There are two basic types of scouring pattern of a sand bed in front of a vertical breakwater under the action of standing waves due to the differences of the sand grain sizes and of the wave conditions. The expressions of the scouring profile, the ultimate maximum scouring depth and the development of the scouring for two scouring patterns based on the experimental data are given in the present paper. The scale laws for the sand grain size and for the scouring depth are derived, thus making it possible to apply the results from the model tests to the prototype conditions. The influence of the scouring patterns on the stability of the foundations of vertical breakwaters is discussed through some typical examples in the paper.