The flow pattern and bed shear stress are important factors to determine the scour potential regions in river bridges. This study has developed a 3D numerical model to simulate the flow around bridge abutments in a compound channel, and study the simultaneous effects of the bridge skew angle and contraction ratio (i.e. the roadway embankment length normal to the flowto the floodplain width) on the velocity distribution and the bed shear stress. After the model performance was verified, 13 cases were considered with different skew angles and contraction ratios. The results showed that the flow was more complex around the flow-splitting embankment than around the flow-guiding one, because of the flow-roadway embankment confrontation. At a zero skew angle, an increased contraction ratio increased the velocity around both abutments significantly; velocity increased by 67% and 40% when the contraction ratio rose from 0.25 to 0.50 and from 0.50 to 0.75, respectively. This velocity increase around the flow-splitting embankment was also visible for 15°, 30° and 45° angles; unexpectedly, the increase was less for the 45° angle than for the other cases. A shear stress study around the flow-guiding embankment showed that in all three cases an increased skew angle reduced the maximum shear stress mildly at contraction ratios of 0.25 and 0.50, and severely at 0.75. The trend of the maximum shear stress variations around the flow-splitting embankment was different for different contraction ratios - at a contraction ratio of 0.25 it was the highest at 45°, but for contraction ratios of 0.50 and 0.75, the maximum was 30°.
This paper investigates the effect of different geometries of approach embankments and guides banks on the flow pattern and bed shear stress values in the skewed bridges in the compound channel using three-dimensional numerical modelling. First, the numerical model was evaluated based on the results of existing laboratory studies. After ensuring its proper performance, the elliptical guide bank and the three types of abutments: vertical-wall, spill-through, and wing-wall, at different skew angles are examined. Investigation of the values of maximum velocity and bed shear stress at the flow-conducting embankment and the flow-splitting embankment showed that in the flow-conducting embankment, the best performance is assigned to the elliptical guide bank. In contrast, the performance of various abutments is different for the flow-splitting embankment depending on the skew angle of the bridge. Then, different patterns based on streamlines for the geometry plan of the guide bank were proposed and studied. The results show that the most suitable pattern for the guide bank reduces the maximum flow velocity by up to 15% and reduces the maximum bed shear stress by up to 80% around the flow-splitting embankments.
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