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Channel Overbank .035 to .045 .050 to .600 .070 .070 depths greater than two to three feet would be common in the overbank areas. overbank velocities would generally range between three to eight feet per second. In the main channel, typical flood depths would be ~ eigttt to ten feet while average velocities would range between ten and fourteen feet per second. FLOOD Table 5 PLAIN ROUGHNESS VALUES Typical Range Maximum At the lower study limit, the flood levels reported in the December, 1971, FlOOd Plain Information for the Big Thompson at Loveland, Colorado, were used as the basis for this study (Ref. 11). A summary of the hydraulic computations completed for ~h~ study is available for review at the offices of the Colorado Water Conservation Board. The major bridges crossing the river and tributaries were considered in the hydraulic analysis.' Smaller culverts and temporary river crossing facilities installed after the July 31, 1976 flood were not considered in the hydraulic analysis, Bridge blockage due to debris catching at each structure and to bridge skew across the flood plain was considered. The maximum blockage of the total bridge flow area was limited to 30 percent. , i , , , i I , FLOOOWAY Super elevations, the rise in water surface elevation due to high velocity flows around curves, WnS ~nnlY7-pd ann included in the laO-year flood profile. Similarly, the damming and blockage effects of side tributary inflow was also included in the lOO-year flood profile. Encroachment into the natural flood plain with fill material, buildings, roads and bridges, reduces the flood-carrying capacity of the stream and increases flood heights, velocities and flood hazards in areas upstream and downstream from the location of the encroachment itself. One aspect of flood plain management inVOlves balancing the assets or benefits of flood plain development against the liabilities of increased ha~ards and deferred public and private costs. The concept of a floadway is used as a tool to assist local communities in this aspect of flood plain management. Under the floadway concept, the area of the 100-year flood plain is divided into a floadway ~one and a floodway fringe or low hazard zone. The floodway zone includes the main channel plus adjacent flood plain areas, which must be kept free of encroachment in order that the lOa-year flood, at a minimum, be carried without increasing potential hazards by increasing flood depths and velocities. A tabulation of the computed flood elevations along each stream is provided in Tables 6-12. The flood plain limits and flood profiles are shown on the Flooded Areas maps and profile charts included in Volume II. The flocdway prepared for this study and delineated on the Flooded Areas maps in Volume II meets the minimum criteria for the floodway ~one as defined in the Model Flood Plain The hydraulic study indicated that most of the canyon bottom would be inundated during a laO-year flood event. Flood -14- -15-