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1. For the given kind of material forming the channel body, estimate the <br />roughness coefficient n (Section 4.5), side elope z (Table 4.3), and <br />the maximum permissible velocity V. <br />2. Compute the hydraulic radius R by the Manning formula (Equation 4.13). <br />3. Commute the water area required by the given discharge and permissible <br />velocity, or A ~ Q/V. <br />4. Compute the wetted perimeter, or P ~ A/R. <br />5. Using the expresaiona for A and P from Table 4.1, solve aimulte- <br />neously for b and y. <br />6. Add a proper freeboard, and modify the section for practicability. <br />6.4.2 Evaluating the Channel for Reasonable Shape <br />Following the design procedure using maximum parmieaible velocity can <br />result in a very shallow, vide channel, as illustrated in the exa~le nt the <br />end of the chapter. This type of cross section ie clearly not desirable since <br />the water would probably not flow uniformly acroea the entire width. Rather, <br />it would tend to concentrate in one area by scouring a ner deeper, narrower <br />channel within the limits of the broader channel. Therefore, consideration <br />• must be given to the computed channel dimensions to insure they represent a <br />practical deaign. F~pirical formslas have been developed that provide <br />guidance in assessing the practicality of a channel deaign. Some of the for- <br />mulas used to evaluate depth of flow or the width-to-depth (b/d) ratio are <br />given below. <br />1. U.S. Bureau of Reclamation procedure <br />d ~ 0.5 /A (6.7) <br />A ~ Area in ft2 <br />and for a trapezoidal cross section <br />b ~ 4 - z (6.8) <br />d <br />2. Irrigation Service Procedure, India <br />d ~ /A/3 (6.9) <br />and for a trapezoidal cross section <br />L I <br />