NEW CYCLE OF DEVASTATING KOSI FLOODS
Dr. AB Thapa
Peoples of our region must be alarmed that we are quickly heading for far worse Kosi flood disasters. We might be very close to the days when the swollen Kosi would permanently abandon its present course and it would be freely roaming over its vast eastern floodplain. The damages, that resulted from the recent bursting of the eastern embankment, would pale in comparison with the great losses of life and property consequent upon the coming catastrophic Kosi flooding. Even the distant West Bengal and Bangladesh lying on the old course of the Kosi might not be completely out of the Kosi flood danger.
In the past the Kosi debouching into the Terai from the mountain oscillated laterally at long time interval. The recent shifting of the Kosi course must be the beginning of a new cycle of devastating Kosi floods.
Causes of Kosi Flooding
Various studies done on Kosi so far come to one common conclusion - the Kosi carries enormous load of sediments that it is unable to transport far beyond. The river, therefore, deposits huge quantity of sediments. In this process of building up an inland delta, the river shifted over 120 km from east to west during the period from 1736 to 1954.
In 1941, Sir Claude Inglis, Director, Water Research Station, Poona identified the problem of Kosi as being due to an excess charge of the sand that Kosi waters carry.
Considering the Kosi behaviour, renowned scientists Leopold and Maddock stated (1954) that a braided stream (like the Kosi) will tend to shift laterally at a rate dependent on the rate of accumulation of materials being deposited. As one course becomes higher than possible adjacent paths, the river would shift.
River Bed Rise
The Kosi is regarded as one of the largest braided streams of the world, though the Kosi River is marked by distinct downstream changes in channel patterns. These patterns are gravel-sandy braided, sandy braided, straight, and meandering.
The alluvial plain of the Kosi River between Chatra and Hanumannagar is triangular shaped. It is steeply sloped with its apex at Chatra. The river has formed a sub-circular mega-fan with a radius of about 60 km downstream of the barrage near Hannumannagar.
In 1966 a study of the delta cone of the Kosi river made by Prof Gole CV Dr. Chitale M. appeared in the ":Proceedings of the American Society of Civil Engineers". The study explains that the delta building of Kosi was still incomplete. The authors have cautioned that the existing measures (embankments) may be short lived.
The studies carried out by Sanyal (1980) and Gohain & Prakash show that in the period between 1963 and 1974 there was high aggradations upstream of the barrage. The area just downstream of the barrage had been marked by a small degradation. But further downstream, there had also been increased aggradations.
Alarming Reduction in Kosi Barrage Capacity
The Kosi barrage had been designed for a maximum flood discharge of 9.5 lakh cusecs. The maximum flood water level was 25 feet above the bed level. Similarly, the normal pond level was 15 feet above the bed level. According to Gohain & Prakash the annual rate of the sedimentation upstream barrage based on observations as of the early 1980s is about 5 cm per annum. It implies that within the last 60 years after the commissioning of the barrage a layer of 3 meters thick sediments might have already deposited on the river bed. Now the maximum flood water depth might be only 60% of the original depth.
We can calculate the present maximum flood discharge capacity of the Kosi barrage assuming that the maximum water depth would be only 60% of the original depth. It can be roughly calculated based on the formulas of French Engineer Antoine Chezi and Irrish Engineer Robert Manning. We will find that at present the barrage might be able to safely discharge a flow only up to 4.0 lakh cusecs. Gohain & Prakash have reported in early 1980s “If the discharge exceeds 8400 cumecs ( or 3 lakh cusecs), which is fairly common, the whole area between the two embankments is submerged.”
Sedimentation Problems Will Worsen
Within the last 60 years twice the flood discharges up to 9 lakh cusecs had been recorded. But after 1968 the maximum Kosi discharge never crossed the 6 lakh cusecs limit. It is now an accepted scientific conclusion that the high flood discharges resulting from heavy rainfalls are the primary cause of increased sediment flow. The following is an excerpt from the Karnali project report prepared under the World Bank’s assistance.
“The sediment load from mountainous areas, such as the Himalayas and the Karakoram, is believed to be derived extensively from localized mass wasting (landslides ) rather than from generalized surface erosion (Carson 1985). Sediment yield should thus be better correlated to precipitation or runoff events which are two of the main factors causing landslides and gully erosion.”
It is seen that within the last 45 years the Kosi flood discharges were far below the maximum discharges already recorded twice within the last 60 years. As a result, we might not be wrong to conclude that within the last 45 years the Kosi barrage project might have been spared to face serious sedimentation problems. However, there is a great probability that in very near future the situation could take an alarming turn.
What Comes Next?
It might still be possible to restore the Kosi river to its original course at a considerable expense. There would be further rises in river bed levels as the years pass and the works to restoring the river course would become too expensive and technically very complicated.
Very recently the Kosi has burst the eastern embankment and shifted its course headed to south when the flood discharge was only about one lack and eighty thousand cusecs. This flood discharge is quite insignificant compared to 9 lakh cusecs. flood discharges, which have already been recorded twice within the last sixty years. Despite the fact that the Kosi has changed its course at such a low flood discharge, Bihar suffered very heavy losses. It is frightening even to think of the horrors if the Kosi would suddenly burst the embankment at a time when the flood discharge is close to nine lack cusecs. There would be a total devastation of the whole north-east Bihar. Even the distant West Bengal and Bangladesh lying on the old course of the Kosi might not be completely out of the Kosi flood danger.
(Dr. Thapa writes on water resources)
