Kosi River - Floods

Floods

The Kosi is known as the "Sorrow of Bihar" as it flows from Nepal to India, as it has caused widespread human suffering in the past through flooding and very frequent changes in course .

The Koshi has an average water flow (discharge) of 2,166 cubic metres per second (76,500 cu ft/s). During floods, it increases to as much as 18 times the average. The greatest recorded flood was 24,200 m3/s (850,000 cu ft/s) on 24 August 1954. The Kosi Barrage has been designed for a peak flood of 27,014 m3/s (954,000 cu ft/s)(2).

Extensive soil erosion and landslides in its upper catchment have resulted in the silt yield of the Kosi of about 19 m³/ha/year (10 cu yd/acre/yr), one of the highest in the world. (2). Of major tributaries, the Arun with its origins in Tibet brings the greatest amount of coarse silt in proportion to its total sediment load. The river is able to transport its heavy sediment load down the steep gradients and narrow gorges in the mountains and foothills where the gradient is at least ten metres per km. On the plains beyond Chatra, the gradient falls below one metre per km, even to as little as 6 cm per km as the river approaches the Ganges. Current slows and the sediment load settles out of the water to be deposited on an immense alluvial fan that has grown to an area of about 15 000 km². This fan extends some 180 km from its apex where it leaves the foothills, across the international border into Bihar state and on to the Ganges. Instead of a single well-defined channel, the river has numerous interlacing channels that shift laterally over the fan from time to time. Without sufficient channelisation, floods spread out very widely. The record flow of 24 200 m³/s is equivalent to water a metre deep and more than 24 km, flowing down the slight slope of the alluvial fan at one metre per second.

The Kosi's alluvial fan has fertile soil and abundant groundwater in a part of the world where agricultural land is in great demand. Subsistence farmers balance the threat of starvation with that of floods. As a result, the flood-prone area is densely populated and subject to heavy loss of life. It contributes disproportionately to India having more deaths in floods than any other country except Bangladesh.

• The Kosi before it flooded in August 2008

• The Kosi during the August 2008 flood