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,II II II II II I I II II II II II II II II II II II II II BIG DRY CREEK FLOOD HAZARD AREA DELINEATION - PURPOSE AND SCOPE - - AUTHORIZATION - This report was prepared to guide local officials in planning the use and regulation of the lOO-year floodplain. The report includes information on past floods and the nature and extent of probable future floods along a 9.1 mil e Study Reach of Bi g Dry Creek. Thi s study was prepared under the current format of the Di stri ct' s Flood Hazard Area Del i neati on Reports. The study scope consists of an analysis of the IO-year and IOO-year flood events. Thi s report was authori zed and contracted through the Urban Drai nage and Flood Control District (UDFCD) as part of a program identifying floodprone areas within the District's jurisdiction. This study identifies floodprone areas along Big Dry Creek between Interstate 25 and l68th Avenue, all within Adams County, Colorado. The floodplain information included in this report is provided by the UDFCD to enable local governments to adopt and administer comprehensive floodplain management regulations. Upon acceptance of this report by the UDFCD, and the designation and approval of the flood hazard areas by the Colorado Water Conservation Board, this study can be used by the City of Thornton and Adams County to regulate future floodplain construction, to guide future drainageway improvements, and to plan flood protection measures along the channel and floodplain of the studied stream. Included in this report are floodplain maps and profile sheets. The floodplain maps show the areas that would be inundated by the IOO-year f1 ood event. The profil e sheets show the IO-year and the IOO-year flood elevations along the channel related to Mean Sea Level (MSL) Datum, also referred to as National Geodetic Vertical Datum (NGVD). The report al so includes information regarding the channel thalweg, existing drainage structures, stream bank elevations where appropriate, typical floodplain cross sections and flood flow characteristics. A floodway was also computed as part of this study based on the criterion specifi ed by the UDFCD (maximum D. 5 foot ri se of the energy grade 1 i ne assuming approximately equal reduction of conveyance on both sides of the floodplain). Floodplain and floodway data, including widths, elevations, velocities and reference data, are presented in Table VII of this report. The floodway data are provided to designate those areas to be kept free from future encroachment and to be reserved for conveyance of the IOO-year flood. Definition of a floodway serves to limit future increases in flood stage and velocity resulting from floodplain development. The flood profiles and floodplain limits presented in this report are based on existing floodplain and channel conditions, with future development in the watershed incorporated in the study hydrology. It is important to note that flood elevations and the corresponding floodplain limits are often altered by road and bridge construction, floodplain development, flood 1