活火山地域の河川の開発・保全 - 東部ジャワ, カリブランタス流域を例として -

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We have conducted our research on the influence of the eruption of the Kelud Volcano on the Kali Brantas, conservation of the river basin, mitigation of the resulting volcanic and flooding hazards, and development process of the basin. For this purposes, we have made geomorphic land classification maps in the Kali Brantas Basin, utilizing the aerial photographs (scale is around 1/40, 000) and LANDSAT Images which were taken in 1972. The Kali Brantas is located in the eastern part of Java, i. e. easternmost part of the Himalaya-Alps Orogenic Zone. The Kali Brantas originates in Mt. Arjuno and round the foot of the Butac and Kelud Volcanos which are active volcanos, and then pours into the Madura Strait. The southern part of the Kelud Volcano consists of the following geomorphic elements: piedmont gentle slope of the volcano, a slightly hilly area on the gently sloping terrace, valley floor plain, marsh, abandoned river course, steep slope, etc. The Kali Brantas plain consists of the following geomorphic elements: natural levees, back-swamps, a delta, lagoon, sand-spits, etc. Natural levees are developed well around the city of Kediri and the river bed is higher than that of the adjacent plain. The Kelud Volcano has erupted approximately once every 15 years. We have a record of its eruption since 1, 000 years A. D. The eruption in 1919 was especially severe, and about 3, 800×104m3 of “Lahar” flowed down to have killed 5, 100 persons. Due to the eruption in the year 1586, about 10, 000 persons were killed. In order to mitigate the damage by “Lahar” a tunnel for decreasing water in the crater was constructed during the Dutch Colonial period. This drainage tunnel reduced the volume of the water from 4, 000×104m3 to 2, 000×104m3 in 1966. After the construction of the tunnel, the lahar damages decreased. However, aggradation as a result of this eruption reduced the depth of the crater about 50m to destroy the tunnel. Then the tunnel was repaired again. After 1970, thre lowering of the river bed has occured partly because of the construction of dams, particularly the construction of “Sabo” dam for “Lahar” and partly the because of the volcanic eruption. Although the lowering of the river bed decreased the possibility of the flooding, it becomes difficult to take the water for irrigation. In February 1990, the Kelud Volcano erupted again. And a lot of volcanic ash was deposited on the southern slopes of the volcano. The tunnel disappered again. So we must look again for the tunnel. Directly after the eruption, torrential rainfall occurred at the upper reaches of the Brantas River. The erosion in the upper reaches was remarkable and a lot of sand and gravel flowed down to the middle reaches and several reservoirs were burned immediately. In conclusion, we would like to indicate that it is very important to predict the geomorphological changes of the river basin in the case of making development plain of the river basin.
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活火山地域の河川の開発・保全 - 東部ジャワ, カリブランタス流域を例として -

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