River channel self-adjustment towards equilibrium: endogenic and exogenic influences on distinctly different river types in the Lake Eyre basin of arid central Australia

Author

Anna Habeck-Fardy, University of Wollongong

Year

2014

Degree Name

Doctor of Philosophy

Department

School of Earth and Environmental Sciences

Recommended Citation

Habeck-Fardy, Anna, River channel self-adjustment towards equilibrium: endogenic and exogenic influences on distinctly different river types in the Lake Eyre basin of arid central Australia, Doctor of Philosophy thesis, School of Earth and Environmental Sciences, University of Wollongong, 2014. https://ro.uow.edu.au/theses/4113

Abstract

The Lake Eyre basin (LEB) of arid central Australia exhibits a variety of river styles, which provides an excellent opportunity to evaluate the H number model, a nondimensional number that demonstrates how energy-efficient a river operates at a given cross section. Due to the paucity of stream gauging in these remote regions, hydraulic parameters are estimated from field surveying and sediment sampling on the muddy Diamantina River in Queensland and sandy Marshall and Plenty Rivers in the Northern Territory. Regional slope estimates, cross-sectional data and sediment size inputs obtained in the field enabled calculations of hydraulic parameters, and consequently, H numbers. Comparison of these to geomorphological observations and other data sources permitted interpretation of the results.

The LEB is a system of nested geological basins with a fluvial history extending back to the Early Cretaceous, and has been the focus of geological and geomorphological investigation for decades. A comprehensive review of this knowledge is provided. In such arid environments, vegetation is attracted to the availability of moisture along the waterways, and consequently can have a profound impact on channel form and process. A detailed review of the rapidly changing appreciation of how rivers and vegetation interact globally is presented.

The H number measures the energy spent overcoming friction from the channel bed and banks for a given sediment load (Huang, H.Q. and Chang H.H., 2006. Scale independent linear behaviour of alluvial channel flow. Journal of Hydraulic Engineering, 132: 722-730):

H = τ/τ_cr-1/W/D-2

When H = 0.30 the energy of alluvial channel flow reaches a minimum for transport of a given water and sediment load, a condition known as the stationary equilibrium state. The H number can be regarded as a quantitative index for measuring the stability of a river and the potential development of complex channel forms in situations where H ≠ 0.30. Along the Diamantina River study reach, H numbers are commonly greater than an equilibrium value of 0.30, suggesting that the channels are in disequilibrium with excess shear. There is, therefore, potential for abundant bedload transport and scour. However, the Diamantina is clearly a highly stable system. It appears that the muddy bank and bed and associated vegetation create narrow deep canal-like channels, well suited for the efficient transport of abundant floodwater charged with little bedload. In this case, obtaining H numbers > 0.30 importantly reveals that the Diamantina has almost no bedload and the channel is bounded by immobile cohesive clay. In fact, this finding challenges the incorrect assumption that only bed material of a particular calibre is capable of being transported.

In contrast, the Marshall and Plenty Rivers under bankfull conditions are operating with H numbers close to 0.30. That both these rivers are at or very close to stationary equilibrium is interesting given that in the study reaches, they run sub-parallel over identical gradients but have bed material of different calibre and different planforms. The Marshall River has a coarser bedload (mean = 1.3 mm) compared to the Plenty (mean = 0.45 mm), and in its steeper upper reaches (slope = 0.0025) can readily move this through a largely single-thread channel. In the study reach where the gradient drops to roughly half (slope = 0.0013), the Marshall needs to anabranch to remain stable. In contrast, the Plenty at the same gradient can move its sand load without anabranching. This supports previous research, indicating that anabranching is a mechanism by which flow efficiency can be enhanced to move bedload where gradients cannot be increased. These results also confirm how vegetation can significantly affect river morphology; Melaleuca glomerata controls the sequential development of the ridges and islands that separate anabranches and Eucalyptus camaldulensis is shown to maintain long linear ridge-form anabranches.

The application and evaluation of the H number model demonstrates that the channels of the Diamantina, Marshall and Plenty Rivers are self-forming systems adjusting towards a stationary equilibrium state. Moreover, this study establishes that riparian vegetation is crucial in enabling channels to achieve this equilibrium state. Specifically, vegetation introduces a degree of cohesion to the channel boundary enabling appropriate energy-efficient width/depth ratios to form. While the three basic flow equations (continuity, flow resistance and bedload transport) and four self-adjusting endogenic variables (width, depth, velocity and slope) can predict the theoretical conditions required for the development of self-forming equilibrium channels, the maintenance of an optimal channel form under natural conditions appears to often require the introduction of vegetation, an exogenic variable that cannot yet be readily included in hydraulic computations. Further, this study shows how the H number model enables river channel equilibrium to be determined directly and more immediately. In the past, geomorphologists have presumed that the equilibrium status of a river would have to be determined by assessing the mass-balance of sediment transport, or adjustments of channel dimensions, over time.