Flooding - reservoirs

Reservoirs are artificially created ponds or lakes that are usually formed by building a wall, known as a dam, across a river. If one of these dams failed, water could escape from the reservoir, resulting in land or properties being flooded. To ensure that reservoirs are properly maintained and to minimise the possibility of reservoir failure, large reservoirs in England and Wales (capable of holding more that 25,000 cubic metres of water) are regulated under the Reservoirs Act 1975. This legislation is enforced by the Environment Agency and requires reservoirs to be routinely inspected and maintained to an appropriate standard. There are approximately 2,000 reservoirs in England and Wales covered by the Reservoirs Act.

If a reservoir is properly maintained, the likelihood of it failing and causing flooding is extremely low. However, in the event of a dam collapse, a large volume of water could be released, quickly flooding a large area and possibly causing significant property damage or even loss of life. Such a scenario is extremely unlikely to occur and reservoirs in the UK have an excellent safety record. Since the first reservoir legislation was introduced in the 1920s, the UK has not had a single reservoir failure resulting in loss of life.

Although reservoir flooding is unlikely, each year there are a small number of incidents (14 recorded between 2004 and 2008) that are deemed serious enough to require emergency drawdown (emptying) of the reservoir, emergency works to take place on the reservoir, and/or evacuation of people in the potential path of flooding. One of the most high profile of these incidents was in the summer of 2007 at the Ulley reservoir in Rotherham. In this case extreme rainfall caused damage to the dam and the risk of reservoir flooding led to a decision to evacuate around 1,000 people near the reservoir and to close main roads (including the M1). This highlighted the risk to communities living in reservoir flood zones and prompted Sir Michael Pitt, in his review into the 2007 floods, to recommend that the Government takes measures to improve reservoir emergency preparedness.

Part of Sir Michael's recommendation was that the Government should produce reservoir flood maps showing the areas that might be at risk from reservoir flooding. They should give these maps to local resilience forums (LRF) so they can use them to prepare emergency plans. In line with this recommendation, reservoir flood mapping of 2,007 reservoirs under the Reservoirs Act was completed in November 2009 and the maps were made available to LRFs shortly thereafter. Since completion of the mapping project, the Environment Agency has been working towards adapting the existing 'What's In Your Backyard' facility on its website to allow members of the public to view a map showing whether any location in England or Wales is in an area that could be at risk of reservoir flooding. Development of this facility is now complete and can be accessed via the Environment Agency website.

The reservoir flood maps available on the Environment Agency website are 'outline' maps, meaning that they show only the areas that might be flooded in a realistic worst case reservoir failure scenario. This means that the extent of the flooding shown is the worst that could realistically happen. In actuality, if reservoir flooding did occur it would most likely be much less severe than the worst case scenario. However, maps showing a realistic worst case scenario are useful to emergency planners who need to know the areas that might need to be evacuated and where it is safe for people to go to.

The maps on the Environment Agency website do not give any information about the depth or speed of the flood waters or the length of time it would take for the flood waters to reach any location. Even in a worst case scenario many areas shown as being at risk of reservoir flooding would be expected to receive no more than one or two centimetres of flood water. Emergency planners do have access to maps containing additional information. However, due to the sensitive nature of the information from a national security perspective, this information is not publicly available.

The reservoir flood maps do not give any indication of the likelihood of flooding occurring, unlike the maps for river and coastal flooding which are also available on the Environment Agency website. Reservoir regulation ensures that these reservoirs are stringently inspected and supervised by qualified civil engineers and that any required maintenance or upgrade works are carried out quickly. This helps ensure that the likelihood of one of them failing remains extremely low.

Most reservoirs in the UK have been there for over a hundred years without any problems. Many communities have therefore been living with the very small risk of reservoir flooding for many years and those communities close to reservoirs have always been aware that a risk existed. Nothing has changed, and the risk of reservoir flooding has not increased. Reservoir legislation and regulation help us ensure that these reservoirs remain safe, and the reservoir flood maps we now have available to us allow us to better estimate the areas potentially at risk and to put in place emergency plans, so we are prepared to respond should reservoir flooding occur.

The Flood Warning Information Service (opens new window) has more information on flood risks from reservoirs.