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<br />cases where the 100- and SaO-year floodplain boundaries are close <br />together, only the 100-year floo~lain boundaries have been shown. <br />Small areas within the floodplain boundaries may lie above the <br />flood elevations but cannot be shown due to limitations of the map <br />scale and/or lack of detailed topographic data. <br /> <br />For the streams studied by approximate methods, only the 100-year <br />floodplain boundary is shown on the Flood Insurance Rate Map. <br /> <br />The 100-year floodplain boundaries for Viele Channel, Davids Draw, <br />Anderson Ditch, and Little Dry Creek, studied by approximate <br />methods, were taken fram the previous Flood Insurance Rate Map for <br />the City of Boulder (Reference 59). <br /> <br />The 100-year floodplain boundaries for Gregory Creek, Bluebell <br />Canyon Creek, Kings Gulch, Sunshine Gulch, upper reaches of Bear <br />Canyon Creek, and Skunk Canyon Creek, were taken fram the Boulder <br />and adjacent county drainageways - Flood Hazard Area Delineation <br />prepared by G&O (Reference 26). <br /> <br />Approxtmate 100-year floodplain boundaries in some portions of the <br />study area were taken directly fram the Plood Hazard Boundary Map <br />for the Town of Nederland (Reference 60). <br /> <br />4.2 Floodways <br /> <br />Encroachment on floodplains, such as structures and fill, reduces <br />flood-carrying capacity, increases flood heights and velocities, <br />and increases flood hazards in areas beyond the encroachment <br />itself. One aspect of floodplain management involves balancing the <br />economic gain from floodplain development against the resulting <br />increase in flood hazard. For purposes of the NFIP, a floodway is <br />used as a tool to assist local communities in this aspect of <br />floodplain management. Under this concept, the area of the <br />100-year floodplain is divided into a floodway and a f100dway <br />fringe. The floodway is the channel of a stream, plus any adjacent <br />floodplain areas, that must be kept free of encroachment so that <br />the 100-year flood can be carried without substantial increases in <br />flood heights. Minimum Federal standards limit such increases to <br />1.0 foot, provided that hazardous velocities are not produced. The <br />floodways in this study are presented to local agencies as minimum <br />standards that can he adopted directly or that can be used as a <br />basis for additional floodway studies. <br /> <br />The floodways presented in this study were computed for certain <br />stream segments on the basis of equal conveyance reduction from <br />each side of the floodplain. Floodway widths were computed at <br />cross sections. Between cross sections, the floodway boundaries <br />were interpolated. The results of the floodway computations are <br />tabulated for selected cross sections (Table 5). In cases where <br />the floodway and 100-year floodplain boundaries are either close <br />together or collinear, only the floodway boundary is ShOWD. <br /> <br />48 <br />