In the performance-based design of a breakwater armor layer, it is often necessary to calculate its damage accumulated over the life cycle of the breakwater. Two methods for calculating the cumulative damage have been proposed; one by Melby and Kobayashi in 1998 and the other by Hanzawa et al. in 1996. In this paper, comparison is made between the two methods for a Tetrapod armor layer. For the damage progression of Tetrapod armor units, hydraulic experiments are made. In the case where a severe damage occurs at the beginning of the life cycle of the breakwater, the two methods do not show significant difference, but in general the latter predicts a larger cumulativeuddamage than the former. For a Tetrapod armor layer, it is recommended to use the average of the two methods.
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