I T WAS in August last year that Bihar witnessed unprecedented floods when a breach occurred upstream of the Kosi barrage and the river suddenly changed its course — converting vast tracts of habitat into a 15 to 20 km wide sheet of water — flowing into some of its ancient channels.

The river was diverted back to the old course through the barrage on January 26, 2009 after restoring an about 2000 m long embankment which had breached.

Rajiv Sinha — a professor of geosciences at IIT Kanpur who has been studying the river for many years — now says that it is only a question of time before there is another breach and flooding from Kosi because the underlying conditions that led to the disaster remain the same.

Sinha says avulsion or sudden change in a river’s course happens when a threshold is crossed — when the cross valley slope is close to or more than the down valley slope.

The Kosi crossed this threshold last year primarily due to excessive deposition of sediment within the channel bed.

The river was literally flowing in a ‘ super- elevated’ position.

It changed its course because it found a new equilibrium.

“ By putting the river back, we have re- created the same ‘ unstable’ condition for the river and it is only a matter of time before the river crosses the threshold!

To the best of my knowledge, no major river training work has been carried out so far to create a more ‘ stable’ and ‘ favourable’ condition for the river to flow along the ‘ forced’ course”, Sinha has opined.

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