17 years since Thredbo landslide
It's been 17 years since a landslide in the small alpine village of Thredbo claimed 18 lives.
It happened not long before midnight in the NSW ski resort, with earth and debris ploughing into ski lodges as people slept. Only one person survived; ski instructor Stuart Diver was miraculously rescued more than two-and-a-half days later.
As Snowy Mountains residents remember family and friends lost, we take a look at what causes landslides and if they can be prevented.
TYPES
There are three main types of landslides in Australia. The closest one to the typical image of a landslide is an earth slide.
This is when the ground moves as one piece, with the surface of that piece remaining flat as it falls.
There are also rock falls, which is when a rock detaches from a cliff face and falls unimpeded.
The third type is a debris flow and is when a mass of soil and rubble moves down a slope collecting more debris as it goes.
NATURAL TRIGGERS
"There are a lot of factors that make land more susceptible to sliding," says Geoscience Australia landslide scientist Libby Metz.
One of those factors is geology. If soil and rocks are not solid enough, there is a higher chance of them sliding.
With rock, such as slate, there are multiple layers that make it weak and easy to fracture and therefore tumble.
The geology of Australia also plays a part. The steeper the slope, the more susceptible it is to landslides.
Rain is another a big factor. Within rocks, Metz explains, there are tiny spaces between the grains that water sits in.
If it rains a lot, those gaps fill up and create pressure that can cause the grains to separate, which can then lead to a landslide, Metz says.
Weathering caused by wind and rain can also weaken rocks and make them more likely to slide.
HUMAN TRIGGERS
Development is a large factor in causing landslides.
From 2000-2011, more than half the landslides causing injury or death were directly or indirectly caused by humans, according to Geoscience Australia.
The Thredbo landslide, which happened at 11.30pm on July 30, 1997, started on the slope below the Alpine Way, running above the resort village.
According to the paper Impact of Landslides in Australia to December 2011 by Marion Leiba, the Alpine Way rested on uncompacted fill and very slow land movement (creep) in the fill had been occurring for years before the landslide.
"The creep caused joints in a water main to leak for at least two months, and possibly as long as 12 months, before the landslide," the paper states.
The uncompacted fill embankment became saturated with water, triggering the landslide.
Although earth can appear stable, once it's built on, it can be under too much pressure and collapse, Metz explains.
This pressure can be caused by the weight of roads as well as by the vibrations of vehicles passing over.
Seismic activity is also one of the main drivers of landslides.
Vegetation can reduce the chances of a slide, says Metz, with the roots holding soil and rock together.
"If humans remove that vegetation, it can cause previously stable slopes to fail because they no longer have those roots."
HOT SPOTS
Parts of Australia are more prone to landslides depending on their rock type, slope, rainfall and tectonic activity.
The central and western areas of Australia are older than other regions and have therefore experienced most of the landslides they are likely to, explains Metz. This is evident in the fairly flat landscape and small hills in those areas.
The east coast, Tasmania, and southern and eastern Victoria are much more susceptible to landslides because there are more mountain ranges, she adds.
"Those are much newer areas of Australia geologically."
MONITORING
Landslides are monitored on a very small, site-specific scale by community groups.
This is done with a piezometer, which measures the water pressure within rocks.
Another way of monitoring is by remote sensing through satellites. Geoscience Australia scientists recently conducted a test during which they used radar imaging to measure small changes in a slope. These changes indicate whether there might be a larger movement in the future.
