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<br />Guide por Approxima~e lone A Are.a <br /> <br />Developing BPBa <br /> <br />If the downstream water-surface elevation is higher than the <br />minimum road elevation, a submergence factor may be considered <br />in the weir flow computation. The submergence factor is <br />dependent upon the D/H ratio, where 0 is the downstream depth <br />of water above the road and H is the upstream energy grade <br />depth above the road, as shown in Figure 24, "Weir Flow Over <br />Road." The submergence factor must be considered when the D/H <br />ratio is more than 0.79. For a non-horizontal road profile, <br />the D/H ratio must be computed for each road segment. The <br />submergence factor, @, can be determined from the curve in <br />"Hydraulics of Bridge Waterways" (Reference 1, Figure 24) and <br />some typical values are given in the table below. <br /> <br />---. --[rm <br />H h <br /> <br /> <br />-~~~=-~/ <br /> <br />~ <br /> <br />----- <br /> <br />_____~r=_ <br />"'----= <br /> <br />Figure 24 - Weir Flow Over Road <br /> <br /> <br />0.998 <br />0.992 <br />0.986 <br />0..976 <br />0.962 <br /> <br />0.944 <br />0.932 <br />0.915 <br />0.889 <br />0.700 <br /> <br />( <br /> <br />Other procedures used in Federal agency backwater computer programs <br />can also be used to determine the submergence factor. <br /> <br />A third cross section may be used to determine a more accurate <br />water-surface elevation upstream of the structure. This may be <br />done by assuming water-surface elevations and calculating the <br />corresponding velocity heads (HV) until an assumed water-surface <br />elevation plus its, velocity head at that elevation equal the same <br />energy gradient elevation obtained from the weir flow equation. <br />The velocity head, HV, can be calculated using the following <br />equation: <br /> <br />HV = a (OJA)2/2g <br /> <br />V-31 <br />