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<br />upstream elevation. If weir flow did not exist, the program would check <br />for piers and then solve for a low flow solution. With piers, the 10w flow <br />solution would be based on the momentum or the Yarnell equation; and <br />without piers, the solution would be computed using standard step <br />calculations. <br />Had the energy elevation required for pressure flow (EGPRS) been <br />calculated, the program would go on to compare the low flow energy el- <br />evation EGLWC with EGPRS to see which controls. The higher of the two <br />controls, as illustrated in the Typical Discharge Rating Curve shown <br />in Figure 4. <br />One exception to the di rect compari sions of the two energy eleva- <br />tions is when the minimum elevation of the top of road (ELTRD) is less <br />than the maximum elevation of the low chord (ELLC). For this type of <br />bridge, a combination of weir flow and low flow can occur. The low <br />flow energy elevation (EGLWC) is compared to the estimated maximum <br />energy elevation f~r low flo~ control (1.5 times depth plus invert <br />elevation), rather than EGPRS, because the low road elevation would <br />cause weir flow to exist prior to the occurrence of pressure flow. <br />Depth is defined here as the difference between the low chord (ELLC) <br />and the invert elevation (EUUN). <br />At critical depth, 1.5 times the depth represents the minimum <br />specific energy that could occur for a rectangular section. If critical <br />depth occurred just at the maximum low chord e1evation, it wou1d produce <br />the maximum possible energy elevation for Tow flow. Therefore, an energy <br />elevation greater than that va1ue would have to be for pressure flow. <br />For the energy range between the low chord and the maximum low flow energy, <br /> <br />13 <br />