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I I I I I I I I I I I I I I I I I I I I. INTRODUCTION A. AUTIIORIZA nON (Reference I). Hydrology for this report was not restudied and the discharges presented in the FrS were incorporated into this analysis. The following is a summary of the scope of work for this project. This report was authorized and contracted through the Urban Drainage and Flood Control District (UDFCD) by the agreement for the Flood Hazard Area Delineation for Plum Creek and East Plum Creck. dated January 21, 200~ (Agreement No. 30-} 0.07). I. Field verify hydraulic structures and parameters; 2. Calculation of water surface profiles for the 10- and IOO-year llood eycnts. 3. Delineation of the IOO-year floodplain boundary on Flood Hazard Area Maps; ..L Development of flood profiles for the 10- and I DO-year flood c\"ents: 5. Detemlination of floodways based on encroachment resulting in a maximum increase in the energy grade line of 0.5 feet and 1.0 feet; 6. Preparation of a report describing characteristics of the study area, flood history of Plum Creek and East Plum Creek, tabulation indicaling pertinent lloodplain and tloodway information. and reduced copies of nood hazard delineation maps and flood profiles. This study identifies flood prone areas along Plum Creek and East Plum Creek within portions of unincorporated Douglas County and the TO\\TI of Sedalia. Along Plum Creek, the study reach extends from approximately 4,000 feet upstream of Airport Road upstream to the confluence \vith East Plum Creek and West Plum Creek, located just downstream of State Highway 67 in Sedalia. Along East Plum Creek, the study reach extends from the confluence with Plum Creek upstream to the limits of the UnFCD boundary, located approximately 1,000 feet upstream of the Burlington Northern and Santa Fe (BNSF) Railroad. The tloodplain information included in this report is provided by the UDFCD to assist Douglas County in floodplain management and regulation along Plum Creek and East Plum Creek. The Iloodplain delineation and flood profiles arc based on existing floodplain and channel conditions. Flood elevations and the corresponding Iloodplains arc often altered by road and bridge construction. floodplain development flood control improvements. or natural processes. Prior to utilization of this report for planning or design purposes. the user is advised to contact the UDFCD or Douglas County to determine if the infomlation presented in this report has been amended. Upon acceptance of this report by the UDFeD and designation of the flood hazard areas by the Colorado Water Conservation Board (C\VCB) and the Federal Emergency [..tallagement Agency (FEMA). this study can be used by Douglas County to regulate future floodplain development to guide future drainageway improvements, and to plan tlood protection measures along Plum Creek and East Plum Creek. C. ACKNO\VL[()GF:i\IF:NTS II, PUHI'OSF: & SCOPE This report was prepared by ICON Engineering, Inc, under contract with the UDFCD. Topographic mapping used for this project was provided by landmark Mapping, Ltd. under a separate contract \vith the UDFCD. The mapping \Va5 prepared according to the UDFCD specifications. Ground control survey tor the topographic mapping was perfonned by !l.lountain Surveying and Mapping, Inc. Topographic mapping was prepared with two-foot contour intervals at a scale of I inch 0= 100 feet on the North American Vertical Datum (NA VO) 1988 (Reference 2). The topography was compiled from aerial photography taken in December 2003. This report \....as prepared to guide county officials in planning the use and regulation of the 100-year floodplain. This report provides information on past flooding cycnts and defines the nature and extent of probable future floods along a 3.2 mile study reach of Plum Creek and a 3.6 mile study reach of East Plum Creek. Hydrologic analysis of Plum Creek and East Plum Creek were determined as part of FE~IA's Flood Insurance Study (FIS) for Douglas County. Colorado. Unincorporated Areas. dated January 5, 1996 The hydrologic analysis was completed under a separate contract \vith FEMA for the Douglas County FIS. Final hydraulic analyses and floodplain delineation was completed by ICON Engineering, Inc.