 50 bridge was surveyed in 1988, and <br />an invert lower than the 1983 Flood Insurance Study profiles was <br />measured. New sections based on 1988 topography were used for the <br />Gunnison River between the mouth of the Uncompahgre River and the <br />U.S. Highway 50 bridge. Upstream of the U.S. Highway 50 bridge, <br />cross sections from a previously effective Flood Insurance Study <br />were used. <br /> <br />10 <br />