Understanding Groundwater Flood Risk

Groundwater is any water which is located in between the pores and cracks of rock and soil beneath the Earth’s surface. 

It is an essential source of water in the UK, contributing to approximately 30% of England’s public water supply – that’s enough to fill 2500 Olympic-sized swimming pools every day. 

During periods of prolonged rainfall, groundwater levels rise as the earth becomes more saturated and water tables (the difference between the ground’s saturated and drier layers) become higher. 

Typically, water tables are higher during winter and reach a peak saturation level in spring. 

Groundwater flooding accounts for around 25% of all property and infrastructure risk, with wide-ranging and costly impacts, including:

• Flooded basements and disrupted buried utilities.
• Ground instability: subsidence, slope failures, and sinkholes.
• Damage to wildlife habitats and farmland.
• New springs, unexpected water flows, and surface flooding.
• Sewer overloads causing pollution and service outages.
• Contaminant spread from brownfield sites.
• Increased construction costs and broader socio-economic disruption.

Climate change may worsen the situation by altering the groundwater recharge season (typically Sept–April). More rainfall in a shorter timeframe could raise groundwater levels more quickly, increasing saturation and flood risk.

Although it can occur pretty much anywhere, groundwater flooding is most frequent in areas of southern and eastern England, parts of the Cotswolds, East Yorkshire and the Eastern Pennines. 

It is particularly prevalent where chalk, sandstone, localised sands or river gravel in the bottom of valleys are present.  

Here at GeoSmart we can produce Groundwater Flood Risk Assessment Reports and Data specifically focusing on groundwater flood risk.

Another useful source for assessing whether you are at risk of groundwater flooding is through the Environment Agency’s flood risk reports and/or the monthly documents they publish on rainfall and river flow volumes. 

If a site has an industrial past, groundwater flooding could cause contaminants in the soil to spread and come into contact with the building or people. 

Equally, groundwater flooding can have an impact on surface water flood management and cause more damage in the event of heavy rainfall.  

Understanding the site’s drainage characteristics and whether a sustainable drainage approach is needed (as determined by your Local Planning Authority) should be considered. 

When groundwater levels are high, water can access the sewage system by pushing through cracks in pipes, joints, manholes or access chambers.

When this happens, the excess water puts pressure on the sewer’s capacity, increasing the risk of sewer flooding and posing a threat to local communities and businesses. 

Sea levels are continuing to rise due to the increasing global temperature and a surge of up to 1m could be seen by the end of the century. 

As a result, groundwater levels located in coastal and estuary locations will also rise, potentially causing damage to inland infrastructure. 

Unexpected consequences could equally be seen if current defence systems become overwhelmed by future groundwater level surges. 

Reducing yours or your client’s susceptibility to groundwater flood risk can be challenging considering it is not widely understood and requires expensive boreholes to measure. 

However, there are several measures which can be undertaken to decrease the associated risks. 

These include: 

Understanding the impact of groundwater on property via a flood risk map

Seek improved insurance cover for groundwater risk

Designing and building property to account for groundwater risk