Tekst: Yngve Vogt, Translated by: Kathrine Torday Gulden
Surprisingly sand and clay landslides can move at a much higher speed under water than above. Landslides can cross ocean floors with a tremendous speed and move several hundred kilometres in more than a hundred kilometres per hour.
"The speed of the landslides can be compared to the airport express in Oslo and the landslides “float” almost without any contact with the sea bed", says Professor Anders Elverhøi from the Department of Geosciences at the University of Oslo to the research magazine Apollon.
He has spent the last ten years studying underwater landslides.
His interest in underwater landslides began already at the end of the eighties, when the oil industry needed knowledge in regard to where they could place installations on the sea bed without there being a risk of them being ruined by landslides.
The oil industry has the last years begun the search for oil in the continental slope which ties together the continental shelf and the deep sea. Huge landslides have been observed in this area.
One example of this is the gas field at Ormen Lange, where the Storegga Landslide occurred approximately 8200 years ago. The landslide resulted in a 10-15 metre tidal wave which lead to the ruin of a Stone Age settlement on the west coast of Norway.
After the Ice Age a similar landslide occurred just outside Bjørnøya. The landslide moved 100 to 200 kilometres across the sea bed, even though the angle of inclination was only between a half and one degree steep, this being no steeper than Denmark.
Landslides in modern times
The most notorious landslide in modern time occurred outside Newfoundland in Canada in 1929. Cables on the sea bed were demolished. When researchers studied the damages they were able to establish that the landslide of one thousand cubic kilometres had moved at a speed of 60 to 100 kilometres an hour.
A thousand cubic kilometres is such a large amount that it could have covered Norway with more than three metres of deposit. The large landslides outside the coast of Norway were even bigger.
Underwater landslides are paradoxical in two ways.
"One paradox is how such huge amounts of deposit can move at such great speed. The other paradox is that the landslides travel across great distances even if the angle of inclination is slight", says Anders Elverhøi.
To be able to reveal the secrets concerning underwater landslides, Elverhøi and Dr. Carl Harbitz at the International Centre for Geohazards have worked together with American researchers at a laboratory in St. Anthony Falls in USA. The researchers have imitated a landslide in a small tank with a six degree angle of inclination. Advanced high speed cameras have documented the experiment by taking 250 video images per second.
The fundamental physical nature of the landslide has been researched, more specifically how the particles distribute and have an impact on each other.
The results of this experiment have been used to create a mathematical model of large landslides at sea.
Deposit landslides travel further in water than in air.
On one of the videos the landslide is similar to a train with its nose pointing upwards. The speed is as intense the entire time.
Due to the pressure which commences in front of the landslide, the landslide points upwards and moves above a thin layer of water, hardly in contact with the sea bed. The phenomenon is called hydroplaning and causes less resistance. This is especially apparent in regard to landslides with a high content of marine clay, such was the case in the notorious Storegga Landslide outside the west coast of Norway.
Stretches into two
Thanks to the experiments at the American laboratory, Anders Elverhøi has calculated that the front part of the Storegga Landslide travelled at three to four times the speed than the behind part.
- When a landslide stretches, the mass is distributed over long distances. This can be compared to a dispersing car queue. The cars in front race forward, while the queue still clutters together behind.
During the stretch, water is mixed into the landslide. This causes less “resistance within” the landslide which results in the landslide moving at a higher speed.
– The combination of hydroplaning up front, that the landslide stretches and that water mixes in to the landslide, has to do with the speed and distance the landslide travels. This also results in the underwater landslide meeting a smaller collected resistance than a landslide would in air. We had not foreseen these results.
In certain cases there is such a large stretch in the landslide that the front of the landslide escapes the back end of the landslide. In these cases the landslide is divided into two.
As of today pictures are only taken of the landslide’s surface.
– We are not sure that the surface of the landslide represents what is actually happening inside the landslide. To be certain of this we feel it is necessary to do research on the particle movement within the landslide.
This is the next step for their research project.
"We can learn more about safety in regard to underwater installations once we understand which mechanisms lead sand and clay to deep water. This understanding is also important to learn what sort of connection there is between landslides and tsunamis and what creates the basis for the oil reservoirs on the continental slope. This isn’t only interesting for Norway, but also for oil searches in other deep sea areas, such as on the outskirts of Nigeria, Angola and Brasil", Anders Elverhøi says.
Anders Elverhøi | alfa
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