Changes in climate patterns and their association to natural hazard distribution in South Tyrol (Eastern Italian Alps)

In Alpine regions changes in seasonal climatic parameters, such as temperature, rainfall, and snow amount have already been observed. Specifically, in the South Tyrol area, meteorological observations indicate that temperatures are increasing and the number of snow days has generally diminished over time with perennial snow line now observed at higher elevations. Changes in rainfall have also been observed with more events associated with higher temperatures in the summer season. Natural hazards - mainly debris and mud flows, landslides, avalanches, rock falls, and (flash) floods - that affect this area every year, damaging population and infrastructures, are either weather or cryosphere-related. While these events have been recorded sporadically since the beginning of the 20th century, a systematic approach of their inventory has been done by local authorities since the 1990s. So far, Earth observation data has not been exploited to complete or complement existing inventories nor have they been used to investigate the influence of climate perturbation on potentially dangerous natural phenomena. The research presented here thus has three objectives: (i) analyse long time series of climate data and hazard occurrence in South Tyrol to examine if these records exhibit a coherent response of hazards to changes in climate; (ii) measure the spatio-temporal evolution of climatic and natural hazard events recorded, and (iii) explore potential relations between meteorological conditions and the hazard occurrence. In this context, in-situ and satellite-based climate data are exploited to study natural hazard triggers while the potential of Earth observation data is evaluated as a complement to the existing historical records of natural hazards. Specifically, Copernicus Sentinel-1 images are used to detect the spatio-temporal distribution of slow earth surface deformations and the results used for checking the completeness of the actual slow-moving landslide inventories. Hazard-related changes in the South Tyrolian landscape have also been analysed in relation to particular meteorological events at a regional scale, assessing trends and anomalies. Results show that: (i) satellite data are very useful to complement the existing natural hazard inventories; (ii) in-situ and satellite-based climate records show similar patterns but differ due to regional versus local variability; (iii) even in a data-rich region such as the analysed area, the overall response of natural hazard occurrence, magnitude, and frequency to change in climate variables is difficult to decipher due to the presence of multiple triggers and locally driven ground responses. However, an increase in the average annual duration of rainfall events and debris flow occurrence can be observed.


Natural hazard event databases
Two event databases on natural hazards supported this research: the "Inventario dei Fenomeni Franosi in Italia" (IFFI) and the "Ereignis Dokumentation 30" (ED30). IFFI is a nationwide project initiated by the National Italian Institute for environmental protection and research (ISPRA) and aims to establish a national database on gravitational mass movements. However, the mapping and maintenance of the IFFI database has been delegated to the regional geological surveys. The IFFI-project was launched in 1995. ED30 is a project launched in 1998 by the Regional Agency for Civil Protection of South Tyrol. Since that year, the database is systematically updated with field reports, photographs and GIS-information related to floods, debris flows, shallow landslides and snow avalanches. In addition, historical events have been reconstructed during an archive study. While the availability two event databases certainly provided a more complete picture of the spatio-temporal occurrence of natural hazards in the region, it required also the implementation of a method that allows the homogenisation of the catalogues and the identification of events that were recorded by both managing authorities and hence appear in both catalogues. Figure 3 shows the workflow that was implemented to homogenize the two databases and to filter identical events. Preceded by a preprocessing step, an event recorded in both databases is identified according to thematical, temporal and spatial criteria. Accordingly, we build an algorithm that: (i) Reduces in a preprocessing step both databases to a period of systematic recordings that overlap in time. For the present case, this was the period from 01.01.1998 to 31.12.2018. IFFI is managed systematically since 1995, and thus had to be reduced to the systematic recording period of ED30, from 1998 on. In addition, the terminology related to event types had to be homogenized; while IFFI is purely focused on different types of landslides, ED30 covers a broader range of event types, including different types of hydraulic hazards and snow avalanches. This required a reduction or exclusion of hazard types in order to make the databases comparable (see Table 3). For the present study, we reduced the broad spectrum of hazard classes to three rather general event types: landslides, debris flows and rock fall. Hazard types that could not be summarized by one of these three classes such as snow avalanches or floods were labelled as "Other". This latter class was excluded from the data analysis.
(ii) Subsets of both databases according to the same event type, selected by the user (e.g. debris flows).
(iii) Searches for matching dates within the subset of the databases of the same event type. The events in IFFI that occur thematically and temporally also in ED30 were extracted from the former one and written into a new data frame.
(iv) Identifies entries of this newly created subset of IFFI that are located within a spatial buffer of 100 m around the points of ED30.
(v) Removes the entries of IFFI that occur thematically, temporally and spatially also in ED30 from the initial IFFI data base and joins it to ED30.