Snow avalanche activity in Żleb Żandarmerii in a time of climate change (Tatra Mts., Poland)

Catena 158 (2017) 201–212

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Snow avalanche activity in Żleb Żandarmerii in a time of climate change (Tatra Mts., Poland)

MARK

Bogdan Gądeka,⁎, Ryszard J. Kaczkaa, Zofia Rączkowskab, Elżbieta Rojanc, Alejandro Castellerd,e, Peter Bebid a

Faculty of Earth Sciences, University of Silesia, ul. Będzińska 60, 41-200 Sosnowiec, Poland Institute of Geography and Spatial Organization, Polish Academy of Sciences, Św. Jana 22, 31-018 Kraków, Poland c Faculty of Geography and Regional Studies, University of Warsaw, Krakowskie Przedmieście 30, 00-927 Warszawa, Poland d WSL - Institute for Snow and Avalanche Research (SLF), Fluelastrasse 11, CH-7260 Davos, Switzerland e Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales IANIGLA, CCT-CONICET-Mendoza, Av. Ruiz leal s/n, Mendoza, Argentina b

A R T I C L E I N F O

A B S T R A C T

Keywords: Climate change Snow avalanches Tatra Mountains

This paper reports from a survey of the occurrence of large avalanches in Żleb Żandarmerii. This couloir is known to be one of the most hazardous avalanche paths in the Tatra Mountains and has one of the longest histories of avalanche observation. This survey looked at the runout distance, return period, dynamics and geoecological implications of avalanches in the context of current climate change. The study took advantage of the longest record of meteorological data available in the Tatra Mountains, as well as archival avalanche observations, topographical maps, orthophotomaps and a high-resolution digital terrain model. Avalanche data were obtained using geomorphological and dendrogeomorphic methods and through modelling with the RAMMS numerical avalanche dynamics simulation software. The largest avalanches reach the foot of its counter slope. Their length, release volume, flow velocity and pressure can exceed respectively 1000 m, 80 000 m3, 45 m/s and 600 kPa. The results of our study suggest that current climate warming has been accompanied by thinning and shortening of the duration of snow cover, as well as by an upward expansion of the timberline (including in the large-avalanche runout zones) of up to 80 m since the mid-1920s. No distinct temporal trend was identified in the large avalanche return period since 1909, but their mass and intensity have declined. Forests and timberline expansion were found to have no influence on the extent of the avalanches in our study, while ground relief could determine both their downward extent and lateral expansion.

1. Introduction Snow avalanches are among the most common natural hazards and most efficient denudational processes in mountainous areas (e.g. Kalvoda and Rosenfeld, 1998). They typically develop in nival and alpine geoecological belts, often cutting into the forest belt and influencing the timberline. Depending on forest structure, topography and snow condition, they can develop on fully forested slopes (e.g. McClung, 2003; Teich et al., 2012a). Avalanche type, frequency, magnitude, intensity and severity are a result of a complex interaction between the local topography, weather and snowpack properties (e.g. Schweizer et al., 2003), while their temporal variability is mostly linked with the variability of meteorological conditions (e.g. Hebertson and Jenkins, 2003; Corona et al., 2010; Peitzsch et al., 2012; Laute and Beylich, 2014). Current climate warming influences the entire geographical

⁎

Corresponding author. E-mail address: [email protected] (B. Gądek).

http://dx.doi.org/10.1016/j.catena.2017.07.005 Received 14 November 2016; Received in revised form 1 July 2017; Accepted 6 July 2017 0341-8162/ © 2017 Elsevier B.V. All rights reserved.

environment, especially in areas where ice/snow are an important environmental component (Vaughan et al., 2013). The relationship between climate and avalanche activity may be described as a complex system of partly interlinked factors (Fig. 1). Changing temperature and precipitation lead to changes in snow depth, the duration of snow cover or/and changes in the occurrence of specific weather patterns which lead to increased or decreased avalanche frequency and intensity or changes in avalanche character (Martin et al., 2001; Lazar and Williams, 2008; Sinickas et al., 2016) and slope morphodynamics (Moore et al., 2013). Relevant current changes in the forest include (i) an increase in the altitude of the timberline and forest density near the timberline, (ii) an increase in forest density in the runout zone, (iii) an increase in disturbances leading respectively to: (i) fewer and smaller release areas, (ii) an increase in snow detrainment or (iii) new and larger release areas and less snow detrainment (Zurbriggen et al., 2014).

