Kosi River Floods
Kosi River is known as Bihar’s sorrow. Almost every year, the river is subject to floods affecting around 20 thousand km² area of fertile lands in Bihar. During floods, its average water flow multiples by as much as 18 times of average water flow, thus creating havoc for the rural economy of Bihar. It caused devastating floods in 1953-54. In 1987, the Kosi floods had affected around 30 districts, claiming lives of around 1400 people and 5500 animals. Another disastrous flood experienced by Kosi was in August 2008 when the river changed its course and created havoc in a densely populated area. The 2008 floods brought wide scale destruction in over 8 lakh acres of agricultural land in the state.
Saptakoshi
Kosi River is one of the largest tributaries of the Ganges and also known as Saptakoshi due to seven of its tributaries. It originates in Tibet. It joins Ganges near Kursela in Katihar district of Bihar.
Kosi and its avulsive shifts
In terms of geography, avulsion refers to rapid abandonment of existing channels and formation of new channels by a river. Avulsive shifts occur as a result of channel slopes that are much less steep than the slope that the river could travel if it took a new course. Although avulsions have a strong impact on river morphology and present a major natural hazard, surprisingly little is known about the factors that control avulsion.
Kosi River is notorious for the meandering behaviour of its east-to-west course. It has a very dynamic regime; very high sediment load and frequent “avulsive shifts.” The avulsive shifts in Kosi River have been well documented and are thought to occur as westward movement of the river channels in last 200 years. But surprisingly, in 2008, the river shifted 120 kilometers eastward and breached an afflux bund at Kusaha in Nepal, 12 kilometers upstream of the Kosi Barrage.
This avulsion diverted the 80-85% of the flow of river to a new course; and has been called as one of the greatest avulsion in any large river system in world in recent times.
Why Kosi Floods are more dangerous?
The Kosi’s alluvial fan has fertile soil and abundant groundwater. The agricultural land in these areas is in great demand. Poverty is prevalent and farmers try to compromise on threat of floods with that of farming in flood prone area. This is the reason that this area is densely populated and subject to heavy loss of life.
Why do rivers change course?
It’s natural for a river to keep shifting its course. It’s a part of its natural evolution. There are several factors that cause change in course of rivers. Firstly, the fast flowing rivers are generally prone to silting up as they surge down the hills and spread out on the plains; thereby allowing sand and suspended matter to deposit in their slower, wider depths. Over time, the deposits create resistance, forcing the river to move to an area of lower resistance. This is why rivers change their courses. Secondly, the nature’s fury such as earthquakes, landslides and hurricanes can also change a river’s course. Thirdly, human activity and climate change are also responsible for change in river course. For example, climate change has triggered the melting of glaciers more quickly thus, the volume of water in rivers has been on the rise, resulting in greater force of flow. Therefore, even relatively lesser amount of rain during the monsoon can lead to a catastrophe.
