Why More Rain Doesn’t Always Mean More Landslides

Why More Rain Doesn’t Always Mean More Landslides
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Why More Rain Doesn’t Always Mean More Landslides by Michael Bishop-Cardiff

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More rainstorms brought about by future climate change aren’t likely to affect the frequency of common landslides.

While climate change projections show the frequency of rainstorms may increase by up to 10 percent, a new study suggests it would produce a long-term increase in shallow landslide frequency of less than 0.5 percent.

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“Landslides pose a major hazard to life and infrastructure, affecting around 12 percent of the world’s population who live in mountain ranges.”

Shallow landslides are the most common type of landslide and are often caused by heavy rainfall. They occur through the collapse of soil, resulting in fast moving debris flows of rock and mud that present a dangerous hazard to anything in their path.

The new findings, published in the journal Scientific Reports, challenge current theories that suggest landslide activity could increase proportionally with increased rainfall.

Instead, the triggering of landslides is much more dependent on the build-up of soil—otherwise known as colluvium—on steep hillslopes, as opposed to rainfall from storms.

Researchers arrived at their results by performing field investigations in the Southern Appalachian Mountains, specifically looking at how the time taken for soil to accumulate on hillslopes affected the landslide triggering rate. They then used computer models to calculate how future landslide hazards may develop as a result of climate change.

The findings show that shallow landslides occur when soil slowly accumulates on a mountainside over a very long time period, from thousands to tens of thousands of years. During a storm, converging ground water flow and the infiltration of rain into the colluvium trigger landslides.

It then takes thousands of years for soil to accumulate once again on the mountainside before a landslide can occur again, so an increase in the frequency of storms during this time would have little effect on the frequency of landslides.

Landslides travel a long way when rocks flow like fluid

“Our results have shown that lots more storms result in very few extra landslides,” says lead author Rob Parker of Cardiff University’s School of Earth and Ocean Sciences. “Though observations tell us that heavy rainfall triggers landslides, it is the process of soil accumulation that happens in the thousands of years leading up to a landslide that can be really important in determining how often landslides occur.

“Though we still expect shallow landslides to continue to be a major hazard in our future wetter climate, we do not expect the frequency of landslides to increase in proportion to the frequency of extreme precipitation events.

“Landslides pose a major hazard to life and infrastructure, affecting around 12 percent of the world’s population who live in mountain ranges. In addition to the direct hazard they pose, landslides are the primary source of sediment in mountain ranges, with significant knock-on effects on river, floodplain, and estuarine systems, as well as playing an important role in global biogeochemical cycles.

“The consequences of landslides are therefore wide-reaching, so it’s vital that we get a better understanding of how they may evolve under future climate conditions.”

Source: Cardiff University

Original Study DOI: 10.1038/srep34438

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