The effects these causes have on the land, the people or buildings and business are devastating. Businesses are destroyed, property destroyed, delayed traffic, death(luckily none were reoperted in Boscastle), crops and fields destroyed and the emergency services are slowed down.
The short, steep valleys of north Cornwall and north Devon are particularly vulnerable to localised summer downpours. They collect water efficiently from the surrounding moors, channel it rapidly into the main stream, and take it all out to sea in a matter of three or four hours. Because of their almost instantaneous response to a sudden cloudburst, these valleys are known in the trade as flashy catchments and they produce true flash floods. Some have speculated that the clearing of natural vegetation from the valley slopes contributed to the abruptness and intensity of the disaster, but it is certain that a destructive flood would have happened anyway. Now we all know what a real flash flood looks like, perhaps our traffic reports and news bulletins will stop using the term to describe 18 inches of water under a railway bridge in Neasden.
The Boscastle flood resulted from a line of convective storms along the north Cornish coast. High-resolution weather forecast models are able to capture this type of development and, in principle, provide several hours warning. One problem is to be able to predict the precise location of the initiation of the most intense storm in the line. Scientists are carrying out research to examine this problem. A first field campaign has taken place this summer in southern England involving state-of-the art measuring techniques with radars, lidars, aircraft and other instruments. The research involves examining where and how convective clouds form and develop into storms. This will help us to better understand the phenomena and develop better ways of forecasting them.
Under conditions of global and regional warming from climate change, a key question is whether the frequency and/or intensity of convective storms leading to flooding will increase or not. Current climate prediction models do not have adequate resolution to describe such local storms explicitly so at the moment there is large uncertainty in our understanding and forecasts of what to expect.
The flood occurred as the result of a prolonged and heavy rainstorm that was almost stationary over the catchment of the River Valency. Both the main river and its tributaries flow in steep sided valleys beneath a plateau rising to some 160m in height. The underlying strata are mostly shales and slate, with some sandstone beds that break down in weathering to give clay-rich soils. These geological features contribute, together with the landforms, to high levels of runoff in storm conditions
While the emergency services have been praised for their swift response to the situation, some people have criticised the Environment Agency for failing to have in place adequate flood warning and protection.
The force of the raging river swept over one hundred cars and three houses away, into the harbour and the open sea. A major rescue operation was launched by the RAF to airlift people to safety.
Also, many people responded with the views on the Boscastle flood on the Internet (left). They expressed they sympathy towards the people who were affected by the flood and how the flood causes great inconvenience and distress for the tourists and the people who live there. They also put forward their opinion of what the causes of the flood were and how well the situation was handled.
Lymouth, North Devon: 15th - 16th August 1952
Fifty years ago, almost to the day of the Boscastle flood, torrential rain caused flash flooding through a number of North Devon towns and villages (known as the Lynmouth Floods) and resulted in the death of 34 people.
15 cm (6 inches) of rain were recorded between the 1st and 14th August in a MET Office rain gauge at Longstone Barrow (Dartmoor) at the head of the West Lyn river. This unsettled weather followed a period of drought affecting most of Southern England.
The catchment area of the Lyn rivers totals over 39 sq. miles, much of which is plateau drained by steep sided combes covered in parts by moorland grasses on wet, peaty ground and by heather and bracken in others. The capacity of the peat to hold water has been reduced over the last century and a half by heavy grazing and burning. In addition there has been much reclamation of surrounding moor and heath in the 19th and early 20th centuries. Since 1947 there have been government grants for agricultural drainage and there is evidence that runoff is more rapid now than before that time. This rapid runoff has been blamed for the apparent increase in flash flooding.
In the 24 hours before the flood, almost 23 cm (9 inches) of rain had fallen on Exmoor. The resulting run off flowed off the moors and into the rivers East and West Lyn which came together as a raging torrent in the steep, narrow valley leading into Lynmouth. Huge bolders exacerbated the flood by creating a dam upstream from Lynmouth, which gave way suddenly causing around 200,000 tonnes of rock to be washed downstream.
