standard Living with the Kosi River floods: Interview with geoscientist Prof Rajiv Sinha

Professor Rajiv Sinha from the Department of Civil Engineering at the Indian Institute of Technology, Kanpur visited IHRR and the Department of Geography as a COFUND Senior Research Fellow based at St Aidan’s College. He is one of the most visible Indian scientists in the areas of fluvial geology, river science and river related hazards. Since the tragic Kosi flood in 2008, he has been the most prominent researcher in this area, pursuing several lines of activity ranging from river dynamics and flood risk to flow modelling and sub-surface stratigraphy. This has led to a series of significant publications both in scientific journals as well as in popular science magazines which have been received very well by the geosciences community.

During the fellowship Rajiv worked on the development of a policy brief for hazard management in the Kosi region in the north Bihar plains in India, building on existing stakeholder engagement at both the governmental and community level. He has also integrated available data to produce flood hazard and risk maps, and design flood mitigation strategies. In collaboration with colleague Professor Alex Densmore, a project leader on the Earthquakes without Frontiers project, Rajiv explored ways to build resilience to flood hazards through community participation.

In this interview Rajiv talks about the unique dynamics of the Kosi River that flows through Bihar (India) and Nepal, and what can be done differently to manage the river and mitigate future floods.

What makes the Kosi River unique?

The first thing to remember is that the Kosi River does not flood in a classical way via typical overbank flooding. Typically a river floods when the banks are full and it over-floods. That’s not what happens with the Kosi. If you look at the historical records, most of the Kosi floods have happened because of a breach in the embankment, at least in the last 30-50 years, which means floods are not always associated with a high discharge event. Because it’s a dynamic system it oscillates from one side to the other. The point where the embankment was breached in August 2008 was vulnerable for some time.

Kosi River barage

Why is it important to ‘know the river’ for managing it?

If you don’t understand the river processes well then you cannot design a sustainable strategy for river management. The river dynamics and the flooding are so intricately interconnected that you have to understand the connection between the two to develop a sustainable strategy. In the case of the 2008 flood, the river flowed into a paleochannel which was low capacity and heavily silted; the discharge was not very high but too large for a two metre wide channel resulting in inundation of large areas which were not flooded for the last 100 years. On the other side of the river there was also a seepage channel that was there for many years. The embankment was eroding on both sides, and there was a significant elevation difference between the river bed and the embankment that created the cross valley gradient to cause the breach.

What about the influence of climate change on the Kosi?

Most rivers will have a much modified hydrograph because of climate change. There will be change in the frequency as well as the magnitude of flooding. Even if the discharge remains the same there will be a shift in magnitude and frequency of flood events.

As the Kosi is located in a region known to be tectonically active how might this affect flooding on the Kosi?

There is no specific evidence of tectonic events affecting the Kosi. Although the area is active tectonically and there were recent earthquakes, including two that happened very close. However, no direct correlation between river dynamics and tectonics. There are tectonic events in the hinterland resulting in frequent landslides and large-scale sediment production that are eventually pushed into the river by heavy rainfall.

How old is the Kosi River?

At this time we do not know its exact age and are still struggling to find out how old the land form is. We have taken sediment cores down to 50m and the data suggests that the river sediments are at least ~50 thousands years old.

What is the significance of the Kosi River’s megafan?

The Kosi megafan is 200 km wide and has been built through frequent lateral migration of the river distributing sediments over large areas. The Kosi megafan has been built over a geological time period through this process and the river has been swinging from one side to the next over historical time period as well. However, the construction of embankments on both sides of the river during 1955-56 has halted the megafan development.

What about local knowledge of the Kosi River?

Before the embankments were made people were living there along the Kosi. They knew how much the river would shift and flood etc. So they knew when to leave, when to come back and so on. They were living with the floods in many ways. Now little of that local knowledge remains today. Since the embankments have been made (10km wide), a false sense of security has been created. Moreover, there are still villages within the embankments and they are under constant threat.

What can be done about the embankment?

You can’t demolish the embankments now because there are big towns where there used to be vacant space. How do you displace all of these people in the river fan? However, the embankment breaching is reasonably predictable and the vulnerable points are known. So, these points need to be monitored well and the embankment as a whole needs to be maintained properly so that the breaches do not occur.

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Village in Supaul district in Bihar near the Kosi River. Photo: Hanna Ruszczyk

How can people monitor the river and mitigate the floods?

You have to learn to live with the embankment but you also have to live with the river, or the floods in some form. The alternative solution is reactivating paleochannels to distribute the river’s flow and time it properly so the river never gets to the point where it breaches the embankment. With remote sensing technology you can monitor the river regularly. Previously the river level was moving close to the top of the embankment but no one took any notice.

Why are the paleochannels particularly important for flood mitigation?

You cannot bring back the pristine condition of the Kosi River but you can at least create a more natural situation. At least 3-4 channels could be re-activated, then you can re-distribute the flow accordingly. That way you don’t need to worry about pressure on the embankment. Creating spillways would help divert the monsoon discharge and assist the local people who have a water scarcity problem. This would benefit populations that do not have access to water at certain times of the year.

What if things continue as usual?

The situation is already bad because these embankments are 60-70 years old. They are not meant to last for more than 30-40 years. They are costing a lot of money to maintain. There are now plans to make an inner embankment, to further channelise the whole river and make a levee to protect the embankment. This could be disastrous. There are plans to continue to dredge the sediments and keep the channel in the centre. But the flux of the sediment is so high that many of these dredged channels don’t last for more than a season. Even if you dredge that sediment, even if you apply all of the machinery to do that, the big question is where do you keep the sediment? Lately they have been dredging the channel and then leaving the dredged sediments close to the river. Some dredging will have to be done if there is no other solution because the channel has to flow centrally, and can’t be allowed to flow too close to the embankments.

What could be done instead?

The first thing is that you need to maintain the embankments properly. You need regular monitoring of the channel position and the dynamics of the river itself, and you need to identify those points along the embankment which are going to be vulnerable through time. You need to keep track of the entire dynamics of the river and then keep maintaining these embankments. At the same time you need to create more and more space within the embankments so the river is allowed to flow. That means that limited and strategic dredging is also important, to keep the river away from the embankments, but sediment management also comes into the picture. You can’t allow this to flow back into the river. There also needs to be a sediment management plan. The sediment could be used to fill up the ground in low-lying areas. This will cost a lot of money, but it has to be done, there is no other solution.

What about the role of local communities?

The government cannot do everything and local communities must develop their own flood defence system. That can only come when you teach or educate them properly and develop some kind of self-defence system. There is an entire generation of people who have not seen the floods, that can’t manage or mitigate floods on their own without support from government. The old wisdom of managing the floods is gone and that needs to be re-activated and expanded in a major way. Educating people to live with the floods is really critical.

People evacuating after breach of Kosi embankment.

Anything else you would like to add?

There is a need to popularise risk maps. Government needs to understand, if something happens tomorrow, which are the priority areas? So risk maps need to be published widely and must be implemented. We need to understand the existence of high and low risk areas. Are there risk maps available at a state or district level? Disaster management cannot be done without having the baseline maps. Information on flood-prone areas needs to be catalogued and available for use. Also, you must have an understanding and some sort of mapping of all of the hazards together. We need multi-hazard mapping, using maps of individual hazards independently then merging them together into a single map. Sometimes even seemingly unrelated hazards like flooding and earthquakes can actually be mitigated and planned in such a way that provides a sustainable solution.

Further Reading

Sinha, R., Cherry, B. and Densmore A. (2014) Living with the Floods: Sustainable Management of the Kosi River, IHRR Policy Brief, Durham University.

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