硫黄島における海岸地形の変動特性 Variations of Coastal Geomorphology at Iwo Jima
Iwo Jima is a small volcanic island located in the Western Pacific about 1, 250 km south of Tokyo. The island, with an area about 23 km<SUP>2</SUP>, is mostly rimmed by sandy beaches exceeding 200 m in width. No ports or shore protection works have been constructed along the coast of this island since it became a territory of Japan. Further, the beach is always subjected to rough seas because it is fully exposed to the open sea. Therefore, a natural sandy beach has been preserved on this island, where we can observe the coastal geomorphology formed by ocean waves. It should be noted however, that there exists quite an unusual upheaval on this island. Since 1982, the author has been conducting various surveys along the entire coast of Iwo Jima to investigate both crustal movements and variations of the coastal geomorphology of this island. In this paper, cross-levelling data are analyzed to check variations of coastal geomorphology on Iwo Jima. Here, cross-levelling has been done at fifty measuring sections distributed along the entire beach of the island at intervals of roughly 300 m. The surveys have been performed 17 times so far since 1982 at each of the fifty sections, resulting in 850 data points. In the analysis, the following five physical terms have been chosen as the terms representing the features of the coastal geomorphology of the island : (1) Variations of the shoreline in a certain period, Δ L<SUB>x</SUB> ; (2) Foreshore slope, I<SUB>fs</SUB> ; (3) Variations of foreshore area in a certain period, Δ S<SUB>fs</SUB> ; (4) Backshore slope, I<SUB>bs</SUB> ; and (5) Foreshore height, Z<SUB>fs</SUB>.<BR>Analyses have been done on the data of each physical term to investigate both time-course and local variations. The following is a summary of the findings.<BR>(a) Shoreline has advanced roughly 50 m on average over the island during the past 14 years since 1982, and is still advancing at almost the same rate.<BR>(b) Foreshore slope is stable at every coast, keeping values which range from roughly 0.15 to 0.22.<BR>(c) Foreshore area has been increased roughly 170 m2 on average over the island during the past 14 years since 1982, and is still increasing at almost the same rate.<BR>(d) Backshore slope is stable at every coast, keeping values which range from roughly 0.08 to 0.18.<BR>(e) Foreshore height ranges from 1.6 m to 4.2 m, although it varies with time and location.