Tsunami scour around a cylinder

TLDR

Pore‑pressure gradients during tsunami drawdown reduce effective stress in sand, explaining the delayed scour, while gravel’s porosity prevents such gradients. The study used a large‑scale sediment tank to mitigate scaling effects in sediment transport. Experiments showed that scour around a cylindrical structure varied with substrate: gravel experienced fastest scour during peak flow, whereas sand scoured most rapidly at the end of drawdown, with vortex structures visualized by video.

Abstract

A series of scale-model experiments investigated the scouring mechanisms associated with a tsunami impinging on a coastal cylindrical structure. Since scaling effects are significant in sediment transport, a large-scale sediment tank was used. Video images from inside the cylinder elucidated the vortex structures and the time development of scour around the cylinder. The scour development and mechanisms differed according to the sediment substrate – sand or gravel. For gravel, the most rapid scour coincided with the greatest flow velocities. On the other hand, for the sand substrate, the most rapid scour occurred at the end of drawdown – after flow velocities had subsided and shear stresses were presumed to have decreased. This behaviour can be explained in terms of pore pressure gradients. As the water level and velocity subside, the pressure on the sediment bed decreases, creating a vertical pressure gradient within the sand and decreasing the effective stress within the sand. Gravel is too porous to sustain this pressure gradient. During drawdown, the surface pressure decreases approximately linearly from a sustained peak at exceeded one-half.