琉球列島久米島,パテノハマ洲島でみられる海岸線変化

Abstract

The Hatenohama sand cays located at the eastern end of Kume Island are the largest coral sand cays in Japan, with a total length of about 6 km and a width at the widest point of 300m. The aim of this paper is to describe the long-term changes in shorelines along the Hatenohama sand cays caused by typhoons. Vertical aerial photographs were taken 5 times in the period between 1962 and 1984. Shoreline changes in the sand cays were measured with a digitizer from these aerial photographs. In order to compare changes of intertidal and subtidal sand areas in the areas surrounding the sand cays, image enhancement techniques were employed on these photographs using a personal computer system and a CCD camera. Typhoons and their wind data recorded at the Kume Island Weather Station over the period 1.962-1984 were analyzed. During those 22 years the number of typhoons wihch approached Kume Island was 126. Wind energy resultant vectors were calculated using the wind velocity and the wind direction at the times when typhoons passed near or over the island. The analysis of sand cay shoreline changes and wind date of typhoons for the period 1962-1984 at Kume Island indicated that the changes of shorelines reflect changes inn the wind-induced waves. Shoreline changes occurring since 1962 were related to the wind-induced waves from the south associated with the occurrence of typhoons. Significant changes have occurred since 1970. Con-siderable quantities of sand have moved, exposing beach rocks on the southern beach and covering formerly exposed beach rocks on the northern beach. Because of the strong south wind, the south sides of the cays experienced ramparts erosion, while the sandy beaches on the northern sides increased in area. It was found that the shoreline changes and changes in the shapes of the sand areas surrounding the sand cays occurred frequently. However, the long-term trends of shoreline erosion/accretion patterns and the changes in mor-phology of sand areas are not clearly understood. This study shows that the eastern and the western ends of the sand cays have been elongated and significant changes have occurred, On the other hand, the middle of the southern beach has suffered little or no shoreline change, with the beach rock being extensively exposed for the length of about 1km. It is assumed that the beach rock supports sandy sediments and protects the shoreline from wind-induced wave erosion.