FLOOD00397 - Laserfiche WebLink

Home Browse Search

at a few cross sections that, although surrounded by flow, will not be inundated by the 100-year flood. Those areas are only shown on the work map where they are large and continuous over several sections. Channel roughness factors (Manning's "n") used in the hydraulic computations were determined by engineering judgment based on field observations of the floodplain areas and by using prediction equations developed by Jarrett (Reference 5). Along the Yampa River, values ranged from 0.043 to 0.060 for the channel and 0.031 to 0.080 for the overbank areas. Along the Elk River, values ranged from 0.045 to 0.062 for the channel and 0.040 to 0.090 for the overbank areas. Water-surface elevations for the 100-year flood were computed by WSPRO, a step-backwater computer program developed by the USGS for the Federal Highway Administration (Reference 6). The starting water-surface elevation for the downstream portion of the Yampa Ri ver was determined using the slope-conveyance method (Reference 6). The starting water-surface elevation for the upstream portion of the Yampa River was determined using the 100-year flood elevation defined by the FEHA (Reference 3). The starting water- surface elevation for the Elk River was determined using the slope- conveyance method (Reference 6). Flood profiles were drawn showing computed water-surface elevations to a plotted accuracy of 0.5 foot for the 100-year flood. The flood profile also shows the streambed elevation, location of bridges, cross sections and tributaries. The hydraulic analyses for this study were based on unobstructed flow. The flood elevations shown on the profiles are thus considered valid only if hydraulic structures remain unobstructed, operate properly, and do not fail. One exception to this was made in determining the floodplain boundaries downstream from cross section D. It was assumed that the rock and earth levee shown on the map just upstream from the railway bridge on the right bank will wash out. All elevations are referenced to the National Geodetic Vertical Datum of 1929 (NGVD). Elevation reference marks used in this study are shown on the maps; the description of the marks are presented in Elevation Reference Marks (Exhibit 3). 4.0 FLOODPLAIN MANAGEMENT APPLICATIONS The NFIP encourages state and local governments to adopt sound floodplain management programs. Therefore, each Flood Insurance Study provides 100-year flood elevations and delineations of the 100- and 500-year floodplain boundaries and 100-year floodway to assist communities in developing floodplain management measures. 7