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<br />The losses COJIIputed by the nonnal bridge routine from cross sections <br />on either end of the bridge do not reflect the shock losses experienced <br />at the entrance and exit of the bridge. These losses may be taken into <br />consideration by using a full river and a constricted cross section <br />immediately upstream and downstrelllll from the bridge and performing normal <br />backwater computations through these cross sections. <br />LIMITATIONS OF THE HEC BRIDGE ROUTINE <br />Little experimental data is available for determining the ~ic <br />force for piers of various shapes in equation 3. While the procedure for <br />detennining pier losses for low-flow control has been verified to a certain <br />extent by Koch and Carstanjen (reference 7) and the Los Angeles District <br />Corps of Engineers (reference 9), additional testing is warranted. <br />The coefficient of freeflow discharge used in the weir equation must <br />be determined largely by judgment. The correction for submergence is <br />based on experimental data for ogee spillways and becomes unreliable for <br />high values of submergence. <br />Some accuracy is probably lost by the assumption that the water <br />surface profile perpendicular to the direction of flow is horizontal. <br />Since the velocity of flow increases from the overbanks to the center of <br />the channel, the water surface elevation drops towards the center of the <br />channel. This condition can cause weir flow in the overbanks and low <br />flow in the channel for a level top of bridge. <br /> <br />'13 <br />