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<br />13 <br /> <br /> <br />l1'J,r"~,,n /-"7 <br />~;; 1_'1 ~_. -J '4 <br /> <br />. <br /> <br />9. RUNOFF AND FLOOD CHARACTERISTICS <br /> <br />a. Sand Creek basin. Floods in the Sand Creek basin are <br />caused by intense short-duration thunderstorms covering limited <br />areas. These storms are caused by unstable frontal .conditions <br />which produce high-intensity short-duration rainfall. Storm <br />centers rarely exceed 10 square miles in area. A number of <br />storm centers may occur in the basin during one storm period. <br />Floods are of a flash-flood nature. Most floods last for only <br />a few hours. The more severe floods clear the basin within 12 <br />hours. Historically, flood-producing storms have centered near <br />Buckley Field. Major floods in the Sand Creek basin have induced <br />flood damages along the South Platte River downstream to Fort <br />Lupton, a distance of about 29 miles. Sand Creek is an ungaged <br />stream and streamflow is generally intermittent. <br /> <br />b. Cherry Creek basin. Major floods in the Cherry Creek <br />basin have resulted from rainfall-producing frontal storms cover- <br />ing a major portion of the basin. Minor floods have been <br />predominantly caused by intense short-duration cloudbursts or <br />thunderstorms covering small areas. The dissected topography, <br />steep stream slopes, and low infiltration rates of the soils <br />contribute to a rapid concentration of runoff. Floods in the <br />basin upstream from the Cherry Creek Reservoir are partially <br />controlled by Soil Conservation Service flood-detention structures. <br />Floods in this area are characterized by a rapid rise, a high <br />maximU!ll discharge, a short duration, and comparatively low volumes <br />of runoff. The Cherry Creek D8lll has substantially reduced the <br />flood potential along Cherry Creek in Denver from runoff originat- <br />ing upstream from the dam. A residual flood problem exists in <br />potential runoff from the area downstream from the dam. Urban <br />expansion and deteriorating channel conditions increase the <br />possibility of flooding. Runoff from this uncontrolled area is <br />typical of urban areas with low infiltration rates and high <br />runoff rates. <br /> <br />10. STORMS OF RECORD <br /> <br />. <br /> <br />a. Sand Creek basin. The heaviest rainfall of record in <br />the Sand Creek basin occurred on 30 May 1948 when an estimated <br />8 inches of rain with hail fell within 4 hours at the storm <br />center. Stapleton International Airport recorded 1.02 inches in <br />a 6-hour period. The storm of 8 and 9 May 1957 centered over <br />Murphy and Toll Gate Creeks. Stapleton International Airport <br />recorded 3.29 inches during a 9-hour period, Which included <br />2.3 inches in 4 hours. Over 4 inches of rain fell at Lowry Air <br />Force Base and at Buckley Field. Rainfall amounts up to 4 inches <br /> <br />"''- <br />