Reservoir Flooding

Primary reference(s)

Defra and EA 2014. Reservoirs: owner and operator requirements. Department for Environment, Food & Rural Affairs (Defra) and Environment Agency (EA). Accessed 19 May 2025

UNEP, 2000. Planning and management of lakes and reservoirs: an integrated approach to eutrophication : a student's guide. UNEP International Environmental Technology Centre. United Nations Environment Programme (UNEP). Accessed 19 May 2025

Annotations

Key relevant UN convention / multilateral treaty

UN Convention on the Law of the Non-navigational Uses of International Watercourses (UN Convention), adopted in 1997 (UNGA, 1997).

Drivers

Heavy rain and increased glacier thaw cause increased inflow of water in the reservoir. As climate change causes increasing rainfall, larger flood peaks may result, which may increase the risk of overtopping (Qiu et al., 2021). 

Impacts

The most significant risk of reservoir flooding is dam failure, which can lead to massive flooding in downstream areas. In August 2018, heavy rains caused the dam breach of the Sheyuegou Reservoir in Xinjiang, China, leading to 20 deaths and 8 missing (Ge et al., 2022) 

Reservoir flooding can cause economic losses, social and environmental impacts, loss of cultural resources and loss of life. Consequences of flooding are dependent on the exposed population and assets downstream, warning times, severity of flood and flood severity understanding (U.S. Department of the Interior Bureau of Reclamation, 2014). 

The effects of flooding on health are extensive and significant, ranging from mortality and injuries resulting from trauma and drowning to infectious diseases and mental health problems (acute and long-term). While some of these outcomes are relatively easy to track, understanding of the human impact of floods is still limited. For example, it has been reported that two-thirds of deaths associated with flooding are from drowning, with the other third due to physical trauma, heart attacks, electrocution, carbon monoxide poisoning and fire. Often, only immediate traumatic deaths from flooding are recorded (WHO, 2013; Wu et al., 2024). 

Morbidity associated with floods is usually due to injuries, infections, chemical hazards and mental health effects (acute as well as delayed) (WHO, 2013). Hypothermia may also be an issue, particularly in children, if trapped in floodwaters for lengthy periods (WHO, 2021). There may also be an increased risk of respiratory tract infections due to exposure (loss of shelter, expo- sure to flood waters and rain). Power cuts related to floods may disrupt water treatment and supply plants thereby increasing the risk of water-borne diseases and may also affect the proper functioning of health facilities, including cold chain (WHO, 2021). Floods can potentially increase the transmission of communicable diseases, including water-borne diseases (such as typhoid fever, cholera, leptospirosis and hepatitis A) and vector-borne diseases (such as malaria, dengue and dengue haemorrhagic fever, yellow fever, and West Nile Fever) (WHO, 2021). 

The longer-term health effects associated with a flood are less easily identified. They include effects due to displacement, destruction of homes, delayed recovery, and water shortages (WHO, 2013).

Multi-hazard context

The figure below summarises common interactions between reservoir flooding 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

Provided a reservoir is properly maintained the likelihood of it failing and causing flooding is low. However, in the very unlikely event of a dam collapse, a large volume of water could be released, quickly flooding a large area and possibly causing significant property damage or loss of life. 

In England, 1.1 million properties are at risk of flooding from the structural failure of large, raised reservoirs and their associated dams. The average age of these structures is 120 years, and the possibility of catastrophic failure may be expected to increase with age. A key factor in avoiding and minimising the impact of such a catastrophic failure is the ability to draw a reservoir down in the event of an emergency. This will reduce the load on the dam structure, reduce the likelihood of failure and, in the very worst outcome, minimise the impacts downstream in the event of failure. Reservoir drawdown is also important to allow inspection and maintenance of the structures retaining the reservoir. (Defra, 2017). 

The optimal operation of reservoir flood control includes various constraints such as upstream and downstream flood control, dam safety, and irrigation needs (Dai & She, 2023). The timing and precision of decisions regarding when to open the reservoir gates are crucial. Any delay in this decision can result in dam damage due to excess water, potentially causing flash floods in nearby areas (Doi et al., 2020).

Monitoring

The section and the table below offer an overview of monitoring reservoir flooding. 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.