Abstract
When artificial alteration is carried out on a coral cay, large topographic changes may be induced because of the imbalance of sand transport around the shoreline of the island. One of the authors considered the impact of the construction of the detached breakwaters to a circular cay as a model case using the BG model (a model for predicting three-dimensional beach changes based on Bagnold's concept). Because such a cay has a closed system for littoral transport, artificial alteration exerted at a location of a cay may affect the entire shoreline of the cay. However, real data concerning this issue was lacking. In this study, taking Minnajima Island offshore of Motobu Peninsula in Okinawa as an example, this issue was considered. It was shown that artificial alteration to enhance the usability of the cay brought the degradation of natural condition of the cay, and influenced to tourism use of the island.