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History
1953.
The worst storm surge of the last
century hit the East Coast on the
31st January/1st February, 1953. It
breached flood defences, knocked out
tide guages between the Wash and Southend
and devastated Canvey Island in the
Thames Estuary. On Canvey alone, 58
people died and 10,000 had to be evacuated.
The flooding extended into Docklands
but Central London was spared.
1953
was a wake-up call for the government.
Immediately a committee was set up
to examine the risk to the capital.
Thirteen months on, the Waverley Committee
issued its report recommending that
a storm surge barrier be built across
the Thames.
Delays.
From that point, things moved more
slowly. Committee succeeded committee.
Six possible locations were investigated
and a variety of designs. With potential
expenditure so huge, it is probable
ministers found it convenient to stall.
Ten years passed and other constructions
claimed available funds. The M1 motorway
was built while the Barrier was on
hold.
In
1965, the newly created Greater London
Council took over responsibility for
flooding in the London area. Public
anxiety was intense by this time and
in 1970 work resumed on the Barrier
project. A site at Woolwich reach
was selected and an innovative rotating
gate design.
Cost-benefit.
An early decision had to be reached
concerning degree of protection. To
what height should the Barrier and
associated defences be built? The
result was a compromise between the
engineers and the economists.
Economists
think in terms of cost benefit analysis.
They balance estimated damage against
the degree of probability of the damage
occurring multiplied by the cost of
preventing it. With the help of data
from the 1962 flooding of part of
Hamburg, the Barrier economists reached
an estimated damage figure for the
tidal flooding of London of £2
billion (1966 values, equivalent to
£20 billion today).
The
Barrier was officially opened in May
1984. At a completion cost of £440
million it had run 75% over budget,
chiefly due to inflation through the
1970's. Added to this sum was the
expense of five minor barriers in
the lower Estuary and a further £300
million for the raising and strengthening
of over 70 miles of banks and defences
downstream.
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Technical
Specifications
The
Thames Barrier consists of a line
of reinforced concrete piers spanning
the river at Woolwich Reach and supporting
steel gates. The exposed pier ends
are disguised in curved housings made
of timber clad with stainless steel.
Their foundations are sunk 17 metres
into the chalk.
Gates.
There are four main navigation openings
of 61 metres with rising sector gates
and a further two 31.5 metre openings
also with rising sector gates, for
the use of smaller craft. To allow
for free flow of tide through the
structure, four more 31.5 metre openings
are provided fitted with simple falling
radial gates. In normal conditions
the rising sector gates lie flat in
concrete sills on the river bed to
allow for free passage of river traffic.
The
rising sector gates are hollow stainless
steel structures, the downriver face
curved to reduce load on the operating
mechanisms. The gates are moved by
means of reversible hydraulic rams
and can be held in four different
positions:
Thames
Barrier Gate Mechanism
Gates
in operation
The
main Control Tower, generators and
workshops are located on the south
bank. As a safety precaution there
is a back-up control room on the opposite
side. Two connecting service subways
run through the concrete sills from
bank to bank under the river, providing
access to all piers. In a final emergency,
gates can be manually operated from
the individual pier engine rooms.
Closure.
A decision to close the Barrier is
taken by the Duty Controller on the
basis of data provided by the Met
Office and the Barrier's own computer
model for the Estuary. Closure is
usually four or five hours in advance
of high water. Before the Barrier
is closed, the Port of London is notified
so that shipping in the area can be
warned. Navigation signals on the
Barrier piers change to indicate closure
and up and down river special signboards
are illuminated.
The
operating sequence is designed to
cause minimum interference with the
normal flow of the river. The four
falling radial gates are closed first,
then the main gates are raised starting
from the outside and working in. Each
gate is independently monitored and
operated from the Control Room.
As
at 29th April, 2002 the Thames Barrier
has been closed 276 times. Sixty-four
to protect London from tidal flooding,
three to assist in preventing fluvial
flooding, one for salvage work on
the Marchioness and one for repair
works following the Sand Kite incident.
The other occasions were routine monthly
closures for experiments and tests.
Click
here for Thames
Barrier -Facts and Figures
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Why
The Barrier Is Too Small
The
ideal Barrier. With hindsight,
the approach the Barrier designers
should have taken is clear. They should
have looked at the two components
of a storm surge independently and
built to withstand the unlikely, but
many fear inevitable, future event.
The combination of the highest surge
residual recorded riding on the back
of a high spring tide.
To
make sense of this, take some of the
figures for surge residuals. Using
the storm of 1953 for comparison,
we see the surge residual as a long
drawn out hump, the highest peak occurring
near the beginning and measuring 2.59
metres at Sheerness/Southend.
This
is high for a surge but by no means
the highest on record. At the same
location, a surge in 1894 reached
2.9 metres; one in 1905, 3 metres;
one in 1921 3.35 metres and one in
1943 a massive 3.66 metres.
Surge
Residual Profiles at Sheerness/Southend
1905, 1921, 1943
Despite their great elevations, none
of these surge residuals resulted
in exceptionally raised sea levels
because they all fell away well before
high water.
