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I I I I I I I I I I I I I I I I I I I prepared at a 2-foot contour interval, and at a scale of 1" = 100' (Reference 1). The final map sheets were reduced in half for printing in this report. The drainage area considered for this study is only the 22- square mile reach from the dam to the South Platte River. An in- tense thunderstorm on this area below the dam has the potential of generating a peak of some 8,000 cfs during a 100-year event. The mapping was prepared according to the specifications of the Urban Drainage and Flood control District. Gound control sur- veys for the topographic mapping were completed by Gingery Associates, Inc. The mapping and the survey control was checked by Geodetic Surveys, Inc., under seprarate contract with the District. The basin below Bear Creek Dam is rapidly approaching a full development condition with the exception of the Bear Valley Park and areas near South Kipling Street. The lower basin is character- ized by dense mobile home parks with the remainder of the basin generally in single family residences. There are some commercial areas located along South Federal, South Sheridan and South Wadsworth Boulevards, most notably the Bear Valley Shopping Center near the intersection of Hampden Avenue and South Wadsworth Boulevard. The hydrology used in this study was completed by the Army Corps of Engineers, Omaha District, as requested by the Colorado Water Conservation Board (Reference 2). The assistance of the Army Corps of Engineers in supplying information in regard to the hydro- logy presented in this report is recognized and appreciated. - BASIN DESCRIPTION - Bear Creek, In its lower reaches, has been altered in the past by channel modifications, such as straightening and enlarge- ment. The channel averages about 80 feet wide and slopes are 5.3 feet per 1,000 feet. Upstream from South Wadsworth Boulevard, the stream is generally in its natural state and averages approximately 80 feet wide ~ with slopes about 7.2 feet per 1,000 feet. Bear Creek generally flows eastward from its headwaters at Mount Evans through the Towns of Evergreen and Morrison until it reaches the metropolitan area of Denver where it is tributary to the South Platte River (Sheet 2). The drainage basin is approx- imately 36 miles long and has an average width of about nine (9) miles. This encompasses approximately 261 square miles of drainage area (Sheet 2). Elevations in the basin range from approximately 14,260 feet at Mount Evans to 5,260 feet at the mouth. Turkey Creek, a major tributary, drains about 52 square miles and enters into Bear Creek approximately two (2) miles downstream of Morrison. Ninety percent of the basin is in the mountains with the remainder draining the foothills and high plains region (Reference 3). - PAST FLOOD HISTORY - The Bear Creek channel has a capacity of approximately 2,000 cubic feet per second (cfs) without causing flood damage. Floods in the past have usually been caused by intense rainstorms in the summer months; however, high flows can occur from snowmelt runoff combined with rain. The completion of the newly constructed Bear Creek Dam just downstream of Morrison has greatly affected the peak discharges of the 8.2 mile Bear Creek study reach. Designed by the Army Corps of Engineers, the dam provides a flood control reservoir that in- tercepts flows from areas in the upper and middle parts of the basin. At the Bear Creek Reservoir, peak flows from the 100-year event have been reduced from 30,000 cfs to approximately 1,000 cfs through storage in the reservoir. Floods in Bear Creek have occurred in May, 1876, May and June of 1894, and in July of 1896. The peak discharge for the 1896 flood was estimated to be 8,600 cfs at the Morrison gage, about two miles upstream of the study reach. On May 9, 1957, high sustained flows throughout the Bear Creek study reach caused severe bank erosion and damage to a bridge that was under construction at South Wadsworth Boulevard, as well as residential lots abutting the stream. _2_