Noosa Landcare’s Farm Forestry Program

Case Study 1. - Erosion mitigation at North Deep Creek

by:   Gary Clarke and Kaara Shaw

Since September 2002 the Noosa Landcare Farm Forestry Program has maintained and increased its services to the community and to individual landholders by undertaking consultancy contracting and total property management services.

Total property management has meant that the program has taken on a wide range of services integrating field operations to gain long term sustainable benefits for the landholders and the environment.

Erosion mitigation projects endeavour to stabilise and repair damage caused by erosion .processes. They often involve engineering solutions followed by stabilising cover crops of grass and then the establishment of long term forest cover.

Some properties are so affected by long term erosion as a result of poor land-use practices that major earthworks need to be carried out.

One such property is North Deep Creek. Clients have contracted Noosa Landcare to reafforest approximately70 hectares of cleared land that has been used (abused) for pineapple production over many decades. After initial surveys it was found that more than 50% of the property had suffered massive soil erosion and was still actively eroding due to water movement across the landscape. There were large land slips, tunnel erosion, gully erosion and severe trenching along the old pineapple bay roads which run up and down the hills every 28 meters (see aerial photograph).  

It would appear that no substantial attempt to control this erosion had been undertaken during the thirty years since it had been cleared. The result is thousands of tonnes of topsoil, subsoil and even the substrate rock has made its way from the slopes down into the gullies further downstream of the property. Even though agricultural production had ceased for some years, the massive erosion patterns were continuing.

Before reafforestation could take place the whole landscape needed to be stabilised and rejuvenated to a point where natural soil process could begin to take place again. The following processes were carried out:

• Extensive survey work and mapping to quantify the problem.
• Consult widely to determine the most feasible and cost effective method of repair
• Put broad ideas to trusted and experienced earthmoving contractors
• Create operational plan
• Put operational plan and budget estimation to clients

The concept in this case was to construct large contour drains with a 2% fall across the slopes to divert potentially damaging flows towards the vegetated gully lines. The soil removed from the drains was utilised to do foundation filling of the trench erosion, negating the need to quarry and transport large quantities of fill back from the gullies to the hillsides. This work was temporarily stabilised with a cover crop of grass applied to minimize soil loss from sheet and rill erosion. The whole area was then to be ‘top dressed’ with top soil salvaged from the silted gullies.

The process continued after the proposal was accepted by the clients:

• Survey contour lines at 2% fall using a laser level and then mark out with pegs for the contractors.
• Construct contour drains
• Stockpile subsoil and shales that are removed during construction of contour drains to await appropriate soil moisture content for distribution along erosion trenches;
• Distribute stockpiles up and down the hill from stockpiles to fill and then track roll trenches;
• Await rain event to check stability and workability of contour drains;
• Construct temporary drainage to minimize soil loss. Spread cover crop of jap millet and carpet grass;
• Create quarries in gullies where silt has been deposited over the years since clearing;
• Utilise 6×6 25 tonne trucks to take top dressing materials for redistribution to repaired trenches and over the surface of the constructed drains;
• Utilise machinery to layout top dressing and complete final trim of drainage channels;
• Reapply cover crop;
• Exclude stock, maintain integrity of drains and erosion works (keep clear of debris);
• Reafforest for long term stability.

Machinery used includes:

• A.T.V.: Access, surveys and spreading of cover crop
• Bobcat: final trim of drains
• Drott: 2nd trim of drains, road construction, spreading topsoil
• Excavators: initial cut of contour drains, filling and compaction of trenches, quarrying of erosion deposits, road construction, loading of trucks, spreading of topsoil and road drainage
• 4×4 loader: transport materials to trench sites that are easily accessible, and to load body trucks.
• Body trucks: transport quarry materials from quarry sites to upper and accessible slopes.

The erosion mitigation project, as a sub-project of the total property reafforestation plan is complicated and represents a large investment by clients to restore this landscape.

The success of a project like this requires:

• Developed skill levels and experience of all staff and contractors involved;
• Good client/project officer communications;
• Good communication between project managers and experienced contractors;
• A combination of practical solutions carried out efficiently to gain effective utilisation of resources e.g. bucks;
• Some luck with coordination of various aspects of project amongst the vagaries of weather;

The moral of the story is to stop erosion before it starts by maintaining forest cover in erodible landscapes. Sustainability is impossible to achieve whilst our soils and hillsides are rapidly making their way towards the coast.

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