Slope failures and movement of driftwoods caused by typhoon nabi in the Nakamata River basin in Kagoshima prefecture [Japan] Tarumizu City, in September, 2005
2006
Teramoto, Y.(Kagoshima Univ. (Japan). Faculty of Agriculture) | Shimokawa, E. | Jitousono, T. | Takaoka, A.
To clarity the characteristics of slope failures and the movement of driftwoods caused by typhoon Nabi in September, 2005 in the Nakamata River Basin in Kagoshima Prefecture, Tarumizu City, field surveys and the interpretation of aerial photographs were carried out. The results are as follows: (1) Thirty slope failures in the Nakamata River Basin were identified as due to typhoon Nabi. The range of area of a slope failure was from 25 square m to 2,600 square m (average 467 square m). The total amount of sediment yield due to slope failures was 4,456 cubic m/square km. (2) The sediment yield due to slope failures in the Nakamata River Basin during the twenty years between 1986 and 2005 was found by determining the area and depth of slope failures by analysis of aerial photographs. The amount of sediment yield during these twenty years was calculated to be 20,309 cubic m/square km, giving a yearly average of 1,015 cubic m/square km. The amount of sediment discharge that had accumulated against the check dams was calculated based on a field survey. During the fifteen years from 1991 to 2005, the amount of sediment discharge that flowed out due to slope failures, debris and mud flows, and floods was 12,010 cubic m/square km. The yearly mean value was 800 cubic m/square km. (3) Driftwoods resulted not only from slopes covered with conifers but also those covered with broadleafed trees. 227 cubic m of the total driftwoods of 630 cubic m yielded by slope failures was caught by the check dams, 176 cubic m was caught in the channel, and 198 cubic m flowed out to sea.
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