Groundwater Flood Risk: Completing the Picture
Groundwater flooding can impact property and infrastructure. It can also become an additional hazard when it emerges at the surface, creating appreciable surface flow and accumulation. In some settings, this surface flooding can cause significant loss and damage for businesses and communities.
In this article, we look at the components of groundwater flood risk and explain how they can affect property, development and infrastructure. We also explore how groundwater flood risk mapping can be used to understand where groundwater may rise, emerge, flow and accumulate, helping to build a more complete picture of risk.
The two components of groundwater flood risk
Groundwater flood risk has two components:
- Subsurface risk
This results from rising water tables exceeding normal levels and leading to the inundation of subsurface infrastructure such as utilities, basements and building foundations. In some places, inundation occurs due to groundwater rising directly into buildings, and this can continue for many days or weeks, leading to disproportionately more damaging consequences than other sources of flooding.
GeoSmart’s GW5 mapping represents subsurface groundwater hazard, combining likelihood and consequences with four classes of risk.
- Surface risk
Surface risk results from groundwater exceeding ground surface elevations, where it can emerge and then route across the land surface, creating flood depths and damage away from the point of emergence. As with subsurface impacts, surface runoff or ponding of emergent water can continue for many days or weeks.
Over the last few years, GeoSmart has developed a sophisticated model within FloodSmart Analytics (FSA GW) that models the surface expression of groundwater emergence and accumulation so that it can be compared with other sources of flooding on a like-for-like basis.
What causes groundwater flooding?
Figure 1. Conceptual groundwater flood mechanisms: recharge-driven high groundwater and river-driven propagation through permeable gravels. © GeoSmart.
Figure 1 illustrates the two main mechanisms that cause groundwater flooding. Bedrock flooding can be observed on the left, which is typical of major aquifers such as limestone when the water table rises to the surface. Permeable superficial deposit flooding, shown on the right, is more typical of river deposits and shallow ground in our riverside towns and cities, when high river levels drive water through the riverbank into low-lying adjacent areas.
Why is groundwater flooding difficult to map?
Groundwater flooding is difficult to map because the hazard is not confined to a single visible process. Consequently, groundwater modelling and mapping have developed more slowly than river and coastal flood risk mapping.
GeoSmart first published maps of the subsurface risk over 12 years ago, and 10 years later we published our first maps showing the surface risk associated with groundwater emergence. We then brought these two hazards together in the first ‘flood risk from all sources’ maps of Britain that enable a consistent national flood risk screening of the risk of flooding from all sources on a 5m grid.
Why both surface and subsurface groundwater risk matter
National mapping shows that for an overall flood risk screening at property level, it is important to consider both aspects of groundwater flood hazard together. Whilst most properties are not at risk, either surface or subsurface risk from groundwater (or both) can locally represent the most significant natural hazard. For development and infrastructure planning in Great Britain, this matters because a site can be exposed to one manifestation of groundwater hazard without the other.
Figure 2 below illustrates the spatial relationship between subsurface groundwater flood risk modelled in our GW5 data and the surface groundwater risk modelled in FSA Groundwater. You will see a typical spatial relationship between subsurface and surface risk areas in this figure that helps show why exposure to one component of the hazard can occur without the other:
Figure 2. An example of flood risk mapping illustrating the relationship between subsurface and surface risk areas associated with a major aquifer setting in England.
How do you map groundwater flood risk?
The traditional question in groundwater flood mapping is: where can groundwater rise sufficiently close to the ground surface to create a hazard? The subsequent question we have addressed with our FSA GW data is: if groundwater emerges, where will the water go, and where will it accumulate?
These questions describe two connected stages of the same hydrological system, and predictive modelling of groundwater systems and surface interactions enables a more complete characterisation of groundwater flood risk that we typically need.
Quantifying subsurface groundwater flood risk
GeoSmart’s latest GW5 Version 3 data is designed as an initial risk screening map to identify the groundwater conditions beneath a location, quantifying subsurface risk. It describes property-specific outcomes at 5m resolution across England, Wales and Scotland, using high-resolution elevation information, bedrock and permeable superficial deposits, refined groundwater catchments and verification against a groundwater flood database.
This risk dataset helps to identify where shallow groundwater can cause consequences even where surface problems may not occur (but subsurface damage can still be extensive). It can enter basements and service voids, affect foundations and excavations, infiltrate sewers, prevent drainage systems from operating and interact with buried infrastructure.
GeoSmart’s FloodSmart Analytics groundwater mapping represents the possible surface expression of groundwater flooding – where groundwater is likely to emerge and accumulate at the surface.
Once water has emerged, topography, flow pathways, depressions and drainage determine its subsequent movement. The surface flood footprint therefore can affect areas that are not themselves at risk from shallow water tables. They are often similar in surface expression to those areas mapped as surface water risk, but prone to longer-duration and hence more damaging flood events.
Figure 3. GeoSmart’s groundwater flood risk maps: GW5 represents subsurface groundwater risk; FSA GW represents surface groundwater risk. The combination identifies the fullest appreciation of groundwater risk at a location.
Can surface and subsurface groundwater risks occur independently?
A groundwater emergence risk is not automatically an inundation risk. Conversely, a surface inundation footprint does not describe every subsurface consequence of a high-water table. The two approaches therefore should be viewed as complementary estimates describing part of the overall risk. They describe different manifestations of the groundwater flood process.
Why groundwater flood risk matters for development and infrastructure
Surface inundation mapping alone may fail to identify conditions relevant to basements, foundation design, buried services, sewer infiltration, construction dewatering, infiltration SuDS, and mobilisation of contamination. A subsurface map alone may identify shallow groundwater without resolving where emergent water subsequently routes and accumulates.
The combination is especially important for linear infrastructure. Roads, railways, sewers and utilities can cross shallow groundwater corridors, emergence zones and downstream accumulation areas within the same scheme. Previous research has also highlighted published evidence of groundwater exposure across several classes of critical infrastructure, reinforcing the need to consider groundwater as an infrastructure hazard rather than only a property flooding issue.
How to assess groundwater flood risk
When considering planning and development projects, the following methodology for assessing groundwater flood risk is recommended:
1. Screen the subsurface groundwater regime. Could groundwater become sufficiently shallow to affect the site, basements, foundations, sewers or buried infrastructure?
2. Examine surface groundwater inundation. Where might groundwater emerge, route and accumulate?
3. Examine interaction with other flood mechanisms. Could high groundwater increase fluvial, surface water, sewer or coastal consequences?
4. Interpret the datasets together. A geospatial analysis bringing all flood risk data together may give sufficient information to support due diligence and make decisions.
5. Where the risk is material, verify locally using geology, borehole records, groundwater monitoring, topography, site investigation and flood history appropriate to the scale and consequence of the development. There is a limit to what risk screening maps can tell you so more site-specific flood risk assessment is often necessary.
Building a complete picture of groundwater flood risk
In conclusion, it is important to recognise that groundwater flood risk has a subsurface expression and a surface expression. GW5 and FloodSmart Analytics groundwater mapping represent these different but connected parts of the process.
Used together and interpreted with site-specific hydrogeological evidence where consequences are potentially material, they provide a stronger basis for assessing how groundwater may affect property and infrastructure.
For flood-risk practice, the implication is clear: where groundwater could materially affect a development or infrastructure asset, both subsurface groundwater risk and groundwater-driven surface inundation should be considered. With our rapidly changing climate, these risks are becoming more material.
