[LMB] Why the London flood barrier is likely to get higher

Tue Aug 7 18:14:14 BST 2007

On topic, because of the link to Miles:

http://news.bbc.co.uk/1/hi/sci/tech/6231334.stm

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 
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.

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."

--------------------------------------------------------------------
INTERFEROMETRIC SYNTHETIC APERTURE RADAR (InSAR)
Graphic: 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 
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).

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 McGregor
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