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<br />MANUAL FOR ESTIMATING FLOOD CHARACTERISTICS <br />OF NATURAL-FLOW STREAMS IN COLORADO <br /> <br />By Jerald F. McCain and Robert D. Jarrett <br />U.S. Geological Survey <br /> <br />INTRODUCTION <br /> <br />This manual contains methods for obtaining flood inforrnation for the 10-, <br />50-, 100-, and 500-year floods at sites on natural-flo'", streams in Colorado. <br />This flood information should be USeful in land-use planning, flood insurance <br />studies, and other studies requiring flood-frequency information. The peak- <br />discharge information is appl icable to a wide variety of st~dies ranging from <br />rapid approximation of flood-prone areas to detailed flood-plain studies based <br />on precise hydraulic field measurements. Conversely, the flood-depth informa- <br />tion is applicable only for rapid approximation of flood-prane areas or as a <br />general guide to flood depths for comparative purposes. This limited usage of <br />the flood-depth information results from the difficulty in ~valuating field <br />conditions, such as channel width, and size of structures, such as bridges <br />and culverts, from existing topographic maps. <br /> <br />The 1 imitations of the methods are discussed first to caution the user of <br />their rang. of applicability. Next, the methods are described and a step-by- <br />step procedure for applying the methods is presented with several examples. <br />Procedures for transferring the depth information to topographic maps are also <br />described. Other sections contain a discussion of accuracy, analytical devel- <br />opment of the methods, and a list of flood data for gaging stations. <br /> <br />i. <br /> <br />Four previous reports have defined the flood frequency of streams in <br />Colorado. These previous reports covel' .the four major river basins in Colo- <br />rado as follows: South Platte River basin (Matthai, 1968), Arkansas River <br />basin (Patterson, 1964), Rio Grande basin (Patterson, 19651, and Colorado <br />River basin (Patterson and Somers, 1966). This report is based on 11 to 15 <br />years of additional data and includes many additional gaging-station records. <br />The flood-frequency analyses for the previous reports are based on the index- <br />flood method described by Dalrymple (1960).. This report uses the log-Pearson <br />Type I II method for fitting a freq~ency curve to gaging-station data and mul- <br />tiple-regression techniques for regional ization or tr'ansferring the results to <br />ungaged basins. More reliable estimates should result from use of information <br />in this report since more basic data and more generally accepted analytical <br />techniques were used. <br />