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<br />of a stream, plus any adjacent flood plain areas, that <br />must be kept free of encroachment in order that the <br />100-year flood be carried without substantial increases <br />in flood heights. As minimum standards, the Federal <br />Insurance Administration limits such increases in <br />flood heights to 1.0 foot, provided that hazardous <br />velocities are not produced. The floodways in this <br />report are presented to local agencies as minimum stan- <br />dards that can be adopted or that can be used as a <br />basis for additional studies. <br /> <br />The floodways for the streams in this study were <br />determined using both the criteria of flood plain <br />encroachment and conveyance reduction, as well as <br />consideration of hazardous velocities and depths within <br />the flood plain. The tributary streams to the Purgatoire <br />River are generally steep and narrow; hence, further <br />encroachment or conveyance ability reduction would <br />result in increased velocities above already hazardous <br />velocity conditions. Hazardous velocities are considered <br />to be average overbank velocities in excess of 2 to <br />3 feet per second. The floodway limits for each of <br />the streams were plotted at the cross section locations; <br />between the cross sections, the floodway boundary <br />has been drawn to represent streamlined limits charac- <br />teristic of floodway limits. <br /> <br />Many factors affect the determination of the encroachment <br />limits on each stream, including sand and gravel mining <br />operations, overtopped bridges, present development <br />within the flood plain, and divided flow conditions. <br />To determine the final floodway limit at each cross <br />section, characteristic velocities were considered <br />along with encroachment limits which cause no more <br />than the maximum 1.0 foot rise in the flood height. <br />Where appropriate, an equal degree of encroachment <br />was permitted on each side of the main channel. The <br />degree of conveyance reduction on each side of the <br />channel was determined by hydraulic limitations. <br /> <br />The results of these computations are tabulated at <br />selected cross sections for each stream studied in <br />detail (Table 1). <br /> <br />As shown on the Floodway Map (Exhibit 2), the floodway <br />boundaries were determined at cross sections; between <br />cross sections, the boundaries were interpolated. <br /> <br />23 <br />