RIVER

16 04 2010

II. ISSUES

NATURE VS. MAN

The Walbrook’s persistent flow, witnessed by explorers of the London Bridge Sewer, who encounter streams breaking through the walls of the sewer, is a reminder that natural systems frequently exist in conflict with systems designed by mankind.  London’s urban fabric initial covering-over, and then total consumption, of the River Walbrook is merely an echo of the greater conflict of a bustling metropolis built on the flood plain of the mighty River Thames.

To begin, London’s unique geology makes water drainage a major issue for human settlement.  Lying in the middle of the London Chalk Basin, which is permeable to water and indeed contains a large aquifer, London sits atop a layer of clay, loam, and other impermeable soils deposited by alluvial activity.  Thus, any water which falls in this area, either by river action or from precipitation, cannot penetrate the sediment and must run along the surface until it reaches the Thames.  A natural landscape covered by soil and plants creates marshland to allow for surface flow, or absorb rain and runoff.  Once this natural landscape is removed and replaced with a city, this natural capacity to accommodate water buildup is destroyed.

Illustration of the sediment beds beneath the ThamesIllustration of the sediment beds beneath the Thames

This leaves London, generally, in a bad position, as its paved surfaces have replaced natural infiltration points, and as much of it sits on London clay, which is largely impermeable to water.  If a flood were to ever occur, the city’s inability to move water back into the river would result in destructive “pooling” in which water remains essentially trapped in one area, soaking into buildings and causing more damage than it would have if it had naturally dissipated after a flood.

FLOOD-PRONE LONDON

Flooding is an increasingly important topic for London.  Global climate change, likely caused by human interruption of natural systems, cycles, and balances, is responsible for rising sea levels, which directly impact the severity of floods in the Thames estuary.  Despite a high projection of 1 meter of sea level rise within the next 100 years, the Thames Barrier, completed in 1982, is expected to protect London from increasingly intense flooding until 2070.

The London flood plainThe London flood plain

Red areas indicate proposed or under-construction housing as part  of Thames Gateway.Red areas indicate proposed or under-construction housing as part of Thames Gateway.

barrier_thames copyGreen areas indicate natural marshland in the Thames floodplain.

However these projections begin to be impacted by human activity yet again.  An initiative to build 120,000 new homes along the Thames, known as the Thames Gateway, will be built almost exclusively within the Thames floodplain.  Much of this area is made up of saltwater estuaries which are crucial buffer zones during tidal storm surges, and construction or other disturbance of these natural flood barriers can have grave consequences for nearby urban areas.  (Americans will recall that development along the estuaries of the Louisiana Gulf was one of the key factors in the devastating flooding of New Orleans in 2005.)  As these ill-concieved building programs get underway, the impact they will have on London’s flood vulnerability remains relatively unknown.

THE LONDON WATER TABLE

Perhaps an even more pressing issue than the possible breach of London by a tidal storm surge, is that of London’s advancing water table.  The water table is the water stored underground in the spaces between particles of soil or sediment, and is effected by rainfall and flow of bodies of water nearby.  Before 1750, London had a water table which was very close to the surface, which is unsurprising as the city was sited on the former marshes and estuaries of the River Thames.  Over the following years, the Industrial Revolution occurred, leading to a boom in factory building in and around London.  The water demands of this industry required large levels of water-removal (abstraction) from the ground water, resulting in a fall in the water table.

Rate of change in groundwater levels, Jan 2000 vs. Jan 2008.Rate of change in groundwater levels, Jan 2000 vs. Jan 2008.

Since the 1960’s, there has been a mass exodus of industry from London, and with the factories went the water demand.  The lack of abstraction led to a precipitous rise in the water table over the following 40 years, leading to major concern in the late 1990’s about the future of the city.  At one point in 1997, some estimates placed the water table at a mere 20 meters below the surface of the City of London, rising at a rate of 2-3 meters a year.

Section through the Thames, showing water table levels.Section through the Thames, showing water table levels.

This would have been disastrous if allowed to continue, but emergency approval of new abstraction licenses led to a number of new pumps being installed in the central London area, mainly open-loop Ground Source Heat Pumps (GSHPs) which exchange ground water’s geothermal heating or cooling properties before returning the water to the ground, quite frequently in the Lea Valley.  While this has temporarily slowed the rise of London’s water table, it does not offer a sustainable strategy for how London can work with – as opposed to against – its water supplies.  It will not be long before the pumping rate falls behind that of the rising water table, or before pumping simply becomes an unworkable financial or technical proposition.

UNCOVERING THE RIVER

Designing the harmonious cohabitation of the city and nature is crucial to London’s survival in the future.  Suffice it to say, this goes beyond banning building in the precious Thames estuary – but cuts to the very attitude with which we treat the natural landscape within the confines of the city.

In this context, it is interesting to consider the proposal by Boris Johnson, and currently in development by the Environment Agency, to uncover and rehabilitate London’s “lost rivers,” with the idea that their presence in the city will increase the Londoner’s quality-of-life, and increase London’s cache in the global real estate market.  While it is undeniable that the restoration of rivers including the Walbrook would have aesthetic benefits, we believe that, executed correctly, the return of London’s lost rivers could provide the city with its own internal, natural landscape which can benefit the city aesthetically, socially, economically, and, most of all, ecologically.

On the “Future” page, we present three different schemes, each with a unique vision of how London might receive its lost rivers into its current urban fabric.  We present them as sketches of possible proposals, in order to gauge the opinions and views of the public, and hope that you will take some time to look them over and give us some feedback.  Thank you.


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