Spatial and temporal occurrence of past debris flows in the Valais Alps: Results from tree-ring analysis

Murgänge sind in den meisten Berggebieten der Welt verbreitete Massenbewegungen, deren plötzliches und unvorhersehbares Auftreten eine Bedrohung für Transportwege und Siedlungen darstellt. Durch eine Ausbreitung der menschlichen Aktivitäten in Gebiete, die dem Einfluss von Murgängen ausgesetzt sind, ist eine detaillierte Gefahrenbeurteilung unumgänglich. Archivdaten zu vergangenen Ereignissen sind jedoch selten und meistens lückenhaft. Jahrringanalysen wurden bisher ebenfalls nur ausnahmsweise für die Untersuchung von vergangener Murgangaktivität eingesetzt. Das Ziel dieser Studie war es deshalb, Murgangfrequenzen für verschiedene Untersuchungsgebiete mit dendroökologischen Methoden zu erarbeiten, um zur systematischen Datenerfassung für Gefahrenbeurteilungen beizutragen. Zusätzlich wurden die räumliche Ausdehnung und räumliches Verhalten von Ereignissen rekonstruiert. Die Ausbreitung von traumatischen, tangentialen Harzkanalreihen (TRD) in von Murgängen verletzten Bäumen wurde ebenfalls untersucht, um die Kenntnisse über Wachstumsreaktionen betroffener Bäume zu verbessern. In der ersten Studie wurden 28 Verletzungen von acht Lärchen (Larix decidua Mill.) untersucht, welche infolge Murgangaktivität im Feergraben (Simplongebiet, Walliser Alpen) verursacht wurden. Ziel der Untersuchung war, das erste Einsetzen sowie die vertikale und tangentiale Ausbreitung von TRD nach einer Verletzung zu bestimmen. Dazu wurden insgesamt 182 Stammscheiben für die Analysen vorbereitet. Diese Studie stellt die erste Arbeit dar, welche die vertikale und tangentiale Ausbreitung von TRD in Bäumen untersucht, welche durch einen geomorphologischen Prozess und unter natürlichen Bedingungen verletzt wurden. Da die Bäume im Oktober 2000 und November 2004 verletzt wurden, also ausserhalb der lokalen Wachstumsperiode, konnten die ersten TRD erst in den Frühholz-Zellreihen des neuen Jahrringes beobachtet werden. Die vertikale Ausdehnung der TRD betrug im Durchschnitt 74 cm, war jedoch oberhalb der Verletzung deutlich ausgeprägter als unterhalb. Auf der Höhe der Verletzung waren TRD in 18% des nach der Verletzung vital bleibenden Bereichs des Umfangs vorhanden. Daneben konnte eine gewisse Verzögerung beim Einsetzen der ersten TRD mit zunehmender Distanz vom Zentrum der Wunde beobachtet werden. Aus diesem Grund sollten bei künftigen Untersuchungen Bohrproben nahe bei der Verletzung gezogen werden, um Fehler in der Datierung zu vermeiden. Für die Rekonstruktion vergangener Murgangereignisse in zwei Einzugsgebieten des Val Ferret (Walliser Alpen, Schweiz) wurden auf den Kegeln der Reuse de Saleinaz und des Torrent de la Fouly 556 Bohrproben von 278 stark betroffenen Lärchen, Fichten (Picea abies (L.) Karst.) und Föhren (Pinus sylvestris L.) gezogen. Jahrringuntersuchungen erlaubten die Rekonstruktion von 39 Ereignissen auf dem Kegel der Reuse de Saleinaz zwischen 1743 und 2003. Entlang der Murrinne des Torrent de la Fouly konnten 30 Ereignissen zwischen 1862 und 2003 rekonstruiert werden. Obwohl die geologischen und morphologischen Eigenschaften im Einzugsgebiet und im aktuellen Gerinne sehr unterschiedlich sind, bestehen kaum Unterschiede in der Murgangfrequenz: In beiden Wildbächen traten Murgänge demnach in der rekonstruierten Zeitspanne durchschnittlich mindestens einmal alle acht Jahre auf. In beiden Einzugsgebieten steht offensichtlich ausreichend Material für die Auslöung von Murgängen zur Verfügung und die Auslösung und das Auftreten von Ereignissen dürfte daher eher transport- als verwitterungslimitiert sein. Das Ziel der dritten Untersuchung war es, räumlichzeitliche Muster vergangener Murgangaktivität auf dem Kegel des Bruchjis (Blatten b. Naters, Wallis) aufzuzeigen. Basierend auf einer detaillierten geomorphologischen Karte (Massstab 1:1000) wurden 401 offensichtlich gestörte Lärchen und Fichten beprobt. Durch die Datierung von 960 Wachstumsstörungen konnten 40 Ereignisjahre zwischen 1867 und 2005 rekonstruiert werden. Durch die Verknüpfung der Jahrringdaten mit jenen der geomorphologischen Karte konnten elf ehemalige Rinnen auf dem Kegel identifiziert werden. Zusätzlich wurden fünf Muster räumlicher Aktivität vergangener Murereignisse aufgezeigt. Während die älteren Ereignisse vornehmlich Bäume im westlichen Teil des Kegels betrafen, haben sich