The equations of motion are developed for an unrestrained revetment block lying on a free-flowing filter layer, subject to harmonic base pressure during wave downrush. The resulting trajectories are calculated numerically and depicted at discrete time intervals. The influence is then estimated of progressive decreases both in the spacing of adjacent blocks and in the filter layer permeability. Some consideration is also given to small-amplitude, continuous oscillations. It is suggested that reductions in filter thickness and permeability are more effective in reducing block uplift than are increases in block thickness.