But
suppose they had not. What if the
peak of the surge of 1943 had coincided
with high water of a mean spring tide,
2.71 metres ODN or more frightening
still one of the highest spring tides,
3.20 metres ODN?
This
would give potential sea levels at
Sheerness/Southend of 6.37 and 6.86
metres ODN. Both hugely in excess
of the 4.69 metres ODN that caused
massive damage in 1953.
The
actual Barrier. It may be that
the ideal approach was just too alarming
or too expensive to consider. Barrier
plans had been shelved often enough
before and the GLC was determined
that this time some form of protection
would be built.
Instead
of looking to the worst combination
of surge residual and astronomical
tide that might happen, the design
team went back to storms that had
actually happened in the past.
Using
historical tide guage records and
a mathematical relationship to obtain
frequency distributions, they extrapolated
probabilities of specified heights
occurring in the future. Based on
these probabilities, they set a height
for the Barrier of 6.9 metres with
adjacent riverbank defences at 7.2
metres.
At
this level, according to the calculations,
the Barrier could be expected to contain
the 1000-year event at least until
the year 2030. The 1000-year event
in the design projections being a
reading of just over 5.5 metres ODN
at Sheerness/Southend.
The
calculations. In addition to the
question mark over approach, there
are concerns over certain features
of the Barrier design calculations.
Three areas in particular merit a
closer look.
1.
Tidal records. Statistics are only
as good as the data they are based
on and historical tide gauge records
are gappy and inconsistent. For the
mouth of the Estuary, crucial to the
frequency distribution on which Barrier
design was based, only 104 'reasonably
complete years' were available between
1819 and 1953.
2.
Date limit of 2030. This relates to
an adjustment for the projected rise
in high water in central London. This
rise has nothing to do with surges.
It is caused by a variety of factors
including change in global-mean sea
level, geological settlement of southern
England due to tectonic movement,
and effects of human disturbance such
as dredging and embanking. Until recently
the rise has been a steady 0.8 metres
per century. As a measure of economy,
Barrier designers added only 0.4 metres
to their calculations to cover the
rise expected over fifty years or
until the year 2030. The joker in
the pack here is global warming.
3.
Allowance for amplification of the
surge as it passes upriver. This funnelling
or 'squeeze' effect is caused by the
trumpet shape of the Estuary. It has
increased with the straightening and
raising of the river banks and scouring
of the bed by currents. Barrier designers
considered they were being generous
in allowing 1.5 metres. A check of
tide tables questions this. While
the difference between high tide levels
at Sheerness/Southend and London Bridge
is normally around a metre, 1.2 metres
is not uncommon for higher spring
tides. Add the 0.3 metre allowance
for wind and wave freeboard and 1.5
metres is already reached. And that
is without the extra input from a
surge.
Taking
the argument one step further, suppose
the allowance is viewed as a percentage
rather than as a finite amount. This
is an equally valid approach and one
which a number of scientists favour.
Rough calculations suggest an amplification
of between 35 and 50%.
Add
35% to the figure to the 5.5 metres
ODN figure for Sheerness/Southend
on which Barrier design was based
and the result is 7.43 metres. Well
over the top of both the Barrier and
surrounding defences.
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Global
Warming
More
unpredictable and extreme systems,
more frequent and violent storms,
more widespread flooding and drought.
Environmentalists have been crying
in the wilderness for years. Now the
world's big insurers are also panicking
about climate change. Premiums against
hurricanes and floods are set to rocket.
Industry statistics show that, compared
to the 1960's, the number of really
big weather disasters has increased
fourfold.
The
culprit is generally accepted to be
global warming, but evidence is hard
to pin down. There is, however, one
specific effect which is readily measurable.
Global mean-sea level. Historically,
global mean-sea level has risen by
around 0.22 metres per century. The
melting of glaciers and polar ice
threatens to accelerate this rise.
Recent forecasts suggest a 0.31 metre
increase. Many environmentalists fear
more.
In
the Barrier design, a global-mean
sea level rise of 0.22 metres was
incorporated in the figure of 0.4
metres for increase in high water
in central London over fifty years.
If the rise turns out to be more than
0.22 metres, Barrier calculations
appear vulnerable.
Taking
the 'most likely' projection, a 0.31
metre rise in global level, the Barrier
design allowance will be exceeded
round about the year 2030. This is
the time when the designers themselves
thought improvements might have to
be made. With the less optimistic
forecast the allowance could be surpassed
as early as 2010.
This
does not mean that flooding is certain,
but it does alter the probabilities
in the design calculation. With the
'most likely' projection, the 1000-year
event becomes a 500-year event by
the year 2050. In other words a doubling
of the probability of occurrence of
an over-Barrier flood in any particular
year. Using the 'worst' projection,
the same state is reached by 2015.
And
one last point: by the year 2070 for
'most likely' and by 2040 for 'worst'
case, the 1000-year event becomes
a 100-year event. This is close to
the original probability of occurrence
of the 1953 flood, deemed an unacceptable
level of risk and justifying construction
of the Thames Barrier.
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