HR: 0800h
AN: H51C-0378    [Abstracts]
TI: Differences of Landslide Occurrences Behavior Due to Slope Aspects in the Amehata River Basin, central Japan
AU: * TANAKA, Y
EM: yasushi@komazawa-u.ac.jp
AF: Department of Geography, Komazawa University, 1-23-1 Komazawa, Setagaya-ku, Tokyo, 154-8525 Japan
AB: The Amehata river basin is a small tributary (about 100km2) of the Fuji River system in the Minobu mountain belt of Southern Japanese Alps, Honshu Island, Japan. This area has one of the highest contemporary uplift and denudation rates in the Quaternary in Japan. It is an appropriate area in which to examine the relation between topography and landslides, because erosion takes place predominantly through shallow collapses (landslides). In this study, I report a geomorphometric examination of the entire Amehata drainage basin about the relation between slope form and its forming processes using high-resolution DEM (5m) and ortho-photographs derived from digital photogrammetry which are valuable for these analysis. South-facing slopes of this area are clearly steeper than north-facing slopes. That is, there is asymmetry of the valleys in an east and west direction, and an axis of symmetry appears to exist in the basin. In detail, slopes facing true south were the steepest, and the average angle decreased as we deviated from this direction. Hence, the slopes can be divided into two categories: north-facing and south-facing slopes. I examined the change in slope angle with altitude for the two categories. The average angle is about 3° greater for south-facing slopes than for north-facing slope. This tendency does not change with altitude. Next, detailed landslide distribution data are made from 1-m resolution ortho-photographs. Landslides in this area are generally collapses (shallow landslides), with the exception of a few very large slides. To quantitatively measure the occurrence of landslides, the relationship between the magnitude of landslides and their cumulative frequency is plotted on a common logarithm scale. This relationship also corresponds to the power function. However, the gradient of the log-log plot are steeper than some former studies. Thus, it shows that shallow collapse is the main agent of slope formation in the Amehata drainage basin compared with surrounding areas. Expecting that landslide behavior differ in relation to slope aspect, I examined the M-F distribution of landslides. Again, I divided landslides into two categories: those that occurred on north-facing slopes and those that occurred on south-facing slopes. Both categories were plotted on a magnitude-frequency distribution chart and linked to the power function. Note the remarkable differences in gradients. Many small landslides occurred on the south-facing slopes, while landslides on the north-facing slopes were less frequent, but larger. Judging from the data, I inferred that the asymmetrical valley in the basin was formed by differences in landslide mechanisms. The differences may be caused by the effect of orographic rainfall or freeze-thaw processes. However, this is also a problem for future research.
DE: 1625 Geomorphology and weathering (0790, 1824, 1825, 1826, 1886)
DE: 1824 Geomorphology: general (1625)
DE: 1825 Geomorphology: fluvial (1625)
DE: 1826 Geomorphology: hillslope (1625)
SC: Hydrology [H]
MN: Fall Meeting 2005