RINA - Démonstrateur d’utilisation de l’intelligence artificielle pour la gestion opérationnelle des risques naturels gravitaires géologiques | FEREC 2020

1.3. COMPARISON BEFORE/AFTER MAJOR PROTECTION WORKS IN 2008 9 Mont Saint-Eynard was set to 24 hours. They show that rainfall frequency during rainfall episodes is 3 times higher than without meteo- rological event. Thus, it is multiply by factor of 7 in the first 25 h and decreases to 1 after 50h. "Considering the mean intensity since the beginning of the episode, the influence factor amounts to 27 if the intensity is higher than 5 mm.h − 1 . Considering the rainfall amount, it amounts to 7.5 if the rainfall amount is between 30 and 40 mm". Therefore, rainfall episode belong to factors which can well explain a part of rockfall hazard and should be take into account in future predictive model. 1.3 Comparison before/after major protection works in 2008 During this period, 139 rockfalls events occurs. If we aggregate them in space, we get only 104 times where 1 or more rockfall occurs. As we can see on Figure 1.6, daily number of rockfall span from 0 to 5. Sample auto-correlation and correlogram show a slight annual pattern in number of rockfall. If we look at times between events, mean time between rockfall is about 25 days and we can see in Figure 1.7, that inter-times distribution is badly fitted by exponential law. So point process of rockfall times is clearly not an homogeneous Poisson Process. Rainfall (Figure 1.6) Daily rainfall quantity span from 0 to 175 mm with. Times between two consecutive rainfall is around 3 days in average. Sample auto-correlation and correlogram confirm the annual seasonality that we could already see in time series. A second significant peak in correlogram is present for 2 cycle/year ie an half-year periodicity. Correlation between rockfall and rainfall (Figures ***) If we superpose daily or monthly time series of rockfall and rainfall, we can observe temporal similarities already observed in [3] .To quantify such dependencies, we plot as well cross-correlation (see chapter 2 for definition). Daily rockfall times series is correlated to rainfall time series at short-term. More precisely, maximum of correlation is for 1 day lag and it decrease till 5 days where it becomes not significantly. Location, size, protection and geological zones (Figure *** ) Location of events are beyond cliff (between PR 1.7 and PR 13). They are unevenly distributed : mainly around PR 6 and PR 12 . Size of events span from 0.2 to 12 000 000 kg. Majority of events have size lower than 100 000 kg (see join distribution location/size). Only 4 events are bigger and all are located between PR 9.2 and PR 10.05.