5 January 2021
Planet Labs high resolution satellite image of the Gjerdrum landslide in Norway
Operations continue with some intensity of the site of the Gjerdrum landslide in the village of Ask in Norway. Sadly, the focus has now changed from rescue to recovery, with an acceptance that there are no more survivors. To date the remains of seven victims have been recovered, leaving three people missing. Operations have been made easier by an improvement in the conditions on the landslide, but finding those still missing will be a difficult task.
My friends at Planet Labs have now managed to capture an excellent SkySat high resolution satellite image of the site – once again can I note my thanks to them, and in particular to Rob Simmon, for their support. This is the first image that I have seen of the entirety of the site.
The crown of the landslide is of course where the losses occurred. The image below shows this area:
The form of this landslide is interesting, with a large main source area, and a smaller area to the north that has affected the houses with such catastrophic consequences. The reason for that morphology is not clear to me – on first inspection it appears that this was a retrogression from the main landslide bowl. If so, it will be important to understand why this happened at this particular location. Others will be better placed to comment on this than me, and I’m sure that the official investigation will provide an explanation.
The Planet Labs image also captured the whole of the landslide, which is very large. This is the image:-
Note the main source on the western side of the image. The main slide moved roughly towards the south, and then followed a very subtle channel towards the east. Mobility was high – the choked channel on the eastern side of the image demonstrates that the landslide moved over 2 2 km. In one location the slide has bifurcated, as seen on the helicopter image. There is a very substantial amount of debris at the toe of the landslide.
There is a great deal of speculation about the trigger of the Gjerdrum landslide. In the past, for example at Rissa, quick clay landslides have been triggered by excavations lower on the slope, which induced rapid liquefaction and expansion of the landslide. There is also some discussion about the role of modification of the topography too – the slope included both a golf course and the housing development. I am not in a position to comment on this, but the Wikipedia article on the Gjerdrum landslide has some details.
Planet Team (2020). Planet Application Program Interface: In Space for Life on Earth. San Francisco, CA. https://www.planet.com/