Testing dendrogeomorphic approaches and thresholds to reconstruct snow avalanche activity in the Făgăraş Mountains (Romanian Carpathians)

Snow avalanches are a widespread natural phenomenon in steep mountain environments, where they modulate landscapes and frequently disturb forest stands. Such disturbances in trees have been used since the 1970s to retrospectively date avalanches, study their extent and reach, as well as to document their triggers. Although virtually every dendrogeomorphic paper is still based on the concepts established by Shroder (1978), important methodological improvements have been achieved in the field ever since and more particularly over the last decade. This study therefore reports on recent methodological progress and employs three different approaches (i.e. Shroder index value and Kogelnig-Mayer weighted index value) and different sets of signals in trees (i.e. inclusion of tangential rows of traumatic resin ducts as evidence of past avalanching) to record snow avalanche activity. Using 238 increment cores from 105 Picea abies (L.) Karst trees which colonize a snow avalanche path in the Romanian Carpathians, we illustrate possibilities and limitations of the different approaches for the period covered by the chronologies (1852–2013). In addition, we sampled 30 undisturbed P. abies trees from a forest stand north of the avalanche path, where no geomorphic disturbance was identified, so as to build a reference tree-ring chronology. The three avalanche chronologies constructed with the disturbed trees allow identification of past process activity, but results differ quite considerably in terms of avalanche frequency, number of reconstructed events and their temporal distribution. Depending on the approach used, 15 to 20 snow avalanches can be reconstructed, with the best results being obtained in the dataset including tangential rows of traumatic resin ducts. The addition of this anatomical feature, formed after mechanical impact enlarges the number of growth disturbances by 43.5%, and can thus explain the increase of reconstructed avalanches by one-third as compared to the results of the chronology using the “conventional” Shroder approach.     

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. Copyright © 2006 John Wiley & Sons, Ltd.     

Diversification of landslide areas as a means for reducing noise in dendrogeomorphic dating

The result of tree-ring-based reconstruction of past landslide events is often the development of a single total chronology. This approach can be very effective for small homogeneous landslides. However, compiling chronological data from heterogeneous (often independent) zones of large complex landslide areas into one chronology can induce over- or underestimation of some events, resulting in lowered reliability of the reconstruction. The solution for elimination of this effect can lie in the diversification of complex landslide areas into homogeneous zones with separate analyses. The aim of this study was to quantify the effect of this separation on detected slope movement events and to define parameters whose investigation could distinguish events (sliding) from noise (creeping).For this purpose, 412 tree-ring series from 206 disturbed common spruce (Picea abies (L.) Karst.) occupying complex landslide areas were dendrogeomorphically analysed. The landslide area was divided into five homogeneous zones using geomorphic mapping, LiDAR-based DEM and geophysical sounding (ERT). Five events (verified in individual zones) were detected in the total chronology. Two extra events in the total chronology (28.6%) were considered noise. Moreover, two zonal events were detected but not recorded in the total chronology. This indicates that the noise in the total chronology of the complex landslide area could reach more than a quarter of dated events. Next, true slide events and noise (caused by creep) were differentiated in the structure of growth disturbances (reaction wood vs. abrupt growth suppression) and their proportion in event reconstruction, spatial patterns of trees containing slope movement signals, and the character of triggers. Thus, for better filtering of noise from signals in tree-ring-based chronologies of landslides, not only observations of dendrogeomorphic index values but also the morphology of landslides and characteristics of dated processes must be considered.     

Modelling late-holocene snow-avalanche activity: Incorporating a new approach to lichenometry

Where snow avalanches descend steeply into large rivers, displacing bedload, avalanche boulder-ramparts may accumulate, retaining a record of late-Holocene snow-avalanche frequency. The age frequency of surface boulders on 12 such features in upper Jostedalen, southern Norway, was investigated using the size-frequency distribution of lichens. A model was constructed to simulate the influence of variations in avalanche frequency since AD 1325 on lichen-size-frequency distributions. Using this modelling approach it is not possible to define a unique pattern of avalanche activity to account for the observed lichen-size-frequency distributions, but it is possible to place strict limits on the range of scenarios that are acceptable. The results suggest that maximum avalanche activity occurred during the 19th century, after the peak of the Little Ice Age. This conflicts with historical records of avalanche damage to property at lower altitude in nearby valleys, which may reflect avalanche activity only during a short period of extreme climatic conditions. Close agreement between the records of snow avalanches and debris flows suggests that both reflect periods of high winter snowfall and rapid spring melting rather than low temperature. Future ‘greenhouse warming’ may therefore result in increased avalanche activity in southern Norway.     

