What Causes Flooding?

• Flooding - the discharge of a river exceeds the capacity of the river’s channel

  • the river overflows and covers the adjacent floodplaiin
  • heavy rainfall is the cause of most floods
• Storm hydrograph - shows how quickly a river responds to a rainfall event

  • water can reach the river by overland flow, throughflow and groundwater flow

  A) lag time

  B) peak flow

  C) rising

  D) recession

  E) overland flow

  F) throughflow

  G) base flow

  
• The shorter the lag time and the steeper the rising limb, the quicker the water reaches the river and the greater the chance of flooding

  • the shape of storm hydrographs vary between river drainage basins and are useful for planning against future flooding and times of drough
• Discharge - the volume of water passing a measuring point or gauging station in a given time, measured in cubic metres per second
• Lag time - the time difference between peak rainfall and peak discharge
• Infiltration capacity - the maximum rate that water enters soil
• Antecedent soil moisture - the amount of moisture present in the soil before a rainfall event
• Impermeable - does not allow water to pass through
• Percolation - the vertical movement of water from the soil into the underlying rock
• Interception - the proess which vegetation prevents rainfall from reaching the surface direcltly

  • intercepted rainfall temporarily heldd as interception storage and then either falls temporarily heldd as interception storage
  • water either falls directly to the ground or is evaporated back to the atmosphere
• Causes of flooding (1)

  • Weather
  • heavy, intense rainfall can exceed the infiltration capacity of the soil and lead to an increase in overland flow
  • stead, prolonger rainfall can saturate the soil and cause the water table to rise, reducing infiltration capacity
  • overland flow will occur if snowmelt is rapid and the ground beneath is frozen
  • Previous weather
  • antecedent soil moisture refers to the amount of water in the soil before a rainfall event
  • the more saturated the soil the less infiltration and the more overland flow
• Causes of flooding (2)

  • Soil and rock type - impermeable soils and rocks e.g. clay or granite, have a low infiltration capacity and percolation rate which leads to greater overland flow
  • Relief - steeper gradients can lead to faster overland flow and water has little time to infiltrate
  • Earthquakes, volcanoes and tropical cyclones - natural hazards can produce tsunamis and storm surges that flood low-lying coastal areas
  • Deforestation - cutting down trees reduce interception and infiltration
• Causes of flooding (3)

  • Urbanisation - concrete and tarmac are impermeable surfaces that lead to more overland flow
  • storm drains speed up the movement of water to the nearest river
  • Agriculture
  • overgrazing and leaving soil exposed reduces interception
  • ploughing down rather than across slopes quickly channels the water downwards
  • heavy farm machinery compacts the soil, making it impermeable
  • Climate change - enhahnced global warming may lead to a rise in sea-levels as well as more rainfall and storms in certain parts of the world