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Flood Hazard Area Delineation For Lower Box Elder Creek Watershed so modeling the area necessitated splitting the channel into two distinct channels. The flow in the two parallel channels commingles at certain cross sections and remains separate at other cross sectionsc A floodway analysis was also completed for this project The floodways defined include the stream channel and that portion of the adjacent land areas required to pass the lOO-year flood discharge without cumulatively increasing the energy grade line at any point more than a designated amount above that of the pre-floodway condition (FEMA, 1995). Two different floodway analyses were done. The computations were done for maximum energy grade line increases of 0.5 and LO foot To the extent possible, the floodway was determined using equal reduction of conveyance on opposite sides of the stream. 4.2 Results The results of the modeling are presented in the following report appendices. Appendix P includes tabular floodplain and floodway information for the 10-, 50-, 100- and 500-year recurrence interval storm events. The floodplain delineation for the 100-year recurrence interval storm event is presented in Appendix Q. The profile information for the 1O-year and 100-year storm events is presented in Appendix Rc Finally, typical cross sections are presented in Appendix Sc The floodplain of the main stem of Hayesmount Creek is generally wide (often over 2,000 feet) and shallow (often less than three feet)c The floodplain is not usually bounded by defined channel banksc The floodway, by contrast, is generally rather narrow (averaging less than 300 feet)c The main stem of Box Elder Creek is often contained within a defined channel/over bank section that is generally about 1,000 feet wide and 10 or more feet deep. The floodway is generally 400 feet wide or less. The floodplain of the Bear Gulch tributaries is limited by narrow channels and relatively steep side slopesc Bear Gulch's main stem changes from a narrow channel with steep side slopes at the upper end of the catchment to a broad floodplain area at the downstream end of the catchment A summary of floodplain and floodway widths is presented in Table 4-L TABLE 4-1 Summary of Floodplain and Floodway Width Floodplain Width Floodway Width Floodway Width (O.S-foot EGL rise) (1.0-foot EGL rise) Ave. Min. Max. Ave. Min. Max. Ave. Min. Max. Box Elder Main Stem 1,120 93 4,492 773 73 2,527 655 73 2,526 Bear Gulch Main Stem 752 89 2,717 462 49 2,716 345 41 1,537 Tributarv BGD 437 95 4,426 338 46 4,426 133 40 1,600 Tributary BGE 138 46 257 71 40 113 53 40 84 Tributary BGG 300 141 887 154 64 550 102 45 325 Tributary BGH 251 150 699 106 50 292 70 40 160 Tributary BGI 314 114 786 137 42 345 90 40 225 Tributary BGJ 524 160 924 212 71 484 171 54 484 Hayesmount Creek Main Stem 994 182 3,377 374 65 1,315 235 43 929 Tributarv HCF 994 185 1,966 439 68 1,272 302 61 600 Tributarv HCX 828 117 2,320 236 22 815 133 22 319 5.0 REFERENCES Adams County. 1989. Storm Drainage and Technical Criteria. AzTec Consultants, Inc. 1998a. Aerial Control and Permanent Monuments, Lower Box Elder Creek Mapping, December 18, 1998. Prepared for the Urban Drainage & Flood Control District AzTec Consultants, Inc. 1998. Bridge/Culvert Geometry, Lower Box Elder Creek Mapping, December 18, 1998. Prepared for the Urban Drainage & Flood Control District Balloffet & Associates, Inc., Clarion Associatesc 1999. Adams County Comprehensive Plan January 1999. Prepared for Adams County Planning and Development Department. Boyle Engineering Corporationc 1985c Colorado Urban Hydrograph Procedure Computer Program-PC Version (CUHPEIPCJ Prepared for Urban Drainage and Flood Control District Boyle Engineering Corporationc 1985. UDSWM/PC Version. Central Colorado Water Conservancy District 1987. Box Elder Creek Rehabilitation Program Bootleg, Klug, and Ireland #5 Reservoirs, Weld and Adams Counties, Coloradoc CH2M-Hill. March 1994c Hydrology Report. Prepared for Urban Drainage and Flood Control District, City of Aurora, Arapahoe County and Adams County. CH2M-HiI1. 1994. Upper Box Elder Creek Outfall Systems Planning Study Hydrology Report. CH2M-Hill. December 1995c Flood Hazard Area Delineation, Coyote Run and Upper Box Elder Creek. Prepared for Urban Drainage and Flood Control District, City of Aurora, Arapahoe County, and Adams County. 991-134.000 Page 9 Wright Water Engineers, Inc.