How to improve dating quality and reduce noise in tree-ring based debris-flow reconstructions

Tree rings have been used to reconstruct the occurrence of debris flows and other mass movements. Identification of past activity was typically based on the presence of growth anomalies in trees, with a focus on scars, stem tilting, trunk burial or apex decapitation. Clear guidelines have been missing so far and the dating of events has only rarely been based on thresholds so as to distinguish signal from noise. In a similar way, the spatial distribution of affected trees has not normally been considered in mass movement reconstructions, and was at best used as a subjective exclusion factor. This study therefore aims at improving dating quality of and reducing noise in debris-flow time series. Based on a dataset of 803 increment cores (385 trees) affected by debris flows, we reconstruct event histories using (i) a classical experts' approach, (ii) a weighted index (W_it) of responding trees as well as (iii) Moran's I and Getis–Ord Local Gi indices. We identify similarities and differences in results and then investigate subsets of the tree-ring sample to define ideal sampling positions on debris-flow cones and guidelines for sample depth.     

Mass and Momentum Balance Model of a Mixed Flowing/Powder Snow Avalanche

This work uses a one-dimensional, depth averaged model to compute the massbalance of a mixed flowing/powder snow avalanche. This model is comprisedof three basic components: the dense flowing avalanche, the powder cloud anda turbulent wake. The dynamics of a mixed avalanche is strongly dependent onthe interaction between the components and also on the snow cover and ambientair, in particular the exchange or entrainment of snow and air mass. Therefore, animportant first step for modelling mixed avalanches is a basic understanding of thenature of these mass exchanges. In this paper, the governing equations of mass,momentum and turbulent energy are briefly presented. Numerical simulations wererun for three avalanche tracks – Aulta, Galtür and Vallée de la Sionne – for whichdata from real snow avalanche events exist. Based on the results, conclusions weredrawn regarding the parameterisation of the mass exchanges. The mass balances forthese three contrasting avalanches are presented.     

Snow mechanics and avalanche formation: field experiments on the dynamic response of the snow cover

Knowledge about snow mechanics and snow avalanche formation forms the basis of any hazard mitigation measures. The crucial point is the snow stability. The most relevant mechanical properties - the compressive, tensile and shear strength of the individual snow layers within the snow cover - vary substantially in space and time. Among other things the strength of the snow layers depends strongly on the state of stress and the strain rate. The evaluation of the stability of the snow cover is hence a difficult task involving many extrapolations.To gain insight in the release mechanism of slab avalanches triggered by skiers, the skier's impact is measured with a load cell at different depths within the snow cover and for different snow conditions. The study focused on the effects of the dynamic loading and of the damping by snow compaction. In accordance with earlier finite-element (FE) calculations the results show the importance of the depth of the weak layer or interface and the snow conditions, especially the sublayering.In order to directly measure the impact force and to study the snow properties in more detail, a new instrument, called rammrutsch was developed. It combines the properties of the rutschblock with the defined impact properties of the rammsonde. The mechanical properties are determined using (i) the impact energy of the rammrutsch and (ii) the deformations of the snow cover measured with accelerometers and digital image processing of video sequences. The new method is well suited to detect and to measure the mechanical processes and properties of the fracturing layers. The duration of one test is around 10 minutes and the method seems appropriate for determining the spatial variability of the snow cover. A series of experiments in a forest opening showed a clear difference in the snow stability between sites below trees and ones in the free field of the opening.     

Dendrogeomorphic chronologies of landslides: Dating of true slide movements?

