FLOOD06232 - Laserfiche WebLink

Home Browse Search

EM 1110-2.1601 1 Jul 91 c. Preliminary layout. A preliminary map or aerial mosaic of the area showing the topography and other control factors to a scale satisfactory for plotting the cen- ter line of the channel should be obtained. A scale of 1 inch (in.) to 100 feet (ft) with 2-ft-contour interval is suggested. although judgment based on local conditions should be used. A preliminary profile should be prepared that will show all pertinent elevations of the ground and existing structures along the banks and along the center line of the proposed channel. d. Preliminary alternative designs. From a study of the preliminary plan. profiles. and available widths, tenta- tive channel cross sections are adopted. These are gener- ally rectangular or trapezoidal sections. Low velocity flows can usually be carried in natural-bottom trapezoidal channels with or without stone-revetted side slopes. High- velocity flows nonnally would be carried in concrete. lined channels. Preliminary hydraulic analyses '-2 of the proposed channels are then made with a view toward establishing the most efficient channel improve- ment from the standpoint of hydraulic efficiency and economic feasibility. e. Economy. Approximate cost estimates are pre- pared, including costs of channel construction, appurtenant works and bridges. and rights-of-way. It may be neces- sary to consider several channel alignments. cross sec- tions. and construction materials before the least-cost design consistent with sound engineering principles is detennined. Assured perfonnance. consistent with project fonnulation based on sound engineering judgment, is a necessary part of economic consideration. With an optimum general design thus tentatively established, and provided the cost is economically feasible for the project as a whole, the detailed hydraulic design is presented in Chapter 2. e . , e . e