Tibetan Rivers

Flash floods from Tibetan rivers: need for caution


FLOODS HAVE been wreaking havoc in many parts of the country every year and the usual exercises of flood warnings, shifting population to safer places, providing flood relief, etc., have been an annual feature for decades. But this year an unusual event caught media attention — a flood warning from China about the impending failure of a lake formed by landslips in the Lang Chen Khambab (Sutlej) catchment in Tibet. The bursting of the lake would have hurtled down an enormous column of water under pressure devastating life and property in many places in Himachal Pradesh as the flood cascades down the valley.

According to reports, a landslip on the Pareechu river (Tibet), which joins the Spiti in Himachal Pradesh, a tributary of the Sutlej, blocked the river at a remote location in the Ali Prefecture of Tibet and water got headed up the lake. The blockage occurred at a place about 35 km from the India-China border. The increased rainfall and water flow from the upper reaches substantially raised water levels downstream. National Remote Sensing Agency's (NRSA) studies on imageries taken on May 10, July 16, July 26 and August 9 showed that while there was no lake observed on May 10, its size grew from 123 ha. on July 16, to 188 ha. on August 9 and to 193 ha. on August 12 with water flowing over the dam. The increase of 5 ha. in the reservoir area in just 3 days caused alarm bells ringing in Shimla and New Delhi necessitating high alert to be sounded in the areas along the river banks. By August 14 it was assessed that there was a good chance of the lake stabilising as the inflow was balanced by the outflow and since the debris wall was situated in a gorge, the water pressure from the swelling lake was being absorbed by the mountain face on either side.
Almost during the same period there were also reports of impending floods in the Brahmaputra due to landslips in the upper catchments of the river (known as Tsangpo in Tibet).
The threat from Tibet for the people and projects in India is not the first to occur. During August 2000, a flash flood occurred in the tribal district of Kinnaur (Himachal Pradesh) causing huge loss of life and property. The water level in the Sutlej rose by 15 m. in a short span of 6 hours discharging a peak flow of 5100 cumecs during the period. During the same year, a lake burst in the upper reaches of the Brahmaputra in Tibet left a trail of destruction downstream in India. Flash floods in these rivers occur due to one or a combination of many factors in the Tibetan Himalayas such as heavy rainfall, temporary blockage due to landslips, breaches in upper lakes, glacial lake outburst flows, dam construction, etc.
In the case of flash floods in 2000, as the sites concerned could not be visited, the reason, on the basis of data available, pointed to a flow triggered by a glacial lake outburst. In the latest case, the Chinese said that local landslips have created a debris wall choking the river gorge thereby forming the lake.
The landslips could be caused by natural factors or man-made due to blasting for specific purposes such as for project construction or for perfecting new dam building techniques. In this connection a news item that appeared in the Scientific American of June 1996 will be of specific interest. It was reported that Chinese engineers had assured the Chinese Academy of Engineering Physics in December 1995 of the possibility of diverting the waters of the upper Brahmaputra to the arid Northwest region and the Gobi desert using new techniques such as nuclear blasting.
The directional blasting techniques, initially pioneered by Russian engineers, help in constructing rockfill dams within a short period of time, particularly in gorges. By planned blasting of mountain slopes on either side of a gorge, the debris thrown up could be directed to fall in pre-determined locations in the gorge to dam the stream, with each layer of rockfill mass getting compacted by the subsequent heaps of debris thrown down by a series of blasts of the hill slopes. The charge required for each blasting, the direction, etc., would have to be planned carefully and technique perfected so as to design such fast construction methods. Since the Chinese have denied any construction activity in the area, the root cause for the occurrence of the slips i.e. whether due to natural causes or experimental blasting, can be confirmed only by site visits by experts.
These virtual liquid bombs ticking away in the Tibetan region pose a permanent threat and strategic concerns to the country's border areas. The risk cannot be removed entirely, but it can help set priorities. For continued vigil and preparedness to meet any eventuality, early warning systems have to be installed at our river stations on the border with state-of-the-art instruments such as Data Collection Platforms equipped with Auto-Weather Stations to monitor meteorological parameters, hydrological sites equipped with sensors to monitor water levels, etc. Instead of scrambling for damage control year after year, a centralised effective disaster management system has also to be put in place so as to address the concerns of the local residents during flash floods. Collapse of dams in the Tibetan region resulting from nature's fury or artificially created, should no more take us by surprise when we have the latest technological inputs. We have already been warned, hence let us be forearmed.

(The Hindu, 14th September,2004)




•