Evaluating the relative contributions of vegetation and flooding in controlling channel widening: the case of the Nepean River, southeastern Australia

• Published in: Australian journal of earth sciences Vol. 57; no. 5; pp. 525 - 541
• Main Authors: Hubble, T. C. T., Rutherfurd, I. D.
• Format: Journal Article
• Language: English
• Published: Taylor & Francis 01-07-2010
• Subjects: bank erosion; Banks; channel widening; Channels; Clearing; Floods; geomechanical modelling; Nepean River; Rivers; root reinforcement; Streams; Vegetation; vegetation clearing; Widening
• ISSN: 0812-0099; 1440-0952
• DOI: 10.1080/08120099.2010.492910

Many lowland stream channels have dramatically widened over the last two centuries. There has been considerable debate about whether this widening was caused by an unusually large flood, by a series of large floods, or by decreased bank stability caused by clearing of riparian vegetation. The relative effects of floods and vegetation can be disentangled in southeastern Australia where streams have undergone both clearing of bank vegetation, and decadal sequences of relatively higher and lower flood magnitude and frequency. Archival aerial photographs of the Nepean River, in southeastern Australia, suggest that banks did not erode during periods of low flood magnitude (drought-dominated regime: from 1901-1949) whether they were cleared or not. However, during periods of flood-dominated regime (1950 to 1970s) only cleared stream banks eroded. Thus, on the upper Nepean River, clearing alone was insufficient to trigger erosion by small floods, and even large floods were unable to erode vegetated banks. The conclusion is that substantial channel widening in this river required both clearing of bank vegetation, and periods of unusually large and frequent floods. This conclusion is supported by geomechanical modelling that examine the reduction in bank shear strength arising from the loss of tree-root reinforcement. The modelling also suggests that bank instability arising from devegetation amplifies the potential for bank failure during the drawdown phase of a flood, leading to channel widening.