Debris flows, an analysis by Prof. Piero Gianolla

Geologist Piero Gianolla, member of the Scientific Committee of the UNESCO Dolomites Foundation, helps us to reassess and place in a broader context what happened on the night between the 15th and 16th of June in Cancia di Borca di Cadore (Belluno), when, once again, heavy rainfall reactivated the debris flow that looms over the village. As stated by the Province Administration office, the mitigation works carried out in recent years have helped to avoid the worst. However, the population has had to reckon with considerable damage to their homes. This event, like many others along the Dolomites, highlights how geological, geomorphological, climate, and human factors all come into play and contribute to both the phenomenon and its impacts.

Prof. Gianolla, the images of the Cancia debris flow clearly show the immense force with which debris flows pour downstream, carrying boulders even of considerable size. Let’s start with definitions: what is a debris flow and why is it a typical feature of Dolomite formations?

A debris flow is a particular type of landslide that occurs when large quantities of debris – i.e. a mixture of water, mud, sand, gravel and even large boulders – are suddenly set in motion and flow downhill with the consistency of a very dense fluid.

Think of a river of mud and boulders moving with impressive speed and enormous destructive force. It is a very fast-moving and dangerous phenomenon, capable of transporting blocks weighing several tonnes even down very steep slopes.

Why is this phenomenon so frequent in the Dolomites, and in particular in places like Cancia, in the municipality of Borca di Cadore?

The answer lies in the very geology of these mountains. The Dolomites are largely made up of carbonate rocks – limestone and dolomite – which tend to fracture easily, and in the presence of faults, these rocks become even more fragmented. This results in large quantities of loose material accumulating along slopes, especially in gullies and erosion-prone areas.

The Alpine climate also plays a role. Sudden, intense rainfall events – now increasingly frequent due to climate change – can rapidly saturate these debris deposits. When the water content is high enough to reduce friction between particles, the entire mass can suddenly lose cohesion and surge downhill as a debris flow.

The situation in Cancia is a textbook example: a well-known, recurring landslide, identified in the geological maps of the area. Several mitigation measures have been implemented in the area due to its unique geological structure and the delicate balance between soil, water, and slope. All it takes is a single severe thunderstorm to break that balance and trigger spectacular and destructive events such as the one in June 2025.

If rainfall is the factor that triggers the flows, what are the factors that cause the accumulation of debris in the first place? Is there an intrinsic reason why some mountains, such as the Antelao, produce a greater accumulation of debris?

Yes, heavy rain is often the immediate trigger for debris flows, but the real “fuel” for these events is the debris accumulated in upstream basins. It is precisely there that the geological and morphological characteristics of the mountain come into play.

For example, let’s consider Antelao, an emblematic and geologically active mountain in the Dolomites, although the same applies to Croda Marcora and nearby peaks. Its tendency to generate large amounts of debris is not accidental, but linked to a number of factors:

• fractured lithology: Antelao largely consists of Main Dolomite rocks and other Jurassic carbonate rocks, which fracture easily, also, as already mentioned, due to the presence of major tectonic dislocations (faults). This natural fragmentation favours the disintegration of the rock into blocks, gravel and sand, which accumulate in the upstream gullies and basins.
• Steep slopes: the slopes of the Antelao and the mountains from Borca di Cadore to Cortina d’Ampezzo characterise the hydrographic left of the Boite torrent and are extremely steep. This accelerates the processes of gravitational erosion and causes material to detach and accumulate more easily in impluvium areas.
• Retreating glaciers and deteriorating permafrost: although the Antelao glacier is now very small, the processes associated with glacial retreat and permafrost melting further destabilise the rock walls. When the ice that held the blocks “stuck” together disappears, cryoclastism is reactivated, slopes become more unstable, and collapses increase. Especially now, with climate change (but I’d rather call it “climate crisis”) raising the temperature and the thermal zero. The southern walls are therefore more prone to instability and collapse.
• Basin and gully morphology: the structure of the mountain is such that it favours the accumulation of debris in basins and gullies which, over time, can fill up to critical levels. When that happens, a heavy rainfall event is enough to mobilise the entire depot.

In other words, due to structural and climatic reasons, some mountains like the Antelao have become real “debris factories”. Upstream basins behave like tanks that are slowly loaded and then abruptly emptied. Understanding these mechanisms is crucial not only for studying the Dolomite landscape, but also for planning risk prevention in the valley.

What is, therefore, the meaning of the word “adaptation”?

The sudden and intense rainfall events that we see increasingly more often are a clear consequence of climate change. In this context, adaptation means accepting that certain phenomena cannot be avoided, but can be better managed by reducing their impacts.

Mitigation works are essential, not only to protect built-up areas, but also to ensure the safety of streams on the valley floor. A debris flow can obstruct the riverbed, divert water and cause damage even in apparently unexposed areas. For this reason, an integrated approach that considers the entire mountain and hydrographic system is essential.

However, despite of all the mitigation works, there is still a percentage of residual risk. Therefore, when planning a project to mitigate these phenomena, it is important to share the choices with those who have always lived in the mountains and cared for them, but without forgetting that what we are experiencing with the climate crisis is something totally new with respect to historical memory.

Adaptation means building a safer and more resilient territory, able to coexist with a changing nature. In short, adaptation means designing with a changing reality in mind, building a more resilient and less vulnerable territory. The Dolomites, with their fragility and beauty, remind us every day how urgent this is.