London's resistance to flooding is deteriorating because of ever increasing development on its flood plains, according to research being carried out at Middlesex University.
In a lecture last week, Dennis Parker, head of the university's school of geography and environmental management, said that although the Thames barrier provides a high level of protection, the docklands redevelopment has poured investment into the Thames flood plain, creating a lot of building.
The conversion of natural surfaces to concrete means that the process of water infiltration has been reduced. The water runs off the concrete and this has significantly increased the chances of flooding in the centre of London.
According to Professor Parker, a member of the university's flood hazard research centre, the London "megacity" grew rapidly during the 1970s and 1980s because of this development, particularly along the M4 and Thames valley towards Reading. In Maidenhead, for example, the number of properties built on the Thames flood plain doubled to more than 3,000.
Professor Parker says that while the risk of tidal flooding has been substantially cut by the Thames barrier, the river and its tributaries are liable to flood after prolonged and heavy rain. Tidal flooding can also coincide with river flooding heightening the risk.
He said: "In the British Isles the frequency of days with thunderstorms is highest in London and there is now quite a high risk of flash flooding caused by thunderstorms." The risk is being exacerbated by rising sea levels which are likely to continue because of global warming. Underground flooding in London has already worsened because of rising water tables associated with industrial change.
He is also concerned that replacement of London's infrastructure is not keeping pace with the rate at which it is crumbling: "This makes London increasingly vulnerable to severe disruption and this is now being frequently highlighted by flooding. The capacity of London's old combined storm drains and sewer system is now insufficient to take the volumes of water generated by thunderstorms."
Severe flash flooding results either along river valleys or almost anywhere where drains are inadequate. "It is difficult to provide a public warning of flooding from thunderstorms over London, and the tributaries of the Thames can rise dramatically within the space of 15 to 30 minutes."
Professor Parker said hazard management needs to be integrated with urban policy, but Government policy on both of these issues is incoherent. He said: "Increased hazard exposure appears to be the dominant agent forcing increased disaster potential in London. Both hazard exposure and vulnerability have been allowed to grow where they might have been checked."