| Co-Investigator(Kenkyū-buntansha) |
NISHIMURA Koichi National Research Inst. for earth Sci. and Disaster Prevention, Nagaoka Inst. of Snow and Ice Study, Chief Researcher, 長岡雪氷防災研究所, 主任研究員 (10180639)
FUSHIMI Hiroji Univ., of Shiga Pref., Dep. of Environmental Science, Prof., 環境科学部, 教授 (10109358)
KOBAYASHI Shun'ichi Niigata Univ., Research Inst. for Hazards in Snowy Areas, Prof., 積雪地域災害研究センター, 教授 (70001659)
HIGAKI Daisuke Hirosaki Univ., Faculty of Agricultural and life Science, Asso. Prof., 大学院・農学研究科, 助教授 (10302019)
HARADA Koichiro Miyagi Agricultural Collage, Lecturer, 講師 (60331279)
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| Research Abstract |
Supra-glacial lakes have been formed on the debris covered ablation area of shrinking large valley glaciers in the southern and northern slopes of Great Himalayas in this half century, maybe due to the recent global warming. Since the lake was dammed by unstable moraines, a glacier lake outburst flood (GLOF) is frequently happened and a serious disaster is generated along down stream. The GLOF newly appears as the new face of natural disaster in the Himalayan countries. Once the lake was formed, its expansion is quite high rate. The lake is not simultaneously formed on all the glaciers : it is born on one glacier but not on the neighbor glacier, though they are situated under the same climatic and morphological condition. For predicting lake formation and its rapid growth, formation and expansion mechanism were studied.
Underground water basin is formed in the ablation area of a valley glacier and bounded by impermeable moraine. After lowering surface arrives at the water table, stable
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ponds appear on the glacier surface expanding ponds connect each other and finally grow up to a lake. Surface lowering rate of the ablation area depends on surface mass balance and surface level change derived from glacier dynamics. The negative mass balance / the emergence velocity in the compressive pressure field, which are common at the ablation area, make the surface lowering / uplifting, respectively. Such a glacier that the surface debris is thinner, the altitude of the ablation area is lower and more numbers of pond and exposed ice beside it, which usually develop on the ablation area, have advantage for the surface lowering because of more negative surface mass balance. More gentle inclination of the ablation area, the less emergence velocity, i.e., less surface uplift. Those conditions are deferent from each glacier. A glacial lake is first created on such a particular glacier as the surface lowering rate is higher than the other glaciers even though they are in the same climatic condition.
It is found that large expansion rate of the lake is attributed to the active calving of the ice cliff, which is formed in the glacier terminus and directly contact with the up-lake end Surface lake water warmed up by sun radiation is transported into the ice cliff by stable valley wind and melts the ice cliff under the water, effectively the cliff scooped out is collapsed and fall into the lake. This mechanism makes the lake expansion and the glacier rapidly retreat, efficiently. On the other hand, bottom ice under the lake melts away by lake water of 2〜3℃ and lake deepens. Investigation of heat budget of the lake proves that heat energy to melt floating ice, produced by calving, and bottom ice is mainly supplied by sun radiation absorbed in lake-surface with low albedo.
Results of the study was already published in 3 English reports and 3 Japanese repots. Less
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