Debris flows on forested cones – reconstruction and comparison of frequencies in two catchments in Val Ferret, Switzerland

Abstract

Abstract. Debris flows represent a major threat to infrastructure in many regions of the Alps. Since systematic acquisition of data on debris-flow events in Switzerland only started after the events of 1987, there is a lack of historical knowledge on earlier debris-flow events for most torrents. It is therefore the aim of this study to reconstruct the debrisflow activity for the Reuse de Saleinaz and the La Fouly torrents in Val Ferret (Valais, Switzerland). In total, 556 increment cores from 278 heavily affected Larix decidua Mill., Picea abies (L.) Karst. and Pinus sylvestris L. trees were sampled. Trees on the cone of Reuse de Saleinaz show an average age of 123 years at sampling height, with the oldest tree aged 325 years. Two periods of intense colonization (the 1850s-1880s and the 1930s-1950s) are observed, probably following high-magnitude events that would have eliminated the former forest stand. Trees on the cone of Torrent de la Fouly indicate an average age of 119 years.As a whole, tree-ring analyses allowed assessment of 333 growth disturbances belonging to 69 debris-flow events. While the frequency for the Reuse de Saleinaz study site comprises 39 events between AD 1743 and 2003, 30 events could be reconstructed at the Torrent de la Fouly for the period 1862-2003. Even though the two study sites evince considerably different characteristics in geology, debris-flow material and catchment morphology, they apparently produce debris flows at similar recurrence intervals. We suppose that, in the study region, the triggering and occurrence of events is transport-limited rather than weathering-limited.

Tree-Ring Reconstruction of Debris-Flow Events Leading to Overbank Sedimentation on the Illgraben Cone (Valais Alps, Switzerland)

“…A large majority of these events does not leave the actual channel and, based on our reconstructions, it seems that debris flows only very exceptionally left the canyon over the last 200 years. This is why the number of reconstructed events might appear quite small as compared to other torrents analyzed with tree-ring analyses in the Valais Alps [17][18][19][20][21][22]. Since the beginning of the systematic debris-flow observation in the Illgraben torrent in the year 2000 [32,33] It is also obvious from the reconstruction that debris-flow surges did not leave the canyon and affect trees located on the cone itself since the 1950s, when several debris flows left GD in trees located on its western part in 1951, 1953, 1955 and 1957.…”

“…Given that a tree is directly impacted by a debris flow, tree-ring dating can pinpoint the year or sometimes even the season in which the disturbance occurred. Previous dendrogeomorphological studies primarily focused on (i) the reconstruction of magnitudes and/or frequencies [15][16][17], (ii) spatial patterns of past debris flows on forested cones [18,19], (iii) recolonization of cones with trees following large and devastating events [20] or (iv) the comparison of reconstructed debris-flow data with archival records on flooding in neighboring rivers [21]. Tree-ring evidence was also used to (v) assess changes in the seasonality of debris-flow activity and to (vi) identify possible impacts of a future greenhouse climate on the frequency or magnitude of events [19,22].…”

“…(3). Growth disturbances of trees affected by a debris-flow event (modified after [17,42]). bifurcation of the abandoned from the currently active canyon.…”

Debris-flow activity in five adjacent gullies in a limestone mountain range

“…Apart from archival data, dendrogeomorphology has often been used to derive temporal and spatial information of previous debris-flow activity (Alestalo, 1971;Strunk, 1992;Baumann and Kaiser, 1999;Jakob, 2010;Šilhán, 2012;Tumajer and Treml, 2013;Stoffel and Corona, 2014). Hereby investigations of growth failures in year rings of trees affected by debris-flows, allowing deduction of information on past events (Hupp, 1984;Strunk, 1991) is combined with information on the spatial distribution of the affected trees to estimate deposition areas of past debris-flows (Bollschweiler and Stoffel, 2007;. Dendrogeomorphology was repeatedly used to reconstruct event magnitudes of (flash) floods based on the analysis of peak flow scars (e.g.…”

Hydrogeomorphic processes and torrent control works on a large alluvial fan in the eastern Italian Alps

“…The dating of the events through dendrogeochronology is applicable on alluvial fans with forests in relatively natural condition. In the European Alps, this requirement is often met on alluvial fans covered by protection forests in the upper valleys (Strunk, 1992;Baumann and Kaiser, 1999;Stefanini and Ribolini, 2003;Bollschweiler and Stoffel, 2007), whereas farming and urban areas often occupy alluvial fans located at lower elevations along the lower valleys. The reconstruction of the frequency of past events from historical archives is particularly fruitful where human activities are present and there is therefore documentation of the damage caused to buildings and land by floods and debris flows (Guzzetti et al, 1994;Barnikel, 2004;Marchi and Cavalli, 2007).…”