Snow avalanches have been studied for decades due to their unpredictability and danger. The influencing factors and dynamic of such events have studied and monitored to ensure an adequate level of protection for the infrastructure and citizens of mountain areas. Among these, the protective effects that the forest exerts on snowfall and snow cover can represent a fundamental component in decreasing the hazard level of avalanche events. Forests are ecosystems that can be cyclically affected by disturbances or alterations due to biotic and abiotic circumstances. One of these factors are windstorm events. The Vaia storm that hit north-eastern Italy at the end of October 2018 damaged large areas of forest, causing the felling of entire wooded areas, with the consequent alteration of ecosystem balances. Forests affected by the event reported heterogeneous distribution of the felled trees. According to various studies, the presence of biomass on the ground in storm-affected areas plays a fundamental role in anchoring the snow on the ground. The stems distributed heterogeneously in the areas, create a rough and jagged surface, decreasing the chances of detachment along the weak layers within the snow cover at the points most exposed to topographic triggering factors. However, the complete covering of the wood on the ground due to snowfall would favour trigger conditions that the standing forest in pre-disturbance conditions would not allow. The present study aims to provide information on the role of ground biomass in avalanche hazard assessment based on the analysis of study areas affected by storm Vaia, regarding the possibility of avalanche triggering as a function of cumulated snow depth. Snow height data are analysed based on historical measurements recorded by two meteorological stations located in the upper Agordino area, province of Belluno. The Vaia-affected test areas selected for the analysis were identified on the similarity of snow conditions to the selected weather stations. For each area, the possible areas of detachment were identified considering the topographical factors favourable to avalanche triggering. The volume of snow capable of storing each area, able to smooth the rough surface, was derived by means of a special algorithm. The historical station data were analysed by means of statistical analysis of the extremes (Gumbel distribution), in the days at the end of February, to search for trends in the value of cumulative snow height at the end of the season with respect to a given return time. For the simulation of possible post-season snowfall, the DH3gg value was derived from the same historical data in the months following March. 11 The values obtained through the analysis of the historical data were regularised by means of an altitude correction, which was necessary due to the elevation difference between the weather station and the study areas. The comparison between the volume capacity of the test areas affected by the storm and the historical data obtained provided the scenarios of possible detachment in the areas most likely to be triggered. The results defined the possibility of detachment in the selected test areas as unlikely due to the presence of trees on the ground. The same result appeared for the individual PRAs within the test areas. The difference in value of the storable volume capacity between the test areas and the reference PRAs showed a marked difference in some cases.
Da decenni le valanghe di neve sono oggetto di studio a causa della loro imprevedibilità e pericolosità. I fattori e le dinamiche inerenti ad esse sono oggetto di studio e monitoraggio per garantire un adeguato livello di protezione alle infrastrutture e cittadini dell’ambiente montano. Tra questi, gli effetti protettivi che il bosco esercita sulle precipitazioni ed il manto nevoso, può rappresentare una componente fondamentale per la mitigazione da fenomeni valanghivi. Il bosco si presenta come ecosistema che può essere colpito ciclicamente da disturbi o alterazioni dovute ad elementi biotici ed abiotici. In particolare, uno dei fattori che maggiormente altera l’equilibrio di questi ecosistemi deriva dalle tempeste da vento. La tempesta Vaia che a fine ottobre del 2018 ha colpito il nordest Italia ha danneggiato ingenti superfici forestali, causando l’abbattimento di intere aree boscate, con la conseguente alterazione degli equilibri degli ecosistemi. Le aree boscate interessate dall’evento riportano legname abbattuto e distribuito eterogeneamente. Secondo differenti studi la presenza della biomassa a terra nelle aree colpite da tempeste ricopre un ruolo fondamentale per l’ancoraggio della neve accumulata al suolo. I singoli fusti distribuiti eterogeneamente nelle aree, infatti, creano una superfice rugosa e frastagliata, diminuendo le possibilità di distacco degli strati di neve più deboli nei punti più esposti ai fattori topografici di innesco. Tuttavia la completa copertura del legname a terra dovuto alle precipitazioni nevose, favorirebbe le condizioni di innesco che il bosco in piedi in condizioni pre-disturbo non permetterebbe. Il presente studio è atto a fornire informazioni inerenti all’importanza della biomassa a terra nella protezione dal rischio valanghivo basandosi sull’analisi di aree studio colpite dalla tempesta Vaia, per quanto concerne la possibilità di innesco di valanghe in funzione dell’altezza della neve al suolo. Le precipitazioni nevose vengono analizzate