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## Journal of the South African Institution of Civil Engineering

*On-line version* ISSN 2309-8775

*Print version* ISSN 1021-2019

#### Abstract

MAHJOOB, A and KILANEHEI, F. **Effects of the skew angle and road embankment length on the hydraulic performance of bridges on compound channels**.* J. S. Afr. Inst. Civ. Eng.* [online]. 2020, vol.62, n.4, pp.44-54.
ISSN 2309-8775. http://dx.doi.org/10.17159/2309-8775/2020/v62n4a5.

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°.

**Keywords
:
**compound channel; contraction ratio; bridge skew; finite volume method; numerical simulation.