What causes Floods?

A car partially submerged in a flooded river

Damage caused by flooding

Flooding occurs most commonly from heavy rainfall when natural watercourses do not have the capacity to convey excess water. However, floods are not always caused by heavy rainfall. They can result from other phenomena, particularly in coastal areas where inundation can be caused by a storm surge associated with a tropical cyclone, a tsunami or a high tide coinciding with higher than normal river levels. Dam failure, triggered for example by an earthquake, will result in flooding of the downstream area, even in dry weather conditions.

Other factors which may contribute to flooding include:

  • volume, spatial distribution, intensity and duration of rainfall over a catchment;
  • the capacity of the watercourse or stream network to convey runoff;
  • catchment and weather conditions prior to a rainfall event;
  • ground cover;
  • topography; and
  • tidal influences.