Dendrogeomorphic reconstruction of past snow avalanche events in Bâlea glacial valley–Făgăraş massif (Southern Carpathians), Romanian Carpathians

Abstract

Snow avalanches are a major hazard in the Southern Carpathians (Romanian Carpathians) and cause damage to transportation routes or infrastructure and losses of human lives. In these mountains, the scientific understanding of avalanches is still poor. The main objectives of this study were to reconstruct spatio-temporal patterns of avalanche events, to complement written records in the Bâlea glacial valley (located in the central part of northern slope of the Făgăraş massif) and to identify the winter weather conditions responsible for avalanche occurrence, using a dendrogeomorphologic approach. The analysis was based on tree-ring series from 293 affected Picea abies trees in four avalanche tracks. Our results from tree-ring records yielded 24 and 17 snow avalanches in the 1867–2012 chronology in first and second track, 24 and 12 snow avalanches in the 1870–2012 chronology in third and in fourth track.The tree-ring based analysis of the 586 extracted increment cores allowed the identification of 853 growth disturbances related to past avalanche activity. The most frequent response is found as reaction wood (64.1%), followed by abrupt growth changes (20.6%), traumatic resin ducts (13%) and callus tissue (2.3%). A total of 77 event-years with AAI > 10% were reconstructed. The reconstruction revealed five high-magnitude avalanches (AAI > 30%), 25 medium-magnitude avalanches (20% ≤ AAI ≥ 30%) and 47 small-magnitude avalanches (10% ≤ AAI ≥ 20%). The return period values vary between 16.7 and 23.6 years. Based on the reconstructed chronologies, our analyses identified 13 cases of synchronicity of events with AAI > 10%. This synchronicity indicates the decisive role of local weather conditions, which are responsible for the occurrence of extreme snow avalanches. The weather conditions triggering extreme avalanche correspond with high amounts of snowfall and consequently thick snowpack, even if the largest snowfalls are not necessarily associated with extreme avalanches.