Abstract.A regional study of snow avalanche processes was undertaken in the SE Pyrenees. Dendrogeomorphology was used to date and reconstruct large-scale snow avalanche events that occurred in the last four decades. Dendrochronological analyses yielded the dates of nine winters when avalanches occurred in the recent past in six studied avalanche paths. Some of these avalanches were already known, but others had not been documented. In one case, the existing avalanche path map was improved with the dendrogeomorphological information of a larger past event. As a result of the dendrogeomorphological analyses, evidence for three regional-scale major avalanche years was identified in the SE

ABSTRACT. Historical data about ancient avalanches are scarce in the Pyrenees.Dendrochronology can provide new data about past avalanches and their return period, but up to now little research has been carried out with this purpose. The Aludex project aims to obtain information about the frequency and extent of extreme avalanches, using a dendrochronological and a nivo-meteorological approach. In this paper, we present the results of a dendrochronological study of the Canal del Roc Roig avalanche path which was affected by two extreme avalanches in February 1996. This first dendrochronological study has permitted us to assess the tree-ring signals due to avalanche events in 53 out of 132 dated trees. The trees presented a variety of responses to the 1996 avalanche events. It is shown that the type of tree-ring signal depends on tree age. The methodology has proved successful in detecting the 1996 and 1972 documented avalanche events, and provided outstanding evidence of undocumented past events such as one in 1930.

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8With the aim of better understanding avalanche risk in the Catalan Pyrenees, the present work focuses on the analysis of 9 major (or destructive) avalanches. For such purpose major avalanche cartography was made by an exhaustive 10 photointerpretation of several flights, winter and summer field surveys and inquiries to local population. Major avalanche 11 events were used to quantify the magnitude of the episodes during which they occurred, and a Major Avalanche Activity 12Magnitude Index (MAAMI) was developed. This index is based on the number of major avalanches registered and its 13 estimated frequency in a given time period, hence it quantifies the magnitude of a major avalanche episode or winter. 14 Furthermore, it permits a comparison of the magnitude between major avalanche episodes in a given mountain range, or 15 between mountain ranges, and for a long enough period, it should allow analysis of temporal trends. Major episodes from 16 winter 1995/96 to 2013/14 were reconstructed. Their magnitude, frequency and extent were also assessed. During the last 17 19 winters, the episodes of January 22-23 and February 6-8 in 1996 were those with highest MAAMI values, followed by 18 January 30-31, 2003, January 29, 2006, and January 24-25, 2014. To analyze the whole twentieth century, a simplified 19 MAAMI was defined in order to attain the same purpose with a less complete dataset. With less accuracy, the same 20 parameters were obtained at winter time resolution throughout the twentieth century. Again, 1995/96 winter had the highest 21 MAAMI value followed by 1971/72, 1974/75 and 1937/38 winter seasons. The analysis of the spatial extent of the different 22 episodes allowed refining the demarcation of nivological regions, and improving our knowledge about the atmospheric 23 patterns that cause major episodes and their climatic interpretation. In some cases, the importance of considering a major 24 avalanche episode as the result of a previous preparatory period, followed by a triggering one was revealed. 25 Key words 26Major avalanche, major avalanche episode, Pyrenees, magnitude, frequency, hazard, risk.

The numerical avalanche dynamics program AVAL-1D, developed by the WSL Institute for Snow and Avalanche Research SLF, was calibrated empirically with real avalanches in the Swiss Alps. For the simulation of avalanches with this program, the SLF recommends the use of two friction parameters obtained for this purpose. Applying these parameters to other regions with different characteristics can lead to inaccuracies if a previous calibration is not performed, but often there is not enough data to do a proper calibration when an avalanche is simulated in a specific avalanche path. For this reason, an investigation to determine the specific parameters to be used in the Catalan Pyrenees was performed. This study was based on back-calculations of well documented events. Twelve dense flow or mixed avalanche events, from small to large size, were selected from nearly 2500 avalanches stored in the Avalanche Database of Catalonia (BDAC). The availability of morphometric and dynamics data, necessary for this purpose, was the critical factor to reject several cases. The result of the study reveals that there is a good fit between the recorded avalanche events and the simulated one's using the SLF recommended parameters.

ABSTRACT. Slushfl ows were first recorded in the Iberi a n Peninsul a on 18 D ecember 1997. Three slushfl ows wer e rel eased at the ski resort of El Port d el Comte, in th e Catala n Pyrenees, northeast Spa in, d uring inte nse rainfa ll. Two of the slu shflows ori gin a ted on the pistes, and th e third affected a noth er pi ste. Three ski lifts were damaged. This p ap er anaIyzes th e h ydrogeological cha rac teristics of the m ass if, th e geom orphic features o f the terrain a nd th e meteorological a nd snow pac k co nditi ons th a t ca used the re lease of th e slushflows. M a n's role in tri gge ring the slushflows by compac ting snow on the pi stes is also considered. Drain age co ntrol for reducing th e haza rd is o utlin ed , taking into co nsiderati on th e low frequency of th e phenomenon.

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