How do gullies develop?
Gullies develop when enough surface water concentrates in a flow line with inadequate groundcover, resulting in scouring of soil until a deeper channel is formed. The inadequate groundcover can be a small starting point: disturbed soil, a livestock or vehicle track, or a stump. Once started, gullies can spread downslope and upslope (gully head erosion).
Severe gully erosion is usually caused by extreme rainfall from storms and cyclones. With the projected changing climate, summer storms in the south-west are forecast to be more common and more severe. The timing of these storms, when most pastures and crop stubbles are dry and groundcover is reduced, increases the risk of water erosion.
In extreme conditions, not all gully erosion can be prevented. We provide the information below to help those who have new or older active gullies.
A gully is any eroded channel deeper than 30cm, and some gullies can be many metres deep.
Should you fill or stabilise?
Filling and shaping large gullies can be expensive. Stabilising may be appropriate where cost is prohibitive.
Stabilising can be done in steps:
- divert surface water from the gully using well-engineered surface water earthworks
- stabilise the eroding gully head
- shape the gully walls
- fence the gully
- encourage grass and other groundcover vegetation growth to help stabilise the gully floor; do not allow shrubs and trees to grow in the gully.
Filling or shaping large gullies assumes that the gully, when stable, can be used as part of a surface water management system on the farm.
This is an essential first step to repair gully erosion. Water moving in the gully needs to be reduced or preferably prevented until the repair is stable, otherwise it will erode any repair work. Diversion can start well upslope of the gully using surface water management options, and include spreader banks immediately above the gully head to divert water to a stable waterway with a safe disposal area. Spreader banks may also be needed on either side of long gullies to prevent surface water entering the repaired area.
Not all gullies erode further at the head. An actively eroding gully head will need a drop structure, which can be a concrete flume, or built from sandbags or gabions (rock-filled cages) with Reno mattresses (rock-filled mattresses) (Figure 1).
Note that gullies in dispersive soils need special management:
- Do not leave rip lines or loose soil if the soil is dispersive.
- Use gypsum treatment on suitable clays.
- Add deeper topsoil to the exposed dispersive subsoil if possible.
- Seed with a suitable annual to get rapid groundcover as early in the season as possible.
The walls of a stable gully floor can be battered back to reduce side-wall erosion (Figure 2). Applying similar design precautions as in best practice guidelines for a waterway may be appropriate.
Filling gullies for crop or pasture
If you intend to crop across the top of a filled gully, you will need shallow approach angles for machine traffic (Figure 3).
Where the area is to remain as permanent pasture, steeper batters are sufficient (Figure 4). The length and slope of the batters will determine the amount of soil disturbed on the shoulder of the gully. There is no need for a drop structure if surface water is to remain permanently diverted from the site.
Your topsoil is important
You can improve vegetation establishment and stability of the gully site by stockpiling the topsoil to spread it back over the reshaped gully. This is particularly important when the subsoil is infertile or dispersive. In these cases, failure to redistribute the topsoil over the site will make it very difficult to stabilise the gully, and it is likely to continue eroding. If dispersive (sodic) clay is exposed in the gully floor, we recommend applying gypsum before spreading topsoil.
For long gullies, graders can strip and stockpile topsoil in windrows parallel to the gully. The gully is then filled from a 'borrow zone' and progressively compacted by the grader. When filled, the grader spreads topsoil over the stripped borrow areas and the filled gully walls and base.
If long stretches of gully are to be filled, it is best to split the gully repair into sections, spreading the topsoil from new sections onto areas previously filled. This minimises the distance topsoil needs to be transported. Topsoil should also be pushed to the head of the gully for later spreading over the gully centre.
Establish vegetation cover as soon as possible after earthworks are finished. Ideally, this means filling gullies in autumn or early winter before heavy rains. We recommend at least 1 year of heavily sown grassy annual pasture on newly repaired erosion scars. The denser cover and matting roots of annuals provide better protection than taller tillering grasses and crops, such as canola and lupins.
In pasture paddocks, it is worth fencing large repaired gullies and any grassed waterways below them to protect the groundcover. The area can be grazed as necessary after groundcover is established.
In cropping paddocks, we recommend that you do not crop across filled gullies until the whole gully is well stabilised, and only use zero tillage.
Remove obstacles in the gully
Do not plant or allow trees and shrubs to grow in the repaired gully because these act as obstacles that can increase erosion by scouring in heavy flows. Dumped machinery, loose rocks and general rubbish in the gully may also cause more scouring.
Machinery management for repairing large gullies
- Many gullies have lateral tunnel erosion, or 'piping'. Unstable gully edges could collapse and make machinery operations hazardous.
- Gully walls may have steep gradients that are a hazard to machinery operation. Take all normal precautions on such sites.
Graders are efficient at moving large volumes of soil laterally when repairing long gullies. Bulldozers are more suited to large and complex gully head erosion.
Graders or bulldozer operations should begin by deep-ripping the gully shoulders to a distance and depth depending on the depth of the gully and optimal batter length. Rip lines should not be present after removing the shoulders and filling, otherwise erosion risk is increased. Guidelines for open deep drains can be adapted for constructing shaped gullies.
The topsoil should then be stockpiled at the gully head. The bulldozer may compact the subsoil during the removal of the topsoil. The area will need to be ripped a second time where this occurs.
Once the topsoil has been stockpiled and the subsoil ripped, the subsoil can be pushed into the gully. It is highly recommended that fill is compacted by machinery regularly to help increase stability, and the amount of fill is increased (by 10–15%) in the gully to allow for settling.
However, if the gully floor is already stable and can be used as a watercourse, do not add fill to the gully floor, and avoid machinery traffic on the floor.
For long gullies, the process above can be done in sections – from the head – and repeated along the length of the gully.
If the gully is small enough, spreading the topsoil over the entire length of the gully in one operation is more efficient (in terms of dozer operation), because the dozer is always moving with a load.