Journal of the Japan Society of Erosion Control Engineering, Vol.59,No.2,2006
Sediment runoff in the rivers of Miyakejima Island
-A four]year portrait of the Miyakejima volcano after the volcanic eruption
in 2000-
Yasuhiro DOI Satoshi TAGATA Takao YAMAKOSHI Katsuo SASAHARA Haruo NISHIMOTO
Abstract
Authors surveyed the sediment runoff actually occurring in the Miyakejima Island
after 4 years have passed since the volcanic eruptions lastly occurred. The survey
was carried out by repeating measuring the freeboards of sediment control dams
newly installed in the rivers. Topographic surveys of cross sections of the rivers
were also carried out in the two rivers. As a result, it was confirmed the tendency
that sediment runoff still persists in the rivers where the more volcanic ash
deposited at the time of the last eruptions. Observing the quality of the runoff
sediment, it can be classified into the two types, that is, scoria and fine sediment.
The result of the topographic surveys in the two rivers, one is the scoria]runoff]river
and the other is the fine]sediment]runoff river, showed that the source of the
scoria was the sediment in the riverbed, while that of the fine sediment was the
sediment consisting of the river banks and rill ditches. The scoria]runoff]river
has abundant erodible scoria on the riverbed still now, while the fine]sediment]runoff
river has solid lava bed such that riverbed erosion will no longer occur.
Key words: Miyakejima, volcanic ash deposit, sediment runoff, scoria
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