In January 2016, an immense mass of rock plunged into the river Kleine Emme near Werthenstein, Switzerland, causing considerable damage. The steep face is still a menace to the surrounding settlement area. Unusual immediate measures were taken to get the situation under control.
It sounds like a debris avalanche coming down in the distance: a gentle trickle, rising to a low roar, and rumbling. Then silence – until it starts up again, and again. Every minute, pieces of rock tumble over the 80-metre-high steep face, pile up into scree at the base, or plummet straight into the roiling water of the river Kleine Emme. It should be a terrifying sight, actually. But what is happening here is a controlled process. The rock is falling not because of forces of nature, but because of two excavators performing a finely coordinated dance on the rock plateau high above. Their mission is to remove around 20,000 cubic metres of rock in 10 weeks. That is approximately the volume of a 50-metre swimming pool – but with a depth of 20 metres. Why are they doing that?
The night of 11 January 2016
The cause for this unusual excavator project is the night of 11 January 2016. Early in the morning, 5,500 cubic metres of rock broke off the Badflue steep face near Wolhusen and tumbled into the river Kleine Emme. The impact of the falling rocks flung debris from the riverbed up to 300 metres into the surrounding area, which hit the buildings in the nearby industrial area like a burst of gunfire. It is a miracle that no one was hurt. The rockfall buried the riverbed almost completely, and the displaced water rushed through the Sandmättli industrial area and over the cantonal road, flooding buildings and undermining a flood protection wall. That night was a catastrophe for the residents of Werthenstein and Wolhusen. How could they stop it from happening again?
Even before the clean-up work was finished, the canton of Lucerne commissioned Basler & Hofmann with the “Integrales Risikomanagement Badflue” (Integral risk management for Badflue) project for the development of a long-term solution. However, even the first geological analyses of the rock face showed that the crisis was far from being averted: more rock could tumble down at any time, causing the same chain reaction as on that night in January. The project team quickly developed immediate measures to get the situation under control. On 24 May, the Lucerne Cantonal Council decided to remove the crest of rock at risk of falling as a preventive measure in order to protect people and property. The excavators started work on the rock plateau in June.
A strict code of conduct
We visit the site manager, Jonas Stettler, on the spot in mid-July. You can see the bare face of the flank of the rock from far away on the cantonal road. At this distance, the excavators look like ants. The site office is on the grounds of the gravel quarry, which was damaged considerably by the rockfall. A strict code of conduct is in force: “Until the rock mass has been removed, these grounds are in the danger zone. People are only allowed outside for short periods of time, and only when absolutely necessary”, Jonas Stettler explains. The security employee standing next to us never lets the rock face out of sight. He makes sure that people follow the rules. And what about the safety of the excavator operators high up on the rock? “The excavators are secured and never come near the edge of the cliff, and the two operators are absolute experts who know all about the rock and this area.” Jonas Stettler is in close consultation with the excavator operators and the geologist the team brought in. “We assess the situation together.”
Rockfall one excavator bucket at a time
Isn’t removing the rock with excavators a little slow? We ask Christoph Rüedlinger, the overall project manager. “Unfortunately, there’s no other way”, says the natural-hazards specialist. “We also looked into blasting the mass of rock, but there are too many major risks involved.” Instead, the rock falls into the river Kleine Emme one excavator bucket at a time. From there, a large excavator transports it to the other bank of the river. In falling, 20,000 cubic metres of rock turn into around 35,000 cubic metres of loose material, which has to be removed by lorry. “It’s certainly an unusual project”, Christoph Rüedlinger agrees. He and his team now have until spring 2017 to come up with a safe, long-term solution for this area. After all, removing the rock only temporarily eliminates the danger. There is no way to stop natural weathering, and the river Kleine Emme is also effectively eating away at the base of the rock face. It is just a matter of time until the next chunk of rock tumbles down. At this point, Christoph Rüedlinger cannot comment on possible long-term measures. “We are taking a very open-minded approach and are really considering every means of sustainably increasing safety in the Sandmättli area.”