Stream channel adjustment in upland swamps, Barrington Tops, New South Wales, Australia

Abstract

The study of stable, channelled, upland swamps is relatively new and the processes by which they attain and maintain stability are of particular interest. Channels can adjust three elements of their morphology: cross-section, bedform and planform. This thesis examines unusual extremes in the adjustment of these elements in several swamp channels at Barrington Tops, New South Wales. In these channels, high bank strength is afforded by dense vegetation that has enabled the channels to achieve unusually low width/depth ratios. The resultant at-a-station hydraulic geometry is such that width barely increases with flow stage, depth increases moderately and velocity increases markedly; the channels have particularly high hydraulic efficiencies. Shear stress calculations suggest that bankfull flows are more than capable of transporting the scant sediment with which they are supplied. This economic use of energy means that, to maintain equilibrium, the excess must be consumed by the remaining elements; bedform and planform. Bedform magnitude (steepness) is strongly linked to flow resistance and there are large variations in the scale of bedforms between the channels studied here. In the smaller but more sinuous Polblue Creek, power is moderate and bed features display moderate steepness values. In contrast, the larger but less sinuous Barrington River and Edwards Creek channels have higher stream powers and their armoured bedforms have developed with much greater steepness. The considerable turbulence associated with these larger features are unable to destabilise the highly resistant channel banks but are sufficient to generate energy losses that reduce grain shear-stresses and prevent bed erosion. Polblue Creek, Barrington River and Edwards Creek have each developed sinuous planforms, with some examples of very tight bends. The reduction in the energy slope associated with planform development and the substantial turbulence that these tight bends generate combine with bedform development to counter the exceptional hydraulic efficiency of the channel cross-sections. The development of extreme bedform and planform morphologies have enabled system stability, despite high shear stresses and limited energy expenditure on sediment transport. This research suggests that bed armouring, high bank strengths and the development of highly sinuous planforms are integral to the maintenance of channel equilibrium, as has been demonstrated by channelled-swamp stability over the last ~1ka.