Emplacement and movement of boulders by known storm waves — Field evidence from the Okinawa Islands, Japan

Abstract

Field observations from previous studies of boulder movements were compiled to augment our own work on boulders that have been displaced by storm waves during recent typhoon events in the Okinawa Islands, Japan to elucidate the ability of storm waves to move boulders. Our observations reveal that recent storm waves displaced by sliding and overturning 100-ton boulders emplacing them on the reef or high cliff tops and that storm waves might also be capable of displacing 200 ton boulders on the reef. The weights of storm wave boulders at the Okinawa Islands are of comparable order to those boulders displaced by historical tsunami origins. Consequently, boulder weight alone is an inappropriate parameter to discriminate between tsunami or storm wave processes. However, these heavy storm wave boulders are close to the reef and cliff edges, while tsunami boulders can be deposited much further inland. Hence, horizontal displacement distance of boulders could be a useful parameter to discriminate boulders deposited by the tsunami and storm waves on the wide fringing reef. The storm wave boulders were characteristically concentrated on the southeastern (Pacific Ocean) sides of each island but large boulders are rarely found on the northwestern (East China Sea) side. This is probably because the storm wave intensities are generally stronger at the southeastern side than at the northwestern side, although differences of reef strength and initial condition of boulders should also be taken into account. Consideration of the high frequency of typhoons at the area suggests that effects of the storm waves and the consequent displacement of boulders on the reef might have contributed to the formation of the reef-moat framework that typifies the Okinawa Islands especially, if the moat is located within the transport limit of the storm wave boulders (approximately 300 m from the reef edge at the islands).