Similarly, the characteristics of the Vertosol soils in the Downs country land type in the BBB have unique properties that may confound gully remediation. Most importantly the soil develops cracks in the dry season which makes banks vulnerable to tunnel erosion in the wet season,
particularly during intense first rains before the soil has swelled to full water-holding capacity. In addition, this cracking clay, unlike the Vertosols of Brigalow country, is friable, crumbles easily and is vulnerable to all forms of erosion. This was illustrated at Site 24( see Figure 31).
Rock chute
Building new bund to drain water towards rock chute
Breach in wing bank
April 2024 October 2024
Breach in new wall
Breach in wall caused by scour
Post January 2025 January 2025
Site 24 – Lessons from a rock chute installation
At Site 24, a rock chute was installed below a series of diversion banks. Over the following four years, the sharp gully edges gradually battered down and vegetated quite quickly. However, while the diversion banks successfully increased the length of the flow path and slowed runoff, this had no significant effect on the overall runoff volume( Hawdon et al., 2025, p. 23). This is likely due to the site’ s shallow soils( less than 2m deep in the lower gullies) and a slope of approximately 6 per cent, both of which limit water infiltration.
Standard twin wing banks were installed either side of the rock chute to direct runoff safely onto the stable gully floor. However, the eastern wing bank retained water instead of draining freely. This led to tunnel erosion and ultimately caused a break in the bank. Interestingly, dead land crabs were found in the dried pond that had formed behind the bank, and may have contributed to the failure.
As a result, the rock chute was bypassed. Sediment buildup in front of the chute’ s crest may also have contributed to this change in flow direction.
Remediation works were undertaken in the 2024 dry season. The eastern wing bank was rebuilt on higher ground to promote free drainage. However, due to limited space and a steep drop into one of the gully branches, the structure became a bund linking the final diversion bank to the chute. Despite being well constructed, the bund was scoured out during the first rains, and much of the material was deposited in front of the chute, leaving it high and dry once again.
It’ s worth noting that the original design by a highly experienced soil conservation officer did not include a rock chute. Instead, the plan featured broad spreaders placed on a firmer ridge.
Key takeaway:
Rock chutes are generally unsuitable for the mobile soils of the Downs country. They are costly, challenging to site correctly, and require ongoing maintenance to remove sediment buildup. Keeping broad, flat flow paths clear is essential to reduce water velocity and erosion. Most importantly, maintaining high vegetation cover remains the most effective strategy for preventing and repairing erosion in these landscapes.
Figure 31. Additional information on the maintenance at Site 24.
48 GULLY MAINTENANCE AND MONITORING TECHNICAL REPORT