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I I I I I I I I I I I I I I I I I I I IY. HYDRAULIC A:'iALYSES Hydraulic analyses for the South Plane River that were conducted as part of this study included three distinct reaches: (a) detailed floodplain mapping was defined for a D.9-mile reach in the vicinity of South Platte. at the confluence "'lth the North Fork South Platte River; (b) detailed floodplain mapping was also defined for a I 3A-mile reach from Nighthawk to just below Wigwam; and (c) an approximate floodplain was delineated for a 1.7-mile reach starting at the upstream end of the main detailed study reach and continuing upstream through the Wigwam area. !\fapping an approximate floodplain through the Wigwam area \\'as necessitated primarily due to the lack of bridge information and. to a lesser degree. the lack of 10\\' flow channel survey data within the property o\\l1cd by the Wigwam Fishing Club. Access to this area for field reconnaissance and field surveying activities was denied by the property O\\l1er. Within the main detailed study reach, the USGS recently completed a detailed hydraulic model of a 3-mile reach from dOVonstream of Trumbull to upstream of Deckers. This model was in part based on the model prepared for the 1978 Flood Insurance Study (FIS) for Douglas County. The scope and budget for this project dictated that the USGSIFIS model would be used for this reach and no additional surwy or field information would be collected. ~.1 Assessment of Channel and Floodplain Roughness Coefficients Field reconnaissance efforts for the study included a site visit by ACE project staff of the study reach on July 12, 200.t in order to assess and document hydraulic conditions present along the river and at bridges. as well as to estimate :vtanning's n values for the channel and overbank areas in the floodplain for use in the hydraulic model. It should be noted that pursuant to the scope of work for this study, only a cursory field assessment and limited photographic documentation was completed through the reach encompassed by the USGS detailed hydraulic model from dO\\l1stream of Trumbull to upstream of Deckers (between Station 73827 and Station 90347). It was initiallv assumed that the roughness coefficients used in the USGS . - hydraulic model would be sufficient for incorporation into the final hydraulic model. Howe\'er, during the preparation of the hydrologic model for the current study. :\lanning's n values through the USGS reach were modified to retlect general field conditions obserwd in 200~ and to promote consistency \\ithin the model. The entire study reach consists of a well defined natural channel that is subject to minor urban encroachment in the vicinity of Deckers, Trumbull. and South Platte. The South Platte River and its owrbank areas can be classified by two distinct terrain types which are interspersed COCWCBOJ_South PlattcJcport doc 8 A....dEfM\ eo\SlhhG E\GhEEl~, he.