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Quick links:
What are flash flooding events? Causes and impacts of flash flooding - links More information about flooding Useful websites:Flood risk & insurance - Environment Agency Climate change in the South West Boscastle flooding update - North Devon District Council Flooding in North Cornwall - MET Office Climate in the South West - MET Office Why flash flooding occurs - BBC Boscastle & other regional floods explained - Wise Weather |
Flash flooding eventsWhat are flash flooding events?As the name implies, flash floods are sudden and often unpredictable events resulting from:
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 places such as Boscastle or Lynmouth, steep-sided valleys accentuated flooding by acting as huge funnels for the run-off and channelled it very quickly down to the sea. See image below for a pictorial representation of how flash flooding occurs:
Flash flooding can be intensified by:
More information about the causes and impacts of flash flooding:
Boscastle, Cornwall (16th August 2004)Images of the Boscastle Flooding
On the run-up to the flood some 200 millimetres (mm) (8 inches) of rain fell in 24 hours. Most of this fell in a 5 hour period on Monday, August 16th with peak intensities of over 300 mm per hour (5 mm per minute). However the storm was very localised and four of the nearest 10 rain gauges - all within a few miles of Boscastle - showed less than 3 mm. Map showing radar rainfall accumulations - North Cornwall 16th August 2004
According to the MET Office, the trigger mechanism for this storm appeared to be the convergence of winds along the coast and the high ground in the local area which also helped to generate heavy showers, which were then exacerbated by the local topography around Boscastle. At the peak of the flood, between 5 and 6pm on August 16th, the flow rate in the River Valency was 140 tonnes per second. The flood water rampaged through the village taking cars, trees and other debris with it at speeds in excess of 4 metres per second (10 miles per hour). An estimated 2 million tonnes (440 million gallons) of water flowed through Boscastle that day. More than 100 people were airlifted by the rescue services from rooftops, trees and on cars where they had clambered to safety. For more information about the Boscastle floods and useful contacts if you have been affected visit the North Devon District Council flooding update pages Lymouth, North Devon: 15th - 16th August 1952Fifty two 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. According to the MET Office, 15 cm (6 inches) of rain were recorded between the 1st and 14th August in a MET Office rain gauge at Longstone Barrow (Exmoor) 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 (Exmoor National Park Authority). 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. More information about flooding
Visit the Environment Agency's Floodline web pages for information about: |
Flash flooding can be very hard to predict
as it often occurs within hours of the rain falling. However, the Environment
Agency's Floodline should be your first stop for flooding news and information.