sulla base dei dati storici di altezza del manto, forniti da due stazioni nivometriche di riferimento nell’alto agordino in provincia di Belluno. Le aree test colpite da Vaia selezionate per l’analisi sono state identificate sulla somiglianza delle condizioni nivologiche rispetto alle stazioni meteo di riferimento. Per ogni area sono state individuate le aree possibili di distacco, ricavate tramite i fattori topografici ritenuti favorevoli all’innesco valanghivo. Il volume di neve capace di immagazzinare ogni singola area, atto a creare le condizioni di innesco valanghivo, è stato ricavato tramite un apposito algoritmo. I dati storici delle stazioni sono stati analizzati tramite analisi statistica degli estremi (distribuzione di Gumbel), nei giorni di fine febbraio, per la ricerca di tendenza del valore di altezza di neve cumulata a fine stagione rispetto a un determinato tempo di ritorno. Per la simulazione delle possibili nevicate post stagione, è stato ricavato il valore del DH3gg dai medesimi dati storici nei mesi successivi a marzo. 9 I valori ricavati tramite le analisi dei dati storici sono stati regolarizzati mediante una correzione di quota, necessaria per la validazione del dato. Il confronto tra la capacità in volume delle aree test colpite dall’evento e dati storici ricavati, hanno fornito gli scenari di possibile distacco, nelle aree più probabili di innesco (PRA). I risultati hanno definito improbabile la possibilità di distacco nelle aree test selezionate a causa del legname a terra. Il medesimo risultato è apparso per le singole PRA all’interno delle aree test. La differenza di valore della capacità di volume immagazzinabile tra le aree test e le PRA di riferimento ha evidenziato una marcata differenza in alcuni casi.
Analisi delle condizioni di innesco di valanghe in zone colpite dalla tempesta Vaia
AGOSTINI, FEDERICO
2021/2022
Abstract
Snow avalanches have been studied for decades due to their unpredictability and danger. The influencing factors and dynamic of such events have studied and monitored to ensure an adequate level of protection for the infrastructure and citizens of mountain areas. Among these, the protective effects that the forest exerts on snowfall and snow cover can represent a fundamental component in decreasing the hazard level of avalanche events. Forests are ecosystems that can be cyclically affected by disturbances or alterations due to biotic and abiotic circumstances. One of these factors are windstorm events. The Vaia storm that hit north-eastern Italy at the end of October 2018 damaged large areas of forest, causing the felling of entire wooded areas, with the consequent alteration of ecosystem balances. Forests affected by the event reported heterogeneous distribution of the felled trees. According to various studies, the presence of biomass on the ground in storm-affected areas plays a fundamental role in anchoring the snow on the ground. The stems distributed heterogeneously in the areas, create a rough and jagged surface, decreasing the chances of detachment along the weak layers within the snow cover at the points most exposed to topographic triggering factors. However, the complete covering of the wood on the ground due to snowfall would favour trigger conditions that the standing forest in pre-disturbance conditions would not allow. The present study aims to provide information on the role of ground biomass in avalanche hazard assessment based on the analysis of study areas affected by storm Vaia, regarding the possibility of avalanche triggering as a function of cumulated snow depth. Snow height data are analysed based on historical measurements recorded by two meteorological stations located in the upper Agordino area, province of Belluno. The Vaia-affected test areas selected for the analysis were identified on the similarity of snow conditions to the selected weather stations. For each area, the possible areas of detachment were identified considering the topographical factors favourable to avalanche triggering. The volume of snow capable of storing each area, able to smooth the rough surface, was derived by means of a special algorithm. The historical station data were analysed by means of statistical analysis of the extremes (Gumbel distribution), in the days at the end of February, to search for trends in the value of cumulative snow height at the end of the season with respect to a given return time. For the simulation of possible post-season snowfall, the DH3gg value was derived from the same historical data in the months following March. 11 The values obtained through the analysis of the historical data were regularised by means of an altitude correction, which was necessary due to the elevation difference between the weather station and the study areas. The comparison between the volume capacity of the test areas affected by the storm and the historical data obtained provided the scenarios of possible detachment in the areas most likely to be triggered. The results defined the possibility of detachment in the selected test areas as unlikely due to the presence of trees on the ground. The same result appeared for the individual PRAs within the test areas. The difference in value of the storable volume capacity between the test areas and the reference PRAs showed a marked difference in some cases.File | Dimensione | Formato | |
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https://hdl.handle.net/20.500.12608/42382