1 Flood risk from groundwater: examples from a Chalk catchment in 2 southern England 3 4 A.G. Hughes1, T. Vounaki1, D.W. Peach1, A.M. Ireson2, C.R. Jackson1, A.P. Butler2, J.P. Bloomfield3, 5 J. Finch4 and H.S. Wheater2 6 1 British Geological Survey, Keyworth, Nottinghamshire, UK 7 2 Department of Civil and Environmental Engineering, Imperial College London, London, UK 8 3 British Geological Survey, Wallingford, Oxfordshire, UK 9 4 Centre for Ecology and Hydrology, Wallingford, Oxfordshire, UK 10 11 12 13 14 Correspondence Abstract A.G. Hughes, British Geological Survey, 15 Groundwater flooding has moved up the policy-makers’ agenda as a result of the 16 Keyworth, Nottinghamshire, UK Email: [email protected] United Kingdom experiencing extensive groundwater flooding in winter 2000/ 17 2001. However, there is a lack of appropriate methods and data to support 18 DOI:10.1111/j.1753-318X.2011.01095.x groundwater flood risk assessment. The implications for flood risk assessment of 19 groundwater flooding are outlined using a study of the Chalk aquifer underlying 20 Key words the Pang and Lambourn catchments in Berkshire, UK. Groundwater flooding in 21 Q2 ’; ’; ’. the Chalk results from the water table reaching the land surface and producing 22 long-duration surface flows (weeks to months), causing significant disruption to 23 transport infrastructure and households. By analyzing existing data with a farmers’ 24 survey, it was found that groundwater flooding consists of a combination of 25 intermittent stream discharge and anomalous springflow. This work shows that 26 there is a significant challenge involved in drawing together data and under- 27 standing of groundwater flooding, which includes vital local knowledge, reason- 28 able risk assessment procedures and deterministic modelling. 29 30 31 Introduction 32 Type 2 – Groundwater flow in alluvial deposits by-passing 33 Groundwater flooding is poorly understood, often confused river channel flood defences, 34 with surface water flooding, and has not been widely recog- Type 3 – Cessation of groundwater abstraction for Public 35 nised as a problem, either in the United Kingdom or Water Supply or mine dewatering, e.g. London Basin and 36 internationally. The UK Government’s Department for the other urban areas, and 37 Environment and Rural Affairs (Defra, 2004) defined Type 4 – Underground structures creating barriers to 38 groundwater flooding as: ‘flooding caused when water levels groundwater flow. 39 in the ground rise up above the natural surface, it will often Groundwater flooding can result in surface water pond- 40 occur when accumulated rainfall over a long period of weeks ing, intermittent stream flow or the anomalous activation of 41 or months is significantly above normal. Groundwater flood- springs, as well as flooding of cellars, basements and 42 ing is most likely to occur in low-lying areas underlain by other subsurface infrastructure, and damage to foundations. 43 permeable strata.’ However, this is a simplification and does Unlike overbank fluvial flooding, groundwater floods 44 not include groundwater interaction with underground tend to be long-lasting, typically of the order of weeks or 45 structures such as cellars and basements, and tunnels. Exam- months. 46 ples of the latter include those used for transport purposes, Examples in the United Kingdom of the second type of 47 such as the London Underground network. flooding include the flood events in south Oxford in 1997 48 It is suggested that the following provides a more (Macdonald et al., 2007, 2008a); and Pilmuir in Scotland in 49 complete description. Groundwater flooding occurs due to 1997 (MacDonald et al., 2008a, b). However, while all of the 50 water table rise. This is characterised by one or more of the above mechanisms can result in significant flooding, it is the 51 following: intense or long duration rainfall that is currently believed to 52 Type 1 – Extreme high intensity and/or long duration be the most important source of UK groundwater flood risk 53 rainfall, (Jacobs, 2004), and is the main focus of this paper. J Flood Risk Management (2011) 1–13 c 2011 The Authors Journal of Flood Risk Management c 2011 The Chartered Institution of Water and Environmental Management (BWUK JFRM 1095 Webpdf:=03/30/2011 12:49:16 3940342 Bytes 13 PAGES n operator=) 3/30/2011 12:49:34 PM JFRM 1095 Dispatch: 30.3.11 Journal: JFRM CE: Deepika JFRM 1095 Journal Name Manuscript No. B Author Received: No. of pages: 13 PE: Thanuja/ananth 2 Hughes et al. 1 Groundwater flooding of this type has a long history in in Europe and the Americas. The south Galway area of 2 the United Kingdom and as defined above has been recog- southern Ireland saw extensive flooding in the late 1980s and 3 nised within communities for a number of decades. For early/mid 1990s (Peach et al., 1997; Johnston and Peach, 4 example, there is documentary evidence of