Experimental Investigation of Downstream Scour at Stepped Gabion and Solid Weirs with and without End Sills

Downstream scour is a critical factor governing the stability and longevity of grade-control structures in alluvial channels. Gabion weirs act as energy dissipation structures, promoting environmental and aquatic sustainability. This study presents a systematic experimental investigation of scour characteristics downstream of three-step gabion weirs (3SGWs) and three-step solid weirs (3SSWs), both with and without a gabion end sill, under clear-water conditions. Experiments in the laboratory were conducted in a rectangular flume using cohesionless bed material, considering 16 weir configurations with varying step geometry and a range of discharges. The effects of the flow, structural porosity, and end sill provision on maximum scour depth, equilibrium scour profile, and scour length were analyzed using dimensional and statistical approaches. Results demonstrate that gabion weirs produce considerably lower scour depths than solid weirs due to enhanced energy dissipation. Inclusion of a gabion end sill further reduces scour depth by 10%–20% by disrupting jet coherence and redistributing near-bed velocities. This study employed nonlinear regression analysis to develop predictive equations for maximum scour depth and scour length with satisfactory accuracy. A generalized dimensionless equilibrium scour profile equation was also developed. The findings offer practical guidance for designing scour-resistant stepped weirs in rivers with sediment-laden conditions.