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Basin’. The London Basin is synclinal (U-shaped), with tens of metres of sands, silts and clays overlaying layers of chalk over most of the central part of London. Further away from the centre of London, the chalk comes to the surface (outcrops) forming the higher ground to the north (Chilterns) and to the south (North Downs). This geology is illustrated in Figure 2.1. 4.27 Flooding from groundwater is a general term that can refer to several different sources of flooding. There are two broad categories: • Groundwater flooding when groundwater permeates above ground through a natural process, usually some time after periods of higher than average rainfall. In such cases, flooding mainly occurs in the ‘dry valleys’ of the chalk outcrop areas in south London. • Rising groundwater, when less water is being abstracted and groundwater returns, or ‘rebounds’, to its natural level. This phenomenon occurs either in the deep Case study | Summer 2007 floods Nationally, more than 55,000 homes were flooded in the summer of 2007. The wettest summer since records began in 1766 caused misery for hundreds of thousands of people and more than £3 billion of insured losses. The wet May and early June meant that the ground was saturated and could no longer absorb rainfall. Extreme rainfall in late June and late July caused flash flooding where it fell and chalk overlain by other geological deposits or in the outcrop areas. In the former case, the rise can cause structural damage to deep foundations or flood underground tunnels and service conduits. In the latter, the rebound can flood properties, although more generally it leads to higher flows in the local chalk springs and streams. 4.28 Following prolonged periods of rainfall, groundwater flooding can typically last weeks, and tends to happen late in the winter when groundwater levels reach a peak. It usually results from poor drainage or where proper attention has not been paid to the nature of the site when constructing buildings. High groundwater levels near sewers can cause sewer flooding or make the groundwater flooding more unpleasant and a potential health hazard. 4.29 Our understanding of groundwater flooding is far from complete. In spring 2006, the Environment Agency assumed a strategic then accumulated in rivers to extend the impact to the floodplain. London did not escape the effects of this wet weather, with 1,400 properties experiencing surface water flooding. Whilst this weather was extremely unusual for summer, climate change is predicted to cause wetter winters with more extreme rainfall events and therefore floods of this scale should be expected in the future. 65
66 The Mayor’s draft water strategy overview for monitoring groundwater flooding. This forms part of its wider strategic role in the new flood risk management framework. As part of the strategic overview, the agency will collate records, as well as assess and monitor the problems associated with groundwater flooding. At the same time, the agency will consider ways to incorporate the risk information into its flood risk mapping strategy. This should improve the awareness and understanding particularly for those at risk, as well as for land-use planners and developers. Rising groundwater 4.30 In the latter part of the 19th century, the chalk layers under London offered a pollution-free source of water. With the growth of industry, abstractions rose steadily resulting in a widespread draw down of the groundwater levels. These abstractions gradually declined after World War II as industry began to move away from London. Some of the large public abstractions ceased in the late 1950s and throughout the 1960s. By the late 1970s, the rebound of chalk groundwater levels was becoming noticeable. 4.31 Until very recently, rising groundwater levels were putting London’s underground infrastructure at a real risk from inundation. In 1999, a team started to investigate how best to resolve the problem. It concluded that abstractions from London’s groundwater should increase by 50 million litres of water a day. Since then, the Environment Agency has granted licences to take the surplus groundwater. It is the Agency’s view that the chalk groundwater levels are now stable and no longer pose a significant threat to the underground infrastructure 4.32 Interest in the use of groundwater for cooling buildings has been growing just at the time when the Environment Agency’s concern has shifted from the problem of rising groundwater levels to the need to stabilise groundwater levels. The Environment Agency has to issue an abstraction licence in order to permit groundwater to be used for cooling a building, and the Agency is increasingly requiring the water to be returned to the aquifer after use in order to achieve stability rather than for it to be drained away or used for other purposes. The engineers responsible for the refurbishment of the Royal Festival Hall 72 originally proposed an extensive distribution network for groundwater on the South Bank but this eventually proved impractical because of the need to maintain stable groundwater levels. 4.33 London Underground’s Cooling the Tube programme use a cycle of abstraction, use for cooling and then re-injection of water, so it has a minimal net impact on the actual level of water resource. However, London Underground is in competition with other potential users for access to this scarce resource for cooling. The Environment Agency’s abstraction licensing process operates on a first come, first served basis, which means that London Underground is at risk of finding that the abstraction limit has already been reached in any area where
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