Floodplain Information Report Volume 3 Gunnison River from Delta to the Confluence with the Colorado River

■ 5.0 INTERPRETATION OF THIS REPORT 5.1 Hydrologic Findings The results of the hydrologic analysis for the Gunnison River are considered accurate. They are ' based in part on existing approved data from detailed studies at the upstream and downstream limits of the study reach. The existing data demonstrate a linear relationship of peak flow versus drainage area for the study reach. 5.2 Hydraulic Findings The hydraulic methods used for this study are the same used in more detailed studies and much more detailed than the typical approximate studies. The 100 -year flood depths range from 11.6 feet in a medium reach of the river to 17.3 feet near the downstream limit of the study. The flood depths are generally consistent throughout the study reach with differences of only a few feet (12 -17 feet for the 100 -year flow depths). ' 5.3 Floodplain Delineations The primary difficulty in presenting precise delineation of the 50 -year and 100 -year floodplains was the limited quality of the available base mapping for the lower Gunnison River. Using USGS topographic quadrangle maps at contour intervals of 20 or 40 feet to portray flood lines ' in unsurveyed areas provides limited information. The margin for interpretation is very great, especially in medium to wide reaches of the river where contours are widely spaced. More detailed techniques will be necessary in areas of interest. This study can be utilized to prepare more detailed floodplain delineations in such areas of specific interest. Those who are interested in greater detail have three choices for using this study: 1) They can refer to the detailed study information for the upstream stream reach near Delta; 2) They can rely on the hydrologic determinations and use other topographic information and hydraulic models; or 3) They can use the depth calculations with other topographic information from field surveys or other maps. 20