Laserfiche WebLink
<br />mation prior to its use for the computation of the water-surface <br />profile. <br /> <br />Manning's roughness coefficients were determined from photographs <br />and field-survey data and ranged from 0.035 to 0.038 for the chan- <br />nel, and 0.038 to 0.080 for the overbanks of the SOuth Arkansas <br />River. Values for Poncha Creek ranged from 0.038 to 0.045 for the <br />channel, and 0.055 to 0.080 for the overbanks (Reference 1). <br /> <br />The water-surface elevations for the 10-, 50-, 100-, and 500-year <br />recurrence interval floods were computed using the U.S. Army Corps <br />of Engineers HEC-2 step-backwater computer program. This program <br />utilizes a solution to the one-dimensional energy equation to deter- <br />mine the shape of the profile between control sections where the <br />water-surface elevation is known or can be assumed. The procedure <br />for a steady flow profile calculation is called the "Standard Step <br />Method." In this method, the distance from a downstream or upstream <br />point, where the conditions are known, to the point, where the <br />backwater effects are to be determined, is divided into reaches by <br />cross sections at fixed locations along the river. Starting from <br />one control point, calculations of the water surface profile proceed, <br />in steps, from one cross section to the next. The HEC-2 program <br />is also capable of handling the effect of the various hydraulic <br />structures which are located across the river (Reference 3). <br /> <br />Locations of selected cross sections used in the hydraulic analyses <br />are shown on the Flood Profiles (Exhibit 1). <br /> <br />The hydraulic analyses for this study were based on unobstructed <br />flow. The flood elevations shown on the profiles are thus considered <br />valid only if hydraulic structures remain unobstructed, operate <br />properly, and do not fail. <br /> <br />All elevations are referenced to the National Geodetic Vertical <br />Datum of 1929 (NGVD). Elevation reference marks used in this study <br />are shown on the maps; the description of the marks are presented <br />in Elevation Reference Marks (Exhibit 3). <br /> <br />4.0 FLOOD PLAIN MANAGEMENT APPLICATIONS <br /> <br />The NFIP encourages State and local governments to adopt sound flood <br />plain management programs. Therefore, each Flood Insurance Study prOduces <br />maps designed to assist communities in developing flood plain management <br />measures. <br /> <br />4.1 Flood Boundaries <br /> <br />To provide a national standard without regional discrimination, <br />the 1 percent annual chance (lOO-year) flood has been adopted by <br />FEMA as the base flood for flood plain management purposes. The <br /> <br />9 <br />