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exi t losses. It was determined remains within the system and that by the system. The Mannlng' s "n" study reach (Reference 32). that the hydraulic grade line the lOa-year flood is conveyed value used was U.U14 tor thlS Sand Creek Cross section data, Manning's "n" values, and starting water- surface elevations were obtained from the 1977 UDFCD Flood Hazard Area Delineation report for Sand Creek (Reference 42). Cross section data were compiled from topographic maps (Reference 59). Manning's "n" values ranged from 0.036 to 0.044 in the channel and from 0.035 to 0.100 in the overbank areas. Backwater computations for determination of the flood profiles were started at the Burlington Ditch Weir, located within the City of Commerce City, where critical depth was assumed. Between Quebec Street and Havana Street, shallow flooding extends north 'from Sand Creek, inundating large areas of the Stapleton International Airport with depths averaging from less than 1 foot to 3 feet. Downstream of Quebec Street, shallow flooding extends west from Sand Creek with depths averaging less than 1 foot. Normal-depth calculations were used to determine depths and extent of shallow flooding areas. South Platte River Valley cross sections for the South Platte River were developed using digitized cross sections from aerial photography flown April 16, 1983. Where appropriate. river cross settions were field surveyed and incorporated into the digitized cross sections (References 60,61,62, and 63). Manning's "n" values ranged from 0.030 to 0.035 in the main channel and from 0.040 to 0.070 in the overbank and split-flow areas. Starting water-surface elevations and channel stationing were obtained from the September 1977 UDFCD Flood Hazard Area Delineation report for the South Platte River in Adams County (Reference 7). Both normal and special bridge routines were used in the HEC-2 computer program to determine bridge Losses. Special split-flow analyses were made for those floodplain reaches where a significant discharge leaves the main channel and flows parallel to the main channel as overland flow until it is forced back into the main channel by the natural topography or a man-made improvement. Separate spl it-flow profi les are presented for the left (west) bank from 56th Avenue upstream to 47th Avenue, for the left (west) bank from West Jewell Avenue upstream to West Iliff Avenue, and for the right (east) bank from Lawrence Street upstream to 8th Avenue. A separate Profile Base Line was delineated for each split-flow area and is shown on the Flood Insurance Rate Map. Westerly Creek The approximate computed using lOa-year the COE elevations along Westerly Creek were HEC-2 computer program (Reference 44). 25