Unusually heavy rainfall tends to cause landslides, sometimes with deadly consequences. In some cases there can be indications a slip has become more likely, providing an opportunity to get away before lives are at risk.
Landslides related to the heavy, long-lasting rain in northern parts of the country in recent days have damaged roads and buildings. The body of Dave Lennard was found after a landslide had brought down his house in the Auckland suburb of Remuera.
The National Emergency Management Agency (NEMA) has a list of warning signs that could indicate an increased risk of a landslide.
The signs are:
- small slips, rockfalls and sinking land at the bottom of slopes,
- sticking doors and window frames, which may mean the land is slowly moving under the house,
- gaps where window frames are not fitting properly,
- steps, decks and verandas moving or tilting away from the rest of the house,
- new cracks or bulges on the ground, road, footpath, retaining walls or other hard surfaces; and
- tilting trees, retaining walls or fences.
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“It’s important to recognise the warning signs and act quickly,” NEMA said.
It also advised people to find out if there had been landslides in their area before, and where they might happen again.
But while there are a limited number of obvious indications land is about to slip, the huge variation in land types can make it hard to know if there’s a high risk of a landslide.
“There are a lot of site-specific characteristics,” Martin Brook, associate professor of applied geology at the University of Auckland, said.
“If you see tension cracks, or some kind of minor subsidence on your land that can often be a precursor, but there’s a lot of uncertainty,” Brook said.
“It really depends on where you live, how far you are from a cliff edge, the slope, if you’ve got vegetation.”
While having trees growing on slopes reduces the risk of landslides, Brook noted many of the landslides during the intense rain in Auckland in recent days had been on land that was covered in trees.
That was an indication of how high the intensity of the rain had been, he said.
In Gisborne, where Brook has been doing research, he had found landslides often happened on land from which trees had been removed. “That allowed water to get into the slope.”
There were databases showing the locations of landslides and information on work that had been done to assess land stability, such as the drilling of boreholes, Brook said.
People could also get an assessment of the stability of their land from an engineering company.
A recent paper co-authored by Brook said the most common slope failures in New Zealand were slides and flows in shallow soil and regolith – unconsolidated loose material on a bedrock layer – triggered by heavy rainfall.
The Landslides NZ website, managed by Auckland Council geotechnical and geological practice lead Ross Roberts, said accurately predicting future landslides was not yet practical.
But past slips were a good indication of the risk, and it was sometimes possible to find if a property was at risk by contacting the local council to find out if there had been slips on or near the property.
The GNS Landslide Database map could also show if there had been recorded slips near a property. But none of these records were complete, as there was no obligation for slips to be reported.
A report prepared by Beca for Nelson City Council said the Australian Geomechanics Society (AGS) had provided guidelines for assessing land instability.
Settings identified by AGS as land potentially susceptible to instability were:
- Natural slopes steeper than 35 degrees (rapid landslides may occur),
- Natural slopes 20-35 degrees (landslide travel possible),
- Steep slopes degraded by recent logging, forest fires, and/or road construction,
- Slopes where geologic and geomorphic conditions are such that sliding is possible,
- Slopes with a history of instability, including large currently inactive landslides subject to undercutting of the toe or reactivation by development.
Factors that could affect the susceptibility of a slope to instability included the rock type, soil type and thickness, watershed size, and vegetation cover.
Triggers for slope movement included rainfall, earthquakes, physical and/or chemical weathering, and land development, the Beca report said.
Rainfall was the most common trigger of instability, but it depended on the duration and intensity of rainfall, along with pre-existing ground saturation and groundwater conditions.
AGS outlined that the best documented historical events had been triggered by high rainfall and involved saturated soils within the upper soil and rock profile (0.5m to 2mm deep) on moderately to steeply inclined slopes and associated with relatively short duration (12- 48hr) high intensity rainfall events.
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