2003 Volume 55 Issue 5 Pages 40-48
After the eruption of Miyakejima Volcano in July 2000, debris flows occurred in torrents whose source areas had been mantled by the volcanic ash. The debris flows in the torrents were generally composed of not only newly deposited fine-ash but also preexisting bold rocks and scoria. In the upper slope of the torrents, numerous gullies and rills had been dissected. A large portion of sediment discharge from the torrents seems to be attributed to the gully erosion. Therefore, it is important to know gully development characteristics to predict debris flow occurrence and its volume at volcanically-disturbed torrents. In order to investigate gully development characteristics, the authors gathered three sets of aerial photographs taken in Aug. 2000, Nov. 2000, and June 2001, and investigated gully development processes in these periods, which were from the onset of the eruptions to Aug. 2000, from Aug. 2000 to Nov. 2000, and from Nov. 2000 to June 2001, respectively. As a result of the surveys, the following were clarified. 1) The development rate of gullies was largest in the first period. The rate rapidly decreased in the second and third period. 2) The gully network in one sectioned slope had been fully developed in the first period. On the other hand, the width of the gullies kept increasing even in the second period. It shows that the gullies get longer at first and subsequently widened in the gullies. 3) Most of gullies had been formed along the topographic depressions recognized in the topographic map made before the eruption. It seems that the gully development is subject to the original topography in the case that volcanic ash deposit is not thick enough to change the original topography. 4) Weak positive correlation exists between width of gully and drainage area. These findings infer the capabilities to predict volume of sediment discharge prior to volcanic eruption.
