High risk of rockslides and floods in Norway: Many people are nervous

27.05.2020

Vast snow volumes will make their way down the Norwegian mountains in the coming weeks. When combined with considerable temperature rises and rainfall, this could lead to an extraordinarily high risk of rockslides and debris floods in May and June.  

Heavy snowfall late in the season, frequent temperature variations and precipitation have characterised the early Norwegian summer of 2020. That combination can cause several types of slides and floods: debris avalanches, debris flood, rockfall and snow avalanches. 

These past days, residents in several Norwegian municipalities have received a letter advising them to clear out their basements. 

“Many people are nervous right now. This year, we are being warned about spring floods and with good reason. We’ve seen up to three times the normal amount of snowfall. In some areas, we haven’t seen snowfall like this for the past 60 years. If sudden melting kicks in, there will be floods,” says Johannes Vik Seljebotn, an expert on avalanches, rockslides and debris floods in COWI. 

Finnmark is already seeing snow melting. Vestlandet has also seen vast amounts of snowfall. In the coming weeks, especially the streams in Østlandet will be vulnerable. The flood warnings around Mjøsa operate with a 90-per cent probability, whereas those for Gudbrandsdalslågen operate with a 75-per cent probability. Last week, Sognefjellsvegen road was under more than one metre of water. 

“What we’re really worried about is a combination of mild weather, which accelerates the melting of snow, and significant precipitation. That would lead to substantial flooding. Rivers can rise by eight to ten metres, leading to major disasters.” 

Not comparable to last year’s torrential rain

Towards the end of last summer, Norway also saw extreme precipitation. That also caused mudslides and rockfall, but not to any large extent, says Seljebotn. 

Johannes Vik Seljebotn is a geologist with COWI. He explains that the combination of massive snowfall and mild weather can lead to a high risk of rockslides and floods in May and June. Photo: Johannes Vik Seljebotn.

“Torrential rain generates vast volumes of water over the course of a couple of hours, which leads to local flooding, but that dissipates relatively quickly. You don’t get huge water volumes in rivers. What we’re facing right now are vast water volumes that need to be transported to the streams. That way, it can last for days and weeks, rather than mere hours.” 

Good weather does not rule out flooding and mudslides

Many do not associate slides and floods with warm spring and summer days. Nevertheless, spring is usually the season when mudslides and rockfall occur at the same time. This is amplified by the vast volumes of snow. 

“When the weather is good, you don’t think about rockslides. But the large variations in temperature resulting from hot days and cold nights cause rockfall. Sudden cooling leads to cracks in mountain surfaces and rocks, triggering rockfall even on beautiful summer days.” 

Local knowledge is key in vulnerable areas

He experienced this himself when visiting a friend at Hardanger Fjord one summer. He was talking to his friend through a window. The sun was setting, the mountain side was cooling down and, all of a sudden, massive boulders were crashing down next to the house.

“My friend jumped out his window in the blink of an eye, and minutes later, the entire valley was covered in a white cloud.”

He says that you cannot underestimate the value of local knowledge in areas prone to slides:  

“When you live in areas with slide issues, you are used to it. The fact that they have this few accidents is down to their long experience, local knowledge and common sense. Once I was out mapping a slide area, local neighbours sat on either side of a valley, talking on the phone, updating each other directly about the state of the mountain side above each other. This local knowledge prevents accidents.”

Debris from debris avalanches can flood roads, thereby rendering infrastructure useless. The photo shows one of the debris avalanches in Voss last summer. Photo: Johannes Vik Seljebotn/COWI.

He refers to the fresh news about the tents in Gudvangen which were almost hit by rockfall as a stellar example of the value of local knowledge. 

“Never ignore warnings from the locals, who know the area like the palm of their hand. Some things should be taken seriously. The risk of getting hit is small, but if you’re out of luck, you could die.”  

High risk of snow avalanches in the mountains – forces of nature need to be respected

Due to the vast snowfall Norway has seen this year, there is also an extra high risk of avalanches. Temperature variations throughout the spring have cause many weak layers in the snow cover: First came the frost, then mild weather and a new layer of snow on the top. As a result, the snow cover has many different properties at the same time.

Seljebotn also believes that more people are willing to take bigger risks than before, because they have better ski gear. 

“We move around more and our ski gear has never been better, and it’s tempting to go places where we should not have been. The randonné trend has resulted in many more summits being visited, but it’s difficult to stay updated on the state of an entire mountain side. And many don’t know enough about the areas they visit.”

He is a volunteer with the Red Cross and has been involved in several rescue operations after slides and floods. He worries that a lot of people overestimate the odds of being rescued in an emergency. 

“Snow avalanches are probably the worst type of slide, with poor survival statistics. Very few survive avalanches for more than 15 minutes. If you are buried deeper than two metres, you’re almost impossible to locate. Furthermore, you jeopardise the lives of the entire rescue crew. You shouldn’t just think of yourself when visiting these areas.”

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.