Lichens: Lichenometric dating of diachronous surfaces

This paper describes software to accompany McCarroll, D. 1993. ‘Modelling late-Holocene snow-avalanche activity: incorporating a new approach to lichenometry’, Earth Surface Processes and Landforms 18 , 527–539; and McCarroll, D., Matthews, J. A. and Shakesby, R. A. 1995. ‘Late-Holocene snow-avalanche activity in southern Norway: interpreting lichen-size-frequency distributions using an alternative to simulation modelling’, Earth Surface Processes and Landforms 20 , 465–471.     

Effects of the Little Ice Age on avalanche boulder tongues in the French Alps (Massif des Ecrins)

Lichens of the subspecies Rhizocarpon geographicum s.l were measured on 25 avalanche boulder tongues in the Massif des Ecrins to elucidate the Little Ice Age history of avalanche activity. Results show: (1) an increase of lichen size from the median to the distal zone of deposits, and a decrease from the edges to the centre; (2) three types of lichen settlement. From the uppermost to the median zone, lichens are absent, because avalanche activity is very active. Down‐slope, lichens occur in two different zones: the median zone is colonized by 5–20 mm size lichens on sides of blocks protected from the abrasional action of avalanches, while in the distal zone lichen diameters are largest (>30 mm) and occur on all sides of the blocks. The spatial distribution of the lichens and their size according to elevation make it possible to distinguish different phases during which avalanche activity has increased. At high elevation, the avalanche activity was at a maximum before ad 1650 and between ad 1730 and 1830. During these two periods avalanches had suf?cient magnitude to reach the basal zone of the deposits. At low elevation since ad 1650 the magnitude and frequency of avalanches have declined. Copyright © 2004 John Wiley & Sons, Ltd.     

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.     

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 Mass Quantification and Avalanche Victim Search by Ground Penetrating Radar

Ground penetrating radar (GPR) systems can be used in many applications of snow and ice research. The information from the GPR is used to identify and interpret layers, objects and different structures in the snow. A commercially available GPR system was further developed to work in the rough environment of snow and ice. The applied GPR is a 900 MHz system that easily reaches snow depths of up to 10 meters. The system was calibrated in the course of several manual snow depth measurements during each survey. The depth resolution depends on the snow type and is around ±0.1 m. The GPR system is carried alongside a line of interest and is triggered by an odometer wheel at regular adjustable steps. All equipment is mounted in a sledge and is pulled by a snowmobile over the snow surface. This setup allows for an efficient coverage of several kilometers of terrain profiles. The radar profiles give a real time two-dimensional impression of structures and objects and the interface between snow and the underlying ground. The actual radar profile is shown on a screen on the sledge allowing the immediate marking of objects and structures. During the past three years the instrument was successfully used for the study of snow distributions, for the detection of glacier crevasses under the snow cover, and for the search of avalanche victims in avalanche debris. The results show the capability of the instrument to detect persons and objects in the snow cover. In the future, this device may be a new tool for avalanche rescue operations. Today, the size and weight of the system prevents the accessing of very steep slopes and areas not accessible to snowmobiles. Further developments will decrease the size of the system and make it a valuable tool to quantify snow masses in avalanche release zones and run-out areas.     

Schmidt‐hammer exposure‐age dating (SHD) of snow‐avalanche impact ramparts in southern Norway: approaches,results and implications for landform age,dynamics and development

Schmidt‐hammer exposure‐age dating (SHD) was applied to the problem of dating the diachronous surfaces of five distal river‐bank boulder ramparts deposited by snow avalanches plunging into the Jostedøla and Sprongdøla rivers in the Jostedalsbreen region of southern Norway. Approaches to local high‐precision linear age calibration, which controlled in different ways for boulder roundness, were developed. The mean age (SHD_mean) and the maximum age (SHD_max) of surface boulders were estimated for whole ramparts, crests and distal fringes. Interpretation was further assisted by reference to R‐value distributions. SHD_mean ages (with 95% confidence intervals) ranged from 520 ± 270 years to 5375 ± 965 years, whereas SHD_max ages (expected to be exceeded by <5% of surface boulders) ranged from 675 to 9065 years. SHD ages from the Jostedøla ramparts tended to be older than those associated with the Sprongdøla, rampart crests were younger than the respective distal fringes, and use of relatively rounded boulders yielded more consistent SHD ages than angular boulders. The SHD_mean ages indicate differences in recent levels of snow‐avalanche activity between ramparts and provide insights into rampart dynamics as boulders are deposited on rampart crests and, in smaller numbers, on the distal fringes. SHD_max ages provide minimum age estimates of rampart age (i.e. the time elapsed since the ramparts began to form) and suggest that at least some of the ramparts have been developing since the early Holocene. Copyright © 2015 John Wiley & Sons, Ltd.     

