EROSIONAL HOT-SPOT GENERATION AT OSHIKIRI COAST IN SHIBUSHI BAY, KAGOSHIMA, JAPAN

摘要

An extension of the breakwater of Shibushi Port has been conducted at the Oshikiri coast in Shibushi Bay, Kagoshima, Japan since 1968 and its outer breakwater has been extended nearly 4.0km. This offshore breakwater created large wave shadow sheltered area behind it and non-sheltered area on the adjacent sandy beach. The typhoon damages had been caused on the non-sheltered sandy beach, thus three groins were built to reduce a deposition by an excess longshore sediment transport into a sheltered area from the non-sheltered adjacent beach. In spite of these shore protection works, one segment of the Oshikiri coast (roughly middle of Fig. 1) was especially overwashed. by the Typhoon 0111(11th Typhoon in 2001). This coastal damage seems not to be caused only by wave sheltering effect of Shibushi Port (wave diffraction effect), but formations of three dimensional sub-aerial bathymetry such as crescentic longshore bar and trough features probably enhanced the overwash damage at a certain location of the beach where a national hospital and local residential area exist. Therefore, this erosional hot-spot-like feature had to be studied in order to plan the emergency shore protection works at the site. The study shows that the erosion in the non-sheltered up drift coast was commenced mainly due to the wave sheltering effect (dH/dx; wave height gradient by wave diffraction) and the generation of 3-D geomorphological features including the crescentic longshore bars and troughs. These nearshore topographies were generated while incident significant wave heights of Typhoons exceeds nearly 6m high on this coast. In addition, it is shown that the average spacing of the troughs is around 600-700m. Finally, submerged breakwaters so called artificial reefs in Japan are recommended to reduce the further coastal damage at the site.