Kosi River, a major and powerful tributary of the Ganga, is made of seven rivers, viz. Sunkosi, Indravati, Bhote Kosi, Doodh Kosi, Arun, Barun and Tamur. Out of these seven rivers, Sunkosi alone supplies 44% water to the mighty Kosi. Sunkosi is born as Mtsang-Tsangpo in Tibet, across Gosainthan mountain range and cuts a deep gorge in southern Tibet to enter Nepal. There it flows in the valley between Mahabharatha range and Himalaya, till it meets the main river. Thus, it is a river that cuts across two countries before entering India.
Recently, on 2nd August a major 500m wide landslide engulfed Sunkosi River and a lake was thus formed. This lake is situated one kilometer upstream from the Sunkosi Powerhouse intake structure and downstream of Bhote Kosi Powerhouse. The landslide swept off the road to China and also affected the habitation downstream.
The dam thus formed was according to experts 409m long, 106m wide and 55m high. The total volume of the lake has been estimated as 8 million cubic meters. The Nepalese authorities quite dexterously punctured the dam in a controlled manner, so as to release water slowly and gradually. Had this dam burst in one go, the results could have been disastrous.
In India, a similar situation had arisen in 1893, when a spur of a hill slipped and fell across Birehi Ganga, at Gohna in Uttarakhand. A dam 11000 feet wide at the base, 900 feet high and 200 feet wide at the top was thus formed. T.H. Holland of Geological Survey of India was thus dispatched to survey and find out the details. Of which, I have already given a detailed account in many stories here in boloji.com (And the Lake burst, Beware of the Himalayan lakes, Landslide devastates Uttarakhand, Tales from the lake sediments etc). Jim Corbett the legendary hunter has written about this dam and lake, in his famous book, Man eater of Rudraprayag, as he had indirect association with it. His great grandfather Capt. Keith Stewart, and his maternal grandfather F.W. Champion had visited this lake, when it was newly formed. Jim Corbett tried to relive their experience by visiting the place. Corbett says that as the lake burst in August 1894, ten billion cubic feet of water were released in a matter of six hours. He says that except a family of five, no one died due to the flash floods, but all the royal palaces and temples at Srinagar were washed away.
The dam at Gohna had burst only partially then. It was in 1970 again when the dam burst completely there was a total mayhem downstream.
Our memories are short lived. During the British days we forgot how the dam at Gohna was formed? Over a period of time Gohna Lake became a fishing retreat for British and they began boating in the lake. The lush forests surrounding caught their attention. Soon the area became a timber merchant’s paradise as well. The greed for timber continued unabated until the second mishap took place.
There are plenty of such stories from the Himalayas. There have been blockades in the path of Sutlej as well and our Nathpa Jhakri Power project had to bear the brunt.
We seem to have forgotten the Hopscotch by Kosi in 2008, when the river decided to go back to its 200 year old course. Such feats are possible even now. We have seen how large the catchment of the Kosi is. Over such a wide extent landslides, cloud bursts cannot be ruled out. In such situations if this mighty river is blocked in its catchment and then the temporary dam bursts the consequences can be disastrous for our country.
Because of the climate change, there are possibilities of extreme events; therefore, such mishaps are bound to occur greater might and frequency. In order to mitigate the problem for now and look for long term management, David Petley, an expert in hazard management at Durham University, UK has suggested evacuation of the population downstream of the dam and controlled release of water from the lake. This is risky, because if there is a sudden outburst, the huge mass of water hurtling down the river can destabilize the slopes by increased toe erosion. If this happens there will be massive landslides all along, bringing further doom for the downstream areas.