Mapping the UK flood risks

The Environment Agency estimates that 2.4 million properties in England are at risk of flooding from rivers or the sea, with a further 2.6 million properties susceptible to surface water flooding.

In addition, there are a substantial number of properties at risk of flooding from groundwater which are not highlighted on current Environment Agency flood maps, but which are shown for the first time on a new National Groundwater Flood Risk Map published by ESI Ltd (ESI), the independent UK-based scientific environmental consultancy specialising in water, land and sustainable development.

With the recent UK floods impacting large parts of southern England, many businesses and infrastructure owners and operators are unfortunately all too familiar with the devastating effects of groundwater flood damage. Previously overlooked by the media in favour of more dramatic cases of flooding, groundwater flooding causes two to four times the damage to buildings and significantly more economic harm as it can have a serious impact on infrastructure and can take some considerable time to recede.

Mark Fermor, expert hydrogeologist and managing director of ESI, explains the science behind groundwater flooding and how infrastructure owners and operators can identify flood risk to reduce the impact on their business or organisation, as well as potential losses:

“Groundwater flooding occurs when sub-surface water emerges from the ground at the surface, or into ‘made ground’ (land or ground created by filling in a low area with rubbish or other fill material) and structures. This could be as a result of persistent rainfall that recharges aquifers until they are full, or may be a result of high river levels or tides driving water through near-surface deposits.

“Compared to surface water flooding, groundwater flooding can last considerably longer, with incidents lasting anything from a week to several months, which is why it can prove substantially more costly to infrastructure projects than other types of flooding.”

Although emergent groundwater tends to be clear and relatively clean compared with muddy fluvial flood waters, it has the potential to be contaminated by sewers and brownfield sites. Groundwater flooding can also be the catalyst for many surface water floods, as it prevents rainfall infiltration.

Groundwater issues can often be a major area of concern for engineers and project managers on infrastructure projects, particularly road and tunnel developments. Over recent years, ESI has provided technical support to those who are planning and developing some of the most significant civil engineering and dewatering projects in the UK. These include the Channel Tunnel Rail Link, the Croydon Cable Tunnel, Thames Water’s Lee Tunnel, as well as a number of other major road and tunnel projects.

Although many organisations across the infrastructure sector are highly reliant on current and accurate flood risk guidance to prevent and predict the costly aftermath of groundwater flooding, until now there has been no national-scale authoritative map of groundwater flood risk.

The previous milestone achievement in this field was the publication by the British Geological Survey (BGS) in 2007 of a national Susceptibility to Groundwater Flooding dataset at a scale of 1:50,000. This indicates areas where geological conditions could enable groundwater flooding to occur and where groundwater may come close to the ground surface. The BGS clearly states this resource was never intended to be used to estimate risk, but in the absence of any other resources, it has been repeatedly relied on for just that purpose.

In October 2013 ESI addressed groundwater flooding issues in England and Wales by publishing the first national authoritative Groundwater Flood Risk Map. ESI has drawn upon the experience from within its team of specialist hydrogeologists to overcome constraints of the previous work available, and has developed its model using best practice algorithms and calibrated risk predictions, using site-specific evidence of real flooding events from many parts of the country to achieve an authoritative national map of groundwater flood risk.

ESI’s Groundwater Flood Risk Map is already proving popular with a variety of infrastructure owners and investors and the new map should see a change in the way that groundwater flooding is considered.

A move to a risk-based model can only be good news for the hundreds of businesses, landowners and developers finally freed from the potential issues which come with being branded a risk in the past and more in-depth and appropriate information can only help with planning projects and infrastructure in the future.

And because we’re working in consultation with dozens of Lead Local Flood Authorities, we will continue to develop and evolve the map as time goes on.

Mark Fermor is a managing director at ESI, which has offices around the UK. For further information please visit www.esinternational.com