Mt Wickham the first trial on a big-scale gully
The Landholders Driving Change Landscape Remediation Activity Area is about trialling different remediation approaches to determine cost-effective solutions that can be transferred to other projects and catchments.
A pilot large-scale gully rehabilitation project has been established at Mt Wickham with the aim to return the catchment to a healthy functioning landscape.
Engineering firms Verterra and Alluvium, principal partners on the project, developed the gully remediation technical design, and project managed works on site. Local contractors completed phase one on-ground works, and will also carry out phase two works in the coming months.
LDC monitors the effect of the work through CSIRO on-ground water quality monitoring equipment to help develop a cost-benefit analysis. Results of the study site, including the smaller gully remediation trials being carried out across the BBB, will inform a strategic investment plan for landscape remediation in the BBB.
Basin 1A before treatment.
Basin 1A after treatment.
A summary of the Mt Wickham project so far
Basin 1A and 1B
Basin 1A is where the most significant erosion took place prior to the start of on-ground works. The area underwent significant earthworks with tunnels excavated and the area reshaped and compacted with a bulldozer to a gradually sloping shape to reduce the velocity of water moving off the site. Hay bales were placed in rows across the basin to slow water and add organic matter to the site.
Works on Basin 1B took a similar approach to 1A on a smaller scale. Both basins were ameliorated with topsoil and seeded.
A series of rock checks have been constructed in Basin 1A to manage water flow down an established flow path. These rock checks were designed to be long and wide with a gradual rise and fall.
They are long so that water will pass over the check rather than go around and out flank; and are wide with a gradual gradient, similar to a speed bump, to decrease the water velocity.
Rock chute 5
Approximately 12 metres long and about 1.5 metres deep, this structure was designed to slow water moving down the main gully channel. The slowed velocity allows sediment particles to ‘drop out’ of the water column at the chute and settle on the gully bed. This area of lower velocity allows the establishment of a new gully floor to reduce upstream in-sizing.
The banks of rock chute 5 were threatened during the heavy January and February rainfall events.
To ensure the chute functioned as designed it was necessary to install temporary diversion banks either side of the chute to spread water across the landscape, rather than over the banks of chute 5.
After the bank stabilisation occurs through increased ground cover, the temporary banks will be removed to allow natural landscape processes to occur.
Gully 10 before treatment.
Gully 10 after treatment.
This alluvial gully was reshaped, filled with stable material, compacted, and capped with ameliorated top soil, checked with rock structures and seeded.
Blue Valley Road runoff diversion banks
Diversion banks were constructed to divert runoff from the road and spread it away from the gully head. Runoff from the road that flowed into a drain was one of the main issues impacting the gully head.
Reducing the volume entering the gully head by spreading water appears to have alleviated the pressure on this area.
An additional rock chute, similar in function to chute 5 will be built near the intersection of the catchment and road drainage, i.e. head of the gully.
This will add further protection to the bed of the main gully system through promotion of vegetation growth in the gully floor, providing future natural sediment traps
Plans are being developed by Alluvium and Verterra for untreated gullies on the northern side of the main gully. They will be excavated and reshaped, and rock chutes will be constructed.
Amelioration involved ripping with a bulldozer and harrows and spreading of an ameliorate mix made up of topsoil, fertiliser, and gypsum. The composition of this mixture was based on soil sample results.
Ripping was done at various depths up to 200mm so as to not disturb the highly dispersive soil but to change the flow of water under the ground surface in an effort to reduce the potential of future tunneling. The different depths also increased water infiltration.
French White Millet made up a quarter of the seed mix and was used as a fast growing sterile cover crop species to provide initial protection and organic matter. Germination has been diverse with a number of different species being identified with a particular abundance of French White Millet, Star Grass, and Gayndah Buffel Grass.
Groundwork at Mt Wickham faced it’s first rain test in January. Click here for all the details. Please note that the site has undergone maintenance since this update.
Government water quality targets provide the context for the project
In response to the ongoing water quality impacts affecting the health of the Great Barrier Reef (GBR), the Queensland and Australian governments have set ambitious targets in the Reef 2050 Long Term Sustainability Plan (Reef 2050 Plan) to reduce pollutants in key reef catchments by 2025 (up to 50% reduction in sediment run-off and up to 80% reduction in nitrogen).
The Queensland and Australian governments also released a new Reef 2050 Water Quality Improvement Plan in July this year that comprises a revised set of catchment targets that can be read here.
Why Mt Wickham was chosen as a demonstration site
The LDC project contracted gully remediation experts to develop a range of criteria to help locate a ‘typical’ large-scale sodic gully rehabilitation study site in the BBB.
Criteria included biophysical traits such as:
- size of the gully;
- soil type; and
- possessing typical erosion features the LDC project is trying to fix.
Other factors include (but not limited to) that the site can:
- demonstrate a range of gully remediation techniques and solutions;
- show the outcome of various remediation treatments, and if they are transferable and cost effective;
- be easily accessible;
- be able to host field days; and
- a willing landholder to host a demonstration site. (We are grateful to the owners of Mt Wickham for allowing the project to undertake remediation works on their property).
Tackling gully remediation in the BBB
Sediment source tracing, erosion mapping and catchment modelling studies show that gully erosion contributes approximately 40 per cent of all fine sediment to the Great Barrier Reef Lagoon.
This makes gully management a high priority for investments to reduce sediment loads.
The principles of gully management success are:
- to increase the efficiency of sediment trapping within gullies;
- improve vegetation cover in gully features to improve stability; and
- to reduce surface runoff into managing gully erosion to improve water quality in the Great Barrier Reef Lagoon
The BBB produces almost a quarter of the total fine sediment load that ends up on the Reef. The LDC Landscape Remediation Activity Area is about trialling various gully remediation approaches in an effort to manage gully erosion to improve water quality, and to determine cost-effective solutions that can be transferred to other projects and catchments.