Numerical Modeling of the Effect of Abutment Length in the Floodplain on Bed Shear Stress and Flow Momentum

Background and Objectives: As the water level rises in the main channel and crosses the floodplain boundary, because of different roughness coefficient and velocity between the main channel and the floodplain, the flow in the main channel move more acceleration than the flow in the floodplains and there will be strong momentum exchange between the flows in the intersection of the main channel and the floodplain. This exchange generates vortices in the intersection of the main channel and the floodplain. Momentum exchange reduces the high velocity flow energy of the main channel and adds to the low velocity flow energy of the floodplain. Momentum exchange also reduces the discharge of the main channel, increasing the discharge of the floodplain and decreasing river transmission capacity. At intersection of the main channel and the floodplain, due to the shear layer, the potential for instability and scour are high. Understanding parameters of the bed shear stress and momentum in the compound channel especially in the intersection of the main channel and floodplain make more accurate calculations related to the protection of the riverbed and determine the protection methods. In this study, the parameters of bed shear stress and flow momentum are quantified in conditions where the compound channel, bridge abutment length in the floodplain and the type of guide-wall simultaneously affect the flow pattern.

Materials and methods: In the present study, three-dimensional numerical analysis is performed to investigate the bed shear stress and momentum exchanging between the floodplain and the main channel in a symmetric trapezoidal compound channel by using FLOW-3D. After validating of the mentioned model, the bed shear stress and momentum exchange between the floodplain and the main channel for different bridge abutment lengths in cases with and without elliptical guide-wall have been investigated.

Results: The bed shear stress in the case with the elliptical guide wall has increased in the main channel and decreased in the floodplain compared to without guide-wall. Shear stress in the main channel has increased to 25.5% and decreased to 36.63% in the floodplain. The momentum exchange in the elliptical guide-wall case has increased up to 78.5% compared to without guide-wall case.

Conclusion: One of the remarkable results is that the bed shear stress in the floodplain decreases in the case of abutment with the elliptical guide-wall which improves the conditions at the bridge abutment. But increase the shear stress in the main channel causes the conditions of the abutment located in the main channel become more critical. Also, in the case of abutment with the elliptical guide-wall, the momentum exchange between the floodplain and the main channel increases compared to in the case of abutment without the guide wall, which increases the shear stress and creates vortices at intersection of the floodplain and the main channel.