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<br />topographic data. See the legend on the Flood Insurance Rate Map <br />for a complete description of the symbols used. <br /> <br />For the streams studied by approximate methods, only the lOO-year <br />floodplain boundary is shown on the Flood Insurance Rate Map. <br /> <br />4.2 Floodways <br /> <br />Encroachment on floodplains, such as structures and fi 11, 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 lOO- <br />year floodplain is divided into a floodway and a floodway fringe. <br />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 lOO-year flood can be carried without substantial increases in <br />flood heights. Minimum Federal standards limit such increases in <br />flood heights to l.O foot, provided that hazardous velocities are <br />not produced. <br /> <br />The floodway in this study is presented to <br />minimum standards that can be adopted directly <br />as a basis for additional floodway studies. <br /> <br />local agencies as <br />or that can be used <br /> <br />The floodways presented in this study were computed for certain <br />stream segments on the basis of equal reduction from each side of <br />the floodplain. It should be noted that various sheet flow areas <br />are present on many of the streams and that the lOO-year floodplain <br />has been delineated to encompass all of these additional areas. <br />The complete filling of these sheetflow areas will increase <br />floodflows downstream, thereby increasing the lOO-year flood <br />hazard. For any channel modification or fill in the sheet flow <br />areas, a detailed review of the improvement should be undertaken to <br />determine the effects downstream. On Van Bibber Creek, a split <br />flow/sheetflow area was encountered between Miller and Oak Streets <br />for which a floodway analysis was not applicable. Therefore, a <br />floodway was not modeled through this area. Fill in this area has <br />the potential for substantially increasing the flood hazard to both <br />upstream and downstream property owners. Floodway widths were <br />computed at cross sections. Between cross sections, the floodway <br />boundaries were interpolated using topographic maps. The results <br />of the floodway computations are tabulated at selected cross <br />sections (Table 2). In cases where the floodway and lOO-year <br />floodplain boundaries are either close together or collinear, only <br />the floodplain boundary has been shown. <br /> <br />The area between the floodway and the boundary of the lOO-year <br />floodplain is termed the floodway fringe. The floodway fringe thus <br />encompasses the portion of the floodplain that could be completely <br />obstructed without increasing the water-surface elevation of the <br /> <br />lO <br />