Physical modeling of local scour development around a large-diameter monopile in combined waves and current

In most of the previous studies on local scour around pile foundations, wave-induced pore pressure response has not been taken into account. The local-scour and pore-pressure responses around a large-diameter monopile in combined waves and current have been physically modeled with a specially-designed flow-structure-soil interaction flume. In the series of experiments, the time developments of the scour-depth and the pore-pressure in the proximity of the model pile were measured simultaneously. Experimental results indicate that the wave-induced upward seepage under the wave troughs may weaken the buoyant unit weight of the surrounding sand, which brings the sand-bed more susceptible to scouring. The superimposition of the waves on a current has much effect on the time-development of local scour and the resulting equilibrium scour-depth, which is particularly obvious when the sand-bed is in the clear-water regime under the current or waves alone respectively. It is observed that the maximum flow velocity at the boundary layer for the following-current case is larger than that for the opposing-current case, which further results in faster time development of scour depth and greater equilibrium scour depth for the following-current case.