Effects of rainfall Intensity-Duration on landslides velocity variations: insights from long-term monitoring of case studies in Emilia-Romagna and South Tyrol (Italy)

What is known, nowadays, is that shallow landslides are mostly influenced by intense short-duration rainfall events while deep-seated ones are mainly affected by long-duration cumulated rainfall. However, the correlation between precipitation and displacement rates, especially for deep-seated landslides, is still poorly investigated on a quantitative basis. In order to understand the mechanisms of acceleration and deceleration of landslides and how they are related to rainfall regimes, long-term, possibly continuous, monitoring of displacement is essential. This contribute aims to present and discuss this issue based on results from about 15 long-term monitored landslides, ranging from earthslides-earthflows to deep-seated rockslides, located in Emilia-Romagna region and South Tyrol. Displacement time series in these case studies have been collected with different in-situ techniques such as principally periodic and continuous GNSS and Robotic Total Stations (RTS), covering periods up to more than ten years. After analysing displacement plots, each identified acceleration event was correlated to rainfall by considering the last significant precipitation event antecedent to the first date of velocity variation, recorded by local meteo stations. Then, Duration (h) and Intensity (mm/h) were retrieved for each event and an Intensity-Duration (ID) plot was built with all data together. It could be observed that the ID-points were distributed along a line with extremely slow deep-seated landslides on one side and rapid earthslides-earthflows on the other, representing the two opposites of the spectrum. Secondly, another aspect that was considered in this framework is the difference between velocity variations of monitored points (such as GNSS benchmarks or RTS prisms) and the velocity of movement propagation along the landslide body. Examples on this topic are presented from Ca Lita and Corvara landslides, located in Emilia-Romagna and South Tyrol, respectively. Landslides response to precipitation events is the result of a complex combination of geological, geomorphological, geotechnical, and meteo-climatic factors. In accordance with ID-points distribution, the lower the surface of movement the lower duration and intensity are needed to enhance instability and displacement rates. On the other hand, the interaction with rainfall is not as immediate for deep-seated landslides, making their interpretation more complex. This study presents (i) a summary of all the recorded velocity variations affecting the proposed case studies, and (ii) an interpretation of their behavior in terms of acceleration and precipitation conditions.