Abstract
Coral cays formation mechanism has various field surveys, experiment, and numerical simulations, but observation data for transport feature of coral gravel and topographic change is almost less. In this study, we conducted field survey in ballast island on northern reef of Iriomote Island which is formed by only coral gravels. Then, we discuss about external force and transport feature of coral gravel in the reef. As a result, we understand that external force of coral gravel transport is caused by the waves than the flow on the reef and Shields number is around 0.03, these results correspond with the past study. The movement of colored coral gravel showed that wave from east and south gathered coral gravel in the reef to ballast island. Then, we measured the amount of gravel which trapped in the sediment-trap on lower end of the reef slope. Thus, coral cays are formed and maintained by transport of gravel to island and flow out to reef slope.