34 people were killed, 28 bridges and 93 houses were totally destroyed or damaged beyond repair. 420 people were also left homeless and 66 cars damaged or washed out to sea.
In what way(s) was the Boscastle flood similar to the Lynmouth flood of 1952?
These two floods are very simlar in many ways. One similarity is that they are both cases of Flash flooding. As the name implies, flash floods are sudden and often unpredictable events resulting from: Massive and sudden rainstorms, a rapid snowmelt in the mountains and Failure of natural or man-made water defences.
Although these events remain relatively rare in the UK, flash floods in the UK do occur. Usually resulting from torrential rain, flash floods arise when the ground becomes saturated with water so quickly that it cannot be absorbed - leading to 'run off' or water running over the soil rather than sinking into it. This run-off can cause localised but severe flooding.
Whilst torrential rain is key to the onset of flash flooding, the drainage and topography of the surrounding area determines the scale and impact of the event. In these places Boscastle and Lynmouth, steep-sided valleys accentuated flooding by acting as huge funnels for the run-off and channelled it very quickly down to the sea.
What (if anything) can be done to reduce the risks of flooding in the future?
How can we prevent it? How can we slow it down? The problem is, we cannot stop it because it is nature, but we can slow it down. How? All we need to do is to not build on floodplains, or if you really have to, build it higher up, say on stilts or with a wall in front of the river. We cannot stop the infiltration in the soil and really we don't want to. But we do want to stop the water levels increasing to reduce the amount of flooding. What we could do for the infiltration is to try and reduce the amount of rainfall. We cannot do this either because we cannot change nature. Because of this flooding, people cannot get anywhere, people's homes are destroyed, businesses are destroyed, the emergency services cannot reach any affected area fast because the floodwater has blocked everywhere and crops, fields and farmer's livelihood are affected.
To limit the damage we may perhaps be able to build a dam in the river and let only a little water out at a time, we, as people, could not build industries on floodplains as the concrete would not absorb anything so therefore the soil would become saturated, people in general could refrain from chopping trees down as they absorb the water through their roots from the banks and rivers. This would decrease the flooding rapidly, but they would still flood, because there may be other rivers nearby.
Risk can be reduced by flood defence schemes, which must also be environmentally sensitive. Flood defence options include:
- Increasing storage capacity (by restoring the floodplain; upstream storage; modifying catchment run-off e.g. with the use of Sustainable Drainage Systems.
- Structures (e.g. flood walls or embankments)
- Increased conveyance (e.g. deepening and widening the channel to enable it to carry more water; adding flood relief channels)
- River diversion
- Land use changes
- Flood warning installation, maintenance or improvement
-
Setting back of river and coastal defences, allowing some areas to flood in order to reduce flooding elsewhere and overcoming the problems of habitat loss through 'coastal squeeze'
- Increasing storage capacity (by restoring the floodplain; upstream storage; modifying catchment run-off e.g. with the use of Sustainable Drainage Systems.
- Structures (e.g. flood walls or embankments)
- Increased conveyance (e.g. deepening and widening the channel to enable it to carry more water; adding flood relief channels)
- River diversion
- Land use changes
Flood warning installation, maintenance or improvement
Graphs on Flood Structure and Flood defences
As can be seen in the graph and table below, the South West has a slightly below average number of defences in fair, good or very good condition (90% compared to the national average of 93.8%). Only the North East performed worse.
Just 10% of all defences surveyed were in poor or very poor condition however, although this was below the national average of 6.1%).
As can be seen in the graph and table below, the South West has the second lowest number of structures in fair, good or very good condition (90%, which is also slightly below the national average of 92.4%). Just 10% of all structures surveyed were in poor or very poor condition (although this is again below the national average of 7.5%).
