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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 …