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<br />I <br /> <br />ua 8cj is <br />( .', <br />\ <br /> <br />. <br /> <br />I propor- <br /> <br />(IV-2) <br /> <br />(IV-3) <br /> <br />./ > <br /> <br />\. <br /> <br />(IV-4) <br /> <br />(IV-Sl <br /> <br />(IV-6) <br /> <br />. <br /> <br />. <br /> <br />EM 1110-2-1601 <br />Appendix IV <br />1 July 70 <br /> <br />3. Backwater Computation. a. All the cross-section hydraulic param- <br />eters necessary for backwater computations are computed in plates IV -1 and <br />IV -3. Computing the same parameters at several water- surface elevations <br />and plotting the results permits ready interpolation for intermediate values. <br />The method is programmed for digital computer use 89,90 if manual computa- <br />tions for a particular project alc'e too time consuming. <br />b. The boundary and hydraulic characteristics of a channel reach are <br />assumed to be those obtained by averaging the conditions existing at each end <br />of the reach. This procedure implies that the roughnesS dimensions k as- <br />signed to the upstream and downstream sections extend to the midsection of <br />the reach. Therefore,> it i~ important that the reach limits be carefully <br />selected. Two different sets of subsection roughness values should be as- <br />signed in cases where the boundary condition changes abruptly such as at the. <br />beginning or end of an improved reach. One set of values would apply in the <br />improved reach and the other in the natural channel. <br />4. Roughnes s Relation. The ruughness dimension k may be taken as <br />equivalent spherical diameter of the average size bed material when the hy- <br />draulic losses in the flow regime are attributable to friction alone. In a flow <br />regime where hydraulic los ses in addition to friction are present, k may <br />still be used if the losses result in a reasonably uniform slope of the energy <br />grade line. In this case. k will be larger dimensionally than the equivalent> <br />spherical diameter of the average size bed material. As Chezy C and <br />( C R1/6 ) <br />Manning's n are equatable 1.486 = --;-- . k may be determined from a <br />knowledge of Manning's coefficient n. While k remains fairly constant <br />with changing R, n varies with the one-sixth power of R. Therefore, it is <br />better to extrapolate from known conditions to unknown by the use of k <br />rather than n. The k must be evaluated for each subsection. Subsections <br />should be chosen with this in mind so that differing bed materials or bed con- <br />ditions producing frictionlike losses, such as ripples. dunes, or other ir- <br />regularities will appear in separate subsections. Hydraulic losses tending to <br />cause breaks in the energy grade line, such as expansion and contraction, <br /> <br />IV -3 <br /> <br /> <br />-- <br />""\or. <br /> <br />.-- <br />