One of Australia’s top science prizes has been awarded to a Tropical Water Quality Hub project team uncovering a previously under-appreciated source of negative impacts on the Great Barrier Reef and how to deal with them.
Assoc Prof. Andrew Brooks from Griffith University heads up a TWQ team that is examining the role played by alluvial gully erosion in the vast Great Barrier Reef catchments of north Queensland’s cattle country and was awarded a Eureka Prize on August 30 for their efforts.
Previous assumptions pointed to surface erosion from grazed hillslopes as the primary source of fine sediment erosion but Prof Brooks and his team used a wide range of both high-tech and low-tech approaches to find huge amounts of fine sediment being scoured from gullies and streambanks and making its way to the Great Barrier Reef.
High-tech approaches used by Prof Brooks and his team included satellite imaging, airborne light detection and ranging (LiDAR), geochemical fingerprinting and radionuclide tracing of cesium-137 in surface soils.
But complicated gadgets don’t always cut the mustard in the harsh conditions of remote Far North Queensland. Prof Brooks and his team hand-built many of the monitoring devices used in the project, such as sediment traps, using material from local hardware stores.
These innovative approaches and a broad range of data sources allowed the team to identify the sources of around a million tons of fine sediment being moved in the Normanby catchment alone.
Prof Brooks said much of the erosion is a legacy issue stemming from the introduction of cattle into the region over 150 years ago. As the animals congregate on the channel banks to feed, their hooves cut into surface soils and allow moving water to concentrate its energy and penetrate into the highly unstable sub-surface soils. This sets in train a feedback-driven erosion process that once started is unlikely to stop of its own accord any time soon, even if cattle are now completely removed from the landscape.
The result is the formation of enormous gullies, some more than ten metres deep, cutting into the alluvial plain and releasing vast amounts of sediment and nutrients into water courses and eventually the Great Barrier Reef lagoon.
Traditional approaches to erosion mitigation have proven ineffective or even counter-productive, as Prof Brooks and his team have found.
Instead, he advocates for using mine-site rehabilitation techniques and full reconstruction of gullies with industrial equipment in the more active gullies which are disproportionately large sources of sediment.
Fortunately, the most serious erosion occurs over a relatively small geographic area, meaning effort and investment can be concentrated for the best results.
The project has already had measurable impacts, informing changes in Queensland government policy toward erosion management.
Prof Brooks, who has been working on the project in various forms for over a decade, said the Eureka Prize was ‘very unexpected’ but a ‘definitely a career highlight’.
“The particular award we received is for a research team, so it is great recognition for efforts of the whole team,” he said.
“I’d like to hope this spurs further interest and keeps that interest in water quality on the Great Barrier Reef – I think in the last 12 months there has, understandably, been a bit of a shift in attention to bleaching and the role of temperature, but sediment and other water quality impacts are still important issues for the GBR ecosystems, and we are only just beginning to tackle gully erosion sources, so we can’t afford to lose sight of this issue.”