FORMATION OF T0MB0L0 AT THE WEST COAST OF IWO-JIMA

Toshiyuki Shigemura, Jouji Takasugi, Yoshihiro Komiya

Abstract


This paper intends to clarify why and how such a huge tombolo having a surface area of 1,700,000 m2 has been formed at the west coast of Iwojima for relatively short period of 33 years after 1945. Analyses are performed on various data obtained through literature survey and field measurements to determine the growth rate of tombolo and variation rate of shore and sea floor surrounding the island. Model tests are also made on the formation of tombolo. The followings are the conclusions derived through the analyses: (1). Source of the sediments is the one produced at the northern part of island where sea floor has been lifting at a rate exceeding 30 cm per year, (2). Waves with dominant direction of N to NE which appear in fall and winter erode the northern coast and currents induced by these waves carry these sediments southward along both coasts of the island. (3). Waves with dominant direction of S to SE which appear in summer and their induced currents carry the sediments northward along both coasts of the island.

Keywords


Iwo Jima; tombolo; tombolo formation

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