Natural hazards

Land instability

Erosion and slope failure is common throughout the Auckland region. Land instability is often made worse by human activities.

Slope instability

The underlying causes of slope instability in the Auckland Region are fairly well known from case studies of specific failures. The causes can often be directly related to the rock and soil composition and the orientation of defects (e.g. bedding inclination, fractures, clay seams) with respect to slope. The degree of weathering and saturation are also significant factors. Some of these may be constant, or changed by new events (e.g. seismic activity or construction activity).

Much of the Auckland region is subject to instability, though the scale of individual slope failures is generally limited in extent. The main risks associated with slope instability:

• economic losses
• reduced capacity along some roads or temporary road closures while debris is removed
• small risk of loss of life.
  

Erosion

Erosion is the action of wind and water wearing away exposed soils or rocks. Eroded soil gets into Aucklands waterways. This causes water pollution and problems for the animals, plants and people that use water. Erosion can also disrupt land use, cause damage to fences, roads and buildings.

50% of the Aucklands agricultural land is prone to erosion. Human activities which modify the land can increase the amount of erosion.

Soil slip and sheet erosion are prevalent on pastoral land in the central Rodney district, 20 minutes north of Auckland (New Zealands largest city). Significant soil loss also occurs as a result of market gardening, common in south Auckland. Pockets of moderate to severe wind erosion occurs on the sand country of the western coastal areas.

Stream-bank erosion can be enhanced by stock or farm animals near stream banks and is a key source of sediment in streams.

A number of agricultural, horticultural and forestry industries rely on the regions soil. Degradation of the soil will lower the productivity of the region, its wealth, and cause negative environmental impacts.

What can be done to conserve soil?

• Plant trees on hill slopes and in gullies.
• Maintain and protect areas of trees, especially older trees. Fence off stream banks - plant trees along watercourses to provide bank stability.
• Dont overgraze pasture, especially in winter or during drought conditions.
• Ensure cultivation of soil at the correct soil moisture content.
• Maintain a good level of soil fertility - apply fertiliser and lime as recommended based on soil testing.
• Improve drainage where possible.
• Match your land use with the soil type and contour of the land - for example, some steep areas may be better retired into farm forestry.