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Landslides happen all over the world, claiming thousands of lives every year, but what causes them, and what are the warning signs?

Water, weakness and the relentless pull of gravity: the science behind landslides


Scott Sutherland
Meteorologist/Science Writer

Wednesday, October 29, 2014, 9:34 AM - With the risk of landslides rising in the Vancouver area, as rain continues to inundate the B.C. coast, let's take a look at the science behind how landslides happen, and the warning signs to watch out for to protect yourself.

By definition, a landslide is when the topsoil of an area essentially 'loses its grip' on the underlying bedrock, and the entire area moves, en mass, down towards lower ground. This can happen very slowly - which is called 'creep' - and end up causing significant damage to houses, buildings, roads and other important parts of our infrastructure over time. The most dramatic events are the ones that happen very quickly, and those are also the most dangerous.

There are two major players (technically 'forces') in the game when it comes to landslides - gravity and friction - acting directly against each other. Any time gravity wins, the earth moves.

Everything on (or above or under) the surface of the Earth is being pulled towards the planet's core by the relentless force of gravity. In some cases, like in rockslides, gravity acting alone is enough to overcome friction, such as the case of precariously-positioned boulder that can finally be torn free of its surroundings by that endless tug, and take a significant portion of the hillside along with it as it bounces towards more level ground.

Often times, though, gravity has a helping hand. Earthquakes can produce an intense jolt that suddenly loosens the grip of friction between the top soil and underlying bedrock on nearby slopes. Volcanic eruptions can do the same. Human activities, such as excavation or construction, can weaken the integrity of the slope and hasten gravity's win as well.

The overwhelming 'ally' for gravity in its fight for supremacy, though, is water.


RELATED: 'Landslides will be a concern' as heavy rain saturates the B.C. coast


The reason that most hillsides remain in place is because of the force of friction acting between all the individual particles of soil, along with the rocks and the tree and plant roots. The tighter the soil is packed together, the stronger the force of friction is between the particles, and the more stable the hillside is going to be. However, add water to the mix and it changes everything.

It can be as simple as falling rain - either over a long period of time in the same area or a particularly strong rainstorm drenching the ground. It can be due to a broken underground pipe leaking water or sewage into the topsoil. It can be melted glacier water after a volcanic eruption (like what happened with Mount St. Helens). However it comes (as that only names a few), the added water trickles through the top soil, soaking the soil grains, especially those near the base of the soil layer, and this weakens the forces of friction between them. Not just that, but the water adds more weight to the ground as well, so not only does it weaken the stability of the soil it also increases the pull of gravity on the whole mass together. 

Different types of soil - clay, sand or silt - have different 'internal strengths', thus have more or less friction between the particles, and different tolerances for water. The presence of trees, with strong roots buried in the soil, can help with stability. Even the roots of shrubbery and grasses can help. At some point in this, though, gravity can always win out. All it takes is a section of the ground near the base of the slope (where gravity is technically strongest and it has the most 'stuff' pushing down on it from above), with its soil consistency turned into something more akin to freshly-mixed cement, giving way, and the rest of the slope can come down with it.

In some cases, where the soil just takes on enough water to barely overcome friction, or the shape of the underlying bedrock is favourable, or both, the debris flow can be fairly limited. It can still take out a roadway or a set of train tracks, or run up against a house or building, but the damage typically isn't too extensive.
As the water content of the soil goes up, though, the potential devastation ramps up along with it. The video below, from National Geographic, shows just a few examples of what's possible when the ground completely gives way.

Warning signs?

In some cases, there just isn't any kind of significant warning that a landslide is going to occur. An unprecedented rainfall can drench an area with far more rain that has ever fallen in such a short amount of time before, and this can overwhelm normally stable hillsides in the region.

There are some signs you can look for though, to check if there's a landslide possible or even imminent in your area.
- Watch for new cracks - in the soil, in sidewalks and roads, in building walls and foundations - especially if near or on a slope. This can indicate shifting soil.
- Also, if doors and windows are sticking (especially if they haven't done so before), that's also a sign of shifting soil.
- Look for unusual bulges in the ground near the base of slopes, since it may be a sign of the base of the slope slumping.
- Check for slanting in trees, telephone poles and fence posts. Even if trees are growing on a slope, they will still grow roughly straight up towards the sky. Poles and fences are built to point straight up as well. If they're slanted, the ground has moved there at some point.
- Rapid changes - increase or decrease - in the flow of local streams, since that can indicate changes in water content in the soil.
- Unusual noises - rumbling, creaking, cracking - especially if they're growing louder, are a strong sign that a landslide may be approaching your location!

If you see any of these signs, contact local officials to pass on the information, or to learn more about the dangers they know about. You can get an assessment of your property for soil stability, and to find out what steps you can take to protect yourself. 

If a landslide is imminent or happening, the best thing to do is leave the area, by the fastest way possible. Given how fast these events can happen, and how quickly they can become very, very bad, don't delay, and don't let belongings slow you down. If you become trapped, getting up to the second story of a building is probably your best bet to protect yourself from what's happening, since it will get you up above the level of the flow.

CLICK BELOW TO WATCH: The most vulnerable locations in Canada for landslides.

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