standard Re-addressing the problem of flooding on the Kosi River

The Kosi River flows through Bihar in North India and Nepal.  In 2008, the embankments of the river were breached by flood waters induced by monsoon rains.  Since then, governments have attempted to prevent future floods through flood protection schemes and engineering methods such as dredging to restore the flow of the river along its central current or pilot channel.  This led to controversy in nearby villages located in the Sansuri district of Nepal who protested the dredging saying it would lead to the inundation of local villages if the water level rose too high, although it has been reported that the elevation of the villages would likely keep them safe from flood waters.  All recent news coverage about the implementation of flood protection of the Kosi seems to assume that dredging or opening the pilot channel would prevent devastating floods now and in the future.  This seems logical — in order to prevent the river from breaching its embankments, re-establish the central flow of the river.  But it’s unclear as to what consequences this would have for the river as a whole.  When the embankments were first built they were intended to hold back rising flood water levels, but since then they have eroded severely and repairing them is costly.  This is why other measures need to be taken to resolve future flooding.

The governments of Bihar and Nepal have persistently went back and forth, each side arguing for or against the digging of an 11.6 km pilot channel of the Kosi River.  One of the main disputes arises from the political boundaries separating both countries — the eastern embankment lies on the Indian side of the Kosi and the western side in Nepal.  Neither country wants to risk raising the water level on their side of the river so the conflict continues while in the meantime the threat of heavy monsoon rainfall lingers and emergency services in Nepal have already been placed on high alert.  Bihar has also issued a statewide alert as many people have already been forced to move from their homes in response to recent flooding.  So far, flood waters have entered more than 100 villages forcing thousands of people to relocate onto high-rise embankments and national highways.  Currently, the eastern embankment is at greatest risk of being breached as the Kosi has changed course putting more pressure on the eastern side.

Not long ago, Dr Alex Densmore was part of a team of researchers who organised a workshop in India on how to prevent future flooding of the Kosi River.  A short report from that workshop, ‘River dynamics and flood hazard assessment with special reference to the Kosi River’, highlights some important details for understanding the Kosi River as part of an entire river system or catchment that drains water from Bihar and Nepal:

The Kosi carries a very high sediment load and the construction of embankments and barrages had resulted in the rise of river bed level, so that the river has been flowing in a ‘super-elevated’ condition at several reaches – in other words, the bed elevation is higher than the elevation of the surrounding plains.

In fact, the Kosi has one of the largest alluvial fans in the world, which contains a large amount of sediment.  Taking into account how much rainfall the region receives, combined with a river that already has high amounts of sediment and has been elevated further by the building of embankments and barrages, and the risk of flooding is too great to ignore.  Obviously, by building embankments along a river there are reduced risks of flooding as well that allow people to develop communities next to the river.  But in this case, if the bed elevation is higher than the surrounding plains it seems that it would make the risk of large-scale flooding even greater.

As such, the river was close to its avulsion threshold at several places and this, combined with poor maintenance of the embankments, led eventually to the disaster of August 2008.

To clarify this a bit, an avulsion is a sudden shift in the course of the river where in this case a new channel is created by flood waters that re-directs the flow of the river.  In 2008, over dependence on the river’s eroding embankments backfired and led to one of the greatest floods of the region in recorded history.

River avulsion

Throughout the workshop, it was emphasized that the problem of Kosi flooding is a problem of sediment. Any solution which can deal with the sediment that the river supplies to the plains of Bihar will also address the issue of unexpected and catastrophic shifts in the river, as occurred in 2008. This means that hard engineering solutions, such as the barrage and embankment scheme, are doomed to fail in the face of such a geomorphically active and dynamic river system. Participants in the workshop emphasized the need to work with the river to allow some degree of dynamic behaviour, but in a way that does not compromise the people and communities on the fan.

