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Alluvial gully erosion rates and processes across the Mitchell River fluvial megafan in northern Queensland, Australia | TRaCK: Tropical Rivers and Coastal Knowledge

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Alluvial gully erosion rates and processes across the Mitchell River fluvial megafan in northern Queensland, Australia

TitleAlluvial gully erosion rates and processes across the Mitchell River fluvial megafan in northern Queensland, Australia
Publication TypeThesis
Year of Publication2011
AuthorsShellberg, JG
Academic DepartmentGriffith University, Australian Rivers Institute, School of Environment
Number of Pages251
UniversityGriffith University
CityBrisbane
Keywordsalluvial gully erosion
Abstract

Gully erosion is the process by which running water cuts new unstable channels into erodible regolith. It causes severe land degradation, is a major component of contemporary sediment budgets, and is a major source of sediment pollution to aquatic ecosystems. In northern Australia, there is widespread gully erosion into unconfined alluvial deposits on active and abandoned floodplains – here defined as alluvial gully erosion. In catchments draining to the Gulf of Carpentaria, alluvial gullies can cover 0.2% to 1.0 % of the total catchment area and locally >10% of the floodplain area. Alluvial gully erosion has been poorly documented and differs substantially from colluvial or hillslope gullying in south-eastern or northern Australia. The objectives of this research were to investigate and quantify alluvial gully erosion processes and rates at a variety of spatial and temporal scales across a pilot study area, which encompasses the Mitchell River fluvial megafan on the Cape York Peninsula in northern Queensland.

Along the Mitchell megafan, alluvial gullies are concentrated along main drainage channels. Their scarp heights and potential energy are highly correlated to the local relief between the floodplain and river thalweg, which is a result of river incision into the megafan since the Pleistocene. Other factors such as floodplain hydrology, soil texture and chemistry, vegetation cover, and land-use disturbance also influence the distribution and propagation of gullies, via changes in the driving and resisting forces. The frequency of river flood inundation of alluvial gullies on the floodplain changes longitudinally according to river incision and confinement.

Near the top of the megafan, flood water is contained in the macro-channel up to the 100-yr recurrence interval (RI) but still backwaters adjacent alluvial gullies. In downstream Holocene floodplains, complete inundation of alluvial gullies occurs beyond the 3- to 5-yr RI and can contribute significantly to total annual erosion. However, a majority of gully scarp retreat is still driven by direct rainfall and infiltration-excess runoff, with the 24-hr rainfall total being the most predictive variable. This direct erosion is enhanced by inherent soil dispersibility and the lack of vegetative cover, with the later if present having the potential to dissipate the effective kinetic energy of rainfall and promote infiltration.

Sediment production estimates from alluvial gullies across the Mitchell megafan suggest that ~ 6.3 Mt/yr were eroded historically, compared to ~3.9 Mt/yr recently. These sediment sources dominate the suspended sediment budget for the Mitchell River, when compared to empirical load estimates from main river and tributary gauges up- and down-stream. At a 33 ha gully, empirical water and sediment yield measurements document high suspended sediment concentrations (10,000 to >100,000 mg/L) and sediment yields (80 to 350 t/ha/yr), which are high by both Australian and world standards. Theoretical modelling of both suspended sediment- and wash-load transport suggests that sediment concentrations are near the ‘transport limit’, which can be modelled using transport-limited equations. Additional field gauge data and modelling efforts should continue to investigate the event, seasonal and annual variability in sediment yield, internal erosion processes, and gully morphology evolution at distributed gully sites.

Trees that have survived or re-colonized after the passage of a gully head cut have the potential to define the timing and rates of gully erosion, through ring counting and age dating. Radium- 226/228 and carbon-14 radionuclide dating of Eucalyptus microtheca trees demonstrates that ring production rates vary between 0.3 and 0.9 rings/yr, depending on local growing conditions. These rates can be used to estimate tree ages from ring counts. Tree age and position along the gully outlet channel can be used to estimate tree colonization rates. Independent erosion rate estimates from historic air photos suggest that time lags and disequilibrium exist between scarp retreat, gully inset-floodplain development, and tree colonization. Thus, tree ages on gully insetfloodplains can only define the minimum time of gully initiation, whereas tree colonization rates can only estimate maximum rates of gully expansion.

Alluvial gully scarp retreat rates are quantified at 18 sites across the Mitchell megafan using recent GPS surveys and historic air photos, which demonstrate rapid increases in gully area of 1.25 to 10 times their initial 1949 area. Extrapolation of gully area growth trends backward in time suggests that the current unprecedented phase of extensive gullying initiated between 1880 and 1950, post-European settlement. This is supported by young optically stimulated luminescence (OSL) dates of gully inset-floodplain deposits, LiDAR terrain analysis, historic explorer accounts of earlier gully types, and archival records of cattle numbers and land management. It is hypothesized that intense cattle grazing concentrated in the riparian zone during the dry season increased the potential for gully erosion initiation in the wet season along steep banks, hollows and precursor gullies. This is a result of reduced native grass cover, increased physical disturbance of soils, and the concentration of runoff along cattle tracks, which were possibly coupled with fire regime modifications, episodic drought, and the invasion of exotic weed and grass species. Thus, land-use change pushed the landscape across a threshold towards instability, which it was intrinsically close to as a result of the evolution of the fluvial megafan over geomorphic time. A conceptual model of the evolution of these alluvial gullies has been developed that describes their initiation, development, and potential stabilization over time. Spatial and temporal projections of gully area growth into the future until stabilization suggest that they will continue to be chronic erosion features on the landscape for several hundred to several thousand years, growing 10 to 50 times their initial 1949 size, unless mitigated by land management intervention.

A paradigm shift in land management and cattle grazing practices is needed to reduce chronic soil erosion across northern Australian floodplains. Cattle should be managed more cautiously in or excluded from the riparian zones and steep banks of river, streams, and other water bodies across large areas of floodplain landscape. This will reduce the initiation of new alluvial gullies, slow gully erosion rates where already initiated, and aid in passive or proactive rehabilitation efforts. Australian and international gully literature indicates that many different biological, chemical, and physical tools and passive or direct management actions are available to rehabilitate gully erosion. However, the failure of many rehabilitation projects is common due to a lack of initial rigorous experimentation and adaptive management. A trial rehabilitation program could determine the most cost-effective, practical, and sustainable land management activities needed to reduce alluvial gully erosion, and prevent future initiation, by targeting the process-based causes of gully erosion rather than the symptoms. Outcomes could be synthesized into best management practices (BMP) guidelines for alluvial gullies that could be utilized by the regional pastoral community for rehabilitation and soil conservation actions.