flooding case studies

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Boscastle Flood- physical causes

Frontal rainfall

physical causes

  • intense rainfall- cumulonimbus clouds
    • 200mm fell in 4 hours
      • 310 gallons overall on 16th of august
    • storm clouds from mid- Atlantic front
  • antecedent rainfall- very wet august
    • soil storage full, causing more saturated overland flow= short lag time
  • steep valleys 
    • less time for soil to infiltrate= more overland flow
  • impermable rock 
    • slate and thin soil- water cannot infiltrate and limited soil storage= more overland flow
  • conlfuence for river valency and river jordan 
    • volume of water increases 
  • very high spring tide 
    • water stays in river as more difficult to flow to sea
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Boscastle Flood- human causes

human causes 

  • no flood control system 
  • bridge in centre of vilage- cars, vegetation and debris get stuck causing a dam
  • sewage and drainage system outdated with a small capacity 
  • buildings positioned close to river channel 
  • surroudning vegetation includes agricultural land with limited interception stores 
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York, River Ouse- physical causes

physical causes 

  • steep gradient valleys 
    • discouages infiltration
  • clay soils 
    • infiltrationn rates 2mm/hr
    • limited soil moisture storage
  • only 4% woodland 
    • less rainfall intercepted
  • high volume of rainfall
  • snowmelt in upand areas 
  • 30% of vegetation moorland
    • less interception can occur and more infilration happens 
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York, River Ouse- human causes

human causes

  • urbanisation
    • majority of urban areas covered in impermable surfaces such as tarmac and concrete 
      • infiltration unable to occur
  • less water intercepted due to removal of vegetation
  • sheep and other livestock trample the ground
    • impermable surfaces- reduced infiltration rates 
  • grip drains to create more grazing land- upper wharfdale 
    • water runs directly into river- short lag time 
  • construction of river flood barrier 
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Bangladesh- physical causes

physical causes 

  • lies on floodplain for ganges 
  • 80% of the country less than 6 metres above sea level 
  • limited amountof soil moisture storage available 
  • snow melt in himalayas 
  • 1800-2600mm of rain a year 
  • 80% of rain betwee june and september
  • syncronised with high tides in bay of bengal 
    • preventing river water flowing to the sea 
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Bangladesh- human causes

human causes

  • deforestation in nepal/ tibet 
    • 50% since 1950s gone
    • soil erosion = 400x faster since trees lost- siltation of river bed by 5cm/year 
  • urbanisation e.g. dhaka 
  • Farraka dam
    • reduced velocity downstream, building up of silt in river channel 
  • no crop rotation, no contour ploughing= soil erosion 
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