IHRR
Institute of Hazard, Risk and Resilience
Policy Brief Living with the Floods:
Sustainable Management of the Kosi River
Sinha, R. (Indian Institute of Technology), Cherry, B. and Densmore, A. (Durham University) Contact: Prof Rajiv Sinha rsinha@iitk.ac.in
Key recommendations for policy • Effectively manage flooding on the Kosi River by moving from a policy of ‘river control’ to a policy of ‘river management’. • Take measures to address the movement of sediment, which is one of the main contributing factors to flooding on the Kosi River, and improve drainage in low-lying areas. • Prepare basin scale GIS interactive flood risk maps based on scientific data and reasoning, historical data analysis and modelling approaches, and link them to an online database and flood warning system. • Trial the preparation of local flood management plans, using a combination of scientific and local knowledge of Kosi River hazards. • Complete a long-term cost benefit analysis on major interventions in the river basin that accounts for their impacts on livelihood and ecology. • Use river paleochannels on the fan surface of the Kosi to divert flood waters away from populations, significantly reducing occurrence of flooding. 1 Institute of Hazard, Risk and Resilience, Durham University 2014
Why have measures taken to prevent flooding on the Kosi River failed? •
The point in the embankment that was breached by the river channel in August 2008 was vulnerable for some time. The breach was mainly due to failure to monitor and maintain the embankment.
• There was a systematic failure to address the dynamic nature of the river, especially the accumulation of sediment in the river channel and the historic movements of the river. This dynamic nature means that failures of the embankments are inevitable. •
The construction of the embankments over time created a false sense of security among local communities near the Kosi River.
Figure 1: Migration of the Kosi River. Too much attention was given to the discharge of water (hydrology) as a driver of flooding, rather than the changing bed elevation and channel morphology of the Kosi River.
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Introduction The Kosi River is one of the most dynamic rivers in the Ganga basin1,2, and one of the most sediment-laden rivers in the world3, making it highly prone to flooding. The construction of barrages, embankments and other structures has restricted the flow and sediment carrying capacity of the Kosi River, with severe consequences for communities that live in the region. Parts of Nepal and north Bihar are prone to two major river hazards from the Kosi River – avulsions and floods – which are highly interconnected. It is widely accepted amongst the geoscientific community that avulsions – sudden changes in the course of the river – are more frequently driven by instability in the river channel due to internal factors such as excessive sedimentation, and changes in the slope of the river and its banks, rather than external factors such as seismic activity or intense, high duration rainfall alone. Importantly, unlike floods, avulsions can occur during a relatively minor increase in water discharge3, as was the case with the 2008 breach in the Kosi River embankment. Thus, effective management of the Kosi River is not merely a matter of developing measures for classic overbank flooding due to rising river levels, but must focus instead on maintaining the river embankments and closely monitoring the river’s behaviour. This can be done using highresolution remote sensing technology (satellite images, LIDAR etc.) now available along with modelling to help predict flood events. For the last 5-6 decades, flooding on the Kosi has usually been triggered by a breach in the embankments originally constructed for flood protection in the 1950s. In most cases, breaches in the embankments are associated with channel instability and poor maintenance. Recent scientific research on the Kosi River from the Indian Institute of Technology in collaboration with the Institute of Hazard, Risk and Resilience and Department of Geography, Durham University, can be used to inform policy to mitigate and prepare for flood hazards, reduce flood disaster risk through sustainable flood management practices, and build both public awareness and resilience to Kosi River flooding. Causes of the 2008 Kosi River flood The breach of the eastern embankment at Kusaha, Nepal in August 2008 took place at a discharge of 144,000 cusecs. Although the river channel could handle a maximum discharge of 950,000 cusecs, this point in the embankment was vulnerable for some time prior to the avulsion. The breach was caused primarily by poor strategies of river management, but also by poor monitoring and maintenance of the embankment, making the event partly a human- 2 Institute of Hazard, Risk and Resilience, Durham University 2014
Kosi River Dynamics The Kosi is a highly complex, dynamic river. Like other major rivers in Nepal and north Bihar, the Kosi originates in the High Himalaya and flows across the Himalayan mountain front into the Ganga Plains. As the river comes out of the mountains it is confined by hard rocks on both sides, but further downstream in the plains it has wide space to avulse or shift its course from one place to the next. Historical shifts of the 1,2 Kosi River over the last 200 years are very well documented , and the major causal factors are: abundant sediment supply from the hinterland as well as from local reworking, leading to channel bed aggradation; flat alluvial terrain and adequate space for the river to swing back and forth; and large fluctuations in discharge. Accumulation of sediment in river bed alters the channel-floodplain relationship in a major way making the river susceptible to avulsion4. As the river changes course it regularly spreads sediments over a large area in the shape of a fan 200 km in length. The construction of embankments along the Kosi in the 1950s has prevented the river from oscillating and from spreading its sediment over the fan surface. This has made its behaviour much more unpredictable and sudden, provoking disaster.