Dendrogeomorphic chronologies of landslide movements are frequently used to investigate past landslide activity. Slide areas are often affected by other slope movements (e.g. creep) simultaneously. Trees growing on landslides record all types of ground movements, which potentially creates significant noise in tree ring based chronologies of landslide movements. The effect of creep movements on dendrogeomorphic landslide chronologies was evaluated in a block‐type landslide in the south‐western foreland of the Orlické hory Mountains. In total, 272 trees (Picea abies and Fagus sylvatica) were sampled (1088 increment cores) on the sub‐horizontal surfaces of rotated slide blocks, which were presumably created only by slide movements, and on the steep internal scarps separating landslide blocks, which were presumably created and affected by a combination of slide and creep movements. Ground movements were dated based on growth disturbances identified in an analysis of eccentric tree growth. The trees growing on the internal landslide scarps separating the rotated blocks exhibited a significantly higher number and frequency of growth disturbances than those growing on the sub‐horizontal block surfaces. All eight dated block surface movements were also identified on the internal scarps. Creep‐based events represented as many as 70% of the dated movement events on the internal scarps. Varying the I_t thresholds did not filter out more than 40% of the noise without significantly reducing the number of true dated slide events. A significant difference was observed between the ability of P. abies and F. sylvatica to record ground movements by eccentric growth. Probably due to its shallower roots (and weaker anchoring of the tree to landslide blocks), P. abies appears to be more sensitive to surficial ground movement, which potentially increases the proportion of dated creep events (noise). Thus, the careful selection of sampled tree species with different physiologies should be considered during dendrogeomorphic field sampling. Copyright © 2017 John Wiley & Sons, Ltd.     

Functioning of the avalanche starting zones which undergo snow-transport by wind: Field observations and computer modeling

For two years, three French and Swiss laboratories have been making field observations and measurements on two high altitude slopes in a Northern French Alps site. The aim of this work is to study the functioning of the avalanche sites which, in their starting zones, undergo snow-transport by wind. The experimental site is located in the French Alps, at 2,800 m, above Grenoble. It is an open area, equipped with an automatic meteorological station and an altitude laboratory. The two slopes that are studied face East. One of them is artificially released but the other has a natural avalanche activity. The investigations concern:

-snow deposition in avalanche starting zones;

-temporal evolution of the snowpack characteristics;

-avalanche release.

For the field observations and measurements, continuous recording of the meteorological conditions on the site, photogrammetrical techniques and two snow depth profiles, as well as stratigraphical snow profiles and video are used. The computer modeling is based on existing computer models developed by the CEMAGREF-Nivologie (ELSA) and the CEN/Météo-France (SAFRAN-CROCUS-MEPRA), which analyse the snowpack and its stability. The field observations and measurements aim at improving snow-transport by wind modeling modules, in order to improve their whole analysis.     

Bayesian optimal design of an avalanche dam using a multivariate numerical avalanche model

For snow avalanches, passive defense structures are generally designed by considering high return period events. However, defining a return period turns out to be tricky as soon as different variables are simultaneously considered. This problem can be overcome by maximizing the expected economic benefit of the defense structure, but purely stochastic approaches are not possible for paths with a complex geometry in the runout zone. Therefore, in this paper, we include a multivariate numerical avalanche propagation model within a Bayesian decisional framework. The influence of a vertical dam on an avalanche flow is quantified in terms of local energy dissipation with a simple semi-empirical relation. Costs corresponding to dam construction and the damage to a building situated in the runout zone are roughly evaluated for each dam height–hazard value pair, with damage intensity depending on avalanche velocity. Special attention is given to the poor local information to be taken into account for the decision. Using a case study from the French avalanche database, the Bayesian optimal dam height is shown to be more pessimistic than the classical optimal height because of the increasing effect of parameter uncertainty. It also appears that the lack of local information is especially critical for a building exposed to the most extreme events only. The residual hazard after dam construction is analyzed and the sensitivity to the different modelling assumptions is evaluated. Finally, possible further developments of the approach are discussed.     

Little Ice Age glaciers in the Balkans: low altitude glaciation enabled by cooler temperatures and local topoclimatic controls

Moraine ridges are present in the highest cirques of the Durmitor massif in Montenegro and post‐date the widespread Pleistocene moraines of this area. Lichenometry suggests that at least eight glaciers were present in the 19th century and correlate with the culmination of the Little Ice Age in the European Alps. Cooler temperatures combined with local topoclimatic controls, including windblown and avalanching snow as well as shading, were crucial for the formation and survival of these glaciers below the regional equilibrium–line altitude. The resultant regional equilibrium line altitude (ELA) was positioned close to the highest peaks between 2400 and 2500 m, with local controls such as avalanche, windblown snow and shading depressing the ELA in the northern cirques to 2130–2210 m. This ELA position was very low for this latitude and lower than for most glaciers in the European Alps at any time during the Holocene, and even equivalent to many Alpine glaciers during the Younger Dryas. Today, one glacier still survives in Montenegro, in a deep northeast‐facing cirque characterized by the largest combined areas of potential avalanche and windblown snow. Copyright © 2009 John Wiley & Sons, Ltd.     

