What Is Reservoir Flooding? Causes, Risks and Planning Implications

Reservoir flooding occurs when water stored behind a reservoir dam or embankment is suddenly released and flows downstream. This can happen if a reservoir structure fails, becomes damaged or is overtopped during extreme conditions.

Although reservoir flooding is considered a very low probability risk, the potential consequences can be significant because reservoirs store large volumes of water. Understanding reservoir flooding is therefore important for property owners, developers and planning professionals located downstream of reservoirs.

How Reservoir Flooding Happens

Reservoir flooding occurs when water escapes from a reservoir in an uncontrolled way.

Reservoirs are engineered structures designed to safely store large volumes of water for purposes such as water supply, flood management or energy generation. These structures are normally carefully maintained and regulated.

If a large volume of water is released suddenly, it can travel rapidly downstream, following valleys and watercourses.

Because reservoirs are often located at higher elevations, water released during a failure event may travel considerable distances and affect areas far downstream.

Where Reservoir Flooding Occurs in the UK

Reservoir flood risk occurs in areas located downstream of raised reservoirs.

Typical locations potentially affected include:

River valleys below reservoirs

Reservoirs are commonly located upstream within river catchments. Areas further downstream along the river valley may therefore lie within potential reservoir flood pathways.

Low-lying land and floodplains

Water released from reservoirs typically follows natural drainage routes such as rivers and valleys, meaning low-lying land may be particularly exposed.

Settlements downstream of reservoirs

Some towns and villages are located downstream of reservoirs that were constructed historically for water supply or industrial purposes.

Infrastructure corridors

Transport routes, utilities and other infrastructure often follow river valleys and may therefore intersect potential reservoir flood pathways.

Although these areas may be identified as potentially affected by reservoir flooding, the likelihood of reservoir failure remains extremely low due to strict safety standards.

How Reservoir Flood Risk Is Mapped

In England, flood risk from reservoirs is mapped nationally by the Environment Agency.

Reservoir flood maps show areas that could potentially be affected if water were released from a large raised reservoir. These maps are based on modelling that assumes a sudden release of water from the reservoir.

Reservoir flooding is different from other types of flood risk because the mapping represents a worst-case scenario, rather than a probability-based assessment.

These maps do not indicate the likelihood of reservoir failure, which is considered extremely low due to regulatory oversight and inspection regimes.

Reservoir flood information is available through government flood risk mapping services and may also be referenced within Strategic Flood Risk Assessments (SFRAs) prepared by local authorities.

As with other national flood datasets, reservoir flood mapping should be considered a screening tool rather than a precise prediction of flooding at individual properties.

Reservoir Flooding and Planning Applications

Reservoir flood risk can be a consideration for development proposals located downstream of large reservoirs.

Planning guidance requires that development proposals consider all sources of flooding, including reservoir flooding where relevant.

Where a site is located within an area that could potentially be affected by reservoir flooding, a Flood Risk Assessment may need to consider the implications for the development.

Potential mitigation measures may include:

• designing buildings with flood resilience measures
• ensuring safe access and evacuation routes during extreme events
• considering site layout to reduce exposure to potential flood pathways

Because reservoir flooding represents a low-probability but potentially high-consequence risk, planning assessments typically focus on ensuring that development would remain safe in the unlikely event of reservoir failure.

Key Things Property Owners Should Know About Reservoir Flooding

Several important points are worth understanding about reservoir flooding.

Reservoir flooding is very rare

Reservoirs in the UK are subject to strict safety regulations and regular inspection regimes, making structural failure extremely unlikely.

Mapping shows potential impact, not probability

Reservoir flood maps show the areas that could be affected if water were released, but they do not indicate the likelihood of such an event occurring.

Flood pathways may extend downstream

Water released from reservoirs typically follows river valleys and natural drainage routes, meaning areas some distance from the reservoir itself may still fall within potential flood extents.

Understanding whether a property lies within a potential reservoir flood pathway can help inform flood risk assessments and development decisions.

Need Advice on Reservoir Flood Risk?

Reservoir flood risk can be complex to interpret because national maps show potential impact areas rather than probabilities of flooding. Assessing the implications for a specific site often requires professional interpretation alongside other flood risk information.

Where reservoir flooding may affect a planning application, property purchase or development proposal, a detailed Flood Risk Assessment can help identify potential impacts and appropriate mitigation measures.