Effect of Grain Size Distribution of Bed Materials on Destabilization of a River Pier Due to Local Scour

As an initial experimental study on the mechanism of destabilization of river piers caused by local scour, the effect of grain size distribution of bed materials on pier destabilization was experimentally evaluated from the viewpoints of river engineering and geotechnical engineering. The results showed that the scour resistance of a non-uniform bed was greater than that of a uniform bed, even if initial mean grain sizes of the two bed materials were almost the same. In addition, the rate of horizontal displacement of the pier toward upstream in the non-uniform bed decreased with time, while the rate increased in the uniform bed. The possible reason for this difference is related to coarsening of the bed surface at the pier nose in the non-uniform materials. Coarsening is assumed to easily dissipate the downflow energy because of larger intergranular gaps. This might have led to reduction of the tractive force acting on the ground immediately below the pier bottom, and thus reduced the rate of horizontal displacement of the pier toward upstream.