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<br />COMBINATION FLOWS <br />A combination flow (pressure and weir, or 10w f'low and weir) may <br />be determined by an iterative prqcess of assuming energy grade line <br />elevations and computing corresponding discharges through the bridge <br />(pressure or low flow) and overbanks (weir flOW) until the total <br />discharge corresponds to the given discharge. The determination of the <br />existence of the various types of flows is illustrated in Figures 6 and <br />7, The combination of low-flow under the bridge and weir flow in the <br />overbanks (condition c) exists when the energy grade line elevation <br />required to pass the discharge under consideration is above the minimum <br />roadway elevation and below the low chord of the bridge. Figure 8 is a <br />flow diagram which depicts the methodology of computing the flow profile <br />for the combination flows using the HEC computer program. <br />BACKWATER WITH CORRECTION FOR BRIDGE DECK <br />A simple method of computing water-surface profiles through bridges <br />(called the normal bridge method) involves performing normal backwater <br />computations, making appropriate corrections for the area and wetted <br />perimeter of the bridge deck. This procedure for non-trapezoidal sections <br />where low-flow controls is superior to the method discussed in previous <br />paragraphs since a trapezoidal channel was assumed. When the discharge <br />is supercritical and the outlet is submerged, the normal method is not <br />applicable, and the method described in the preceding paragraphs should <br />be used, since critical depth should not be crossed in normal backwater <br />computations. However, if the downstream tailwater is also over the road- <br />way, tben critical depth is not crossed and the normal bridge method is <br />appropriate. <br /> <br />12 <br />