Murgänge während der 1930er-Jahre in den östlichen Teils des Kegels verlagert. Im letzten Artikel wurden zwei unterschiedliche dendroökologische Ansätze für die Rekonstruktion vergangener Muraktivität kombiniert. Durch die klassische Datierung von Wachstumsstörungen mit dendrogeomorphologischen Methoden konnten auf dem Kegel des Grosse Grabe (Mattertal, Walliser Alpen) 49 Ereignisse zwischen 1782 und 2005 bestimmt werden. Die räumliche Ausbreitung der Ereignisse konnte durch die Positionierung der geschädigten Bäume auf der geomorphologischen Karte rekonstruiert werden. In den Sektoren, in welchen keine Bäume ein Ereignis überlebt haben und neue Bäume auf den Ablagerungen aufwuchsen, wurden die ältesten “Post-Event“-Bäume beprobt und ihr Alter durch das Zählen der Jahrringe ermittelt. Die Anzahl fehlender Jahrringe, die teilweise durch das Verfehlen des Baummarks während der Beprobung oder durch die Beprobung oberhalb des Wurzelanlaufs auftraten, wurde abgeschätzt und im Einzelfall dem errechneten Alter hinzugefügt, um so den Keimungszeitpunkt der Bäume so genau wie möglich zu bestimmen. Die Kombination der beiden Ansätze – dendrogeomorphologische Ereignisrekonstruktion und Bestimmung des Alters von Pionierbäumen – erlaubte eine Abschätzung des minimalen Zeitrahmens, der seit dem letzten Ereignis verstrichen ist, für insgesamt 23 der 29 Murrinnen auf dem Kegel. Es fällt auf, dass die Zeit seit dem letzten Ereignis mit wachsender Distanz von der heutigen Rinne stetig zunimmt. Abschliessend kann festgehalten werden, dass diese Dissertation neue Erkenntnisse zu den Möglichkeiten und Grenzen von Jahrringanalysen in der Murgangforschung liefert. Das Auftreten und die tangentiale und vertikale Ausbreitung von TRD in von Murgängen betroffenen Bäumen konnte erstmals aufgezeigt werden. Daneben wurden Frequenzen vergangener Ereignisse für vier Wildbäche in den Walliser Alpen rekonstruiert. Dekadenfrequenzen deuten darauf hin, dass die Muraktivität vor allem während der warm-feuchten Phase von 1916 bis 1925 überdurchschnittlich hoch war. In Gegensatz dazu konnte seit 1996 eine klare Abnahme in der Anzahl Ereignisse festgestellt werden, die jedoch teilweise auch durch bauliche Massnahmen in den Rinnen verursacht wurde. Während die vorliegende Arbeit wichtige Grundlagen für die Gefahrenbeurteilung in mehreren Wildbäche liefert, fehlen zuverlässige Angaben zur Grösse vergangener Ereignisse weiterhin. Hier sind weitere Studien nötig, um nebst den Veränderungen in der Frequenz auch den Einfluss des sich ändernden Klimas auf die Magnitude künftiger Ereignisse abschätzen zu können.Debris flows are common mass-movement processes in most mountainous regions of the world, where their unpredictable and sudden occurrence represents a major threat to transportation corridors and settlements. Increased anthropogenic activity in regions exposed to debris-flow risk renders a detailed hazard assessment inevitable. However, archival data on past events remains scarce and, most of the time, fragmentary. Similarly, tree-ring analyses have been used only exceptionally to investigate past debris-flow activity. It is therefore the aim of this PhD thesis to reconstruct debris-flow frequencies for different torrents within the Valais Alps (Switzerland) using dendroecological methods in order to (i) contribute to the systematic acquisition of data on past events for hazard assessments, and (ii) to reconstruct the spatial extent and behavior of previous events. Also, the extension of tangential rows of resin ducts (TRD) was assessed in trees injured by debris-flow activity in order to improve knowledge regarding growth reactions of impacted trees. In the first paper, 28 injuries from 8 European larches (Larix decidua Mill.) wounded during debris-flow activity in the Feergraben (Simplon region, Valais Alps) were investigated. The aim of the study was to assess the onset of TRD after wounding, as well as their vertical and tangential extensions. Consequently, 182 stem discs were prepared for analysis. This study represents the first fundamental research on the vertical and tangential extension of TRD in trees that have been impacted by a geomorphic process under natural conditions. As the trees were injured in October 2000 and November 2004, i.e. after the end of the local growing season, TRD could only be observed in the earlywood cell layers of the new growth ring. The