Laserfiche WebLink
<br />For the streams studied by approximate methods, only the lOO-year floodplain boundary is <br />shown on the Flood Insurance Rate Map (Exhibit 2). <br /> <br />The approximate lOO-year boundaries for the flows along Sutherland Creek downstream of <br />Cross Section A were delineated using topographic maps at a scale of I: I ,200, with contour <br />intervals of 4 and 20 feet (Reference 40). <br /> <br />The 100-year flood boundaries for the remaining streams studied by approximate methods <br />were delineated using topographic maps enlarged to a scale of I :4,800, with a contour <br />interval of 40 feet (Reference 42). <br /> <br />Approximate 100-year floodplain boundaries in some portions of the study area were taken <br />directly from the Flood Hazard Boundary Map for the City of Colorado Springs, City of <br />Fountain, and the unincorporated areas of EI Paso County (References 52, 53, and 54). <br /> <br />4.2 Floodways <br /> <br />Encroachment on floodplains, such as structures and fill, reduces flood-carrying capacity, <br />increases flood heights and velocities, and increases flood hazards in areas beyond the <br />encroachment itself. One aspect of floodplain management involves balancing the economic <br />gain from floodplain development against the resulting increase in flood hazard. For purposes <br />of the NFIP, a flood way is used as a tool to assist local communities in this aspect of <br />floodplain management. Under this concept, the area of the lOO-year floodplain is divided <br />into a flood way and a floodway fringe. The floodway is the channel of a stream, plus any <br />adjacent floodplain areas, that must be kept free of encroachment so that the lOO-year flood <br />can be carried without substantial increases in flood heights. Minimum Federal standards <br />limit such increases to 1.0 foot, provided that hazardous velocities are not produced. The <br />flood ways in this study are presented to local agencies as minimum standards that can be <br />adopted directly or that can be used as a basis for additional flood way studies. <br /> <br />The floodways presented in this study were computed for certain stream segments on the basis <br />of equal conveyance reduction from each side of the floodplain. Floodway widths were <br />computed at cross sections. Between cross sections, the floodway boundaries were <br />interpolated. The results of the floodway computations are tabulated for selected cross <br />sections (Table 5). In cases where the flood way and lOO-year floodplain boundaries are <br />either close together or collinear, only the floodway boundary is shown. <br /> <br />Hazardous velocities would occur during flooding on the upper reaches of Monument Creek <br />near the Town of Palmer Lake and on Monument Creek Tributary under existing-channel <br />conditions (Reference 6). Encroachments, islands, and flood boundaries are subject to <br />extreme erosion when veiocities are so high. Therefore, the lOO-year flood boundary was <br />designated as the floodway as shown on the Flood Insurance Rate Map (Exhibit 2). <br /> <br />Because of high-velocity conditions, the floodways were developed for Fountain Creek, <br />Ruxton Creek, Sutherland Creek, and Williams Canyon by applying the allowable rise to the <br />energy gradeline instead of the water-surface elevation. Floodways were also identified by <br />the criteria that allow a maximum hazardous velocity of 3 feet per second at the edge of the <br />floodway fringe. These hazardous velocity criteria proved to be determinant in the floodway <br />delineation. For Williams Canyon and Ruxton Creek, the floodway boundary is the same as the lOO-year flood boundary. A floodway fringe has been computed for some areas along <br /> <br />36 <br />