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<br />Guide For Approximate Zone A Areas <br /> <br />Developing BFEs <br /> <br />where: <br /> <br />Ku Ke, KR' KT <br /> <br />= <br /> <br />conveyance for left overbank, <br />channel, right overbank, and <br />total conveyance, respectively <br />flow area for left overbank, <br />channel, right overbank, and <br />total flow area, respectively <br /> <br />Au Ac, AR' AT <br /> <br />= <br /> <br />Hydraulic depth, h, is computed by using the following <br />relationship: <br /> <br />A <br />h = ...I <br />T <br /> <br />where: <br /> <br />T = top water-surface width at the normal depth <br />AT = Total Flow Area <br /> <br />If the velocity head is greater than or equal to one-half the <br />hydraulic depth, the flow is supercritical. <br /> <br />For prismatic channels, the following equation can be used to <br />determine the critical depth: <br /> <br />02 = A' or 0 = ..jgA'/1 <br />g T <br /> <br />For a series (3 or more) of water-surface elevations, compute <br />the corresponding total area, A, water-surface topwidth, T, <br />and the critical discharge, Q, using 0 = ..jgA'/T. Compute the <br />value of right hand side of the above equation. Plot the <br />water-surface elevations and the corresponding discharge <br />values on graph paper. The critical water-surface elevation <br />and, therefore, critical depth, can be determined from this <br />graph for a range of discharge values. <br /> <br />For rectangular channels, critical depth can be computed <br />directly from the above equation and is expressed in the <br />following relationship: <br /> <br />[ 0 ]"'" <br />Dc = (5.67T) <br /> <br />The energy is minimum at the critical depth. For irregular <br />cross sections, critical depth is determined from the <br />relationship between the water-surface elevation and the <br />energy. The energy is computed by adding the water-surface <br />elevation and the corresponding velocity head (or energy grade <br />elevation) . For irregular cross sections, the velocity <br />coefficient, a, must be considered when computing velocity <br />head (HV). Several water-surface elevations should be assumed <br />and corresponding energy grade elevations computed. These <br />values are then plotted on a graph of water-surface elevation <br /> <br />V-27 <br />