A month-long stretch of record rainfall and unusually warm temperatures have triggered the worst landslides in Longyearbyen since 1972, forcing the closure of roads near hillsides and prompting safety officials to warn people not to travel near mountainsides in wet weather until after freezing temperatures have set in.
Longyearbyen has received nearly 150 millimeters of precipitation since July 1, roughly twice the average for that time span, said Ole Humlum, an adjunct professor of physical geography at The University Centre in Svalbard. Temperatures have been two to three degrees Celsius above normal during much of that timespan, so the typical freezing of the ground that begins in October has not started yet – and it will be a long time before the underlying permafrost is full solid.
“Perhaps by Christmas and New Year’s everything will be frozen up,” he said.
Humlum offered his observations about Longyearbyen’s landslide situation during a presentation Tuesday evening at UNIS. Much of it was based on a report he and other researchers assembled at the request of city officials after an emergency alert was issued for a rainstorm that hit Oct. 14 and 15.
The storm dumped 18.3 millimeters of precipitation on Longyearbyen between 5 p.m. Oct. 14 and 11 a.m. Oct. 15, with a peak rate of about 2.8 millimeters an hour around 3 a.m., according to the report. Humlum said researchers believe – but are not certain – that’s when the largest of the landslides from the storm occurred.
“That amount was apparently enough, or perhaps the intensity was – that’s an open question – was enough to release all these slides,” he said. “This would not have happened in any other type of climate.”
Temperatures were also well above zero, soaring to as high as seven degrees Celsius on the morning of Oct. 15.
The road between Huset and the old museum was closed Oct. 15 after about 5,000 cubic meters of rock and mud fell from just below the summit of Platåberget, spreading across the road next to Longyearbyen Cemetery. Although the road was not heavily covered, the debris layer was up to five meters high in other spots.
Although the road opened temporarily a few days later, the city announced Monday it would be reclosed indefinitely until freezing weather sets in due to the occurrence of additional slides during rainstorms.
The landslide was the largest since July of 1972, when 30.8 millimeters of rain fell during a 12-hours, resulting in more than 80 debris flows on both sides of the lower Longyeardalen valley, Humlum said.
“A previous investigation (based on that storm) suggested a rain intensity of two mm/hour as a critical value for the release of active-layer detachment slides, debris flows and mudflows in the permafrost environment around Longyearbyen,” the UNIS report notes.
This month’s storm caused numerous other landslides and debris flows in the area, including one in Bjørndalen that forced the closure of the access road after the storm and the area below the Svalbard Satellite Station next to Svalbard Airport.
The Governor of Svalbard issued a series of warnings urging caution near mountainsides, and to avoid hiking and excursions until the soil stabalizes. Humlum, in his presentation, said the odds of a landslide when it is not raining are greatly reduced,but “it’s better to be more prudent on the safe side.”
He offered a number of safety tips, including:
• If caught in a rainstorm, check to see where landslides may have already occurred since there is likely to be less hazardous material still contained there.
• Avoid gullies as much as possible since falling debris will be redirected there.
• Be wary of fine-grain sediment, which is often covered with vegetation.
• Pause long enough to think, then act if you hear a noise. “Take a look at it for one to three seconds, quite calm, and then decide which direction to run.”
The UNIS report recommends, among other things, implementing a monitoring system for landslides.
“The lack of continuous, automated precipitation measurements at the elevations of primary concern for mass wasting and snow avalanche hazard assessment is currently a critical limiting factor in the development of accurate and reliable hazard forecasts for the Longyearbyen area,” the report states.
Weather readings at Svalbard Airport can offer some guidance, but since the airport is at sea level it cannot provide accurate data about mountainside conditions, the report adds. In addition, since data is updated every 12 hours it may not be sufficient during a storm intense enough to trigger slides.
“We suggest installing online slope stations to measure rain precipitation, snow depth, water content, and ground temperatures through the active layer and into the top permafrost,” the report states. “In addition, ground ice content, sediment grain size and other basic geotechnical and geological data should be obtained from cores drilled at the sites.”