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<br />e <br /> <br />e <br /> <br />tit <br /> <br />Evaluation of project effects. - - Many items must be considered in <br />evaluating project benefits, among these are: <br /> <br />1, The stage-discharge relationship of the project channel, including <br />overbank-flow, should be determined for "before" and "after" project <br />condi tions. <br /> <br />2. Stage-discharge curves for the outlet should be developed by computing <br />the water surface profile through two or more cross sections below <br />the outlet using the existing roughness coefficient. <br /> <br />3. The effect of project improvements on stages in the outlet should <br />be analyzed for storms of at least two recurrence frequencies. <br />Storm frequencies selected for the analysis should be that used <br />for project design and one other significantly different from the <br />design discharge. <br /> <br />Procedures outlined in National Engineering Handbook, Section <br />4, Hydrology, Part I, Watershed Planning, shall be used for the <br />analysis. <br /> <br />4. Where downstream effects of channel improvement are significant <br />(stage increases will cause damages below the termination of the <br />project channel) an analysis to determine effects should be carried <br />downstream to the point where effects have been dissipated. <br /> <br />5. Geologic and soil investigations to determine the effect of project <br />improvements on the stability of the outlet should be made as outlined <br />in Chapter 3, "Site Investigations." <br /> <br />Special outlet conditions. - - <br /> <br />1. Tidal Influence. Where channel improvements discharge into rivers, <br />estuaries, bays, and sounds, which are subject to tidal influence, <br />the effect of the tides on discharge from the channel should be <br />determined. This is true if the outlet is a tidegate, which opens <br />and closes according to the relative elevations of the tide and the <br />hydraulic grade of the channel; or if the channel discharges directly <br />into tidewater, without a tidegate. <br /> <br />The characteristics and types of tides are discussed fully in "Tidal <br />Datum Planes."11 Annual editions of "Tide Tables, High and Low <br />Water PredictiOns" are available from the Coast and Geodetic <br />Survey, U.S. Department of Commerce. <br /> <br />2. Pumping. Where the project is provided with pumps to discharge <br />the runoff ftom the watershed, the area may be protected by levees. <br /> <br />1/ <br />- Numbers refer to numbered references at end of this technical release. <br /> <br />1-3 <br />