Local scour and pore-water pressure around a monopile foundation under combined waves and currents

A series of experiments were conducted in a large flow-structure-soil interaction flume to analyze the scour development and pore-water pressure response around a monopile foundation under the action of combined waves and currents. In the experiments, the scour depth and pore pressure response around the pile were measured simultaneously. The experimental results indicate that the maximum equilibrium scour depth due to waves plus currents is greater than a linear sum of those caused by waves and currents respectively. This nonlinearity effect is particularly obvious when the sand-bed condition under currents or waves alone is in clear-water regime. The maximum equilibrium scour depth normalized with pile diameter is closely dependent on the Froude number with increasing the wave-induced water particle velocity meanwhile the current velocity keeping constant. The wave-induced pore pressure gradient around the monopile under the wave trough weakens the buoyant unit weight of the surrounding sand and induces the sand-bed more susceptible to scouring.