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<br />determination of the flood profiles were started at the Burlington Ditch Weir, located <br />within the City of Commerce City, where critical depth was assumed. <br /> <br />Between Quebec Street and Havana Street, shallow flooding extends north from <br />Sand Creek, inundating large areas of the Stapleton International Airport with depths <br />averaging from less than 1 foot to 3 feet. Downstream of Quebec Street, shallow <br />flooding extends west from Sand Creek with depths averaging less than 1 foot. <br />Normal-depth calculations were used to determine depths and extent of shallow.T... <br />flooding areas. <br /> <br />South Platte River <br /> <br />Valley cross sections for the South Platte River were developed using digitized cross <br />sections from aerial photography flown April 16, 1983. Where appropriate, river <br />cross sections were field surveyed and incorporated into the digitized cross sections <br />(References 60, 61, 62, and 63). Manning's "n" values ranged from 0.030 to 0.035 in <br />the main channel and from 0.040 to 0.070 in the overbank and split-flow areas. <br />Starting WSELs and channel stationing were obtained from the September 1977 <br />UDFCD Flood Hazard Area Delineation report for the South Platte River in <br />Adams County (Reference 7). Both normal and special bridge routines were used in <br />the HEC-2 computer program to determine bridge losses. <br /> <br />Special split-flow analyses were made for those floodplain reaches where a <br />significant discharge leaves the main channel and flows parallel to the main channel <br />as overland flow until it is forced back into the main channel by the natural <br />topography or a man-made improvement. Separate split-flow profiles are presented <br />for the left (west) bank from 56th Avenue upstream to 47th Avenue, for the left <br />(west) bank from West Jewell Avenue upstream to West Iliff Avenue, and for the <br />right (east) bank from Lawrence Street upstream to 8th Avenue. A separate Profile <br />Base Line was delineated for each split-flow area and is shown on the FIRM. <br /> <br />Westerly Creek <br /> <br />The approximate I-percent-annual-chance elevations along Westerly Creek were <br />computed using the USACE HEC-2 computer program (Reference 44). <br />Manning's "n" values and cross-section data in some portions of Westerly Creek <br />were obtained from the 1977 UDFCD Flood Hazard Area Delineation report for <br />Westerly Creek (Reference 64). Additional cross-section data were obtained from <br />construction drawings for recent channel improvements (Reference 65). Starting <br />WSELs were determined in the 1977 Major Drainageway Planning report for <br />Westerly Creek (Reference 11). <br /> <br />Coon Creek and Other Tributaries <br /> <br />Elevations for Coon Creek, First Creek, First Creek Tributary, Sloans Lake Basin, <br />and Marston Lake North Drainage Basin, which are presented in this FIS as <br />approximate study areas, were determined previously as part of the following <br />UDFCD reports: the Flood Hazard Area Delineation report for Dutch Creek, Lilley <br /> <br />24 <br />