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<br />~ <br />~ <br />c <br />~ <br /> <br />flash flood (probability of occurrence is less than 1%). The <br />typical situation is a large thunderstorm cell or cells moving <br />slowly and affecting several adjoining basins simultaneously. <br />The impact on local Colorado River levels would be similar to the <br />isolated flash flood, however, this type of storm would result <br />in a slightly higher peak and continue for a longer duration. <br />Consequently, its effect would extend further downstream. <br /> <br />c <br /> <br />C. Regional Storm <br />This situation would be caused by a surge of moisture from <br />the remnants of a tropical storm. Tropical storms that could <br />possibly affect the Lower Colorado River basin are common, but <br />usually dissipate before reaching the area. However, the historic <br />record shows such storms can occur. A storm of this type occurred <br />in September of 1939 and produced 4 to 5 inches generally over the <br />Lower Colorado River basin. That storm seems to have generated <br />about 50,000 acre-feet over an area of 1800 square miles above <br />Parker Dam. This magnitude of runoff could occur anywhere in the <br />Lower Colorado River basin below Hoover Dam and produce an additional <br />10,000 to 15,000 cfs in the River for a period of 12 to 36 -hours. <br />The impact could be significant, depending upon the available <br />storage space in the reach. <br />The hazard from a regional storm can be diminished by main- <br />taining about 50,000 acre-feet of space in Lake Havasu. <br /> <br />D. General Storm in the Upper Basin <br />This storm is also of a tropical nature, with 2 to 3 inches <br />of precipitation widespread over the basin and a center as high as <br />8 to 10 inches. A storm of this nature occurred in September of <br />1927. This storm would have produced an inflow into Lake Powell <br />of 1.5 million acre-feet above the base flow, had Glenn Canyon Dam <br />been in place. If a storm of similar magnitude occurred this <br />year, inflow close to 2.25 million acre-feet would result. The <br />impact in the downstream areas would depend enterly upon available <br />storage in Lake Powell and Lake Mead. <br /> <br />Critical Flood Areas- <br />The critical flood areas have not changed as a result of high <br />water in the Colorado River. These include the Needles, California <br />area and the Bullhead City area, the Parker Strip, and the Yuma <br />area on the Arizona side which have been previously studied and <br />mapped as part of the FEMA Flood Insurance Study. Two new <br />factors adding to the local flooding hazard are the potential im- <br />poundment of floodwaters behind temporary protective dikes which <br />have been constructed to keep out Colorado River waters, and <br />the snagging and ramming of these temporary protective measures <br />by floating debris in the Colorado River itself. <br /> <br />Existing Flash Flood Proqram- <br />Duri~e past three years, the National Weather Service has <br />eS~k1Shed a series of flash flood systems and lor local raingage <br />,~etworks used to assist forecasters in the issuance of flash <br />"/ flood warnings. <br />Throughout Yuma and La Paz Counties in Arizona and eastern <br />Imperial and Riverside Counties in California is a 45 station <br />observer network. Each observer within the network has been <br /> <br />E-2 <br /> <br />. <br /> <br />~,,) <br />;~~ <br /> <br />. <br /> <br />t~~ <br />~'l <br />