Blatten
(Orthophoto: copyright swisstopo)
A 4.0 million m³ rock mass detached from the Kleines Nesthorn, transforming into a rock–ice avalanche that entrained snow and glacier material, ultimately depositing approximately 9.0 million m³. The event buried most of Blatten, dammed the Lonza River, and led to the evacuation of the village with one person missing.
To simulate the catastrophic event near Blatten, the release area was divided into three distinct source zones, each with characteristic material properties and staggered failure timing:
- Glacier Front: The initial release involved approximately 530,000 m³ of material, composed of 80 percent glacier ice, 10 percent rocky debris, and 10 percent water. Failing with a height of 25 m, this detachment triggered a fast-moving ice avalanche.
- Middle Glacier Section: Roughly 30 seconds later, the central portion of the glacier collapsed. This section contributed a further 2.7 million m³ of material, consisting of 40 percent rock, 50 percent ice, and 10 percent water – marking a major increase in both mass and flow complexity.
- Upper Glacier Section: Very shortly (~ 2 s) after the middle failure, the upper glacier – overlain by loose rock from the preceding instability – also gave way. This final release introduced 1.2 million m³ of material, comprising 60 percent rock, 30 percent ice, and 10 percent water.
Video 1: RAMMS::RockIce simulation of the Blatten rock-ice avalanche.
Figure 1: RAMMS::RockIce simulation of the avalanche core and powder cloud 45 seconds after release. The initial avalanche, triggered by the collapse of the glacier front, consists predominantly of ice and rapidly accelerates downslope, reaching the alluvial fan and generating an intense powder cloud. Following closely behind is the second, more massive flow, released from higher on the glacier and composed of a dense mixture of rock and ice. This follow-up avalanche contributes significantly to the total mass and momentum of the event, reinforcing the destructive force as both flows converge in the fan area.
The initial, ice-dominated avalanche accelerated rapidly in the steep terrain, reaching the alluvial fan before the slower, denser rock–ice masses. Peak velocities approached 90 m/s (roughly 290 km/h). As the fragmented ice dispersed within the avalanche core, it enabled the formation of a powerful powder cloud. Spreading laterally across the fan, the avalanche overtopped the protective dam and surged directly toward Blatten.
As it descended, the avalanche entrained an additional 4.5 million m³ of material, including deposits from previous avalanches. Upon reaching the opposite valley wall, it surged 200 m upslope, destroying swathes of forest. During the reversal on the counterslope, the retreating ice mass collided head-on with the second wave of rock–ice flow. This high-energy interaction helped redirect the dense mass away from the village and back down the valley floor.
RAMMS::RockIce simulations show that the downslope deposition zones became increasingly rock-rich, while the upper fan deposits – which directly impacted Blatten – retained a higher ice content.
The entire event unfolded in less than three minutes. The final deposits formed a chaotic, interlayered mix of rock, soil, and glacier ice, while the accompanying powder cloud inflicted widespread damage on the counterslope forest.
Figure 2: Maximum flow velocities within the avalanche core, composed of rock, soil, ice, and water. Peak speeds – reaching up to 90 m/s – are observed near Blatten, just before the flow surges up the opposing slope. The simulation captures the remarkable run-up onto the counterslope, illustrating the immense momentum and energy of the event.
Figure 3: In the RAMMS::RockIce simulation of the Blatten event, forest shapefiles were integrated to represent the mountain forest cover surrounding the valley. This figure highlights the extent of forest destruction caused by both the dense avalanche core and the expanding powder cloud. Areas of complete deforestation trace the path of high-impact flow zones, illustrating the combined force of mechanical entrainment and air-blast pressures on the landscape (1: destroyed by core, 2: destroyed by powder cloud, 3: destroyed by both).
Figure 4: Simulated volumetric ice composition (up to 50 percent) of the rock-ice avalanche deposits. The results highlight the heterogeneous and dynamically evolving nature of the flow material.
