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<br />. <br /> <br />range land except for the downstream 10 miles, which is urbanized. <br /> <br /> <br />The principal tributary is Toll Gate Creek, which drains an area <br /> <br /> <br />of 39 square miles and empties into Sand Creek about 8 miles <br /> <br /> <br />upstream from the South Platte River. The ungated emergency spill- <br /> <br /> <br />way of Cherry Creek Dam is designed to discharge into West Toll <br /> <br /> <br />Gate Creek. Since discharge through the spillway would require a <br /> <br /> <br />flood in excess of standard project magnitude into Cherry Creek <br /> <br /> <br />reservoir, the probability of spillway discharge is remote. However, <br /> <br /> <br />the potential for damage along Toll Gate and Sand Creeks from an <br /> <br /> <br />occurrence of this nature is significant. A map of the Sand Creek <br /> <br /> <br />basin is presented on plate 45. <br /> <br />Flood History. Available data indicate that major flood <br /> <br /> <br />events in the Sand Creek basin are caused by runoff from intense <br /> <br /> <br />thunderstorm-type rainfall. Although no gaging records are avail- <br /> <br /> <br />able, at least 28 events of this type have been identified in the <br /> <br /> <br />basin since 1896. Descriptions of two of the largest events are <br /> <br /> <br />presented below. <br /> <br />. Flood of 8 and 9 May 1957. A heavy rainstorm occurred over <br /> <br /> <br />the Sand Creek basin and adjacent areas on 8 and 9 May 1957. Over <br /> <br /> <br />4.0 inches of rainfall occurred at the storm center. Major flooding <br /> <br /> <br />resulted on Sand and Toll Gate Creeks. Runoff from the Sand Creek <br /> <br /> <br />basin caused severe flooding along the South Platte River. The <br /> <br /> <br />floodwaters in the Sand Creek basin receded in less than 12 hours, <br /> <br /> <br />but high flow conditions along the South Platte River continued <br /> <br /> <br />for several days as higher than average snowmelt lasted almost to <br /> <br /> <br />the end of the month. On Sand Creek, near Stapleton International <br /> <br /> <br />Airport, the flow was estimated to be 25,000 cubic feet per second <br /> <br /> <br />and upstream from Toll Gate Creek, the Sand Creek discharge was <br /> <br />. <br /> <br />39 <br />