10 years of groundwater flood maps: Helping to evaluate flood risk from groundwater sources
A decade on from the launch of our groundbreaking GB-wide groundwater flood map in 2015, we reflect on the ongoing benefits the product continues to provide.
Understanding groundwater flood risk: Causes and consequences
Groundwater flooding is the term used to describe flooding caused when groundwater ingresses into underground infrastructure or emerges at the ground surface and subsequently ponds or flows downslope, affecting property or roads.
It typically occurs following periods of prolonged or heavy rainfall, which drive elevated groundwater levels. Regions with permeable geology are particularly susceptible to groundwater flooding. For example, a wet winter across chalk downlands can cause groundwater flooding in early spring, which can last for weeks or months.
Understanding how groundwater flooding occurs and how it differs from other types of flood risk is crucial for developing effective mitigation strategies and ensuring communities are better prepared. It can affect areas far from rivers or coastlines, making it a hidden hazard for property owners, planners, and insurers. Standard methods of flood protection, such as sandbags, are much less effective against flooding from groundwater, and the prolonged duration is particularly problematic.
- Local Authorities – for strategic flood risk assessments (SFRAs).
- Planners and developers – to guide site selection and development.
- Insurers – to assess property flood risk.
- Infrastructure Managers – transport and utilities.
- Environmental Consultants – for risk modelling and mitigation planning.
A pioneering first for groundwater flood mapping
JBA was the first organisation in the UK to produce probabilistic flood maps for groundwater. In 2007, we released the first-ever flood outlines for chalk areas, subsequently upscaling this to a national extent after the 2013/14 winter floods.
Consistently mapping groundwater flood risk on a national scale for a range of different aquifer types had never been done before. We first demonstrated proof of concept using a coarse-scale approach based on a 50m grid resolution, which was not without its challenges. However, we successfully translated our method to an increased 5m resolution, making it not only the first of its kind but also incredibly advanced in its output.
A distributed hydrogeological model underpins the map, which was used to estimate the groundwater table position under different flood conditions. The model is spatially variable, accounting for topographical and geographical influences on groundwater levels, including the depth and transmissivity of near-surface superficial deposits and the characteristics of bedrock. Model parameters were refined by calibration using observed levels from a range of boreholes across the country.
To reflect different flood conditions, the model is loaded with varying amounts of rainfall. For instance, our 1% annual probability groundwater flood map corresponds to the rise in the groundwater table that is expected when the 1% yearly probability winter rainfall event occurs. Rainfall probabilities are based on statistical analyses of reliable long-term rainfall records and vary spatially to reflect regional and local rainfall variations.
Delivering UK groundwater flood mapping to shape decisions
The advanced 5m-resolution groundwater flood maps are available for use at national, regional, or local scales.
Each 5m square is assigned a risk rating related to the scale of groundwater flooding, making the mapping easy to interpret for a wide range of user sectors, including transport, planning, insurance, and water utilities. We continually enhance the flood mapping software to provide robust flood risk insights at a price that supports widespread adoption.
Available in the following formats:
- ArcGIS shapefiles or Geodatabases
- ASCII raster’s
- MapInfo files
- Interactive PDF maps
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