Reconstruction of the most intense avalanches of the last 40 years in the Catalonian Pyrenees

Dendrochronology applied to the study of landslides has allowed scientists from the University of Barcelona (UB), and from the Geological Institute of Catalonia to date and map landslides that occurred over the 20th century, up to the present day.

The study, which has recently been published in the Natural Hazards and Earth System Sciences journal, reveals, with dendrochronological analysis and meteorological records, nine winters with avalanches during the second half of the 20th century. More severe landslides occurred in the periods 1971-72, 1995-96, and 2002-03. The study is currently ongoing in the Aragonese Pyrenees in collaboration with the University of Zaragoza.

"In a landslide area, avalanches fall which may have very different dimensions." The most frequent are usually small and do not cause damage, but some can cause damage to ground cover, infrastructure, transportation or buildings," Elena Muntán, the main author of the study and researcher at the Ecology Department of the UB, said.

The team of researchers selected six landslide areas in the Catalonian Pyrenees as a sample of the entire territory. In total, 15,000 avalanche areas have been identified in Catalonia alone.

Trees as landslide indicators

The scientists took samples of wood from living and dead trees from the landslide area to ascertain, by looking at their rings, the time at which disturbances in their growth occurred. "Snowfall causes inclination of tree trunks, damage and breakage of branches or the crown, uprooting, clearance of old or dead trees of the specimen," Muntán states.

According to the researcher, these effects are represented by identifiable ring marks, as happens with mountain pine (Pinus uncinata) in the high mountains of the Pyrenees, the inclination of which changes the anatomy of subsequent rings and gives them a more intense colouration on one of the sides.

Scientists predict that it will be possible to reconstruct the last 150 years of landslide events from this mountain range using dendrochronology (study of geomorphological processes by means of dendrochronology). "The best strategy for studying past landslides is a combination of techniques that estimate the frequency of landslides and the dimensions of the largest landslides (which are usually catastrophic) through historical documents, surveys of the local population, winter monitoring, collection of meteorological and snow-meteorological data, geomorphological mapping and dendrochronology," the scientists points out.

Avalanche danger

Dendrochronology and the research of historical documents can be useful to learn about past avalanches given the fact that humans are now more exposed to the risk of landslides. "In principle, the risk of avalanches has not increased, but the exposure of people and property to this natural phenomenon has, due to the increased number of people frequenting mountain areas during the winter. Faced with a situation of increasing urbanisation in many valleys of the Pyrenees, it is necessary to clarify which area is exposed and to what extent, and to work accordingly," Muntán states.

Although most avalanches occur in remote areas high in the mountains, accidents involving people do also occur, particularly due to the increase of people taking part in mountain sports during the winter in the Pyrenees. The dendrochronologist stresses that "landslides do not have to be very large to injure or affect people."

However, finding about about meteorological conditions and snow cover that might be triggered may help to prevent avalanches. "It is possible to give a certain degree of advanced warning about the possibility of spontaneous triggering of avalanches, although past events must be studied to learn about these conditions ," Muntán affirms.

Mitigation is done by installing special devices in avalanche areas (passive defence) once the area has been studied, or by artificial triggering of avalanches (active defence).