Reservoir Flooding

Primary reference(s)

National Geographic, 2020. Reservoir. National Geographic Encyclopaedia. Accessed 30 October 2020.

Additional scientific description

Reservoirs are artificially created lakes that are usually formed by building a dam across a river. When a dam fails, a large volume of water is suddenly released from the reservoir, resulting in downstream land or properties being flooded (Cheshire East Council, 2020). Metrics and numeric limits Not identified.

Metrics and numeric limits

Not identified.

Examples of drivers, outcomes and risk management

Reservoir operation is an effective tool for water supply, hydropower generation, flood control and environmental or ecological enhancement. Dams are designed to safely pass floodwater, however floods exceeding the design capacity of the dam spillways cause uncontrolled release of water over the dam crest, which is accompanied by a high risk of failure. Uncontrolled water passing over a dam crest can compromise the structural integrity of the dam, such as through erosion or cavitation, which can rapidly escalate to complete failure (US Department of the Interior Bureau of Reclamation, 2014).

As climate change causes increasing rainfall, larger flood peaks may result, which may increase the risk of overtopping (Qiu et al., 2021).

Reservoir flooding can cause economic losses, 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 weak. 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).

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, exposure 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 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).

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. Cheshire East Council in the UK provides guidance on its website for access by all on reservoir flooding. This includes finding out where a reservoir is in your area; showing how you can prepare for reservoir flooding; information about evacuation when seeking refuge from floodwaters upstairs would be unsafe and you may need to evacuate the area; and guidance on evacuation centres, routes and road closures (Cheshire East Council, 2020).