Soil Erosion in Tasmania

Location and extent of erosion in Tasmania

Table: Private Freehold Land at Risk of Erosion (Ha)

The occurrence and extent of land degradation in Tasmania has been reported on a land system basis by Grice (1995)

Splash, Sheet and Rill Erosion

Splash and sheet erosion are widespread in Tasmania with 15,500 ha classified as having severe to extreme splash, sheet and rill erosion risk. The combined risk from splash, sheet and rill erosion is highest for cropping and forestry operations in the north of the state, particularly on steep slopes. In particular, extensive areas of severe erosion risk exist on stable soils between Devonport and Wynyard, and in the south of the state on north-facing dolerite slopes.

• Mostly in the North-east, far North-west and the Central Midlands
• Cropping and forestry operations on moderate to steep slopes
• On gentle slopes, moderate sheet and rill erosion occurs on intensively cropped (vegetables) land
• In the South, severe sheet and rill erosion also occurs on grazed, steep north facing slopes.

Gully Erosion

Gully erosion is not appreciable or only occurs at a minor level throughout most of Tasmania. Gully erosion is absent from large parts of King island, north west Tasmania, the northern midlands, upper Derwent Valley and Central Plateau. The main areas of severe gully erosion are south of Launceston, the southern midlands and east of Hobart near Buckland.

• Minor occurrence in Tasmania
• Severe gully erosion is restricted to areas south of Launceston, Southern Midlands, and east of Hobart (Buckland)
• Associated with dispersive soils originating from Permian Mudstones or Triassic sandstones.

Tunnel Erosion

Tunnel erosion is largely confined to the south, almost exclusively on soils derived from Permian mudstone, Triassic sandstone or Quaternary redeposition of these sediments. A less invasive form of tunnel erosion is also observed on steep black cracking clays derived from Jurassic Dolerite or Tertiary Basalt in high rainfall areas.

• Restricted to south of the state ~ relatively shallow
• Occurs on dispersive soils including, Permian Mudstones, Triassic sandstones and Quaternary re-deposition of Permian and Triassic sediments.

Wind Erosion

Despite lying in the path of the 'Roaring Forties' and having higher average wind speeds than the mainland states, Tasmania is not overly susceptible to wind erosion. Wind erosion occurs in many coastal areas due to poor vegetation cover and inappropriate land use. Cropping on sandy soils in the Tamar valley, Midlands and Derwent valley has resulted in moderate to severe erosion. Across the state, approximately 16,000 ha of private freehold land are recognised as containing a severe wind erosion hazard.

• Associated with sandy soils in low rainfall areas near the coast
• Occurs in coastal land systems south of cape Portland and east of Weymouth
• In-land: cropping land systems with sandy soils in the Tamar valley, midlands and Derwent valley.

Mass Movement

In Tasmania landslips occur on the geologically immature basalt soils along the north west coast, and the steep Jurassic dolerite and sedimentary slopes of the southern midlands and Huon channel. Landslips or mass movements are most frequent on slopes above 25⁰ with little vegetation and high annual rainfall.

• Occur on Basalts scarps of the north-west coast
• Steep upper slopes on Jurassic dolerite and sedimentary rocks in the Huon Valley.

Comparison with Other States

Rates of soil erosion have been determined by measuring the proportion of Caesium 137 in topsoils at representative sites across Australia.

Table: Grazing or Forestry/Unused




Table: Cropping and Cropping/Grazing Rotations

• Rates of erosion are less in Tasmania than in other States, under both grazing and cropping systems.
• Erosion in cropping and grazing systems is greater than the rate of soil formation (0.3 t/Ha/yr) in all states.