Laserfiche WebLink
<br />The computational methods and techniques are generally accepted for <br />hydrologic analyses and produced results considered reasonable for <br />Mesa County. Unexpected results were not encountered in carrying <br />out the hydrologic analyses for this Flood Insurance Study. <br /> <br />Peak flows in areas studied by approximate methods were determined <br />from lOO-year peak flow envelope curves. These curves were <br />developed by plotting the largest recorded peak flows in the region <br />for both rain and snowmelt flood events against drainage area. <br /> <br />Peak discharge-drainage area relationships for the streams studied <br />by detailed methods are shown in Table 3. <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 section data for the backwater analyses of the Colorado River <br />detailed study reach were obtained from topographic maps compiled <br />from aerial photographs. The photogrammetric subcontractor, Air <br />Photo Survey, Inc., flew the study area on December 8, 1988, during <br />low flow conditions (Reference 17). Cross sections were digi tized <br />from 1:4,800 scale maps with 2-foot contour intervals. <br />Approximately 20 cross sections from previous studies were used, <br />after adjustments based on new maps. <br /> <br />Field surveys were conducted to obtain soundings in the Colorado <br />River that defined the channel geometry below the water line. A <br />total of eleven sections, or one section approximately every 3 <br />miles, were taken. <br /> <br />Based on these 11 sections below the water line, the channel bottom <br />along the study reach was defined and the channel geometry on the <br />digitized sections were modified accordingly. <br /> <br />Information on Colorado River bridge geometry and elevation was <br />obtained from the Colorado Department of Highways, Mesa County <br />records, and field surveys. <br /> <br />Cross sections used in the backwater analyses for the downstream <br />portion of Little Salt Wash were obtained from the 1976 COE Flood <br />Hazard Informat ion Report (Reference 15). The remainder of the <br />cross sections for Little Salt Wash and all of Big Salt Wash were <br />digitized from USBR topographic maps at a scale of 1:4,800 with a <br />2-foot contour interval (Reference 18). <br /> <br />Cross sections for the backwater analyses of Leach Creek were <br />obtained by field survey and by use of 2-foot contour orthophoto <br />maps at a scale of 1:2,400 (References 19). All bridges, dams, and <br />culverts were field surveyed to obtain elevation data and <br />structural geometry. <br /> <br />22 <br />