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What the reservoir flood maps show, how they were created and how to use them for your assessments.

More detailed maps are available on ResilienceDirect for category one or 2 responders.

Reservoir flood maps show where water may go in the unlikely event of a dam or reservoir failure.

There are 2 flooding scenarios shown on the reservoir flood maps. They are a ‘dry-day’ and a ‘wet-day’. The ‘dry-day’ scenario predicts the flooding that would occur if the dam or reservoir failed when rivers are at normal levels. The ‘wet day’ scenario predicts how much worse the flooding might be if a river is already experiencing an extreme natural flood.

You can find more information on these scenarios in the ‘assumptions made as part of the modelling section’.

To see the reservoir flood maps, you should:

Once you have found the location, you can view the flood extent of a ‘wet-day’ and ‘dry-day’ scenario.

If you are considering development in an area of reservoir flood risk, you should contact the reservoir owner(s) to help you understand:

You can find these by searching for a flood risk summary for a particular postcode. This shows the:

The risk designation for a reservoir is in the geographical information system (GIS) datasets. These are available via the Defra Data Services Platform, or by contacting the Environment Agency. There are 3 categories of reservoir risk designation. They are:

high risk – which means that in the very unlikely event the reservoir fails it is predicted that there is a risk to life

not high risk - which means that in the very unlikely event the reservoir fails it is predicted that there would not be a risk to life

not determined yet - a determination has not yet been made

More than one reservoir could affect one location at the same time. You can see the flood extent from each reservoir separately on the Defra Data Services Platform in:

To search the platform, you need to know one of the following:

This will help you identify which reservoir impacts your location of interest. All the reservoir flood extents are also available as a web mapping service.

The ‘maximum fluvial extent’ shown on the pdf maps is the extent of the river flooding we used in the model of the “wet-day” scenario. It is not the same as flood zone 2 or 3 shown in the Flood Map for Planning but is considered to be an extreme flood. Use the ‘fluvial-only extent’ to see the impact of the reservoir flooding.

If you are proposing a new development that is at risk of flooding from a reservoir and preparing a flood risk assessment, you should:

See the following guidance for more details:

You should refer to the preparing on-site plans for reservoir dam incidents guidance.

Refer to these maps when preparing a SFRA and local plan, but do not use them in isolation. If you are considering allocating land for development in areas at risk from reservoir flooding, you should:

See the following guidance for more details:

The Environment Agency made a model of each reservoir using computer software. A laser recorded the height of the ground around each reservoir. We added this to the computer software to produce a model of the ground.

For each reservoir, we calculated the volume and speed of water flow out of the reservoir in the unlikely event that it fails. We added this ‘flow’ to the ground model of the reservoir and allowed it to spread onto the area downstream. The computer recorded the largest predicted flood extent.

The models used to create the maps are available for use in an emergency. Contact the Environment Agency to request the specification used to create the models.

In both a wet-day and dry-day reservoir failure scenario we assume:

In a dry-day scenario we’ve assumed that just before reservoir failure the:

In a wet-day scenario we’ve assumed that just before reservoir failure:

In some places the floodwater from one reservoir flows into another reservoir. This is called a ‘cascade’. These have been modelled to fail when they meet set criteria.

The Environment Agency regulates reservoirs in England.

For reservoir flood information for Scotland, visit the Scottish Environment Protection Agency website. For Wales, information can be found via the Natural Resources Wales website.

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