How GeoSmart’s GW200 screening map works, and why it’s deliberately precautionary
GW200 is a 200m grid screening version of GeoSmart’s high-resolution groundwater flood risk data (GW5), free for public bodies to use and publish. This article explains how it is built and why we made the design choices we did.
What GW200 shows
GW200, the Groundwater Flood Risk Screening Map of Great Britain, presents the boundary between the “negligible” and “low” risk zones of our 5 m GW5 map. Every 200 m cell is shown in one of two classes:
- Negligible risk of groundwater flooding.
- Potential risk of groundwater flooding in the 200 m cell.
Each cell takes the maximum risk level that GW5 models anywhere within it. If GW5 models Class 3 (Low), Class 2 (Moderate) or Class 1 (High) risk anywhere inside a cell, the whole cell is classed as potential risk. The data is supplied as a GeoPackage (.gpkg) file for use in GIS software, together with a PDF copy of the licence. It is a general-purpose indicative screening tool, intended to give a useful first view for a wide range of applications. It is also deliberately a pointer: it shows where groundwater flood risk may need attention, and that more authoritative spatial data is available when it does.
Problems associated with choice of mapping schema
Choices made about mapping schema have consequences. When detailed map data is presented on a coarser grid, each grid square often represents the average risk score. Part of the square may not be at risk, so the coarser grid inevitably produces some “false positive” findings. It is therefore important to include a statement that there could be flood risk within the square, flagging it for more detailed assessment.
GeoSmart’s approach is to flag whether possible risk exists within the square, without indicating how high that risk may be, which reduces the false-positive effect. We consider this consistent with the precautionary principle enshrined in tiered environmental risk assessment practice. Research for the Environment Agency published in 2021 highlighted the need to consider groundwater flood risk in land use planning, including the effects of climate change, and the importance of making precautionary assumptions at initial screening level so that serious hazards are not missed.
Why a maximum-occurrence rule matters with groundwater
Groundwater flood hazard is often concentrated in narrow zones, frequently under 300 m wide, following valley floors, spring lines and permeable deposits. A screening map that averages risk over a coarse grid can dilute these zones out of existence.
GeoSmart raised this concern with the Environment Agency in 2014 about the classification schema used in the Areas Susceptible to Groundwater Flooding (AStGWF) maps, which score each 1 km square by the percentage of its area that is susceptible. Narrow but genuinely high-risk corridors can end up classed as low or negligible. Comparing groundwater flood risk mapping with the AStGWF maps revealed that:
- 90% of the areas on the Environment Agency map showing some class of risk or susceptibility are not at risk;
- the highest-risk areas in Kent are mostly shown incorrectly on the Environment Agency map as “Negligible Risk”, which is not a conservative approach and is not consistent with normal risk screening principles; and
- the classification scheme leads to high-risk locations being classed as negligible, not because of any indication of lower risk, but because they are narrow zones compared with the chosen 1 km grid.
GW200 is built the other way round. If any part of a 200 m cell carries modelled risk, the whole cell is flagged.
An example: Brighton
The map below shows the GW5 groundwater flood risk classes for an example area in Brighton, from high to negligible, at a vulnerable setting on the highly permeable Chalk aquifer, alongside the GW200 map of the same area, showing the larger area flagged at initial risk screening level.
GW200 offers the advantage of freely available groundwater flood risk screening data that LLFAs and other public authorities can make available to members of the public for non-commercial uses. The consequence is a conservative screen. It will flag more ground than the full-resolution data does, and some flagged 200 m cells will contain mostly unaffected land. That is the intended behaviour for a first-pass tool: it should rarely miss a place where groundwater risk could be material, and it hands the question of “how much, and where exactly” to more detailed data or a site-specific assessment.
What is under the map
GW200 inherits its method from GW5. The groundwater flood risk model brings together:
- a digital terrain model (DTM), built from LiDAR from all the agencies (Environment Agency, SEPA and Natural Resources Wales) and infilled, where LiDAR is not available, with the Ordnance Survey Terrain 50 DTM;
- British Geological Survey 1:50,000 scale geological maps; and
- British Geological Survey groundwater level data.
GeoSmart’s in-house hydrogeological and risk models process these inputs, and a national database of recorded groundwater flooding events, collated by GeoSmart from a range of sources, is used to calibrate and validate the output.
The two mechanisms it represents
- Bedrock flooding. After extreme rainfall, groundwater levels in places rise to intercept the ground surface, sometimes on top of already above-average aquifer levels. Water tables typically reach the surface first in dry river valleys; once the drainage channel’s capacity is exceeded, a surface flood begins. This is often called “clearwater” flooding because the water has been filtered by flow through the rock, but the reality is more challenging, because it is more often contaminated by sewage and other near-surface contaminants.
- Permeable superficial deposit (PSD) flooding. Where groundwater in permeable superficial deposits is in good hydraulic contact with a river or the coast, flooding can occur during high river levels or tides. It can also occur where the connection is through the super-connected subsurface conduits that are widespread beneath our older towns and cities, where a network of old drains, sewers and service routes can bring river water to flood property even when it is protected from overland river flooding. This flooding may occur more frequently than river flooding.
What GW200 does not show
A screening product needs clear edges, so these are stated plainly:
- It is not a property-level tool. It provides no address or property details, and together with its scale that means it should not be used as a definitive tool for assessing groundwater flood risk, or to determine risk at an individual property or precise point.
- Water table rebound is not modelled. This includes, for example, rebound beneath cities, mineral extraction areas or around water supply boreholes where abstraction is reduced.
- It shows subsurface risk, not surface emergent flow. GW5, and therefore GW200, shows where groundwater may flood property and infrastructure directly. Where groundwater emerges and then flows, accumulates and ponds, creating a hazard downstream, it is mapped separately in FSAGW, our 5 m surface groundwater flood risk data. The two datasets are complementary parts of one picture: a site can be exposed to one without the other.
GW5 groundwater flood risk and FSAGW emergent flow mapped together for an example site.
More detailed assessment is needed wherever significant assets, community impacts or other serious consequences could result from flooding.
What “negligible risk” means
Where GW200 shows negligible risk, GeoSmart’s national groundwater flood risk model is not showing a risk at that location. It does not mean that there is no risk of groundwater flooding. Some local groundwater conditions are not modelled or mapped, either because there was not enough data or because the mechanisms are too local to be seen in the data used to develop the map. Groundwater flooding can therefore occur in areas shown as negligible risk. Where the consequences of a decision are important, for example, building a home or a valuable asset, or locating a basement or critical infrastructure, always obtain a site-specific groundwater assessment, and do not rely on GW200 alone. This applies equally to the Negligible class (Class 4) of GW5.
We say this plainly because it is true of any national screening data.
Using and publishing the GW200 screening map: a guide for public bodies explains what a site-specific assessment involves, and what to include when you publish the map. You can download GW200 free of charge.
GeoSmart Information Ltd is a flood risk specialist modeller and developer of flood risk maps for all sources of flooding. To obtain GW5 data or to learn more about groundwater flood risk, visit www.geosmartinfo.co.uk, or contact enquiries@geosmartinfo.co.uk or 01743 298100.
