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I I . . . . . I . I I I I :1 :1 I '. \1 More recently, minor flooding occurred on July 23 and 24, 1965. Twenty-five families had to be evacuated, 15 basements were flooded and several bridges were damaged (Reference 2). Heavy rains from May 4 through May 10, 1969, caused the greatest flood o~ record in the study reach. A total storm rainfall record of 11 inches at Morrison caused extensive flooding from South Lowell Boulevard to the confluence of the South Platte River. Many people in a mobile home park and in nearby houses had to be evacuated. Flood water filled the basements and the first floors of many homes. (Reference 3). - FLOOD RELATED STUDES - A detailed flood study of Bear Creek in the Denver metropoli- tan area was published by the Army Corps of Engineers in December of 1972. The publication, entitled "Special Flood Hazard informa- tion Report for Bear Creek," was prepared for the Colorado Water Conservation Board (Reference 3). This study includes a hydrologic analysis of Bear Creek using channel conditions and development in 1970. The report contains Flood Hazard Delineation maps along with water surface profiles for the Standard Project Flood (approx- imate 500-year flood event) and the Intermediate Regional Flood (approximate lOO-year flood event). The study was completed before the construction of the Bear Creek Dam. Consequently, discharges and flood elevations arc much higher than those presented in this report, and do not reflect present day flooding conditions. "- \}-)\.r~ .;.. ",' , - HYDROLOGIC AND HYDRAULIC DETERMINATIONS - Hydrology Analysis A hydrologic analysis was completed by the Omaha District of the Army Corps of Engineers to establish the peak discharges for the 10-, 50-, 100-, and 500-year recurrence intervals in Bear Creek below Bear Creek Dam. ~ The newly constructed Bear Creek Dam and Reservoir intercepts flows from 239 square miles of the total 261-square mile drainage basin. This flood control reservoir, with some 110,000 acre feet of storage behind a 190-foot high embankment, has drastically re- duced the flood potential for lower Bear Creek. The Special Flood Hazard Information Report for Bear Creek (Reference 2) shows a lOa-year discharge of 30,000 cfs near the reservoir site prior to reservoir construction. Recent calculations using a release prob- ability curve developed from a theoretical operation scheme esti- mated a lOa-year release of 1,000 cfs from the reservoir. The 22-square mile drainage area below Bear Creek Dam still has the potential of generating damaging runoff flows during a cloudburst event Over the lower basin. No applicable runoff records are available through this reach because of the recent con- struction of the dam. Therefore, discharges for the 22-square mile drainage area below the dam were developed by using the Environmenal Protection Agency's Storm Water Management Model (SWMM) (Reference 6) with modifications by the Missouri River Division of the U.S. Army Corps of Engineers. These discharges were computed by assuming full basin development. The amount of impervious area was based on field investigations in 1977, which indicated that about 12.7 percent of the lower basin would be impervious. Infiltration rates were assumed to 0.5 inch per hour, and depression storage was esti- mated at 0.18 inch for pervious areas. These are values determined 1 from previous studies in the Denver area. The rainfall depth for the 100-year event was obtained from the NOAA Atlas for Colorado. A one-hour duration was considered critical with a rainfall depth of 2.37 inches (Reference 5). The results of this hydrologic analysis are presented in Table I. Although the lOa-year frequency discharge has been largely re- duced by the construction of Bear Creek Reservoir, the discharge generated in the lower basin alone, for this event, is sufficiently -3 -