Zugspitze rockfall – measurement results: How at risk is the Reintal valley near Garmisch?

Last week, on 22 September, 36,000 tonnes of Wetterstein limestone broke away from the north face of the Zugspitze in Tyrol, near the German border. The first block broke off at 7 am; the second plunged down into the valley an hour later at speeds of up to 200 kilometres per hour. Thankfully, the 13,800 cubic metres did not crumble over a wide area. Most of it was caught by rocky outcrops. However, the collapse created a gigantic fountain of dust that lingered in the air for several hours.

Rockfalls are generally not uncommon on the steep slopes of the Wetterstein massif, but this major rockfall took even researchers and scientists by surprise. A team of geologists from the Technical University of Munich has since surveyed the site of the rockfall using drones and analysed seismic data. According to their findings, more rock has come loose from the 80-degree steep rock face than originally assumed: the rockfall site lies at an altitude of 2,800 metres (within the permafrost zone), is 100 metres long, 40 metres wide and three metres thick.

The summer heat is now seeping into the permafrost

The trigger was this summer’s extreme heat. It caused the permafrost on the north face to thaw to such an extent that the rock masses came loose. This time lag can also be observed in various other rockfalls in the Alps. They always occur some time after a heatwave, because the heat seeps only slowly deeper into the rock and thaws the permafrost – the ‘glue’ within the rock. This causes hydrostatic pressure to build up within the rock, which can have an explosive effect weeks later. According to measurements, the point of failure currently shows frozen rock with a temperature of minus 0.7 degrees.

The temperature in the resulting rock niche is now being measured using infrared cameras. In addition, a laser scanner is to be installed which can detect even the slightest rock movement within a four-kilometre radius. This would enable a warning to be issued in good time before a further rockfall, as such rockfalls give warning days or even weeks in advance. This is already the case in the Partnach and Höllental gorges, where this technology is already in use. It provides highly precise 3D data that pinpoint the exact location of rockfalls and rockfall activity and detect early warning signs.

How threatened is the Reintal?

The cycle tour to the Reintalangerhütte is one of the most popular routes in the German part of the Wetterstein massif. It climbs from Garmisch-Partenkirchen up the steep northern slopes of the Zugspitze and ends in the gorge-like valley at the hut. There have been repeated rockfalls in that area in recent years. The two swimming pools originally described by Elmar Moser were buried by rockfalls decades ago. The last major rockfall, in August 2005, which blocked the famous ‘Blaue Gumpe’ with scree, is regarded as ‘historic’.

For this reason, too, the Reintal valley is repeatedly the focus of meticulous measurements by the slope specialists at the Technical University of Munich. According to their findings, the permafrost in the high-alpine cliffs above the Reintal valley is disappearing at a dramatic rate, and their stability is said to be seriously at risk. Long-term scientific studies show that rock temperatures here have risen by around 1 degree over the past decade and that the permanently frozen surface in the measurement areas has receded by almost 40 per cent.

Following the rockfall on 22 September, further investigations will now be carried out to gain a better understanding of how temperature propagates through the rock strata. The laser scanner is therefore due to be installed shortly so that the entire width of the Zugspitze North Face can be monitored and warnings of rockfalls and landslides can be issued in good time.

The Eibsee Rockfall

Scientists now also attribute the islands in what is now Lake Eibsee to a massive rockslide that is thought to have occurred 4,000 years ago. At that time, the summit of the Zugspitze – which once stood over 3,000 metres high – broke off, and fragments of it are thought to have landed in Lake Eibsee: the islands in the lake today.