On the basis of actively retreating glaciers and other criteria, the potentially dangerous glacial lakes were identified using the spatial and attribute database complemented by multi-temporal remote-sensing data sets. Medium- to large-scale aerial photographs were used for detailed geomorphic studies and evaluation of the active glaciers and potentially dangerous lakes.
In general, based on geomorphological characteristics, glacial lakes can be grouped into three types: glacial erosion lakes, glacial cirque lakes, and moraine-dammed lakes. The former two types of glacial lakes occupy the lowlands or emptying cirques eroded by ancient glaciers. These glacial lakes are more or less located away from present-day glaciers and the downstream banks are usually made of bedrock or covered with a thinner layer of loose sediment. Both of these glacial lakes do not generally pose an outburst danger. On the other hand, the moraine-dammed glacial lakes have the potential for bursting. A standard index to define a lake that is a source of potential danger because of possible bursting does not exist. Moraine-dammed glacial lakes, which are still in contact or very near to the glaciers, are usually dangerous. In most of the literature/reports, the term ‘glacial lake’ is used for such lakes, and the term ‘glacial lakes’ used for glacier erosion lakes and glacier cirque lakes. The present study defines all the lakes formed by the activity of glaciers as ‘glacial lakes’. Moraine-dammed glacial lakes are usually dangerous. These glacial lakes were partly formed between present-day glaciers and Little Ice Age moraine. The depositions of Little Ice Age moraines are usually about 300 years old, form high and narrow arch-shaped ridges usually with a height of 20–150m, and often contain dead glacier ice layers beneath them. These end moraines are loose and unstable in nature. The advance and retreat of the glacier affect the hydrology between the present-day glacier and the lake dammed by the moraines. Sudden natural phenomena with a direct effect on a lake, like ice avalanches or rock and lateral moraine material collapsing on a lake, cause moraine breaches with subsequent lake outburst events. Such phenomena have been well known in the past in several cases of moraine-dammed lakes, although the mechanisms at play are not fully understood. The criteria for identifying the potentially dangerous glacial lakes are based on field observations, processes and records of past events, geomorphological and geo-technical characteristics of the lake and surroundings, and other physical conditions. The potentially dangerous lakes were identified based on the condition of lakes, dams, associated mother glaciers, and topographic features around the lakes and glaciers. Rise in lake water level In general the lakes which have a volume of more than 0.01 km3 are found to have past events. A lake which has a larger volume than this, is deeper, with a deeper part near the dam (lower part of lake) rather than near the glacier tongue, and has rapid increase in lake water volume is an indication that a lake is potentially dangerous. Activity of supraglacial lakes Groups of closely spaced supraglacial lakes of smaller size at glacier tongues merge as time passes and form bigger lakes. These activities of supraglacial lakes are an indication that the lakes are becoming potentially dangerous. Position of lakes The potentially dangerous lakes are generally at the lower part of the ablation area of the glacier near to the end moraine, and the mother glacier should be sufficiently large to create a potentially dangerous lake environment. Regular monitoring needs to be carried out for such lakes with the help of multi-temporal satellite images, aerial photographs, and field observations. In general, the potentially dangerous status of moraine-dammed lakes can be defined by the conditions of the damming material and the nature of the mother glacier. The valley lakes with an area bigger than 0.1 sq.km and a distance less than 0.5 km from the mother glacier of considerable size are considered to be potentially dangerous. Cirque lakes even smaller than 0.1 sq.km associated (in contact or distance less than 0.5 km) with steep hanging glaciers are considered to be potentially dangerous. Even the smaller size steep hanging glacier may pose a danger to the lake. Dam conditions The natural conditions of the moraine damming the lake determine the lake stability. Lake stability will be less if the moraine dam has a combination of the following characteristics:
A moraine-dammed lake, which has breached and closed subsequently in the past and has refilled again with water, can breach again. Ayaco Lake in the Pumqu Basin burst out in 1968, 1969, and 1970, and in 1980s it was refilled again with water and poses danger. In fact, there is a less water volume than before, based on the ASTER image of Feb. 21 and Oct. 13, 2001. So, regular monitoring of such lakes is necessary using multi-temporal satellite images. Condition of associated mother glacier Generally, the bigger valley glaciers with tongues reaching an elevation below 5,000 masl have well-developed glacial lakes. Even the actively retreating and steep hanging glaciers on the banks of lakes may be a potential cause of danger. The following general characteristics of associated mother glaciers can create danger to moraine-dammed lakes:
Physical conditions of surroundings Besides moraines, mother glaciers, and lake conditions, other physical conditions of the surrounding area as given below may also cause the lake to be potentially dangerous:
Major Glacial Lakes Associated with the Glaciers For identification of potentially dangerous glacial lakes, the glacial lakes associated with glaciers like supraglacial lakes and/or dammed by lateral moraine or end moraine with an area larger than 0.02 sq.km have been considered and they have been defined as major glacial lakes. The area of the inventoried glacial lakes is larger than 0.003 sq.km. There are 824 such glacial lakes in the Himalayan-China Region. Among these lakes, 199 glacial lakes having an area larger than 0.02 sq.km. Most of the major glacial lakes are in contact with or at a distance of less than 500m away from the glaciers and some of them are 1,500m away from the glaciers. Major glacial lakes associated with the glaciers were shown in the Tables (from Table11.1, to Table 11.8)
Potentially Dangerous Glacial Lakes Based on the analysis of inventory data using different criteria and the study of satellite images, 77 glacial lakes are identified as potentially dangerous lakes in the Himalayan-China Region. Out of these there are three glacial lakes (i.e. Gelhaipuco, Ayaco, and Jinco) with past outburst events. The identified potentially dangerous lakes are recommended for further detailed investigation and field survey to understand their activity (Table 11.9).
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