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:
- the West Coast south of Port Waikato
- the King Country
- the Coromandel Peninsula
- numerous other hill country districts.
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:
- Rock - hard mass that was in its natural place before movement.
- Debris - soil material where a range of sizes is represented.
- Earth - soil material with mostly small particles.
Landslide movement types:
- Rotational rock slump.
- Translational debris slide.
- Earth block slide.
- Complex slides (which combine any of the above).
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 or storm events
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.
- Earthquake activity
Earthquakes can often create enough movement to dislodge material, creating a slide, or liquefying a lower layer, creating movement at the surface.
- Human activities
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.
Prevention and minimisation
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:
- Discouraging building and other development in ‘at risk’ areas (the cheapest option).
- Warning signs where the landslide risk cannot be reduced.
- Slope drainage to reduce pressure from ground water.
- Removing material to reduce the angle of slope, making it less likely to slide.
- Removing unstable material (although this may not be an option if it involves taking away the bulk of the hill area).
- Leaving the slide debris in place to help the slide stabilise.
- Other engineered options such as rock-bolting, shot-crete (a type of concrete used to cover the slide face, or potential slide face) and adding weight to the toe of the slide where possible.
Measuring and monitoring
The Institute of Geological and Nuclear Sciences has been working on landslides as part of its Geonet Project. 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.
- We are participating in the Landslide Register by providing information to the Institute of Geological and Nuclear Sciences on landslides that are triggered within the Waikato region.
- We provide information on preventing or reducing the effects of landslides.
- We are involved in hazard identification. For particular sites Waikato Regional Council can plant the slide or intercept surface water to increase stability.
- We develop and carry out risk mitigation plans, which minimise the effect of natural hazards on the Waikato economy and community.
For policy information on natural hazards, see our Regional Policy Statement.
- Waikato Regional Council - Natural Hazards
- Waikato Regional Council - Civil Defence and Emergency Management
- Waikato Regional Council - Subsidence
- Waikato Regional Council - Earthquakes
- Waikato Regional Council - Geothermal Activity
- The New Zealand GeoNet Project - real-time monitoring and data collection for rapid response and research into earthquake, volcano, landslide and tsunami hazards.
- United States Geological Survey - National Landslide Information Centre – has landslide information, photos, diagrams, teacher/student resources, recent events and frequently asked questions.