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<br />Flood Hazard Area Delineation <br />For Lower Box Elder Creek Watershed <br /> <br />1.0 <br /> <br />INTRODUCTION <br /> <br />4. <br /> <br />Perform step backwater hydraulic calculations using the HEC-RAS version 2.2 computer model for <br />the 10-, 50-, 100- and 500-year recurrence interval storms for future development (Le. future <br />percentage imperviousness) conditions and existing (Le. without improvements) stream conditions. <br /> <br />1.1 <br /> <br />Authorization <br /> <br />This report has been prepared for the Urban Drainage and Flood Control District (UDFCD) and the project <br />sponsors: Adams County and the City and County of Denver. Work is being carried out under the <br />Agreement Regarding Outfall Systems Planning and Flood Hazard Area Delineation for Lower Box Elder <br />Creek Watershed, UDFCD, Agreement No. 99-01.12, August 13, 1999. <br /> <br />1.2 <br /> <br />Purpose <br /> <br />The purpose of this report is to provide floodplain information so that the Denver International Airport <br />(DIA), Adams County and the City of Aurora can implement floodplain zoning ordinances and/or <br />floodplain regulations and other land use controls, as needed, to reduce potential flood damages and <br />unwise development in the floodplainc <br /> <br />1.3 <br /> <br />Scope <br /> <br />The scope of work for this project includes preparation of an Alternative Evaluation Report, preparation of a <br />Flood Hazard Area Delineation (FHAD) and preparation of a Preliminary Design Reportc The scope of <br />work for the FHAD portion of this project includes the following items: <br /> <br />1. <br /> <br />Assemble and review pertinent information about the watershedc <br /> <br />2. <br /> <br />Carry out field observations in order to collect data relevant to the hydrologic and hydraulic <br />modeling for the watershed. <br /> <br />3c <br /> <br />Define the hydrology of the area using the CUHP95 and UDSWM95-PC computer programs for the <br />10-, 50-, 100-, and 500-year recurrence interval storms under existing and future development <br />watershed conditions. Results of this portion of the study were reported in the Hydrology Reportfor <br />Lower Box Elder Creek Watershed Outfall Systems Planning and Flood Hazard Area Delineation <br />(Wright Water Engineers, Inc. [WWE] 2000). <br /> <br />5. <br /> <br />Perform floodway encroachment calculations using the HEC-RAS version 2.2 computer model for <br />the 100-year recurrence interval storm for future development (Lee future percentage <br />imperviousness) conditions and existing (i.e. without improvements) stream conditions using target <br />energy grade line increases of 0.5 foot and 1.0 foot <br /> <br />6. <br /> <br />Delineate the floodplain for the 100-year recurrence interval storm with future watershed <br /> <br />. . <br />ImpervIOusness. <br /> <br />7c <br /> <br />Prepare water surface profiles for the 10- and 100-year recurrence interval storms with future <br />watershed imperviousnessc <br /> <br />8. <br /> <br />Prepare tabular floodplain information (water surface elevation, floodplain width, and average <br />velocity) and floodway information (right and left floodway width, floodway water surface <br />elevation, and average velocity) for the 10-, 50-, 100- and 500-year storms with future watershed <br /> <br />. . <br />ImpervIOusness. <br /> <br />1.4 <br /> <br />Mapping <br /> <br />The UDFCD provided mapping (I "=200' scale, 2' contour interval) for the FHAD study areas lying outside <br />the boundary of the DIA. DIA provided mapping (1 "=200' scale, 2' contour interval) for all areas within <br />DIA's boundariesc The mapping from the two sources was combined by Pacific Western Technologies. <br />The mapping was prepared using Colorado State Plane Coordinate System, Central Zone, NAD 83, NA VD <br />88. Information for each survey monument is available from the UDFCD (AzTec Consultants, 1988a). <br /> <br />1.5 Sponsors and Stakeholders <br /> <br />The sponsors for this study include Adams County and DIA. These two entities attended the project kickoff <br /> <br />meeting and each of the regularly scheduled progress meetings that were held prior to finalization of the <br /> <br />991-134cOOO <br /> <br />Page I <br /> <br />Wright Water Engineers, Inc. <br />