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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />II <br /> <br />13 <br /> <br />Flood Plain and Floodway Data <br /> <br />channel would generally range between 4 to 8 feet per second <br /> <br /> <br />for a 100-year event in the two study areas. Overbank velocities <br /> <br /> <br />would range between 0.5 to 3 feet per second in the study areas. <br /> <br />The limits of future flooding for Second and Third Creeks are <br /> <br /> <br />graphically outlined on the attached Flood Hazard Area Delineation <br /> <br /> <br />Maps and tabulated in Tables 5 and 6, respectively, Flood Plain <br /> <br /> <br />and Floodway Reference Data. The tables list the potential flood <br /> <br /> <br />elevations, flood plain and floodway widths, floodway location, <br /> <br /> <br />floodway surcharge elevation and thalweg elevation at each cross <br /> <br /> <br />section studied. <br /> <br />Water flowing at a rate of 6 to 8 feet per second or greater will <br /> <br /> <br />cause severe erosion of stream banks and is capable of transport- <br /> <br /> <br />ing large rocks. Velocities in the range of 6 to 8 feet per <br /> <br /> <br />second could erode fill around bridge abutments. Water flowing <br /> <br /> <br />at about 2 feet per second or less will deposit debris and silt. <br /> <br />As mentioned, floodways have been computed for each stream. A <br /> <br /> <br />floodway represents a part of the flood plain which is required <br /> <br /> <br />to pass a 100-year flood event without raising the flood profile <br /> <br /> <br />more than an acceptable amount and also represents that part of <br /> <br /> <br />the flood plain most hazardous to personal safety and welfare. <br /> <br /> <br />In this study, the floodway was defined as being the 100-year <br /> <br /> <br />flood plain area in which average flow velocities exceeded two <br /> <br /> <br />feet per second, flow depths exceeded two feet or the point of <br /> <br /> <br />encroachment to which the natural 100-year flood profile is not <br /> <br /> <br />increased more than 0.5 feet. Floodway widths, which represent <br /> <br /> <br />the maximum limits of encroachment into the flood plain, are <br /> <br /> <br />tabulated in Tables 5 and 6. <br /> <br />- SUMMARY - <br /> <br />This report has identified the probable flooding limits of a <br /> <br /> <br />potential 100-year flood event in the Second and Third Creek <br /> <br /> <br />drainage basins. The flood plain limits presume planned, future <br /> <br /> <br />development in the basin, existing, mapped flood plain conditions, <br /> <br /> <br />no peak flood flow reduction or interception by the irrigation <br /> <br /> <br />canals, and minor blockage at the existing culvert and bridge <br /> <br /> <br />structures. Even with these assumptions, floods of greater <br /> <br /> <br />magnitude can and will occur in which the limits of flooding <br /> <br /> <br />will exceed those shown in this report. <br /> <br />Flood Velocities <br /> <br />Presently, the majority of potential flood damages on these <br /> <br /> <br />streams exists in the lower reaches, generally downstream of <br /> <br /> <br />U.S. 85. In this area, there has been development within the <br /> <br /> <br />100-year flood plain. Upstream, flood damage will result to <br /> <br /> <br />agricultural lands. Most of the road and irrigation canal <br /> <br />Average flow velocities during a 100-year flood event on Second <br />or Third Creek are shown on the profile portion of the Flood <br />Hazard Area Delineation Drawings. The flow velocity in the <br />