Note Number: LC0095
Published: November, 1999
Landslips have been occurring since before European settlement. The mass movement of soil is one of the many forces shaping the land in areas that are still geologically active.
Landslips inhibit farm production by loss of accessibility,exposure of infertile subsoil, germination of noxious weeds on disturbed soil, and the loss of stock and capital items.
What causes landslips?
Figure 1. Earth slump slip commonly found on sedimentary parent material.
The loss of forest cover has a destabilising effect on the soil. Extensive root systems which bind the soil have largely gone and excess water formerly used by trees now remains in the soil.
Excess water in the soil profile is considered to be the prime cause of landslips and their incidence is directly related to rainfall, although geology, soil type, and topography are all contributing factors.
Conditions which contribute to excess water or excess soil water pressure include poor drainage, badly sited dams, and the removal of deep rooted perennial vegetation. Landslips can occur on either volcanic or sedimentary soils. They most frequently occur on slopes above 25 degrees, but also occur on much gentler slopes, especially on older existing slips.
How to manage a landslip
To manage a landslip site the water in and around the affected area must be managed. Two main methods of managing water are by erecting physical works designed to enhance drainage and increasing water use by increased vegetation cover.
1. Drainage works
Where possible use surface or sub-surface drains to redirect water flow away from the slip. Construction of small diversion banks above the slip is one way of diverting water. Water should be diverted to a well vegetated stable site away from the slip area to help minimise further erosion activity.
Often the soil surface is severely broken up immediately after a landslip has occurred. Where grading is possible it will help reduce infiltration, assist surface drainage, prevent ponding and allow for revegetation works.
If possible, batter back head escarpments and steep faces which are prone to further slipping. Excess material available from the 'head' after grading, could be added to the toe of the slope to provide added support. Establish a good grass cover over the disturbed area. Support structures at the toe of the slip can be constructed if needed, but engineering advice may be required.
4. Stream bank stabilisation
Streams may undercut the toe of the slip and remove supporting material. Diverting or piping the stream at this point, or reinforcing the bank with rock or other material, may be necessary. Advice should be sought from the Catchment Management Authority in your area or this department.
1. Deep rooted vegetation
To further reduce excess water the use of deep rooted perennial grasses (eg. Cocksfoot, Tall Fescue, Phalaris, Kangaroo Grass) is recommended. Plant deep rooted trees and shrubs on the active slip area and exclude stock. Planting in the catchment above the slip will maximise water use before it reaches the slip.
A mixture of trees and pasture ideally suits the area above a slip. Government or private plantation schemes can provide valuable assistance and information for establishing forestry or agroforestry programs.
How to prevent landslips
It may not be possible to prevent landslips entirely, but with good land management a reduction in their extent and frequency can be achieved. Although the risk of slips occurring will vary with climate, soil type and topography, some basic guidelines need to be followed.
1 Diverting water away from slip-prone slopes
Improve drainage by diverting surface water away from landslip prone slopes using diversion banks or interceptor drains. Ensure safe disposal of excess water to well vegetated sites to prevent further erosion. Grassed drainsmay be sufficient in non-porous soils but in basalt soils, plastic or concrete drains may be needed. It is important to drain springs or soaks which contribute excess water to landslip prone slopes.
2 Land classing
Fence off slip-prone areas so that they can be managed differently to the rest of the farm. This can be achieved by undertaking a land management plan or whole farm plan.
3. Water usage
Increase absorption by planting deep rooted perennial grasses or trees. Take in as much land above the slip-prone area as can be spared rather than restricting works to the landslip prone area.
Avoid structural disturbances. Roads should be constructed along ridges rather than across slopes where destabilisation may be caused as a result of removing supporting material. Excavation may also expose the soil to more infiltration increasing groundwater problems. Ensure that runoff from roads does not contribute to problems caused by excess water.
5. Streambank vegetation
Protect and maintain streamside vegetation since an eroding stream may act to destabilise the toe of a slope.
6. Be wary of dormant or 'old fossil' slips
Dormant or old fossil slips are to be treated with caution. They are characterised by long uneven hummocky slopes. Smaller third or fourth generation slips are likely to occur on these slopes after periods of heavy rainfall.
7. Cracks and fissures
Cracks and fissures often appear before a landslip occurs. Investigate the underlying cause and where possible smooth over and plant out to prevent excess water entering the subsoil.
8. Adopt a 'whole catchment' perspective
The underlying cause of landslips often originates beyond property boundaries. In such cases, the co-operation of neighbouring landholders or Landcare groups will need to be sought.
Important points to remember
- Don't construct dams on old slips or slip-prone hillsides as this will increase water pressure in the soil.
- Maintain a well managed pasture and do not overstock.
- Avoid excess cultivation of slip-prone areas as this can adversely affect soil structure and organic matter levels and lead to greater erosion risk and increased infiltration.
- The aim of landslip control is to see a return to stability and productivity of the area. A combination of short term solutions (such as drainage works) and long term remedies (such as planting deep rooted trees/pastures) may well be the best approach.
- For effective landslip control the cost of the works, their likely success rate and off-site benefits must all be evaluated.
Soil creep or terracing (often mistaken for stock tracks across a hill face) is another form of mass movement. Although soil creep is hardly noticeable, it still represents a loss of soil from the farm, creates management problems and needs to be addressed.
Again, excess soil water, topography, geology, and overstocking are all contributing factors. As terracing occurs on only the steepest slopes it is advisable to fence these into different land classes so that they can be managed separately.
Demonstration sites showing techniques for treating landslips are established at various locations in South Gippsland. For information on these sites contact us.
In some cases financial assistance may be available to landholders for erosion control works.
Working your property along the lines of a land management plan (whole farm plan) can assist greatly in both preventing and treating land degradation problems and may involve some of the preventative strategies mentioned above.
This brochure is a general guide only. For further advice and information contact us.
This Information Note was prepared by David Ziebell and Penny Richards (Leongatha)
This document was initially produced for the Farmcare Program with assistance from the National Soil Conservation Program.