Natural hazards in the Alps

Imagine the authorities knock on your door: "You have to leave your village – the mountain above you could collapse." This is exactly what happened in May 2023 to the roughly 90 residents of Brienz/Brinzauls in Graubünden. Around five weeks later, during the night leading into 16 June 2023, around 1.2 million cubic metres of rock and debris really did thunder down into the valley – and came to a halt just a few metres from the village.

Living in the Alps means living with natural hazards. The main ones are:

• Avalanches – masses of snow that break away from a slope
• Floods and debris flows – water, mud and rock debris that flood valleys
• Rockfalls and rock avalanches – rock that breaks away from cliff faces

In this module, you will find out how these hazards arise, how Switzerland protects itself with engineered structures, protection forests, warning systems and hazard maps – and why climate change makes all of this worse.

Most dangerous avalanches are slab avalanches: an entire layer of snow breaks away at once and races down into the valley. They mainly occur on slopes steeper than about 30 degrees – especially after heavy snowfall or when wind redistributes the snow.

Case study: the avalanche winter of 1999: In February 1999, several metres of snow fell in the higher parts of the Swiss Alps within just a few weeks. Around 1200 avalanches caused damage, claiming 17 lives in buildings and on transport routes. Evolène in Wallis was hit hardest: twelve people died there on 21 February 1999. Total damage amounted to around 600 million francs.

This is how Switzerland protects itself today:

• Warning systems: The WSL-Institut für Schnee- und Lawinenforschung SLF in Davos publishes an avalanche bulletin once or twice a day in winter (at 17:00, with an update at 08:00 if needed). It uses the European avalanche danger scale with five levels – from level 1 (low) to level 5 (very high).
• Engineered structures: Steel supporting structures in the avalanche release area hold the snow on the slope before an avalanche can even start.
• Protection forests: Trees stabilise the snow cover. Around 42 per cent of Switzerland's forest area serves as protection forest against natural hazards – the cheapest "barrier" of all.
• Spatial planning: Building is not allowed in high-hazard zones in the first place.
• Emergency measures: When danger is imminent, roads and ski areas are closed or avalanches are triggered using controlled explosions.

Flooding occurs when heavy rain, long periods of rain or snowmelt supply more water than rivers and soils can absorb. Straightened rivers and floodplains that have been built on make the problem worse, because the water has nowhere else to go. The floods of August 2005 were Switzerland's costliest single natural event to date: six people died and damage amounted to around 3 billion francs. Today, flood protection therefore relies not only on embankments but also on river restoration, which gives rivers more space again.

A particularly dangerous form occurs in steep mountain streams: a debris flow – a fast-flowing mixture of water, mud and rock debris that shoots down the stream bed into the valley like liquid concrete.

Case study: Bondo (GR), 23 August 2017: Around 3 million cubic metres of rock broke away from Piz Cengalo (3369 m above sea level). The rock avalanche fell onto a glacier, swept up ice and water, and triggered huge debris flows that travelled through Val Bondasca into the village of Bondo. Eight hikers who were walking in the valley lost their lives. There were no deaths in the village itself: an automatic alarm system gave a timely warning, the residents were evacuated, and a retention basin held back some of the material. Even so, houses were destroyed and roads were buried – Bondo shows that warning systems can save lives but cannot always prevent damage.

Brienz stands on a slope that has been slowly sliding down into the valley for centuries. After the residents returned in summer 2023, the village had to be evacuated again in November 2024 – its future is uncertain. A drainage tunnel deep inside the mountain is intended to drain water from the slope and slow its movement.

Climate change makes many natural hazards in the Alps worse:

• Permafrost thaws: Permafrost is permanently frozen ground. In Switzerland, it mainly occurs higher than about 2500 m above sea level and covers about 5 per cent of the country's area. The ice in cracks in the rock acts like cement, holding the rock together. When it thaws, rock faces become unstable – thawing permafrost played a role in the rock avalanche at Bondo.
• More heavy precipitation: Warmer air can hold more moisture. Heavy rainfall – and therefore floods and debris flows – becomes more likely.
• Glaciers melt: They expose loose rock debris and destabilise slopes that were previously supported by ice.

Switzerland cannot prevent such events – but it can reduce the risks. Hazard maps are a key tool: experts map the areas in each municipality where avalanches, floods or landslides pose a threat.

Zone	Hazard level	Meaning
Red	high	New homes must not be built
Blue	medium	Building is allowed only under certain conditions, e.g. reinforced walls
Yellow	low	Indicates a possible hazard

Other measures include monitoring (sensors, radar), warning systems, engineered structures and protection forests. Sometimes the only option left is to evacuate in time – as in Bondo and Brienz.