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<br />EM 1110-2-1601 <br />1 Jul 91 <br /> <br />c. Preliminary layout, A preliminary map or aerial <br />mosaic of the area showing the 1Opography and other <br />control fac10rs 10 a scale satisfactory for plotting the cen- <br />ter line of the channel should be obtained. A scale of <br />I inch (in.) 10 100 feet (ft) with 2-ft-con1Our interVal is <br />suggested. although judgment based on local condilions <br />should be used. A preliminary profile should be prepared <br />that will show all pertinent elevations of the ground and <br />existing structures along the banks and along the center <br />line of the proposed channel. <br /> <br />d. Preliminary alternative designs, From a study of <br />the pre1iminary plan. profiles. and avai1able widths. tenta- <br />live channel cross seclions are adopted. These are gener- <br />ally rectangular or lrapezoidal seclions. Low velocity <br />flows can usually be carried in natural-bottom trapezoidal <br />cl1:mnels with or without stone-revetted side slopes. <br />High- velocity flows normally would be c:uried in <br />concrete,lined channels. Preliminary hydzauIic analyses <br /> <br />1-2 <br /> <br />~ <br /> <br />of the proposed channels are then made with a view <br />toward establishing the most efficient channel improve- <br />ment from the standpoint of hydrnuIic efficiency and <br />economic feasibility. <br /> <br />e, Economy. Approximate cost estimates are pre- <br />pared. including costs of channel construction. appurtenant <br />works and bridges. and rights-of-way, It may be neces- <br />sary to consider seveJ3l channel alignments. cross sec- <br />lions. and construction materials before the least~ost <br />design consistent with sound engineering principles is <br />detennined. Assured perfonnance. consistent with project <br />formulation based on sound engineering judgment. is a <br />necessary part of economic considet3tion. With an <br />optimum general design thus tentatively established. and <br />provided the cost is economically feasible for the project <br />as a whole. the detailed hydraulk design is presented in <br />Chapter 2. <br />