Reservoir Flood Risk: What The Maps Show And How Likely It Actually Is
Reservoir flooding is unlikely. The reservoir flood maps show where water could go if a dam failed completely, a worst-case scenario rather than a prediction. No one in the UK has died because of a reservoir failure since 1925.
What Is Reservoir Flooding?
Reservoir flooding is flooding caused by water released when a dam or reservoir fails. The Environment Agency's maps show where water may go in the unlikely event of a dam or reservoir failure, so they describe the area that could be affected, not how likely that is.
Two facts put the maps in context:
- The safety record. The Environment Agency's reservoir safety research strategy states there has been no loss of life from reservoir failure since 1925.
- The scale of what is mapped. The same strategy notes: "Around 2.6 million people live or work in areas that could be affected by a major reservoir failure in England." That figure describes where water could reach, not where it is expected to.
Unlike flooding from rivers, the sea and surface water, reservoir flooding has no annual chance band. Natural Resources Wales says the same of its own maps: "We do not provide a risk level for flooding from reservoirs. We show the worst-case scenario."
What Causes Reservoir Flooding?
Reservoir flooding is caused by the failure of a dam or embankment, which releases stored water downstream quickly. It is not caused by rain alone; a full reservoir that does not fail does not produce the flooding shown on the maps.
The maps model two situations:
| Scenario | What it shows |
|---|---|
| Dry day | Flooding if the dam failed when rivers are at normal levels |
| Wet day | How much worse the flooding might be if a river is already in an extreme natural flood |
In both scenarios, the model assumes that a void occurs through the full height of the dam, the most severe way a dam could fail. The Environment Agency also notes the age of the stock: reservoirs in England and Wales have an average age of over 130 years.
Where In The UK Is Reservoir Flooding Most Common?
Reservoir flood areas lie downstream of large reservoirs, so they follow river valleys below dams rather than any region. The Environment Agency has mapped flood extents for 1,865 large raised reservoirs in England.
Actual reservoir flooding is rare everywhere. The places on the maps are those that would be affected in a failure, not places where flooding is expected.
For reservoir flood information in Scotland and Wales, the Environment Agency directs users to SEPA and to Natural Resources Wales.
How Do You Find Out If A Property Is At Risk Of Reservoir Flooding?
A property's reservoir flood information in England is shown on the Environment Agency's long term flood risk service by choosing "flood risk from reservoirs", or through the checker on this site. The result shows whether the property is inside a mapped reservoir flood area.
The search result also gives a flood risk summary that names:
- the reservoir,
- its owner, where this is not a private individual,
- its grid reference,
- the lead local flood authority.
Each reservoir also has a risk designation, available in the Environment Agency's GIS datasets:
| Designation | Meaning |
|---|---|
| High risk | In the very unlikely event the reservoir fails, it is predicted there is a risk to life |
| Not high risk | In the very unlikely event the reservoir fails, it is predicted there would not be a risk to life |
| Not determined yet | A determination has not yet been made |
A "high risk" designation describes the consequence of a failure, not its likelihood.
Does Reservoir Flooding Appear On A Flood Map?
Reservoir flooding appears on its own reservoir flood maps, which are separate from Flood Zones and from the flood risk bands for rivers, the sea and surface water. Those maps show dry-day and wet-day extents, with no annual chance attached.
| Map | Reservoir flooding shown? |
|---|---|
| Flood Map for Planning — Flood Zones | No |
| Check your long term flood risk — reservoirs | Yes, as a mapped extent without a chance band |
| Reservoir flood maps GIS data | Yes, dry-day and wet-day extents and risk designations |
| Natural Resources Wales flood maps | Yes, as a worst-case scenario |
The maps are designed mainly for planners, emergency responders and reservoir owners. The Environment Agency lists their uses as strategic flood risk assessments, community flood plans, planning decisions and reservoir owners' on-site emergency flood plans.
How Are Reservoirs Kept Safe?
