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Dry Creek Flood Review Pueblo, Colorado - Jfuly 1996 Background On July 9, 1996 an intense thunderstorm occurred over the Dry Creek watershed which resulted in a major flood event. The area of flooding occurred in the southeast sector of Pueblo, Colorado, The Dry Creek basin encompasses 5.4 square miles, with the headwaters :located near Southern State University, Dry Creek, which is an ephemeral stream, originates at the southern extremity of Baculite Mesa in the high plains northeast of Pueblo and flows southerly to discharge into the Arkansas River within the City of Pueblo, The stream flows result from rainfall runoff from early spring to late fall. Floods in Dry Creek normally originate from runoff of the entire watershed due to the limited areal extent of the drainage, The steep slopes and short time of concentration are conducive to fast rises and high peak flows of short duration, The average slope of the watersh,:d is approximately 114 feet per mile (approximately 2,2%). Analysis of climatological data and inspection of the flood history show that the most severe flood- producing storms occur during the late spring or summer months~ The spring season is the transitional period between the occurrence of the instability and frontal activity and result in heavy spring snow storms in the higher elevations or high intensity rains ov.cr the plains region, These storms and the physical features of the area can produce the rapid concentration of runoff that characterizes flash floods of high peak flows, small volumes, and short duration, Problem Description The intense thunderstorm event on July 9,1996 caused flooding problems throughout the basin, However, a serious flood problem occurred in the reach of Dry Creek downstream of the Missouri- Pacific Railroad (See Attachment A for the locations), Floodwaters were not contained within the main charmel and circumvented a levee located just downstream of the county road crossing, The diverted flow then traveled "behind" the levee causing serious flood danlage to homes, landscaping, and other features, ,-..., Hydrologic and Hydraulic Determinations The hydrologic and hydraulic characteristics and parameters are taken from the Flood Insurance Study entitled "Flood Insurance Study, Pueblo County, Colorado Unincorporated Areas - September 29, 1989," This report was designated by the Colorado Water Conservation Board as floodplain resolution No~ 96-415, The lOO-year discharge for the su~ject stream reach is 2,900 cfs (See Table I on Attachment B). The CWCB staff visited the site on May 12, 1997 and measured cross-sectional areas and high water marks (HWM's) to estimate the July 9,1996 flood discharge, The measurements were performed using conventional field survey techniques. Good HWM's did exist at the subject site, The staff