Event scale processes

Extreme river floods are caused by a complex web of processes across space and time scales. The main driver usually is precipitation, sometimes mixed with snow melt. The meteorological situation is therefore key to producing precipitation that may induce extreme floods. But there are feedbacks between extreme precipitation and land surface processes which are not very well understood:

  • As the event magnitudes increase, threshold processes may occur on the land surface when the catchments get saturated, resulting in much faster flood response with bigger magnitudes than for small and medium floods.
  • The initial state of soil moisture affecting this saturation depends strongly on the season and the previous weather conditions, so the timing of the flood controls whether there is ‘resonance’ of high soil moisture with high precipitation or not.
  • In the river system threshold processes may occur when flows exceed bank full discharge.
    All of these processes translate into a complex pattern affecting the probability distributions of flood peak discharges, in particular at the upper tail of the distribution, i.e. for extreme events.

Key science questions

  • What were the hydro-meteorological causes of extreme floods in the past and how have the interactions with the catchment and rivers systems materialised?
  • What parts of the coupled system have dominated with respect to the flood magnitudes?
  • What was the role of anthropogenic effects such as river training in these events?
  • How do the event characteristics change as one moves from small to extreme events?