Flood Risks In The UK

Flood Risks in the UK

Flooding in the UK can be caused by:

• Watercourse Overflow - when a river or stream overspills its normal course onto surrounding land
• Flash Flooding/Surface Run-off - when sudden excessive rainfall exceeds the capability of the land or existing drainage/river systems
• Breaches in man made obstacles such as sea defences, dams, dykes or levees
• Storm Surge - when intense storms have the effect of raising the sea level. These often cause flooding when they coincide with spring tides
• Spring Tides - the phases of the lunar cycle when tides are at their highest
• Tsunamis - tidal waves

Watercourse Overflow

Historically, settlements on which almost all our inland towns and cities are founded were located by rivers. Rivers were important as transport routes, trade arteries and river crossings were important strategically. This means that at least part of almost every town in the UK is under threat.

Periodic floods occur naturally on rivers, covering an area known as the flood plain. These floods usually result from heavy rain, sometimes combined with melting snow, which causes the rivers to overflow their banks. This is part of the natural annual cycle of a river’s behaviour.

Traditionally, farmers would graze their livestock on river meadows in the spring, take a hay crop from them in summer whilst moving their animals to higher ground for winter when most flood plain inundation occurs.

However, over the last 250 years we have sought to restrict the flow of rivers by building embankments, in some cases straightening their natural courses. In addition we have built on and continue to build on the flood plains.

It is only in the last 50 years that we have recognised the futility, like Canute, of trying to tame these natural processes and the Department for Environment, Farming and Rural Affairs (Defra) now issues planning guidelines that discourages development on river flood plains. But without any teeth to enforce these recommendations they are largely ignored and flood plain development continues at pace.

Flash Flooding

Flash flooding occurs when a sudden large amount of rainfall occurs beyond the capabilities of the ground and water courses. These floods often occur when storms hit already saturated ground so that all the water runs off into the drainage or river systems.

Surface run-off is exacerbated by the increasing proportion of our cities and towns which are covered by concrete, tarmac and roofing and as such unable to absorb any rainwater.

Flash flooding can also occur when a water main bursts, although this type of flood will mostly happen in basements and cellars if the the normal drainage systems are blocked or unable to cope.

On 15 and 16 August 1952, a storm of tropical intensity broke over south-west England, depositing 229 mm (9 inches) of rain within 24 hours on an already waterlogged Exmoor. Debris-laden floodwaters cascaded down the north escarpment of the moor, converging upon the village of Lynmouth.

The river Lyn through the town had been culverted to gain land for business premises; this culvert soon choked with flood debris, and the river flowed through the town itself with much of the debris being boulders and trees.

Overnight, over 100 buildings were destroyed or seriously damaged along with 29 bridges, and 38 cars were washed out to sea. In total, 34 people died, with a further 420 made homeless.

Exactly 52 years later, on 16 August 2004 over 7 inches (181mm) of rain fell over the high ground just inland from the village of Boscastle.In Boscastle itself, 89 mm (3.5 inches) of rain was recorded in 60 minutes. The torrential rain led to a 2m rise in river levels in one hour. A 3m wave — believed to have been triggered by water pooling behind debris caught under a bridge, and then being suddenly released as the bridge collapsed — surged down the main road, travelling at an estimated 40 miles per hour Two million tonnes of water flowed through Boscastle that day.

Around 50 cars and 6 buildings were washed into the sea, along with uprooted trees and other debris. In an operation lasting from mid-afternoon until 2:30am, a fleet of seven helicopters rescued about 150 people clinging to trees and the roofs of buildings and cars. Amazingly, no major injuries or loss of life were reported.

In addition to six buildings being washed away, many other buildings suffered serious flood damage, were reported unsafe and later demolished. The ground floors of other buildings were covered with inches of mud washed in by the flood waters.

Breaches

If man made water defences are breached, then the flood is more destructive than if the water had gradually inundated the land beyond because the build-up of pressure and volume of water creates an explosive flood of high volume and velocity.

One of the biggest man-made disasters in British history, the breach and collapse of the Dale Dyke Dam on 11th March, 1864 released 650 million gallons of water which roared down the Loxley valley and into Sheffield, killing 270 people and uncounted numbers of livestock. It destroyed 415 dwelling houses, 106 factories/shops, 64 other buildings, 20 bridges and 4478 cottage/market gardens.

