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<br />The 100- and SOO-year floodplain boundaries are shown on the Flood <br />Boundary and Floodway Map (Exhibit 2). In cases where the lOa-and <br />SOO-year floodplain boundaries are close together, only the <br />lOa-year ftoodway boundary has been shown. Small areas within the <br />floodplain boundaries may lie above the flood elevations but <br />cannot be shown due to limitations of the map scale and/or lack of <br />detailed topographic data. <br /> <br />For the flooding sources studied by approximate methods, only the <br />lOa-year floodplain boundary is shown. <br /> <br />Approximate lOa-year floodplain boundaries in some portions of the <br />study are were taken directly from the Flood Insurance Rate Map <br />for the City of Broomfield (Reference 9) and the Flood Insurance <br />Study for Jefferson County, Colorado (Reference 10). <br /> <br />The approximate flooding in the vicinity of the sewage disposal <br />plant was taken from the 1979 UDFCD report (Reference 2). <br /> <br />4.2 F100dways <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. One <br />aspect of floodplain management involves balancing the economic <br />gain from floodplain development against the resulting increase in <br />flood hazard. For purposes of the NFIP, a f100dway is used as a <br />tool to assist local communities in this aspect of floodplain <br />management. Under this concept, the area of the lOa-year <br />floodplain is divided into a f100dway and a f100dway fringe. The <br />f100dway is the channel of a stream, plus any adjacent floodplain <br />areas, that must be kept free of encroachment so that the lOa-year <br />flood can be carried without substantial increases in flood <br />heights. Minimum Federal standards limit such increases to 1.0 <br />foot, provided that hazardous velocities are not produced. The <br />f100dways in this study are presented to local agencies as minimum <br />standards that can be adopted!direct1y or that can be used as a <br />basis for additional f100dway1studies. <br /> <br />The f100dways presented in this study were computed on the basis <br />of equal conveyance reduction from each side of the floodplain. <br />The results of these computations are tabulated at selected cross <br />sections for each stream segment for which a f100dway is computed <br />(Table 3). <br /> <br />The f100dways at the confluence of the Nissen Reservoir Channel <br />and the City Park Channel incqrporate sheetf10w diversion areas <br />common to both channels. The;f100dways were delineated through <br />these sheetf10w areas after analyzing two HEC-2 computer runs: <br />(1) with the total flow confined to the channels, and (2) with <br />partial flow confined to the channel. If the change in <br />water-surface elevation between these computer runs was greater <br /> <br />14 <br />