Erosion is a natural process but is often intensified by human land use practices.

Erosion is the processes that loosen sediment and soils and move it from one place to another. This occurs when the energy of water, wind and gravity overcomes resisting forces (e.g. roots of vegetation, cohesion of rocks) or where resisting forces are absent (i.e. bare earth).

Understanding the different types of erosion helps land managers to apply appropriate land practices and to better plan for remediation of existing soil erosion sites.

Types of Erosion

Water Erosion

Our high rainfall wet seasons can pose a significant risk to soil stability. This is driven by two main processes, direct impact of raindrops hitting the soil surface and the accumulation of surface run-off.

The vulnerability of soils to water erosion depends on:

Rainfall intensity (erosivity): high intensity rainfall creates serious risk as heavy drops on bare soil causes the soil surface to seal,

Nature of the soil (erodibility): clay soils vary in their ability to withstand raindrop impact or water flow,

Slope length: if a slope is long, water running down the slope becomes deeper and moves faster, taking more soil with it

Slope steepness: the speed of runoff increases on steep slopes, which increases the power of water to break off and carry soil particles.

Resisting forces: What type of resisting forces are in place to withstand the water impact.

Types of water erosion

Raindrop-detached soil transported in uniform sheets of surface water across the landscape.

Removal of soil through concentrated water flow in small well-defined channels along drainage lines.

Tunnel erosion occurs when surface water moves into and through dispersive subsoils. Dispersive soils are poorly structured, so they erode easily when wet.

The tunnel starts when surface water moves into the soil along cracks or channels or through rabbit burrows and old tree root cavities

This is a collective term that describes the type of channel in which the erosion is occurring, rather than the erosion mechanism.

The causes of gully erosion can be similar to those found for waterway erosion.

Most gully erosion starts as a form of lateral bank erosion but then continues through a process of head-cut erosion, followed by bank scour, bank slumping and bank undercutting.

Bed scour- the direct removal of material from the channel bed as a result of high-velocity flows detaching and entraining the soil or sediment into the flow.

Bed scour also includes the following forms of channel erosion:

  • head-cut erosion where a small erosion head (waterfall) migrates up the bed of the channel, and
  • scour holes where isolated holes are formed in the channel bed or floodplain

Bank scour – the direct removal of material from the channel bank as a result of stream flows detaching and entraining the bank material into the floodwater.

Bank undercutting – occurs when the actions of bank scour are concentrated on the lower areas of a bank.

Bank slumping – is the mass movement of bank material, primarily through the actions of gravity.

Lateral bank erosion – form of bank erosion that mimics the erosion processes that commonly occur in head-cut erosion.

Wind Erosion

This is the detachment and movement of soil by air moving at least 20km/hr. W his occurs in two main ways suspension and saltation. In suspension, fine soil particles are lifted high into the air, often resulting in dust storms.

In saltation, larger particles are briefly lifted and bounced along the ground, contributing to sand movement.

Wind erosion is most common in areas with low rainfall, especially when soil moisture is at or below the wilting point. However, any soil affected by drought is vulnerable.

Dust storm effecting Brisbane City (Courier Mail).

Mass Movement

It typically occurs on slopes steeper than 25 degrees, especially in areas with sparse vegetation and annual rainfall exceeding 900 mm. These movements are most common after intense storms, when the soil becomes saturated with water and heavy.

There are several types of mass movement, including soil creep, earth flow, slumps, landslips, landslides, and avalanches.

Factors that increase the likelihood of mass movement include erosion or excavation at the base of a slope, added weight from buildings or embankments, and the removal of vegetation, which eliminates stabilising root systems.

Mass movement along the Mary River.

Preventing and managing erosion on your block

Head over to our ‘Managing erosion on your block’ page to explore the principles to reducing soil degradation and erosion and access useful resources to manage and remediate soil erosion on your block.