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Groundwater Floodingbirmingham, Nottingham and Liverpool UK Geohazard Note May 2012 lowered. This form of groundwater level rise, known as ‘groundwater rebound’, is a longer-term problem affecting some of the UK’s major cities e.g. London, Groundwater floodingBirmingham, Nottingham and Liverpool. What is the impact of groundwater flooding? Abnormally high groundwater levels can also cause infrastructure and transport routes. Where high groundwaterthe flooding of levels basements cause theand reactivation low-lying property, of areas down-gradient where the drainage networks aredormant not adequate. springs this High can groundwater result in the levels flooding can ofcause the mobilisation of pollutants in the ground. This is a particular issue in urban areas which suffer from groundwater rebound. In these situations, there may also be engineering effects from impacts such as the reduction in the bearing capacity of foundations and the shrinking and swelling of clays. Compton Village, Berkshire: groundwater flooding in a chalk catchment in the winter of 2003. Image © NERC. What is the cost to the UK economy? Why does groundwater flooding occur? and economic disruption. It can affect infrastructure andGroundwater services, suchflooding as underground can cause significant trains, sewers, social Groundwater levels naturally rise and fall seasonally and there are many places, such as wetlands, where groundwater levels are close to the ground Overview groundwater rises above the ‘normal’ level at a surface. Groundwater flooding has an impact when location where it does not usually reach the ground groundwater emerges above the ground surface. • surface,Groundwater or into flooding man-made occurs ground when (e.g. This tends to happen when long periods of sustained building basements or buried services), in high rainfall occur when groundwater levels are areas away from perennial river channels. already high. It has been estimated that over 1½ million properties in England and Wales are lowered by pumping may also rise over many years • whenGroundwater the amount levels of which groundwater have been being artificially pumped susceptible to groundwater flooding. in an area reduces. For example, this can happen susceptibility maps for England, Wales and with the closure of water-intensive industries • ScotlandBGS has produced and also provides groundwater information flooding to in urban areas. Water levels then gradually rise, environmental regulators to help assess the returning to their natural state, inundating building basements and foundations and sub-surface infrastructure constructed when the levels were risk of groundwater flooding. www.bgs.ac.uk [email protected] UK Geohazard Note May 2012 properties and road and rail networks. Flooded water to emerge into streets, gardens and homes. Thesewers economy can overflow is hit by causing insurance contaminated claims, loss of productivity and emergency services costs. In England and Wales alone it has been estimated that over 1.6 million properties are susceptible to properties located on the outcrop of the Chalk in the groundwater flooding, in particular around 400 000 south and east of England (Jacobs, 2004). UK Examples Little data existed on the occurrence or impact the winters of 2000/01 and 2002/03. During of groundwater flooding in the UK prior to on the outcrop of the Chalk in the south and these periods groundwater flooding occurred major rivers. For example, the Patcham area ofeast Brighton of England was andaffected in the by floodplains groundwater of reactivation of a dormant Chalk spring. The flooding in December 2000 as a result of the railways and both rail and road journeys were affected.flooding causedThe event extensive is estimated damage to haveto the cost the local economy £800 000. Groundwater flooding in a garden in Oxford, July most recently in 2007 in which approximately 2007. Image © NERC. 200The cityproperties of Oxford were suffers affected from by recurrent a 1 in 20 floods,year eitherevent. theA significant sole cause number or the initial of these cause properties prior to inundationwere flooded by by river rising waters. groundwater Groundwater which levels was casesThe duration taking months of flooding for levels from groundwaterto return to normal. can be rose initially due to the high rainfall but in the much greater than surface water flooding, in some following days, further rise in groundwater This has significant economic implications. Susceptible locations levels was caused by river flood waters whichinfiltrating would into otherwise the underlying have been aquifer. protected The high lying areas underlain by permeable rocks (aquifers). groundwater levels resulted in flooding in areas TheseGroundwater may be floodingextensive, is regionalmost likely aquifers, to occur such in low-as the Chalk in the south and east of England, or may from river flooding. The impact here of be localised sands and gravels in valley bottoms fewgroundwater low-lying floodingproperties was were primarily also affected. on houses underlain by less permeable rocks where there is a with basements although the ground floors of a good hydraulic connection with a major river. www.bgs.ac.uk [email protected] UK Geohazard Note May 2012 any assessment of the probability of groundwater UK’s major cities where groundwater levels recover followingGroundwater a reduction flooding in may long also term occur groundwater in some of the high groundwater level are used to assign degrees of pumping in urban areas. flooding actually happening. Ranges of the depth to groundwater flood susceptibility. Scientific detail Secondary hazards Monitoring and measurement Secondary hazards resulting from groundwater The datasets that are used by BGS to map susceptibility toflooding move belongingsare similar orto thoserelocate from and other this hastypes risk of maps, a commercial licensed digital elevation associatedflooding. All with forms it. Floodingof flooding also can causes cause stress people and modelto groundwater and a national flooding groundwater are 1:50 000 level scale dataset. geological A a longer-term impact on health, particularly for vulnerable sections of society. within the geological maps. The national groundwater In the short term the emergence of contaminated levelpermeability dataset hasindex been is applied produced to theusing rocks a combination classified water above the ground surface that can result of point measurements, digitised maps of previously contoured levels and interpolations using river levels (presumed to be an expression of the water table). includingfrom groundwater the integrity flooding of walls, brings and health can compromise risks. The interpolations take into account the permeability subsurfaceGroundwater infrastructure, flooding can foralso example damage the property, sewerage system, and low-lying areas of the transport groundwater levels are relatively high. Assumptions network. areindex made of the about geology. the settings The dataset within reflects which periods groundwater when identify the depth to groundwater in these areas. flooding occurs and the datasets are combined to The BGS dataset has been produced based on Monitoring and response by BGS which has been developed through research in a numberconceptual of case understanding study areas. of In groundwater addition, BGS flooding receives andResponsibility environment for regulatorsaction during such times as the through its Enquiries Service and through interaction Environmentof flooding lies Agency with local in England authorities and Wales, withanecdotal clients information and customers on groundwater that allows itflooding to validate rather than BGS. However, BGS advised the dataset and improve its understanding of the the Environment Agency on the potential settings where groundwater flooding occurs. anfor eventgroundwater occurs, BGSflooding aims in to the utilise winter data of is updated every two to three years to take into 2007/08 following the July 2007 floods. If The groundwater flood susceptibility dataset collected to improve its understanding and its subsequent updates to the permeability index account new 1:50 000 geological mapping and BGS has produced national groundwater groundwater flood susceptibility mapping. groundwaterdataset. This canlevels affect in these the areas areas identified and therefore as being the and Scotland and has collected data for the susceptible to groundwater flooding as well as the caseflood study susceptibility areas in whichmaps forit has England, focussed Wales its research. It has also developed groundwater susceptibility classification. models in these areas, which it has used to How is the hazard characterised? improve its understanding of groundwater The BGS dataset indicates where groundwater flood dynamics. flooding could occur but does not incorporate www.bgs.ac.uk [email protected] UK Geohazard Note May 2012 Scenarios for future events Outside the BGS, there is also expertise in • (e.g. the Centre for Hydrology and Ecology be recognised as it does have a serious economic andgroundwater universities) flooding and specialist in research environmental organisations impact.The significance Without thisof groundwater recognition floodingengineered needs to consultancies. BGS has collaborated with a number of other researchers and has advised which could be circumvented by the movement of a range of consultancies during their work for groundwaterflood defences through may continue the permeable to be constructed subsurface. government departments and the environment
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