Since the work by Ahrens and McCartney (1975) and Bruun and Johannesson (1976), it has been accepted that, for given slope, unit type and damage level, the stability number, N_s, and the so called K_d factor depend on Iribarren numbe, I_r, and that the worst stable conditions are related to collapsing wave breakers. The plotted values of N_s and K_d from laboratory tests, as a function of I_r, show a large scatter. The scatter is larger for large values of I_r. It is shown here that the scatter almost disappears once the data are plotted against kh, or I_rt, usign the total wave height at the toe of the structure instead of the incident wave height. The total wave height, H_t, results from the itneraction of the incident and reflected wave trains on the slope and depends not only on the magnitude of te reflection coefficient but also on its phase.
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