Hydrological processes and topographic change on the slopes of Unzen volcano

This study examined topographic changes on the slopes of Unzen Volcano, which were devastated during the 1990 to 1995 eruption. Aerial photographic measurements and hydrological observations showed that: during the early period of the volcanic activity, pyroclastic flows buried valleys and made convex bottoms, hence forming new valleys along the edges of the convex bottoms. Later, deposition of pyroclastic flows and gully erosion occurred along these new valleys. The occurrence of Hortonian surface flow, which caused erosion of sediments, was dependent on the sediment structure, especially the coverage of the surface by rocks. The slopes outside the gullies resist surface erosion, hence the topographic changes due to erosion are limited mostly to inside the gullies.