vertical extension of TRD averaged 74 cm, but was much greater above rather than below the injury. At the height of the wound, TRD were present in 18% of the circumference remaining vital after the impact. In addition, a certain delay in the onset of the reaction could be observed with distance from the centre of the impact. Therefore, increment cores should be sampled close to the wound in future studies in order to avoid dating mistakes. For the reconstruction of past debris-flow events in two catchments located in Val Ferret (Valais Alps, Switzerland), a total of 556 increment cores from 278 heavily affected European larches, Norway spruces (Picea abies (L.) Karst.) and Scots pines (Pinus sylvestris L.) were sampled from the cones of Reuse de Saleinaz and Torrent de la Fouly. Treering analyses allowed reconstruction of 39 events for the period 1743 to 2003 at Reuse de Saleinaz. Along the debris-flow channel of the Torrent de la Fouly, 30 events were reconstructed for between 1862 and 2003. Although the catchments and channels of the two torrents evince considerable differences in geology and morphology, debris-flow frequencies are very similar with, on average, one event every eight years for the period reconstructed. In both catchments, material is apparently readily available and the triggering and occurrence of events thus seems to be transport- rather than weatheringlimited. The aim of the third study was to assess spatiotemporal patterns of past debris-flow activity on the cone of the Bruchji torrent (Blatten b. Naters, Valais). Based on a detailed geomorphic map (scale 1:1000), 401 obviously disturbed European larches and Norway spruces were sampled. In total, 960 growth disturbances identified in the samples allowed assessment of 40 event years for the period 1867-2005. The combination of tree-ring analysis with geomorphic mapping allowed identification of eleven previously active debris-flow channels. In addition, five patterns of spatial behavior of past events could be assessed. While older events preferentially affect trees in the western part of the cone, the flow regime apparently changed during the mid-1930s towards the eastern part of the cone. In the last part of the study, two different dendroecological approaches were combined for the assessment of past debris-flow activity. Classical dating of growth disturbances with dendrogeomorphological methods allowed reconstruction of 49 events between AD 1782 and 2005. The spatial extent of events was determined by a positioning of disturbed trees on the geomorphic map. For sectors where survivor trees were absent, the oldest post-event trees were sampled and their age assessed counting the number of growth rings. Tree rings were added when the pith was absent on the increment core and to account for missing rings at sampling height. As a result, we were able to approximate the real age of trees with reasonable accuracy. The coupling of two different dendroecological approaches – dendrogeomorphological event reconstruction and assessment of germination dates of successor trees – allowed estimation of the minimum time elapsed since the last debris-flow activity for 23 of 29 channels on the cone. The time elapsed since the last event seems to increase with distance from the current channel. In conclusion, this PhD thesis provides new insights into the possibilities and limitations of tree-ring analyses in debris-flow research. The onset, as well as the tangential and vertical extension of TRD, could, for the first time, be assessed in trees impacted by debris flows. In addition, frequencies of past events could be reconstructed for four torrents in the Valais Alps. 10-year frequencies indicate times of high torrential activity in the past, especially during the warm-wet period between 1916 and 1925. In contrast, a decrease in the number of events can be observed since 1996. However, it needs to be said that a part of this decrease in frequency should be considered as being the result of anthropogenic interventions in the channels, rather than the effect of changing climatic conditions. While this thesis provides valuable data for hazard assessment, reliable data on the magnitude of past events remains scarce. Consequently, further research needs to determine not only changes in the frequency, but also the influence of a changing climate on the magnitude of future events

