What is a floodplain? (3 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 and part 2 covered ecological processes. In part 3, I’ll present the structural elements formed from these processes.

Ecosystem structures

The periodic flooding and related processes of erosion and deposition (as described in Part 2) determine, to a considerable extent:

• the shape and features of the floodplain

• the depth and composition of soils

• the type and density of vegetation

• the richness and diversity of wildlife habitats.

Identification and mapping of these structural elements is another approach to defining floodplain extent.

Floodplain Features

The morphology of floodplains is intimately linked with the form and behaviour of the river channels that shape them. The key processes that create floodplain features are:

• lateral accretion from sediment deposition in the channel. This is how floodplains are formed on meandering rivers. The outer bend erodes and sediment is deposited on the inside bend. Through this process of erosion and deposition, the channel gradually moves sideways across the landscape. Typical floodplain features resulting from lateral accretion include: point bars; billabongs; and floodplain scour routes.

• vertical accretion from floodwater deposition. When a particular flood event exceeds the capacity of a waterway, floodwaters overtop the banks and deposit sediment on the floodplain. Coarser particles (sands and gravels) drop along the banks, where they sometimes build up natural levees. Finer material is carried further from the channel where the rate of flow is slow enough to permit silt to be laid down as backswamp deposits.

Whilst fluvial forms on floodplains (e.g. wetlands, billabongs) are important ecosystem structures, they do not occur over the entire floodplain. As such, their presence/absence is not a reliable indicator of floodplain extent. However, care should be taken to ensure that major floodplain features are not excluded from any definition of floodplain extent.

Floodplain Soils

By strict definition, soil is formed in situ by chemical and physical modification of some parent material (rock or sediment), under the influence of groundwater movement, atmospheric agents, organisms, and decomposing organic matter of both plant and animal origin. This may take hundreds or possibly thousands of years. However, sediment deposition on floodplains enables soils to be virtually ‘ready-made’, consisting of loose and often fertile organic material which can immediately carry plant life. The establishment of vegetation on the floodplain furthers the growth of floodplains by aiding deposition and preventing erosion.

Whilst the soils of floodplains are distinctive, the existing data are often not sufficiently detailed to describe floodplain boundaries. In addition, issues such as soil disturbance and the exporting and importing of soil materials may also prove problematic.

Floodplain Vegetation

The array of plants on floodplains includes species that are adapted to dry, almost terrestrial, conditions, to aquatic conditions, and to the various intermediate conditions. These plants can be grouped under different communities based on factors such as floristic composition and structure. In Victoria, plant communities are described as Ecological Vegetation Classes (EVCs). Each EVC includes a collection of floristic communities (i.e. groups based on co-occurring plant species) that occur across a biogeographic range, and although differing in species, have similar habitat and ecological processes operating.

Pre-1750 floodplain EVCs are based on a range of ecological variables (e.g. landform, geology, soils, altitude, aspect) and provide a consistent approach that encompasses the majority of floodplain environments. However, the pre-1750 vegetation mapping across Victoria is not precise and instead depicts broad patterns of vegetation across a landscape. In addition, it does not account for land use changes since European settlement that may have altered the vegetation community structure or composition.

Floodplain Habitats

Floodplains include both aquatic and terrestrial habitats, making them highly productive and diverse ecosystems, often supporting large and diverse populations of plants and animals. For example:

• floodwaters retain and replenish wetlands, supporting the flora and fauna of floodplains and river systems

• following inundation, some floodplains become very important breeding areas (and their proximity to water promotes significant roosting and nesting areas at other times)

• floodplains with geomorphic features that retain water (e.g. billabongs) have several patches where surface water is prolonged, providing habitat for fish and birds (e.g. piscivores)

• underlying aquifers provide an alternative water source for deep-rooted species such as most floodplain trees and some shrubs.

Sources and further reading:

Charlton, R. (2007). Fundamentals of Fluvial Geomorphology. Routledge.

Costermans, L. (1989). Native Trees and Shrubs of South-Eastern Australia.

Gould, L. and Morris, J. (2005). Riverways: Shortcuts to River Management Information in Australia. Greening Australia.

Millar, C.E. Turk, L.M. and Foth, H.D. (1965). Fundamentals of Soil Science.

Morisawa, M. (1968). Streams: Their Dynamics and Morphology.

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.

Parkes, D., Newell, G. and Cheal, D. (2003). Assessing the Quality of Native Vegetation: the ‘Habitat Hectares’ Approach. Ecological Management and Restoration, Vol 4.

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