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SECTION 1.0 INTRODUCTION 1.3 1.0 Authorization This study was authorized by the Urban Drainage and Flood Control District (UD&FCD) and Douglas County under UD&FCD Agreement No. 95-06.10, dated May 23,1996. Greenhorne & O'Mara was retained to complete the scope of work summarized in Subsection 1.2. This Flood Hazard Area Delineation Report (FHAD) comprises one of the final report deliverables for the overall study. The other major deliverable is the Preliminary Design Report showing the selected Outfall System Plan for the study area. 1.4 1.2 Purpose and Scope This report was prepared to guide County officials in planning the use and regulation of the 100-year floodplain. The report includes information on past floods and the nature and extent of probable future floods along the study reaches. The report and technical addendum presents the results of a hydrologic and hydraulic analysis performed for the Willow and Little Willow Creek Watersheds. The hydrologic analysis was completed using the UD&FCD's Colorado Urban Hydrograph Procedure, Version F (CUHPF/PC) (Reference I) to determine peak flows for subwatersheds. The peak flows for the subwatersheds were routed along the existing streams and conveyance elements using the UD&FCD's Storm Water Management Model 386 (UDSWM386) (Reference 2). Summary tables presenting the results of the hydrologic analysis for the 10-, 50-, 100- and 500-year storms for future development conditions are included in the Appendix to this report. Water surface profiles were computed for each of the streams being studied using the U.S. Corps of Engineers' HEC-2 Water Surface profiles program (version 4.6.2). Channel cross section data was taken from topographic mapping developed for the project as digital, three- dimensional mapping at 2-foot contours. The study area includes the Willow Creek, East Willow Creek, and Little Willow Creek Watersheds. These watersheds are located in a transitional zone between the mountain foothills and the plains in Douglas County, Colorado. Figure 1.1 shows the general vicinity of the study area. The Willow Creek watershed encompasses an area of 8.27 square miles and is directly tributary to the South Platte River. East Willow Creek drains into Willow Creek just upstream of the South Platte River and includes a tributary area of 1.37 square miles. Little Willow Creek drains into the High Line Canal just upstream of the Platte Canyon Reservoir and the South Platte River. The Little Willow Creek Watershed extends upstream to the west of the Dakota Hogback and includes an area of 4.12 square miles. Mapping Mapping was provided by the UD&FCD. Mapping consisted of I-inch to 100-feet, 2-foot contour interval mapping of the FHAD study limits. This mapping was produced by Benchmark Mapping Services, Inc., under a separate agreement with the UD&FCD. The mapping was compiled using aerial photogrammetric methods from aerial photography flown February 9, 1996. Mapping for the remainder of the study area consisted of United States Geological Survey (USGS) quadrangles. Background Information Background information provided by the sponsors and others was used to develop the hydrology and hydraulic analyses. Information provided is identified below. Section 5 contains a complete listing of References. . Master Plan - Executive Summary, Douglas County, Douglas County Planning Commission, January, 1992. . Master Drainage Plan For Willow Creek Ranch at Roxborough Park East, Douglas County, Colorado, Holland Corporation, November 7, 1984; Revised January 25, 1985. . Final Drainage Plan For Willow Creek Ranch at Roxborough Park East, Volumes I and 2, Holland Corporation, June 28, 1958; Revised August 20, 1985. . Phase II Drainage Study For Roxborough Village Filing No. 14 (Parcel H), Paragon Engineering Consultants, Inc., September 22, 1995; Revised July II, 1996. . Roxborough Village - Little Willow Creek Existing 100- Year Floodplain Sheets I through II, Holland Corporation, June 14, 1985. . Willow and Little Willow Creek Mapping Ground Control, Coordinate List and Control Diagram, Field Notes, and Closure Computations, Nolte and Associates, March 14,1996.