Resource Management and Geography - Theses
Permanent URI for this collection
Search Results
1 - 1 of 1
-
ItemGeomorphology of small arid zone streams in the Pilbara (Western Australia) and implications for design of mine river diversions( 2022)This thesis is concerned with an under-recognised human impact on rivers: river relocations (called diversions herein), in which a section of river is diverted into an entirely new channel for part of their length. Relocated channels present a consistent set of physical and ecological challenges, often related to accelerated erosion and deposition. I develop a classification of river diversions and present a series of case studies that highlight some of the key issues with river diversion construction and performance. Changes to channel dimensions and materials, alongside changes to flow velocity and channel capacity, lead to a consistent set of problems, such as heightened erosion or deposition, hanging tributaries, vegetation loss, water quality issues, and associated ecological impacts. Diversion channels often suffer engineering failures. This thesis reviews river diversions as a global phenomenon, and then focuses on a major class of diversion in which streams are diverted around mine sites. Mining river diversions are constructed to avoid flooding of the mine site and to allow access to ore. During mine operation, river diversion channels are designed to convey large floods with an emphasis on channel stability and effective flow conveyance. After mining has ceased, the expectation is increasingly that river diversion channels eventually behave more like a natural river system and imitate characteristics of surrounding watercourses. However, in many regions there is limited guidance on how to incorporate the natural geomorphic and environmental attributes of the neighbouring watercourses into these diversion designs. The Pilbara region in Western Australia has many open-pit mines and river diversion channels relocating small headwater channels but generally there is a poor understanding of the regional watercourses within this semi-arid landscape. I examine the geomorphology of small arid zone streams in the Pilbara to provide improved guidelines for geomorphic criteria for river diversion designs. This work also fills a basic knowledge gap around the geomorphology and hydrology of headwater channels in the dryland Pilbara. Geomorphic processes occur on a continuum of timescales, where landforms are influenced by a series of imposed controls (e.g. climate and geology) and flux controls, those that adjust over geomorphic timescales (e.g. vegetation, bedforms). After a review of river diversions in general, this thesis uses an integrated approach to combine geomorphology, hydrology, hydraulics and ecohydrology to address three key questions about Pilbara streams: 1) What is the geomorphology of headwater streams in the Pilbara and what are the key sedimentological, hydrological, and hydraulic controls? 3) What are the conditions required to allow natural channel morphology to develop within river diversion channels? 4) How long does it take for a target morphology to develop in river diversion channels? Based on extensive field surveys, this thesis develops the first classification of dryland headwater channels in the Pilbara, describing the variation in channel form and the range of geomorphic features found in them. Vegetation contributes 35-54% of channel roughness (resistance) in these dryland channel types and can increase channel roughness by 110% in low-slope anabranching sandy channels. This thesis highlights the importance of incorporating appropriate roughness in diversion channels through the presence and distribution of vegetation. To better understand sediment flux in these catchments, I assessed the long-term denudation within catchments using cosmogenic nuclides (26Al and 10Be) using a nested catchment approach to quantify dominant sediment pathways from slopes to larger channels. Denudation rates in the Upper Fortescue catchments are among the lowest recorded between 0.94 - 4.04mMyr-1 and channel sediment have a complex exposure history. This is attributed to sediments undergoing prior-burial for a minimum of a few hundred thousand years, and/or b) sediments being largely derived from below the surface from cliff faces through spalling and slab fragmentation. This thesis is the first comprehensive geomorphic study of headwater channels in the Pilbara, generating a series of guideline hydraulic values for different channel types. Two-dimensional hydrodynamic modelling was used to estimate peak flood flows in lieu of adequate stream gauging. Before this research, there have been limited attempts to determine the best regional flood frequency estimates for small headwater catchments in the Pilbara, leading to confusion about the most appropriate approaches to calculate peak discharges. Finally, the thesis integrates the new geomorphic knowledge to produce a series of guideline hydraulic criteria for river diversions across a range of flood intervals. The developed hydraulic guidelines for the different channel types can help engineers and managers design a permanent river diversion that replicates conditions found within local headwater channels. The results also indicate a wide range of hydraulic values within each channel type, highlighting the importance of localised fluctuations in velocity, streampower and basal shear stress in maintaining channel form and complexity.