Gully erosion

Major forms of soil erosion include:

  • gully erosion, which is discussed below
  • wind erosion
  • sheet and rill erosion
  • streambank erosion.

All forms of soil erosion have on-site and off-site effects, including:

  • reduced yields and income from crops and pastures due to loss of nutrients and organic matter
  • sedimentation
  • damage to, and degradation of, aquatic ecosystems.

The cost of gully erosion is high, with environmental, health and economic impacts.

Gully erosion process

Gully erosion is an obvious form of soil erosion consisting of an open, incised and unstable channel generally more than 30 centimetres deep. It occurs where surface water flow has become trapped in a small concentrated stream, and begins to erode channels in the ground surface.

Specific on-site effects of gully erosion include:

  • reduced access to and on properties
  • a reduction in the area of arable and other agricultural land, which becomes divided into smaller parcels and leads to increased farming costs
  • major changes to the patterns of overland flow causing sedimentation in watercourses and leading to bank erosion problems
  • damage to underground utilities such as communication cables, pipes and power cables
  • increased rates of erosion where more subsoil material is exposed.

Further economic losses from soil erosion are incurred by landholders and the wider community from off-site effects, such as:

  • sedimentation and increased flooding affecting fences, farms and public roadways, railways, culverts and bridges
  • sedimentation of waterways and water supplies
  • increased turbidity and nutrient loads in streams, rivers and water storage areas
  • increased pollution from agricultural chemicals and animal effluent in incised waterways.

 

Causes of gully erosion

Causes of gully erosion include:

  • a history of poor ground cover due to clearing, high stocking rates, repeated cultivations, or decimation by fire or rabbits
  • seasonal and cyclic drought
  • concentrated runoff from steep lands flowing into cleared drainage depressions
  • unstable soils in drainage lines
  • intense rainfall
  • elevated runoff caused by factors such as low levels of tree cover and poor soil infiltration
  • the specific hydrological characteristics of the catchment where high runoff rates exist.

Large eroded gullies in the midst of farmland

Gullies can remove significant areas of land from high value cultivation. Photo: B. Peasley/DECCW

Where is gully erosion?

Gully erosion is generally most highly developed where the contributing effects of land use, climate and slope interact. The western slopes of NSW feature many ’hotspots‘ of erosion on susceptible soils. High rainfall also contributes to the development of many serious gullies on the eastern slopes of the Great Divide.

The following table is extracted from the NSW Land Degradation Survey 1987-88. The extent of gully erosion is always assessed on an ad hoc basis. Although the figures are relatively old, they are still indicative of what is happening today.

The table estimates the number of gullies found in each catchment management authority (CMA) area.

 

CMA Name

Total gullies per CMA

Border Rivers/Gwydir

13 983

Central West

12548

Hawkesbury/Nepean

2858

Hunter/Central Rivers

10221

Lachlan

13201

Murray

2090

Murrumbidgee

15005

Namoi

16553

Northern Rivers

4007

Southern Rivers

6885

Western

Not Available

Lower Murray Darling

Not Available

 

Map showing gully erosion extent in the Clarence and Coffs Coast - Linear erosion

Example map showing the extent and location of gully erosion.

The number of gullies in a region varies according to geography. Studies have found that significant areas of gully erosion exist in a variety of catchments. For example, the Macleay River Catchment on the mid-north coast of NSW has almost 97 kilometres of gully erosion whereas the Clarence River Catchment has 580 kilometres. Each catchment contains large amounts of erosion in a small area.

The Department of Environment, Climate Change and Water (DECCW) is the custodian of several datasets that show the location and relative severity of gully erosion. DECCW can prioritise these areas for treatment based on their severity or intensity.

DECCW has several maps showing the extent and location of gully erosion. To obtain maps contact your local DECCW office.

Minimising or preventing gully erosion

The most important aspect in preventing gully erosion is to maintain good ground cover. The combination of occasional heavy rainfall and a reduced ground cover caused by cropping, fire or high stocking rates result in most erosion damage.

Data indicate that anything under 70% of ground cover affects runoff and soil loss. The percentage of ground cover affects the frequency and the amount of soil loss, and major rainfall accounts for most of the runoff. These factors underline the necessity to keep ground cover in place at critical times of the year if possible.

A stubble cover of 30% is required on cultivated areas to halve erosion rates when compared to the erosion rates from 10% stubble cover obtained after burning.

Treating gully erosion

The two components to the successful treatment of gully erosion are:

  • treatment of the catchment to reduce or redirect runoff
  • stabilisation of gully heads, floors and walls by engineering, earthworks and vegetative techniques or fencing.

Areas affected by extreme levels of gully erosion are best treated by changing the land use, sometimes in combination with minimal structural works. In some cases, the area surrounding the gullies must no longer be used for cropping or grazing, and must be allowed to revert to native vegetation. In other cases, it may be necessary to stop any further cultivation and keep the areas only for occasional grazing.

Areas affected by less severe gully erosion can be treated with gully control earthworks. Sound crop, pasture and soil management practices are essential to complement the earthworks.

The techniques for treating deep, extensively branched gullies are:

  • diversion of runoff to a safe disposal area
  • gully checks
  • covering the heads with water
  • diversion banks.

However, the use of these techniques may be severely restricted by the steepness of land, possibility of soil erosion and the lack of a safe disposal area. In such instances:

  • use vegetation and earthworks in gully floors and on sidewalls
  • use structures to allow safe re-entry of runoff into drainage lines, such as concrete, rock, grass and geotextile flumes
  • assist rehabilitation by reducing runoff and peak flows.

On cropping lands, properly designed banks and waterways are effective and economical. On some types of grazing land, economic factors, landscape and gully characteristics mean that earthworks cannot be used. In these circumstances, protect the catchment using pastures, tree and shrub cover, and manage grazing and fire.

Gully fill is not recommended in deep gullies with soils that are highly susceptible to erosion. The most successful methods of treating deep gullies are to:

  • fence them to exclude stock
  • plant trees, shrubs and grasses
  • construct structures in the gully to encourage deposition of sediment.

Rounding edges, heavy fertiliser applications and revegetating bare areas have also been successful techniques in some of these gullies.

Cost of land degradation

Costs of gully erosion are assessed on an ad hoc basis. Although the figures given below for the cost of gully erosion are relatively old, they are indicative of what is happening today.

A joint Commonwealth/States collaborative study in 1975–77 determined that more than one-half of the land in Australia used for grazing or the growing of crops needed treatment for land degradation. This area totalled more than 2.6 million square kilometres. A more recent study estimated that by 1983, two-thirds of Australia's farm land needed soil conservation treatment. The collaborative study concentrated on soil erosion, salinity and vegetation degradation. Other forms of land degradation, such as soil acidity, waterlogging and declining soil fertility and structure, were not fully assessed, although these are now estimated to be causing greater economic losses to land holders than soil erosion and dryland salinity.

It has been estimated that every minute, 2.5 tonnes of salt flow over the South Australian border in the waters of the Murray River.

The Institute of Foresters of Australia estimated that for every dollar dryland salinity cost:

  • water and wind erosion cost $5
  • soil acidification cost $25
  • soil structural decline cost $125
  • soil nutrient degradation cost $625.

There is no doubt that gully erosion is preventing land holders from maximising the potential of their land, leading to economic losses to the land holders but also to NSW in terms of lost agricultural production.

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Page last updated: 26 September 2013