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.
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ItemCauses of incision of Gippsland river channels during the period of European settlement, with particular reference to Bruthen Creek( 1987)Historical evidence shows that many of the Gippsland river channels which are now deeply incised were formerly shallow, meandering and swampy. The change has occurred within the historic period, beginning in most cases between 1870 and 1900. This thesis investigates the changes along Bruthen Creek, near Yarram, using a variety of historical sources, including early maps and air photos, and archival material from the Department of Crown Lands and Survey (now the Department of Conservation, Forests and Lands), Victorian Railways (now V-line) and the local Shire offices. Together these provide sufficient evidence to support a model of development of incision through headward migration of a series of five distinct erosion sequences, each of which originated in a different part of the channel system. Possible causes of instability are investigated. It is concluded that although there appears to have been a climatic change towards increased annual rainfall in the area, dating from the mid 1940s, as proposed for coastal New South Wales, there is no evidence that this is causally related to channel instability. Similarly, although there has been extensive deforestation of the catchment, it is argued that this has not had sufficient impact on run-off to have caused channel erosion. The thesis concludes that human interference has been the critical factor leading to incision, particularly the efforts to drain the riverain swamps by channelising flow within them, and limiting the extent and frequency of flooding. This hypothesis is presented for Bruthen Creek, and evaluated in the light of the author's studies of eroded channels in other parts of Gippsland. Many of these studies have already been published, and are presented with this thesis as supporting papers. Some evidence is also included from the preliminary studies by the author, not yet published, of channel instability in a group of tributaries of the Tarra River, the catchment of which adjoins that of Bruthen Creek. It is concluded that all the Gippsland channels which were shallow and poorly defined at the time of settlement and have subsequently incised have been subject to similar attempts to channelise flow. Much of the work was carried out on a small scale by individual farmers soon after land settlement, and the only record of their activities is in the original land selection records. Entrainment of flow to ensure that floodwaters passed under newly constructed road and rail bridges has been a contributory factor, but most of this, particularly in the case of roads, is only poorly documented. The importance of individual action, and the paucity of early road records, has meant that the extent of human interference has often been underestimated, because much of it took place long before the establishment of a government department with a specific interest in river management.