Basics

What is a flood?

To put simply flooding is water where it is not wanted.

In November 2011, the Australian Government introduced a standard definition of flood for certain insurance policies. For this purpose a flood is defined as:

The covering of normally dry land by water that has escaped or been released from the normal confines of: any lake, or any river, creek or other natural watercourse, whether or not altered or modified; or any reservoir, canal, or dam.

Floods can have both positive and negative impacts. They can bring welcome relief for people and ecosystems suffering from prolonged drought, but also are estimated to be the most costly natural disaster in Australia.

Why do floods occur?

Flooding occurs most commonly from heavy rainfall when natural watercourses do not have the capacity to carry excess water. However, floods are not always caused by heavy rainfall. In coastal areas, water inundation can be caused by a storm surge as a result of a tropical cyclone, a tsunami or a high tide coinciding with higher than normal river levels. If a dam fails, triggered for example by an earthquake, the downstream area will flood, even in dry weather conditions.

Other factors which can contribute to flooding include:

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

Where do floods occur in Australia?

Riverine flooding occurs in relatively low-lying areas adjacent to streams and rivers. In the extensive flat inland regions of Australia, floods may spread over thousands of square kilometres and last several weeks, with flood warnings sometimes issued months in advance. In the mountain and coastal regions of Australia, flooding can happen rapidly with a warning of only a few hours in some cases.

The Great Dividing Range which extends along the length of eastern Australia provides a natural separation between the longer and slower westerly flowing rivers and the shorter, faster easterly flowing coastal rivers. In some cases, natural blockages at river mouths, including storm surge and high tides, can also cause localised flooding of estuaries and coastal lake systems.

Flash flooding, otherwise known as overland flooding, can occur almost anywhere there is a relatively short, intense burst of rainfall such as during a thunderstorm. As a result, the drainage system has insufficient capacity or time to cope with the downpour. Although flash floods are generally localised, they pose a significant threat because of their unpredictability and normally short duration.

Just because an area has not flooded in the past, does not mean it will not flood in future. Similarly, just because an area flooded during one event, does not mean that it will necessarily flood during the next one.

The Bureau of Meteorology maintains the Australia Rainfall and River Conditions which contains up-to-date rainfall and river information for all catchments within Australia.

Talking about floods

The use of consistent terminology is important for improving the quality and consistency of flood information. Geoscience Australia uses the following flood related terms when talking about floods and flood research.

The most costly summer for floods in Australia was 2010-11, with extensive flooding in the Lockyer Valley, Ipswich and Brisbane in January 2011. This flooding resulted in a cost of A$6.64 billion (2013 Australian dollars, including deaths and injuries but excluding most indirect losses). There were 35 deaths and 20,000 people were made homeless. Between 1967 and 2013, the average direct annual cost of flooding has been estimated at A$943 million (excluding the cost of deaths and injuries).

References:

Ladds M, Keating A, Handmer J and Magee L (2017) How much do disasters cost? A comparison of disaster cost estimates in Australia. International J of DRR. 10.1016/j.ijdrr.2017.01.004

Ladds, MA, Magee, L, Handmer, J (2015) AUS:DIS - Database of losses from disasters in Australia 1967-2013. https://github.com/liammagee/sealand

John Handmer, Monique Ladds and Liam Magee (December 2016), Disaster losses from natural hazards in Australia, 1967-2013. (Report with AGD)