Kosi Floods- Nature induced or Man-made?

- Dr. Ashok K. Ghosh, Prof-in-Charge, Dept. of EWM, A.N.College, Patna&

Dr. Nupur Bose, Head, Dept. of Geography, A.N.College, Patna

The Kosi river system is the only Indian river whose hydrology is deeply influenced both by the regional geological complexities and inputs of annual precipitation and Himalayan glacier melts. This antecedent drainage system is notorious for its migratory trends, resultant flooding, and a huge detrital load of boulders and sand. Entire Kosi fan belt is etched with palaeo-channels that demarcate the east-to-west and back swing of the river from the Epic Age onwards. The recent flooding caused by a breach in the Kosi embankment in Nepal has highlighted the increasing role of geological processes like plate motions, neotectonics, and local isostatic equilibrium in the Himalayan region and the adjacent Ganga basin. Presently, the Kosi has abandoned its westward curving channel for an almost direct north-south flow from the Himalayan base upto its confluence with the Ganga.

Modis Image, Aug. 22, 2007

Remote Sensing Image of Kosi Basin- August 2007

Modis Image, Sept.02, 2008

Remote Sensing Image of Kosi Basin- September 2008

Remote sensing imageries and geology of the Kosi Fan reveals a number of facts that must be taken into account in future management of the river. These are:

The breach of the river embankment occurred on its left bank, which is the cut-off slope of an elbow bend of the river, implying the already existing pressure of the turbulent water descending steeply from the Himalayas.

The previous curved shape of the channel in its middle reaches roughly coincided with the underlying Barauni fault, the present flow being parallel and mostly above the eastern Bhawanipur fault located in the basement complex beneath the sediments. It implies that horizontal plate movements of the region is also being accompanied by vertical or isostatic adjustments of the rock layers, thereby making the entire region not only flood-prone but also seismically volatile [ earthquake-prone].

Fears of tectonic changes having caused the breach in the Kosi embankment has been rendered true following report of an earthquake measuring 6.8 on Richter scale that occurred in Tibet on 25 August, 2008. Kosi river originates in the Tibetian Himalayas. Any disturbance in the Tibet Plateau has the potential to impact the river’s course on account of elastic rebound of the affected rock structures. The stress built up in the last 200 years, may have induced the eastern migration of the channel due to displacement of the crust along the main and transcurrent faults below the river bed.

The researchers had presented papers in Australia [2004] and UK [2007] which highlighted the possibilities of the eastward migration of the Kosi river resulting from upliftment of the north-western Bihar and subsidence in the north-east as evinced from remote sensing studies. They had reported first the changing trends of surface water configurations over a period of last two decades, followed by resultant changing land use patterns.

It would be extraneous to state that current climate change has been largely responsible for the sudden change in Kosi’s direction of flow, in the absence of similar trends in the remaining parallel Himalayan streams in the Indian Plains. Kosi is a highly unstable river, whose detritus is dominated by sand that renders the soil infertile. Historical records of living with the floods in the plains would suggest adjustment of infrastructure and livelihoods to the annual inundation by the Kosi System, by allowing the river to settle into palaeochannels during rainy season and leave the interfluves dry. It requires immediate attention by earth scientists to see the emerging trends of the river Kosi influenced by geological causes, and then decide upon appropriate action. The authors will be presenting their findings in the proposed annual conference of Association of American Geographers in 2009.________________________________________________________________________