Flow Field around Semielliptical Abutments

Abstract

Hydraulic structures in rivers employ abutments of different shapes. An abutment causes a scour hole around the bottom of the structure in the river. The scour hole grows no further when the turbulence fluctuations and bed shear stress are unable to move sediment particles. Although there have been a multitude of studies on the rate of scour depth around abutments of different shapes, no research work seems to have been reported regarding the flow structure of local scour around semielliptical bridge abutments. This study experimentally investigated the flow field around semielliptical bridge abutments and compared the results with those around wing-wall and semicircular abutments. Results showed that the abutments with small dimensions (width and length) produced higher scour depth due to strong vortices and downflow than those with large dimensions. For the same aspect ratio, small and large abutments cause different patterns of the velocity field in streamwise and vertical directions. The abutment size (width and length) significantly influences flow separation and reattachment, manifesting larger separation and deeper scour hole in small size abutments. The equilibrium scour depth depends on the abutment size, with stronger circulation and vorticity in small abutments than in large ones. The internal boundary layer around small abutments plays a more significant role in the flow structure than that around large ones, generating a larger strong vortex trail with more scour in the vicinity of abutment and causing the failure of the structure. Unfavorable pressure gradients with nonlinear distributions of turbulence intensities and Reynolds stress at the scour hole upstream of the abutment are the primary factors causing the development of vortex trail and the scouring mechanism.