Altering the course of the Kosi in the past appears to have led to flooding problems today as Alex Densmore elaborates in a previous post on this blog:

The problem is that this programme took no notice of either (1) the sediment load carried by the Kosi, which is now piling up between the levees and making an avulsion more and more likely, and (2) the needs and desires of the local populations, who were effectively abandoned to their fates once the levee system was completed. In the 2008 flood which resulted from this neglect, 3.3 million people – residents of the poorest state in India – were displaced from their homes, and many areas of the fan remained under water for more than 9 months before the river finally drained away.

If mostly relying on embankments to control flooding of the Kosi River is the wrong way to go, what alternatives exist?  One is using what are called ‘paleochannels’, which are basically old, inactive river channels that could be used to mitigate flooding:

The paleochannels on the Kosi fan area may provide an effective solution to dissipate and distribute the excess discharge during high flows. If these paleochannels can be reactivated by connecting them to the main channel using regulation structures and can be maintained as natural channels, this will allow both water and sediments to be distributed across a large area and would reduce the flood risk as well as the siltation of the Kosi river. Some research and surveying will be necessary to identify the potential paleochannels for this purpose and to compute the flows which they can carry.

Finally, finding solutions to environmental problems of this sort involving science and engineering often requires democracy because the problems are so complex, and input from other sources such as the communities who experience the problems firsthand can aid researchers.  If anything, projects for mitigating or preparing for flooding and other hazards in developing countries can set an example for how science can be used to deal with these highly politicisied issues as intelligently as possible.  Many potential solutions exist, but without close scientific evaluation and public input they could likely create greater problems for the future, in attempt to temporarily resolve problems of the present.

Further Reading

The Kosi River floods: towards a set of solutions –

Rising Koshi threatens to flood eastern Nepal –

Report: Mitigating flood hazards on the Kosi River, India –

Latest News on the Kosi

Nepal warns of repeat of 2008 flooding disaster –

Rising Kosi River Threatens Bihar –


  1. To revive the old channels of Kosi is a fairy tale solution. Those advocating it are innocently ignoring two basic issues- 1. there are people and habitations and infrastructure on these channels, developed over the last sixty years, which will be required to be dismantled. 2. Most important- the very design of the existing kosi project cannot be complimented with this solution, in other words – you need a completely new project.

    thanks you very much

    Dr. Onkar Mittal
    tel: +91-9818110784

    1. Hi Onkar, thanks for your comment. I’m interested in what ways you think the ‘existing Kosi project’ is developing a sustainable solution to the flooding problems on the Kosi and by sustainable I mean long-term, if you think this is the case at all. How do you think it could be improved? And what direction do you think flood alleviation efforts should take and why? It’s clearly a complicated issue.

  2. Hi BreTT, I have gone through the above article, Onkar’s comment and your reply. Dr Onkar has rightly raised important issues regarding reviving old channels of kosi river. I shall entail relocation of a vast population and put tremendous pressure on land inhabited by a dense population – mostly belonging to economically lower strata. In my opinion, solution lies in channelization of the river course between two embankments separated by 12 – 16 kms from each other on both banks of river. River banks need to be strengthened against scouring/erosion. I too agree it is a complicated issue that may require years of sustained efforts.
    J K Singh

    1. Thanks very much Janak. The points you both make are valuable and I think helpful in understanding the complexity of this problem. With more research and efforts perhaps a ‘middle way’ can be found.

  3. Hello everyone,
    I am from Delhi but my grandfather is from bihar (Supaul). Where in 2008 flood was happened. At that time my grandfather was in village and he was in the flood for more than 20 days. That time I had decided to do something for my village…mainly related to the flood problem…..then I have chosen a course that can help me to do something for my village…Now I am a graduate student in Earth Science and I have decided to do my research on Kosi river…so I can help the villagers.
    Now I am reading about the kosi river and what type of work and research have done in this field…and I have found that there is lots of work is already done.
    I will try to do some extensive work that can help public.
    My aim is to use my Science in the Public interest.

    1. Sounds excellent. Thank you for sharing. There has been some active research going on the Kosi here working with scientists who have been dealing with this issue firsthand.

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