抄録
Miyakejima Volcano has resumed its eruptive activities since June 26, 2000. The series of eruptions thickly deposited volcanic ash on slope of the volcano. As a result, sediment began to discharge seriously from almost all torrents of the volcano. Previous studies on sediment discharge from newly erupted volcanoes have shown that condition of slopes changes significantly in the first year after eruptions. To know the change in the characteristics of the ash deposits that would drastically alter the characteristics of sediment discharge, the authors did field investigations in Aug. 2000 and Aug, 2001. As a result of the investigations, no significant change was found in grain size distribution, bulk density, and final infiltration capacity. On the other hand, topographic surveys showed gullies increased rapidly and ratio of the area occupied by the gullies have increased to about 10%. The gully formation results in exposure of permeable under-layer. It is considered to be possible that sediment discharge from the volcanically disturbed watersheds decreases without increase in the infiltration capacity of the volcanic-ash deposit in case that the original surface exposed by the gully formation is permeable enough to exhaust water discharged from the ash-covered source area.
