2011年新燃岳噴火による降灰があった山地森林流域からの土砂流出

抄録

This study sought to clarify the characteristics of sediment discharge from a forested-mountain watershed where volcanic ash was deposited during the 2011 eruptions of Shinmoe-dake at Kirishima volcano, Kyushu, Japan. Quantitative estimates of sediment discharge were determined by measuring sediment inputs at Sabo dams, bedload transport using hydrophones, and sedimentation at a local reservoir. The Sabo dams were distributed within a 10-km radius from the crater source. The hydrophone stations and the reservoir were located 11 km and 40 km, respectively, downstream from the crater. Post-eruption specific sediment yields (SSY) estimated from the sediment input at the Sabo dams were maximum ten times greater, and the SSY obtained from measurements of reservoir sedimentation were two to six times greater than pre-eruption figures, with an increase of up to 10^３m³ km^－2 yr^－1 order. Bedload transport increased threefold after the eruption. Given that the characteristics of sediment discharge might change before and after the eruption, the high sediment discharge volumes studied were caused not only by intense hydrological events, but also by the removal of volcanic products that originated from the eruption. Processes of sediment discharge including transportation of volcanic products from the mountain watershed to alluvial channels and the downstream movement of sediment waves can be explained by correlating the timing of intense hydrological events with the increase in sediment discharge. Because high sediment discharges corresponded with hydrological events and continued after the eruption, and since the side of the watershed adjacent to the volcano is still covered by large amounts of deposited volcanic products, the continual monitoring of sediment transport remains important in terms of watershed management and future disaster risk mitigation.