*4.4. Nivological Analysis*

By accounting for limits derived from the available nivological data, as previously reported in the Materials and Methods section, the following can be stated:


**Figure 9.** Seasonal fresh snow trend for the timespan 1986–2009 (**a**) and snowfall regime (**b**) at the Prati di Tivo gauge (1450 m a.s.l.).

In order to provide a complete and updated description of the nivometric trends, the historical series and datasets available at the Rifugio Il Ceppo gauge (1340 m a.s.l.) were analyzed. This weather station was taken into account since it shows snowmaking very similar to that of Prati di Tivo, even if it is located on the eastern side of the neighboring Laga Mountains at a distance of about 20 km from the study area.

**Figure 10.** Daily snowfall (**a**) and snow cover thickness (**b**) at the Prati di Tivo gauge (1450 m a.s.l.).

As graphically reported in Figure 11, the nivometric trend spanning over a 40-year time period (1979–2019) confirms the increase of snow precipitation. However, it is less marked than that evident for the Prati di Tivo area (Figure 8a), with a minimal difference in the recent interseasonal variations (2.1 cm vs. 3.1 cm/season), whose trends are increasingly marked.

**Figure 11.** Seasonal fresh snow at Rifugio Il Ceppo (1340 m a.s.l.). The blue dashed line represents the 5-year moving average.

#### *4.5. Snow Avalanche Hazard Assessment*

A stepwise methodological approach allowed us to perform a complete snow avalanche hazard assessment, also taking into account the study area's main physiographic and geomorphological features. The avalanche-prone areas along the main N–S-oriented avalanche tracks characterize the northern escarpment of the Corno Piccolo ridge, which alternate with several rock gullies. The area shows elevation values ranging from 1320 to 2270 m a.s.l. As confirmed by the previous data and analysis [123,124], the avalanche paths largely affected residential structures and ski facilities, causing significant damages in recent times.

Firstly, a snow avalanche inventory analysis was carried out. The geodatabase retrieved from the State Forestry Corps of Italy and the Abruzzo Region stored and collected almost 800 avalanches over the whole Abruzzo Region from 1957 to 2013. The yearly number ranged up to 70 in the last decades, and about 40 events were recorded in the previous years covered by the catalog, with a poor direct correlation with the snow thickness. As graphically reported in Figure 12, for the Prati di Tivo area, the database reports 131 snow avalanches: 10 of which accounted as slab snow avalanches, 26 as glide snow avalanches, 54 as powder snow avalanches, 9 as loose snow avalanches, and 32 as mixed or not classified snow avalanche.

Additionally accounting for the main geomorphological features, a preliminary analysis of the spatial distribution of snow avalanches over the study area shows the northern escarpment of the Corno Piccolo ridge almost totally affected by avalanche phenomena whose detachment areas are located at elevations ranging from 1700 up to 2550 m a.s.l. These phenomena mainly involved several N–S-oriented rock gullies and trails, often anastomosed, allowing the snow movements to extend heterogeneously, depending on the type and amount of snow involved.

Moreover, to provide a complete inventory, official avalanche data of the Abruzzo Region were integrated with the literature data, local chronicles, eyewitness reports of past avalanche events, and studies of previous events recorded in various historical and technical archives.

**Figure 12.** Historical snow avalanche map of the study area (1957–2013 period).

The study area has suffered a rapid and intense urban development for sport tourism purposes since 1965, although several avalanches have occurred in the past, according to local chronicles and eyewitness reports. These events were also integrated into the snow avalanche inventory analysis, enabling the mapping of the maximum extent of avalanches in the Prati di Tivo area. They are chronologically reported as follows:


•3 March 1999—A large avalanche hit the study area, causing several damages to the ski facilities (e.g., pylons, intermediate station, and ticket office).

The resulting data highlight a scarce spatial characterization of snow avalanches along the study area. Given the proximity of different detachments sites, the reconstruction of past avalanche activity remains quite difficult, distinguishing events occurring from neighboring detachment areas. Consequently, considering the absence of an updated Probable Avalanche Location Map—CLPV (Carta di Localizzazione Probabile delle Valanghe in Italian) [44], an analysis of snow avalanches' paths was performed to identify areas likely to be exposed to avalanche hazard. This "static" approach [105,125] was based on the analysis of morphological features for delineating the predisposition to snow avalanche occurrence within the whole northern escarpment of the Corno Piccolo Ridge, also considering the spatial distribution and the recurrence of the main phenomena. Six main avalanche paths were selected (Figure 13) that are considered the most likely to occur and affect houses, roads, and sporting infrastructures. Moreover, most of the chosen sites were devoid of terminology, and according to their proximity to isolated buildings or specific sites/localities, it was decided to provide detailed descriptions for some of them.

**Figure 13.** Snow avalanche paths recorded and mapped in the present study.

The exceptional snowfall events generating the snow avalanche disaster involving the Rigopiano Hotel in January 2017 [75,126,127] caused several collateral events in the surrounding areas, including the Prati di Tivo area. They generated a wide snow avalanche along the Vallone della Giumenta (Figure 13) and determined acute injuries to the Prati di Tivo residence (Figure 14).

**Figure 14.** Detail of damage caused by the Vallone della Giumenta avalanche to the Prati di Tivo residence on 18 January 2017 (source Il Martino, 2017). For the avalanche path's location, see Figure 12.

This event led to the definition and realization of the first Avalanche Hazard Map of the entire Central Appennines area within the Avalanche Hazard Exposure Zones Plan— PZEV (Piano delle Zone Esposte a Valanghe in Italian). At the same time, the PIDAV was presented to develop prevention, mitigation, and managemen<sup>t</sup> activities of ski facilities directly exposed to avalanche dynamics [100]. In this context, the nivometric dataset available at Prati di Tivo gauge (1450 m a.s.l.) was widely analyzed to derive the required parameters for dynamic snow avalanche modeling (Table 3). This analysis allowed us to define the possible return times of snowfall events at different periods. In detail, the relationship between the maximum height of the snow cover (Hs) and the increase of the snow cover height over three consecutive days (Dh3gg) revealed that snow events (i.e., January 2017) that created a Hs > 3 m can be statistically considered as significant outliers (std > 3) with potential return times far exceeding 300 years.

**Table 3.** Main nivometric parameters required for the return time estimations, recorded at the Prati di Tivo gauge (1450 m a.s.l.). N.B.: Hs is the height of the snow cover and DH3gg is the increase of the snow cover height over three consecutive days.


A thematic map elaborated following the AINEVA criteria [31] and integrated with a detailed nivological analysis is graphically shown in Figure 15. This map clearly shows the main areas exposed to avalanche hazards. Considering the spatial distribution of the snow avalanche paths, as reported in Figure 12, it is possible to delineate the transit and invasion zones, marked with different colors according to the estimated avalanche hazard and the potential return periods (such as T = 30, 100, and 300 years). The analysis of the map highlights different scenarios: with a return time equal to 30 years, all tourist, sporting, and residential facilities can be affected by significant snow avalanche phenomena characterized by paths that reach the Prati di Tivo area at elevations of 1450 m a.s.l., while avalanches with a return time equal to 300 years can ge<sup>t</sup> a wider spatial extension (downward to 1370 m.a.s.l.) comparable to the maximum extent of the historical snow avalanches previously described (dashed blue line in Figure 12).

**Figure 15.** Avalanche hazard map of the Prati di Tivo area. Areas affected by snow avalanches with a return time equal to 30, 100, and 300 years are shown, respectively, in red, blue, and yellow.
