The main forms of erosion are:
- surface erosion
- fluvial erosion
- mass-movement erosion
- streambank erosion.
When rain, wind or frost detach soil particles from the surface, the particles are washed or blown off the paddock.
This occurs when rain falls on bare or sparsely covered soil, loosening fine particles (silt, clay and humus) that are carried downhill in surface run-off. Sheet erosion lowers the fertility of the soil, because it removes the most productive layer, which has usually been enriched by fertiliser.
The wind can remove the valuable fine soil on the land surface. Seasonally strong winds hit many areas, such as sand dunes, the Central Volcanic Plateau, Wairarapa, Hawke’s Bay, Canterbury and Otago. If covering vegetation has been grazed or disturbed, wind erosion can be severe.
This occurs when running water gouges shallow channels or deep gullies into the soil.
On sloping land, particularly if cultivated, water run-off may gather in small V-shaped channels or rills. These are particularly evident in pumice soils or those formed from loess (wind-blown dust), but can occur on all hill soils.
Gully erosion occurs on unconsolidated subsoils. These are generally deep and generate a lot of sediment, which often feeds into rivers.
Gullies tend to erode at their head, eating back into the landscape. They are widespread throughout the country. In the 1950s, there was serious gully erosion in sandstone at Pohangina in the Manawatū. Severe gullying also developed in pumice soils on the Volcanic Plateau when pastures were first sown.
When water enters the soil on a slope, usually down cracks after a dry period, it scours out the subsoil to form a tunnel. The soil often later collapses inwards, and an open gully forms.
Loess soils (fine dust) are very prone to this kind of erosion, especially on Banks Peninsula and in Marlborough. Wither Hills near Blenheim had many severe tunnel gullies in the mid-1950s. Once the surface was reshaped by bulldozers, some of this land has been used for growing grapes and olives. Although somewhat localised, tunnel gullies also occur in volcanic pumice and Northland’s weathered sandstones.
When gravity combines with heavy rain or earthquakes, whole slopes can slump, slip or slide.
The Tarndale slip is the largest in the southern hemisphere. It lies on the mudstone banks of the Waipaoa River, inland from Gisborne, and adds a considerable amount of sediment to the river. Forest has been planted on nearby former farmland to slow the growth of the slip.
Slips are one of the main forms of erosion in the North Island. They are a direct result of the change from forest to pasture.
Slips occur when the soil (topsoil and subsoil) on slopes becomes saturated. Unless held by plant roots to the underlying surface, it slides downhill, exposing the underlying material.
Slips are common where the underlying rock is mudstone (papa) or siltstone – as in the hill country of Gisborne, Taihape, Whanganui, Taranaki and King Country. The Wairarapa hill country, where loess lies on north-facing slopes, is also susceptible.
In the South Island, soil slips are widespread throughout the hill country, but mostly north of Christchurch.
Awash with it
During a February 2004 storm in Manawatū, there were 62,000 landslides, totalling 1,000 million tonnes of soil. About 50% came from hill-country farms. Flooding caused $100 million worth of damage. Around 210 million tonnes of soil are washed into New Zealand rivers annually: about 10 times the global average. This is a mix of natural and accelerated erosion.
These can be spectacular, but are not widespread. Saturated soils move downwards en masse, and may include underlying rock. However, the surface vegetative mat stays intact, and forms humps and hollows.
This occurs in steep or mountainous areas, where the underlying rocks are greywacke (in both the North and South islands) or schist (South Island). At high altitude, gravity pulls material downhill into a fan shape, which can be quite spectacular.
There is debate over whether this is accelerated or normal erosion. What is clear is that screes can continue to move if vegetative cover is reduced, or stock track across their slopes.
This is common throughout New Zealand, especially in rivers and watercourses with periodic flooding. Although not dramatic, it is significant because most of the valuable farmland lies beside main rivers, on alluvial terraces.