The photo shows water-logged volumes in a debris flood, which consists of soil, vegetation and rocks. Photo: Johannes Vik Seljebotn/COWI.
According to him, a 30-degree slope is the most common angle for avalanches. Therefore, you should always keep away from areas where the terrain slopes more than 30 degrees.
“Plus, it’s much more enjoyable to ski on a slope smaller than 30 degrees. You are more in control of the situation, which makes you look way cooler,” says Seljebotn.
Human intervention often increases the risk of slides
A large part of mudslides and rockfall are a result of changes to the natural landscape. For instance, new road and residential projects.
“Infrastructure development is responsible for more slide accidents than the weather. Before, when building a house, you would choose a location that was safe. Now, you want a great view. In some places, huts are being constructed in old slide areas, because the amount of snowfall has been decreasing steadily. But then there are years like 2020, with snow volumes that are significantly bigger than normal. And your property may not be safe anymore,” says Seljebotn.
The same can be said for building new roads.
“A common rockfall angle is a 45-degree slope, which is often used in road crossings. You don’t need large crossings for this issue to materialise. This time of year, you can see a lot of new slide debris in ditches along the roads of Vestlandet.”
Johannes Vik Seljebotn stresses that Norway is an expert in rockfall and slide protection, and that local assessments are carefully integrated in new projects.
“In the future, as winters become milder, this type of rockfall will become more common, as the shift between cold and warm weather results in more frequent frost bursts.”
Most frequent types of slides and floods in Norway:
- Debris avalanche: Usually a result of intense precipitation or vast volumes of melting snow. The debris includes coarse and/or fine matter and usually some vegetation, and the avalanche occurs as water destabilises loose masses. Debris avalanches can also be caused by other types of slides and floods, such as rockfall, or as a result of human intervention, e.g., by increasing the weight load on a slope. Often occurs at a slope of 25-30 degrees.
- Debris flood: Is a water-laden flow of masses that occurs along creeks, streams or ravines. Consists of all particle size fractions, and picks up materials on its way, increasing its flood volume. Can occur as a result of debris avalanches or unintended damming in rivers. Debris floods have a vast range, and the finest masses have the longest range.
- Rockfall: Rockfall covers individual blocks measuring up to hundreds of cubic metres. Is triggered in areas with a slope of more than 45 degrees. Frost and thaw in the spring and the autumn can trigger it, as can increasing water pressure in cracks during precipitation. Can also be caused by crack formation in connection with sudden cooling of mountains.
- Rockslide: Occurs in the same way as rockfall, but its volume ranges from one hundred thousand to millions of cubic metres.
- Slush avalanche: A special type of avalanche where the material consists of snow with a high water content. Usually occurs in connection with vast snow volumes melting in the spring or heavy rainfall on snow-covered slopes.
- Snow avalanche: Most often occurs in terrain that slopes more than 30 degrees, in areas without vegetation that are sheltered from the predominant wind direction. Slab avalanches are triggered along weak zones in the snow layers. Human activity can trigger snow avalanches.