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Fig. 1. Conceptual model of the impact of climate change on snow avalanche activities. Symbols (+) and (−) indicate respectively positive (increase) or negative (decrease) feedback in snow cover, forest and snow avalanches.

regional avalanche activity is most frequently related to propitious climatic conditions and snow avalanches of great magnitude (Germain et al., 2009; Butler et al., 2010; Castebrunet et al., 2012). Over severaldecade long periods, however, it may be expected that the trends of natural activity along each avalanche path will be compatible with overall climatic trends. The problem in confirming such expectations lies in the scarcity of sufficiently long, homogeneous and long-term data records (Eckert et al., 2013). Żleb Żandarmerii in the Tatras is one of the avalanche paths with the longest and best quality history of observation. Known as highly active and hazardous, this couloir spans the alpine belts and the forest belt. A tourist path and a road leading to a mountain refuge with > 100 years' history and thousands of winter visitors cut across its path. Hence, the main goal of this research is to study the activity of this chute in terms of avalanche frequency, runout distance, return period, dynamics and geoecological implications (including timberline modifications and slope morphodynamics) in the context of current climate change. The study employs the longest record of weather data available in the Tatras spanning the last 90 years, archive data on avalanches observed in the couloir (from 1909), topographical maps from the 19th and 20th centuries, orthophotomaps and a high-resolution digital elevation model (DEM). Information about avalanche events was also obtained using geomorphological and dendrogeomorphic methods (e.g. Stoffel et al., 2013, Voiculescu et al., 2016) and through numerical simulations of extreme avalanches (e.g. Bartelt et al., 1999; Christen et al., 2010a). We took advantage of these extraordinary data sets on the Żleb Żandarmerii avalanche path in order to address the following questions:

Poor availability of coherent and reliable data from direct avalanche observations is an impediment in research on the impact of climate change on avalanche activity (Laternser and Schneebeli, 2002). Nevertheless, Eckert et al. (2013), in their study covering the period 1946–2010, found that in general the annual number of avalanche occurrences in the French Alps had been going down since 1978 and their runout, altitude, and return period were going up. In the Swiss Alps a negative trend in the occurrence of favourable weather conditions for forest avalanches (i.e. avalanches with starting point in forested terrain) was also identified during the same period (Teich et al., 2012b). Together with coinciding trends of forest expansion and increasing density of forest cover, this negative trend is likely to result in fewer avalanches in forested landscapes (Teich et al., 2012c). The climate-warming related change in the snow cover varies widely from region to region (e.g. Kotljakov, 2006), which may lead to differences in avalanche hazard. Such factors as distance to the sea (e.g. McClung and Schaerer, 2006) and atmospheric circulation (e.g. Casteller et al., 2011) may also play a role alongside landform and altitude. Air temperature has a greater impact on snow cover in lowerlocated terrain where zonal advection also plays a greater role (Falarz, 2007). Beniston (1997) demonstrated that the influence of climatechange on long-term snow cover variability diminished with increasing altitude. In this context, there are interesting results of research into the climatic aspects of snow cover variability across the vertical profile of the Polish Tatra Mountains. It was found that, in the second half of the 20th century, a relationship between the sums of daily snow depth on the one hand and snowfall and air temperature on the other was only confirmed at the foot of the mountains and at the timberline level. Higher up, only snowfall was significant (Falarz, 2002). Recent studies, however, have revealed an influence of the quantity and kind of precipitation (snow vs. rain) taken together with air temperature on the development of snow cover in this area (Gądek, 2014). The question of how to translate these climatic phenomena into specific avalanche hazards remains a challenging research topic with relevance to both mountain administration bodies and avalanche practitioners across Central Europe. Avalanche events tend not to be synchronous across a region or even a slope. They depend on the ground relief and land coverage, as well as on the local conditions for snow cover development and its physical properties (Schweizer et al., 2008). Nevertheless the widespread

1. How can weather data, remote sensing, dendrogeomorphic methods, measurement of debris displacement and avalanche modelling be combined in order to reconstruct avalanche history? 2. Do changes in forest cover affect avalanche runout? 3. Is there an indication of trends in the frequency and/or intensity of avalanches which can be related to climate change?

2. Study area The avalanche path in question is found within Dolina Rybiego Potoku, a valley on the Polish side of the Tatra Mountains (49°12′ N, 202

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Fig. 2. Location of the study area and the measurement sites: HG – Nival Research Stationat Hala Gąsienicowa, KW – synoptic station at Mt. Kasprowy Wierch, ŻŻ – Żleb Żandarmerii, 1 – trees damaged by an avalanche sampled for dendrochronological dating, 2 –pipes, 3 – stripes for the measurement of debris displacement, 4 – grids for the measurement of debris displacement. Base orthophotomap made using aerial photos taken in 2012.

(avalanche cone) is overgrown with Pinus mugo bushes, while the bottom section (valley floor) is occupied by relatively young spruce forest (Picea abies (L.) H. Karst). The busiest tarmac road of the Tatras cuts across the bush zone and carries ca. 1 million tourists walking along it every year, including in winter. The oldest mountain refuge on the Polish side of the mountains (since 1874) is located just 1 km from the couloir. For this reason the avalanche path has been one of the spots covered by the longest frequent monitoring by the refuge personnel, tourists, the forest and National Park authorities and the mountain rescue services. The climate of the Tatra Mountains is transitional with both maritime and continental influences. Air masses mainly arrive from the south-west (Niedźwiedź, 1992). During the period 1954–2010, the mean air temperatures of the year and of the cool half-year at Mt. Kasprowy Wierch (Fig. 2) were, respectively, 0.51 °C and − 5.9 °C. The mean number of days with snow cover, the maximum snow depth and the seasonal sum of the daily snow depth were 240, 205 cm and 20,345 cm, respectively (Gądek and Leszkiewicz, 2012).

