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I I I I I I I I I I I I I I I I I I I 16 VI HYDRAULIC ANALYSIS overbank. Selected "n" values ranged from as low as 0.02 for street crossings and parking lots to as high as 0.08 for over- bank areas restricted by buildings or debris. At a typical channel section, consisting of a natural channel with natural overbank areas, the generally selected "n" values were between 0.030 and 0.035 in the main channel, and between 0.045 and 0.050 in the overbanks. In overbank areas which were crowded by build- ings, "n" values of 0.08 were used when it was felt that a flow of water could occur through the developed area. When the flow conveyance capability of an overbank was considered to be neg- ligible, the area was eliminated from the cross section and modeled essentially as a wall. Hydraulic computations were made along the channel of Lakewood Gulch to determine the extent of flooding which would occur during the predicted 100-year flood. The principal tool used in the hydraulic analysis was the computer program developed by the U.S. Army Corps of Engineers entitled "HEC-2, Water Sur- face Profiles" (Ref. 2). Data relating to channel slope, rough- ness, cross section shape, and existing drainage facilities were developed from the topographic maps, facilities plans, and field reconnaissance. These data were then utilized in the computer program to calculate flow depths, widths, and velocities at selected cross sections along the channel. Typical channel cross sections and water surfaces are shown on each of the FHAD sheets. The depths of flooding in the 100-year flood were de- termined at each cross section, and the topographic maps were used to interpolate flooded outlines between cross sections. The initial water surface at the downstream end of Lakewood Gulch was established as the elevation of a 10-year flood on the S. Platte River at its confluence with Lakewood Gulch. In several locations the predicted flood-waters would diverge into two or more separate flow paths. In general, one of the paths would convey only a minor amount of water, and is described as "indeterminate shallm. flooding" on the FHAD sheets. The course and extent of such flooding is predicted based solely upon the contour mapping and engineering judgment. Hydraulic computations are made only for the main channel flow. In the channel reach between Decatur Street and the South Platte River, the flow splits into at least three specific flow routes, each of which conveys a relatively large portion of the total flow. This required that separate and detailed hydraulic computations be performed along each route. The routes were divided from each other by the imposition of artificial walls. The cross Values of Manning's "n" were selected in order to model the roughness of the stream bed and overbank areas. Each cross section or group of cross sections was evaluated and assigned an "n" value for the left overbank, the channel and the right