Objectives
The WP study the relationship between rockfall onset and meteoclimatic conditions such as: rainfall extremes, antecedent rainfall, snow- and ice-melt, temperature extremes and temperature fluctuations. The relationships will be studied by analysing past events collected in WP1 with multivariate statistical techniques (multiple regression, logistic regression, discriminant analysis) relating the rockfall events (dependent variable) with available geological, morphological and triggering event data (independent variables). This analysis allows for defining functional relationships among meteoclimatic conditions and rockfall triggering that can help simulating the effect of future climatic changes on rockfall triggering probability.
Results
The analysis are conducted according to the method proposed by Paranunzio et al. (2015) for 50 rockfall events in the Lombard Prealps between 2009 and 2018, considering temperature and precipitation values for different time intervals prior to the collapses. The method is aimed at identifying anomalies in the behavior of certain variables for the years in which events occurred. The greater the degree of anomaly, the greater the control that a certain variable seems to have exercised on the triggering of events in those specific years.
The figure shows the box plot of the climatic variables associated with the event, discretized by lithological macro-groups characterizing the area of the phenomenon. Where the data is more consistent, there is an absence in the link with the variables under examination. For foliate rocks, where the sample is not very numerous, an influence can be observed for the terms of daily precipitation (P P1 day), weekly (P P7 day), monthly (P P30 day) and quarterly (P P90 day) (positive anomalies) and daily temperatures (P T1 day) (negative anomaly). The same analysis is done for altitudinal classes in order to look for possible relationships with temperature oscillations.
From the overall analysis of the data it is possible to conclude that:
1) The variable most related to collapses is 1-day precipitation (P1day)
2) There is no significant change with elevation. This is probably due to the fact that the investigated events do not affect high altitudes and are mainly located along the communication routes at medium and low altitudes.
3) The different lithologies do not show appreciable variations expect for phyllites and mica schists that appear more sensitive to precipitation than the other lithologies.
We recently defined an Extremely Energetic Rockfall (EER) as the process of a boulder striking the ground with high kinetic energy after a free ballistic flight or after rolling (a process that normally does not take away much energy) of several hundred meters. The ensuing process of disintegration gives origin to a sequence of phenomena involving some different hazardous conditions. The figure below shows how this process takes place. When the block hits the ground at high energy, it is completely disintegrated. The air is highly perturbed to the point of sustaining a shock wave, while the lager fragments have a sufficient kinetic energy to be shot in the air. The finer part forms a suspension (dust cloud) which after moving from its source area downslope, finally settles.
The figure below shows some previous events where the formation of a dust cloud and its downward motion is evident. Such cloud may prompt the facilities closure, since it reduces the visibility. Closer to the impact point, the fragments of higher dimension abrade the surfaces and, hence, are hazardous to the building, objects and in some cases cause losses of human lives.
The figure below illustrates an electronic microscope image showing the composition as well as the shape and size of the particle constituting the powder.
It is possible to interpolate the granulometric curves of the powder particles with many distributions and, thereby, to assess the amount of generated powder, its deposition surface as well as the energy occurred to generate the fragmentation (fragmentation energy).