20°03′ E). The area consists of granitic rocks (Nemčok et al., 1994), has a well-formed post-glacial relief (Rączkowski et al., 2015) and is legally protected as a national park and as an international biosphere reserve (UNESCO). Żleb Żandarmerii dissects the eastern slopes of Mt. Opalony Wierch (Fig. 2). It is ca. 1.3 km long, has a drop of ca. 600 m and an average slope of 29.5o. The largest avalanches can reach the foot of its counterslope provided by a lateral moraine from the Würm period. Within the avalanche path, the timberline crosses the valley at 1340 m. The funnel-shaped starting zone of the avalanche path is located at 1950–1630 m and the slopes at ca. 35°. Its relatively large and unfragmented surface is smooth and almost entirely covered by grassland. Erosion scars expose waste rock where full-depth avalanches or other processes have stripped the grass. A marked tourist path crosses a bottom section of the starting zone. The avalanche track has a gutter shape and the corresponding map contours are curved at between 80° and 90°. Its bottom slopes evenly at ca. 20° and is lined with debris, while the slopes are covered by grass, shrubs and the dwarf form of Mountain pine (Pinus mugo ssp. mugo). The runout zone slopes at between 15° and 0°. Its upper section 203

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Fig. 3. Changes in air temperature and snow cover at Hala Gąsienicowa (the air temperature data gap is a result of World War II); p – the level of statistical significance, r2 – the coefficient of determination.

regarded as representative of the Polish Tatra Mountains (Niedźwiedź, 1992). The daily air temperature and the snow depth data covered the period 1927–2015 (with gaps in 1940–1947). They were used to calculate the average air temperatures in the hydrological half-year (November–October) periods, the number of days with snow cover ≥1 cm (duration of snow cover) and the seasonal maximum and sum of daily snow depth. Next, the study determined the maximum and minimum values, the arithmetical means with standard errors, standard deviations and the 11-year moving averages. Trends in the seasonal and annual values, described by a normal distribution, were determined using the linear regression method (least squares) and their statistical significance was t-tested (significance level α ≤ 0.05). In each case, there have been calculated the level of statistical significance (p) and the coefficient of determination (r2), which indicates to what degree the regression equation explains the variability of the dependent variable.

Table 1 Summary of natural large avalanches in Żleb Żandarmerii (observation data). Data 1909 1910 1911 1946 1948 1956 1982 2005

May 11 April 04 January 28 April March 03 December 29 March 13

Length (m)

Type

Source material

– Approx. Approx. Approx. – Approx. Approx. Approx.

Wet Wet Mixed – Wet (?) Slab Powder Wet

Zaruski, 1910 Sawicki, 1910 Zaruski, 1911 Kłapa, 1959 Kłapa, 1959 Kłapa, 1959 Sałyga-Dąbkowska, 1986 Chowaniec, 2005

1100 1450 1450 1450 1450 1000

3. Data and methods Both the archival data and the methods of obtaining new data were selected with an eye to ensuring a coverage of all the main elements of a system involving the natural environment and the avalanche.

3.2. Archival snow avalanche data The avalanche event data were obtained searching for: (i) published research into snow cover, avalanches, forests and morphodynamic processes in the Tatras; and (ii) Tatra nature journals published from the mid-19th century. Other sources included reviews of: (iii) the Zakopane parish records 1848–1890, (iv) the records of the Tatra Mountain Rescue (from 1908); and (v) the avalanche catalogue in the Polish Tatras kept by the Nivology Section of the Institute of Meteorology and Water Management.

3.1. Meteorological data and analysis The study employed weather data from the Hala Gąsienicowa weather station, the only nival research station of the Polish weather service (Institute of Meteorology and Water Management) in the Tatras. The station, located above the timberline, at 1520 m, is just 5.5 km away from Żleb Żandarmerii (Fig. 2). The local climatic conditions are 204

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Fig. 4. A full-depth avalanche in Żleb Żandarmerii on 13 March 2005 (photo by P. Budzyna).

Fig. 5. Avalanche events in Żleb Żandarmerii and their maximum extent determined using growth disturbances (scars and decapitations) recorded in the sample trees. Base orthophotomap made using aerial photos taken in 2012.

scar surface area (Rączkowska et al., 2016). This task was carried out using the ArcGIS 10.4 package from ESRI with the DEM-assisted PRA which was field-corrected.

3.3. Cartographic and remote sensing data Cartographic material included topographical maps of the Tatras at a scale of 1:25 000 from 1898 and at a scale of 1:20 000 from 1934. The former is the result of plane table surveys carried out in 1876 and in 1895–97, while the latter was drafted using stereo-photogrammetric photos mostly taken in 1925–1927. Alongside these historic sources the study also employed digital orthophotomaps based on aerial photos from 1955, 1977, 1999, 2009 and 2012; a high resolution IKONOS-2 satellite image from 2004 (from GIS resources of the Tatra National Park); and a digital elevation model (DEM) with one-metre resolution (LiDAR) purchased at Centralny Ośrodek Dokumentacji Geodezyjnej i Kartograficznej, Poland's central cartographic resource.

