School Creek Landslide at Taroona High one of thousands monitored across Tasmania

Taroona High School is slowly sliding into the River Derwent. (ABC News: Luke Bowden)

Across Tasmania there are almost 10,000 recorded landslides, which are monitored by the state’s geologists.

A lot aren’t active and may date back to the last glacial period, but government body Mineral Resources Tasmania (MRT) maps the areas to minimise the effects of landslide hazards.

Chief government geologist with MRT Andrew McNeill said landslides were a natural part of many areas of the Tasmanian landscape.

“One of the issues we have is knowing which ones are going to be active and which ones are old features that are unlikely to be active again,” Dr McNeill said.

He said changes to the climate could trigger landslides.

“One of the big triggers that we see around urban areas is human disturbances such as housing developments,” Dr McNeill said.

“They can re-activate or start new landslides which then damage property and infrastructure.”

Landslide damage

Since the 1950s, 150 buildings have been damaged or destroyed by landslides, including the loss of 75 houses.

The state has paid $10 million in compensation for damage to dwellings.

The Hobart suburb of Rosetta has a live landslide issue, which has destroyed nine houses and damaged 13 in the past.

“There are properties there that may claim at some stage for damage due to landslide,” Dr McNeill said.

There are many different types of landslides, and they differ from fast-moving debris flows caused by heavy rainfall where rocks and trees get pushed downhill.

Dr McNeill said while slow-moving landslides weren’t a risk to life, they could get to a stage where houses became uninhabitable.

In 2019, five families from Deviot in the West Tamar sought compensation for damage to their properties from a landslide that re-activated after heavy rainfall in 2016.

Nearby Legana has experienced similar problems.

MRT’s mapping and data is designed to assist town planners to avoid unstable areas and minimise damage to property and infrastructure.

How landslides are monitored

From the 1940s, geologists relied on air photos to monitor how the land had changed over time, but now 70 per cent of the state is covered by Lidar mapping.

Lidar stands for Light Detection and Ranging and uses laser technology to give a detailed picture of the land.

“It’s really helped us see the landslides across the state,” Dr McNeill said.

The landslides are tracked on a database, and geologists will look at the images and, if needed, examine the site by foot to determine rock types and provide further information.

A report in 2009 identified 2,300 landslides on the database, but further advanced mapping has seen that figure grow to almost 10,000.

Taroona High ‘sliding down the hill’

One monitored landslide is known as the School Creek Landslide, located at Taroona High School.

It is a deep-seated landslide that affects the school and 10 nearby houses.

It is believed to have been active since 1946 and has been monitored by MRT since the 1970s.

“We are doing a lot of monitoring on it,” Dr McNeill said.

“It’s what we call an earth-creep landslide because it occurs at very low rates in the order of millimetres per year.”

He said the landslide was moving on average at 3–4 millimetres per year. The school sits on the main part of the landslide, which means it moves with it and the damage occurs at the edge of the landslide.

“It’s enough to damage the road and there’s a vacant block that was taken out,” Dr McNeill said.

“They’re basically just sliding down the hill with the landslide.”

He said MRT had new techniques for monitoring the landslide, and were working with a Canadian university using a satellite-based system to get measurements of movement over time.

“We build that up into a time series and we can see at what rate the landslide is going,” he said.

Dr McNeill said during La Niña, the landslide sped up.

“Now things are starting to dry up. Things are slowing down,” he said.

He said the landslide was not a huge risk due to the slow speed it was moving.