Erosion management and avoidance in northern Australia

This project used the Gilbert-Einasleigh region of Queensland as a case study to demonstrate how new irrigation developments can be planned and implemented to minimise the threat of developing new alluvial gully erosion or exacerbating existing erosion (both associated with alluvial gullies and stream bank erosion).  The study highlights the need for high resolution soil mapping and landscape analysis prior to development.

The extreme variability of rainfall and prevalence of dispersible soils in northern Australia means that most large floodplain rivers in the region are highly prone to soil erosion, particularly ‘alluvial gully erosion’.

Alluvial gully erosion occurs when running water concentrates on roads, tracks, and cattle pads, cutting new unstable channels into floodplains, degrading the land and threatening infrastructure such as roads, fences, dams and other irrigation infrastructure.

Once initiated, gullies can migrate across floodplains up to ten metres each year, consuming large areas of otherwise productive land. In extreme cases the land may become unusable for pastoral and agricultural use, so this form of erosion represents a real risk to any new development in northern Australia. The best way to manage it is to ensure gullies are not initiated in the first place, but this involves the collection of high resolution soils and topography data as a basis for precision planning.  Regional scale synoptic mapping is not sufficient for this task.

Many of the floodplains that have been identified for new agricultural development in the Gilbert Catchment in Queensland are prone to alluvial gully erosion.  By mapping the areas at risk of this form of erosion, millions of dollars in lost infrastructure and land degradation can be saved.

The research team reviewed existing mapping and remote sensing data on gully density and erosion risk, integrated new data layers, and consulted widely with stakeholders in the catchment to determine the most likely development scenarios and how to avoid instigating or exacerbating erosion.

A spatial database was built, drawing together all the available mapping and data on gully density and erosion risk. This information was cross-checked in the field to validate its accuracy and map the progress of gully erosion since original surveys in 2006-7.

The project team also worked with industry, landholders and NRM groups to identify:

• The areas that are likely to be cleared and intensively farmed.
• Potential water extraction points from major rivers.
• The location of irrigation channels.
• The location of off-stream water storages.

The Gilbert Catchment’s alluvial stream and river channel network and existing riparian vegetation were also mapped to identify areas that should remain undisturbed regardless of the proposed development scenario. The project also identified the areas of land that will be most beneficial to rehabilitate.

The project outputs include:

• Maps identifying areas that currently subject to gully erosion, and that are likely to initiate new gully erosion should they be further developed
• Mapping to highlight areas that should be excluded from clearing or further development in order to minimise the acceleration of alluvial gully and stream bank erosion.
• A map of land that is highly degraded and could be suitable for rehabilitation as part of an offset program.

New data on gully erosion rates – with rates > 3000 t/ha/yr documented.

The Gilbert Catchment was selected as a case study because of the high degree of interest and investment focused on its development. The study area around Georgetown is approximately 300km inland between Cairns and Townsville.