Disentangling the impacts of exogenous disturbances on forest stands to assess multi-centennial tree-ring reconstructions of avalanche activity in the upper Goms Valley (Canton of Valais,Switzerland)

The purpose of dendrogeomorphic analyses is to amplify the signal related to the geomorphic process under investigation, and to minimize the noise induced by other signals in the tree-ring series. Yet, to date, no study accounts specifically for interferences induced by climate conditions or exogenous disturbances and which can, potentially, affect the quality of tree-ring based process reconstructions. In this paper, we develop a specific procedure allowing evaluation of the quality of reconstructions in five avalanche paths at Oberwald (Swiss Alps). The study is based on possible interferences between snow avalanches, climatic conditions and ecological signals in the tree-ring series. Analysis of past events was based on tree-ring series from 564 heavily affected, multi-centennial European larch trees (Larix decidua Mill.) growing near or next to the avalanche paths. A total of 2389 growth disturbances, such as scars, tangential rows of traumatic resin ducts, compression wood as well as abrupt growth suppressions or releases, were identified in the samples, indicating 43 destructive snow avalanches since AD 1780. At the same time, 31 potential events, which were detected with the conventional Shroder index value, were rejected from the final reconstruction due to potentially strong interferences between the different signals. This high rejection rate underlines the necessity to systematically–and carefully–discriminate ecological and climatic noise from avalanche-related disturbances. This discrimination is even more so crucial as a significant proportion of dendrogeomorphic studies in the Alps are based on L. decidua trees which are cyclically affected by larch budmoth outbreaks.     

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.     

Measurements of powder snow avalanche- nature -

Snow cloud growth rates of powder snow avalanche were obtained with analysing the pictures recorded in Ryggfonn, Norway. Although results showed wide scattering, as far as the data obtained in runout zone are concerned, they roughly agreed with the water tank experiments by Beghin and Olagne (1991).Air movement in snow avalanche cloud was measured with an ultra-sonic anemometer in Kurobe Canyon, Japan. It showed rising current existed near the front and downward at the trail. Comparing with drifting snow threshold and particle suspension criterion, entrainment, suspension and deposition of snow particles in the snow cloud were discussed.     

Large debris avalanches and associated eruptions in the Holocene eruptive history of Shiveluch Volcano, Kamchatka, Russia

 Shiveluch Volcano, located in the Central Kamchatka Depression, has experienced multiple flank failures during its lifetime, most recently in 1964. The overlapping deposits of at least 13 large Holocene debris avalanches cover an area of approximately 200 km² of the southern sector of the volcano. Deposits of two debris avalanches associated with flank extrusive domes are, in addition, located on its western slope. The maximum travel distance of individual Holocene avalanches exceeds 20 km, and their volumes reach ∼3 km³. The deposits of most avalanches typically have a hummocky surface, are poorly sorted and graded, and contain angular heterogeneous rock fragments of various sizes surrounded by coarse to fine matrix. The deposits differ in color, indicating different sources on the edifice. Tephrochronological and radiocarbon dating of the avalanches shows that the first large Holocene avalanches were emplaced approximately 4530–4350 BC. From ∼2490 BC at least 13 avalanches occurred after intervals of 30–900 years. Six large avalanches were emplaced between 120 and 970 AD, with recurrence intervals of 30–340 years. All the debris avalanches were followed by eruptions that produced various types of pyroclastic deposits. Features of some surge deposits suggest that they might have originated as a result of directed blasts triggered by rockslides. Most avalanche deposits are composed of fresh andesitic rocks of extrusive domes, so the avalanches might have resulted from the high magma supply rate and the repetitive formation of the domes. No trace of the 1854 summit failure mentioned in historical records has been found beyond 8 km from the crater; perhaps witnesses exaggerated or misinterpreted the events. Received: 18 August 1997 / Accepted: 19 December 1997     

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.     

Dry Granular Flow Modelling Including Erosion and Deposition

In this paper, we considered dense avalanches made of dry and cohesion-less snow as gravitational granular flows. We used an effective friction law derived from the recent progress on granular behaviour research to represent momentum loss. We formulated a simple erosion and deposition model where the eroded and deposited mass fluxes are estimated from the knowledge of the starting and stopping conditions on a given slope angle. We adopted the shallow water hypothesis and introduced the new formulation of erosion and deposition processes. We solved the equations thanks to an accurate finite volumes solver scheme, using a non-structured mesh. This allows an easy integration of defence structures and singularities. Each component of this model was tested using analytical solutions and experimental laboratory data. Finally, we used the February 1999 Taconnaz avalanche event to test this model on a real snow avalanche case. These tests gave promising results.     

