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<br />Channel and overbank roughness factors (Manning's "n") were assigned <br />on the basis of field inspections and aerial photography. Roughness <br />values vary from 0.03S to 0.040 for the channel, and from 0.070 to <br />0.110 for the overbanks. <br /> <br />Flood Profiles were drawn showing computed water-surface elevations <br />to an accuracy of O.S foot for floods of the selected recurrence <br />intervals. Starting elevations were developed by the slope-area <br />method. All elevations are referenced to the National Geodetic <br />Vertical Datum. Elevation reference marks used in this study are <br />shown on the maps. <br /> <br />For study purposes herein, it has been assumed that limited clogging <br />of the main river channel will occur, and the levee and all bridges <br />will remain intact. <br /> <br />4. FLOOD PLAIN MANAGEMENT APPLICATION <br /> <br />A prime purpose of the National Flood Insurance Program is to encourage <br />state and local governments to adopt sound flood plain management programs. <br />Each Flood Insurance Study, therefore, includes a flood boundary map <br />designed to assist communities in developing sound flood plain management <br />measures. <br /> <br />4.1 Flood Boundaries <br /> <br />In order to provide a national standard without regional discrimina- <br />tion, the 100-year flood has been adopted by the Federal Emergency <br />Management Agency as the base flood for purposes of flood plain <br />management measures. The SOD-year flood is employed to indicate <br />additional areas of flood risk in the community. For each water- <br />course studied in detail, the boundaries of the 10~-year and the <br />SOO-year floods have been delineated using the flood elevations <br />determined at each cross section; between cross sections, the <br />boundaries were determined using topographic maps at a scale of <br />1:4800, with contour intervals of 2 feet (Reference 5). In cases <br />where the 100-year and 500-year flood boundaries are close together, <br />only the 100-year boundary has been shown. <br /> <br />The absence of well-defined watercourses in the overbank areas of the <br />Rio Grande makes flood paths unpredictable. Because of this unpre- <br />dictability, most of Alamosa is designated as a sheet flow area. <br /> <br />Numerous small areas within the flood boundaries may be above the <br />flood elevations and, therefore, not be subject to flooding; owing <br />to limitations of the map scale, such areas are not shown. <br /> <br />" <br /> <br />REVISED: 11/9/82 <br /> <br />10 <br />