Sediment organisation along the upper Hunter River,Australia: a multivariate statistical approach

摘要

Freely adjusting gravel-bed rivers are subject to recurrent input of material andfrequent, but irregular, reworking of bedload materials. In contrast, many Australianrivers are characterised by low-bedload input and infrequent mobilisation of bedloadmaterials. In this study, the influence of these boundary conditions upon surface bed-material organisation are analysed through application of extensive field sampling andmultivariate statistics based on a range of bed-material parameters along an 8 kmreach of the Upper Hunter River in New South Wales. The Hunter River is a mixed-load, gravel-bed river that has been subjected to significant disturbance since Europeansettlement. Since European settlement, the channel in the study reach has been trans-formed from a passive meandering river to a configuration in which the low-flowchannel locally adjusts around various bar and bench features. Channel capacity hasenlarged locally by a factor of 3, channel alignment has changed via cut-offs (bothnatural and human-induced), and channel–floodplain relationships have been altered.The multivariate statistical analyses performed in this study provided a very useful toolfor identifying surface facies and the way in which sediment is organised. Bedloadsurface facies do not demonstrate an equivalent degree of organisation to thatdocumented for fully self-adjusting rivers. A conceptual model is developed that re-lates the variability and spatial arrangement of differing classes of bed material to thetype of geomorphic unit, elevation above the low-flow channel and ease of reworking(i.e., frequency of potential mobilisation). Spatial variability in the sediment mix of thestudy reach, appraised in light of long-term changes to sediment flux, prompts the needfor refinement of bedload transport models for this kind of river.