Reservoirs in England are kept safe by law under the Reservoirs Act 1975, enforced by the Environment Agency. Owners of large reservoirs must register them, and those designated high risk must be supervised and inspected by qualified engineers.
The main requirements:
| Requirement | Detail |
|---|---|
| Registration | A large raised reservoir, one that holds or has the potential to hold 25,000 cubic metres of water above ground level, must be registered with the Environment Agency |
| High-risk designation | Given to reservoirs where an uncontrolled release of water could put people's lives at risk |
| Supervision | High-risk reservoirs must appoint a supervising engineer |
| Inspection | An inspecting engineer carries out an inspection at least once every 10 years and sets deadlines for safety measures |
| Emergency planning | It is a legal requirement for all undertakers to prepare an on-site emergency flood plan for all large raised reservoirs in England |
The Environment Agency regulates reservoirs in England. Owners who do not comply can be prosecuted.
Does Home Insurance Cover Reservoir Flooding?
Most home insurance policies cover flood as a standard peril, but we found no published source that sets out how insurers treat properties inside reservoir flood areas. Check the policy's definition of flood and ask the insurer if you are concerned.
What is known:
- Flood is a standard peril. The ABI states that Home insurance covers you against damage or loss to your property caused by a variety of perils such as fire, storm, flood, subsidence, burst pipes and water damage.
- The maps carry no chance band. Unlike rivers, the sea and surface water, there is no annual probability for insurers or homeowners to read from the reservoir maps.
- Insurers set their own terms. Flood Re notes that Prices, excesses and terms for policies are set by insurers and not Flood Re.
How flood cover and Flood Re work is explained in flood insurance.
What Should You Do If Your Home Is In A Reservoir Flood Area?
If your home is in a reservoir flood area, you do not need to take special action day to day, because failure is unlikely. Know how you would be warned and where you would go, as you would for any emergency.
Sensible steps:
- Check the other sources too. A home in a reservoir flood area may also carry river or surface water risk, which does have an annual chance band; see check the flood risk of any UK postcode.
- Sign up for flood warnings for river and sea flooding through Floodline on 0345 988 1188.
- Know what to do in an emergency. Follow the emergency services' instructions and call 999 if you are in danger. General flood preparation is in what to do in a flood.
- For development near a reservoir, the Environment Agency advises contacting the reservoir owner to understand the risk and whether the development could affect the reservoir's risk designation.
Sources (Verified 17 Sep 2026)
| Claim | Source |
|---|---|
| Unlikely event; dry-day and wet-day; model assumptions; risk designations; flood risk summary; users; contact the owner; EA regulates England; Scotland and Wales | Environment Agency, Reservoir flood maps: when and how to use them |
| 25,000 m³; registration; high risk; supervising and inspecting engineers; 10 years; on-site plan; prosecution | Defra and Environment Agency, Reservoirs: owner and operator requirements |
| No loss of life since 1925; 2.6 million people; average age over 130 years | Reservoir safety research strategy |
| 1,865 reservoirs mapped | Environment Agency news |
| NRW worst-case | NRW, Different types of flood risk |
| Flood as standard peril | ABI, Home insurance |
Frequently Asked Questions
How Likely Is Reservoir Flooding In The UK?
Reservoir flooding is unlikely. The Environment Agency's maps show the area that could flood if a dam failed, not a forecast, and there has been no loss of life from reservoir failure in the UK since 1925.
Why Does My House Show A Reservoir Flood Risk?
Your house is inside the area water could reach if a nearby dam failed completely. The map shows a worst case, with no annual chance attached, so it does not mean your home is likely to flood.
What Is A Large Raised Reservoir?
A large raised reservoir holds, or could hold, at least 25,000 cubic metres of water above ground level, about 10 Olympic swimming pools. It must be registered with the Environment Agency in England.
How Often Are Reservoirs Inspected?
A high-risk reservoir in England must have a supervising engineer and be inspected by an inspecting engineer at least once every 10 years. The inspecting engineer can set deadlines for safety work.