Flood
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Since time immemorial, coastal areas
of eastern England and the Netherlands have been inundated repeatedly.
Storm winds have raised sea levels and generated huge waves. Coastal
defences have failed. Agricultural land has been flooded. People
and their livestock have perished. |
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In the south-west of the Netherlands on 18 November 1421, water
from the North Sea swept through 72 villages and 10,000 thousand
people died. Again in 1570, 1825, 1894, 1916 and 1953, disastrous
breaches of Dutch coastal defences occurred. For the people
of the Netherlands, these defences have always been vitally
important: 40 per cent of their country lies below mean sea
level.
Along the coast of eastern England, too, from the Humber
to the Thames, there have been many failures of coastal
defences.
In a storm in 1897, for example, a kilometre and a half of
the shingle spit at Orford Ness in Suffolk was washed away.
And on 6-7 January 1928, a northerly gale raised water levels
in the Thames Estuary so much that disastrous flooding of London
occurred. At several places in the City, Southwark, Westminster
and Hammersmith, water overtopped the embankments and low-lying
riverside districts were flooded. When a section of the embankment
near Lambeth Bridge collapsed, water rushed into the basements
of nearby houses so quickly that people were unable to escape
and 14 were drowned.
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Fig 1: The
coast of eastern England
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The deviation of the observed tide at a given place and time
from the tide that would occur if there were no meteorological
influence is called a surge.
A surge is positive if the water level is higher than the tide
caused only by astronomical forces, negative if lower. Positive
surges occur when water is driven towards a coast, negative when
it is driven away.
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Surges are caused mainly by the action of wind on the surface of
the sea, with barometric pressure a secondary factor. When pressure
decreases by one millibar, sea level rises by one centimetre. Thus,
a deep depression with a central pressure of about 960 mb causes sea
level to rise half a metre above the level it would have been had
pressure been about average (1013 mb). When pressure is above average,
sea level correspondingly falls.
The effect of a strong wind coupled with very low pressure can
be to raise sea level in eastern England more than two metres.
Fortunately, though, large positive surges tend to favour mid-tide.
They rarely coincide with high water.
The strong winds that create surges also generate large waves. Embankments
are usually high enough and other coastal defences sound enough to
protect against all but the highest of surges. However, waves wash
away protective dunes, and batter sea walls relentlessly, weakening
them until they fail. They break over coastal defences, too, undermining
the foundations on the landward side, until structural failure occurs.
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Fig 3:
Waves breaking on the beach. Photo© Environment Agency |
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The greatest surge on record for the North Sea as a whole occurred
on 31 January and 1 February 1953. Its amplitude reached 2.74
m at Southend in Essex, 2.97 m at King's Lynn in Norfolk and 3.36
m in the Netherlands.
Almost 100,000 hectares of eastern England were flooded and 307 people
died. In the Netherlands, 50 dykes burst and 1,800 people drowned.
The flood covered nine per cent of all Dutch agricultural land and
three per cent of the dairy country. The sea reclaimed over 200,000
hectares of polder country.
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Fig 4:
The flooding of 1953. Photos © Environment Agency
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To some extent, the disastrous surge of 1953 was predicted successfully
by the Met Office and the Dutch Surge Warning Service, in that
forecasts of dangerously high water levels were issued several
hours before they occurred. Nevertheless, the committee appointed
by the British government to inquire into the disaster recommended
that a flood-warning organisation be set up. This recommendation
was implemented, the name 'Storm Tide Warning Service' being adopted
later. Its Dutch counterpart had been established soon after the
great surge of January 1916, when the dykes of the Zuyder Zee
were breached in many places and vast areas of the Netherlands
inundated.
The storm that caused the disastrous surge at the end of January
1953 was among the worst to visit the UK in the 20th century.
Hurricane-force winds had blown down more trees in Scotland than
were normally felled in a year. A car ferry, the Princess Victoria,
on passage from Stranraer in Scotland to Larne in Northern Ireland,
sank with the loss of 133 lives. Only 41 of the passengers and
crew survived. From Yorkshire to the Thames Estuary, coastal defences
had been pounded by the sea and given way under the onslaught.
During the afternoon of 31 January, the shingle spit of Spurn
Head in Yorkshire was breached. Soon after darkness fell, Lincolnshire
bore the brunt of the storm. Sand was scoured from beaches and
sand hills, timber-piled dunes were breached, the landward slopes
of embankments were eroded, concrete sea walls crumbled, the promenades
of Mablethorpe and Sutton-on-Sea were wrecked, and saline water
from the North Sea flooded agricultural land.
Later that evening, embankments around The Wash were overtopped and
people were drowned in northern Norfolk. Fifteen died in King's Lynn
and another 65 between there and Hunstanton. At Wells-next-the-Sea,
a 160-ton vessel was left high and dry on the quay.
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Fig
5: Boats washed ashore by
high seas. |
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Surges travel counter-clockwise around the North Sea basin, first
southwards down the western side of the basin, then northwards
up the eastern side. They take about 24 hours to progress from
north-east Scotland to south-west Norway.
