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Significant coral bleaching was observed in summer in Shiraho reef, a fringing reef in the east coast of Ishigaki Island, Okinawa, Japan. To find relationship between coral bleaching and thermal stress in the reef and its governing factors, we have examined data obtained by field surveys in July 2003 and 2007 at Shiraho reef and the corresponding numerical simulation results for spatio-temporal variations of water temperature. The results of analysis showed that the spatial distribution of the degree of coral bleaching in the reef is closely related with that of the mean water temperature, which is strongly governed not only by local topography but also by meteorological conditions, especially latent heat flux.

Atoll islands with low-lying and little resource have high vulnerability against the sea-level rise due to the global warming. The countermeasures concerning the future land maintenance against the sea-level rise should be considered immediately. However, the studies on the quantitative information or numerical models of atoll island changes are quite few. Thus, in this study, measurement of beach profiles in Laura islet located in the western part of Majuro Atoll, Marshall Islands were carried out on the same profiles which were measured in 1997 and 1998 by SOPAC. Comparing our measurement results with the previous SOPAC's profiles revealed the quantitative profile changes during the last decade: from 1997 to 2007. It also revealed that the lagoonal coastline in Laura islet were stable in the last decade except some coast lines. We also reproduced these profile changes by the numerical model which was developed in this study based on longshore sediment transport system. The results of numerical calculation and the measured geographical changes showed a good agreement.

Coral cays are low-lying islands formed by coral gravels deposited by the waves and currents on the coral reefs. Cases have also been reported that the coral cays were formed in the short time scales when high waves with storm surges. Once the formation of coral cays can be controlled, which can contribute to the protection, environment and use of the coast on coral reefs and the national land conservation of the Pacific Ocean island nations in danger of inundation by the sea level rise. In this paper, the formation mechanism of the coral cays is verified through hydraulic movable bed model tests and the numerical simulation aimed at Ballast Island in the north of Iriomote Island. As a result, we confirmed that the wave breaking on the reef edge, wave setup, currents, and gravel supplies on the coral reefs exert influence on the formation of coral cays.