Geomorphological analysis of the historic landslide of Sottrù (Badia Valley, Italy) reactivated on December 13, 2012

The historic landslide of Sottrù, located in the Badia Valley (Autonomous Province of Bolzano), was reactivated on 13 December 2012 after a long period of dormancy. According to historic documents, the previous activation dates back to June 1821. At that time, the slope was mobilised after a period of persistent rain and snow melt causing the damming of the Gadera stream. After snowmelting, in June 1827 the natural dam collapsed flooding meadows and villages down valley. In December 2012, the landslide was partly reactivated, showing the first slow movements on the 13th and more rapid displacements on the 14th. A detailed geomorphological survey of the landslide was carried out during summer 2014. A surficial highly permeable level (in-situ or reworked till) has been found laying on top of a thick sequence of clays, shales and marls, both over the landslide body and the surrounding areas. From the landslide scar uphill, the hydrographic pattern is extremely irregular, made of discontinuous drainage lines indicative of higher infiltration rate than superficial run off. The comparison between the current morphology and the pre-2012 one has not evidenced any movement within the historic source area, while relevant displacements occurred at two lateral zones whose dynamics caused the partial collapse of the central part of the landslide, originating a second, lower crown. Since the previous activation (1821), the landslide has been dormant for quite a long period, i.e. 191 years, and its last activation occurred during a season where landslides are usually not frequent. Temperature and precipitation time series, recorded at the nearby meteorological station of La Villa, have been analysed for the last 27 years. It has been found out that November 2014 had many rainy days, three of which with precipitation amounts ranging from 30 to 50 mm. The rainiest day (November 29) was suddenly followed by a dramatic drop below 0°C of maximum and minimum air temperatures, lasting until the 13th of December i.e., the landslide activation day. This probably caused the freezing of the soil which at that time was snow free and completely saturated. Moreover, it is possible that the 13-day period below 0°C enabled also the freezing of a spring located in the lower part of the slope nearby the village of Sottrù thus blocking groundwater drainage, leading to the onset of a confined-aquifer condition and, ultimately, to the rapid increase of pore water pressure inside the clay-rich landslide material. This hypothesis has been made also in consideration of similar events occurred in the Dolomites (e.g., in Zoldo Valley in 1991). However, in order to confirm this hypothesis, geophysical and geotechnical analysis would be needed.