River Erosion and Accretion

Primary reference(s)

Lawler, D.M., 1993. The measurement of river bank erosion and lateral channel change: a review. Earth surface processes and landforms, 18(9), pp.777-821. Accessed 13 February 2025.

Hasanuzzaman, M., Islam, A., Bera, B. and Shit, P.K., 2024. Quantifying the riverbank erosion and accretion rate using DSAS model study from the lower Ganga River, India. Natural Hazards Research, 4(4), pp.550-561. Accessed 7 February 2025

Islam, A., & Guchhait, S. K. (2020). Characterizing cross-sectional morphology and channel inefficiency of lower Bhagirathi River, India, in post-Farakka barrage condition. Natural hazards, 103(3), 3803-3836. Accessed 21 Nov 2024.

Annotations

Key relevant UN convention / multilateral treaty

United Nations Convention on the Law of the Non-Navigational Uses of International Watercourses (Watercourses Convention): Adopted in 1997, this convention provides a framework for the use and protection of international watercourses, promoting cooperation and equitable utilization of shared water resources.

Convention on the Protection and Use of Transboundary Watercourses and International Lakes (Water Convention): Adopted in 1992, this convention aims to ensure the sustainable use of transboundary water resources, prevent and control water-related hazards, and protect ecosystems.

Sendai Framework for Disaster Risk Reduction 2015-2030.

Drivers

River erosion is primarily driven by the bank erodibility and water erosivity. River erosion is accelerated by flood events, however antecedent conditions contribute to conditioning of the riverbank (Darby et al., 2007). Conditions such as antecedent flow, (e.g. high water levels cause saturated banks which are less stable), or periods of low flow (which can result in unstable dry and cracking banks) and the condition the riparian zone (i.e. bedrock, or the presence and condition of riparian vegetation which provide reinforcement of the banks and reduce near bank velocities) contribute to the resistance of the bank to erosion. Erosion is also accelerated by activities such as sediment extraction, river modifications and livestock grazing (Wilkinson et al. 2022). In areas with cold climates, the movement of river ice during the spring thaw can result in river erosion. 

River erosion and accretion are associated with a few triggering hazards. For example, river flooding leads to bank collapse; ground shaking also induces the riverbank failure due to vibrating actions. Liquefaction through groundwater movement can trigger the weakening of the bank materials, leading to erosion. Moreover, some man-made hazards like deforestation, sand mining and dam failure trigger river erosion and accretion. Deforestation increases surface run-off potential with higher potential riverbank erosion with greater sediment loads to the river. Sand mining destabilizes the riverbed and bank causing the collapse of the bank. Dam failure results in the huge release of water from the reservoir triggering the swelling of the downstream river courses causing bank failure. Soil erosion or artificial removal of the topsoil induces the collapse of the bottom layer of the soil adjacent to the riverbank. Moreover, sediment flux from the soil erosion also escalates the river accretion process and develops channel bars due to the transport-limited conditions of the river (Islam and Guchhait, 2020).

Impacts

River erosion can result in significant land loss and damage to infrastructure. In South Asia, for example, Bhuiyan et al. (2017) reported that the rivers of Bangladesh are responsible for cumulative annual erosion of up to 10,000 hectares of land. In addition to floodplains and settlements, Bangladesh also loses several kilometers of roads, railways, and flood control embankments each year (Bhuiyan et al., 2017). Widespread land loss, declining per capita income, livelihood crisis, and community conflicts are often reported in the lower Ganga plains (along the Bhagirathi-Hooghly River) in India (Islam and Guchhait, 2017).   

In addition to the loss of land and infrastructure, the consequences of stream erosion and riverbank mass wasting are increased suspended sediment loads in streams, which impacts on water quality with consequential implications for human and ecological health (Grove et al., 2015). This includes detrimental impacts on aquatic, terrestrial and semi-aquatic ecosystems (Zaimes et al., 2021). 

River accretion can create navigation issues, increase flooding risks, and disrupt natural habitats. 

Sea level rise in the wake of climate change is expected to exacerbate the inundation of low-lying areas with swelling of the river courses and bank failure. The foremost visible loss due to riverbank erosion is the financial shock induced by the agricultural land loss, engulfment of settlements and creation of the homelessness and environmental refugees in the areas of the frequent erosions and accretion (Islam and Guchhait, 2024). The emergence of sand bars due to the deposition of riverine sediments can become the source of social conflicts and violence (e.g. Zaman, 1991).

Multi-hazard context

The figure below summarises common interactions between river erosion & accretion and other hazards. This information should be used with caution and not be solely relied upon in Disaster Risk Management, particularly as some interactions may not have been included. Note that hazardous events occurring together or locally in space or time may not necessarily cause, amplify, or be otherwise related to each other. Specific examples of multi-hazard context can be found in the ‘Hazard drivers’ and ‘Impacts’ sections above.

Multi-hazard diagram

Risk Management

River erosion and accretion monitoring and assessment requires a good understanding of these processes; however, they can be highly temporal and spatially complex (Koutalakis et al., 2024). Various methods and tools can be applied to study and monitor stream bank erosion and accretion including field mapping, remote sensing, erosion pins, and laser scanners (see Koutalakis et al., 2024 for more details).   

Riverbank vegetation, for example mangroves, contributes to riverbank resilience to erosion, as do alluvial sediments. Mitigation of the impacts of bank erosion includes planning and avoidance, and soft and engineered protection or renaturing. Some examples are presented by the Scottish Environment Protection Agency (SEPA, 2020) and the Reef Trust, Commonwealth of Australia (Wilkinson et al, 2022). 

Monitoring

The section and the table below offer an overview of monitoring river erosion & accretion. This information can be used for forecasting within a national early warning system (EWS). Since EWS capacities and processes differ across countries, the most current and specific information regarding EWS should be obtained from the appropriate national or regional agency/authority responsible for disaster management.