In 1953, because many telephone lines in Lincolnshire and Norfolk
had been brought down by the wind, virtually no warnings of the storm's
severity were passed to counties farther south until it was too late.
Suffolk and Essex suffered most.
By midnight, Felixstowe, Harwich and Maldon had been flooded,
with much loss of life. Soon after midnight, the sea walls on
Canvey Island collapsed and 58 people died. At Jaywick in Clacton,
the sea rose a metre in 15 minutes and 35 people drowned.
The surge travelled on. From Tilbury to London's docklands, oil
refineries, factories, cement works, gasworks and electricity
generating stations were flooded and brought to a standstill.
In London's East End, 100 metres of sea wall collapsed, causing
more than 1,000 houses to be inundated and 640,000 cubic metres
of Thames water to flow into the streets of West Ham. The BP oil
refinery on the Isle of Grain was flooded, and so too was the
Naval Dockyard at Sheerness.
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In the early hours of 30 January, the storm that was to wreak so
much havoc was an unremarkable depression with a central pressure
of 996 mb located a little to the south of Iceland. Such a depression
here was not unusual. During that day, however, the depression deepened
rapidly and headed eastwards.
By 1800 UTC on 30 January, it was near the Faeroes, its central pressure
980 mb By 1200 UTC on 31 January, it was centred over the North Sea
between Aberdeenshire and southern Norway and its central pressure
was 968 mb
Meanwhile, a strong ridge of high pressure had built up over the
Atlantic Ocean south of Iceland, the pressure within being more than
1030 mb In the steep pressure gradient that now existed on the western
flanks of the depression, there was a very strong flow from a northerly
point. Winds of Force 10 were reported from exposed parts of Scotland
and northern England and a gust of 56 m/s was measured on the Orkney
Islands. The depression turned south-east and deepened to 966 mb before
filling. By 1200 UTC on 1 February, it lay over northern Germany,
its central pressure now 984 mb.
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Fig 6: East coast floods
of 1953
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In the deep water of the open ocean in
the northern hemisphere, winds drive water 45° to the right of the
wind. In the southern hemisphere, they drive it 45° to the left.
In shallow water, the angle between wind direction and resulting
current is considerably less. The deviation is caused by the effect
of Earth's rotation, the so-called 'Coriolis effect', through which
moving objects are deviated to the right in the northern hemisphere,
left in the southern. In western parts of the southern North Sea,
where the water depth is around 15-25 m, the angle of deviation
is 20-30°. |
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Fig
7: The variation of a wind-driven
current with depth (after Ekman 1905) |
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All day on 31 January, winds blew from the north over western
parts of the North Sea, with a strength of Force 10 or 11. They
drove water south-south-westwards, and generated waves more than
eight metres high. The surge originated in the waters off the
north-east coast of Scotland and was amplified as it travelled
first southwards along the eastern coasts of Scotland and England,
and then north-east along the coast of the Netherlands. It reached
Ijmuiden in the Netherlands around 0400 UTC on 1 February.
Since 1953, there have been other large surges in the North Sea,
among them one, on 12 January 1978, that caused extensive flooding
and damage along the east coast of England from Humberside to
Kent. London came close to disaster, escaping inundation by only
0.5 m, and the enormous steel and rubber floodgates designed to
protect the major London docks were closed for the first time
since their completion in 1972.
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Concern over rising sea levels, isostatic subsidence of south-east
England and the appalling consequences of a major flood in central
London led to the construction of the Thames Flood Barrier near
Woolwich. This was completed in 1982.
Incidentally, the earliest record of a flood in London, dated 1099,
is found in the Anglo-Saxon Chronicle: 'On the festival of St Martin
(11 November), the sea flood sprung up to such a height and did so
much harm as no man remembered that it ever did before'.
Over the years, coastal defences in the Netherlands and eastern
England have been raised and strengthened continually to protect
against storm surges. Our coasts and estuaries are safer now than
they have ever been. Nevertheless, surges remain a threat, as
complete protection against the most extreme can never be guaranteed.
At least the likelihood of being taken by surprise is now rather
low, because weather and surge forecasting systems have improved
greatly in recent years, and the Storm Tide Forecasting Service
has established clear and effective procedures for alerting the
authorities when danger threatens.
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(using terminology in the latest version of the National Geography
Curriculum)
Geographical enquiry and skills
- Changes in coastal environments
- Extension of geographical vocabulary
Knowledge and understanding of places
- How and why changes happen in places
Knowledge and understanding of patterns
and processes
- Relevant to all of this section of the curriculum
Knowledge and understanding of environmental
change
- Describe and explain environmental change and recognise ways
of managing it
Breadth of study
- Geomorphological processes
- Causes and effects of a hazard and human responses to it
- Knowledge of weather and climate
- Issues of topical significance
Locational knowledge
- Parts of the UK
- The seas around the UK
- Reference to another country in Europe
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