Equilibrium scour depth at offshore monopile foundation in combined waves and current

Unlike the pier scour in bridge waterways, the local scour at offshore monopile foundations should take into account the effect of wave-current combination. Under the condition of wave-current coexistence, the water-soil interfacial scouring is usually coupled with the pore-pressure dynamics inside of the seabed. The aforementioned wave/current-pile-soil coupling process was physically modeled with a specially designed flow-structure-soil interaction flume. Experimental results indicate that superimposing a current onto the waves obviously changes the pore-pressure and the flow velocity at the bed around the pile. The concomitance of horseshoe vortex and local scour hole around a monopile proves that the horseshoe vortex is one of the main controlling mechanisms for scouring development under the combined waves and current. Based on similarity analyses, an average-velocity based Froude number (Fr (a)) is proposed to correlate with the equilibrium scour depth (S/D) at offshore monopile foundation in the combined waves and current. An empirical expression for the correlation between S/D and Fr (a) is given for predicting equilibrium scour depth, which may provide a guide for offshore engineering practice.