Rockfall onslaughts intensified by land clearing

Boulders loosened by earthquakes and tumbling down steep slopes travel further today than millennia ago because humans have cleared vegetation, a new study has shown.

A trio of scientists from New Zealand and Australia studied two generations of boulders dislodged from Mount Rapaki in southern Christchurch, New Zealand – one triggered by a 6,000 to 8,000-year-old earthquake event, and another that struck Christchurch in 2011. 

They found without vegetation, the modern rocks travelled as much as 200 metres past their furthermost prehistoric counterpart.

The work was published in Science Advances.

“What makes this an exciting bit of science, which I think in some ways is quite novel, is that we’ve literally compared the locations of thousands of boulders statistically,” study co-author Mark Quigley from the University of Melbourne says.  

Vegetation acts as a barrier to slow rockfalls as boulders bounce down slopes, smash into tree trunks and leave impact scars. 

The area they studied in Christchurch was once home to a dense native forest. The prehistoric rockfall there would have been clogged up in the trees and unable to travel far.

But when humans settled in the area, they cleared the trees and left the boulders obstacle-free. 

Quigley suggests one way the hazard could be reduced is by letting self-regenerating native forests grow and absorb the brunt of any potential boulders’ force. 

“Once the forest grows enough, it could be a very good long-term strategy,” he says. 

While it’s already known planting vegetative barriers can slow plummeting rocks, this study showed how using events from the geologic record to assess risks doesn’t always work when the landscape has changed so drastically.

If scientists wanted to gauge the rockfall hazard before the 2011 earthquake, they may have downplayed the risk by mapping the prehistoric rocks as a guide, as those rocks didn’t break into urban areas, Quigley says. 

But this time round, the boulder onslaught from Mount Rapaki travelled up to 770 metres downslope, with 26 of them plummeting into Rapaki village.

The hazard, though, doesn’t end with the earthquake and its aftershocks: weakened cliffs are still a risk.

One way Christchurch fought the residual rockfall risk was to set down shipping containers to temporarily protect pedestrians and road users. And only last month – in a fresh sign of recovery – it was announced the battered and dented containers would at last be removed from coastal suburbs. 

“We can’t dismiss the possibility that there won’t be any future events in the short-term, or that other regional earthquakes won’t cause the odd rock to fall off,” Quigley says.

“But what we are confident about it that the earthquakes in Christchurch were really worst-case scenarios for rockfall generation at the site we studied.”

Study co-author Josh Borella from Christchurch’s University of Canterbury says despite the “knee-scraping, ankle-twisting field work”, the results were gratifying. 

“[The] most exciting aspect of the project is being able to show quantitatively the impact that humans are having on the expression of geologic phenomena, and specifically in this case, rockfall hazard, which hasn’t been done before.” 

He adds the results from the post-earthquake rockfalls were crucial. “Put simply, if you had gone out to Rapaki before the 2011 Christchurch earthquakes and mapped the prehistoric rockfall […] you would probably conclude that during future rockfall events the Rapaki community was safe because none of the prehistoric boulders reached the village.

“However, this would be wrong because changes to the landscape, in this case human-induced deforestation, have not been factored into the equation. 

“When they are, you come to a very different conclusion, and recognise that the rockfall risk could be significantly higher in future rockfall events, as we tragically learnt during the 2011 Christchurch earthquakes.”