A debris flow originated from a wet avalanche in South Tyrol (Italian Alps) on January the 9th, 2018, early in the morning, on a North facing slope. The mass movement was triggered by a large front of wet snow which started sliding on a frozen soil (meadow), where a layer of melting water acted as lubricant. The frozen soil has a low hydraulic conductivity that did not allow the water from snowmelt to infiltrate, therefore the snowmelt flowed as surface runoff between the base of the snow layer and the meadow covered slope surface. The surface runoff had also been fed by intense rainfall the night before. Due to abormal high temperatures (far above the average), the snowpack was extremely wet.
An abrupt change in slope and the availability of lose material turned the process into a debris flow, which entrained trees and boulders along the way, increasing its size. In certain sections the sliding mass eroded the soil layer up to the bedrock. The moving mass went all the way down to the valley floor. Clear signs of high force impacts were observed (deep tree scars). The deposition area was mainly concentrated at the toe of the slope. This required urgent mitigation measures to mitigate the risk for the infrastructures running through the valley.
