Prioritising BBB gullies for remediation

LDC and Griffith University mapped gullies in the BBB to help prioritise gully rehabilitation projects. Ground verification work has also been completed to keep the project on track to tackle gully hotspots.

NQ Dry Tropics’ understanding of where and how to remediate gullied landscapes in the dry tropics has grown from a low base five years ago, to now having a significant body of guidance. 

Work arising out of the LDC includes:

  • transferable procedures for the remote sensed mapping and characterisation (including load estimates) of gullies;
  • an extensive, spatially-explicit, GIS dataset that identifies, characterises and prioritises 22,311 active gullies within the BBB area;
  • a framework for site selection that incorporates a range of biophysical and socio-economic considerations; and
  • a spatial prioritisation based on cost-effectiveness that incorporates the biophysical characteristics and socio-economic considerations.

Prioritising work is important because it identifies the most sediment yielding gully systems to guide investment to remediate gully hotspots. 

In November last year, NQ Dry Tropics and Griffith University carried out ground verification work to ensure the initial mapping undertaken was robust.

The methodology has been validated and this means the prioritisation work can be relied on by investors as a valid approximation of gully sediment yield.

Water quality in the BBB – the challenge

Fine sediment and nutrients from agricultural land are a significant threat to the Great Barrier Reef.

The Burdekin catchment is the largest contributor of anthropogenic fine sediment loads of the 35 catchments that drain to the Great Barrier Reef. It delivers, on average, 3.3 million tonnes per year; more than double the load of any other catchment. Most comes from gully erosion in grazing areas. 

The Bowen Broken and Bogie (BBB) river catchment encompasses an area of 11,718km2 and is part of the much larger Burdekin River system (130,000 km2). Despite its relatively small size (less than 9 per cent of the Burdekin, and less than 3 per cent of the total catchment area draining into the Great Barrier Reef), the BBB catchment generates almost a quarter of the total fine sediment load into the GBR lagoon. 

About 65 per cent of that fine sediment comes from gully erosion. A total of 22,311 active gullies were mapped within the BBB study area (~3,500 km2), with a cumulative active area of 4,620ha.

This makes the gully remediation in the BBB catchment a focus for investment activity. Gully remediation in this landscape is challenging. The area is vast, sparsely populated and poorly serviced. 

The climate exhibits a pronounced wet and dry season with most rain falling in intense falls between November and April. Average annual rainfall is 715mm (28.1 in), with significant year-to-year variability. The window for undertaking large-scale earthworks is often narrow, and revegetation is constrained by water availability.

Many of the gullies are a legacy of historical land management, and unrelated to current land management practice or current owners. Many gully works involve high capital cost and deliver little private benefit to the landholder. Characteristics vary considerably between gullies, requiring bespoke treatment plans for each gully.


Reef regulations - grazing, standard conditions

  1. For land in good or fair condition (more than 50 per cent ground cover at 30 September), continue using measures to maintain land condition.
  2. For land in poor condition (less than 50 per cent ground cover at 30 September), steps must be taken to improve land condition.
  3. For land in degraded condition (less than 20 per cent ground cover at 30 September), steps must be taken to improve land condition OR prevent areas from further degrading or expanding.
  4. Keep records of measures taken and also of agricultural chemicals, fertiliser and mill mud or mill ash applied to land.
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The LDC project monitors four gully sites (represented in this table) with gold standard equipment and analysis, carried out by CSIRO.

Results have been compiled in a preliminary report from Bartley et al (2020), with the final report expected to be released by the end of 2020. The preliminary report shows all four sites have indicators of improvements, notably the Strathbogie and Mt Wickham sites.

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*The Exploring New Incentives activity area has provided an opportunity for graziers to adopt improved land management practices through a range of activities. For some of these properties, it was the first time they signed contracts for on-ground works.

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Figure 1. Total fine sediment reduction by project type and erosion source. Inset shows the proportion of the total project area for each project type.

These estimates have been calculated using two methods: 

1) The pollutant reduction component of the Alluvium/Great Barrier Reef Foundation (GBRF) investment tool for hillslope and streambank erosion management projects; 

2) The Reef Trust Gully Toolbox method for gully erosion management projects. The LDC Water Quality Report 2020 (Waterhouse et al., 2020) highlights that a number of assumptions underlay these calculations, therefore these figures should be treated as the best available estimate of sediment reductions to date.

Preventing sediment from reaching the Great Barrier Reef

Each wet season sediment is washed into local waterways and out to the Great Barrier Reef (GBR). 

Most sediment is very fine, and can stay suspended for a long time and can travel great distances. Valuable topsoil is lost from production, and increased concentrations on the reef can be harmful to seagrasses and corals. 

Landholders in the BBB have completed 69 on-ground water quality practice changes, and it is estimated that these have contributed a fine sediment reduction of 6,154 tonnes per year from reaching the GBR. 

Of this, approximately half of the sediment savings are attributable to grazing land management changes on hillslopes and streambanks, and the other half as a result of gully remediation treatments across a broad range of scales, as shown in the graphs above. 

The table below also highlights the relatively small area of intervention in the gully management projects compared to the large sediment savings that these can achieve - 60 per cent of the sediment savings over only 4 per cent of the project area.

Table 1. Estimated sediment reductions (tonnes) from projects completed in the LDC Project to date.

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*GLMWW = Grazing Land Management Wire and Water

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