Figure 3: Avulsions of the Kosi River that have occurred near Kusaha.
induced disaster3. The major river avulsion caused by the breach shifted the course of the river 120 km to the east where it re-occupied a pre-existing channel that had a much lower water carrying capacity. As a consequence, the flow was 30 km wide and 150 km long, inundating populated areas in Nepal and north Bihar and affecting a population of more than 3 million in total. In the last 30-50 years all major flood events of the Kosi River have been caused by breaches in the embankments. Breaches are not always associated with a high discharge, as was the case for the 2008 avulsion. Factors that lead to breaches in the embankment and require further attention include the internal dynamics of the river as well as the very high seasonal variability in discharge, including excess discharge during the monsoon season5. If these factors were taken into account, the breach in 2008 could have been prevented. The Kosi River channel builds up large amounts of sediment over time, which substantially raises the bed and thus raises flow levels at any given discharge6. The increased elevation of the river bed, relative to areas outside of the embankment, makes avulsions much more likely – irrespective of the river discharge. This sediment accumulation has worsened markedly since the construction of the embankments along both sides of the river. Thus, measures of flood control that have been implemented since the 1950s have not only failed to address these essential factors that determine the movement and flood potential of the Kosi River, but have actively made the problem worse. Instead of concentrating on controlling the river to prevent flooding, efforts should be made to manage the floods by identifying causal factors and vulnerability, working with local communities to design flood mitigation strategies, and harmonising the relationship between humans and the natural environment.
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Figure 4: How the embankments confine the Kosi making the occurrence of a large impact flood more likely.
Flood vulnerability and planning While it could be argued that the implementation of the embankments and barrage to prevent future flooding on the Kosi was warranted at the time, today the embankments are in desperate need of repair and little has been done to prevent future breaches. The combination of the poorly maintained embankments and the dynamic processes of the Kosi have made local communities extremely vulnerable to flood disaster. This lack of attention and foresight in understanding the behaviour of the Kosi has rendered the embankments and other past measures of flood control ineffective. It must also be understood that prior to the construction of the embankments people were accustomed to sudden changes in the course of the river, and were aware of how much the river could shift during avulsions. Since the construction of the embankments, which are now 10 km wide, villages have formed around them, making it difficult for people to move if there is a flood. Now, people must learn to live with the embankments as well as the river flooding. However, current plans to construct an inner embankment would in effect do the opposite of what is needed to manage the river sustainably. This is because the river is already highly restricted, and further restriction from an inner embankment would lead to severe build-up of sediment that would lead to even greater likelihood of avulsion, and greatly aggravate future flood disasters. It needs to be appreciated by managers in charge of the Kosi that the river cannot simply be controlled using the same engineering methods that have failed in the past. If anything, what the 2008 breach revealed is that these forms of intervention could lead to yet another catastrophe. Water logging Another unplanned consequence of embankment construction, and the subsequent rise in river bed, has been obstruction of many of the smaller tributaries that used to join the Kosi along the river bank. This obstruction, combined with unplanned construction of roads and smaller embankments, has resulted in drainage congestion, backflow and waterlogging outside of the embankments5. Water logging in the Kosi plains is therefore a by-product of flood mitigation measures and unplanned development. It causes severe agricultural damage by inundating crops for a much longer duration than usual; such damages are irreparable, and leave communities that are dependent on agriculture for their livelihoods with no option. This calls for a sustainable and integrated management of water-related hazards in this region. Sustainable river management Along with maintaining the existing embankments, regular monitoring of the channel position is also required. Something also must be done with the abundant amount of sediment that the river produces. The river channels can be dredged, but this is costly and the sediment must be transported away from the river rather than simply left on the embankment, which could lead to a much bigger problem if the embankment is breached. In order to maintain the embankment properly the following must be taken into consideration:
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Strategic dredging should be used to help keep the river away from the embankments to prevent it from eroding and leading to another breach.
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More space should be created within the embankments to allow the river to flow that would help make the river more manageable in the long-term.
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A long-term sediment management plan is needed that should include catchment treatment as well as a proper disposal strategy for dredged sediments.
