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Last modified
11/23/2009 12:50:35 PM
Creation date
5/15/2007 10:43:00 AM
Metadata
Fields
Template:
Floodplain Documents
County
Statewide
Stream Name
Colorado River
Title
Colorado River Basin Probable Maximum Floods, Hoover and Glen Canyon Dams
Date
9/1/1990
Prepared By
US Department of the Interior Bureau of Reclaimation
Floodplain - Doc Type
Floodplain Report/Masterplan
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<br /> <br />130,000 <br />160,000 <br />190,000 <br />230,000 <br />260,000 <br />320,000 <br />360,000 <br />450,000 <br /> <br />5 <br />10 <br />20 <br />50 <br />100 <br />500 <br />1,000 <br />10,000 <br /> <br />20.00 <br />10.00 <br />5.00 <br />2.00 <br />1. 00 <br />0.20 <br />0.10 <br />0.01 <br /> <br /> <br />control space for the dam. Flood frequency analyses were also <br />completed for the dam site. The following paragraphs contain <br />information that was presented in Debler's report. <br /> <br />Prior to construction of Hoover Dam, high flows along the lower <br />Colorado River occurred annually. The 1884 flood had a peak flow <br />between 250,000 and 300,000 ft3/s, as estimated by the Geological <br />Survey and Bureau of Reclamation, respectively. These estimates <br />were based on gage heights at Grand Junction and Yuma, a flood <br />observation at Lees Ferry, newspaper accounts, and high water marks <br />in Black Canyon. Between 1878 and 1929, peak flows were estimated <br />to exceed 100,000 ft3/s 23 times and 200,000 ft3/s three times (1884, <br />1920, and 1921) at Black Canyon. <br /> <br />Newspapers of 1884 contain numerous references to heavy snows and <br />the late arrival of spring, but only one precipitation station was <br />available in the upper basin. The station was located in the San <br />Juan basin at Fort Lewis and showed precipitation 40 percent above <br />normal from October to May, with temperatures below normal in the <br />spring. <br /> <br />Flood waters of the Colorado River appeared in Salton Sink in 1828, <br />1840, 1849, 1852, and 1867: however, this fact does not necessarily <br />indicate floods greater than 1884. Some evidence indicates the 1867 <br />flood was probably a high flood, but may not be larger than the <br />210,000 ft3/s which occurred in 1921. Changes in stream bed <br />elevation greatly affect gage heights recorded for some of these <br />event s . <br /> <br />Debler considered the 1884 event a "near maximum flood." He <br />indicated that with 9,500,000 acre-feet of flood control capacity, <br />reservoir outflow could be limited to 75,000 ft3/s. The spillways <br />were each sized to pass 200,000 ft3/s, for a total capacity of <br />400,000 ft3/s. The outlet works were sized to pass 100,000 ft3/s, <br />and the flood control space was set at 9,500,000 acre-feet. <br /> <br />The 1884 flood was determined to be about a 500-year flood. <br />Table 1.3 displays the flood frequency analysis conducted as <br />a part of Debler's study. <br /> <br />Table 1.3.--1930 flood frequency analysis for Hoover Dam <br /> <br />Peak flow <br />(ft 3/ s) <br /> <br />Frequency <br />(year) <br /> <br />Probability <br />(percent) <br /> <br />9 <br />
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