Dendrogeomorphic reconstruction of past debris‐flow activity using injured broad‐leaved trees

Tree‐ring records from conifers have been regularly used over the last few decades to date debris‐flow events. The reconstruction of past debris‐flow activity was, in contrast, only very rarely based on growth anomalies in broad‐leaved trees. Consequently, this study aimed at dating the occurrence of former debris flows from growth series of broad‐leaved trees and at determining their suitability for dendrogeomorphic research. Results were obtained from gray alder (Alnus incana (L.) Moench), silver birch and pubescent birch (Betula pendula Roth and Betula pubescens Ehrh.), aspen (Populus tremula L.), white poplar, black poplar and gray poplar (Populus alba L., Populus nigra L. and Populus x canescens (Ait.) Sm.), goat willow (Salix caprea L.) and black elder (Sambucus nigra L.) injured by debris‐flow activity at Illgraben (Valais, Swiss Alps). Tree‐ring analysis of 104 increment cores, 118 wedges and 93 cross‐sections from 154 injured broad‐leaved trees allowed the reconstruction of 14 debris‐flow events between AD 1965 and 2007. These events were compared with archival records on debris‐flow activity at Illgraben. It appears that debris flows are very common at Illgraben, but only very rarely left the channel over the period AD 1965–2007. Furthermore, analysis of the spatial distribution of disturbed trees contributed to the identification of six patterns of debris‐flow routing and led to the determination of preferential breakout locations of events. The results of this study demonstrate the high potential of broad‐leaved trees for dendrogeomorphic research and for the assessment of the travel distance and lateral spread of debris‐flow surges. Copyright © 2010 John Wiley & Sons, Ltd.     

Geomorphic influence on small glacier response to post‐Little Ice Age climate warming: Julian Alps,Europe

The evolution of glaciers and ice patches, as well as the equilibrium‐line altitude (ELA) since the Little Ice Age (LIA) maximum were investigated in the Julian Alps (south‐eastern European Alps) including ice masses that were previously unreported. Twenty‐three permanent firn and ice bodies have been recognized in the 1853 km² of this alpine sector, covering a total area in 2012 of 0.385 km², about one‐fifth of the area covered during the LIA (2.350 km²). These features were classified as very small glaciers, glacierets or ice patches, with major contribution to the mass balance from avalanches and wind‐blown snow. Localized snow accumulation is also enhanced in the area due to the irregular karst topography. The ice masses in the region are at the lowest elevations of any glaciers in the Alpine Chain, and are characterized by low dynamics. The ELAs of the two major LIA glaciers (Canin and Triglav) have been established at 2275 ± 10 m and 2486 ± 10 m, respectively, by considering the reconstructed area and digital elevation model (DEM) and using an accumulation area ratio (AAR) of 0.44 ± 0.07, typical of small cirque glaciers. Changes in the ELA and glaciers extension indicate a decoupling from climate. This is most evident in the smallest avalanche‐dominated ice bodies, which are currently controlled mainly by precipitation. The damming effect of moraine ridges and pronival ramparts at the snout of small ice bodies in the Julian Alps represents a further geomorphological control on the evolution of such ice masses, which seem to be resilient to recent climate warming instead of rapidly disappearing as should be expected. Copyright © 2016 John Wiley & Sons, Ltd.     

Sensitivity of the French Alps snow cover to the variation of climatic variables

In order to study the sensitivity of snow cover to changes in meteorological variables at a regional scale, a numerical snow model and an analysis system of the meteorological conditions adapted to relief were used. This approach has been successfully tested by comparing simulated and measured snow depth at 37 sites in the French Alps during a ten year data period. Then, the sensitivity of the snow cover to a variation in climatic conditions was tested by two different methods, which led to very similar results. To assess the impact of a particular “doubled CO₂” scenario, coherent perturbations were introduced in the input data of the snow model. It was found that although the impact would be very pronounced, it would also be extremely differentiated, dependent on the internal state of the snow cover. The most sensitive areas are the elevations below 2400 m, especially in the southern part of the French Alps.     

Experimental Determination of Constitutive Equations for Dense and Dry Avalanches: Presentation of the Set-Up and First Results

In order to study dry-snow avalanches, an intermediate scale device was built atthe “col du lac blanc”, a pass near the Alpe d'Huez ski resort, in the French Alps.It consists of a flow channel that can be artificially fed with snow through a hopper.The channel has been instrumented so as to measure height, normal and shear stressesat the bottom, and velocity profile within the flow. Through the variations in channelinclination and feeding rate, access to a wide range of slopes and heights is available.These measurements have been taken for a few dry snow flows at the end of the2001–2002 winter. The first results are presented here.     

