On August 16 2004 the harbour village of Boscastle in North Cornwall was hit by flash floods caused by a downpour of biblical intensity. Massive damage was caused to property there, though miraculously nobody was killed in the disaster. Strangely, just a few miles away next-to-no rain fell that day.
The figures tell the story. During the afternoon of August 16 more than 200mm of rain fell in the area. During a normal August the entire rainfall would be about 75mm, yet in Boscastle itself 89mm of rain fell in one hour. Near the village the storm was equally dramatic, during one 15 minute period 24mm fell two miles away up the valley. At one point a wave 3m high swept through the settlement when debris held up by a bridge gave way to free the floodwater that had built up behind the inadvertent barrage – it is calculated that 140 tonnes of water per second flowed through the village when the storm was at its most powerful.
The flood waters washed about 80 cars into the sea. Boats were ripped from their moorings and hurled miles from land. Many buildings were demolished, several caravans likewise. RAF and Navy helicopters lifted scores of people to safety, locals and visitors plucked from trees and roofs where they had sought safety as the waters hurtled through the town. And of course the less dramatic damage caused by mud seeping into homes and businesses meant that ordinary life was impossible until the clean-up operation was completed. Whilst no loss of life occurred the local economy, based on the summer tourist season, was for a time devastated.
Boscastle is no stranger to flooding: previous inundations occurred there several times in the last century. Since the great flood of 2004 another lesser event has hit the place, with a minor flood in 2007.
The geography of the village goes some way to explaining the events, as it is at the end of a natural funnel system of steep sided valleys, but it is thought to be the unique local meteorology that brings the flood waters down in the intriguingly named Brown Willy effect.
The Brown Willy effect is named after a nearby hill on Bodmin Moor . Meteorologists posit that damp Atlantic westerlies bring water northwards up the Cornish coast when they hit the South Cornish shores, while those that hit the North Cornish shores are drawn southwards, converging and being forced upwards as they do so, this effect being intensified by the high land on Bodmin Moor. This phenomenon is made yet more powerful by the low pressure over the warm summer land dragging in cooler damp breezes from the sea on either side of the peninsula. The rapid air movement means rainwater is deposited quickly, often starting over the high hill of Brown Willy, then moving across the land bringing highly localised downpours and flash-flooding.
An alternative explanation is the concept of a blocking high, an area of high pressure that does not move, preventing other weather systems from tracking with the result that all the rainfall from a blocked storm is dumped in a particular area.
Whatever the explanation, the work of Boscastle’s people to restore their village has been amazing, and the coordinated efforts to design-in flood avoidance measures are impressive. This is one of the country’s most naturally beautiful areas. It is to be fervently hoped man can now control the more extreme consequences of that beautiful nature.
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From Carola on 7th January 2012
I would like to know when this meteorological phenomenon was first named the "Brown Willy effect."