Cretaceous length of day perturbation by mantle avalanche

These last 10 years, numerical models of mantle convection have emphasized the role of the 670 km endothermic phase change in generating avalanches that trigger catastrophic mass transfers between upper and lower mantle. On the other hand, scientists have emphasized the concomitance of large-scale worldwide geophysical and tectonic events, which could find their deep thermal roots in the huge mass transfers induced by the avalanches. In particular, the paleontological records show two periods of length of day (l.o.d.) shortening between 420 and 360, and 200 and 80 Myr BP. This last event is synchronous with a strong true polar wander and a global warming of the upper mantle. In order to study the potential effects of the avalanche on the main component of the Earth’s rotation, the Liouville equation has been solved and the l.o.d. evolution has been calculated from the perturbations of the inertia tensor. The results show that the inertia tensor of the Earth’s is mainly sensitive to the global transfers through the 670 km discontinuity. The l.o.d. perturbations will be synchronous with the global thermal effects of the avalanche. These theoretical results allow proposing a self-consistent physical mechanism to explain periods of the Earth’s rotation acceleration. Within this context, the l.o.d. shortening during the Cenozoic and Cretaceous brings one more clue to the possible participation of a mantle avalanche in generating the concomitant large scale events which have occurred during this very particular period of the Earth’s history.     

Geology, failure conditions, and implications of seismogenic avalanches of the 1944 eruption at Vesuvius, Italy

Widespread avalanching occurred at Mt. Vesuvius during its 1944 eruption, the latest activity of this volcano. The 1944 avalanche deposits display many of the morphological and structural features shown by common slides of the slump-earth flow variety, including levees, transverse ridge-and-trough topography, and preserved stratigraphy. The longest avalanche travelled 1.3 km, with an estimated volume of slightly more than one million cubic meters. Avalanches came to rest on moderately-inclined slopes. Internal structure includes low- and high-angle shears and tensional fractures. Deposits are poorly consolidated. Two lithologic types are observed; avalanches composed of both blocks and ash, with blocky rubble forming a capping layer, and avalanches composed almost wholly of ash. Block-and-ash avalanches were triggered where slopes of loose tephra had been preloaded with lava flows. Ash avalanches formed where heavy accumulations of ash were deposited by prevailing winds. Seismic activity accompanying eruption served as a trigger for avalanching.     

Late-holocene snow-avalanche activity in southern norway: Interpreting lichen size–frequency distributions using an alternative to simulation modelling

Recent work by McCarroll (1993, 1994) suggests that a new approach to lichenometry can be used to date diachronous surfaces. Size–frequency distributions of largest lichens obtained from the diachronous surfaces of avalanche boulder ramparts comprise two elements: an equilibrium population growing on boulders deposited throughout the Holocene, and overlapping populations growing on groups of boulders deposited by events during the last 300 years. In this study, almost 10000 lichens were measured on two groups of closely similar ramparts. The number of very large lichens is used to predict the form of the equilibrium distribution, which is then removed, leaving a corrected distribution which reflects the age-frequency of surface boulders deposited during the ‘Little Ice Age’. The results confirm that maximum avalanche activity occurred during the 19th century, but a minor peak at one site suggests that avalanche activity was enhanced during the mid-18th century.     

Varying Dam Height to Shorten the Run-Out of Dense Avalanche Flows: Developing a Scaling Law from Laboratory Experiments

Dry and dense snow avalanches are considered as dry granular flows. Theirinteraction with dams is the main objective of this paper. We studied howvarying the dam height could shorten a granular avalanche run-out thanksto a set of simple laboratory experiments carried out on different scales.Shortening the run-out was expected to depend on two effects: (i) storageof the granular material upstream of the dam, and (ii) local energy dissipations.A scaling law is highlighted. As suggested by dimensional arguments, thecontribution of the local energy dissipation was shown to be more dominantthan the storage effects.     

Generation of pyroclastic flows and surges by hot-rock avalanches from the dome of Mount St. Helens volcano,USA

Several hot-rock avalanches have occurred during the growth of the composite dome of Mount St. Helens, Washington between 1980 and 1987. One of these occurred on 9 May 1986 and produced a fan-shaped avalanche deposit of juvenile dacite debris together with a more extensive pyroclastic-flow deposit. Laterally thinning deposits and abrasion and baking of wooden and plastic objects show that a hot ash-cloud surge swept beyond the limits of the pyroclastic flow. Plumes that rose 2–3 km above the dome and vitric ash that fell downwind of the volcano were also effects of this event, but no explosion occurred. All the facies observed originated from a single avalanche. Erosion and melting of craterfloor snow by the hot debris caused debris flows in the crater, and a small flood that carried juvenile and other clasts north of the crater. A second, broadly similar event occured in October 1986. Larger events of this nature could present a significant volcanic hazard.