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I I I I I I I I I I I I I I I I I I I concerning the definition of channel and overbank I\fanning"s n values is provided in the field reconnaissance notes included in Appendix D. ~.2 Description of Hydraulic :\Iodels Three hydraulic models were prepared In support of this study using the U.S. Army Corps of Engineers' River Analysis System computer model (HEC-RAS Version 3.1.2, dated ApriI200-t). The first hydraulic model analyzed the IOO-year (I-percent annual chance) flood and the I-foot rise flood way for the D.9-mile reach of the South Platte River in the vicinity of the community of South Platte. The second hydraulic model analyzed the IOO-year flood and the I-foot rise flood way for the 13.4-mile reach of the South Platte River from Nighthawk to just dO\mstream of the Wig".mm area. As mentioned previously. this hydraulic model incorporated the existing USGSIFIS hydraulic model for a 3-mile reach from dov.l1stream of Trumbull to upstream of Deckers. The third hydraulic model evaluated the IOO-year flood for the 1.7-mile reach of the South Platte River in the vicinity of Wig\'.:am. Although this reach \',:as studied using a detailed model. the floodplain is defined as approximate primarily due to the lack of data for the three private bridges located within this reach. In addition. channel survey data was not available for a number cross sections in this reach. Cross sections were defined at the three bridge locations in the \Vigwam area based on a combination of LiDAR data representing the top of road pronle across the bridges. remote site observations and ground photographs. as well as existing data for the upstream bridge. 4.3 Cross Section Geometry Cross section locations along the two detailed study reaches. as well as the approximate reach. were selected to represent changes in geometry along the channel. including bridges. while also attempting to provide adequate detail for defining accurate hydraulic conditions along the channel. In all cases. the overbank ponion of each cross section was defined using the 2-foot contour interval topographic information identified in Section 2.1. The main channel of the river was defined using one of three data sources. In most instances. the field survey data identified in Section 2.2 was used to define the main channel. including the helm\' water ponion of the cross sections. In a few cases. main channel information was taken from cross sections surveyed by the USGS in suppon of an unpublished COCWCBO,,_SOlnh PlatttJeportdlX: 12 A\dER'ioO\ eo\..drhG hGi\URS, hc.