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at a location three-quarters mile downstream of the original loca- tion and operated until October 1957. Reestablishment of the station was again made for a 2-year period from October 1962 to October 1964. Station USe has again been discontinued. 3.2 Hydraulic Analyses Analyses of the hydraulic characteristics of flooding from the sourceS studied were carried out to provide estimates of the eleva- tions of floods of the selected recurrence intervals. Water-surface elevations of floods of the selected recurrence intervals were computed through use of the COE HEC-2 step-backwater computer program (Reference 7). Cross sections for the backwater analysis of the North Fork Republican River were obtained from 2- and 5-foot contour topo- graphic mapping (References Band 9) developed from aerial photo- graphs at a scale of 1:2,400 (Reference 10). Locations of selected cross sections used in the hydraulic analyses are shown on the Flood Profiles (Exhibit 1) and on the Flood Boundary and Floodway Map (Exhibit 2). Channel and overbank roughness factors (Manning's Un") were assigned on the basis of field inspection of the flood plain areas. Typical values for channel roughness factors were 0.055 and 0.07 for over- bank coefficients. A detailed description of the hydraulic analysis is included in the Technical Addendum of the COE Flood Plain Infor- mation Report (Reference 6). Starting water-surface elevations for all floods on the North Fork Republican River were calculated using normal-depth analysis. Obstructions to flood flows within the study reach include natural obstructions, such as brush and other vegetation growing along the streambank, and manmade obstructions, such as bridges, buildings, and culverts. During floods, debris COllecting on bridges can decrease the river's carrying capacity and caUSe greater depth upstream of the structures (backwater effect). As floodflows increase, masses of debris may break loose and cause a "wall" of water and debris to surge downstream until another obstruction is encountered. In some instances, debris may collect to the point where structural capability is exceeded and the bridge is destroyed. Because the OCCUrrence and amount of debris are indeterminate factors, only the physical characteristics of the structures were considered in computing the water-surface profiles. Similarly, maps of the flooded areas show the backwater effect of inadequate bridges and culverts, but do not reflect increased water-surface elevations that could be caused by debris collecting against the structures or the deposition of silt in the stream channel under structures. 10