The case study of the Avalanche of “Testa Grigia Orsio” of 11/24/2019 - Gressoney La Trinité – Valley– A. Debernardi 1; N. Durand 1; V. Segor 2; A. Plater 2

1 Fondazione Montagna sicura, Villa Cameron, Villard de la Palud 1, 11013 (AO), Italy – [email protected] 2 Snow and avalanche office, Hydrogeological structure of mountain basins, Region, Amérique 33, 11020 Quart (AO), Italy – [email protected]

AIM OF THE WORK : The Aosta Valley Region is located The Gressoney Valley is to analyze the nivo-meteorological conditions that caused in the north-western part of Italy bordered to the north by the “Testa Grigia Orsio” avalanche of 11/24/2019, to and borders with and , to the west with the France. More than 60% of its Gressoney Ayas valley, to the south-east describe its dynamics and related damage, with the aim of territory is above 2000 m a.s.l.. La Trinité with the Dora Baltea valley, adapting and relocate the avalanche barriers that have been Several large and very large to the south and east with destroyed. The study will be supported by the historical avalanches take place in the the provinces of Biella and region, infact, the 17% of the Vercelli and is crossed by the information collected in the regional Avalanche Cadastre and regional territory is affected by river , which flows from by “good advice” given by the local avalanche committee. avalanches. the homonymous glacier.

1 -NIVO-METEREOLOGICAL SITUATION : manually measured data MOD1 AINEVA From the second half of November, different cold fronts caused severe snowfalls in At the Gabiet dam, an nd historical manual the eastern valleys. The most important occured between November the 22 till th measurement site, near the 25 , which brought a lot of snow even at the bottom of the valley. the studied area, from The currents were southerly and affected the valleys on the border with November the 22 nd till where they were measured amounts of 190-250 cm of new snow. the 24 th early morning, Moderate winds accumulated more snow on the northern slopes. at 2300m a.s.l., 190 cm of new snow were Two main types of avalanches occurred during this period: measured. A. Wet and glide avalanches below the treeline –

(combination of non-frozen soil and mild temperatures 2 - DYNAMICS OF THE AVALANCHE: Pentespetz with rain on snow up to 2000 m and wet snow even up The avalanche triggered at 2970 m a.s.l. from the rocky ridge (3136m a.s.l. to 2600 m). that descends from Pentespetz (3136 m) on the right B. Dry surface slab avalanches at higher altitudes - hydrographic side of the Gressoney Valley. In the falling path the avalanche reached high velocities thanks also to a vertical thanks to the wind and cooler temperatures. CONCA SORRELET drop and entrained the wettest snow present below 2400 m. Some of these avalanches reached the valley floor Here the avalanche destroys many avalanche barriers built in and municipal and regional roads. 1978.

The avalanche descending towards the Salto di roccia valley uprooted or Vertical mountain wall crushed several large CONCA PARADISO conifers. The avalanche, divided into several branches, reaches the Lys river and the 3 - HISTORY OF THE AVALANCHE SITE meadows at an altitude Avalanche barriers The event of 03/10/1975, the largest in size of the deposition area, was up for the of 1630 m. The powder construction of the avalanche barriers in 1978. cloud of the avalanche Obre Eselbode 1630 m a.s.l.. From the many historical photos avalaible, however, it’s not possible to precisely managed to "plaster", determine a triggered area going up to the height of the rocky bars. without causing damage, some houses in Obre Eselbode located on the opposite side.

2.1 - SOME NUMBERS: the Blue line: historical avalanche perimeter width of the release zone was 650 m including the main and secondary slab. The avalanche covered an altitude difference of about 1350 m and a path of 2300 m. The very large final deposit was around 300.000 Damaged avalanche mc. barriers area 2.2 – DAMAGES: of the 900 m of avalanche barriers intalled, 250 m have been destroyed, mainly concentrated in the most central part of the basin under the vertical drop called "Conca Paradiso". 11/24/2019 avalanche area 5- WORK IN PROGRESS During the summer of 2020, the regional administration has foreseen the repair of some avalanche barriers and the installation of 68 new “snow-umbrella” structures at the “Conca Paradiso” the main avalanche released zone and also the original installation site. The cost 4 - COMPARE BETWEEN PAST EVENTS AND THE ONE OF 11/24/2019 ofFor the the intervention upper part ofwill the be basin around (“Conca € 350.000. Solleret”), at least one winter season of monitoring, The main question is the location of the release zone. In fact, before the last event, held by the local avalanche committee, is expected to better understand wind effects on the for the Avalanche Cadastre, the triggering area coincided precisely with the upper snow slab formation. This precious information, along with all the data collected after the limit of the rocky bar at 2550 m a.s.l (blue perimeter on the map), but in November 2019 event, will allow a better definition of the upper release area for eventual November 2019 the avalanche released zone can be much further upstream. new protection measures. This work will be affected by the nature of the lithological Unfortunately, even in this case, the bad weather immediately canceled the traces substrate, by steepness of the slopes (safety of the workers) and by the “real size” of the of the slabs. avalanche basin. However, thanks to the direct testimony of the Local Avalanche Committee, it was 6 – CONCLUSION : Unity is strength! partially possible to determine the To better understand this case of study, the collaboration of location of the release zone. different professionals dealing with avalanches was important. Several on-site inspections were Avalanche forecasting combined with local risk monitoring and necessary to “reconstruct” the real management are summarized during the design phase of the event. Thanks to a joint work (technicians avalanche barriers. and local guides) different trigger 7 - ACKNOWLEDGMENTS - Thanks to the mountain guides Frachey Davide and Welf Arnoldo of the scenario have been proposed, with still Gressoney local Avalanche Commission (CELVA – Consorzio Enti Locali della Valle d’Aosta ), for their availability and some open questions. competence!