The Calculation Of Bridge Backwater

Because of the constriction of the bridge, flow area under the bridge would be smaller than before, and the streamline in the upstream would be constricted while expand in the downstream, then there will be head loss because of the local resistance of the river. So the elevation of the flow between upstream and downstream would be different, which called backwater. The backwater isn't obvious when the discharge is small but significant when the discharge is large. The existence of backwater increases the elevation and the flow area . If the discharge is very large, the flood would overflow the bridge, even more serious, make the bridge damaged. When designing bridge, the computation of the scour, the design of the bridge spans, and the choice of the scheme all are based on the computation of bridge hydraulics. The computation of the backwater is one of the main content of bridge hydraulics. The rising elevation of upstream affect the elevation of the embankment design and the design of bridge opening. So the backwater computation is very important in bridge design. Backwater in shallow rivers was computed in this paper. River in which the cross-section chang slightly along the flow direction were regarded as uniform rivers. If there were no bridge in the river, the flow pattern was considered as uniform flow. So the elevation of different sections could be calculate if given the discharge. Then the results were put into the model built with software SMS. In the model ,a bridge was built. Then we could obtain the unknown factors in the model. After the calculated date were compared with the original data, the coefficient could be determined. Backwater in different floods were calculated in this model ,then the results were compared with the test data and the reason why they were different was analysed.Moreover, backwater caused by piers had also been studied in this paper independently.