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I I I I I I I I I I I I I I I I I I I 13 A detailed, technical appendix of hydraulic calculations is available at the offices of the Urban Drainage and Flood Control Di strict. VI HYDRAULIC ANALYSIS After development of the hydrology described in the preceding section, hydraulic analyses were made to determine the water surface elevations which would occur in a ten-year and a 100- year flood. The primary tool used in the analyses was the compu- ter program developed by the U.S. Arny Corps of Engineers entitled "HEC-2, Water Surface Profiles" (Ref. 2). Bell Happing Company, in addition to preparing the topographic maps, provided digi- tized cross sections of the channels. These were supplemented by sections taken directly from the maps. The maps and sections were used first in the computation of the water surface profiles, and second in the description of the flooded outlines in the 100-year flood. VII DESCRIPTION AND DELINEATION OF THE lOO-YEAR FLOOD HAZARD The 100-year Flood Presented at the end of this Section are the Flood Hazard Area Delineation (FHAD) Sheets illustrating the estimated extent of flooding which would occur on Dry Gulch, North Dry Gulch, and Colfax Gulch in the event of a 100-year flood. No records are available to indicate whether or not floods of the magnitude described herein have occurred on Dry Gulch. However, it is statistically indicated that a rainfall equal to the 100-year storm will occur at some time. It is indicated by the hydrologic and hydraulic analyses that such a storm will generate the flood depicted in this report. Estimates of the channel roughness coefficients were made based upon field observations. Values of Manning's "n" used in this report were as low as 0,017 for streets and parking lots, and as high as 0.100 for over-bank areas crowded with buildings or debris. In general, areas of natural channel section with nat- ural over-banks were assigned "n" values of approximately 0.045 and 0.080, respectively. Where buildings represented signifi- cant obstructions to the channel flow, they were represented in the cross sections as total blockages. Table 3 presents a tabulation of data relating to the antici- pated flooding in a 100-year storm. FHAD Sheets 3 through 20 illustrate the thalweg (valley bottom) profile of the channel, the water surface elevations of the 10-year and 100-year floods, and the areas which would be inundated by the 100-year flood. Table 3 also presents data describing a "floodway" along each channel. The floodway is that portion of the channel which carries the majority of the flood flow. It is defined as the