BOSCASTLE FLASH FLOOD, CORNWALL, 2004
Summary
Heavy rainfall during the afternoon of the 16th August 2004 dumped 75mm of rain upstream from Boscastle in just two hours. The village sits immediately downstream of the confluence of the Rivers Valency and Jordan, whose water levels rose by 2 metres in a single hour. A further 3 metre high wave was released down the valley after a bridge, which had trapped debris and water, collapsed. The resulting wall of water, which the met office believed to be in the region of two billion litres, travelled at a speed of 4m/s causing structural damage to buildings, and sweeping cars out to sea. Miraculously, nobody died in Boscastle that day, and on top of the expenses for repairing the damage, a further
£10 million was spent on protecting the village from future floods. The flood has been described as a
1-in-400 year event.
Heavy rainfall during the afternoon of the 16th August 2004 dumped 75mm of rain upstream from Boscastle in just two hours. The village sits immediately downstream of the confluence of the Rivers Valency and Jordan, whose water levels rose by 2 metres in a single hour. A further 3 metre high wave was released down the valley after a bridge, which had trapped debris and water, collapsed. The resulting wall of water, which the met office believed to be in the region of two billion litres, travelled at a speed of 4m/s causing structural damage to buildings, and sweeping cars out to sea. Miraculously, nobody died in Boscastle that day, and on top of the expenses for repairing the damage, a further
£10 million was spent on protecting the village from future floods. The flood has been described as a
1-in-400 year event.
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Physical factors contributing to the flood
- Heavy rainfall, nearly twice the monthly amount in one hour according to the BBC (130mm, monthly average 60-70mm)
- The water table was already high, which encouraged groundwater to flow sooner.
- A total of three rivers, the Valency, the Jordan and the Paradise converge in close proximity to the village of Boscastle.
- Each of the three rivers flow through steep sided valleys, meaning there is less soil to hold moisture, and increases surface run-off.
- The drainage basin was small for the volume of water received. Just 40 square kilometres, which struggled to hold over 2 billion litres of water.
- The bedrock in the area is largely impermeable, which increases surface run-off. To the north of Boscastle, the Boscastle formation is predominantly impermeable mudstones, the Yeolmbridge formation to the the south is semi-permeable sandstones and limestones and the Tredorn Slate formation to the south west of the village is also mostly impermeable.
- The heavy rain coincided with high tide in the bay which restricted the rate at which the water could exit the river.
Human factors contributing to the flood
- Boscastle is built on a very narrow floodplain, and has grown much bigger than the original settlers could have envisaged when they were first attracted to its natural harbour and constant fresh water sources. At the time of the flood, Boscastle had a population of around 1000.
- Debris had been trapped by a bridge in the village which created a dam. Once the pressure of the water build up became too much, the bridge collapsed and a second wave of water flooded through the village.
- The village and its flood defences were drastically under-prepared for an event on such a large scale as this, as it was rare and unexpected. Flood defences were due to be upgraded in the October of 2004 but work on them had yet to start at the time of the flood.
- The sewage and drainage systems in Boscastle at the time were antiquated and not suitable for an event of this magnitude.
Impact of the flood
ECONOMIC
- Early estimates after the flood put the damage at about £50 million.
- £10 million was spent on upgrading the village's flood defences.
- 150 cars swept away by the torrent, along with damage to 58 buildings, six of them being dragged out to sea.
- The village did not reopen to residents or visitors until May 2005, 8 months after the flood, which in the short term lead to a complete loss of business and tourism.
- £400,000 was raised in the immediate aftermatch for the villagers after a fundraising campaign started due to visits by Prince Charles and MPs.
- Some homes and businesses were unable to be rebuilt following an assessment from the environment agency.
- Was not all bad news for the village though, as in the years following the reopening of the village, there has been an increase in tourism in the village.
'Boscastle has used a natural disaster and awareness of the power of nature and turned it around to use tourism in a very positive way,' says Jason Freezer, sustainability project manager of English tourist board, Enjoy England.
POLITICAL
ENVIRONMENTAL
- Boscastle has transformed itself into a green tourism destination in a bid to raise awareness of the effects of climate change.
- The flood has been cited as a reason for Southwest Water to build new sewage treatment plants in the area as part of their £2 billion 'clean sweep' programme in the South West region.
- The Environment Agency has set out internal guidance on rapid responses for future floods in the wake of the 2004 flood.
- The Met office have invested in new methods of predicting small scale heavy rainfall in an effort to better predict the likelihood of events like this happening in the future.
ENVIRONMENTAL
- A widened river channel which is more prepared and can handle a 1-in-75 year event.
- Coastal pollution at the time of the event due to increased deposition of sediment, along with the debris of trees, cars, buildings, sewage.
- The force of the water created new river channels where previously, gullies existed.
- Bedload transport from the flood deposited seemingly alien granite boulders to areas where the bedrock is dark flints.
5 Key points
- No fatalities, the emergency response team was quick and 100 people were evacuated during the flood.
- The response has been to build flood defences at a cost of £10 million to protect from future events.
- The village was rebuilt and reopened fairly quickly, during the space of 8 months. Many floods in LEDCs take much longer for repairs to be completed.
- A combination of physical factors quite unique to Boscastle contributed to the flood.
- The problem was amplified by human factors including the bridge that blocked debris and water.
LINKS
- http://therivermanagementblog.wordpress.com/2013/04/03/the-story-of-boscastle-2004-a-geomorphic-perspective/
- http://www.theecologist.org/News/news_analysis/290033/boscastle_after_the_flood_a_green_future.html
- http://www.independent.co.uk/news/uk/home-news/boscastle-flood-of-2004-the-tenth-anniversary-of-freak-weather-striking-a-cornish-town-9673334.html
- http://news.bbc.co.uk/1/hi/england/cornwall/4077795.stm
- http://www.metoffice.gov.uk/education/teens/case-studies/boscastle