Reservoir/dam collapse

Impact 5
upper risk error bar
upper likelihood error bar
risk indicator
lower likelihood error bar
lower impact error bar
Impact & Likelihood
Impact key
5 Catastrophic
4 Significant
3 Moderate
2 Limited
1 Minor
Likelihood key
5 >25%
4 5-25%
3 1-5%
2 0.2-1%
1 <0.2%


The collapse or breach of a reservoir or dam can be sudden and result in the uncontrolled release of fast-flowing water into a populated area. Potential causes of this may include climate-related land instability, internal erosion or an earthquake. There have been no catastrophic failures of dams in the UK since 1925. However, the incident at the Toddbrook Reservoir in 2019 highlights the significance of this risk and the need to integrate effective preventative measures. This has led to better flood mapping and flood management plans to improve preparation by local resilience forums.


The reasonable worst-case scenario is based on a sudden collapse of a reservoir without warning. This would result in flooding, with a substantial quantity of water moving at high speed. There would be casualties, fatalities and significant mental health impacts. Utilities (water, energy, communications) to nearby homes and businesses would be lost, with significant economic impacts resulting from property damage. Recovery operations would be hazardous among collapsed infrastructure and debris.

Key assumptions

That the dam collapse occurs with little or no warning and with no time to evacuate local properties. It is assumed that the impact on essential services and communities would be significant and require long recovery times.


Variations to the risk include the amount of warning time prior to the dam collapse, the size and location of the reservoir and a deliberate incident.

Response capability requirements

Since 2020, the Department for Environment, Food and Rural Affairs has made it a legal requirement for all owners of large raised reservoirs to have on-site emergency flood plans. Reservoir owners, local authorities and local resilience forums (LRFs) have emergency plans and produce locally specific off-site flood plans from reservoir flood maps.

The Environment Agency leads operational preparedness and response to flood impacts and during local-level operation responses and would work as part of a multi-agency team, coordinated through the LRF drawing on resources including the National Flood Asset Register, which has over 100 specialist flood rescue teams on standby to be deployed across the country.

The joint Department for Environment, Food and Rural Affairs/Cabinet Office National Flood Response Centre would coordinate the national UK response.


There would be major economic, environmental, infrastructure and humanitarian implications. Key aspects of immediate recovery would involve searching for missing people buried by rubble, debris and sediment, evacuation and shelter of populations, temporary accommodation and clean-up of contaminated urban and agricultural land and environmental damage. There would also need to be long-term repairs to damaged infrastructure (motorways and energy infrastructure) and buildings.