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.
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.
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.
Avalanche hazard maps of high accuracy are difficult to produce. For land-use planning and management purposes, a good knowledge of extreme run-out zones and frequencies of avalanches is required. In the present work, vegetation recognition (especially focused on Pinus uncinata trees) and dendrochronological techniques are used to characterize avalanches that have occurred in historical times, helping to determine both the extent of large or extreme avalanches and their occurrence in time. Vegetation was studied at the Canal del Roc Roig (eastern Pyrenees, Spain) avalanche path. The avalanches descending this path affect the railway that reaches the Vall de Núria resort and the run-up to the opposite slope. During winter 1996, two important avalanches affecting this path were well documented. These are compared with the results of the vegetation study, consisting of an inventory of flora, the recording of vegetation damages along eight transverse profiles at different altitudes on the path and a dendrochronological sampling campaign. The data obtained contributed to a characterization of the predominant snow accumulation in the starting zone, the 1996 avalanches and the range of frequencies of large avalanches. Also, traces of avalanches that increase the path mapped in the avalanche paths map published by the Institut Cartogràfic de Catalunya in 2000 were identified, improving the initial existing information.
Slushflows were first recorded in the Iberian Peninsula on 18 December 1997. Three slushflows were released at the ski resort of El Port del Comte, in the Catalan Pyrenees, northeast Spain, during intense rainfall. Two of the slushflows originated on the pistes, and the third affected another piste. Three ski lifts were damaged. This paper analyzes the hydrogeological characteristics of the massif, the geomorphic features of the terrain and the meteorological and snowpack conditions that caused the release of the slushflows. Man's role in triggering the slushflows by compacting snow on the pistes is also considered. Drainage control for reducing the hazard is outlined, taking into consideration the low frequency of the phenomenon.
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