The pattern of scouring due to waves around offshore breakwaters may be correlated well with the distribution of water particle velocity and mass transport velocity.
　Equations of the mass transport velocity just beyond the bottom boundary layer for diagonal, partial standing waves are obtained as eqs. (32) and (33). To be concrete,the secondary non-linear velocity averaged over a period of waves is first developed on the assumption that the primary linear velocity just beyoud the layer is equal to the bottom velocity of small amplitude standing waves. Ultilizing the non-linear velocity stated above, the mass transport velocity is derived following the technique shown by Longuet-Higgins (1953). Then, the results of numerical calculation are presented.
　Experiments have finally been done for the purpose of verifying the results. Other tests with a movable bed have been made in order to investigate the correspondence between the distributions of the mass transport velocity derived above and the experimantal patterns of scouring.