Methodology for determining floodway / flow conveyance extent in Australian floodplains

Maintaining the flood function of the floodplain is essential to ensuring that floodplains can perform their natural functions of flow conveyance and storage. The delineation of the floodplain based on its function during floods is a useful tool for determining the impact that development activity within the floodplain may have on flood behaviour. Breaking down the floodplain according to these functions identifies areas of the floodplain where flood behaviour is most sensitive to change. Traditionally in Australia, floodplains have been broken down into areas according to their hydraulic function. In most states, areas of the floodplain are classified as floodway or flow conveyance areas, flood storage areas or flood fringe areas. This hydraulic categorisation of the floodplain is typically applied to areas of the floodplain up to the Defined Flood Event (DFE), which historically has been assumed to correspond to the 1% Annual Exceedance Probability (AEP) event. However, consideration must also be given to flood function across areas of the floodplain between the DFE and the Probable Maximum Flood (PMF), which defines the true extent of the floodplain. This area is often neglected as it sits beyond the domain of typical planning controls, but may for some floodplains be an area of critical flood function due to risk and safety issues associated with the stability of buildings and people when exposed to floodwaters. Floodways are defined as those areas where a significant discharge of water occurs during floods. However, there are no specific procedures for identifying floodways or for defining their extent. The methodology for defining floodway extent is typically left to practitioners to determine. As a result, the outcomes are often subjective and can lead to conjecture due to the legal implications of land being classified as floodway. Over the last decade, there has been considerable advancement in the tools used to simulate flooding. Accessibility to improved and more detailed topographic data has also occurred, as has the capacity to more rigorously interrogate flood characteristics derived from computer modelling using this data. In particular, the increasing use of this data in 2D hydrodynamic models has allowed more meaningful representation of flood flow across floodplains. A range of data-sets such as depth, velocity, velocity-depth product, distribution of flow and unit stream power, can now be readily exported from flood models. This data can be combined with reliable topographic data to facilitate the hydraulic categorisation of floodplains, including the identification of floodways. At the same time, the modelling tools can be more easily adapted to test the impact of floodplain encroachment and confirm initial estimates of floodway corridors. This paper outlines a methodology that can be applied to all river and floodplain systems to determine floodway or flow conveyance area extent. It also documents examples of major and minor river systems where this methodology has been applied and considers the potential link between stability criteria and flow conveyance.