3.5. Measurements of debris displacement The measurements were performed by vertically planting a 0.5″ (1.27 cm) steel tube 0.5 m in length into each of six erosion scars (shallow and devoid of vegetation) found in the starting zone. In one of the scars, in the transit zone, the rock debris was marked by painting coloured strips across the direction of the avalanche path. Additionally, in two places in the accumulation zone and in one in the transit zone, test plots of one square metre each were set up. Before and after each winter season of the period 2012–2015, the length of the overground sections of pipe were measured, as well as the displacement of the painted debris and the volume of rock debris in the test plots. Fig. 1 illustrates the location of the measurement sites.

3.4. Cartometric measurements Using the digital elevation model (slope inclination, curvature, roughness and size) and the orthophotomaps (terrain coverage), the study determined the following: (i) the potential release area (PRA) (Bühler et al., 2013), (ii) the timberline (Kaczka et al., 2015b), (iii) the dense, grouped and open forest areas (Feistl, 2015) and (iv) the erosion

3.6. Dendrogeomorphic methods An increment borer was used to take 1040 samples from 439 trees 205

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Fig. 6. Variations in the timberline near Żleb Żandarmerii between the mid-1920s and 2012. Base orthophotomap made using aerial photos taken in 1955.

Additional measurements included: (i) the circumference of the damaged tree, and the damage (ii) height and (iii) size. The samples were prepared in a standard manner to assure sufficient visibility of the ring boundaries and other anatomical features of the wood (Stoffel and Bollschweiler, 2008). The cores were scanned at a resolution of 1200 dpi or 2400 dpi and the tree-ring widths were measured with the CooRecorder 7.7 software (Cybis Elektronik & Data AB). A stereoscopic microscope was used to analyse the presence of anatomical anomalies in the wood: traumatic resin ducts (TRD) and reaction wood (Stoffel and Corona, 2014). All samples were checked for any missing rings through cross-dating process (Schweingruber, 1996) using a local reference chronology established using 60 spruce trees from the same valley, but outside of the avalanche paths. The data obtained from cross-dating of the X sample (scar) was accepted for further analyses only if it coincided with the date of first appearance of TRD of the same injury. The study of wood anatomy resulted in the selection of those scars which were created during winter: (i) the last ring of the X sample included complete latewood and (ii) corresponded TRD were located in the early wood of the following year. Dating of crown damage (decapitation) involved the identification of acute and persistent reductions of growth (Butler and Malanson, 1985). Together we sampled > 250 scars and 190 decapitations from which respectively 222 and 132 were dated successfully. To determine the main avalanche events a

Table 2 Surface area of erosion scars in the release zone of Żleb Żandarmerii during the period 1955–2009 based on remote sensing data. Year

Area (ha)

1955 1977 1999 2004 2009

0.65 0.20 0.17 0.17 0.63

growing in a zone where the avalanche path coincides with forest. The selected trees, mostly Norway spruce (Picea abies L. Karst.), bore damage marks to their crowns (decapitations) and trunks (scars), which are among typical growth disturbances employed to date snow avalanches (Butler and Malanson, 1985; Lang et al., 1999; Butler and Sawyer, 2008; Stoffel et al., 2006; Luckman, 2010; Corona et al., 2012). Decapitated trees were sampled beneath the damage marks, while scarred trees were sampled at three places: directly at the location of the scar (X), above the damage (Y) to obtain traumatic resin ducts, and in the undamaged part of stem (Z) (Kaczka et al., 2015a). The location of the sampled trees was recorded using a precision GPS receiver.

Fig. 7. Year-to-year change in the height of erosion scars (aggradation/degradation). Measurement plots within the release zone in Żleb Żandarmerii.

