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I I I I I I I I I I I I I I I I I I I floodway represents that portion of the flood plain where the 100-year flood velocities exceed 2 to 3 feet per second or the water surface profile would not increase more than 0.5 feet after encroachment. Floodway widths, repre- senting the limits of encroachment into the floodplain at each point in the stream, are tabulated in Table C-l of Appendix C. The water surface eleva- tions to be expected after encroachment are also tabulated. - SUMMARY - This report has identified the probable flooding limits of the IOO-year flood. As mentioned previously, floods of greater magnitude can occur and flooding beyond the extent of the limits shown in the report can also occur due to other factors. such as debris blockage. Flood Velocities Both structural and non-structural methods may be used to alleviate the flood problems. The non-structural measures include floodproofing, building permits, flood insurance. land use regulations and flood plain zoning. Average flow velocities during a 100-year flood for each drainageway are provided on the Flood Hazard Area Delineation Drawings. The drainageway velocities would average between 0.,9 and 11.1 feet per second for a 100- year event in the Basin area. Water flowing at a rate of 6 to 8 feet per second or greater will cause severe erosion of streambanks and is capable of transporting large rocks. Velocities in the range of 6 to 8 feet per second could erode fill around bridge abutments. Water flowing at about 2 feet per second or less will de- posit debris and slit. Presently Sloans Lake Drainage Basin is considered to have a moderate flood damage potential. The preservation and protection of the flood plains and adjoining lands will serve to maintain and even reduce the present flood haza rd. Floodway Evaluation Computer output data from HEC - 2 Water Surface Profile Computer Program fo r the existing channel conditions and for the floodway determination were ex- amined, to determine the distribution of flow, velocities and depth of flows in the fringe areas. The floodway limits were then adjusted to smooth the floodway boundaries and to reduce the rise in water surface elevation where existing improvements are involved. Adjustments have been made to the floodway limits to minimize the impact of the 100-year flood water surface profile ,ise in areas whe,re improvements are within the 100-year flood plain. In Reach J-K between stations 18+45 and 45+00 a floodway has not been defined because of the shallow I~O-yr flood depths, development in the flood plain, and lack of a well defined drainage course. 12