Flash: Huge rockslide imminent in Swiss village of Brienz

The rockslide above Brienz in eastern Switzerland marked by a white surface bare of vegetation. Credit CHRISTOPH NÄNNI, TIEFBAUAMT GR, SWITZERLAND via the BBC

On 9 May 2023 the authorities of the Albula/Alvra municipality in the Swiss canton of Graubünden informed people living in the small village of Brienz that they must evacuate the area by 18 May as the threat of rock falls from the mountain beneath which they live had triggered a red alert. By 13 May all 130 dwellings had been abandoned.

The danger is posed by an estimated 5 million tons of rock associated with a developing landslide that is now estimated to be moving at around 32 m per year. The village itself had long been creeping down slope at a few centimetres each year, but recently its church spire had begun to tilt and buildings became riven by cracks. Seemingly, engineering attempts to mitigate the hazards have been unsuccessful, and large boulders have already tumbled into the vicinity of Brienz.

Being situated beneath a crumbling scree slope devoid of vegetation that had been developing since the last glaciation, the geological risk to the village comes as no surprise to its population and local authority. The local geology has a thick limestone resting on the thinly bedded Flysch – a metamorphosed sequence of fine-grained turbidites – from which groundwater escapes very slowly, thereby becoming lubricated. A curved (listric) failure zone has developed beneath the exposed mountainside, hence the danger. Acceleration on the listric surface began about 20 years ago.

At least the people of Brienz have been moved to safety, unlike 144 school children and adults in the mining village of Aberfan in South Wales. On 21 October 1966 they were crushed to death by coal-mining waste that suddenly flowed from waste tips on the steep valley side above the village. In that case no warning was given by the National Coal Board authorities who allowed  the tipping witout a thought for its geological consequences.

See also: Petley, D. 2023. The very large incipient rockslide at Brienz in Switzerland. The Landslide Blog (10 May 2023)