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<br />23. EFFECT OF lHLLOW RENOVAL <br /> <br />. <br /> <br />Removal of. the willows and vegetation from the channel would <br />increase the channel capacity by various amounts, depending chiefly <br />on the density and extent of the willow growth. In the test reach, <br />the water surface profiles of various streamflows were .computed, <br />assuming that the willows were removed and that the coefficient of <br />roughness throughout the full channel width was that of the clear <br />portion. The improvement in the test reach was estimated to be 36 <br />percent of the present capacity. This increase was applied to the <br />other reaches of the river, considering, also, the extent of willow <br />growth within the channel lines; and improvements ranging from 1,600 <br />second-feet to 3,700 second-feet "ere estimated to result from <br />willow removal. In vie" of the erodibility of the islands once they <br />are denuded, it is probable that the river would develop a more <br />efficient channel section, but estimated improvement did not include <br />this factor since it is difficult to forecast how much time this <br />adjustment would require. Reference is made to the detailed dis- <br />cussion of the efficacy of channel clearing in Appendix IV: Hydrol- <br />ogy and Hydraulics. <br /> <br />24. BENEFITS FRON WILLOW RENOVAL <br /> <br />The benefits which would accrue from the removal of channel <br />willows would arise from the reduction of flooding as more water <br />would be carried in the improved channel and less would be forced <br />into overbank flow. The residual annual flood damage was computed <br />for each reach assuming that the existing and authorized flood <br />control projects were in operation. The further reduction in flood <br />damage attributable to the channel improvement was computed by the <br />frequency method and the forecast of future benefits was developed <br />from an economic base study. The results were summarized for the <br />three main reaches, which will probably be the basis for project <br />sponsorship; namely, Trenton Dam to Harlan County Reservoir; Harlan <br />County Dam to the Nebraska-Kansas State line; and the State line <br />to Milford Reservoir. The benefit values, together with the annual <br />project costs and the resulting economic ratios are shown in <br />table 8. Reference is made to Appendix III: Economic Analysis for <br />a more detailed explanation of the benefit computations. <br /> <br />. <br /> <br />28 <br />