University report will inform landscape remediation plans

GRIFFITH University has delivered a report that characterises and prioritises the highest sediment producing gullies within the BBB catchment. Clusters of high and moderate-yielding gullies have been identified to focus sediment savings efforts.

A total of 22,300 actively eroding gullies in the BBB were mapped, providing a blueprint for future investment in large-scale gully remediation within the catchment.

Importantly, the report identified 73 priority “clusters” of gullies. Clusters of high and moderate-yielding gullies are most likely to be the most cost-effective places to focus resources for rehabilitation to achieve major sediment savings to help meet end-of-catchment water quality targets as identified in the Reef 2050 Water Quality Improvement Plan.

The report notes that there is a small number of high yielding gully clusters disproportionately contributing a significant proportion of the catchment’s sediment yield.

The report, Gully Rehabilitation Prioritisation in the Bowen and Bogie Catchments, was led by Griffith University Associate Professor Andrew Brooks and the university’s Precision Erosion and Sediment Management Research Group, in conjunction with the Australian Rivers Institute, James Cook University’s TropWATER,and engineering firms Alluvium and Verterra.

The final report has broken new ground in technical methods and approaches to gully work nationally and internationally, and has undergone technical review. 

LiDAR (Light Detection and Ranging) was used to characterise and prioritise the highest sediment producing gullies within the BBB catchment. 

LiDAR is a method for measuring distances with laser light (from a fixed wing aircraft in this instance) and measuring the reflection with a sensor. Differences in laser return times and wavelengths can then be used to make digital 3-D representations of the target, and high resolution maps from which erosion and sediment losses can be calculated across time. 

The current footprint of LiDAR capture was directed at the known “hotspots” for active gullies identified through a preliminary mapping exercise undertaken at the start of the LDC project.

This final report is the result of many years of effort including an initial gully prioritisation report in July 2018, which provided the basis for the initial large scale gully remediation efforts for the LDC project at Mt. Wickham and Glenbowen. 

A technical report was produced in December 2018 on a methodology for mapping large-scale gullies.  Detailed site plans and remediation concept plans for large scale gully complexes across five properties were completed during 2019 and early 2020, with this final report completed in June 2020.

Here is more information about how LDC is piloting a suite of gully repair approaches in the BBB catchment area.

Griffith University Associate Professor Andrew Brooks.

A gully at Mt Wickham.

LiDAR helps to produce 3D maps

 

LiDAR (Light Detection and Ranging) allows scientists and mapping professionals to examine natural and man-made environments with accuracy and precision.

Airborne LiDAR is a remote sensing method that uses light in the form of a pulsed laser to measure distance to the ground.

These light pulses combined with other data recorded by airborne systems generate precise, three dimensional information about the shape of the landscape and its surface characteristics.

The data, combined with historical air photos, can be used to reconstruct gully erosion rates and sediment yields.

This information is critical for determining sediment savings and the cost-effectiveness of rehabilitation treatments.

LiDAR will also provide information used in the design of on-ground remediation 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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*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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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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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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