What is a floodplain? (2 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. Part 1 looked at the 1% AEP, in part 2 I’ll cover ecological processes.

Ecological processes

Whilst the 1% AEP flood extents are typically used for planning purposes, they do not represent the true spatial extent of the floodplain. In addition, flood mapping is often limited in coverage to the major waterways, leaving many smaller waterways (with floodplains) unmapped.

So what about the processes that formed and continue to shape floodplains?

Floodplains are fluvial depositional environments formed over long periods from sediments transported by rivers in flood. The rate of floodplain formation is dependent on the prevailing flow regime in the river and the nature of sediment delivery from the upper catchment.

Various documents have defined the spatial extent of floodplains from an ecological perspective. For example:

• defining floodplains from a hydrological perspective:

• Area of land adjacent to a creek, river, estuary, lake, dam or artificial channel, which is subject to inundation by the probable maximum flood.

• Any area susceptible to inundation by floodwater from any source.

• That area of relatively flat land covered by water during a major flood.

• defining floodplains from a geomorphological perspective:

• Temporary alluvial storage areas adjacent to the river channel.

• That portion of a drainage basin that is covered by stream transported sediment that was deposited in or near a stream channel.

• Fluvial depositional environments formed over long periods from sediments transported by rivers in flood.

Unfortunately, the identification of ecological processes within the landscape can be difficult (especially considering that flood inundation and sediment deposition are both episodic and dynamic). Furthermore, some floodplains may be under-represented due to past and present intervention that has dramatically altered the extent and frequency of floodplain inundation e.g. channelisation and levee banking - but they are still floodplains.

A suitable surrogate may be to identify the structural elements that are formed from these processes. This will be my topic in part 3.

Sources and further reading:

Californian Department of Water Resources (2005). Natural Floodplain Functions and Societal Values. Revised Draft.

Mussared, D. (1997). Living on Floodplains.

Roberts, J. Young, B. and Marston, F. (2000). Estimating the Water Requirements for Plants of Floodplain Wetlands: A Guide. Occasional Paper 04/00.

Thoms, M. David, N. Rayburg, S. and Southwell, M. (no date). Busting some myths of Australian floodplain river ecosystems.