| Budget Amount *help |
¥1,500,000 (Direct Cost: ¥1,500,000)
Fiscal Year 2004: ¥600,000 (Direct Cost: ¥600,000)
Fiscal Year 2003: ¥900,000 (Direct Cost: ¥900,000)
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| Research Abstract |
Based on morphometric analysis and field survey, the relief condition, debris production and transport processes of Yaku Island, Southern Japan are discussed. Mean altitude and mean relief derived from Japanese mountains show a strong positive relationship. The highest point on Yaku Island plots near this regression line indicating that the relief of the island is high enough for its altitude. The relief in the central area of Yaku Island, which is the area with the highest altitude, is lower than in other regions. Yaku Island has very high relief in lower altitude areas and very low relief in higher altitude areas. In the high relief areas of the coast, there are deep valleys with steep slopes. In the low relief areas of the central part of the island, low relief surfaces are distributed on the ridges. In the upper-middle reaches through the large relief area, huge gravels, whose maximum diameters are 3-10m, fill on the riverbed. Gradual downstream decrease of maximum diameter according to the decrease of channel slope shows that these huge gravels were transported being affected by sorting process. There are many landslide scars in the tributaries which join the main stream in these reaches. At the junctions these tributaries develop alluvial cones on the valley floor of the main stream. The deposits of the riverbed and the alluvial cones include several meters size gravels. The distinctive relief conditions of Yaku Island can be attributed to the uplift characteristics of the island. Huge block production owes to the geological characteristics. These blocks sometimes formed landslide dams at the tributary junctions with the main stream. Large debris flows or torrential floods, caused by landslides in the tributary heads or burst of the dams, transported such huge gravels.
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