Experimental Investigation on Flow and Bed Sediment Transport around a Semi-Circular Abutment in the Presence of Irregular Vegetation

Abstract

This study investigates local scour, a significant cause of bridge damage, by examining the effect of vegetation as a natural method to mitigate erosion around bridge abutments. Laboratory experiments were conducted on semi-circular abutments under conditions with and without vegetation. A total of 65 velocity profiles were obtained using acoustic doppler velocimetry (ADV), enabling the calculation of 3D velocity, turbulence intensity, and Reynolds stresses. The experiments revealed that the presence of vegetation decreased the time to reach scour equilibrium by up to 40%, and reduced scour depth by up to 33%. Vegetation significantly reduced shear stresses near the bed and around the abutment, with turbulence intensity values becoming more uniform in the streamwise and transverse directions and larger than those in the vertical direction. Flow events at specific angles showed distinct patterns. Without vegetation, at a 90-degree angle, ejection events dominated near the bed while sweep events were prevalent near the water surface. At a 130-degree angle, sweep events dominated near the bed, and ejection events were dominant from the middle depth upwards. With vegetation, at a 90-degree angle, all four events (sweep, ejection, inward interaction, and outward interaction) were close to each other in the initial one-third near the bed, with significant decreases in sweep and ejection events. At a 130-degree angle, vegetation showed no significant difference in dominant events compared to the non-vegetation case. These findings highlight the effectiveness of vegetation in reducing scour around bridge abutments and provide valuable insights into the flow-vegetation interactions that influence scour processes.