## 5 Results and Discussion
### The Characterisation of the Croso Stream Slope
It is classified in the IFFI system as quiescent, since the processes may undergo reactivation in a short time, in this morphoclimatic context. The effects of the intense meteorological events are very significant also in relation to the presence of abundant loose and unstable debris, representing the predisposing factor. It is a self-feeding process, since during the events, additional material along the thalweg and along the lateral slopes is mobilised too (Figure 6a). Moreover, debris contributions derive from the nearby Fontana stream, an incision draining the slope very close to the San Domenico village. The Croso debris flow is hence classified as a complex, hyper-concentrated flow, possibly extremely rapid (velocity \(>\) 5 m/s; [57]) flowing potentially for 600 m of altitude difference. At the confluence with the Caiirasca stream, an alluvial fan, characterised by 20-25% of steepness, has generated (Figure 5a). Potentially, the interruption of the Caiirasca stream could occur due to alluvial fan accretion. Along the stream, human interventions like check dams for mitigating the risk are present, but during the major events they are often overwhelmed by debris. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Date**} & \multirow{2}{*}{**Typology**} & \multirow{2}{*}{**Causes**} & \multicolumn{2}{c}{**Affected Stream or**} & \multirow{2}{*}{**Effects**} \\  & & & & **Slope Portion** \\ \hline \multirow{4}{*}{19–20 August 1958} & \multirow{4}{*}{Rapid debris flow} & Intense meteorological event & & \\  & & & & \\  & & & & \\  & & & & \\  & & & & \\
16 June 1979 & Rapid debris flow & Intense meteorological event & & \\  & & & & \\  & & & & \\  & & & & \\
1 January 1993 & Slide & Intense meteorological event & & \\
20 August 1996 & Rock fall & & Slope behind San Domenico village & \\
26 May 1997 & Flood & & Croso stream and Fontana stream & \\
7 July 1997 & DSGD with diffuse & & The slope portion interested by active DSGD is \\  & mass wasting episodes & & \\  & & & & \\ \end{tabular}
\end{table} 
Table 3: Hydrogeological instability and mass wasting events affecting the Croso stream and the tributary Fontana stream, located immediately westerly to San Domenico village, before the 2000 event [28]. Concerning the historical archive analyses [28], the hydrogeological instability and mass wasting events that occurred along the investigated slope and around the San Domenico village before 2000, are reported in Table 3.  
The first event analysed in this research, that activated recently along the Croso stream incision, affecting the down-valley road, dates back to 13-16 October 2000 (Table 3). During the 2000 event, mass wasting, river incision and lateral erosion processes affected extensively the whole Po hydrographic basin. In the adjoining Divedro Valley, for example, the intense and prolonged rainfalls induced, among the others, a landslide causing 15 casualties [58]. The Caiirasca Valley was deeply affected by soil slips and debris flows, among which the Croso and Fontana stream debris flow was the most relevant in the area (Figure 6c) and rapid debris flow induced erosion along the thalweg, and damages to check dams and the road. The second, more recent, event occurred on the 12 August 2019 when a series of debris flows affected the slopes of the Caiirasca Valley, provoking temporarily interruptions along the Caiirasca stream. Along the Croso stream incision, the most powerful one occurred, whose effects were similar to those registered in 2000 (Figure 7). The videos recorded immediately after the event using a drone is retrievable on the web (courtesy of [[https://www.localteam.it](https://www.localteam.it)]([https://www.localteam.it](https://www.localteam.it))) at [[https://www.youtube.com/watch?v=](https://www.youtube.com/watch?v=)]([https://www.youtube.com/watch?v=](https://www.youtube.com/watch?v=)) OCPL63 CHxQ. The bridge, slightly damaged, was completed overwhelmed and filled below with the debris and the organic material, mainly wood (Figure 7a,b).