And the Lake Burst!

The curiosity to know what is behind the mountains has always driven humanity to explore the remotest parts of the Himalayas. What was known in the history as the southern land of darkness, now a mountain kingdom Bhutan has always been an enigma for the explorers. The scenic beauty, clean air, blue waters and above all happy people of Bhutan have fascinated everyone since times immemorial. 
Like all the mountain states, Bhutan also has its share of natural hazards. There are some hazards like an earthquake which can not be predicted. Then there are others like landslides which are usually a by-product of excess rains and of course often triggered by human interference in destabilizing the slopes. Yet there are some unfortunate hazards triggered due to a misbalance between the three elements of our environment, viz. the atmosphere, lithosphere and the hydrosphere. Yes it is true; these three principle elements are normally in a state of balance. But if one of them begins to act abnormal, things go haywire. Usually it is the atmosphere which gets affected faster than the others which leads to excess rain, excess heat and prolonged drought. Yet another impact of rise in temperatures is excess melting of ice.
Before delving deeper in the issue it is significant to know that the glaciers produce huge quantities of rock flour and rocks of assorted sizes are also carried by the glaciers as moraines. It is normal to find a stream of water coming out of the glacier and flowing through the valley carved by the retreating glacier. This stream in its initial passage is flanked by the moraines. There have been instances when the stream passage has been blocked by landslides damming the stream. In addition it is quite normal to find glacial lakes left by the retreating glaciers in the Himalayas. Water in these lakes is held captive by the morainic dams.
Lunana in Gasa district of Bhutan is one such area which is extensively glaciated, say Dr. O.N. Bhargava and S.K. Tangri of Geological Survey of India (GSI), A.K. Choudhary of Hydroelectric Project Authority, Bhutan and Y. Dorji of Bhutan Government in one of their publications. A richly glaciated terrain no doubt is endowed with great beauty, but it has in its own hidden hazards. The lakes left by the retreating glaciers are often prone to bursting. This becomes inevitable if the rate of melting of ice increases within a short span of time due to increase in temperature.
Sudden bursting of lakes leads to flash floods in the downstream areas. Often lakes are formed if a gigantic landslide blocks a river. Readers will recall ‘Beware! The landslides’, where a massive landslide on Birehi Ganga blocked the river at Gohna. Piles of material rolled into the river from both the sides. On the eastern side an entire spur of a hill, 11900 feet high, rolled down in to the river. Consequently, a dam was formed on the spot. This incident took place in 1893 and fortunately GSI was already there. All such incidents are investigated by the GSI to find out the cause and remedies to avoid such mishaps in future. This was no exception.
T.H. Holland of the Geological Survey of India undertook the survey of this slide on 2nd March 1894. He recorded that a 1000 feet high dam was formed across the river at Gohna, 160miles north of Haridwar. The blockade gradually submerged the area and a four-mile long and one mile wide lake was formed. The observations of Holland were so precise that not only he calculated the rate of submergence of the land, but also predicted the period by which the water of the artificial lake will overflow. As per that on the night of 25/26 August 1884, the top 380 feet of the dam was washed away. The breach in the dam released 10,000 cubic feet water in merely four and half hours causing devastating floods downstream. The magnitude of the floods can be realized from the fact that downstream at Chamoli the water level rose by 160 feet and at Haridwar it rose by 13 feet.
Gohna incident took place in 1894, when population was much less and giant hydroelectric projects in the river valleys of the Himalayas had not even been conceived. The impact of such accidents now can be far reaching and economically quite damaging.
Well aware of hazardous consequences of such mishaps in the Himalayan terrain, GSI in collaboration of the Royal Government of Bhutan undertook the survey of the lake burst that took place in Lunana area leading to flash floods in Pho Chu River on 7th October 1994. It was a severe hot summer of 1994 that led to excessive melting of glaciers say Bhargava et. al.
It is interesting to note that this area has some of the major glaciers of Bhutan. They are in order of their enormity, Tshoju, Chhunami, Raphstreng and Luggye north glaciers. Amongst these Chhunami has suffered maximum says the report of Bhargava and his team. Raphstreng glacier receding at the rate of average 23 m per year debauches in to a proglacial lake. In addition there are many other glaciers which keep adding their melt water in to the streams leading to Pho Chhu River. These glaciers encompass an area ranging in altitude from 4200 meters to more than 7000 meters. Common sense says water flows in the direction of slope. The slope available to water here is enormous.
The pressure of water in the glacial lake caused its moraine dam to breach leading to floods. These floods destabilized the slopes on either bank of Pho Chhu near Tshopda Tsho outlet. A terrain traversed by several glaciers is prone to heavy erosion and the lose material on the slopes remains in a kind of static balance. The moment water begins to flow through the material, their ‘meditating posture’ gets disturbed and the material begins to slide. That is how the slopes at Tshopda Tsho got disturbed.
The hazard in a glaciated terrain is compounded by ice avalanches. Such avalanches often occur in Raphstreng area. These avalanches can cause considerable damage to the body of the glacier and a sudden surge of the glacier can damage the rim of the lake in front. Obviously the lake will not be able to hold water and a sudden release can cause flash floods. The GSI team well aware of all such possibilities took up a detailed study.
There can however, be no uniform pattern of mitigation of such hazards. Each area has its unique geological, geomorphological, geotechnical, microclimatic and hydrographic characters. These need to be studied minutely before arriving at a conclusion.
The mitigation measures suggested by the team included engineering solutions to physically check sudden flooding of the lakes and also partial draining of the lakes, to lower their water level. Amongst the long term measures suggested were constant monitoring of the slopes, plantation along the slopes, regular monitoring of the lakes and seismic and micro-climatic observations.
The Royal Government of Bhutan implemented the suggestions in the years 1996-97 and since then the lakes have remained intact report Bhargava and his co-workers.
The type of investigation carried out by Bhargava and his team in Bhutan attains significance in the present days. Himalayas are a big source of Hydroelectric Power. The Himalayan region holds an estimated share of 64,653 megawatts of hydroelectric power potential. In fact country's 73% demand of power can be met from the Himalayan Rivers alone. The power generating potential of The Rivers of Ganges system in the Himalayan regime is estimated to be 19,000 megawatts. The Indus river system also has almost a similar capacity.
Alas the climate change has threatened even the existing hydroelectric projects of the Himalayas. Therefore it is pertinent to study in detail the hazard proneness of the hydroelectric projects in the Himalayas and avoid loss of life and structures. The instances of lake bursts are likely to increase with the rising temperatures.
Himalayan Ice is not that cool. It can spell doom if it melts fast!   

Images (c) Gettyimages.com
1) Chomolhari base camp, Jangothang, Bhutan
2) Satellite view of Bhutan.
3) Mount Jichu Drake reflected in Sophu Lake


More by :  V. K. Joshi (Bijji)

Top | Environment

Views: 3472      Comments: 2

Comment Thanks Dr Bhargava.

22-Dec-2010 11:31 AM

Comment Good reading

29-Nov-2010 00:10 AM

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