[LMB] Why the London flood barrier is likely to get higher
alayne at twobikes.ottawa.on.ca
alayne at twobikes.ottawa.on.ca
Tue Aug 7 18:14:14 BST 2007
On topic, because of the link to Miles:
Thursday, 12 July 2007, 21:05 GMT 22:05 UK
London's small but relentless dip
By Jonathan Amos
Science reporter, BBC News
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
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.
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
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
"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."
INTERFEROMETRIC SYNTHETIC APERTURE RADAR (InSAR)
1. Envisat beams radar signal to Earth. Analysis relies on an InSAR
variant known as Persistent Scatterer Interferometry
2. Persistent scatterers are typically roofs, metal & concrete objects.
They can be consistently identified on a sequence of images
3. Comparing the changed components of the return signals - the 'phase
shift' - from many passes reveals the land movement
Change is relative to start of European satellite data archive (1991)
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
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).
HOW LONDON'S DEFENCES HAVE RESPONDED TO RISING WATERS
Thames wall (Environment Agency)
1. Lowest section of wall constructed as a result of 1879 Flood Act
2. Update to Flood Act before end of 19th Century raised wall further
3. 1928 flood and subsequent 1930 Flood Act lifted defences again
4. Interim addition after 1953 flood while Thames Barrier was built
alayne at twobikes.ottawa.on.ca
"He that would make his own liberty secure, must guard even his enemy from
opposition; for if he violates this duty he establishes a precedent that
will reach himself." -- Thomas Paine
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