Michelle Schneuwly-Bollschweiler

Possibilities and Limitations of Dendrogeomorphic Time-Series Reconstructions on Sites Influenced by Debris Flows and Frequent Snow Avalanche Activity

Abstract Past debris-flow and snow avalanche activity was assessed for the Reiselehnrinne (Tyrol, Austria) using growth disturbances in growth-ring series of 372 Norway spruce (Picea abies (L.) Karst.) trees. Determination of events was performed by analyzing (a) the number and (b) intensity of growth disturbances within tree-ring series and (c) the spatial distribution of affected trees. Differentiation of debris flow from snow avalanche events was based on the intra-annual position of scars, callus tissues or tangential rows of traumatic resin ducts, and on the spatial distribution of trees with simultaneous reactions in the tree-ring series. We introduce a weighting factor to substantiate the dating of past process activity in a comprehensive way and to compare individual events as to their intensity and total number of tree-ring responses. The accuracy of the dendrogeomorphic assessment was then evaluated by comparing the reconstructed event frequency with chronologies available for the Reiselehnrinne. Comparison of tree-ring with historical data demonstrated clearly that the reconstructed event frequency contains the majority of past debris flow and snow avalanche events in the Reiselehnrinne, but that dating of events is not always possible, especially if they are clustered in time or have a limited spread on the cone.

Atmospheric Forcing of Debris Flows in the Southern Swiss Alps

This article addresses the role of large-scale circulation and thermodynamical features in the release of past debris flows in the Swiss Alps by using classification algorithms, potential instability, and convective time scale. The study is based on a uniquely dense dendrogeomorphic time series of debris flows covering the period 1872–2008, reanalysis data, instrumental time series, and gridded hourly precipitation series (1992–2006) over the area. Results highlight the crucial role of synoptic and mesoscale forcing as well as of convective equilibrium on triggering rainfalls. Two midtropospheric synoptic patterns favor anomalous southwesterly flow toward the area and high potential instability. These findings imply a certain degree of predictability of debris-flow events and can therefore be used to improve existing alert systems.

Dendrogeomorphology – Tracking Past Events with Tree Rings

For a realistic hazard assessment, knowledge of past events is of crucial importance. As archival data is generally fragmentary, additional information sources are needed for an appraisal of past and contemporary as well as for the prediction of potential future events. Tree rings represent a very valuable natural archive on past torrential activity as they may record the impact of events in their tree-ring series. In the past few years, dendrogeomorphology has evolved from a pure dating tool to a broad range of applications. Besides the reconstruction of frequencies, tree rings allow - if coupled with spatial positioning methods - the assessment of spread, runout distance, breakout locations or preferred flow path. Similarly, the wide field of applications includes the identification of magnitudes and triggers of debris-flow events if meteorological data is included.

How fast do European conifers overgrow wounds inflicted by rockfall

The capacity of trees to recover from mechanical disturbance is of crucial importance for tree survival but has been primarily investigated in saplings using artificially induced wounds. In this study, mature Larix decidua Mill., Picea abies (L.) Karst. and Abies alba Mill. trees growing on alpine slopes that were wounded by naturally occurring rockfall were analyzed to determine their efficiency in overgrowing wounds. In total 43 L. decidua, P. abies and A. alba trees were sampled. First, 106 samples from 27 L. decidua and P. abies trees were analyzed to reconstruct yearly and overall overgrowth rates. Cross sections were taken at the maximum extension of the injury and overgrowth rates were determined on a yearly basis. Results clearly showed that L. decidua overgrew wounds more efficiently than P. abies with an average overgrowth rate of 19° and 11.8° per year, respectively. The higher on the stem the injury was located, the faster the wound was closed. Young and small trees overgrew wounds more efficiently than older or thicker trees. In contrast, no correlation was observed between injury size or increment before/after wounding and wound closure. Second, cross sections from 16 L. decidua, P. abies and A. alba (54 injuries) were used to assess closure rates at different heights around the injury. Overgrowth was generally smallest at the height of the maximum lateral extension of the injury and increased at the upper and lower end of the injury. The efficiency with which L. decidua closes wounds inflicted by rockfall makes this species highly adapted to sites with this type of mechanical disturbance.

Susceptibility and Triggers for Debris Flows: Emergence, Loading, Release and Entrainment

Debris flows generally form when unconsolidated material becomes saturated and unstable, either on a hillslope or in a stream channel. The process is defined as a moving mass of loose mud, sand, soil, rock, water and air that travels down a slope under the influence of gravity. Flows can carry material ranging in size from clay to boulders, and may contain a large amount of woody debris. Volumes of material delivered by single events vary from less than 100 to more than 100,000m3. Generally three factors are necessary for a debris flow to develop: water, sufficient inclination, and abundant sediment supply.

Dating Methods Overview: Checklist for Practitioners

Practitioners need data on past events for hazard and risk assessments or for planning of mitigation measures. The dating methods described earlier in this book are appropriate to provide at least part of the data required by practitioners. To give an overview on the type of data that can be obtained through the different methods and to display and compare the advantages and limitations of each method in a structured way, an overview (“checklist for practitioners”) is presented.

Frequenz und Fliesshöhen der Plattlaui (Uri), rekonstruiert mithilfe von Jahrringen

Frequency and flow heights in the Plattlaui avalanche couloir (Uri) – a reconstruction based on tree growth rings The Maderaner Valley (Uri) is one of the regions in Switzerland most heavily affected by snow avalanches, and in the past has also regularly been ravaged by devastating events. The present article is intended to contribute to the database of known avalanche events, and to improve understanding of past avalanche activity. In order to do this, the annual rings of spruce trees (Picea abies [L.] Karst.) were investigated for growth disturbances which could indicate the frequency and flow height of past avalanches in two adjacent couloirs of the Plattlaui avalanche track. The dendromorphological analysis was based on growth ring samples taken from a total of 42 spruce trees on a transect at an altitude of 1570 m. The occurrence of past snow avalanches was primarily reconstructed from evidence of injuries and the associated tangential rows of traumatic resin ducts, but also included abrupt growth su...