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Gully erosion is the removal of soil along drainage lines by surface water runoff. Unless steps are taken to stabilise the disturbance, gullies will continue to move by headward erosion or by slumping of the side walls.
It's far easier and more economical to do repair work in the early stages of newly formed gullies. Large gullies that have been left unchecked are difficult and costly to repair.
The cause of gully erosion
Gully erosion occurs when water is channelled across unprotected land and washes away the soil along the drainage lines. Under natural conditions, runoff is moderated by vegetation, which generally holds the soil together, protecting it from excessive runoff and direct rainfall.
Excessive clearing, inappropriate land use and compaction of the soil caused by grazing often means the soil is left exposed and unable to absorb excess water. Surface runoff then increases and concentrates in drainage lines, allowing gully erosion to develop in susceptible areas.
Some of the problems caused by gully erosion include:
- dissection of the property causing access and management difficulties
- loss of productive land (gullies often occur in the most productive area of the catchment)
- reduced amenity and property values, including destruction of farm improvements, such as fences or tracks
- discolouration of water supply and sedimentation of waterways, dams and lower paddocks
- provides a harbour for vermin.
Preventing gully erosion
In most cases gullies can be prevented by good land management practices aimed at maintaining even infiltration rates and a good plant cover.
Strategies for preventing gully erosion include:
- maintaining remnant vegetation along drainage lines and eliminating grazing from these areas
- increasing water usage by planting deep-rooted perennial pastures, trees, or an appropriate mixture of both thus maintaining healthy, vigorous levels of vegetation
- identifying drainage lines as a separate land class in which vegetation needs to be protected
- immediate stabilisation of sheet or rill erosion
- vermin control
- ensuring runoff from tracks is evenly distributed across paddocks ton dissipate its energy
- maintaining high levels of organic matter in the soil
- avoiding excessive cultivation (fence and manage the land according to its capability)
Control measures for gully erosion
To be effective, gully control needs to be tackled in two ways:
- fixing the problems in the catchment
- stabilising the gully itself.
The objective of catchment works is to reduce and divert the flow of water into stable drainage lines. This can be achieved by:
- increasing infiltration rates and water uptake by plants
- by diverting and storing water.
A practical way to begin is to subdivide the catchment into appropriate land classes and then apply grazing and cropping practices most suited to each class. The development of a land management or whole farm plan is an ideal way of identifying these issues.
Strategies for stabilising the catchment include:
- cooperatively tackling the problem by the formation of a landcare group — this could be be the most effective method where the source of the problem is spread over several properties
- diversion of water away from erosion prone gullies (for example, with diversion banks) — this disperses the erosive power of the water over well-vegetated areas
- contour cultivation where possible to slow down runoff and spread the water over a wide area
- maintaining farm tracks and culverts so that drainage is evenly dissipated and prevented from concentrating along any section
- using trees and deep-rooted perennial pastures to assist in both utilising excess water and reducing runoff.
The objective is to divert and modify the flow of water moving into and through the gully so that scouring is reduced, sediment accumulates and revegetation can proceed. Stabilising the gully head is important to prevent damaging water flow and headward erosion.
A variety of options can be used to get the water safely from the natural level to the gully floor, such as:
- grass chutes
- pipe structures
- rock chutes
- drop structures.
Structures might also be required along gully floors since some grades can be quite steep and allow water to rush down under peak flows, ripping away soil and vegetation. These may take the form of rock barrages, wire netting or logs across gullies.
Sediments held in the water will then be deposited along the flatter grades as a result of slower water flow, allowing vegetation to re-establish.
If erosion control and revegetation work is undertaken, then damaged areas should be fenced off from stock, until restoration is complete.
Dams can also be constructed to slow the flow of water into the gully head, but special care needs to be taken to get the overflow water back into the gully floor safely.
Don't fill gullies with solid objects
Don't fill eroded gullies with solid objects such as old drums, car bodies or concrete. This only creates further erosion by directing water around such objects and removing more soil.