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Origins
Storm
surges are huge elevations in sea
level which strike unpredictably during
the winter months. Those travelling
down the North Sea pose a particular
threat to the East Coast of the U.K.
and Thames Estuary. The Met Office
Storm Tide Forecasting Service records
around twenty East Coast surge events
a year.
Most
of these storm surges arise around
the Grand Banks, off the coast of
Canada. The warm Gulf Stream meets
the cold Labrador current in this
region and areas of low atmospheric
pressure (depressions) form.
Beneath
a depression the sea is sucked up
into a hump, small at first but stretching
over a footprint of perhaps a thousand
miles diameter. Winds associated with
the depression drive the hump eastwards
across the Atlantic, the dynamic effect
magnifying the height of the hump
as it travels.
Storm
surges often pass unnoticed between
Iceland and Scandinavia but ccasionally
one veers into the confined space
of the North Sea. With north westerly
winds blowing on its flank the hump
is forced between the converging coastlines
of England and continental Europe.
This funnelling effect further increases
the height.
Thrown
by the rotation of the earth on to
the East Coast, the storm surge batters
sea defences from Scotland down to
the mouth of the Thames. Entering
the trumpet-shaped Estuary it is squeezed
once again as it roars up the river
to London.
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Composition
The
height of a storm surge, as measured
by a tide gauge, is made up of two
components: the normal astronomical
tide for the location plus the extra
hump of the surge.
Tide
is controlled by gravitational forces
and has a twice daily rhythm of highs
and lows. At any given location, the
levels of high and low tide are not
constant. The highest of spring tides
occur every fifteen days, at or near
the new and full moon, the lowest
or neap tides when the moon is in
its first or third quarter. The highest
tides of all are alternate spring
tides when the gravitational forces
of sun and moon are working together.
The
worst storm surge of the 20th century,
31st Jan - 1st Feb, 1953
The
hump of the surge is known as the
surge residual. It is best visualised
as a graph, rising to a peak over
time and then falling away. Time is
shown in terms of hours before high
water and a surge of long duration
may last over two or more high waters.
The
interval between surge peak and high
water is absolutely critical. A surge
peaking at low tide is much less likely
to reach danger level than one peaking
at or near high tide. The nature of
the tide is also important. A neap
tide is less likely to cause problems
than a spring tide.
The
nightmare scenario is the exact coincidence
of a large surge peak with the highest
of spring tides.
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Surge-Tide
Interaction
The
odds are against the peak of a surge
coinciding with high water. The normal
pattern is for the peak to arrive
around four hours beforehand. This
is due to an effect known as surge-tide
interaction which is thought to be
brought about by friction, although
the mechanism is not well understood.
Politicians
and others in authority are apt to
speak of surge-tide interaction as
a proven fact. This is dangerous.
It does not always occur.
A
search through the records turns up
numerous examples of surge peaks arriving
nearer than expected to high tide.
In 1994 the Met Office recorded a
peak at Sheerness just ninety minutes
before high water. 'Surge tide interaction',
the Annual Report clearly states,
'did not appear to be in evidence
on this occasion.'
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Forecasting
The
Storm Tide Forecasting Service.
STFS is part of the Environment Monitoring
and Response Centre at the Met Office.
Its responsibilities include the forecasting
of sea levels and the issuing of primary
alerts to the Environment Agency regions
when danger levels are threatened.
The EA regions then operate a secondary,
local, warning service. Where the
Thames Barrier is concerned, the Met
Office signs off at Southend and Barrier
Control, with its specialized knowledge
of the Estuary, takes over.
The
first step in forecasting is the network
of forty five tide guages round the
country. These are maintained by the
Proudman Oceanographic Laboratory
at Liverpool and are linked by an
interactive data logging and transmission
system to which the STFS has access.
The gauges relevant to East Coast
forecasting go from Wick in Scotland
down to Southend at the mouth of the
Thames Estuary.
Tidal
forecasts are based on a computer
model developed for the Met Office
by the Proudman Laboratory. The model
is run four times a day using forcing
data on winds and pressure fields
as well as information from the tide
gauge network. Its reliability is
constantly improving with increased
computing power and refinements in
the collection of forcing data. There
is also an international exchange
of data with other North Sea countries.
'The
tide that did not go out'. Occasionally
a surge does not behave as predicted.
This is a cause of real concern to
the STFS because it upsets forecasts
and leaves EA regions unprepared.
An
example is the East Coast surge of
19th to 21st February 1993. At Aberdeen
the surge was already beginning to
exceed the model. It continued to
do so at Newcastle, Hull and Immingham.
By Cromer in Norfolk it was almost
double its forecast height and this
pattern continued past Felixstowe
in Essex.
At
the last moment the surge began to
conform. When it reached Southend,
the gateway to the Estuary, it was
back to forecast levels. Still massive
but a near miss, not a disaster.
Progress
of East Coast Surge, 19th-21st February,
1993
(click
for larger graph)
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