About floods
What is a flood?
A flood occurs when water inundates land that is normally dry. Floods can be caused by a number of processes, but the dominant cause in Australia is rainfall. Floods are a natural process, but activities of people can affect flooding. Floods occur at irregular intervals and vary in size, area of extent, and duration. Floods can occur suddenly or slowly.
Sudden, heavy and intense rainfall can cause floods to quickly rise in the minutes or hours that follow. These are known as flash floods and are typically associated with relatively small catchment areas.
Floods can occur slowly in large catchment areas, where rainfall can build up over hours, days or weeks. The runoff from this rainfall may create significant floods that inundate large areas of land for days, weeks or months.
What factors contribute to floods?
Rainfall is the most important factor in creating a flood, but there are many other contributing factors such as the amount of rain within a period of time, where the rain has fallen, previous rain that may have caused the ground to be saturated, and release of water from dams (controlled or uncontrolled).
When rain falls on a catchment, the amount of rainwater that reaches the waterways depends on the characteristics of the catchment, particularly its size, shape and land use. Some rainfall is 'captured' by soil and vegetation, and the remainder enters waterways as flow. River characteristics such as size and shape, the vegetation in and around the river, and the presence of structures in and adjacent to the waterway all affect the level of water in the waterway.
What are the consequences of floods?
Floods impact on both individuals and communities, and have social, economic, and environmental consequences. The consequences of floods, both negative and positive, vary greatly depending on the location and extent of flooding, and the vulnerability and value of the natural and constructed environments they affect.
How do we forecast floods?
Weather forecasts can provide advance warning of a flood, and seasonal forecasts can alert of a heightened chance of flooding in the coming months. Forecasting river levels and flood extent (the size of the flood and its reach) is a complex process that is continually being improved. The Bureau of Meteorology (BoM) is the lead national agency for flood forecasting and warning services in Australia, working in partnership with agencies at the state and local government levels. The BoM provides forecasts of the water level in rivers at critical locations during flood events. Local governments and emergency agencies may further interpret the river water level forecasts and provide advice on flood inundation extent. The BoM also provides severe weather warnings that include risk of flash flooding. In addition, the BoM provides forecasts of rainfall and river flow for the coming three months. These seasonal forecasts may help alert agencies and the public of entering a period of heightened chance of flooding.
How do we estimate the chance of a flood occurring?
Understanding the chance of different sized floods occurring is important for managing flood risk. The chance of a flood event can be described using a variety of terms, but the preferred method is the Annual Exceedance Probability (AEP). AEP is the likelihood of occurrence of a flood of given size or larger occurring in any one year, and is expressed as a percentage (%). The likelihood is based on how often flooding has happened historically based on modelled data, and includes the size of the flood and how often a similar sized flood is possible.
A flood with a 1% AEP has a one in a hundred chance of being exceeded in any year. Currently, the 1% AEP event is designated as having an 'acceptable' risk for planning purposes nearly everywhere in Australia. However, good planning needs to consider more than just the 1% AEP flood.
How do we manage flood risks?
Flood risk includes both the chance of an event taking place and its potential impact. Land use planning informed by floodplain management plans can reduce risk for new development areas. Flood risk is harder to manage in existing developed areas; however modification measures such as dams or levees can change the behaviour of floodwaters. Similarly, property modification measures can protect against harm caused by floods to individual buildings, and flood modification measures (such as flood mitigation dams, levees, and waterway or floodplain modifications) can help communities deal with floods.
Floods - what does the future look like?
Australia's growing population and changing climate patterns imply that the characteristics of the floods we experience will change in the future. Better future land use planning and floodplain management can mitigate the impacts of flooding. Appropriate urban design can reduce the severity of flood impacts. Catchment and waterway revegetation can reduce the impact of flooding. Climate change is likely to affect flooding patterns. Emerging technologies can improve our ability to predict and manage floods.