Experimental Study of Wave-Induced Pore Pressure Gradients around a Sandbar and Their Effects on Seabed Instability

The position and morphology of offshore sandbars are highly dependent on wave conditions; however, the mechanisms driving sand movement by water waves remain elusive to scientists and coastal engineers. This study presents a series of experiments conducted in a wave flume to investigate the impact of wave-induced pore pressure gradients on seabed instability around a sandbar, observed in the Benin Gulf of Guinea. The Froude-Darcy similitude principle was developed to ensure the similarity of hydrodynamics and seepage forces between the experiments and field conditions. Pore pressure gradients and free surface elevations were measured using three arrays of pore pressure transducers and eleven wave probes, respectively. The results indicate a rapid increase in both the horizontal pressure gradient and the maximum downward pressure gradient during the shoaling process. Conversely, the maximum upward pressure gradient decreases prior to wave breaking. Wave-induced pressure gradients significantly influence seabed instability and sediment transport. The effective weight of sand particles is reduced by up to 52% due to the upward pressure gradient during the shoaling process, and momentary liquefaction is triggered by the horizontal pressure gradient near the breaking point based on the liquefaction criterion. When liquefaction occurs, shear granular flow forms on the seabed surface.