River Flooding: Boscastle

• Overview:

  • August 2004
  • Boscastle - town on the North coast of cornwall.
  • 0 deaths
  • River Valency and River Jordan both burst banks.
• Physical Factors:

  • Relief.
  • Steep v-shaped valley.
  • Little time for infiltration.
  • Increased surface runoff.
  • Geology
  • Granite and Slate are impermeable.
  • Both reduce percolation and increase surface runoff.
  • River Valency
  • 7-8km long.
  • Short lag time.
  • Flashy hydrograph.
  • Shape of drainage basin
  • Circular
  • Flashy response
  • All water arrives at roughly the same time.
  • Antecedent Rainfall
  • Bodmin Moor received 125% more than average.
  • Soil became saturated.
  • High intensity precipitation
  • 7 hours of storm.
  • 200mm
  • Relief rainfall.
• Human Factors:

  • Deforestation during Bronze Age.
  • Less transpiration and interception.
  • Increases surface runoff.
  • Global warming makes extreme weather events occur more frequently.
• Environmental Impacts:

  • Soil erosion due to surface runoff.
  • Sedimentation in the river channels and into harbour created a delta.
  • Bank erosion due to high velocity of the river and high discharge.
  • Bed erosion due to high velocity and discharge meant heavier boulders were rolled down the river.
  • Ecosystem impacts such as trees being uprooted and vegetation being drowned.
• Socio-economic Impacts:

  • Commercial buildings destroyed.
  • Cafes, hotels, shops and tourism buildings.
  • Houses damaged, people became homeless.
  • 116 cars destroyed, 84 removed from harbour.
  • Large increase in cost of flood insurance.
  • Fall in house prices of 50%.
  • Reduce in revenue in tourism industry.
  • 'Dark Tourism' visiting where tragedy happened.
• Hard Engineering Solutions:
• Hard Engineering Solutions:

  • Channelisation.
  • Rivers widened and deepened to increase channel capacity.
  • Enough to protect against a once in 75 year flood.
  • 2004 flood was one in 400 years.
• Soft Engineering Solutions:

  • Afforestation
  • Increase interception and transpiration rates.
  • Reduce channel runoff.
  • Flood Plain Zoning
  • Leaves the areas close to the rivers empty.
  • Building Adaptations
  • Using floodboards to stop water entering through the doors.
  • Electric points raised above ground level.