Sedimentation and succession pattern in regenerating primary vegetation governing gully formation in the area devastated by the 2000 eruption of the Mount Usu volcano, in Hokkaido, Japan

摘要

The eruption of the Mount Usu volcano in 2000 devastated about 71 ha of forest in southern Hokkaido, Japan. The pyroclastic debris ejected caused secondary damage to lives and property. The sedimentary patterns, erosion processes, and effects of re-vegetation on erosion within the devastated area were monitored from 2002 to 2003 with a view to predicting and controlling future secondary damage. Examination of the sediment profiles established that deposits near the crater were massive and unstructured, whilst airborne tephra settling away from the crater was deposited in specific layers with heavy particles settling first. Rill and gully erosion occurred throughout the devastated area, and their abundance and dimensions varied with slope, the surface vegetation cover, and the particle size distribution of the sediment. Flat areas ( <3 ) had fewer rills and gullies, compared to steeper areas with slopes of 12-16.5% irrespective of vegetation cover.The number and depth of rills and gullies were higher in the sandy steep area (16.5°) with only 3% vegetation cover. On the other hand, in similar areas with 35 to 65% vegetation cover, the depth of rills and gullies increased less rapidly over a one year period, pointing to the significance of re-vegetation in frustrating rill and gully development. Sedimentary characters, surface grain-size distribution, topography, and vegetation all play important roles in erosion processes and are indispensable to erosion control for predicting future sediment behavior and related secondary disasters.