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I I I I I I I I I I I I I I I I I I I 10 Flood Plain and Floodway Data SUMMARY The limits of future flooding for the South Platte River are graphically outlined on the Flood Hazard Area Delineation maps and tabulated in Tables I and II, Flood Plain and Floodway Reference Data. The tables list the potential flood elevations, flood plain and floodway widths, floodway location, floodway surcharge elevation and thalweg elevation at each cross-section studied. This report has identified the probable flooding limits of a potential IOO-year flood event in the South Platte River drain- age basin. A floodway represents a part of the flood plain which is re- quired to pass a 100-year flood event without raising the flood profile more than an acceptable amount and also represents that part of the flood plain most hazardous to personal safety and welfare. In this study, the floodway was defined as being the limit of encroachment into the flood plain where existing flood profiles are increased no more than one-half of a foot or where velocities exceed two feet per second or where depths are in ex- cess of two feet. Floodway widths, which represent the maximum allowable limits of encroachment into the flood plain, are tabu- lated in Tables I and II. The flood plain limits are based on (1) planned, future develop- ment in the basin, (2) existing and mapped flood plain condi- ~ions, and (3) no blockage at the existing culvert and bridge structures. Even with these assumptions, floods of greater mag- nitude can and will occur in which the limits of flooding will exceed those shown in this report. Flood Velocities Presently, the South Platte River in Adams County, is considered to have a high flood damage potential to agriculture and road crossings. The preservation of the flood plain will serve, at least, to maintain the present level of damage potential while any improvements will minimize or reduce the flood damage po- tential. Reducing the flood hazard potential can be accomplished through structural improvements to bridges and channels, flood proofing and an operational and effective flood plain management program. Average flow velocities, characteristic of a 100-year flood event for the South Platte River are shown on the profile portion of the Flood Hazard Area Delineation drawings. The flow velocity in the channel ranges generally between 3 and 15 feet per second for a 100-year event in the study area. Overbank velocities range between 0.5 and 5.0 feet per second. Water flowing at a rate greater than 6 to B feet per second will cause severe erosion of stream banks and is capable of transport- ing large rocks. Velocities in the range of 6 to B feet per second could erode fill around bridge abutments. Water flowing at about 2 feet per second or less will deposit debris and silt.