Snow avalanches in the Pyrenees: dendrochronological dating, dendrogeomorphological mapping and detection of past snow-avalanche seasons at a regional scale = Allaus de neu al Pirineu : datació dendrocronològica, cartografia dendrogeomorfològica i detecció d'allaus del passat a escala regional
[eng] In recent decades, information about past snow avalanches in the Pyrenees is being acquired by searching in different sources. One of these sources are tree rings. In this Ph.D. thesis, dendrogeomorphological techniques are used to study the snow avalanche process in the Pyrenees. The methodol...
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| Tipo de recurso: | tesis doctoral |
| Estado: | Versión publicada |
| Fecha de publicación: | 2016 |
| País: | España |
| Institución: | Universidad de Barcelona |
| Repositorio: | Dipòsit Digital de la UB |
| OAI Identifier: | oai:diposit.ub.edu:2445/111207 |
| Acceso en línea: | https://hdl.handle.net/2445/111207 http://hdl.handle.net/10803/403058 |
| Access Level: | acceso abierto |
| Palabra clave: | Dendrocronologia Allaus Cartografia Pirineus Dendrochronology Avalanches Cartography Pyrenees |
| Sumario: | [eng] In recent decades, information about past snow avalanches in the Pyrenees is being acquired by searching in different sources. One of these sources are tree rings. In this Ph.D. thesis, dendrogeomorphological techniques are used to study the snow avalanche process in the Pyrenees. The methodology to date snow avalanches using wood samples from trees of the genus Pinus L. is developed, and external signs of snow-avalanche effects and reactions on these trees are identified. Dendrogeomorphology is applied to date snow avalanche events, to reconstruct local event chronologies, and to map major avalanches. All these findings are subsequently used to estimate process frequency at specific avalanche paths. At a regional scale, by studying the snow avalanche events in several avalanche paths along the territory, dendrogeomorphology contributes to determine major avalanche winters. Calibration and validation of the methodology was done in Roc Roig avalanche path (Nuria, Catalonia). Evidence recorded in trees from the well- documented 1995-1996 avalanche season was used to establish fieldwork strategy and dendrochronological analyses to date past snow avalanches. Once the connection between external evidence and tree-ring reactions to the known avalanche events in 1995-1996 winters was understood, previous avalanche events were found out in the same avalanche path. In this site two extreme avalanches took place in 1996. Pinus uncinata trees disturbed during this avalanche season allowed to assess the associated tree-ring signals. Trees displayed a variety of responses to these events. The methodology was proved successful in detecting the 1996, 1986 and 1972 documented avalanche events, and provided evidence of undocumented past events such as the one in 1929-1930, which was a major avalanche as in 1996. To obtain high-quality evidence it is necessary to correctly recognize external signs from snow avalanches on trees, to know the corresponding tree reactions and to distinguish associated tree-ring signals. The characterization of snow-avalanche effects and reactions in Pinus uncinata trees was carried out with a recent avalanche event in Barranco de las Fajas avalanche path (Sallent de Gallego, Aragon). Data was systematically gathered from all trees in selected transects across the avalanche path and tree structure was described. In the 2007-2008 avalanche clearing, shapes from trees affected by the event are compared to shapes from trees affected by earlier events. The distribution of tree morphologies along the path is discussed. Describing and quantifing disturbed tree morphologies should contribute to discriminate among different geomorphological hillslope processes. By applying these findings to a set of six avalanche paths distributed throughout the territory, evidence for three regional-scale major avalanche years was identified in the SE Pyrenees from 1971 to 2004: 1971-72, 1995- 96 and 2002-03. In the total count, dendrochronological analyses yielded the dates of nine winters when major avalanches occurred in the recent past in the six avalanche paths. Some of these avalanches were already known, but others had not been documented. Also in this chapter, an example on how dendrochronological evidence of a past major event on trees was used to map an extreme runout. This was done in an avalanche path descending from Pui de Linya (Espot, Catalonia). In this case a major avalanche occurred in 1971-1972 which had not been previously documented. Thus, the map done according to conventional techniques was improved by using dendrogeomorphological evidence. The combination of information yielded by dendrogeomorphology together with data from the other sources is the best way to improve our knowledge on the snow avalanche process. In the Pyrenees, though still very scarcely applied, dendrogeomorphological analyses play an important part in the zonification of selected snow avalanche paths to prevent avalanche hazard where forest or trees are present. |
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