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3.7. Avalanche modelling Calculations of the dates of snow avalanches recorded in trees were performed using the RAMMS numerical avalanche dynamics simulation software (Christen et al., 2010b). This tool, developed by the WSL Institute for Snow and Avalanche Research SLF, allows the modelling of two-dimensional runout distances, flow velocities, flow heights, impact pressures, flow paths of large-scale avalanches and the braking effect of forests. The inputs included: (i) DEM, (ii) PRA, (iii) areas of dense, grouped and open forest, as well as data on (iv) the species composition and (v) the average diameter of the trees. The K-values, representing the braking power per square metre that the forest exerted on the avalanche flow, were determined using data on the forest type, crown coverage and the roughness of the ground, small vegetation and dead wood (Feistl, 2015). Numerical avalanche simulations were performed of: (i) the avalanche that descended down in 1956 (the best documented) the path almost devoid of mature trees and (ii) its theoretical extent and tree destruction in the current forest conditions. For these simulations, we used a depth-averaged energy equation accounting for the R kinetic energy associated with particle velocity fluctuations (Bartelt et al., 2012). Since only dry and dense avalanches were being modelled the value of the static Coulomb friction parameter (μ0) was assumed to be 0.55 m/s2 while the speed-dependent friction parameter before fluidisation (ξ0) was assumed to be 1800 m/s2. The density of the snow released was assumed to be 250 kg/m3 and the density of the snow deposited in the runout zone was approximately 400 kg/m3. The α parameter, which controls the production of R random fluctuation energy, was 0.06. The parameter β, which determined the dissipation mechanisms of the fluctuation energy, was 0.8 (Bartelt et al., 2012; Bartelt et al., 2015; Vera Valero et al., 2015). The model was calibrated using data on: (i) the location of trees damaged during the event being modelled, (ii) the type and (iii) age of that damage, as well as (iv) orthophotomaps based on archival aerial photos and (v) reports from direct observations. The calibration exercise involved the selection of a set of snow parameters (including the average depth, temperature and cohesion of the snow released) and the specifications of the entrainment snow layer so that the calculated surface area of tree destruction would include the trees broken during the event being modelled. The value of the dimensionless entrainment coefficient κ (erodibility) was assumed to be 0.35 (new snow) or 0.2 (old snow). The assumed snow temperatures were compared with the air temperature measured at the Hala Gąsienicowa station on the avalanche day and over a preceding month, corrected for the altitude difference. The model calibration process involved DEM resolution optimisation over the range from 1 m to 10 m. 4. Results and interpretation 4.1. Changes in air temperature and snow depth The average air temperature in the hydrological winter half year (MWAT) at Hala Gąsienicowa ranged from − 5.3 °C (1929) to − 0.3 °C (1930). The arithmetic mean was − 2.8 (standard error ± 0.12) °C and the standard deviation was 1.1 °C. The mean summer half-year temperature (MSAT) ranged from 5.9 °C (1936) to 9.8 °C (2012). The average was 7.9 ( ± 0.09) °C with a standard deviation of 0.79 °C. From the second half of the 20th century, the mean moving 11-year MWAT values followed a cycle that brought them higher with each successive cycle. The moving MSAT values declined until the mid1970s, after which they gradually increased again (Fig. 3). The rate of temperature increase accelerated considerably during the last 30 years (1986–2015). However, only the MSAT trend was statistically significant (p < 0.01) and linear regression explained it at 37%. The number of days with snow cover at Hala Gąsienicowa ranged from 139 (2001) to 231 (1937). Their arithmetical mean was 182

Fig. 8. Geometry and power of an avalanche that descended down Żleb Żandarmerii and Głęboki Żleb on 3 March 1956 (results of numerical simulation using RAMMS software).

simple index of avalanche events was calculated consisting of the ratio of the number of trees damaged by avalanches in a given year to the number of trees growing in that year (Shroder, 1978; Corona et al., 2010; Germain et al., 2010). The rather low (Butler and Sawyer, 2008) cut-off value of 5% was adopted to identify avalanche events (Lempa et al., 2016).

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Fig. 9. Results of numerical simulation of Żleb Żandarmerii avalanche extent under different snow conditions: D – the average thickness of the snow released.

valley floor and the counterslope was destroyed in 1946 and 1956 (Kłapa, 1959). The most exceptional period occurred in early March 1956, when a series of large natural avalanches occurred on several slopes reaching locations previously considered safe, destroying shepherds' huts and a mountain refuge in Dolina Goryczkowa. The results of sample analysis from decapitated trees taken at the edges of Żleb Żandarmerii demonstrate that the increment reduction time varied between 2 and 12 years and depended on the extent of the damage, as well as on the age and general condition of the tree affected. The broken trees were mostly young and averaged 25 years of age. The decapitated samples came from the period 1878–2012 and the peak damage (12%) was dated to the year 2010. The scarred samples date from between 1880 and 2010, with 1962 producing the greatest share of damage (10%). When compared to the account of avalanche events only tree damage from the winter seasons of 1945, 1946, 1948, 1956 and 1983 was qualified as avalanche related (Fig. 5a). The location of these trees suggests that the avalanche of 1948 came to a rest in the middle of the valley floor while the remaining events reached the counterslope (Fig. 5b). The events of the winter seasons 1946, 1948, 1956 and 1983 also feature in the archival observation materials. The lack of any account of an avalanche in 1945 may be explained by the fact that this was still in the Second World War and the Morskie Oko refuge was closed, but it is more likely that the date is one year too old. The other unaccounted event, the wet ground avalanche of March 2005, caused no tree damage (Fig. 4) and could not be read from treering anomalies. Any avalanche episodes that occurred before 1913 were excluded from the dendrochronological analysis due to a risk of low reliability in view of the small degree of sample replication (between 5 and 13).

