Saturday, January 23, 2010

Experts’ bid to unravel causes behind erosion

Ajit Patowary

GUWAHATI, Jan 21 – Experts are trying to determine the factors mainly responsible for river bank erosion in Assam, particularly in its Brahmaputra Valley for several decades, as, erosion has been causing more havoc for the State’s people compared to flood. Five experts have, in a joint paper, pointed out to some vital factors, which have been triggering erosion in the State.

It is estimated that around 7.4 per cent of land areas of the Brahmaputra Valley has been lost due to bank erosion ‘in the recent history of observation.’

The five experts in a joint paper have maintained that in general, the losses due to erosion show an increasing trend. They have said that the reports available with the Assam Water Resources Department indicate loss of 3,860 square kilo meters (sq kms) of State’s land since 1954 due to this factor. The rate of loss is estimated to be about 80 sq kms per year.

The erosion of the Brahmaputra wiped out more than 2,500 villages and 18 towns, including sites of cultural heritage and tea gardens, affecting the lives of nearly half a million people.

About 130 river reaches are presently classified as being under moderate to severe erosion and 25 reaches as very severe. A comparison between the value of land lost to riverbank erosion and flood damage, between 1954 and 1969, indicates that the costs of riverbank erosion were 35 per cent to 85 per cent of the losses caused by the ravages of flood, said these experts.

In their paper – River Bank Erosion and Restoration in the Brahmaputra River in India – Dr Arvind Phukan, a former Professor in the University of Alaska, Rajib Goswami of the State Water Resources Department, Dr Deva Borah of the Woolpert Inc, Portsmouth, Ananta Nath of the Big Cypress Basin, South Florida Water Management District, and Dr Chandan Mahanta of the Guwahati IIT, made the above observation.

These experts, in their above paper, have tried to throw light on the factors responsible for riverbank erosion. To them, the rate of rise and fall in river water level, number and position of the major channels which remain active during flood and the angle at which the thalweg approaches the bank line, are the factors responsible for bank erosion.

Besides, they found the amount of scour and deposition that occurs during the flood, variability of the cohesive soil in bank material composition, formation and movement of large bed forms, intensity of bank slumping and progression of abandoned river courses to present-day channel also to be responsible for river bank erosion in the State.

They further maintained that due to the braided characteristics, the main stem river consists of variable number of different sized channels and sand bars which change their locations and sizes each year.

The most significant bank line modifications take place during the falling stages when excess sediment is deposited as bars within channel, causing a change in local flow direction and migration of thalweg.

During floods, because of change of river hydraulics (mainly depth, velocity and shear stress), inducing variable sediment transport characteristics and erosive forces, the channel starts shifting at some vulnerable reaches, they observed.

The key factors that cause extreme instability in the Brahmaputra river at many vulnerable reaches, such as – Nagaghuli, Maijan, Majuli, Bhairabpur, Balikuchi, Kaziranga, Howlighat and Palasbari – are aggradation of the riverbed, intense braiding, large water discharge and heavy sediment load since the 1950 flood, said these experts.

Moreover, there is a tendency of the river to shift southward within the valley reach. The tendency has become more prominent after the Great Earthquake of 1950, which raised the whole landmass of the northeastern part of the valley, particularly the north of the river including the Himalayan foothill region by 3 to 4 metres.

This southward thrust has initiated widespread erosion in the south bank near the Dibrugarh town and is still continuing at different reaches in spite of the implementation of aggressive bank protection measures. The Assam Water Resources Department has identified 25 such vulnerable and severe river bank erosion sites.

Moreover, they said, the records of the last century show a general trend of widening of the Brahmaputra in Assam. The widening trend of the river is clearly visible when compared with the erosion and accretion rates over different periods. Long-term observations on width changes of the river are available, although data from different authors are not directly comparable, they said.