降雨流出•土砂流出に影響する火山噴出物の特性: 2011年新燃岳噴火の事例

摘要

We examined effects of juvenile pyroclastic materials produced by the 2011 eruption of the Shinmoedake Volcano in the Kirishima Mountains on runoff and sediment discharge. The four characteristics of juvenile pyroclastic materials are thickness, grain size, porosity, and solidification. Pyroclastic materials consist of fine ash and coarse pumice, distributed mainly around the east and southeast areas of Shinmoedake, respectively. The thickness of the fine ash and the pumice deposits is more than 1 cm (maximum 3 cm) and more than 10 cm, respectively. The pumice particles are vesicular. Solidification materials were not reportedly found in the ash fall, so it is unclear whether the ash fall contained any solidification materials. Hyetographs and hydrographs of the eight rainfall events corresponded well. Although direct runoff related to total rainfall of less than about 100 mm was low in all the watersheds, direct runoff related to total rainfall of more than about 200 mm increased in the two watersheds whose headwater is located in the Shinmoedake and whose runoff coefficient was higher than that of the other watersheds. The two watersheds with a high runoff coefficient had a relatively thick ash fall deposit because of their closer proximity to the Shinmoedake Volcano compared to the other watersheds. In addition, these two watersheds were almost completely covered by fine ash. These facts suggest that the infiltration capacity in the watersheds temporarily decreased, and hence, there was a high level of discharge during rainfall events. The sediment movement after the 2011 eruption of Shinmoedake was only due to the high rainfall intensity (＞30 mm/h). Because the pyroclastic materials were mainly porous pumice and the deposit of fine ash was thin, decrease in infiltration capacity was small. Therefore, sediment discharge was not triggered by low rainfall intensity events. To evaluate the potential for sediment discharge after an eruption, it is essential to investigate the infiltration capacity and the thickness of the ash fall, as well as the characteristics of juvenile pyroclastic materials considered in this study.%火山噴火による降灰後の土石流発生メカニズムは，「火山の噴火や火砕流の発生に伴って細粒の土砂が地面を覆うと雨水の浸透能が小さくなり，表面流が発生しゃすくなるため」と考えられている(池谷ら，1995)。この土石流発生の1つの要因である斜面の浸透能は，細粒物質の含有量，空隙率が大きく関係しており，細粒物質が多いほど，そして空隙率が小さいほど，浸透能は低下する(寺本ら，2004)。