London's Small But Relentless Dip

By Princessa

A new assessment of land and sea level changes in London and the Thames estuary has been made by scientists.
Their study - based on tide gauge, GPS, gravity, and satellite measurements - shows a general pattern of subsidence of 1-2mm a year.

With waters rising in the region by about 1mm a year, the combined effect is a 2-3mm a year rise in sea level with respect to the land.

The study has been conducted for the Environment Agency.

The information is critical to the planning of London's sea defences in the face of climate-driven ocean rise. The region is home to 1.3 million people and has a property value put at more than £80bn.

These numbers are set to increase substantially as the capital, together with the estuary counties of Kent and Essex, look to expand development ahead of, and beyond, the 2012 Olympics.

The 300km of tidal defences including embankments, walls, gates and barriers will, at some stage, have to be adapted or moved, or new types of defences created that make better use of the natural floodplain.

London's key defensive installation, the Thames Barrier at Woolwich, also faces upgrading.

The new housing and business developments in the tidal floodplain, behind those defences, are also challenged to be located, designed and built to manage the increasing risk of flooding.

Engineers would like to know where improvements should be prioritised and on what timescale. "Monitoring of the estuary will give us a really good understanding of the likely trajectory in terms of risk," said Owen Tarrant, from the Environment Agency's TE2100 Project.

"The way that risk evolves through the century will not only affect the timing of the implementation of the options, but it will also affect the identification of the preferred options," he told BBC News.


See how defences have been raised in the past
Finer scale

The new assessment of land and sea level changes has been led by Dr Richard Bingley, from the Institute of Engineering Surveying & Space Geodesy at the University of Nottingham.

He has recruited researchers from a range of institutions and disciplines.

The team's intention has been to draw together data sets from different measurement approaches, to get a fuller picture of how the Thames region is moving over time.

Dr Bingley's own area of expertise is with the UK's scientific Global Positioning System (GPS) stations, which can, after much processing and analysis, sense millimetric changes in land movement.

Their data has been combined with readings from the absolute gravimeters run by the Proudman Oceanographic Laboratory to give detailed point trends. And this information has then been further combined with an InSAR analysis by Nigel Press Associates (NPA) of radar measurements from Europe's Envisat and ERS satellites.

The result is a broad picture of land deformation across the Thames region as whole.

The investigation confirms geologic studies that show the Earth's crust is still responding to the loss of the heavy ice sheet which covered much of Britain more than 10,000 years ago - with southeast England, including London, slowly sinking.

"Britain as a whole was already quite well understood," explained Dr Bingley. "We knew the north was rising and the south was subsiding; but without the work we've done we'd only have had a single figure for the Thames Estuary.

"Through the use of InSAR we can extrapolate from a few scattered GPS stations to almost a million points spread throughout the region so we see things on a much finer scale; we can show domains of movement and how - in some respects - they are restricted to quite close to the estuary, but of course that's where the flood defences are going to be."

The land subsidence - of the order of one or two millimetres per year - has to be combined with the measurements taken by tide gauges to give a true picture of sea level rise. Dr Bingley and colleagues have now done this for the Thames - and it equates to a year-on-year 2-3mm increase.

The new maps of land movement have been analysed by geologists to assess which rocks and sediments are likely to experience further descent. Some are relatively easily explained, such as the continued settlement of recent, or Holocene, deposits that line the river.

Some dips relate to water extraction by pumping stations, and it is even possible to see the settlement of land above underground construction projects such as the Jubilee Tube line extension and an electricity tunnel between Battersea and Putney.

But there are also some surprises, with a land rise evident in particular around Northolt in the northwest of London.

"London lies at the junction of three deeply-buried geological terrains," explained Dr Don Aldiss from the British Geological Survey.

"In the northwest, deep under Northolt, is part of what we call the Midlands Microcraton. These are among the oldest rocks in England. The uplift around Northolt is not massive - less than half a millimetre per year - but it's real. It seems to be some kind of edge effect or bulging where the rocks from the south meet the microcraton."

Tracing the millimetric trend in land movement has been an extremely challenging task, especially given the far larger day-to-day movements that can occur.

London itself will rock by 10mm, twice a day, with loading from ocean tides. The seasons also alternately load and unload the ground, making the Earth's crust "breathe" up and down over a longer period.

All of these confounding variables have to be taken into account - something that has proved especially testing when using GPS to sense millimetric changes in land movement.

"Within the GPS data you have to model loading effects and also account for atmospheric effects on the GPS signals. We have done this and have not only reduced the errors, but we now understand better what's in those error bars," explained Dr Norman Teferle from the University of Nottingham.

The full scientific report (including the images presented here) has been published as Defra/Environment Agency Joint R&D FCERM Programme R&D Technical Report FD2319/TR and can be downloaded from the Defra/EA Joint R&D FCERM Programme website (see internet links).