Between 10,000 and 20,000 cubic metres of snow tumble down the avalanche path at Ryggfonn, NGI's full-scale field test site on Strynefjellet, taking more snow with it down the track. It has been ten years since new measuring instruments were installed. Since then, the researchers have had to wait patiently for the perfect conditions for the perfect avalanche test.

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“The test exceeded our expectations, and we are very pleased with the results. We estimate that the avalanche had a breaking edge more than 200 metres long. On a scale from 1-5, the avalanche was strength 4. In other words, this was a big slide - just what we want for these tests. The radar measurements indicate that the avalanche moved at up to 50 metres per second, i.e. 180 km/h,” says project engineer Henrik Langeland, NGI's avalanche expert in Stryn.

NGI’s unique role

Ryggfonn is one of two avalanche paths in the world that can conduct this type of instrumented large-scale experiment. The other is in Switzerland.

“The avalanche path, from the loosening area to the bank with the last measuring instruments, has a vertical drop of 900 metres and a length of more than 1.5 kilometres. So we are dependent on the right snow conditions for the avalanche to be big enough,” says Langeland.

The results of these avalanche tests will provide a better basis for planning safety measures in the development of infrastructure and buildings. With the help of successful experiments, the researchers will be able to develop improved computational models for avalanche loads, a task which involves advanced physics.

An avalanche has to be planned

A lot of things need to be in place before an avalanche can be triggered manually, and the researchers only have a small timeframe in which the experiment can be carried out under optimum conditions. The snow must be unstable enough, and there must be sufficient visibility and light to be able to see and film the avalanche, and ensure safety.

“This is not a cordoned-off area, so before every blasting operation we have to fly over the area to make sure there are no people or animals nearby. Strynefjellet is a popular hiking area and reindeer may also enter the area at certain times,” explains Langeland.

When the avalanche is triggered, the speed and pressure are measured using several robust installations down the slide path. Before this is done, a drone survey is carried out to create a surface model of the terrain, and the same drone survey is carried out after the avalanche has been triggered and stopped at the valley floor. The difference between the models gives the detailed avalanche volume and the area affected by the avalanche, i.e. the run-out distance and avalanche width.

“After the avalanche has been triggered and data collected, the big job of analysing the data starts. The analyses can be compared with earlier avalanche data from Ryggfonn and will be systematised to provide learning about the phenomenon with improved models for risk assessments for buildings and infrastructure, for example,” says Langeland.

NGI was given the responsibility for avalanche research in Norway in 1972

Until the 1970s, the organisation of avalanche prevention work in Norway was inadequate. Several committees were created to determine how such an organisation could best be set up. This resulted in White Paper No 9 in 1972, whereby the NGI was given the responsibility for avalanche research in Norway, a role it still retains. Avalanche research is now financed through a direct grant from the Norwegian Parliament, which is administered by the Norwegian Water Resources and Energy Directorate.

The Norwegian Geotechnical Institute (NGI) is a leading international centre for research and consulting within the geosciences. NGI develops optimum solutions for society, and offers expertise on the behaviour of soil, rock and snow and their interaction with the natural and built environment. NGI works within the markets Offshore energy; Building, construction and transportation; Natural hazards, and Environmental Engineering. NGI is a private foundation with office and laboratory in Oslo, branch office in Trondheim, and daughter companies in Houston, Texas, USA, and Perth, Western Australia. NGI was established in 1953.

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