Tree-ring reconstruction of past debris flows based on a small number of samples—possibilities and limitations

Tree-ring analyses have often been used in the past for the reconstruction of spatiotemporal patterns of previous debris-flow activity, often yielding very precise and extensive data for torrents where information on former activity was largely missing. Unless dendrogeomorphology is slated for multimillion Euro developments, the large sets of tree-ring series that are usually used in these studies render analysis time-consuming and not necessarily very cost-effective. In this study, we present results on past debris flows obtained with 35 Larix decidua Mill. trees growing on the cone of the Torrent de Pétérey (Zinal, Valais, Swiss Alps). It is concluded that studies based on a limited number of samples may yield valuable data on past events, but that the reconstructed frequency remains widely incomplete and indications on the spatial aspects of past events are only fragmentar

Rainfall characteristics for periglacial debris flows in the Swiss Alps: past incidences-potential future evolutions

Based on observational meteorological data since A.D. 1864 and tree-ring records of debris-flow activity, this paper assesses changes in rainfall characteristics and their impact on the triggering of geomorphic events in a high-elevation watershed of the Swiss Alps since the end of the Little Ice Age. No trends are visible in the frequency of heavy rainfall events, but we observe a reduced number of heavy, short-lived rainfalls in summer and a concentration of advective storms is recorded in late summer and early fall since the late 1980s. These changes in triggering meteorological conditions resulted in a cluster of debris flows in the early decades of the twentieth century and a lowering of debris-flow activity since the mid 1990s, and may be mirroring the observed changes in persistent high-pressure systems over the Alps. We also observe intra-seasonal differences in debris-flow system response reflecting the state of the permafrost body in the source area of debris flows, allowing for very small debris flows to be released by limited rainfall inputs (<20mm) in June and July. The same quantities of rain will not trigger debris flows in August or September, when a thick active layer of the permafrost body is capable of absorbing water. With the projected amplitude of climatic change, seasonality, return intervals and volumes of debris flows are likely to be altered. RCM projections based on the IPCC A2 scenario suggest a decrease in heavy summer rainfalls which will most likely result in a (further) reduction of the overall frequency of debris flows, leaving more time for sediment to accumulate in the channel. Such an increase of channel accumulation rates along with the projected destabilization of the steep rock-glacier body is likely, in turn, to exert control ultimately on sediment volumes released from the source areas during future events. Observations from adjacent catchments suggest that extremely large debris flows, beyond historical experience, could occur at the study site and in similar debris-flow systems of the Valais Alps originating from periglacial environment

Spatio-temporal variability in debris-flow activity: a tree-ring study at Geisstriftbach (Swiss Alps) extending back to AD 1736

Past debris-flow activity on the forested cone of the Geisstriftbach torrent (St. Niklaus, Valais, Swiss Alps) was assessed from growth disturbances in old conifer trees, providing a much improved record of past events. The study of 633 tree-ring sequences sampled from 252 European larch (Larix decidua Mill.), Norway spruce (Picea abies (L.) Karst.) and Silver birch (Betula pendula Roth.) trees allowed reconstruction of 53 debris-flow events since AD 1736. The spatial analysis of trees affected during particular events on the geomorphic map allowed for a spatial representation of individual events and a reconstruction of four flow patterns. Based on our results and Siegfried maps, we believe that before the formation of a dogleg near the cone apex in the late 1890s, debris flows preferentially used the channels located in the west-southwestern part of the Geisstriftbach cone. This study contributes to our understanding of debris-flow processes on cones and provides an example of how dendrogeomorphic techniques may help in the reconstruction and understanding of debris flows in Alpine area

