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Open Access Publications from the University of California

The Effect of Islands on Surface Waves


The interruption of the progress of waves by an island produces a zone of "wave shadow" to the "wave lee" of the island. Observations have demonstrated that the shadow is never completely devoid of waves and that on occasion relatively high waves may occur in this lee region. An investigation is made of factors influencing the wave conditions in the shadow. The characteristics and mean direction of approach of the incident waves are assumed to be known. The penetration of wave energy into the region to the lee of the island is determined by the following factors: (1) the effect of underwater topography off the island's shores in refracting wave energy into the lee, (2) the effect of currents near the island in refracting energy, (3) the diffraction effect resulting when a barrier interrupts wave fronts, and (4) the effect of variability in direction of wave travel in limiting the extent of the shadow. The quantitative results indicate that the important effects in the penetration of wave energy into the lee are generally the result of refraction by underwater topography and variability in direction. The factors discussed are adequate for explaining quantitatively the wave conditions observed in the lee of the Island of Sicily, San Clemente Island, and Santa Catalina Island. The methods may be of general interest in the prediction of wave heights near the lee shores of islands or on coastal shores which are in the shadow of offshore islands.

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