What is gully erosion?
Gully erosion is one of the most common and visual forms of erosion on agricultural land.
Gully erosion is the removal of soil from surface and sub-surface, creating permanent channels more than 30cm deep.
Once a gully forms, erosion can continue in three ways:
- The gully flood can deepen until rock is exposed or the grade becomes flat enough that the flow velocity is too slow to erode the soil any further.
- The banks undercut causing them to collapse and wash away. Sidewalls can also simply erode or melt away in very dispersible soil types.
- The gully head can cut back up the slope or other gully heads form on the side of the gully (lateral gullies) as water flows over the edge.
In some gully systems these processes can occur at the same time.
How gully erosion starts
Gully erosion often begins as a small ‘nickpoint’. This is a shallow and discontinuous cut in the land that is below the natural base level.
This cut starts the waterfall erosion process if there is enough run-off water flowing over the nickpoint. These points slowly get larger until they begin to join up, creating larger, continuous gullies.
Knickpoint's often occur where groundcover has been disturbed by any number of natural or unnatural events.
The most common causes of nickpoints are vehicle tracks, fencelines and livestock paths.
Gullies develop most aggressively where incoming run-off water is the strongest. Run-off water can be the strongest due to decreased groundcover from overgrazing. This allows run-off water to flow unimpeded.
Groundcover is the key to stabilisation. A reduction in groundcover can expose bare soil leading to higher run off volumes and velocity of these flows. For example, stock tracks reduce both the protective groundcover and concentrate water flow.
Maintaining groundcover at 60% (or 40% for rangelands in Western NSW) improves water infiltration, slowing run-off water which in turn slows the formation and expansion of gullies.
Property layout should consider the location of vehicle tracks, fence lines, watering points and gateways with respect to their influence on stock movement and the potential to initiate erosion and gully development.
This gully head is moving upslope and dehydrating the most productive part of the landscape.
Gully head formation
Gully heads are the vertical faces at the top of gullies. They actively erode and move upslope, driven by run-off water from above the gully head.
As the water falls over the vertical face it plunges into a hole. This causes undercutting of the vertical face eventually causing the ground to collapse and the process starts again.
The vertical faces always head toward the strongest incoming run-off water, expanding as they move upslope.
Factors influencing the rate of gully erosion
The rate of gully development depends on several key factors:
- Groundcover: less groundcover increases runoff and erosion.
- Soil type: dispersible or highly erodible soils are more vulnerable.
- Rainfall and runoff: higher rainfall and flow velocity increase erosion.
- Slope and topography: steeper slopes promote faster water flow.
- Land use: cropping and grazing can disturb soil and speed up erosion.
Other factors can speed up gully erosion, including:
- agricultural activity
- construction (road and rail)
- development
- forestry.
Impacts of gully erosion
Gullies can interfere with many aspects of farming including:
- cultivation
- mustering stock
- general access
- water supply
- infrastructure damage and maintenance
- soil loss
- draining the landscape
- changing the aesthetic of the landscape.
Gullies are usually found in the most productive landscapes on a property. These productive areas are in low-lying drainage tracts which receive and hold water. They grow the most groundcover, dry out last and stay green longer. This makes them important for agricultural productivity and for biodiversity.
When gullies encroach into these productive areas, the broader landscape becomes less resilient and more susceptible to the negative impacts of drought.
Gullies have a de-hydrating effect on the landscape. Gullies provide an easy way out for raindrops that have fallen on the land surface. They suck water to them that would otherwise have gone into the soil to grow groundcover.
Gullies also steal water that would have recharged local aquifers. As a result, the surrounding country becomes degraded and bare.
Gully erosion eats away at the soil and there is no easy way of replacing this soil. Halting this soil loss is the main motivation for stabilising gully heads.
The soil removed from gullies is eventually deposited somewhere. It may bury more productive soils, create sand bogs on vehicle tracks or silt up ground tanks and waterholes.
Sediment transported from gullies may be deposited in dams leading to a reduction in capacity. This also has impacts on drainage and infrastructure such as the blocking of pipes and culverts.
Sediment mobilised in drinking water catchments by erosion can reduce water quality, while also degrading the habitat for aquatic plants and animals.
How groundcover helps
Ensuring you have adequate levels of groundcover is the most effective way to minimise run-off and reduce erosion.
By reducing run-off, more water is available for plant and pasture growth.
By reducing erosion, the soil, nutrients and organic matter are retained in place and siltation problems are minimised.
Your next steps
Take action to prevent or repair gully erosion on your property:
Contact Local Land Services NSW
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