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I I I I I I I I I I I I I I I I I I I Purpose and Scope This report was prepared to guide local officials in planning the use and regulation of the floodplain areas such that flood hazards and future flood damages are minimized. It includes information on histor- ical floods, existing factors which influence the flood hazard, and the nature and extent of probable future floods. The report data includes flooded area maps delineating the 100-year flood boundary and flood profiles showing the water surface elevations obtained from a U.S. Geological survey publication entitled "Floods in Colorado," Water Supply Paper 997 (Reference 2). Federal Insurance Administration flood insurance studies were completed for 80ulder County in 1979 (Reference 3). A report entitled "Floodplain Information Report, Lefthand Creek, Volume I, Mouth to Foothills Highway" was prepared for Boulder County, the City of Longmont and the Colorado Water Conservation Board in 1981 (Reference 4). Acknowledgements Differences between this study and the previous floodplain information contained in References 1 and 3 can be attributed to updated survey data which includes more detailed topographic mapping, land use changes, additional years of historic flood data, and updated hydrologic and hydraulic methodology. This has resulted in significant changes in both discharges and water surface elevations from those previously reported in References 1 and 3. Information in Reference 4 is for Lower Lefthand Creek and is totally compatible with information herein. for the 10-, 50-, 100-, and SOO-year floods. Supplemental material to this report in the form of technical back- up includes the supporting engineering and hydrologic data which may also be used in the location and design of roads, bridges, and channel modifications. The assistance and cooperation of the individuals, agencies, firms, and organizations listed below was appreciated and is gratefully Related Flood Studies Maps and Surveys Detailed topographic mapping and cross section data were provided by the Colorado Water Conservation Board. 80th the mapping and cross sec- tions were compiled by Bell Mapping Company of Denver, Colorado, using photogrammetric methods from aerial photographs taken April 21, 1978. The topographic mapping was developed on a modified state plane coor- dinate system at a scale of 1:2400 and a contour interval of 2.0 feet. U.S. Geological SUrvey 7.S-minute quadrangle mapping with 10-foot contour intervals at a scale of 1:24,000 was also used in this study. acknowledged. Bell Mapping Company - Denver, CO Larry Buss - U.S. Army Corps of Engineers, Omaha District Larry Cieslik - U.S. Army Corps of Engineers, Omaha District Larry F. Lang - Chief, Flood Control and Conservation Section, Colorado Water Conservation Board D. Randolph Seaholm - Water Resource Specialist, Colorado Water Conservation Board Mike G. Serlet - Public Works Department, Boulder County, CO Information on past floods and some bridge data were obtained from a report entitled "Floodplain Information Lefthand Creek, Volume I, Boulder County, Colorado" (Reference 1). Additional past flood information was Bridge data was supplied to the Corps of Engineers by Boulder County and the Colorado Water Conservation Board. Additional surveys, where 2