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Tunnel erosion | LC0061 |
David Cummings, Melbourne
November, 1999 |  |
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This note is to be used in conjunction with local experience for managing areas with tunnel erosion in Victoria.
Introduction
Tunnel erosion is initiated by water movement along channels and cracks in dispersive subsoils. Clay particles can be readily picked up by the water and move off with the water. Larger soil particles (silt and fine sand) will then fall into the flow and steadily build a tunnel. The first indication of tunnel erosion is usually the occurrence of a fine sediment fan at the outlet point of the developing tunnel. Eventually parts of the tunnel roof collapse producing potholes and gullies.
Susceptible soils typically have dispersive subsoils (ie contain clay particles which readily separate into individual mobile particles when in direct contact with water) and hard setting surface soils. Hilly, cleared land with an average rainfall of 300-650 mm (12 - 25 inches) is typically affected.
Figure 1. Collapsed tunnels
Predisposing conditions
Tunnels will develop in susceptible soils when the following conditions are met:
- The land slope allows sufficient hydraulic head to drive free water through the soil;
- Surface runoff accumulates at places where it is able to move through the surface soil and gain direct access to the subsurface soil;
- Subsurface soil has interconnected cracks or channels through which this runoff can move (however slowly to start -with);
- Subsurface water is able to find an exit point through the surface soil at a point lower in the landscape.
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Contributing factors
Hence a susceptible soil in a susceptible location can be set-up for tunnel erosion by:
- Reducing vegetative cover and developing a surface seal causing increased runoff
- A decrease in rate of water usage from the soil profile by tree removal, change of pasture composition, and reduction in pasture cover
- Exposure of root channels following the death of trees and other deep rooted vegetation, rabbit burrowing, and indiscrete ripping or trenching.
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Aims of reclamation
Reclamation of tunnel erosion therefore aims to:
- Break down existing channels and tunnels
- Prevent water concentrating into flow paths on the soil surface
- Reinstate a vigorous vegetative cover to prevent exposure of bare soil, increase organic cycling, increase soil water use and increase infiltration.
- Modify dispersibilty of clay in critical sites through the incorporation of by-product gypsum.
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Figure 2. Formation of a tunnel
Treatment
The tunnelled area must be completely broken up and reconsolidated to interrupt channelled flows of water and then stabilised with vegetation. A soil test for the appropriate fertiliser mixture and responsiveness to gypsum is a worthwhile investment. The following points outline a general approach for reclamation:
- In autumn, break up the tunnels by deep ripping the. area up and down the slope.
- Deep rip across the slope making sure that these rip lines are deeper than the bottom of the tunnels.
- Compact using rubber wheeled tractor on areas of shallow tunnelling. Tunnels deeper than the rippers are best excavated and repacked.
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Figure 3. Diversion of runoff from a treated area
- Divert surface run off where possible away from the area for at least 12 months. This helps prevent any sheet erosion while a vegetative cover is establishing. Divert water to a safe disposal site. Small "eyebrow" banks can be constructed across the treated area to dispose of surface runoff from the area itself (See diagram).
- Apply by-product gypsum at the recommended rate if required.
- Cultivate the area for sowing.
- Sow the area with a seed and fertiliser mixture suitable for your locality. It is essential to use a deep rooted species such as Phalaris and/or Cocksfoot as part of the seed mixture as, once established, they will use water from deep in the soil profile.
- It is important when sowing the area, to use an adequate initial application of fertiliser to obtain a quick vegetative cover on the disturbed ground to protect it from heavy rain. Most land on which tunnel erosion occurs is lacking in some essential nutrients. A soil test will be useful to determine an appropriate fertiliser mixture.
- Temporarily fence the area out of production for a period of 12 months to allow a dense vegetative cover to develop prior to grazing. Use a rabbit-proof fence in most cases. It is important that the area be only lightly grazed for the first 2-3 years to allow the pasture to develop and produce a strong root system which will bind the soil together and maximise water use.
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Maintenance
- Watch the area closely for any sign of recurrence of the tunnelling. It is possible to miss small tunnels when carrying out the initial ripping program. Small tunnels could possibly be repaired by hand.Ensure rabbits do not burrow in the diversion banks or the treated area.
- Continue regular top dressing of the area with fertiliser for at least three years. There are few sites where maintenance is not required. A'90% control' after the first year is an average outcome.
- Continually check the discharge points of any diversion banks for possible erosion. Include these areas in the fertiliser maintenance program to ensure a good grass cover.
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General comments
If the tunnels are not too deep it should be possible to rip the area with a rabbit ripper on the linkage of a standard farm tractor. Deep tunnels will require ripping with a bulldozer. In some instances the area will have to be dug out and re-compacted. The common practice of filling-in tunnels and gullies with solid objects like old drums, rocks or concrete may only create further erosion when water is diverted around them. Tackling erosion problems using a whole farm plan can be beneficial because the works are undertaken in a logical order which will reduce the overall costs of treatment and prevention.
Further information
This pamphlet is only meant to be a general guide. If you have tunnel erosion on your property detailed advice should be sought from the local office of the Department of Sustainability and Environment.
Your local officer may be able to tell you of similar works in the area.
This note replaces note number SC0001.
The advice provided in this publication is intended as a source of information only. Always read the label before using any of the products mentioned. The State of Victoria and its employees do not guarantee that the publication is without flaw of any kind or is wholly appropriate for your particular purposes and therefore disclaims all liability for any error, loss or other consequence which may arise from you relying on any information in this publication.
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