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Nepal case study : Catastrophic Floods

  ちょうど現地調査中だった1977年9月3日、東ネパール・クンブ地域のドゥドゥ・コシ川流域のミンボ谷上流域から氷河湖決壊洪水が発生した。この洪水で浸食された地形をたどって、アマダブラム峰(6814m)南面の空の氷河湖にたどりつき、湖を堰きとめていたIce-Cored Moraine (氷塊を含む氷河モレーン堆積物)が決壊し、氷河湖決壊洪水が発生したことをつきとめた。下記の参考文献は、ネパール・ヒマラヤの氷河湖決壊洪水に関する初めての報告である。

INTRODUCTION
OCCURRENCE
DISCUSSION
参考文献
Neapl case study; Catastrophic floods (1985) Techniques for prediction of runoff from glacierized areas, International Association of Hydrological Sciences, 149, 125-130.

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Fig. 6 Imja Glacial Lake, 1 km wide and 2 km long, in the down-stream part of the Imja Glacier, Dudh Kosi region.

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INTRODUCTION

  Hagen (1963) reported on outburst floods of glacial lakes in the Manaslu region. Central Nepal, and other similar events in Rhutan were mentioned by Gansser, (1966). Since the mountain people of the Great Himalayas live in the periglacial as well as in the glacial regions, glacier outburst floods pose a large threat along with other glacial disasters such as avalanches, glacial mud flows and glacier surges.

  On 3 September 1977 an outburst flood from a moraine-dammed lake occurred in the Mingbo Valley. Dudh Kosi region. East Nepal (Figs. l(a) & (b)). The flood wave was recorded at the Rabuwa Hydrological Station. 70 km downstream Erom the Mingbo valley (Fig. 2).

  This paper reports the occurrence of the outburst flood, estimates the flood volume and discusses the possibility of similar glacial disasters.


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PictureFig. 3 Newly formed terraces along the Mingbo Valley.

OCCURRENCE

  Since a Saturday market was to be held. people were walking toward

  Namche Razaar, and some stayed at hotels along the banks of the Dudh Kosi river. In the early morning of 3 September, the flood-wave destroyed houses built on the river terrace several metres higher than the present river bed. There were several fatalities.

  Construction materials stored more than 10 m above the river for a small water-power station for the Everest View Hotel near Namche Bazaar were taken out by this flood (Miyahara. 1982). Small wooden bridges across the Dudh Kosi were also carried away. At least six bridges were destroyed near Namche Razaar. This seriously disrupted communications in the region of the upper Dudh Kosi (Khumbu region).


  The topographical characteristics of newly formed terraces (Fig. 3) indicate where the outburst flood from the moraine-dammed lake occurred in the Mingbo Valley. Fig. 4 shows the ice-cored moraine that formed the moraine-dammed lake. The outburst flood was caused by the collapse of this moraine. By using a hand transit. the former moraine-dammed lake was mapped as shown in Fig. 5, and the volume of water in the lake was estimated as 4 x lo5 m3, which is consistent with the calculated volume of the flood wave recorded at Rabuwa.

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Fig. 4 Ice-cored moraine (A) that caused the outburst flood of the glacial lake in the Mingbo Valley.

DISCUSSION

  Figure 2 shows the precipitation at Shangboche about 15 km to the west of the Mingbo Valley. The collapse of the ice-cored moraine has no relationship with marked rainfall around 3 September. It is possible that melting of the ice-cored moraine was a reason for the collapse which initiated the outburst flood.

  Most of the large glaciers in the Nepal Himalayas have ablation areas covered with thick debris (Fig. 1) and are classlEied as debris-covered glaciers (Moribayashi L Higuchi. 1977). The debriscovered glacier is structurally composed of two ice bodies: one is stagnant in the down-stream and the other active in the upper-stream parts (Fushimi. 1977). Since there are many glacial lakes formed by ice-cored moraines in stagnant ice bodies such as the Imja Glacier (Glacier No. ED320 in Fig. 1) or the Dudh Kosi region (Fig. 6). further studies are needed to predict glacier-outburst floods in the Great Himalayas. It will be necessary for such purposes to monitor expansion of the glacier lakes by taking aerial photographs and landsat imagery of the glacierized area.

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REFERENCES

Fushimi. H. (1977) Structural studies of glaciers in the Khumbu region. Seppyou 39. Special Issue. 30-39.
Gansser, A. (1996)  Geological research in the Bhutan Himalaya. The Mountain World 64/65. The Swiss Foundation for Alpine Research. 88-97.
Hagen. T. (1963) The evolution of the highest mountaln in the world. In Mount Everest, Oxford Univ. Press. London. 1-96.
Miyahara. T. (1982) Himalaya no hi (in Japanese). Bungeishunju. Tokyo. 261 pp.
Moribayashi. S. b Higuchi. K. (1977) Characteristics of glaciers
in the Khumbu region and their recent variations.Seppyou 39. Special Issue. 3-6.