Story: Soil erosion and conservation
Page 8 – Mechanical control of erosion
Several mechanical methods are used to control and prevent erosion.
Flumes are artificial channels that control the flow of water down a slope and release it into an area where its impact is reduced. They are often placed at the head of gullies to prevent the backward erosion of the headwall by water flowing over the top.
Debris dams are sited in the floor of gullies. Built of wood planks or tyres, they trap material moving down the gully floor. Often this technique is used in conjunction with pair planting.
Detention dams are small dams on farms or sites such as ephemeral waterways which, under heavy rainfall, can create erosion within the waterway. The dams are designed with a wide spillway that allows some storage of water, and in flood conditions allows a steady and slow release of water over the spillway.
Where an earthflow has occurred, land smoothing is used to stabilise the soil. Bulldozers smooth the surface of the earthflow, so water will run off rather than pond and saturate the unstable soil. This technique is expensive.
Infilling can be used where tunnel gully erosion has occurred. The gully edges are pushed into the centre, which is compacted. The contour of the land is then shaped to spread run-off. This method was used successfully in the early 1960s at Wither Hills near Blenheim.
Pasture furrows were introduced in the 1950s, notably in Canterbury’s cultivated downlands, to control run-off and prevent sheet and rill erosion.
In the pasture phase of crop rotation, small channels are ploughed about 10 metres apart across the slope. These divert run-off to grassed waterways, which then feed into natural streams and rivers.
A variant of pasture furrows are graded banks, which are much wider and further apart. These were used in Northland.
Conservation tillage is where crop-growing soils are left, after harvest, covered in crop residues. This acts as a mulch, protecting the soil from wind erosion and raindrop impact.
With contour cultivation, all cultivation is done across the slope. This creates a series of mini-barriers to the downward flow of water.
Direct drilling is a method where pasture seeds or crops are drilled straight into the soil, under pasture. The advantage is that being unploughed, the soil is not vulnerable to erosion.