L6 - Flood hydrographs

• a hydrograph shows how a rivers discharge changes in response to a precipitation event.
• the horizontal axis measures time, usually in hours or days. the bars represent rainfall and the line graph shows discharge
• the vertical axis on a hydrograph measures precipitation (usually rainfall) in millimetres and discharge in cubic metres per second (cumecs)
• peak rainfall

  the highest amount of rainfall per time unit (the highest bar)
• rising limb

  shows how quickly the discharge rises after a rain storm (the first part of the line graph)
• peak discharge

  the highest recorded discharge following a rainfall event (the top of the line graph)
• lag time

  the time difference between peak rainfall and peak discharge (measure the horizontal difference between the top of the highest rainfall bar to the top of the discharge line and note the difference in hours
• falling limb

  shows the reduced discharge once the main effect of runoff has passed (the last part of the line graph which is going down)
• base flow

  the normal flow of a river when its water level is being sustained by groundwater flow (usually shown as a separate line)
• bankfull discharge

  a horizontal line marking the level of discharge above which flooding will occur as the river will burst its banks
• the shorter the lag time and the steeper the rising limb, the greater the flood risk
• a river with a long lag time and a very gentle rising limb will have a very low flood risk
• on any hydrograph, the rising limb will be steeper than the falling limb.
• the rising limb is fed by surface runoff, which reaches the river quickly over impermeable surfaces
• the flashy response hydrograph is associated with sudden flooding called 'flash floods'
• several conditions may result in a flash flood for example:

  • prolonged rainfall causing the ground to be saturated
  • long drought so the soil is baked hard
  • clay soil, which means water cannot infiltrate
  • steep sided-valley floor where water cannot drain easily
  • small river basin so tributaries soon link with the main river to swell its discharge
  • little vegetation to intercept precipitation
  • urban area with lots of impermeable surfaces
  • rural area with poor farming practices, like ploughing downhill
• on a slow response hydrograph an identical rainfall event will result in a less steep rising limb. the peak discharge is lower and the lag time longer. on this of hydrograph the flood risk is low