Tunnel Erosion (DPI Vic)

November, 1999 Tunnel erosion LC0061 David Cummings, Melbourne ISSN 1329-833X

This note is to be used in conjunction with local Predisposing conditions experience for managing areas with tunnel erosion in Tunnels will develop in susceptible soils when the Victoria. following conditions are met: Introduction • The land slope allows sufficient hydraulic head to drive free water through the soil; Tunnel erosion is initiated by water movement along channels and cracks in dispersive subsoils. Clay particles • Surface runoff accumulates at places where it is able can be readily picked up by the water and move off with to move through the surface soil and gain direct access the water. Larger soil particles (silt and fine sand) will to the subsurface soil; then fall into the flow and steadily build a tunnel. • Subsurface soil has interconnected cracks or channels The first indication of tunnel erosion is usually the through which this runoff can move (however slowly occurrence of a fine sediment fan at the outlet point of the to start -with); developing tunnel. • Subsurface water is able to find an exit point through Eventually parts of the tunnel roof collapse producing the surface soil at a point lower in the landscape. potholes and gullies. Susceptible soils typically have dispersive subsoils (ie Contributing factors contain clay particles which readily separate into Hence a susceptible soil in a susceptible location can be individual mobile particles when in direct contact with set-up for tunnel erosion by: water) and hard setting surface soils. Hilly, cleared land • Reducing vegetative cover and developing a surface with an average rainfall of 300-650 mm (12 - 25 inches) is seal causing increased runoff typically affected. • 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. 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 Figure 1 Collapsed tunnels the incorporation of by-product gypsum.

Figure 2 Formation of a tunnel

• Divert surface run off where possible away from the Treatment area for at least 12 months. This helps prevent any The tunnelled area must be completely broken up and sheet erosion while a vegetative cover is establishing. reconsolidated to interrupt channelled flows of water and Divert water to a safe disposal site. Small "eyebrow" then stabilised with vegetation. A soil test for the banks can be constructed across the treated area to appropriate fertiliser mixture and responsiveness to dispose of surface runoff from the area itself (See gypsum is a worthwhile investment. The following points diagram). outline a general approach for reclamation: • Apply by-product gypsum at the recommended rate if required. • In autumn, break up the tunnels by deep ripping the. • Cultivate the area for sowing. area up and down the slope. • Sow the area with a seed and fertiliser mixture suitable • Deep rip across the slope making sure that these rip for your locality. It is essential to use a deep rooted lines are deeper than the bottom of the tunnels. species such as Phalaris and/or Cocksfoot as part of • Compact using rubber wheeled tractor on areas of the seed mixture as, once established, they will use shallow tunnelling. Tunnels deeper than the rippers water from deep in the soil profile. are best excavated and repacked. • 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.

Figure 3 Diversion of runoff from a treated area

Maintenance bulldozer. In some instances the area will have to be dug out and re-compacted. • Watch the area closely for any sign of recurrence of the tunnelling. It is possible to miss small tunnels The common practice of filling-in tunnels and gullies with when carrying out the initial ripping program. Small solid objects like old drums, rocks or concrete may only tunnels could possibly be repaired by hand.Ensure create further erosion when water is diverted around them. rabbits do not burrow in the diversion banks or the Tackling erosion problems using a whole farm plan can be treated area. beneficial because the works are undertaken in a logical • Continue regular top dressing of the area with order which will reduce the overall costs of treatment and fertiliser for at least three years. There are few sites prevention. where maintenance is not required. A'90% control' after the first year is an average outcome. Further information • Continually check the discharge points of any This pamphlet is only meant to be a general guide. If you diversion banks for possible erosion. Include these have tunnel erosion on your property detailed advice areas in the fertiliser maintenance program to ensure a should be sought from the local office of the Department good grass cover. of Sustainability and Environment. Your local officer may be able to tell you of similar works General comments in the area. 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

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