Drought variations in Almaty (Kazakhstan) since AD 1785 based on spruce tree rings

While dendroclimatic studies have extended the knowledge of drought variations in Tien Shan, these have been almost exclusively based on tree-ring data from Tien Shan in China. We present a drought reconstruction for Almaty based on a tree-ring width chronology developed from sites of the Schrenk spruce in Tien Shan, Kazakhstan. The drought reconstruction, spanning AD 1785–2014, was developed by calibrating tree-ring series with the mean August to January standardized precipitation evapotranspiration index (SPEI). The drought reconstruction was verified with independent data and accounts for 41.9 % of the actual SPEI variance during the common period. The drought reconstruction compares well with some tree-ring-based drought/precipitation reconstructions from Western Tien Shan and reveals the large-scale drought signals of Western Tien Shan. The wavelet analysis indicates the existence of some decadal (60 and 11 years) and interannual (2.0–4.0 years) periodicities, which may potentially be the fingerprints of large-scale land–atmosphere–ocean circulations. This study provides the first long-term drought reconstruction and drought assessment for Almaty and will aid in future plans to address climate change of Kazakhstan.     

Dendrogeomorphic dating of rockfalls on low‐latitude,high‐elevation slopes: Rodadero,Iztaccíhuatl volcano,Mexico

Dynamics and rates of rockfalls have been repeatedly studied in mountain environments with archival records as well as lichenometric, radiocarbon or dendrogeomorphic approaches. In this study, we test the potential of conifers growing at a low‐latitude, high‐elevation site as a dendrogeomorphic tool to reconstruct to calendar dates associated rockfall activity. Analysis is based on tree‐ring records of Mexican mountain pine (Pinus hartwegii Lindl.) growing at timberline [~4000 m above sea level (a.s.l.)] and at the runout fringe of a north–northeast (NNE)‐facing slope of the dormant Iztaccíhuatl volcano (Mexico), which is subject to frequent rockfalls. The potential and limitations of tree‐ring data are demonstrated based on 67 rockfall impacts dated in the increment‐ring series of 24 trees since ad 1836. While findings of this paper are site‐specific, the study clearly shows the potential of dendrogeomorphic approaches in extra‐Alpine, low‐latitude environments and for the understanding of rockfall processes in space and time. Copyright © 2011 John Wiley & Sons, Ltd.     

Dating of rockfall damage in trees yields insights into meteorological triggers of process activity in the French Alps

Rockfall release is a rather unpredictable process. As a result, the occurrence of rockfall often threatens humans and (infra)structures. The assessment of potential drivers of rockfall activity therefore remains a major challenge, even if the relative influence of rainfall, snowmelt, or freeze–thaw cycles has long been identified in short-term monitoring projects. In the absence of longer-term assessments of rockfall triggers and possible changes thereof, our knowledge of rockfall dynamics remains still lacunary as a result of the persisting scarcity of exhaustive and precise rockfall databases. Over the last decades, several studies have employed growth disturbances (GDs) in tree-ring series to reconstruct rockfall activity. Paradoxically, these series were only rarely compared to meteorological records. In this study, we capitalize on the homogeneity of a centennial-old reforestation plot to develop two reconstructions – R1 including only growth suppressions, and R2 based on injuries – with limited biases related to decreasing sample size and changes in exposed diameters back in time. By doing so, our study also and quite clearly highlights the large potential that protection forests have in terms of yielding reliable, multidecadal rockfall reconstructions. From a methodological perspective, we find no synchronicity between R1 and R2, as well as an absence of meteorological controls on rockfall processes in R1. This observation pleads for a careful selection of GDs in future reconstructions. In terms of process dynamics, we demonstrate that summer intense rainfall events (>10 mm day⁻¹) are the main drivers for rockfall activity at our study site. Despite the stringency of our detection procedure, correlations between rockfall activity and meteorological variables remain comparable to those reported in previous studies, as a result of the complexity and multiplicity of triggering factors. We therefore call for a more systematic coupling of tree-ring analysis with rockfall and microclimatic monitoring in future studies. © 2020 John Wiley & Sons, Ltd.