( ± 2.25) while the standard deviation was 21. The maximum snow depth ranged from 78 cm (2014) to 353 cm (1944) and the corresponding arithmetical mean was 152 ( ± 4.93) cm and the standard deviation was 46 cm. The seasonal sum of daily snow depth varied from 2886 cm (2014) to 27,263 cm (1938) with the corresponding mean value of 12,295 ( ± 536.71) cm and the standard deviation of 5006 cm. In the light of the analysis of moving 11-year means, the number of days with snow cover followed cycles and has been declining since the mid-1990s. The moving means of the maximum snow depth were typically close to 150 cm. Only in the late 1920s and early 1930s and again at the turn of the 21st century were these values markedly higher, but they have been gradually declining over the last decade. The moving means of daily snow depth, on the other hand, have been declining since the late 1970s. Over the last three decades, three measures, i.e. the variability of days with snow cover and the maximum depth and sums of the daily snow depth, all followed linear trends showing a decline (Fig. 3). Only the first and the last of the three were statistically significant (p < 0.5). Linear regression described these trends at 19% and 13%, respectively. 4.2. Avalanche observations vs. dendrochronological data Archival avalanche observations in Żleb Żandarmerii go back to the mid-19th century, since when eight large natural events have been recorded which reached either the valley floor or the counterslope (Table 1). The two events with the best geometry and extant data happened in 1910 (Sawicki, 1910) and 1956 (e.g. Kłapa, 1959). There is, however, only one photograph of a large avalanche that includes the start, transit, and deposition zones. It was taken in March 2005 (Fig. 4) and depicts an avalanche similar to the one described by Sawicki in 1910. From the observational data collected during the last 100 years, it can be observed that the large avalanche return period varied from one year to 35 years and followed no apparent trend. The forest between the

4.3. Variation in the timberline and erosion scar size Various cartographic and remote sensing documents suggest 208

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the mid-1920s. The process by which the timberline altitude increased was most rapid in 1977–1999. Within the avalanche path the timberline was pushed down to 1340 m, on the valley floor. Unlike on the topographical map dating from 1934, most of the valley floor is currently forested, but avalanches are known to have often destroyed the forest up to the counterslope. The most recent such periods are dated back to before 1955 and in 1956–1977. This would suggest that the powder avalanche of December 1982 (Sałyga-Dąbkowska, 1986) did not cause much damage. Therefore, it can be concluded that the results of this cartographic and remote sensing study are compatible with direct observations and dendrochronology data. Changes in the size of the erosion scars in the release zone should reflect the activity of ground avalanches, including ones that have not reached the forest. Measurements of aerial and satellite-based orthophotomaps from 1955 to 2012 (Table 2) suggest that during the period 1955–1977 the erosion scars in the release zone shrank by as much as 70%. This is largely explained by the abandonment of pastoral activity. At the turn of the 21st century, the erosion scars had an overall area of approximately 1700 m2. Then, between 2004 and 2009, this increased by a staggering 370% to again reach the levels of the 1950s. This increase could be explained by the large ground avalanche that occurred in March 2005, an exceptional event at the scale of the last six decades. 4.4. Present-day slope morphodynamics Between 2012 and 2015, the overground height of the reference pipes inserted in the erosion scars within the avalanche starting zone changed by between 0.001 m and 0.035 m (Fig. 7). These changes were more often linked to the deposition of fine-grained mineral material (68%) than to its depletion (32%). In all likelihood this was the result of melt water action, even if soil creep and freeze-thaw processes cannot be entirely ruled out. The deposition vs. depletion change at each reference point was chaotic. A very few painted rock fragments were displaced in the transit section of the couloir, typically at a distance of 10–20 cm and no > 3.5 m. This would suggest that mineral material was solely displaced by melt water and/or runoff water. No new mineral/organic material was found in the accumulation zone. These results suggest that there were no ground avalanches (whether large or small) in Żleb Żandarmerii over the last three years. This finding is compatible with a lack of any reports of such avalanches during the study period. 4.5. Ground relief and forest influence on the extent of large avalanches The best dendrogeomorphological documentation and archival reports cover the avalanche event of 3 March 1956. Aerial photos taken in the autumns of 1955 and 1977 provide a spatial reference to the extent and density of the forest on the eve of the event and 21 years later, after a period without a forest-intruding avalanche. In the light of the observation material, a DEM analysis, and a site visit, the avalanche had two release zones, both facing East: i) in the upper section (known as Gładkie) of Żleb Żandarmerii and ii) in the upper section (known as Opalony Upłaz) of the nearby Głęboki Żleb couloir. Their overall surface area was ca. 7.78 ha. The calibrated dynamic model of this avalanche suggests that the depth of the snow layers released was 1.4 m (Gładkie) and 1 m (Opalony Upłaz). This means that they included not just freshly fallen and blown-in snow, but also snow deposited earlier. A layer of depth hoar or buried surface hoar was the weak layer (Kłapa, 1959). The calculated release volume was ca. 80 500 m3. The avalanche reached the counterslope where it came to a rest 1450 m from the starting point on a lateral moraine of Würm-age. The avalanche core reached its maximum depth (ca. 15.5 m) in an upper section of its transit zone within Żleb Żandarmerii, while its maximum velocity (47 m/s) and pressure (647 kPa) were reached in the middle and lower sections of that zone (Fig. 8). While its power was sufficient to continue

