Gully prioritisation – developing a catchment-wide investment plan

The BBB generates almost half of the fine sediment exported to the Great Barrier Reef lagoon. For this reason, it is a target hotspot for sediment reduction, which is why gully remediation is a key component of the LDC project.

Historically, the prioritisation and selection of gullies for rehabilitation in the Great Barrier Reef (GBR) catchments was based on the ad hoc use of satellite or aerial imagery and local knowledge. Consequently, the prioritisation and selection of gullies for remediation intervention was not as systematic. 

Prioritising work is important because it links to improvement in water quality leaving the landscape (properties) by identifying those gullies where most sediment can be stopped.

Up-to-date monitoring shows half of the sediment from the BBB comes from just 6 per cent of the gullies in that area, emphasising the need to prioritise.

One of LDC’s goals was to produce new mapping and prioritisation methods to help identify active and cost-effective gullies for remediation. This included a long-term rehabilitation investment prioritisation plan for the BBB.

LDC contracted Griffith University to deliver a report that characterised and prioritised the highest sediment producing gullies in the BBB catchment. This research work was carried out between 2018 and 2021.

The results from this body of research has significant implications for policy settings around the ongoing rollout of water quality improvement strategies for the GBR over the coming decades to achieve the GBR water quality 2025 and 2050 targets.

The data shows that significant inroads can be made into the required sediment yield reductions by focusing on the highest yielding alluvial gullies, while being mindful of cost effectiveness.

In October, Griffith University carried out further ground verification work for consistency with gully classifications. The data collected will confirm the accuracy of the LiDAR data previously used and will help refine the data informing the gully prioritisation work. 

Data capture included gully characteristics, erosion activities, soil profiling and sampling.  Results from this validation work will provide NQ Dry Tropics and investors the confidence that investment in landscape remediation will be made where the most reduction in sediment yield will be achieved.

Which gullies should be considered as a priority?

There are three primary features of gullies that determine whether rehabilitation will deliver a cost-effective return on investment:

  1. Rate of erosion: gullies that are active and have a high rate of fine sediment generation.
  2. Position in the landscape: includes factors such as distance to the end of the catchment, rainfall patterns and sediment delivery ratio which may be affected by soil type, slope and trapping mechanisms such as dams.
  3. Opportunities for efficiencies: rehabilitation of densely situated gully features may prove to be viable through clustering of efforts.

 

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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