Investigating the Effect of Six-Legged Element Placement Density on Local Scour at Wing-Wall Bridge Abutments

The focus of this study is to investigate the application of six-legged elements around wing-wall abutments experimentally. Six-legged elements have extensively been used for river engineering purposes for scour protection. The experiments were done in a 1m wide laboratory flume with uniform sand material. Four baseline tests were performed in different ratios of flow velocity to critical velocity values (0.65-0.95). The six-legged elements were then placed in three different placement densities on the bed; open (T₁), medium (T₂), and dense (T₃). The amount of created coverage for T₁, T₂ and T₃ densities were 54, 76 and 97 percents, with 63, 76, and 97 pieces respectively. Each alternative was tested under different flow conditions. Generally, the results proved that six-legged elements can considerably reduce the scour depth so as it reduced the maximum scour depth of abutment nose up to 100%. This maximum reduction of scour depth has been obtained when the elements were placed densely. However the extent and intensity of edge failure increased with higher density placements. Displacement of the maximum scour depth from the abutment tip was witnessed during the experiments especially in high Froude numbers. Hence each placement density could leave different effects on the bed topography in the vicinity of the abutment and on the downstream of the bridge; this gives alternatives to the designer for selecting six-legged elements placement densities with respect to the features of each placement density and conditions of the site.