The Surfing Rock of Monte Spico

On 24.04.2023, an approximately 3 m³ large platy boulder detached itself from the steep slope above the valley station of the Monte Spico ski resort in Taufers (I). Despite the low slope inclination of 10-20°, the boulder slid down the slope at high speed and without any rotational movement, missed a nearby restaurant by only 5 metres and finally came to a stop in the middle of the Ahrntal road [1].

We explain the unusual sliding motion of this rockfall event by two main factors:

• Friction between rock and soil is the main driving force that causes a rock to enter a rolling motion. As friction acts at the contact surface instead of the center of mass, the eccentricity of the force causes the rock to rotate. In this case, the contact friction between rock and soil was very low due to rainfall after a long dry period [1]. Therefore, there was very little force eccentricity that could cause the rock to rotate
• The platy shape of the rock further enhances this effect. Platy rocks have a higher moment of inertia about the "wheel" axis, causing them to roll down a slope like an upright wheel if the rotational speed is high enough. In this case, the rock cannot reach the required rotational speed because of the low friction. Therefore, the rock remains on the flat side where rotational movement is even more difficult to achieve due to its shape.

If this hypothesis is true, we can model this sliding rockfall behaviour with RAMMS::Rockfall. To this end, we use a platy rock from the RAMMS rock library (Real_Flat_1.8) with a volume of 3 m³ [1]. We set the release point at the location of the lift mast no.4 that gets hit by the rock before it slides down the ski slope. As seen in the video footage, the rock has a velocity close to zero after this impact. We therefore do not define any initial velocity. The initial orientation of the rock has close to no influence on the simulation. Ideally, we place the rock with its flat side down as seen in the video. But even if we choose an initial upright position, it will just tip to its flat side and start sliding, because the friction is too low to start a rolling movement. Lastly, we drastically reduce the friction coefficient to: \(\mu_{min} = \mu_{max}=0.01\). We choose \(\mu_{min} = \mu_{max}\) because as seen in the video there is no soil accumulation in front of the rock during the scarring process that would cause the friction to increase while the rock is sliding down.

As expected, the rock only moves with these custom friction parameters. In a second simulation with default parameters recommended by RAMMS, the rock remains stuck at its release position due to the low slope inclination.