Pressure-Induced Subsurface Sediment Transport in the Surf Zone

The horizontal pressure gradient associated with the steep front of forward-leaning breaking waves is transmitted into the porous bottom to depths far exceeding the depth of the shear-stress-induced surficial transport sheet flow layer. The horizontal pressure gradient induces a flow within the bed which exerts a seepage force on the soil-skeleton. If sufficiently large the seepage force will cause a failure of the soil matrix and displace subsurface sediment grains, i.e. result in a subsurface sediment transport. This paper investigates the pressure-induced subsurface transport mechanism experimentally by photographically recording the displacement profile of subsurface lightweight sediment grains during passage of the steep front of breaking waves. A simple theoretical model for the dynamic response of subsurface sediments in failure is developed and provides predicted displacement profiles in reasonable agreement with measurements. The potential importance of the subsurface sediment transport mechanism is discussed.