Floodplain Information Report Volume 4 Colorado River Mainstem from Rifle to the Utah Stateline

Home Browse Search

0 k- I 0 5.0 INTERPRETATION OF THIS REPORT 5.1 Hydrologic Findings The results of the hydrologic analysis for the Colorado River mainstem are considered accurate. They are based principally on existing approved data prepared by federal agencies. The availability of detailed information for almost 70% of the study reach provided accurate relationships with the unstudied reach findings. 5.2 Hydraulic Findings The hydraulic methods used for this study are similar to those used in more detailed studies. They are much more detailed than the methods used in typical approximate studies. The 100 - year flood depths for the previously unstudied reaches range from 19.2 feet in a medium reach of the river to 26.5 feet near the downstream limit of the study (where the drainage area essentially doubles at the Gunnison River confluence). The majority of the flood depths are generally consistent throughout the previously unstudied portions of the study reach with differences of only a few feet (18.2 -26.5 feet for the 100 -year flow depths) . The variation between the 50 -year and 100 -year flood depths is generally less than one foot. To find flood depths for the reaches where there has already been a detailed study, interested parties are directed to the flood profiles in those detailed studies. 5.3 Floodplain Delineations The primary difficulty in presenting precise delineations of the 50 -year and 100 -year floodplains for the previously unstudied reaches was the limited quality of the available base mapping for the unstudied reaches of the Colorado 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 topographic information will be needed in areas of specific 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 consult the existing detailed studies, if they pertain to their site, to better ascertain the flood status of the site in question; 2) They can rely on the hydrologic determinations in this report and use other topographic information and hydraulic models; or 3) They can use the depth information in this report with more detailed topographic information to modify the floodplain delineation. 22