Strathmore work begins | Landholders Driving Change

Work starts on fourth large-scale gully

Work has started on LDC’s fourth large-scale gully on Strathmore Station near the Bowen River rodeo grounds, 1.3km from the Bowen River Bridge.

The location was selected by the Griffith University project team as a high value site to reduce fine sediments in the Great Barrier Reef lagoon. The gullies being remediated have a collective area of about 7,650 m2 and it is estimated that these gullies output on average approximately 922 t/y of fine sediment to the reef.

The design has been led by engineering firms Verterra and Alluvium Consulting as an Unincorporated Joint Venture. They are managing the works which are being carried out by local companies Pattle Contractors and Beauchamps Excavating.

The works consist of two sites within the Strathmore Station boundary, and are referred to as the Northern zone (gully reshaping) and Southern zone (bund and chute).

The selected gullies are on a small eastern tributary which joins Red Hill Creek about 2km upstream from its confluence with the Bowen River.

Due to high visibility and accessibility, this site lends itself to becoming a demonstration of gully rehabilitation.

Rehabilitation is expected to be completed this month (December).

Remediation approach:

Northern zone

The northern gullies (combined) are being remediated by reshaping the upper active erosion to form a gently sloping basin with a single channel at the northern extent. The crest will be reshaped to drain away from the basin and direct flows to a stable gully in the south. 

The single channel will be reshaped and existing tunnels will be excavated to the base and collapsed and contaminated waste is to be removed prior to the reshaping works starting.

Topsoil is being stripped, stockpiled and ameliorated. Uncontaminated cut material will be stockpiled and ameliorated to be used as capping material of ameliorated topsoil.

Modification of the northern zone is reducing the catchment area and reducing flows leaving the site downstream of the landform.

Flows are directed around the western perimeter of the land formed area to the stable gully at the south. The landform works receive only rainfall falling within the reshaped area.

Southern zone

Remedial works in the southern zone involve constructing a contour bund that redirects overland flow to a single rock chute to prevent flows entering the active erosion sites in the southern zone. 

The contour bund and drain redirects flow from the table drains to a stable area.

The works consist of the following:

  • Construction of a contour bund that will redirect table drainage to a chute and into a stable gully;
  • Installation of a grade control rock chute;
  • Soil amelioration of disturbed areas (sub-surface and surface);
  • Soil amelioration and revegetation of contributing catchment areas (surface);
  • Installation of fencing around the works.

A cultural heritage survey and impact assessment was completed at the gully sites in consultation with the Birriah People which you can read about here.

Excavation works for a rock chute at Strathmore.

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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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