Post-Eruption Characteristics of Rainfall Runoff and Sediment Discharge at the Miyakejima Volcano, Japan

摘要

The Miyakejima Volcano, which is located to the south of Tokyo, erupted voluminous tephra in 2000. Deposition of tephra changed the hydrologic properties of the river basins around the volcano and intensified erosion processes and storm runoffs have occurred even at the time of low-intensity rainfall in the rivers that had hardly ever generated runoff before the eruption. In this study, post-eruption hydrology and erosion properties at the Miyakejima volcano are studied through field hydrologic measurements. The results indicate that runoff rates of the study slopes were different from each other depending on the conditions and properties of the deposited tephra and that the runoff rates have declined in the 4 year study period. Fine sediments still run off from the upper slopes, while gullies developed rapidly for a few months and subsequently their development has decelerated. Runoff rates of all the river basins were also different from each other depending on the conditions and properties of the deposited tephra in the upper slopes. Almost no sediment discharge is observed in the rivers where the solidified lava has been exposed on the riverbeds. On the other hand, the significant sediment discharge continues in the rivers whose riverbeds are composed of the erodible sediment such as scoria.