Tunnel Erosion
Tunnel erosion commonly occurs on slopes that have subsoil
easily dispersed by water and a surface soil that is much
less erodable due to its inherent structure, chemical composition
or the presence of plant roots. As the subsoil begins to wash
out, tunnels form under the more stable surface soil and extend
up the slope, increasing in size after each bout of heavy
rain. If the tunnels become large the surface soils may collapse
forming a small gully.
Hidden tunnels are dangerous to stock and for people on horses,
bikes or tractors. The gullies make access around the slopes
impossible and concentrate water flow, which can lead to further
erosion. Although the initial cause of tunnel erosion may
be the presence of rabbit burrows or old tree root channels,
it may also simply be a result of excessive subsoil waterlogging.
The first sign is usually springs of muddy water spewing from
the base of the slope during heavy rain.
Dense plantings of trees above and over the affected site
may reduce waterlogging and help bind the soil. Interestingly,
tunnel erosion is often evident within native forests and
can be seen weaving around the roots of large eucalypts. Alternative
species which form more dense root mats and are able to sucker
from exposed roots may be more effective in controlling tunnel
erosion than eucalypts. Planting trees or perennial pasture
on any well-drained soils above the affected area may also
help reduce waterlogging in the affected area.
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