Alluvial gully erosion in Australia’s tropical rivers: a conceptual model as a basis for a remote sensing mapping procedure

Alluvial gully erosion is a likely key sediment source into many large rivers in tropical Australia. This type of gullying differs fundamentally to the generally accepted gully model from Southern Australia, in that it is found exclusively within alluvium and can propagate entirely as a result of basal sapping. The process is found to varying degrees within alluvial river types along most Gulf rivers, as well as rivers in the Northern Territory and Northern WA. It is more extensive in the larger, steeper gradient Gulf rivers like the Mitchell, Leichhardt, and Nicholson Rivers. In places along the Mitchell River, gullies of up to 5m or more in height form continuous scarps along both riparian margins for tens of kilometres. Specific sediment yields of 1250 T/ha have been recorded from individual gullies in these areas yields that are comparable with the highest recorded global rates from the Chinese Loess plateau. New conceptual models of the processes driving these gullies, and controls on their morphology and spatial distribution are required if this process is to be adequately parameterised into existing sediment budget models, such as SedNet, in northern Australia. It is likely that similar features exist in eastern draining tropical rivers such as the Burdekin and Fitzroy, suggesting existing SedNet models may need to be adjusted to account for this different style of gully erosion. This paper presents a typology of alluvial gully erosion in northern Australia, and an initial model of the key factors we think are driving the process.