Reservoir Flooding

There are approximately 400 large raised reservoirs in Wales which hold 10,000 cubic metres of water, or more above natural ground level.


Any reservoir (above ground level) that contains over 10,000 cubic metres falls under the Reservoirs Act 1975. The Act ensures that the owners of a reservoir carry out necessary inspections and regular maintenance. Reservoir flooding could be very different from other forms of flooding. It could happen with little or no warning and you may need to evacuate immediately.


Reservoir flooding may be similar to river or surface water flooding if water escapes slowly, but in the unlikely event there is a catastrophic failure of the dam wall then a large volume of water could escape very quickly. Reservoir owners or operators should have on-site emergency plans in place to help prevent this happening.


Across the country, the likelihood of individual dam failure is very low with no loss of life since the 1920's, however there are some regular smaller incidents most years.


If you know your property is in an area that could flood, buy protection equipment and prepare your property well in advance to reduce the risk of flood water getting inside.


Remove flood protection equipment once the water has gone to help dry out your property.

Call the Natural Resources Wales Incident Hotline on 03000 65 3000 if you discover an apparent fault at a reservoir.


Keep yourself Safe

  • Keep yourself and others clear of danger. Move away from the reservoir, keeping to higher ground, if you can do so safely.
  • Make sure 999 has been called if people are injured or if there is a threat to life.
  • Don’t walk or drive through floodwater.
  • Follow the advice of emergency services in the area.
  • Try to remain calm, think before acting and try to reassure others.

You can see the possible extent of flooding from reservoirs on Natural Resources Wales website: Geocortex Viewer for HTML5 (