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HoC Environment Food and Rural Affairs Committee - Future Flooding Prevention

Following on from the floods that hit the UK over Winter 16/16 the House of Commons Environment, Food and Rural Affairs Committee have launched an inquiry into Future Flooding Prevention. Details of the inquiry can be found on the committee website. The submission produced by the Geological Society can be found below:

Submitted 15 March 2016

1. The Geological Society (GSL) is the UK’s learned and professional body for geoscience, with about 12,000 Fellows (members) worldwide. The Fellowship encompasses those working in industry, academia, regulatory agencies and government with a broad range of perspectives on policy-relevant science, and the Society is a leading communicator of this science to government bodies, those in education, and other non-technical audiences. Of our 12,000 Fellows, more than 2,000 work in the area of Hydrogeology (groundwater science) and related specialisms. We have not answered the questions set out in the terms of reference but instead want to highlight the hazard of groundwater flooding and the role of hydrogeological processes in flooding within permeable catchments.

2. The terms of reference for this inquiry do not refer to the contribution of groundwater to surface flooding or the hazards of groundwater and sewer flooding. We are very concerned that the prevailing practice of treating flooding mechanisms independently has led to a fragmented approach to regulation and management of flood risk, and we hope that your inquiry will consider this fundamental issue. In particular, omission of the contribution of groundwater to flood management and planning prevents adequate estimation of flood risks as the basis for planning sensible responses. This has also resulted in a lack of capability in the flood risk management community to fully consider or communicate the potential risks posed by groundwater behaviour to the public. Hydrogeological expertise, concerned with the uses and function of water occurring underground and the interactions between surface water and groundwater within catchment areas where the subsurface is permeable to water, is key to flood risk management.

3. Water is present everywhere underground in the form of groundwater and it flows readily in those geological formations with appreciable permeability. Groundwater is water that has travelled from the surface as rainfall to the subsurface through a process of infiltration. The depth at which the rock becomes saturated with water is known as the water table and the rock below this depth is called the ‘saturated zone’. Groundwater flows within the saturated zone in the direction of the point of discharge which can be a spring, a river or the sea. Much of the flow of a river is made up of discharging groundwater and thus understanding groundwater activity is a significant component of surface flood management. It is our view that flood risk management requires a holistic catchment-based approach which fully encompasses flooding, groundwater flooding and the different ways in which surface and subsurface flows intersect. It is important that policy making, implementation of policy and regulation fully incorporate all of these aspects on a catchment basis if flooding is to be accurately predicted and the impacts effectively managed.

4. Groundwater flooding occurs as a result of water rising up from underlying rocks or from water that flows from abnormal springs. The risk from groundwater flooding has long been known to specialists in the field but was raised in the minds of the public and policymakers during the groundwater flooding events of winter 2013/2014, although the role of the subsurface in surface flooding remains much less well appreciated even today. That winter saw exceptional rises in groundwater levels which in turn reduced the extent to which water could infiltrate the surface and drain into the subsurface. This combination of factors therefore not only resulted in several instances of groundwater flooding but also reduced the ability for high rainfall and surface water to infiltrate into the subsurface which exacerbated the surface flooding events. This gap in awareness of the consideration of groundwater and of the potential risks it poses is often due to the fact that the makeup of the subsurface and the processes it supports are simply not visible to people. For that reason it is important to highlight the important contribution of the subsurface in the area of flood risk and prevention in order to build a consideration of groundwater and groundwater flooding and the need for more integrated catchment assessment and management into policy making in a more consistent way.

5. Described as a ‘hidden threat’, it was estimated in 2004 that 1.6 million properties in England and Wales were at risk of groundwater flooding. More recently the British Geological Survey (BGS) reviewed the events of 2014 and concluded that the risks may be more localised. However, it is important to note that in addition to such properties affected directly by groundwater emergence, groundwater is a significant risk driver in flooding that has been classified under other headings. Groundwater flooding can cause the flooding of basements and subsurface infrastructure as well as causing the emergence of groundwater at the ground surface. It tends to occur after long periods of sustained high rainfall, unlike some surface flooding which is a more immediate reaction to short periods of very high rainfall. This is due to the longer lag time associated with groundwater caused by the time it takes for water to infiltrate into the subsurface, fill the aquifer and raise the water table. High groundwater then tends to flow to where there is lower topography, and in these areas the water table is usually at shallower depths. This movement of water to areas that are already saturated causes groundwater to flood at the surface. Locations that are the most vulnerable to this type of flooding are in areas of low-lying chalk such as Hampshire. Chalk is a common aquifer rock (a body of rock which can contain or transmit groundwater). A notable instance is the chalk aquifer from which Greater London extracts much of its drinking water. Understanding the distribution of vulnerability to groundwater flooding and the potential emergence points for groundwater in any area is an important part of effective flood management.

6. As described in the Environment Agency’s ‘Flooding from Groundwater’ pamphlet, unlike surface water flooding, groundwater flooding can occur days or even weeks after heavy or prolonged rainfall and so it is important those affected are made aware of the potential risks from groundwater so they can plan accordingly and maintain vigilance even after the initial surface flooding has subsided. Groundwater Flooding may also continue for a long time, often lasting for weeks or months. It can have a particular impact on subsurface infrastructure, basements and low lying houses. In those situations, groundwater is most likely to flood properties and infrastructure up through the floor rather than coming in through the doors and windows as with surface flooding. Being aware of the different sources and impacts of groundwater flooding is significant not only for homeowners but also for effective planning and implementation of building codes, building infrastructure and for competent risk management. There are also knock-on effects with drains and sewers being overwhelmed by the contribution of rising groundwater and surface water.

7. Detail on planning and awareness of groundwater flooding is absent from important policy documents. Respondents to the draft flood risk management plans that were compiled last year by the Environment Agency noted the omission of groundwater flood risk and the underrepresentation of groundwater in the draft plans. The consultation received a number of comments about the lack of detail included on risks of groundwater flooding, the combined risk from multiple sources of flooding and the absence of groundwater flooding warning systems. Many respondents thought that all types of flooding should be covered in one plan including better recognition of the interaction of groundwater with other drainage systems such as sewage flooding. It would be helpful to include more consistently the risks of groundwater flooding and its role in flooding in high level policy documents.

8. There is also more that could be done both in terms of research of groundwater mechanisms and making the public aware of the potential risks. There should also be easy access to publically available informative materials which communicate the impacts of groundwater flooding and ways for homeowners to prepare and be vigilant of potential flooding events.

9. With regards to research, the BGS has undertaken research to assess the mechanisms for groundwater flooding in two of the most vulnerable settings in the UK – areas where Chalk crops out at the surface, such as in Brighton, and areas where river valleys are underlain by permeable deposits as seen in Oxford. It would be valuable for the government to commission research in other vulnerable areas in the UK.

10. The Environment Agency produces monthly water situation reports based on data provided by themselves, the Met Office and water companies. These include, among other things, the amount of water stored below ground in aquifers as well as the outlook for groundwater flow over the coming months. In the most recent report, at the time of writing, groundwater levels were described as having ‘increased at almost all indicator sites and were classed as normal or higher for the time of year at all but 2 sites’. This type of information is very useful in the context of prediction and planning for groundwater flooding and it is imperative that this type of information is effectively communicated to those in areas vulnerable to flooding. We agree that a key factor in improved management of groundwater flooding is awareness, both with the public but also with end users of the flood management plans. Consistent attention to groundwater behaviour and its impact within a more holistic catchment context is required to raise awareness and protect against the potential damage to people, property and the environment.