11 March 2022

Main Arm: a significant rainfall induced landslide in Australia

Posted by Dave Petley

Main Arm: a significant rainfall induced landslide in Australia

The eastern side of Australia has in recent days suffered a rainfall event of extreme severity, causing extensive floods and landslides, claiming at least 22 lives.  It is hard to understand the magnitude of this event, and the degree to which it has varied from the long term record.  The graph below, produced by the Guardian, shows the 2021-2022 cumulative rainfall to 8 March 2022 versus the long term trend for Brisbane.  In this case, “very wet” is the 90th percentile:-

The daily cumulative rainfall for Brisbane for 2021-22 v the median, 10th percentile (very dry) and 90th percentile (very wet) of historic daily cumulative rainfall values.

The daily cumulative rainfall for Brisbane for 2021-22 v the median, 10th percentile (very dry) and 90th percentile (very wet) of historic daily cumulative rainfall values. Historical data is from 1900 to 2021. Last updated 8 March 2021. Image from The Guardian.

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The event of the last week or so is extraordinary, taking cumulative rainfall to double the 90th percentile value for the time of year.  This is a La Nina event, turbocharged by global heating, in action.

Unsurprisingly, this rainfall had triggered landslides. An area that has been severely affected in Main Arm in the Northern Rivers region in the northern part of New South Wales.  The landslide below was triggered by the rainfall (image tweeted by Liam Mendes – @liammendes):-

The landslide at main Arm in New South Wales, Australia.

The landslide at main Arm in New South Wales, Australia. Image tweeted by Liam Mendes (@liammendes).

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@Liammendes has also tweeted another view of the landslide:-

The landslide at main Arm in New South Wales, Australia.

The landslide at main Arm in New South Wales, Australia. Image tweeted by Liam Mendes (@liammendes).

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This landslide appears to have occurred in deeply weathered residual soil or regolith.  Note the house in the lower right side of the image, which has been destroyed by the landslide.

This is only one of a number of landslides in the area.  Recovery from the disaster has been severely impacted by damage to the roads caused by landslides and washouts. Fortunately there have been no reported fatalities in Main Arm.