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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 Related Flood Studies flood damages are minimiled. It includes information on historical 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 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 Boulder County and Longmont in 1979 and 1977, respectively (References 3, 4). Differences between this study and the previous floodplain information report can be attributed to updated survey data which included topographic mapping, additional years of historic flood data, and updated hydrologic and hydraulic methodology. and regulation of the floodplain areas such that flood hazards and future floods, existing factors which influence the flood hazard, and the nature and extent of probable future floods. The report data i nc 1 udes flooded area maps deli neat i ng the lOO-year flood boundary and flood profil es showi ng the water surface e 1 evat ions for the 10-, 50-, 100-, and 500-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. Bell Mapping Company - Denver, Colorado Larry Buss - U,S. Army Corps of Engineers, Omaha District Larry Cieslik - U.S. Army Corps of Engineers, Omaha District James M. Landeck - City Engineer, Longmont, Colorado Larry F. Lang - Chief, Flood Control and Conservation Section Colorado Water Conservation Board Parker and Associates - Denver, Colorado Neal Renfroe - Director of Public Works, Longmont, Colorado D. Randolph Seaholm - Water Resource Specialist, Colorado Water Conservation Board Mike G. Serlet - Public Works Department, Boulder County, Colo. Maps and Surveys Detailed topographic mapping and cross section data were provided by the Colorado Water Conservation Board. Both the mapping and the cross sections were compiled by Bell Mapping Company of Denver, Colorado, using photogrammetric methods from aerial photographs taken April 21, 1978. Parker & Associates of Denver, Colorado prepared supplemental mapping near the confluence with St. Vrain Creek from aerial photography taken March 24, 1979. The topographic mapping scale is 1:2400 and the contour interval is 2.0 feet. U.S. Geological Survey 7.5-minute quadrangle mapping (Reference 5) with 10 foot contour intervals at a scale of 1:24,000 was also used in this study. Bridge data was supplied by the Corps of Engineers and Boulder County. Additional surveys were furnished by the City of Longmont, Boulder County, and the Colorado Water Acknowledgements The assistance and cooperation of the individuals, agencies, firms, and organi zat ions 1 i sted below was appreci ated and is gratefully acknowledged, Conservation Board. Survey control monuments (Table 8) exist in the field for use in relating information contained in the report to actual ground conditions, 2