Fig. 10. Numerical simulation of avalanche extent and tree damage within the avalanche paths Żleb Żandarmerii and Głęboki Żleb on March 3rd 1956 and of the same event under the assumption of current forest conditions. The black and white dots mark the location of trees decapitated or scarred by avalanches in 1955/1956.

considerable temporal variations of the timberline in the area over the last 150 years (Fig. 6). The current timberline in the vicinity of Żleb Żandarmerii runs at 1550–1580 m, which is up to 80 m higher than in 209

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Fig. 11. Scheme of integration of weather data, remote sensing, dendrogeomorphic methods, measurements of debris displacement, and mathematical modelling in order to reconstruct avalanche history.

of large avalanches (level of the danger ≥3) is going down, but this trend is not statistically significant (Gądek et al., 2016). Due to the long return period of large avalanches (e.g. McClung and Schaerer, 2006) the study period may be insufficient to capture their trend of change. It must be noted, however, that the continuing rise of the timberline and the regrowth of vegetation on the runout zones of large snow avalanches is a common phenomenon across the Tatras. These ecological changes are primarily related to decrease of snow depth and increase of summer air temperature and partly to land use changes. Increase of summer temperature in the Tatras results in: (i) increase of the spruce increment growth but in the same escalate their susceptibility to bark beetle infestation, (ii) rise of the forest density and (iii) its vertical extent, (iv) increase of the growth of mugo pine and (v) extension of the mugo pine coverage (Kaczka et al., 2015b; Kulakowski et al., 2016). In their study Martin et al. (2001) used avalanche and snow data, a simple climate scenario, and modelling to demonstrate that avalanche danger might also be decreasing in the Alps, slightly in winter and more so in spring. They also noted an increase in the relative proportion of wet-snow avalanches. In the snow-cover structure on the Tatra Mountains the proportion of melt-related forms is increasing, but due to the shrinking depth and duration of snow cover, this has not translated into an increase in wet-snow avalanches (Gądek et al., 2016). The recent increase in the size of the erosion scars was a result of a single wet ground avalanche. In this context the monitoring of the release zone provides interesting morphodynamic information (Rączkowska et al., 2016), especially within erosion scars. This line of research would be worth continuing in order to measure debris displacement in avalanche paths with different altitude, exposure, morphology and size.

uprooting mature trees close to the floor of Dolina Rybiego Potoku, any damage to trees at the mouth of Żleb Żandarmerii had been caused in winter 1946. Avalanche runout of the 1956-avalanche was influenced by the terrain characteristics of Dolina Rybiego Potoku, but not by extend of the forest cover and recent increases of timberline. An increase in the thickness of snow released from Żleb Żandarmerii does not result in extending the length of the avalanche, but only in expanding its width, mainly in the direction of the valley incline (Fig. 9), and the affected area of forest. The extent of large avalanches released at Gładkie and Opalony Upłaz are not mitigated by the currently elevated timberline ca. 300 m below the release area. Indeed, a comparison with simulations under current forest cover suggest that recent changes in forest cover and increases of timberline would not significantly impact the size and geometry of an avalanche. However, the area of forest damaged would have increased from 5.5 ha (simulated for the 1956conditions) to ca. 8.3 ha (Fig. 10). 5. Discussion The weather data suggest that the Tatra climate is warming. The accompanying shrinkage of the depth and duration of its snow cover is reflected in changes to the avalanche hazard. During the last 25 years, the number of days with level 2 of avalanche danger (the 5-level avalanche danger scale: www.avalanches.org) was declining while that with the lowest level (level 1) of the danger was growing. This would be expected to produce a decline in the occurrence of natural small and medium-sized avalanches. Also the number of days with the highest risk 210

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the tree and forest damages, for modeling of snow released movement helps to estimate the release volume, velocity, pressure and avalanche extent in case of the lack of the snow data. 2. The extent of extreme avalanches is influenced by the ground relief, which can also control their lateral expansion, but not by forest cover and timberline increases, which plays no significant role in our case study. 3. The current climate warming in the Tatras is accompanied by a shrinkage of the depth and duration of the seasonal snow cover and by an increasing altitude of the timberline, including within large snow avalanche runout zones. However, due to long return period of these avalanches and therefore relatively low overall number in the data series, no statistically significant trends were detected in the highly avalanche prone Żleb Żandarmerii.

