Title
Elizabeth Thornton
ABSTRACT
Increasingly researchers and catchment managers are investigating trans-scale processes and
the implications of these for the modelling and management of river networks. This study
examines several aspects of channel morphology at both network and local (reach) scales. The
primary aim of the study was to determine whether downstream hydraulic geometry
relationships are an appropriate method of predicting alterations in channel morphology with
increasing scale in a network context. The study was carried out in the Kangaroo Valley,
NSW, where the stream network is characterised by coarse grained bed sediment. A
secondary objective of the study was therefore, to determine the influence of coarse bed
sediment on hydraulic geometry and whether the channels of this cobble bed system adjust
their hydraulic geometry variables, to alterations in discharge, in a similar manner to the way
in which gravel bed rivers do. The second scale of observation was carried out at the reach
scale to determine spatial variability of channel morphology and the way in which it scales up
with the network.
To achieve the objectives of the study, channel form, velocity and bed and bank sediment
particle size were studied intensively at five river reaches of different scales within a single
network. Analytical techniques included the computation of hydraulic geometry relationships
for different discharge regimes as well as statistical analyses to determine the spatial
variability of channel morphology within and between reaches.
With regard to the up scaling of channel form within the network, the majority of hydraulic
relationships were found to be significant, the notable exceptions were between vbf and Q, vbf
and A, and s and Q*. Of the channel morphologic variables a stronger relationship was
observed between reach averaged bedform length and scale variables than reach averaged
bedform elevation and scale variables. In addition, the spacing of bedforms are not constant
proportions of bankfull width and depth. At the local scale a velocity reversal was predicted
for pool riffle sequences demonstrating the potential of this system to sort bed sediment
within reaches. The data for bed sediment exhibits no strong up scale trend in sediment
particle size, but does exhibit some evidence of sorting within reaches.
A similarity between downstream and network hydraulic geometry relations was
demonstrated, implying the potential for modelling adjustment in channel form to discharge
regimes across the network. Further analysis of the applicability of this technique is required
to determine the effects of complex geologies, vegetation covers, landuses, and where
peculiarities in individual network links may complicate these relations. It was also
demonstrated that the coarse grained sediment in this network acts to restrict the channels
ability to increase depth as channel scale increases. The analyses undertaken for the Kangaroo
Valley have provided useful insight to the behaviours of coarse grained river network which
enhances the potential of catchment wide models to accurately predict change.
ABSTRACT
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