Glen Bowen update | Landholders Driving Change

Rehabilitating gullies to improve water quality

Phase two of gully remediation at Glen Bowen has been completed.  

The two alluvial gullies, called Gully 2 and Gully 4, are shown (right).

Engineering firm Neilly Group developed the gully remediation technical design for these two gullies, and project managed the works. Local contractors completed the on-ground works.

This work follows the remediation of Gully 1 that was carried out on Glen Bowen in 2019, read about it here.

Site overview: Gully 2 and Gully 4.

Gully 2

Gully 2 is a typical alluvial gully formation where its catchment is largely itself. It is eroding primarily through direct rainfall alone, which indicates that the soils are highly dispersive. 

This has been confirmed by soil testing results presented in the assessment report (Griffith University, 2019).

The actively eroding footprint of Gully 2 has a perimeter of about 1km, with an internal area of 1.1ha. The banks of the gully scarp are on average 2.5 – 3m high. 

The drainage lines within the gully all converge to a single drainage line that discharges into the Bowen River.

The aerial photographs show the footprint of the gully.

Gully 2

The Gully 2 remediation consists of the following works:

  • Reshaping of the northern and central complex gully networks into free draining basins. Reshaping work involved bank battering and reshaping through cut to fill and compaction earthworks operations.
  • Reshaping of the southern complex gully network into a stabilised and free draining basin, and reshaped through cut to fill and compaction earthworks operations.
  • Construction of three rock cut-off walls at the downstream extent of each of the basin reshaping works.
  • Construction of rock check dams on 1m contours in the beds of the reshaped basins.
  • Treatment of all the reshaped surfaces consist of:
    • application of gypsum to all reshaped and disturbed surfaces;
    • spreading of topsoil stripped from the footprint of the works; and
    • application of soil ameliorants, fertiliser and drill seeding.

Gully 4

Gully 4 has an upstream catchment about four times the size of the gully’s footprint. 

The main linear drainage line of the gully appears to have developed due to the concentration of overland flow from the upstream catchment, resulting in linear head cut gullying. 

The adjacent alluvial gullies developing off this main gully have no contributing catchment beyond their own footprint and are eroding primarily due to direct rainfall alone. This indicates the soils are highly dispersive, which has been confirmed by soil testing results presented in the assessment report (Griffith University, 2019).

The actively eroding footprint of Gully 4 has a 1.2km perimeter with an internal area of 4.8ha. The banks of the gully scarp are on average 0.6m high. 

An aerial photograph which encapsulates the entire footprint of the gully is shown (right).

 

 

Aerial photograph of Gully 4 looking downstream from southern extents.

The Gully 4 remediation consists of the following works:

  • Construction of a rock chute on the head cut located on the main drainage line with a bund to divert overland flows and control their entry into the gully.
  • Construction of a rock chute on the head cut located on the smaller drainage line to the east with a bund to divert overland flows and control their entry into the gully.
  • Reshaping of the actively eroding scarp into a free draining basin to redirect water flow through the reshaped basin and into the main drainage line. 
  • Reshaping of a steep sloped alluvial gully on the north-eastern extents of Gully 4 into a single stabilised and free draining basin. 
  • Reshaping of 13 smaller scale alluvial gullies into individual stabilised and free draining areas.  Construction of multiple rock check dams, on 0.3m contours in the beds of the reshaped areas as well as throughout the existing main drainage line and a smaller drainage line located to the east. 
  • Treatment of all the reshaped and regraded surfaces consisting of a minimum of:
    • application of gypsum to all reshaped, regraded and disturbed surfaces;
    • spreading of topsoil stripped from the footprint of the works; and
    • application of soil ameliorants, fertiliser and drill seeding.

 

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KEQ #1

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.

KEQ #2

*GLMWW = Grazing Land Management Wire and Water

KEQ #3

*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.

KEQ #4

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.

KEQ #5

KEQ #6

KEQ #7
KEQ #8

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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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