Four-dimensional growth response of mature Larix decidua to stem burial under natural conditions

This paper illustrates the effects of abrupt stem burial (burial depth ~0.5m) on tree growth in mature (46±8years) European larch (Larix decidua Mill.) trees. In contrast to the previous research, which was mostly carried out with saplings and on experimental sites where regular aggradation occurred through the transport of sand, this work focuses on the impact of natural, abrupt burial of mature trees with rocks contained in a sandy-silty matrix typical for debris flows in mountainous headwater catchments. The effect of burial is assessed radially and axially within the stem and over almost three decades after burial. The analysis of growth disturbances and their intensity was based on the 143 cross sections (572 growth series) taken at 10-cm intervals from 6 Larix decidua Mill. The results show quite clearly that abrupt burial causes massive suppression of radial growth as compared to pre-event conditions (mean 77%, min 38%, max 92%, SD 7.2%). The trees sampled were unable to resume pre-burial growth rates even after 25years, but recovered to reference growth conditions (as measured in undisturbed, local reference trees) after 15years (min 3years, max 25years, SD 9years). The results differ only insignificantly between different heights along the tree axis and suppression is equally well expressed at different radial positions within the ste

Possible impacts of climate change on debris-flow activity in the Swiss Alps

This study uses a long dataset of past debris flows from eight high-elevation catchments in the Swiss Alps for which triggering conditions since AD 1864 have been reconstructed. The torrents under investigation have unlimited sediment supply and the triggering of debris flows is thus mainly controlled by climatic factors. Based on point-based downscaled climate scenarios for meteorological stations located next to the catchments and for the periods 2001-2050 and 2051-2100, we study the evolution of temperature and rainfall above specific thresholds (10, 20, 30, 40 and 50mm) and durations (1, 2 or 3days). We conclude that the drier conditions in future summers and the wetting of springs, falls and early winters are likely to have significant impacts on the behavior of debris flows. Based on the current understanding of debris-flow systems and their reaction to rainfall inputs, one might expect only slight changes in the overall frequency of events by the mid-21st century, but possibly an increase in the overall magnitude of debris flows due to larger volumes of sediment delivered to the channels and an increase in extreme precipitation events. In the second half of the 21st century, the number of days with conditions favorable for the release of debris flows will likely decrease, especially in summer. The anticipated increase of rainfall during the shoulder seasons (March, April, November, December) is not expected to compensate for the decrease in future heavy summer rainfall over 2 or 3days

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

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 debris- flow 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

Differentiating past events on a cone influenced by debris-flow and snow avalanche activity - a dendrogeomorphological approach

Dendrogeomorphology was used to investigate past events on a cone affected by both debris flows and snow avalanches. We report on results of 520 cores from 251 injured Larix decidua Mill. and Picea abies (L.) Karst. trees sampled on the Birchbach cone (Swiss Alps). Detailed analysis of tree-ring sequences allowed dating of 561 growth disturbances in individual trees for a 252 yr period, extending from 1750 to 2002, which could be attributed to 30 different event years. We then localized the position of rows of traumatic resin ducts (TRDs) within the tree ring so as to assess the intra-seasonal position of damage. In agreement with data on the local growth period, TRDs located at the beginning of the new growth ring were considered the result of avalanche impacts that occurred during the dormant season or in earliest earlywood between late October and early May. In contrast, TRD found in late earlywood or within latewood were considered the result of periglacial debris-flow activity, as these layers of the tree ring are locally formed between July and early October.For nine out of the 30 reconstructed event years, the intra-seasonal timing of TRDs indicated that reactions must be the result of past snow avalanche activity. In 19 other event years, TRDs showed that damage has been caused between July and early October and, thus, through debris flows in the Birchbach torrent. Finally, the spatial patterns of trees showing reactions as a result of particular events were assessed so as to approximate the extent of past debris flows and snow avalanches