groundwater 1998; Lees et al., 1998). This was due to the overtopping of 5 flooding in the 1930s (McMahon, 2000), and, in areas where turloughs, which are lakes fed or drained by karst ground- 6 communities were aware of the phenomenon, house build- water systems. Groundwater flooding has also been ob- 7 ing practices were adapted to accommodate this (e.g. Morris served in the permeable sediments in the Danube flood 8 et al., 2007). However, only in recent years has it been plain. An example of groundwater flooding circumventing 9 considered an important hazard by government agencies flood defences has been reported in Hungary (Vekerdy and 10 (e.g. Defra, 2004; Jacobs, 2004; Pitt, 2008), an importance Meijerink, 1998). In the United States, groundwater flood- 11 prompted by several significant flood events. In the 1990s ing has been recognised by the United States Geological 12 groundwater flooding came into sharp focus with the flood- Survey (USGS) as occurring in glacial deposits in Washing- 13 ing of the city of Chichester in winter 1993/1994 (Taylor, ton state during the winters of 1993/1994 (Visocky, 1995) 14 1994), which resulted in significant disruption to the trans- and 1996/1997 (USGS, 2000), where flooded depressions 15 port network, closing a major trunk road (the A27), and resulted in road closures. Groundwater flooding in the 16 flooding homes and businesses. Concerns were reinforced winter of 1993/1994 resulted in the production of 100-year 17 by extensive flooding of Chalk catchments in SE England in return period groundwater flood risk maps (Visocky, 1995). 18 the Autumn/Winter of 2000/2001. Near Henley, Berkshire, An attempt was also made to undertake groundwater flood 19 floods generated during the winter of 2000/2001 affected the risk mapping in southern France (Najib et al., 2008). This 20 basement and ground floors of houses until June, when work assessed groundwater flooding in karst systems using 21 remedial work was undertaken (Robinson et al., 2001; other estimates of flood return periods based on groundwater 22 examples of the flooding of homes, including cellars and hydrographs. Marechal et al. (2008), using an empirical 23 basements are Oxford in 1997, Macdonald et al., 2007). transfer function modelling approach, proposed that thresh- 24 Significant disruption was caused by closing roads (e.g. the olds of cumulative 25-day rainfall could be used to predict 25 A338 in Berkshire 2000/2001; Robinson et al., 2001) and the occurrence of groundwater flooding in karst systems. 26 railways (e.g. London-Brighton line in November 2000; 27 Binnie Black and Veatch, 2001). The cost and disruption UK policy context 28 caused by groundwater flooding, while not as much nation- 29 ally as fluvial or marine flooding, can still be significant. For Before the Making Space for Water initiative, Defra commis- 30 example, the estimated cost of the relatively localised sioned a study to determine the extent of groundwater 31 2000 Brighton groundwater flooding was d800 000, exclud- flooding (Jacobs, 2004). This study concluded that the 32 ing the cost of the railway closure (Binnie Black and Veatch, Chalk aquifers of south and east England demonstrated the 33 2001). most important manifestation of groundwater flooding in 34 Flooding in the Pang and Lambourn catchments in England. This study developed predictive Groundwater 35 Berkshire during the winter of 2000/2001 is the main focus Emergence Maps based on the proximity of groundwater 36 of this paper, and is discussed in detail below. The 2000/2001 levels to the ground surface in a winter hydrologically 37 flood was paralleled in Chalk catchments in north-west similar to 2000/2001. This study was followed by a specific 38 France. In the Somme catchment, large areas were flooded report on the Chalk aquifers (Jacobs, 2006). 39 for several months by groundwater. This was studied as part The UK flood events of summer 2007 (Marsh and 40 of the EU FLOOD1 project (Adams et al., 2008), which Hannaford, 2007) and their impacts on infrastructure 41 included the Brighton area. Various efforts to model the resulted in the British government commissioning a review 42 flood have been made (e.g. Pinault et al., 2005; Korkmaz (Pitt, 2008). The Pitt review examined all aspects of surface 43 et al., 2009) and recently Habets et al. (2010) published a flooding, and significantly, recommended that the Environ- 44 comparison of four models to reproduce flood conditions in ment Agency of England and Wales (EA) should take a 45 2000/2001. This comparison showed that the models all national responsibility for all flood risk, including ground- 46 overestimated the heads during and after flooding. This was water flooding (Pitt, 2008). Following this recommenda- 47 attributed to an overly simplistic representation of the flow tion, the EA published an assessment of flood risk for 48 processes in the deep unsaturated zone.