Glen Bowen work begins | Landholders Driving Change

Work begins at Glen Bowen Station large-scale gully remediation

WORK has started at Glen Bowen Station, the LDC’s second large-scale gully remediation site.  Machinery was mobilised in November, and earthworks are expected to be completed by mid-December. 

Neilly Group Engineering was commissioned to design the remediation measures to be carried out at Glen Bowen.

The site that is being remediated, and known as Gully 1, is an alluvial gully and is eroding primarily through direct rainfall which indicates the soils are highly dispersive.  This was confirmed through soil testing.

The actively eroding footprint of Gully 1 has a perimeter of approximately 1.1km, with an internal area of 3.36ha. The banks of the gully scarp are on average 2 – 2.5m high. 

This aerial photograph (below) encapsulates the entire footprint of the gully.  The drainage lines within the gully all converge to a single drainage line that discharges into the Bowen River.

The design

The design at Glen Bowen involves the reshaping of complex gully networks which will form two stabilised, free-draining basins. The reshaping will involve bank battering and topography reshaping through cut and fill, and compaction earthworks operations. 

On-contour rock check dams will be constructed across the bed of the drainage basins to assist in slowing the flow of water and improving the soil condition by enhancing the deposition of fine sediment, nutrients and seed. 

The design also includes reshaping three incised linear gullies and one gully scarp which will also be reshaped to create a free draining surface which is less undulated and ties into the existing surface.

The existing earthen bund around the head of the gully will be reinforced to manage overland flows and utilise excess cut from reshaping works.

The reshaped surfaces will be treated including:

  • application and incorporation of gypsum to all reshaped and disturbed surfaces;
  • spreading of topsoil stripped from the footprint of the works or borrowed from an alternative source on the property;
  • application and incorporation of mulch;
  • sowing or broadcasting of seed and application of fertiliser; and
  • application of coir net matting over the designated batters and upslope areas of the reshaped surfaces.

The cost of the remediation is $415 per ton of sediment yield.

Following remediation we expect:

  • improvements in vegetation, cover and biomass, and soil condition;
  • reductions in erosion and gully retreat rates; and 
  • improvements in water quality (sediment concentrations) because less material is being removed and more sediment is being trapped.

Glen Bowen is being monitored and evaluated using a range of practices for the effectiveness of vegetation, runoff and water quality measures.  

CSIRO has installed monitoring stations at two gullies – the treatment site that is undergoing remediation and a control site for reference.

Longer term, it is expected there will be improvements in runoff and sediment loads, however this is also influenced by rainfall, so trends will take longer to detect.

Tackling gullies in the BBB

THE BBB produces almost a quarter of the total fine sediment load that ends up in the Great Barrier Reef lagoon.

About 65 per cent of fine sediment comes from gully erosion which makes gully management a high priority for investment, and is why gully remediation is a key component of the LDC project.

The Landscape Remediation program is trialing large and small-scale gully remediation approaches to improve water quality, and to determine cost effective solutions that can be transferred to other projects and catchments.

By the end of 2020:

  • Work will be complete on three demonstration sites showingcasing different methods of treating large-scale gully erosion; and 
  • Multiple small-scale erosion sites will be remediated, in partnership with landholders.

The LDC goal for the Landscape Remediation program is to produce a high resolution spatial map of active gullies, and a long-term rehabilitation investment prioritisation plan for the BBB.

This will mean having multiple shovel-ready projects available to take advantage of future funding opportunities.

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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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