Shoreline Change Characteristics of the Ogimi Coast: Correlation with Other Coasts Around Okinawa Island

Recently around the world, shoreline retreat is a serious problem. Monitoring and projections of shoreline change are thus one of the essential tasks for sustainable coastal management. Not like beaches with quartz sand grains supplied from the land, coral beaches have a particular feature in its bed materials. The Ogimi coast is one of such coral beaches suffering significant shoreline retreat in recent years. In order to find a way to mitigate coastal erosion, it is essential to understand the mechanism behind. As a part of analysis of such mechanism, this study investigates characteristics of shoreline changes at Ogimi coast and other coasts around Okinawa Island. CoastSat was applied to extract temporal variations of shoreline locations of 20 coasts around Okinawa Island from 2000 to 2022. Based on the obtained shoreline locations, empirical orthogonal function (EOF) analysis was performed. Next, similarities between each coast were investigated by comparing Eigenfunctions (leading modes) of EOF analysis and the radius of curvatures and shore-normal directions of each coast. As a result, beaches with convex profile had a trend of coastal retreat and beaches with concave profile were almost stable or had a trend of coastal advance. Retreating coasts showed no clear correlation with the shore-normal direction while advancing coasts were mostly facing north to northwest.