Background
The Kosi River's upper basin in southern Tibet and eastern Nepal drains some 60,000 km² of mountainous terrain, a region that tectonic forces are elevating by about 1 cm a year. If erosion keeps pace with geologic uplift, an estimated 600 million cubic meters of sediment is potentially carried downstream in an average year. However, empirical measurements of the river's sediment load have yielded estimates of 100 million cubic meters annually, indicating that the area is rising.
River gradient ranges from more than 10 meters/km for major upper tributaries in the mountains to as little as 6 cm/km as the lower Kosi nears the Ganges. As the gradient decreases on the plains, current slows and turbulence that holds sediments in suspension diminishes. Sediments settle out and are deposited on the riverbed. This process eventually raises a channel above the surrounding terrain. The river breaks out, seeking lower terrain, which it again proceeds to elevate by deposition. This creates a cone-shaped alluvial fan. The Kosi alluvial fan is one of the largest in the world, covering some 15,000 km² and extending 180 km from the outermost foothills of the Himalayas to the Ganges river valley.
Flood waters naturally spread out over the surface of this cone. Flows over 25,000 m³/s have been measured where the Kosi exits the Himalayan foothills, enough to create a flow of water 30 km wide. At this rate, in one week enough water would accumulate to cover the entire megafan to a depth of 1.5 meters.
Preventative flood control measures include upstream reservoirs that can also serve irrigation needs and produce hydroelectric power. However, in Nepal these are mostly in the planning stages. The flood control measures mainly consist of downstream embankments meant to confine the river to a fixed channel. In theory, the faster flow along this channel would carry high flows away and keep sediments in suspension.
On 18 August 2008 one of the man-made embankments failed. The river reverted from the prescribed western channel to an old channel near the centre of its alluvial fan. The river spread out widely and inundated towns, villages, and cultivated fields on the densely populated alluvial fan. Recurrent flooding on the lower Kosi contributes largely to India's history of suffering more flood deaths than any other country except Bangladesh, and has earned the Kosi the epithet "The Sorrow of Bihar".
Read more about this topic: 2008 Bihar Flood
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