School of Geography - Theses
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ItemThe resistance of herbaceous vegetation to erosion: implications for stream form( 2006)Vegetation growing on the banks of a stream channel influences geomorphological processes operating in that channel. A large body of research has investigated the role of vegetation in controlling the erosion of stream channels, but the majority of this research has considered riparian trees and other woody vegetation. Consequently, our understanding of the influence of herbaceous vegetation on channel erosion is limited. The overall aim of this thesis was to address this knowledge gap by establishing the resistance to erosion of herbaceous vegetation in terms of shear stress and the length of time (duration) of exposure to shear stress, then comparing it to the shear stress and duration of exposure that occur in a number of streams in Victoria, Australia. The study focuses on the influence of herbaceous vegetation on the fluvial entrainment of sediment from horizontal surfaces. Horizontal surfaces are a particular subset of geomorphological features that occur in alluvial streams that include bars and benches. The erosion resistance of herbaceous vegetation directly sampled from horizontal surfaces in streams was estimated using a custom-built laboratory flume that generated very high bed shear stresses. The influence of stem length and substrate size on erosion resistance was tested in the flume study: mature herbaceous vegetation with long stems withstood high levels of shear stress for several days of continuous exposure. Erosion resistance was found to be inversely related to substrate size and positively related to stem length. The maximum erosion resistance of herbaceous vegetation in the flume study was greater than values reported in the literature for herbaceous vegetation growing in artificial drainage channels and on hillslopes. The shear stress exerted in stream channels is often estimated using methods based on cross-sectional average hydraulic conditions. Comparison of cross-sectional mean shear stress and the local shear stress exerted on horizontal surfaces estimated at six study sites in Victoria using high-resolution two-dimensional hydraulic modelling indicated that the erosion of horizontal surfaces is unlikely to be accurately predicted by mean shear stress. It is also possible that mean shear stress will not accurately predict the erosion of other sub-cross-section-scale geomorphological features. The frequency of erosion of horizontal surfaces covered with herbaceous vegetation was estimated at the study sites using erosion prediction analyses that combined the erosion resistance data from the flume study, estimates of local shear stress and exposure duration of horizontal surfaces at the study sites. The exposure duration was estimated by analysing the long-term sub-daily discharge records. The erosion prediction analyses indicated that mature herbaceous vegetation had sufficient resistance to erosion to withstand the shear stress and exposure duration at all study sites, and consequently would not be eroded. Immature herbaceous vegetation, however, would be eroded at all study sites; the frequency of erosion varied and was dependent on the stem length. The erosion prediction analysis method was applied to investigate the variation in the effectiveness of herbaceous vegetation in stabilising horizontal surfaces at the catchment-scale. Although the results were not conclusive, it appears that the influence of herbaceous vegetation is greatest in the upper catchment, which contrasts with previous research that identified a mid-catchment peak in fluvial entrainment of bank material. The duration of exposure to inundation is likely to be the main driver of herbaceous vegetation degradation in the lower catchment, as hydrograph attenuation leads to long duration events that cause oxygen deprivation stress on the herbaceous vegetation. A number of conceptual models of the temporal variation in erosion resistance of horizontal surfaces for different management scenarios were developed. Further data on the influence of vegetation other than herbaceous vegetation on the erosion resistance of horizontal surface are required, but based on the outcomes of the thesis and previous research it is clear that stream restoration designs that aim to increase channel stability with woody vegetation should be carefully considered. It is possible that rather than stabilising the channel, the impact of the maturing woody vegetation on the herbaceous vegetation understorey will lead to a reduction in overall stability.