Macrochannels and their significance for flood-risk minimisation, West Dapto, New South Wales

A prominent characteristic of streams draining catchments in West Dapto, New South Wales, are well developed macrochannels that have formed within alluvial terraces in mid-catchment zones. A detailed hydraulic modelling study using HEC-RAS, HEC-GeoRAS and Arcview GIS indicates that these macrochannels are scaled to accommodate high magnitude floods. They offer a significant degree of natural protection from flood events up to and in excess of 100 years recurrence interval, essentially by operating as 'bankfull' channels during such events. Macrochannel landforms can be clearly distinguished and mapped on fine-scale digital elevation models (DEMs) and other GIS data sources such as rectified aerial photography, offering the opportunity to integrate analyses of fluvial landforms and channel processes into hydraulic modelling studies, and ultimately, flood-risk avoidance strategies. Such an approach has the potential to improve on traditional flood risk avoidance methods that are focussed primarily on design-flood heights by enabling the interpretation of hydraulic modelling outputs in the context of fluvial landforms that exert a significant control on flood behaviour.