Tunnel erosion remediation through soil condition and deep ripping

Tunnel erosion is an insidious form of soil erosion that can lead to paddocks becoming untrafficable, dangerous and costly to repair. Sediment from tunnel erosion sites can also affect water supplies and nearby soil quality.

Sodic (dispersible) sub soils on sloping land are most prone to tunnel erosion and are common across Victoria. Tunnel erosion may be initiated by uneven saturation or concentrations of run-off into the subsoil.

This can arise through poor ground cover or cultivation or excavation in- or near- susceptible areas. Run-off then moves as lateral flows in the sub soil, entraining clay particles down the slope and leaving behind voids or tunnels.

The first indications of tunnel erosion are usually the discovery of patches of pale sub soil deposited on the soil surface of the paddock near an outlet.

This signifies that sub surface tunnels have already formed nearby, which may not be evident from the surface. As run-off continues to mobilise sub soils, these tunnels enlarge and eventually slump or collapse. This can lead to large and deep erosion gullies.

The Heal family’s 1400-hectare sheep farm near Nagambie in Victoria had a paddock with 4 hectares of tunnel erosion. The site also had sheet erosion (where topsoil is lost), leaving little opportunity for pastures to grow.

Farmer Ken Heal had tried several times to address the problem, including the conventional approaches of deep ripping, chisel seeding and tree planting, all with limited success.

With advice from Agriculture Victoria and working with the Gecko Clan Landcare Network, a new approach was taken, with funding through the Goulburn Broken Catchment Management Authority as a demonstration site.

Such remediation doesn’t come cheaply — in this case $1900 per hectare.

Firstly, a soil test was taken to identify soil constraints.

The site was deep ripped with a bulldozer to collapse the tunnel network and reduce preferential pathways for water to re-enter and re-instate tunnels. Deep ripping needs to occur just beyond tunnel depth, working carefully to ensure tunnels are cross ripped and not just up and down slope.

The bulldozer then completed the earthworks by creating an even soil surface to help reduce ponding which can potentially concentrate water and start the process again. To mitigate the cost and risk of earthmoving, it is essential to use an experienced contractor for this work.

Gypsum and lime at 2 t/ha were spread to help improve the soil structure and condition, along with superphosphate to increase fertility. Chicken manure was spread to increase organic matter. The site received subsequent applications of these fertilisers to build conditions to promote plant growth.

Such remediation doesn’t come cheaply: in this case $1900 per hectare.

Soil test information from tunnel erosion site.

Breakdown of treatment costs across the site (approx. 4 ha)

The key to managing risks of tunnel erosion is to maintain a dense and productive cover of grasses, which create a dry soil buffer to soak up rainfall and reduce saturation into tunnel-prone soils.

Ken’s site was first sown down to a mixture of Tetila Ryegrass, Ryecorn, Saia Oats and sub clovers at 40kg/ha.

The grasses provided quick cover to the bare soil and protected it from erosive rains.

Grasses at the site were allowed to go to seed for several seasons, and the perennial grasses are now colonising the site.

Dense planting of trees to address tunnel erosion rarely works. Such dense tree plantations often feature areas of bare ground, with grasses finding it hard to establish.

In such cases rain falling beyond the canopy still reaches the ground and can continue to erode at-risk soils.

Importantly, Ken fenced the site to better control grazing at the site which is now strategically managed.

He is very happy with the progress of the paddock, whilst there are some limitations in returning the area to normal production.

However, there is less sediment reaching waterways and dams and smothering adjacent pastures. The additional costs of pasture establishment and fencing have not been provided here.

For further information search for tunnel erosion at Victoria Resources Online