More recently we can all remember the effect of a breach in the Mississippi levee caused by the 9 metre Storm Surge associated with hurricane Katrina that flooded 80% of New Orleans in August 2005. 1,836 people are known to have died, a further 705 are still listed as missing and even by November 2006 sections of the city were still not open to residents.

Storm Surge

A storm surge is an onshore gush of water associated with a low pressure weather system. Storm surge is caused primarily by high winds pushing on the ocean's surface. The wind causes the water to pile up higher than the ordinary sea level. Low pressure at the center of a weather system also has a small secondary effect, as can the depth of the body of water. A drop in atmospheric pressure of one millibar can raise the sea level one millemetre.

It is this combined effect of low pressure and persistent wind over a shallow water body which is the most common cause of storm surge flooding problems. In the UK storm surges rarely rise above 2 metres and coastal flood defences are designed to cope with these surges, but in the floods of 1953 water reached 3.3 metres (see Spring Tides below).

Spring Tides

Around the time of a new and full moon when the sun, moon and earth form a line (a condition known as syzygy), tidal forces, due to the sun, reinforce those of the moon. Tidal ranges are then at their maximum: this is called a spring tide.

The North Sea flood of 1953 and the associated storm combined to create a major natural disaster which affected the coastlines of the Netherlands and England on the night of 31 January 1953 – 1 February 1953.

A combination of a high spring tide and a severe cyclone caused a storm tide; a tidal surge of the North Sea up to 3.36m, which overwhelmed sea defences and caused extensive flooding.

In the UK, 307 people were drowned. Thirty thousand people were evacuated and over 20,000 homes were flooded and even more were left homeless. Nearly 100,000 hectares of Eastern England were flooded and over 40,000 head of livestock were lost as coastal defences were breached in over a thousand places all the way from Lincolnshire to Kent, including parts of London. The surge came perilously close to overtopping the embankments in London which would have flooded the tube system. It took nine months to drain all the flood water and make temporary repairs to the sea walls.

Tsunamis

Although they are rare events, the UK is not immune from Tsunamis. Some 7-8000 years ago a 20 metre high wave hit Scotland caused by a landslip in Norway, and the balance of probability is that the Great Flood of 1607 that devastated the coasts of the Bristol Channel and caused 2,000 deaths was a tsunami triggered by an earthquake in a seismic fault off the coast of southern Ireland.

Eyewitness accounts of the disaster, published in six different pamphlets of the time, told of "huge and mighty hills of water" advancing at a speed "faster than a greyhound can run" and only receding 10 days later. Cardiff was the most badly affected town. The coasts of Devon and the Somerset Levels as far inland as Glastonbury Tor, 14 miles from the coast, were also affected. There remain plaques up to 8ft above ground level to show how high the waters rose on the walls of surviving churches.

In their 2001 paper published in the American Geophysical Union, Steven Ward and Simon Day suggested that during a future eruption, the Cumbre Vieja volcano in La Palma may experience a catastrophic failure of its west flank that would trigger a tsunami capable of travelling across the Atlantic arriving at the East coast of the United States as a 10-25 metre tidal wave. What their model also shows is that, triggered by the same event, a tidal wave of up to 7 metres would reach the southwest of the UK, which, when funnelled by the topography of the Severn estuary could devastate the same areas that were hit in 1607.

Following the tsunami triggered by the Sumatran earthquake on 26 December 2004, Defra commissioned a study into the tsunami risk to the UK which concluded that the risk of a tsunami higher than storm surge levels (2 metres) was extremely low and that although further study and upgrading of warning systems were recommended, no specific tsunami flood defences were required.

More articles about historical floods in the UK:

Floods of 1953: A useful background synopsis about the flood.

The Great Flood of Sheffield: An interesting and comprehensive account by Mick Armitage.

Bryant and Haslett: 1607 coastal tsunami? Archaeology in the Severn Estuary 13 (2002)

Great Flood of 1607: Mike Kohnstamm

The threat posed by Tsunami to the UK: A study commissioned by Defra Flood Management, June 2005