Alluvial Gully Erosion Rates and Processes Across the Mitchell River Fluvial Megafan in Northern Queensland, Australia

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.