Researchers studying the influence of climate change on snow avalanche activity are facing a serious obstacle in the general unavailability of long duration and complete regional records on avalanches (Laternser and Schneebeli, 2002). Dendrogeomorphologic methods can provide valuable options to fill this gap (Stoffel et al., 2013). Their weaknesses, however, include a total dependence on the availability of trees that record avalanche events in their increments with a dating accuracy of less than one year. To mitigate these problems researchers can resort to supplementary data sources, including archival remote sensing materials, which document spatial variability in the timberline and in erosion scars, and direct morphodynamic measurements on the surface of the release zone. In the light of this, the information gathered from direct observations of extreme avalanche events that occurred in Żleb Żandarmerii over the last 100 years seems complete. The array of methods applied in this study, including dendrogeomorphological, remote sensing, geomorphological and mathematical modelling, together offer the kind of data that can largely be regarded as a good alternative to the results of long-term monitoring of large avalanches. Fig. 11 summarises ways of integrating various data sources verified in this study. Results of the direct observations and dendrogeomorphic study suggest that the mass and intensity of the extreme avalanches in Żleb Żandarmerii has been declining in recent years. Due to the limited number of these events this trend is not statistically significant. It can be noted, however, that during this period climate was warming and the duration and depth of snow cover were decreasing (Fig. 3). In the light of the numerical simulations, forest was ruled out as a significant factor in determining the extent of large avalanches. This contradicts to other studies which have found effects of forest cover and timberline increases on avalanche runout (Teich et al., 2012a; Zurbriggen et al., 2014) and can largely be explained by the relatively large distance between avalanche release and timberline in the Żleb Żandarmerii case study. Ground relief, on the other hand, does play a role, as illustrated by the fact that all extreme avalanches stopped at the lateral moraine at the foot of the counterslope at the runout of the couloir. As the mass of released snow increased, the avalanche tended to expand laterally and was partly controlled by the relief of the underlying ground. Another difficulty faced by the researcher deprived of a direct observation/ measurement is how to determine the depth of the released snow. The estimation method offered in the Swiss Guidelines (Salm et al., 1990), based on weather data and the slope altitude and incline, is only a rough estimation, because the release depth of a large avalanche may be greater than that of the new snow. For this reason the depth of the snow layer released in Żleb Żandarmerii was determined by calibrating the model with the use of dendrochronology data. This still leaves the question of how to determine the real snow cover entrainment rate (Bartelt et al., 2012; Vera Valero et al., 2015). In this respect studies intended to develop a full parameterisation of the process would be very valuable.

Acknowledgements We wish to thank P. Bartelt, M. Christen, Y. Bühler and T. Feistl (WSL Institute for Snow and Avalanche Research SLF) for introducing to an expanded version of the RAMMS software. The assistance of M. Lempa and K. Janecka (University of Silesia) in dendrochronological research carried out in both field and laboratory is greatly acknowledged. We are also grateful to the anonymous reviewers for their useful comments and suggestions. The study was supported by the National Science Centre project No. 2011/03/B/ST10/06115 and University of Silesia. References Bartelt, P., Salm, B., Gruber, U., 1999. Calculating dense snow avalanche runout using a Voellmy-fluid model with active/passive longitudinal straining. J. Glaciol. 45 (150), 242–254. Bartelt, P., Bühler, Y., Buser, O., Christen, M., Meier, L., 2012. Modeling mass-dependent flow regime transitions to predict the stopping and depositional behavior of snow avalanches. J. Geophys. Res. Atmos. 117, F01015. Bartelt, P., Vera Valero, C., Feistl, T., Christen, M., Bühler, Y., Buser, O., 2015. Modelling cohesion in snow avalanche flow. J. Glaciol. 61 (229), 837–850. Beniston, M., 1997. Variations of snow cover depth and duration in the Swiss Alps over the last 50 years: links changes in large-scale climatic forcings. Clim. Chang. 36, 281–300. Bühler, Y., Kumar, S., Veitinger, J., Christen, M., Stoffel, A., Snehmani, 2013. Automated identification of potential snow avalanche release areas based on digital elevation models. Nat. Hazards Earth Syst. Sci. 13, 1321–1335. Butler, D.R., Malanson, G.P., 1985. A history of high-magnitude snow avalanches, Southern glacier national park, Montana, U.S.A. Mt. Res. Dev. 5, 175–182. Butler, D.R., Sawyer, C.F., 2008. Dendrogeomorphology and high-magnitude snow avalanches: a review and case study. Nat. Hazards Earth Syst. Sci. 8, 303–309. Butler, D.R., Sawyer, C.F., Maas, J.A., 2010. Tree-ring dating of snow avalanches in Glacier National Park, Montana, USA. In: Stoffel, M., Bollschweiler, M., Butler, D.R., Luckman, B.H. (Eds.), Tree Rings and Natural Hazards: A State-of-the-Art. Springer, Berlin, Heidelberg, New York, pp. 35–46. Castebrunet, H., Eckert, N., Giraud, G., 2012. Snow and weather climatic control on snow avalanche fluctuations over 50 yr in French Alps. Clim. Past 8, 855–875. Casteller, A., Villalba, R., Araneo, D., Stöckli, V., 2011. Reconstructing temporal patterns of snow avalanches at Lago del Desierto, southern Patagonian Andes. Cold Reg. Sci. Technol. 67, 68–78. Chowaniec, J., 2005. Złamane bariery. Tatry TPN 2 (12), 7. Christen, M., Bartelt, P., Kowalski, J., 2010a. Back calculation of the In den Arelen Avalanche with RAMMS: interpretation of model results. Ann. Glaciol. 51, 161–168. Christen, M., Kowalski, J., Bartelt, P., 2010b. RAMMS: numerical simulation of dense snow avalanches in three-dimensional terrain. Cold Reg. Sci. