Groundwater Flood Risk Explained – Why It’s the Most Overlooked Planning Constraint

6-minute read

Written by Annabel Elleray, Senior Project Consultant

Groundwater flooding can be a major issue to development proposals, which is why it must be assessed as part of a planning application. However, it’s often missed at an early stage as the requirements for submitting a planning application are limited to the more well-known flood sources, which can cause issues later in the planning process.

In this article, we’ll delve deeper into what groundwater flooding is, why it is important to consider and how it is assessed as part of a planning application.

What is groundwater flooding?

Groundwater describes the water that we find in the rocks beneath our feet. A proportion of rain falling on the ground will seep through the soil layer and collect within permeable rocks, known as aquifers, to form large groundwater systems. When rainfall levels are higher, the amount of water in these aquifers increases, causing the top of the system – known as the water table – to rise, peaking in winter and early spring (February to April).[JB1] 

During particularly wet periods, groundwater levels can rise significantly and cause flooding. This is most common in the chalk valleys of South-East England, where groundwater can emerge in springs and form streams.

Another major area at risk of groundwater flooding is river valleys, which are often filled with permeable sand and gravel deposits. Groundwater collects within these deposits, particularly when they are underlain by clayey bedrock, and rising water levels in these deposits can cause flooding in low points at the same time as rising river levels.

How does groundwater flooding differ from river, sea and surface water (pluvial) flooding?

All sources of flooding are interconnected, so understanding how the groundwater system works also helps us to predict where other types of flooding will occur.

River flooding, when high flows on a river cause it to burst its banks and inundate the surrounding area, is often preceded by a rise in groundwater levels, as many rivers are supported by a significant groundwater base flow. Groundwater levels close to the coastline fluctuate in line with tidal conditions, which means that a storm surge can have an effect further inland, even in defended areas. Meanwhile, surface water flooding, where high rainfall overwhelms drainage systems, can be made worse when groundwater levels are high, as it’s common for groundwater to infiltrate into sewers and reduce the capacity of drainage systems.

Much of the time, groundwater flooding can occur at the same time as river, sea and surface water flooding, so distinguishing between the different sources can be hard. That’s why it’s important to understand whether a development is at risk of groundwater flooding and account for it appropriately at an early stage.

Why does groundwater flood risk need to be considered during the planning process?

The potential for groundwater flooding must be considered at planning stage. The reasons for this are clear: many developments, such as on the chalk aquifer in South-East England, are vulnerable to changes in the water table, and once these have been constructed providing appropriate mitigation can be tricky. Compared to other sources of flooding, rises in the water table can be prolonged and unpredictable, so damages can add up over time. Below ground assets such as basements, sewers and pipework are particularly susceptible, even in areas where widespread groundwater flooding does not occur.

Groundwater flooding is predicted to become more severe with climate change in much of the country, as more intense periods of rainfall and sea level rise can cause variability in groundwater levels. National guidance recommends that flood risk is assessed over the lifetime of a proposed development, so development proposals should also account for the potential for groundwater flooding to last for longer periods or be more extreme in the future.

Also, shallow groundwater can pose an issue with certain Sustainable Drainage (SuDS) features such as soakaways, which needs to be considered when developing the drainage strategy for a new development.

How is groundwater flood risk assessed within a planning application?

Typically, in areas where there is an identified river, tidal or surface water flood risk, local planning authorities require a Flood Risk Assessment to be submitted as part of the planning application. Whilst being in an area at risk of groundwater flooding isn’t an automatic trigger for assessment, local authorities reserve the right to request an FRA in areas with known groundwater flooding issues, and groundwater flood risk is a key component of any analysis.

The potential impacts on the groundwater system also form a crucial consideration for basement development. This is particularly important in London, where there has been significant basement development and much of the land is underlain by shallow sand and gravel aquifers. Local authorities often request this to be considered in the form of a Basement Impact Assessment.

How does GeoSmart’s data help us to assess groundwater flood risk?

Figuring out whether a site is at risk of groundwater flooding requires consideration of many different sources, including geological mapping, topography and other nearby flooding sources, which can be difficult for a non-specialist to interpret. Additionally, much of the available mapping of groundwater flood risk is at a large scale (using 1 km grids) or only covers part of the country.

 To make initial decisions surrounding groundwater flooding clearer, GeoSmart has developed the Groundwater Flood Risk Map (GW5). This mapping models the risk of groundwater emergence across Great Britain at a 5 m resolution, categorising the risk based on how shallow the water table will be following an extreme rainfall event. It’s a great option for screening the groundwater flood risk at an early stage and identifying sites where further analysis is needed.

Another key tool is FloodSmart Analytics, which considers the risk of flooding from all sources, including groundwater. This shows the possible groundwater flood depths during extreme across the country at a 5 m resolution, based on where groundwater is most likely to emerge and accumulate. This mapping is used to support a range of sectors, including asset management, insurance and finance.

GeoSmart uses both sources of data when preparing Flood Risk Assessments for planning, and the results of the mapping are verified by referring to site-specific information, including local topography, nearby boreholes and ground investigation. This provides a robust assessment of groundwater flood risk to satisfy national and local requirements during the planning process.

Summary

If a Flood Risk Assessment is required as part of planning submission, groundwater flood risk will need to be considered and mitigated for. It’s crucial that this flood source is properly assessed, particularly when proposing a basement or when developing in known at-risk areas.

Our team of consultants has the necessary expertise to advise on how groundwater flooding could affect a development proposal, backed up by our industry-leading mapping. Please don’t hesitate to contact us if you have a project you’d like to discuss further.

GeoSmart Information Ltd (Technical Advisors) 
enquiries@geosmartinfo.co.uk 
01743 664124 
www.geosmartinfo.co.uk