Gully erosion is an advanced stage of rill erosion
where surface channels have eroded to the point where they can not be removed by tillage operations. Gully erosion is responsible for removing vast amounts of soil, irreversibly destroying farmland, roads and bridges and reducing water quality by increasing the sediment load in streams. Gully initiation is thought to be a response to excessive water in the local environment caused by the removal of perennial vegetation.
A gully head forms as rill erosion deepens and widens creating a characteristic nick point or headwall. Most gullies extend up slope as a result of headwall migration. However, it is the collapse and slumping of the sidewalls which usually contributes the greatest proportion of soil loss.
- Water running into the gully either scours the face or undercuts the head wall resulting in gully migration.
- Widening of gully sides occurs by undercutting or slumping.
- Gully head shape indicates if erosion is due to scouring (forward slope) or dispersion (undercut).
- Believed to be a response to changed hydrological conditions
Prevention and Control
- Minimise tree clearing.
- Ensure drainage from roads, buildings and stock routes is not concentrated into gullies.
- Fence off stock from susceptible areas.
- Maintain contour banks and waterways.
Once established, gully erosion can be difficult to control. In most cases a combination of approaches, including the use of vegetation, fencing, diversion banks and engineering structures are required.
Vegetation is the primary, long-term means by which gully erosion can be controlled. All gullies need to be fenced from stock and revegetated along the gully floor, sidewalls and surrounding areas. Establishing vegetation on gully sidewalls is often difficult due to moisture stress. Consideration should be given to supplying irrigation to get vegetation established.
- Revegetate gully floor with rapidly growing grasses
- Revegetated sidewalls and surrounding areas with trees and grasses
- Irrigation, hydroseeding, and mulching may be required
- Revegetate catchment above gully (increase water usage).
Gully erosion may be able to be controlled if runoff can be diverted and safely disposed of.
- Design to divert water away from the gully head with contour drains and diversion banks
- Requires safe disposal of water
- Risk of transferring instability from one area to another.
Properly designed and installed engineering structures may be the only means to stop headward advance in many gully systems.
- 'Safely' carry runoff to the gully floor
- Dissipate energy
- Inherent risk of being bypassed
- Requires specialist engineering advice.