5.5

• Meteorological causes of flooding

  Flash floods happen very quickly and often without warning and they can be caused by:

  • Intense rainfall caused by severe storms or tropical storms
  • Prolonged rainfall – e.g a series of mid-latitude depressions – most of river flooding in the UK is caused by depressions passing over an area.
  • Features of a drainage basin
• Monsoon – a seasonal change in the direction of the prevailing wind of a world region. It involves a wet and dry season and occurs in sub-tropical regions. The monsoon climate of India + SE Asia has the wet season between April and September – low pressure caused by the ITCZ over the sub-continent draws air from the Indian ocean bringing humid conditions and torrential rain.

  Snow melt – a sudden rise in temperature in spring can cause rapid snowmelt and lead to flooding. Also jokulhaups caused by volcanic activity under ice sheets can create flooding. 
•  Human actions that can exacerbate flood risk

  • Growing population – more people forced to live in flood risk zones + deforestation/vegetation removal
  • Urbanisation – impermeable surfaces and storm drains increase surface runoff, bridges can trap debris and act like dams in times of flood
  • Removal of wetlands – wetlands are drained for farming and for settlements
  • Flood management – flood protection in one area may lead to flooding further downstream causes the water to move quickly to another location.