Rainfall induced landslides on the rise

07 April 2022

Australia's heavy rainfall events are becoming more intense and could lead to an increase in landslide events.

That's according to CQUniversity expert Dr Raj Sharma who has studied the impact of landslides caused by transient rainfall.

The Senior Lecturer in Civil Engineering said that despite the long-term trend of total annual rainfall is decreasing' Australian rainfall is highly influenced by El Nino and La Nino' which brings significant variation in interannual rainfall.

"The potential of a rainfall event to create a landslide depends on the magnitude' duration and intensity of rainfall and existing moisture on the ground.

"Climate change is likely to impact all these parameters. Hence' the likelihood of rain-induced landslides in a particular location will change'" Dr Sharma said.

"When rainfall intensity is more than the capacity of the ground to percolate the water slowly' water will choke close to the surface' and the pore-water starts to build up from there.

"This water table at a shallow depth can cause shallow landslides."

He added that when this pore-water pressure is inside hillslopes' the risk is increased.

"Slopes are constantly pulled down by gravity but are held in place due to the resistive forces of their mass'" he said.

"During rainfall events' rainwater infiltrates the ground through the soil pores' joints and cracks and fills up the soil pores.

"This action lubricates the soil material and increases the pore water pressure' reducing the slope's resistive force against gravity. When the resistive forces become smaller than the gravitational force' a landslide occurs."

Dr Sharma warned that even built-up areas are susceptible to landslides and people should be vigilant during and after rainfall events.

"Landslide occurs on a sloped landscape' so steep slope angles mean the area is more susceptible to a landslide'" Dr Sharma explained.

"Urban areas in sloped lands are also vulnerable to slope instability. These areas are developed by modifying the natural drainage paths and are landscaped by cutting and filling. Cutting and filling are not as strong as the natural terrain."

"Additionally' leakage from the sanitary sewer and stormwater pipes could unintentionally increase the groundwater table' increasing the landslide risk of the area."

He said that changes to vegetation' such as clearing to make room for roads' buildings' and other construction make the soil in urban areas less stable.

"Sparse vegetation reduces natural soil stabilisation and makes the ground susceptible to more erosion and the formation of soil pipes.

"Trees are effective at stabilising soil mass and increase evapotranspiration' decreasing the likelihood of landslides on milder slopes.

"However' being in a heavily wooded area doesn't eliminate the chance of a landslide. Trees on steep slopes may increase landslide risk due to their weight."

As shallow landslides are more widespread and can be a threat to communities' Dr Sharma encouraged everyone to know some possible signs of an impending disaster.

"Even though shallow landslides are fast' you may see some warning signs on the house or surrounding area due to uneven settlement or movement of the foundations. Sudden jams of doors or windows could be one of the signs.

"Similarly' people may notice that new cracks appear in walls. Widening of existing cracks on the ground or formation of new cracks' break of underground utilities' unusual sounds of tree cracking are other possible signs."