Boscastle 2004 Flash Floods

With a 1 in 400 probability of occurring, Boscastle’s 2004 flash flood is considered an extreme weather event. A flash flood is is a rapid flooding of geomorphic low-lying areas – washes, rivers, dry lakes and basins.

This youtube link provides us with a clear image of what the flood was like:

Location: Boscastle, a small village in Cornwall, South West of England.

Water stores: There are two rivers which flow into Boscastle and they are the River Jordan and the River Valency.
Date: 16th August 2004
Probability: 1 in 400 but Boscastle is a village which regularly floods, more recently there was one in 2007.
What Happened:
  1. 184mm of rain fell in 24 hours – most of it in a five hour period on 16th August.
  2. Peak intensify of rainfall was over 300mm per hour (which is big!)
  3. An estimated 2 million tonnes of water flowed through Boscastle that day.
  1. Weather + climate– A depression formed in the Atlantic shortly before the flood with the remains of Hurricane Alex which slowed down on the land making the rainfall more immense. The storm was localised (this meant it stayed in one place and surrounding areas received a mere 3mm of water) because there was a trough situated right on top on Boscastle.
  2. Winds – A convergence of the prevailing South-Westerly winds and the path of the depression cause a vertical uplift of air. This lead to the creation of cumulonimbus clouds and more rainfall.
  3. Topography/relief – Boscastle is at the bottom of a steep hill so like a funnel it attracts more overland flow.
  1. Land use – The upper part of the village has been developing (urbanisation!) Also, hedges have been removed to make fields bigger.
  2. Lack of any flood control system – in the form of either raised banks around the river channel or emergency drainage ditches to catch overflowed water.
  3. The sewer & drainage systems- Boscastle had old sewage systems which had a small capacity. This encouraged surface runoff.
Hydrograph and brute facts…
  • The lag time of the flood is 4.5-5 hours (This is a short lag time)
  • The peak discharge is 48 cummecs
  • The peak rainfall is 55mm
  1. 58 proporties flooded and 4 were swept away by the flood
  2. Fortunately no-one died but there was one injury – a broken thumb!
  3. 32 cars were swept away and never to be seen again
  4. A range of infrastructure was badly damaged
  5. Sewer pipes were blocked and washed away so raw sewage contaminated flood water which caused a severe health risk.
  6. Four footbridges along the Valency were washed away.
  7. Long term disruptions and a major re-building project needed to take place.
  8. Long-term stress and anxiety to people traumatised by the incident.
  1. Damage to buildings and services was £2 million.
  2. Rescue operation including helicopters, police, fire service was costly.
  3. Loss of tourism a major source of income to the village of Boscastle.
  1. Damage was caused to local wildlife habitats.
  2. There was costal pollution as debris and fuel from cars flowed out into the sea.
Hard engineering defences systems were used; costing £4.6 million
  1. The environmental agency made a decision to lay a large relief culvert that would carry excess rainwater. It is twice the size of the old one.
  2. The River Valency is being widened and lowered from the lower bridge to the car park, so that it has a bigger capacity and can carry more water.
  3. The height of the car park is being raised using stone removed from from the river bed. Also barriers are being made for the car park so that if it floods it takes much longer for the cars to be swept away.
  4. There are plans to demolish the lower bridge near the harbour and replace it with a higher bridge further downstream. This will give the river more capacity and help to reduce flooding.
  1. The Environmental Agency : Builds, maintains and inspects flood defence for rivers (including Valency and Jordan).They monitor water levels and flows. They issue warnings, forecasts and implement major incident plans.
  2. The local authority : They deal withs some flooded culverts and roads. Also, they support police in arranging evacuation and providing rest centres. They also arrange emergency accommodation and medical care.
  3. Police: The co-ordinate the emergency response in major floods and helps rescue lives and properties,
  4. Firefighters : Rescue people trapped by floodwater and can pump water our of some buildings.

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