flooding case studies

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

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• sewage and drainage system outdated with a small capacity 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 

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 

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 

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 

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