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I I I I I I I I I I I I I I I I I I I -7- computed using the HEC-2 Water Surface Profile Computer Program, developed by the U. S. Army Corps of Engineers, Hydraulic Engineering Center in Davis, California (Ref. 8). Valley cross-sections used in the analysis were taken from the 1" = 100' scale, 2' contour topographic mapping, prepared for this study and discussed earlier in the report. The geometry of the various structures within the flood plain areas, such as irrigation crossings and road crossings, was measured in the field at the time the mapping control work was completed. channel - 0.027 to 0.060 overbank - 0.035 to 0.100 the flood profile by more than an acceptable amount and also represents that part of the flood plain most hazardous to personal safety and welfare. In this study, the floodway has been defined as being the limit of encroachment into the flood plain where existing flood profiles would not be increased by more than 0.5 feet. This criteria is consistent with that recommended by the District and also is compatible with the flood plain and floodway regulations of Jefferson County and the City of Arvada. Hazardous velocities within a flood plain also would require that an area be designated floodway for the purposes of personal safety and welfare. Thus, the floodway has also been delineated to include those areas within the lOa-year flood plain where velocities exceed 3 fps. The floodway widths computed represent the recommended maximum limits of encroachment into the flood plain. The lOa-year flood discharges, elevations and floodway data for each study cross- section are tabulated in Table IX, included in the Appendix of this report. Values for channel and overbank roughness factors, Manninq's "n", were estimated based on photographs of the existing channel and a field inspection of the entire flood plain area. The hydraulic calculations completed to determine the flood prOfiles and flood plain limits plotted on the plan and profile drawings utilized Manning's "n" values of: In the undeveloped areas of Jefferson County, cross-sections were spaced approximately 600 feet apart. Supplemental cross- sections were added at bridges and culverts to allow for deter- mination of backwater effects at each structure. All cross- section locations, except those taken along the center of existing roads,are shown on the attached maps. Damage-Benefit Potential Floodways have also been computed for Van Bibber Creek and the South Tributary. A floodway represents a part of the flood plain which is required to paSS a lOa-year flood event without raising As part of the Phase A study, the damage potential along Van Bibber Creek was analyzed. The study theory for the flood damage analysis is outlined in the text, "Economics of Water Resources Planning" (Ref. 3). The damage analysis included estimated of structural damage to homes, damage to home contents, and damage to streets, irrigation and utility crossings. In addition, indirect damages were considered. Indirect damages include dollar costs and expenses which would be incurred as a result of flooding. Examples of such are the loss of business, profit, taxes, added expense of daily living and operation. Intangible effects of floods were not considered in the analysis because of the inability to assess a dollar value to such damages. Such items would include the loss of environmental The hydraulic analyses did not consider any flood flows to be carried from the drainageway, along or within irrigation ditches. The geometry of each ditch crossing was considered, however, from the standpoint of hydraulic backwater effects.