From a geological point of view, the landslide that hit Niscemi in the province of Caltanissetta in Sicily yesterday after the heavy rains of storm Harry caused 1000 evacuees, damage and closed schools. it is a sliding landslide: an underground rupture in the slope on the edge of the town has separated two layers of rocks, the upper one of which has slipped onto the one below pushed by the weight of the water accumulated by the heavy rains of recent days, with the separation between the two layers lubricated by that same water. This is a large landslide (with a detachment of 15-25 meters along a front approximately 4 km long) in an area with a high risk of landslides and already fragile from a hydrogeological point of view (a smaller landslide had already occurred on 16 January).
The causes of a sliding landslide like that of Niscemi in Sicily
As the name suggests, a sliding landslide occurs when a layer of rock falls by sliding over another layer of rock along a failure surface at depth. This surface can be curved (rotational surface), flat (planar surface) or a mix of the two.
But how do two layers of rock slide past each other? Gravity certainly helps, but even intuitively we realize that the friction forces between the two layers must be very impressive. Typically, sliding landslides require a lubricant between the two surfaces. In this case we don’t have to look far: just think of the torrential rainfall that hit Sicily for days due to Storm Harry. The rain infiltrates the soil and soaks it, until it reaches a clayey layer. Clays, as we know, are impermeable rocks, so here the water stops and accumulates: a lubricating layer is formed. But not only that: the water accumulates above the layer of clay, weighing down the rocks and soil above.
At this point we have a strip of land that becomes increasingly heavier and rests on an increasingly slippery surface: this is the recipe for creating a sliding landslide.
What does the speed of a landslide depend on?
In landslides (in which a mass of rock falls to the ground) or in mud flows, the speed of fall is very high: everything can be resolved within minutes, even seconds.
Slide landslides are slower. In Niscemi, the layer of collapsed rocks took hours to fall about 20 meters. The rock is probably still continuing to descend and the landslide is still ongoing. Slide landslides can also last months, years, sometimes centuries. This certainly doesn’t make them any less dangerous.
Here are the main factors on which the speed of a landslide depends.
- Slope: intuitively, the more inclined a slope is, the greater the speed of fall.
- Saturation: The water content. The higher the water saturation in the clay layer, the lower the friction between the sliding rock layers. This increases the speed of the landslide event.
- Material: More compact rocks tend to fall faster, while loose materials or fine debris tend to flow more slowly, as part of their energy is spent deforming the material itself.
- Volume: Large landslides tend to be slower than small ones. Therefore, generally, the deeper the sliding surface, the greater the volume involved and the lower the speed of the landslide.
