Flood alert!

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.

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.

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.

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.

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.

In the early hours of 30 January, the storm that was to wreak all this 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.

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.

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.