隆起楕円形島における海浜形成と底質の淘汰 : 鹿児島県新島（燃島）の例
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Shinjima Island is located in Kagoshima Bay, Japan. It was formed by raising of the soil from the sea bottom ofAira Caldera area due to the volcanic eruption of Mt. Sakurajima in the year 1778. Since then, cliff erosion has occurred at the west, north and east coasts of the island. On the other hand, sandy beach (spit) has been created at the south side of Shinjima Island by eroded materials from the cliff. The shape of the island becomes elliptic due to the predominant northern incident waves. This relatively new island is one of the ideal locations to study the formation process of natural beach and sediment sorting by waves in a natural manner, as well as to investigate the speed ofclifferosion. In Kagoshima Prefecture, a number of coasts are bounded by sea cliffs composed by Sirasu sand which is an unconsolidated volcanic debris and is a commonly found soil material in southern Kyushu in Japan. The study shows that a rate ofclifferosion at north coast of Shinjima Island is nearly 1.55m/yr and the cliff has been retreated over 289m since the rise. A sand spit created by the cliff material migrates to southeastward and southwestward directions depending on the seasonal change in predominant wave angle. A size of the spit recently tends to decrease due to the reduction of cliff erosion by revetments and breakwaters set in front of the cliff. Forty percent of the tarus sediment material is smaller than 0.5mm in front of the cliff, in contrast six percent of sediment material on the sandy spit is smaller than 0.5 mm. Most offiner portion of the supplied cliff material was dispersed into deep water while it was moving along the island to the southern tip. Coarse material which is larger than 5mm is left relatively close to the cliff(supply site) and a component of coarse material decreases to a down-drift direction. As a result, a sorting of transported sediment is increased in down-drift direction.
- 2007-12-25
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