Ground water flooding

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 – the an independent scientific environmental consultancy specialising in water, land, and sustainable development.
    
With the recent UK floods impacting large parts of southern England, many Government agencies and local authorities 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.
    
For Government agencies, local authorities, and their partner organisations, the impact of a flood is measured not only in damage to property and infrastructure, which can be partially offset by insurance of course, but also the potentially serious knock-on effects in terms of business interruption and the effect on lives.
    
Mark Fermor, expert hydrogeologist and Managing Director of ESI, explains the science behind groundwater flooding, and how Government agencies and local authorities can now assess flood risk to mitigate against flooding and reduce the impact on their operations, as well as potential financial losses.

The science
Groundwater flooding occurs when sub-surface water emerges from the ground at the surface, or into Made Ground 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 to 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.

Getting a clear picture
Despite many Government agencies and local authorities being 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 considerable 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.

Case Study: City of Oxford
The River Thames flows directly through the city of Oxford where there is a long history of flooding. Groundwater plays a significant role in this, and with urbanisation putting increased pressure on land availability, development has been driven towards the floodplain. This has resulted in more than 3,600 homes, roads and other critical infrastructure being located in the River Thames floodplain to the west of the Old City.
    
By introducing the probability of a flood event (based on the 1 in 200 year groundwater levels) and severity of the flood event consequences, ESI’s new Groundwater Flood Risk Map can determine the risk of groundwater flooding. What’s more, the model has identified that the areas at risk from groundwater flooding are considerably less than previously flagged by others – a finding that will potentially have a major impact on those with responsibility for land management and development, as well as those currently living and working in seemingly ‘at risk’ areas.

In the case of Oxford, the majority of the historical city can now be recognised as lower risk, helping infrastructure owners and planners to focus on other priorities, whereas in the high risk areas resources can be focused on suitable development to avoid costly problems arising, or early warning and mitigation measures to reduce impacts in the case of existing property.

Case Study: Compton, Berkshire
The village of Compton in Berkshire is located in the catchment area of the River Pang and has a long history of inundation. This can lead to overflowing sewers and basement flooding which has led some residents to install pumps in their properties as a mitigation measure.
    
The new National Groundwater Flood Risk Map can help identify properties in Compton that are at risk so that residents can plan accordingly and water companies can best prepare staff and partners serving the area. The obvious benefit of the map is that it allows the user to get a large scale understanding of the groundwater flood risks of a region and an indication of the potential flood risks at a given site. This is a powerful screening tool in the early stages of planning for all major projects, including utility connections.  In the case of villages like Compton, the map assists with clarification of how much risk is faced in different parts of the district, helping those living and working locally to plan ways to avoid exposure to flooding wherever practical, and use of the predicted groundwater level frequency distribution helps those designing infrastructure projects when used within their geographic information systems (GIS).
    
Already proving popular with Government agencies, local authorities, and a variety of organisations allied to the public sector, the new Groundwater Flood Risk Map and related GIS data should see a step-change in the way that groundwater flooding is considered. In my opinion, a move to a risk-based model with more in-depth and appropriate information can only be a positive thing when planning for the future.
     
By providing the very best and most accurate data, ESI is able to allow Government agencies, local authorities, and their partners to mitigate and manage potential issues as they wish. Groundwater flooding issues are certainly more localised than previously thought, but projects within risk zones need groundwater to be properly considered in order to avoid problems.
    
In the face of the recent prolonged flooding in southern England, I would recommend all municipal professionals revisit their flood risk management and continuity plans.     
    
When floods occur it’s often too late to make effective plans, which is why good preparation is key. Maps of groundwater flood risk zones can be obtained now for the first time from sources such as landmark.co.uk at a low cost to provide initial indication of the level of risk faced at a property or site. Advice and guidance is also available through specialist firms who understand flooding risks and their management.

Specialist help
At ESI we work alongside a number of local authorities UK-wide on groundwater and flooding issues, in particular with Lead Local Flood Authorities who have primary responsibility for developing, maintaining and applying a strategy for local flood risk management in their areas and for managing the risk of flooding.
    
We have offered to licence the Groundwater Flood Risk Map free of charge to Lead Local Flood Authorities in England and Wales who join our User Group and commit to providing feedback and data to help improve the national database and future editions of the map.
    
The ESI Local Authority User Group will provide Lead Local Flood Authorities with access to vital information which will further help them to fulfil their responsibilities in respect of the Flood and Water Management Act 2010. The resource can be exploited by participating Authorities internally, and will also enable members of the public concerned about groundwater flooding to consult the map at their local Council offices.

Further information
www.esinternational.com