Experimental investigation of local scour around two submerged short square piles under tandem, side-by-side and staggered arrangements in steady current

This research investigates the effects of the arrangement of two submerged square piles on water flow induced local scour topography at the sand-covered bed in steady current under clear water scour condition. The height of each pile is the same as its boundary length. Experimental tests were conducted for G/D = 1 to 5 under the tandem arrangement and G/D = 1 and 2 under side-by-side (SBS) and staggered arrangements, where G is the gap between the two piles and D is the boundary length of the square piles. Results showed that in the tandem arrangement, the scour of the upstream pile is similar to that of a single pile, but the scour of the downstream pile goes through three stages: scour, backfill and re-scour. The backfill stage is caused by the downstream motion of the sand dunes generated from the upstream pile. In the SBS arrangement, the evolution of scour of each pile is similar to that of a single pile. However, the scour rates at the two inner upstream corners of the piles are slowed down by the sand dune between the gap at G/D = 1. In the staggered arrangement, the maximum scour depth of the downstream pile is greater than that of the upstream pile because the flow is accelerated twice from the corners of the two piles, respectively. The maximum scour depth at the SBS and staggered arrangements at G/D = 1 and 2 are slightly greater than that of a single pile, while the maximum scour depth of the tandem arrangement at G/D = 1 to 5 is similar to that of a single pile.