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<br />The 100- and SOO-year flood plain boundaries are shown on the Flood <br />Insurance Rate Map (Exhibit 2)~ On this map, the lOa-year flood <br />plain boundary corresponds to ~he boundary of ~he areas of special <br />flood hazards (Zones AE and AO), and the SOO-year flood plain bound- <br />ary corresponds to the boundary of areas of moderate flood hazards. <br />In cases where the 100- and SOD-year flood plain boundaries are <br />close together, only the lOa-year flood plain boundary has been <br />shown. Small areas within thei flood plain boundaries may lie above <br />the flood elevations but cannot be shown due to limitations of 'the <br />map scale and/or lack of detailed topographic data. <br /> <br />For the streams studied by app~oximate methods, only the lOa-year <br />flood plain boundary is shown on the Flood Insurance Rate Map (Exhi- <br />bit 2). <br /> <br />Approximate lOa-year flood pla~n boundaries in some portions of <br />the study area were taken directly from the Flood Hazard Boundary <br />Map (FHBM) for Pitkin County (~eference 24). <br /> <br />4.2 Floodways <br /> <br />Encroachment on flood plains, .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 itself. <br />One aspect of flood plain management involves balancing the economic <br />gain from flood plain develop~ent against the resulting increase <br />in flood hazard. For purposea of the NFIP, a floodway is used as <br />a tool to assist local communities in this aspect of flood plain <br />management. Under this concept, the area of the lOa-year flood <br />plain is divided into a flood~ay and a floodway fringe. The flood- <br />way is the channel of a stre~, plus any adjacent flood plain areas, <br />that must be kept free of encroachment so that the lOa-year flood <br />can be carried without substantial increases in flood heights. <br />Minimum Federal standardslim!t such increases to 1.0 foot, provid- <br />ed that hazardous velocities ~re not produced. The floodways in <br />this study are presented to local agencies as minimum standards <br />that can be adopted directly or that can be used as a basis for <br />additional floodway studies. <br /> <br />The floodways presented in this study were computed for certain <br />stream segments on the basis df equal conveyance reduction from <br />each side of the flood plain. Floodway widths were computed at <br />cross sections. Between cross sections, the floodway boundaries <br />were interpolated. The resul~s of the floodway computations are <br />tabulated for selected cross aections (Table 3). The computed <br />floodways are shown on the Flqod Insurance Rate Map (Exhibit 2). <br />In cases where the floodway aqd lOa-year flood plain boundaries <br />are either close together or qOllinear, only the floodway boundary <br />is shown. <br /> <br />16 <br />