Study on formation mechanism and expansion mechanism of a moraine-dammed glacier lake

近半个世纪以来，在大喜马拉雅山脉南坡和北坡的大型山谷冰川萎缩的碎片覆盖的消融区形成了超冰川湖，这可能是由于最近的全球变暖。由于冰湖被不稳定的冰碛筑坝，经常发生冰湖溃决洪水，给下游造成严重灾害。GLOF最近成为喜马拉雅国家自然灾害的新面孔。湖泊形成后，其扩张速度相当快。这个湖不是同时在所有的冰川上形成的：它是在一个冰川上形成的，而不是在相邻的冰川上形成的，尽管它们处于相同的气候和形态条件下。为了预测湖泊的形成和快速增长，研究了湖泊的形成和扩展机制。地下水盆地形成于山谷冰川消融区，以不透水的冰碛为界。下沉面到达地下水位后，稳定…冰川表面出现更多的池塘，扩大的池塘相互连接，最终成长为湖泊。消融区地表下降速率取决于冰川动力学引起的地表物质平衡和地表水位变化。在烧蚀区常见的负质量平衡和压压力场的涌现速度分别使表面下降和上升。表面碎屑较薄、消融区海拔较低、消融区周围通常发育有较多池塘和露冰的冰川，其表面负质量平衡较多，有利于降低地表。消融区倾角越缓，涌现速度越小，即地表隆升越小。每个冰川的条件都不一样。冰川湖首先形成于某一特定冰川上，即使它们处于相同的气候条件下，其表面下降速率也比其他冰川高。研究发现，湖的大膨胀率是由于冰崖的主动崩解所致，冰崖形成于冰川末端，与湖上端直接接触，地表湖水被太阳辐射加热后，被稳定的谷风输送到冰崖中，融化了水下的冰崖，有效地将铲出的冰崖塌陷并落入湖中。这一机制使得湖泊扩张，冰川迅速、有效地退缩。另一方面，湖底冰被2 ~ 3℃的湖水融化，湖水变深。湖泊热收支调查表明，冰裂产生的浮冰和底冰的融化热能主要由低反照率的湖面吸收的太阳辐射提供。这项研究的结果已在3份英文报告和3份日文报告中发表。少

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 … More 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