WATCH ABOVE: An aerial view of the devastation from the Washington state mudslide.
TORONTO – Two days after a mudslide killed at least eight people and flattened 30 homes in a rural area northeast of Seattle, scores of people were still unaccounted for.
Authorities said the slide–which happened in an area with a history of unstable land–was caused by ground water saturation from recent heavy rainfall. A slide also happened in the same area in 2006, according to the local emergency management officials.
What triggers a mudslide
Prolonged rains are the primary trigger. When the soil becomes waterlogged, it’s unstable. Most happen on a slope, similar to an avalanche, said former applied geology professor Marcos Zentilli.
“But in this case it’s the soil that gets saturated with water, normally in an unusually strong range so the water can’t really drain,” said Zentilli, a Professor Emeritus at Dalhousie University.
The soil becomes very heavy, and suddenly the strength of the material in the soil isn’t enough to hold it. And then it goes.
“And the moment it goes, any motion in it makes it transform itself into a complete liquid – it’s like a quicksand.”
“So materials that are normally standing on it … sink into it because the density of the material becomes very, very low because it’s full of water trying to escape,” he said.
Another trigger is cutting roads in forested areas, since that alters the normal drainage pattern. Zentilli said forest industry workers often look for ways to make sure drainage is adequate before and after construction, but this is becoming more common as we increasingly invade natural environments.
Signs to look out for
Close to the top of slopes, there may be signs of cracks opening where material is starting to slide before a mudslide begins. Zentilli said this starts very slowly but accelerates to a speed so fast it’s not possible to run ahead.
“People look for those tell-tale signs of cracks, some indication that the soil is starting to extend –like stretchmarks on the soil,” he said, noting some areas for potential mudslides have been identified by studies up water from Whistler in British Columbia in recent years.
How to survive a mudslide
Zentilli said the danger is the speed of a mudslide that’s transporting a lot of material that otherwise would not be transported by a river: Trees, homes and very large rocks, for example.
“It’s like quicksand-–you could sink in it,” he said. “Then when it stops, that material in the water escapes and the material settles into a very thick mud…after a few hours, you can actually drive a car over it.”
The best tactic is to run laterally away from where the slide is coming–don’t run downhill (you’ll never outrun it) and don’t run towards it.
When it comes to taking shelter, Zentilli advises against hiding in a basement as you would for other types of natural disasters.
“Probably a bathtub would be a good idea–something that would protect you or try to keep up, because once you get covered by that mud, it solidifies very quickly.”
He added even if your legs were in the mud and the rest of your body was free, it would be very difficult to remove yourself since the mud solidifies once the water escapes.
“It clamps you down.”
Best prevention methods
Two of the best ways to prevent mudslides from destroying homes and taking lives are building adequate drainage systems and responsible building of homes.
“Drainage seems to be the main way to avoid accumulation of water within especially the lower slopes of these areas,” said Zentilli.
“Best is to not build houses in places that are susceptible to mudslides, but we are foolish enough to build in places where they do.”
He said few people consider building location as a danger, but with changing weather patterns, it should be a priority.
“We have to consider that these unusually strong rains will continue to happen and that a consideration should be before anybody invests in building and invading natural environments, one should be looking at the evidence for preventing things like landslides or floods.”
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