Temporal Evolution of Clear-Water Local Scour at Bridge Piers with Flow-Dependent Debris Accumulations

Previous research has shown that local scour at bridge piers is severely increased by the accumulation of woody debris around piers. However, due to the unavailability of accurate information regarding the characteristics of formed debris jams, the shape and dimensions of accumulations tested in previous laboratory experiments have had to be assumed. This article provides an assessment of debris-induced scour based on recently available knowledge about the relation between the potential dimensions of debris accumulations, the characteristics of flow, and debris elements. Clear-water scour experiments (with and without debris accumulation) were conducted using debris models with shape and size that correspond to the particular flow characteristics of each experiment. The results showed that scour depths obtained with flow-dependent debris accumulations were larger than without accumulations by a factor ranging from 1.18-2.19. The analysis of the scour depths affected by the accumulations suggested similar characteristics as well as dependence on the flow intensity, blockage area ratio, and depth ratio.