Guwahati, Wednesday, July 06, 2011
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Changes in State riverbed feared
Ajit Patowary
 GUWAHATI, July 5 – Under the impact of the resultant flow reduction due to any possible future diversion of the Yarlung Tsangpo to acute water scarce regions in northern part of China, the river morphology in Assam is likely to undergo further intense braiding along with harmful rise in average stream bed level. As a consequence, even for moderate kind of monsoon rains, there will be much more magnified incidence of flood events than what is experienced in the present, since flood carrying capacity of the aggraded channel section will progressively decrease.

This is the view expressed by Professor Nayan Sarma, Head of the Department of Water Resources of the IIT, Roorkee. He was talking on the hydel power projects planned on the Yarlung Tsangpo and diversion of the river planned by the Chinese authorities. Prof Sarma as connected with various research works on the Himalayan rivers, including the Brahmaputra, the Ganga and the Koshi.

Prof Sarma said that besides the 510- MW Zangmu dam, five more dams, downstream of Zangmu, are under active consideration of the Chinese authorities. And these include the largest of all the hydel dams of the world—the about 38,000 MW Motuo dam.

Proposal for another massive dam Daduqia, with an installed capacity of 42,000 MW, just near the Big Bend on the river, close to the Indian border is also under active consideration of the Chinese authorities to meet the 40,000 MW shortfall of power in China, said Prof Sarma.

To analyze the impact of any possible future plans for flow diversion of the Yarlung Tsangpo by constructing storage or diversion dams or by simply making dams at a very high altitude (not at foothills) for run-of-the-river hydel project, it has become highly pertinent to delve into the potential environmental, socio-economic and safety concerns in the immediate downstream Assam valley of the Brahmaputra from a strict technical standpoint, he said.

At the present juncture, this has become an imperative necessity in the absence of any water sharing agreement amongst the riparian countries of the trans-boundary river, he maintained.

About 44 per cent of the average flow of the Brahmaputra at Pandu is contributed by the Dihang or Siang main stream, which is known as Yarlung Tsangpo in Tibet. While during monsoon season, the balance 56 per cent flow of the river comes from the tributaries in Assam, the bulk of the non-monsoon dry weather base flow of the Brahmaputra is generated from the source glaciers in the Tibetan high Himalayas, he said.

It should thus be apparent that any reduction of stream flow in the Yarlung Tsangpo, because of climate change or artificial flow diversion in coming decades, will have a very profound impact on the ultimate quantum of water resources in downstream areas of the Brahmaputra.

As a consequence, even for moderate kind of monsoon rains, there will be much more magnified incidence of flood events than at present, since flood carrying capacity of the aggraded channel section will progressively decrease.

In the possible event of any flow diversion of the Tsangpo in future, besides stream flow three major sectors of water uses namely – ecology, inland navigation and drinking water will be very severely hit especially in the non-monsoon low flow period.

Due to greatly reduced water depth in the very wide braided river sections coupled with widespread sedimentation, the prime aquatic creatures like dolphins, turtles, big fishes and other mega fauna will practically become extinct due to irreversible process of habitat shrinkage.

The inland navigation fairways will become inoperable due to loss of required navigation draught. The supply sources catering to rising water demand for municipal, industrial and pollution control from the Brahmaputra such as for Guwahati city, refineries etc. will run dry during low flow season. Even abstraction of ground water is likely to be much more arduous and expensive due to resultant depression in sub-soil water levels.

Lastly, leaving aside the attractive future plan for a potential mega project on the Tsangpo (as tentatively identified near Namche Barwa at Motuo to generate a huge block of 38,000 MW), even the under construction 116-metre high Zangmu Dam on the Tsangpo, at an altitude of 3,260 metres in Eastern Tibet, may pose terrible hazard for the plains of Assam in the event of a major earthquake.

It is a universally followed practice that before planning any dam – whether run-of-the-river or storage based, especially on such a very high earthquakeprone zone, it is a statutory obligation of the planners to conduct comprehensive dam break analysis so as to predict and evaluate the possible consequences in the downstream areas resulting from any dam failure.

This is highly essential for the case of a trans-boundary river like the Brahmaputra. There are good reasons to believe that such a statutory dam break analysis has not been conducted for the under construction Zangmu and other dams proposed on the river. And thus the safety concerns of the life and property in the downstream areas in Assam are totally under-rated, said Prof Sarma.

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