Figure 5: Several potential sites of future avulsions apart from the Kusaha where the August 2008 avulsion occurred. Reaches of the Kosi River in red are most critical.
Recent research has found that it is possible to predict future avulsions of the Kosi River. This analysis shows several potential sites of future avulsions other than Kusaha where the 2008 avulsion occurred5. Such data and information are crucial to monitor vulnerable sections of the river to avoid or allow suitable time to prepare for similar disasters. Reactivating paelochannels Old river channels of the Kosi should be used to divert flood waters and sediment to prevent future hazards. The embankment has cut off the Kosi from these river channels, but only small measures would need to be taken to reactivate them. In fact, based on preliminary modelling, more than 20 per cent of the average annual flow could be diverted to three of the Kosi’s paleochannels. This would be a far better approach to mitigating flooding on the Kosi than allowing the river to flood through one channel. While it would not prevent flooding as a whole it would make floods far more manageable and less likely to cause loss of life or severe damage. To divert the river’s flow into the paleochannels would require the construction of low spillways within the embankments (Figure 6) that would transfer water once the river reaches a particular level. These spillways could be used to divert monsoon discharge as well as channel water to populations who are experiencing drought.
Figure 6: How the paleochannels could be used to divert flood waters. Creation of spillways for gates to control flooding and flow of sediment through river.
Institute of Hazard, Risk and Resilience, Durham University 2014
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Public awareness and education The local knowledge and wisdom of managing floods on the Kosi River is nearly lost, and therefore, increasing educational awareness about the Kosi needs to be prioritised by government. Communities that live on the Kosi River need to be educated about how to live with future floods, as no authority in place will be able to protect everyone from harm during a flood event. Flood risk maps must also be used and popularised in order to take seriously the existence of high and low risk areas. Disaster management cannot be done without having up to date maps that show flood-prone areas, and preparation and dissemination of such maps must be prioritised to save lives and reduce damage to physical infrastructure. To ensure the effectiveness of a flood management strategy at a local and national scale, an integrated approach is needed between government departments responsible for managing floods and the local communities affected by them. Floodplain regulations also have an important role to play in order to prevent them from being inhabited and relocating people from floodway areas where possible6. Flood mitigation plans inclusive of the needs and values of communities who live on or near the Kosi River should: •
Create a culture of local flood management by educating communities about how they can live with the floods and avoid disaster.
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Encourage direct participation from communities in flood planning and mitigation activities through engagement, to help them take responsibility and initiative when flood events occur.
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Empower communities to involve themselves in developing their own flood defence systems and in preparing for future flood events.
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Tap into local knowledge resources of the Kosi River where available, focusing on ways to reduce damages and loss of life from flooding, including prioritising vulnerable areas.
Preparing for multiple hazards The Kosi River has been difficult to manage because it requires a process-based understanding of various water-related hazards associated with this river. A multi-hazard framework needs to be adopted that accounts for flooding, water logging and the dynamics of the river, which are all interconnected. These must be addressed as part of the basin-scale management of the river system rather than solely viewed as a local problem. Mapping hazards together can result in more effective approaches to managing the Kosi to help resolve the complex dilemmas that communities face when confronted with river avulsions, flooding and water logging. Managing hazards in an integrative way would not only provide a sustainable solution to mitigate them, but would also help to develop resilience amongst the affected community. References: 1. Gole and Chitale, 1966. 1966. Inland delta building activity of Kosi River. J. Hydraul. Div., ASCE 92, 111-126. 2. Wells and Dorr, 1987. Shifting of the Kosi River, northern India. Geology, 15, 204–207. 3. Sinha, R. 2009. The Great avulsion of Kosi on 18 August 2008, Current Science, 97, 3 10 August 2009 5. Sinha et al., 2014. Avulsion threshold and planform dynamics of the Kosi River in north Bihar (India) and Nepal: A GIS framework. Geomorphology, 216, 157–170 6. Sinha, R. 2009. The Great avulsion of Kosi on 18 August 2008, Current Science, 97, 429-433 7. Sinha, R., Bapalu G.V., Singh, L.K., Rath, B. 2008. Flood Risk Analysis in the Kosi River Basin, North Bihar Using Multi-Parametric Approach of Analytical Hierarchy Process (AHP)J. Indian Soc. Remote Sens. (December 2008) 36, :293–307 8. Raymahashay, B. and Sinha, R. Flood disasters and management: Indian Scenario. Indian Institute of Technology Kanpur
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