Reservoir flood maps: when and how to use them
What the reservoir flood maps show, how they were created and how to use them for your assessments.
Applies to England
This guidance is for:
- local authority planners preparing a strategic flood risk assessment (SFRA) for a local plan
- parish councils when preparing community flood plans
- local authority planners determining planning applications for development
- developers completing site-specific flood risk assessments for planning applications
- reservoir owners and operators when preparing on-site emergency flood plans
More detailed maps are available on ResilienceDirect for category one or 2 responders.
What the maps show
Reservoir flood maps show where water may go in the unlikely event of a dam or reservoir failure.
Dry-day and wet-day scenarios
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’.
Viewing the maps
To see the reservoir flood maps, you should:
- select ‘flood risk from reservoirs’ in the drop-down menu
- use a place or postcode to search the maps
- see a flood risk summary of the area in the search result
- zoom in or out to see an area of interest
Once you have found the location, you can view the flood extent of a ‘wet-day’ and ‘dry-day’ scenario.
Find the reservoir name, owner, location and risk designation
If you are considering development in an area of reservoir flood risk, you should contact the reservoir owner(s) to help you understand:
- the flood risk in more detail
- if the development could affect the reservoir’s risk designation, it’s design category or how it is operated
You can find these by searching for a flood risk summary for a particular postcode. This shows the:
- name of the reservoir
- owner of the reservoir, where this is not a private individual
- National Grid Reference of the reservoir
- lead local flood authority
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
Flood extents from one reservoir
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:
- the portable document format (pdf) maps
- GIS compatible datasets
To search the platform, you need to know one of the following:
- reservoir name
- the reservoir owner’s name
- the lead local flood authority the reservoir is in
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.
Maximum fluvial extent
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.
Preparing a flood risk assessment
If you are proposing a new development that is at risk of flooding from a reservoir and preparing a flood risk assessment, you should:
- consider whether you need to make your own model to understand the flood risk from the reservoir, to complete the assessment
- refer to the specification for the reservoir flood maps as a starting point if creating your own model
- consider whether the development may have an impact on the reservoir or reservoir owner
- refer to the local planning authority’s (LPA) SFRA - for information on reservoir risk and recommendations for how to address it
- contact the LPA for pre-application advice
- contact the LPA to work out if you also need to consult with their emergency planning team and with the reservoir owner
See the following guidance for more details:
- preparing a flood risk assessments
- planning practice guidance for flood risk and coastal change
- flood risk emergency plans for new developments
Preparing an on-site emergency flood plan
You should refer to the preparing on-site plans for reservoir dam incidents guidance.
Informing strategic decisions
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:
- consult with the local resilience forum for advice on emergency planning
- consult with relevant reservoir owners - to understand the impact development may have on their reservoirs
- use your SFRA to check whether proposed development will be safe throughout its lifetime
See the following guidance for more details:
- how to prepare a SFRA
- flood risk emergency plans for new development
- preparing a local plan and emergency planning in the planning practice guidance on flood risk and coastal change
How the maps were created
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.
Assumptions made as part of the modelling
Wet-day and dry-day scenarios
In both a wet-day and dry-day reservoir failure scenario we assume:
- a void occurs through the full height of the dam and stops at the base of the dam
- the base of the dam is the bed of the watercourse just downstream, if there is one
- features, such as road and rail embankments, may collapse when under water that is flowing quickly
- openings or culverts in embankments of smaller than 2 metres are blocked to represent floating debris
- wave walls are not considered as part of the model
- a reservoir on a different tributary would not fail at the same time
- that cascade reservoirs which are not regulated by the Environment Agency have an average depth of 1 metre
- that flood defences in the model are included in the model where the ground levels have been recorded by laser
- that where there is a boundary with the coast, that water will leave the model based on the slope of the land and is not restricted by the sea
In a dry-day scenario we’ve assumed that just before reservoir failure the:
- water level in the reservoir is the same as the spillway level or the underside of the roof for a service reservoir
- watercourses upstream and downstream of the reservoir are at a normal level
In a wet-day scenario we’ve assumed that just before reservoir failure:
- the water level in the reservoir is higher than the top water level and is consistent with the probable maximum precipitation
- there’s already an additional flow in the downstream watercourse that represents an extreme flood in the present day
Cascade reservoirs
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.
Reservoir flood information for Scotland and Wales
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.