The Flash Flood Problem
Climate change scenarios predict that average summer rainfall will reduce, but that summer rainstorms could become more severe, resulting in an increasing number of summer floods. Most of these will be caused by the failure of urban drainage systems, as in Bishopbriggs in July 2002, but the most severe freshwater floods seem to have several attributes in common:

•  Steep sloping catchments

•  Convergence of two or more weather fronts

•  Static thunderstorms

Some examples of severe flash floods which were all greater than the 1 in 100 year return period: Lynmouth, 1952; Martinstown (Dorset), 1955; Ffestiniog, 1973; Hampstead, 1975; Halifax, 1989; Boscastle, 2004.

In August 2002, Fylingdales, North Yorkshire , had 115mm of rain in 24 hours. In the same month, there were localised floods in the London area, with some places receiving 60mm of rain in just one hour, resulting in the closure of several railway stations. Severe summer rainfall events are quite common, but are they becoming more frequent, as climate change experts predict?

The European Environment Agency (EEA) recently published a report which examines the impacts of Europe 's changing climate. This suggests that between 1975 and 2001, the annual number of flood events increased. According to the report, climate change is likely to increase the frequency of extreme flood events, in particular the frequency of flash floods.

Flash floods occur where torrential rain falls and it cannot be soaked up or drained away. This leads to run-off, with water running over the ground, rather than through it, resulting in flooding.

The Met Office was recently quoted as saying:

"In general terms the number of 'intense' rainfall events during the summer is likely to decrease, but the intensity of the rain on the days that it does so will probably increase. Thus episodes of severe flooding may become more frequent, despite a general trend towards drier summer conditions."

Here are two examples of flash floods:

Lynmouth, 16th August 1952

This was one of the most extreme British weather events of the 20 th century. The catastrophic floods at Lynmouth, north Devon , were as a result of heavy rain on Exmoor on the 15-16 th when an unprecedented 225 mm of rain fell in 22 hours at Longstone Barrow, draining into the West Lyn river (275 mm is estimated to have fallen over parts of Exmoor ). The East and West Lyn rivers reached record levels where they converge near the seafront in town. It rained most of the day over most of Devon , with a seven hour long intense downpour from late afternoon. The flood came in darkness, with a sudden surge of water. There was a loss of 34 lives, 93 buildings were destroyed or severely damaged, 420 people were made homeless, 130 cars were washed into the Bristol Channel , and 28 bridges were swept away. The water moved large rocks, trees, telegraph poles, and cars.

There were subsequent allegations that the severity of the flood was due to government cloud seeding tests. This has never been proved, but cloud seeding experiments in China are reported to have been very effective (so much so, that cloud seeding near Beijing resulted in the city being cut off by heavy snowfall in winter 1999/2000.)

Boscastle, 16th August 2004 .

Boscastle is a picturesque village, popular with tourists, on the north coast of Cornwall , England . Not only was the date the same as at Lynmouth, 52 years earlier, the circumstances of this flood were similar to Lynmouth, in that it was caused by a heavy localised rainstorm over a steep catchment. While the rainfall was not nearly as extreme as at Lynmouth, provisional estimates suggest it was a 50 year return period event. The heavy rain was caused by Westerly winds carrying the remnants of Hurricane Alex , which had picked up vast amounts of water from the Atlantic on its journey towards the UK . Winds hit the peninsula of Cornwall from the sea to the North and South of the peninsula, converging over the hills in the centre of the peninsula, pushing clouds up to 40,000 feet high. This produced a prolonged, stationary thunderstorm resulting in 130mm (6 inches) of torrential rain in six hours (the monthly average is 70 to 90mm).

Much of Boscastle is within a steep and narrow valley leading to the sea. The River Valency, flows through the valley which is just downstream of a confluence of the River Valency and the River Jordan. On the afternoon of the 16 th August, a torrent of water was reportedly initially dammed by a car which became lodged under a bridge, but then the water suddenly broke through and smashed into the village without warning. Within 15 minutes, the water was inside riverside properties and the road bridge in the centre of the village had been swept away. One holiday maker, Wayne Grundy, was quoted as saying ". I actually saw a caravan and two cars washed over the top of a bridge. . (the) tea shop just filled with water. One minute they were ankle deep and the next they were up to their chests."

120 people were rescued from rooftops, trees and vehicles by a fleet of seven helicopters. Over 50 cars were washed into the sea. The harbour's seafront was partly destroyed.

Up to 1,000 people were affected in Boscastle and the neighbouring village of Camelford , as the River Camel also burst its banks. Tintagel, and Crackington Haven were also affected. Fortunately there were no deaths or serious injuries, probably mainly because the event happened during daylight and near an RAF base, a Royal Navy base, and an HM Coastguard base each with rescue helicopters and highly trained search and rescue crews who were at the scene within 20 minutes of the first notification. They did an amazing job. However the effects of shock after such an event could be long lasting.

A report on the Boscastle floods contains many stories of heroism and community spirit, it also shows some interesting features:

•  It was pouring with rain and many people went to the Long Bar of the Wellington Hotel. Few were aware that this 400 year old hotel is built on top of a culvert containing the River Jordan near its confluence with the Valency. The owner and a former employee did know and checked the culvert. They decided to evacuate the crowded bar and the rooms upstairs, but people were not keen to go out into the rain. Meanwhile the back wall of the bar had become a dam, and the water was rising fast. Minutes after the hotel was evacuated, thousands of tons of water came through the first floor windows at the back and suddenly filled the bar area with mud and water 4 metres deep. Many could have lost their lives.

•  The village was full of tourists, and some 50 cars floated away in the current, not only that, witnesses spoke of a strong smell of petrol from the waterlogged cars, so there would have been a fire risk.

•  There were some mini skips used for collecting bottles, clothing etc for recycling. These floated away and could have caused serious damage. A giant oven from the bakery also floated away, smashing through the walls of the shop.

One resident remembered the 1958 flood and when she saw the river rising she evacuated people from her shop, deployed flood boards and sandbags and went to higher ground in good time. Her shop which was 300 years old, crumbled and was completely washed away. If she had not acted so quickly, many could have drowned

Insurance aspects

Each of these major summer floods happened with very little warning and involved rivers described as "Flashy" meaning prone to flash floods. This is why the author's insurance template (See Appendix 1) calls for a 500 year return period standard for the location of new properties near flashy rivers.

«back to top«