What is a floodplain? (1 of 3)

A key aspect in managing floodplains is to define their spatial extent i.e. the area influenced by floods. Typically, floodplains are defined based on flood events but other approaches can consider ecological processes or ecosystem structures. I’ll present each of these approaches in a separate post; starting with the 1% AEP.

The 1% AEP

Floods occur when flows generated by rainfall overtop the banks of a river, creek or constructed channel, or in urban areas when the amount of runoff exceeds the capacity of stormwater drains.

Floodplain management is based on making trade off decisions about the economic, social and environmental costs and benefits of allowing a broad range of activities to take place in areas that flood i.e. floodplains. It involves the application of a range of measures including:

• best practice guidelines, strategies and processes to manage the flood risk

• studies, flood maps, land use planning controls, building regulations and research to understand the flood risk, help prevent future flood damages and to minimise threats to life, health and safety

• community awareness, education and training to improve preparedness when a flood occurs

• structural measures to reduce the risk of flooding to existing properties

• flood warning and emergency response measures to manage the flood risk when floods overwhelm existing measures.

Whilst the above measures require consideration of a range of flood events, it is not generally considered economical or feasible to provide ‘full’ flood protection up to the most extreme rainfall flood event (i.e. the probable maximum flood).

As a result, lesser flood events are typically adopted for planning and development purposes (i.e. defined flood events) and represent a compromise between the level of protection that can be afforded and the risk a community is prepared to take with the consequences of larger floods.

Most Australian States and Territories have adopted the 1% AEP* flood event as the appropriate design flood event for planning and development purposes. The area covered by the 1% AEP is therefore often referred to as the floodplain.

* AEP - Annual Exceedence Probability. This is the statistical likelihood of occurrence of a flood of a given size or larger in any one year, usually expressed as a percentage.

Sources and further reading:

DELWP (2016). Victorian Floodplain Management Strategy. Victorian Government Department of Environment, Land, Water and Planning, Melbourne, Victoria

Emergency Management Australia (1999). Managing the Floodplain. Manual 19. Australian Government.

Melbourne Water (2015). Flood Management Strategy. Port Phillip and Westernport.

Natural and Beneficial Functions of Floodplains Task Force (2002). The Natural and Beneficial Functions of Floodplains. Reducing Flood Losses by Protecting and Restoring the Floodplain Environment. Report for Congress.

SCARM (2000). Floodplain Management in Australia: Best Practice Principles and Guidelines.