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<br />elevat ions 1 and the use of topographic maps at a scale of <br />1:24,000 with a contour interval of 10 feet (Reference 23). <br />Planimetric features for Cherry Creek Spillway and Unnamed <br />Creek were obtained from the City of Aurora Quarter Section <br />Plat Book. Some areas Qf shallow flooding were interpolated <br />using topographic maps at a scale of 1:24,000, with contour <br />intervals of 10 feet (Reference 26). <br /> <br />The 100- and SO a-year floodplain boundaries are shown on the <br />FIRM (Exhibit 2). On this map, the laO-year floodplain <br />boundary corresponds to the boundary of the areas of special <br />flood ha",ards (Zones A and AE); and the SOD-year floodplain <br />boundary corresponds to the boundar; of areas of moderate <br />flood hazards. In cases where the 100- and 500-year <br />floodplain boundary are close together, only t~e 100-y~ar <br />floodplain boundary has been shown. Small areas wi thin the <br />floodplain boundaries may lie above the flood elevations but <br />cannot be shown due to limitations of the map scale and/or <br />lack of detailed topographic data. <br /> <br />For the streams studied by approximate methods, only the 100- <br />year floodplain boundary is shown on the FIRM (Exhibit 2). <br />The top widths were obtained from elevation-discharge curves <br />and were plotted at selected cross section locations and the <br />100-year floodplain boundaries were interpolated using <br />topographic maps (Reference 26). The approximate lOO-year <br />floodplain boundary for First Creek was delineated from the <br />information contained in the Phase A report, Aurora Water and <br />Draina2e of First Creek, (Reference 29). <br /> <br />4.2 Floodways <br /> <br />Encroachment on floodplains, such as structures and fill, <br />reduces flood-carrying capacity,. increases flood heights and <br />velocities, and increases flood hazards ~n areas beyond the <br />encroachment. One aspect of floodplain management involves <br />balancing the economic gain from floodplain development <br />against the resulting increase in flood hazard. For purposes <br />of the NFlP, a floodway is used as a tool to assist local <br />conununities in this aspect of floodplain management. Under <br />this concept,. the area of the 100-year floodplain is divided <br />into a floodway and floodway fringe. The floodway is the <br />channel of a stream, plus any adjacent floodplain areas, that <br />must be kept free of encroachment so that the IOO-year flood <br />can be carried without substantial increases in flood heights. <br />Minimum Federal standards limi t such increases to 1.0 foot, <br />provided that hazardous velocities are not produced. The <br />floodways in this study are presented to local agencies as <br />minimum standards that 'can be adopted directly or that can be <br />used as a basis for additional floodway studies. <br /> <br />22 <br />