Most of significant harmonic in typical strong ground motion have a period below 3 sec. Thus for T = 3 sec, the error e < 0.05 if h > 158.5m. For a reservoir depth 100 ft, e < 0.05 if T < 1.32 sec. Because the natural periods of the system are very small (first natural period for 100 ft depth of water is 0.085 sec), the contributions to the total response from harmonic components of ground motion with the periods longer than 1.32 sec will be small. This is particularly so because we are dealing with an fluid. Hence, it may be concluded that errors introduced by neglecting surface wave is on the order of 0.05. Similar conclusions can be derived for other depth between 100 ft and 520 ft. On this basis, it seems that the effect of surface waves can be ignored with little loss of accuracy.

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