Landslides occur when unstable rock and soil on steep slopes are disturbed by earthquakes, heavy rain or activities such as mining or road construction. The diverse nature of the Waikato region’s landscapes produces different types of landslides in different areas.
A landslide is a mass movement of rock, soil and other earth material down a slope. They can be very large (such as a landform at Te Kauri on the West Coast believed to be a large ancient landslide), or small, affecting a limited area.
Landslides are more likely to occur on areas of land steeper than 45 degrees. In our region these areas include:
These areas have an increased chance of slope failure if other factors such as high rainfall, accelerated soil erosion, unstable basement rock structure or earthquakes increase the risk.
In the past, landslides caused by the geothermal alteration of the soils in the Hipaua Cliffs near Little Waihi have covered the area, while larger cliff failures have caused soil and rock flows that created over eight hectares of new land at the southern edge of Lake Taupo.
Landslides can be divided into groups according to their type of movement and type of material.
Types of material:
Landslide movement types:
Rocky coastal cliffs and cliffs in gorges above streams are more likely to experience rock ‘falls’ and ‘topples’. Soil slopes more likely to slide by ‘flow’ or ‘slide’.
Landslides can also threaten ‘lifeline’ services such as water, power, telecommunication and transportation networks. Find out more about managing Lifelines to deal with unexpected emergency or natural hazard events.
Landslides are usually set off by one or more ‘triggers’:
Heavy rain often adds weight to the soil surface. The underlying material can’t support the increased weight and fails. This is the sort of damage seen after cyclones and storm events that bring heavy rain and saturated soil conditions. Landslides also expose an area to further erosion by wind and water.
Earthquakes can often create enough movement to dislodge material, creating a slide, or liquefying a lower layer, creating movement at the surface.
Human activities such as mining, quarrying and road construction can remove the base or toe of a slope or old slide. Once the support from this is gone, sliding may occur in a heavy rainstorm. Other activities, including explosions and the use of heavy equipment, can also initiate landslides. The removal of vegetation can increase the rate of erosion or increase the rate at which the soil absorbs water, raising the ground water level and de-stablising the slopes.
Sites where previous landslides have occurred also pose a future risk, as landslides often occur at the same place more than once.
It is difficult and often expensive to prevent or minimise landslides. We need to identify the reasons behind the movement and reduce risks to human life through engineered options or changes in land use.
Planting trees can help stabilise an area, but if planted in the wrong place they can add weight, which may initiate a slide. Trees can have a beneficial effect by drawing up some of an area’s ground water, however there is the potential to dry out the site too much. Vegetation also helps slope stability as plant roots bind up soil particles, making them less prone to erosion – although the effectiveness of this may vary depending on the type of vegetation.
Other options that may help include:
The Institute of Geological and Nuclear Sciences has been working on landslides as part of its Geonet Project(external link). This involves collecting technical information as well as mapping the distribution and occurrence of landslides in a Landslide Register.
In some areas prehistoric landslides can still be seen, as it may take many years for evidence of a landslide to be removed through natural processes. These ancient landslides can be used to measure and identify potential risk for land use and development.
Attempts have been made at modelling landslides to predict their behaviour. Modelling is difficult because different rock and soil types have different strengths and weaknesses.
For policy information on natural hazards, see our Regional Policy Statement.(external link)