Case study 1: Boscastle
A flash flood occurred in Boscastle on the 16th August 2004. 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.
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.
More information is available from the Environment Agency publication - Living with the risk
Information relating to the susequent flood defence scheme are available on the Environment Agency publicaion scheme
Case study 2: Lynmouth
On the 15th and 16th August 1952 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 is available on the BBC pages
