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<br />the program will compute the energy elevations for low and weir flow and <br />pressure and weir flow. The higher of the two energy elevations will con- <br />trol. Energy elevations below the maximum low chord are for low flow or <br />low and weir flow for this type of bridge. <br />Based on the previous checks, the bridge routine has differentiated <br />between low flow and pressure flow. With either type of flow, the pro- <br />gram checks against the minimum top of road elevation (ELTRD) to deter. <br />mine if weir flow also exists. If the energy elevation is greater than <br />ELTRD, a trial and error solution is made to determine the distribution <br />of flow. The computed weir flow is listed under QWEIR and the flow <br />under the bridge is given under QPR regardless of whether it is low flow <br />or pressure flow. The flow diagram for computing the combination flow <br />solution is shown in Figure 5. Up to 20 iterations are made to balance <br />the total discharge to within 1% of the given discharge. <br />Important parameters in the decision logic of the special bridge <br />routine are the tno test elevations ELLC and ELTRD. Because they play <br />such an important role in the bridge routine, it is recommended they <br />always be coded as input on fields 4 and 5 of the X2 card. <br />Input Losses. One other method of computing water surface profiles <br />through bridges is to input the bridge loss. The loss used could be <br />just the .structure" loss, or it could be the total loss between any <br />two adjacent cross section. Differences in water surface elevations <br />can be read on the X5 card for each discharge profile. The field read <br />on the X5 card is called by variable INQ on the second field of the Jl <br />card. <br /> <br />15 <br />