River Floods and Hydrographs

• Storm hydrographs


  A way to show flooding on a graph
• Lag time


  The difference between peak rainfall and peak discharge
• Bar graph


  • Shows rainfall in mm
• Line graph


  • Shows discharge in cumecs
• Rising limb


  • River discharge level increasing
• Falling limb


  • River discharge level returning to base flow
• Physical factors affecting hydrographs

  • Geology
  • Soil type
  • Vegetation
  • Slope
  • Drainage basin shape
  • Antecedent rainfall
• Geology


  More resistant rock absorbs less water and therefore encourages runoff
• Soil type


  More impermeable soils absorb less water and encourage runoff
• Vegetation


  Plants encourage interception and this creates less runoff
• Slope


  Steeper slopes cause faster surface runoff
• Drainage basin shape


  A wide basin with lots of tributaries close together encourages a steeper rising limb and short lag time
• Antecedent rainfall


  When the ground is already saturated with water any extra water will create runoff
• Human factors


  • Urbanisation
  • Deforestation
  • Land-use change
  • Building on floodplains
• Urbanisation


  Cities growing and creating impermeable surfaces
• Deforestation


  Less vegetation leads to less interception and more runoff
• Land-use change


  Fields that used to drain water and store water become impermeable and therefore runoff occurs
• Building on floodplains


  Humans building on areas susceptible to flooding
• Factors increasing flood risk

  • Climate change
  • Increased population
  • Increased urbanisation
• Climate change


  Increased frequency and intensity of storms, increased dry periods making the soils impermeable and increasing cold periods, which increases surface runoff
• Increased population


  More people living by flood plains
• Increased urbanisation


  More construction of impermeable surfaces which increase surface run-off