Role of Grain Size Distribution and Pier Aspect Ratio in Scouring and Sorting around Bridge Piers

Several bridge piers were visited, and their scour hole was characterized into three zones: scour, transition, and mound zones. The observed onsite sorting and scouring patterns were recreated through flume experiments with varying pier aspect ratio (1:1, 1:2, 1:4 and 1:6) and sediment geometric standard deviation (σg). The experiments showed that maximum scour depth decreased with non-uniformity; 52.7 mm, 20.2 mm, and 16.6 mm are the respective maximum scour depth of beds with σg = 1.4, 2.5, and 5.2. While aspect ratio has minimal effect on maximum scour depth values, it has noticeable effects on scour shape and asymmetry. The location of the lowest point of the scour shifted further downstream as σg increased. The number of occurrences where the location of the lowest point is outside the upstream cylindrical hemisphere is zero (0) for σg = 1.4, three (3) for σg = 2.5, and five (5) for σg = 5.2. The observed processes of upstream-to-downstream propagation of scour may explain the gradual decrease in fines on the surface, as well as the asymmetry of the scour, confirming the role of combined effects of grading of the bed material and piers’ aspect ratio in the scour-forming process.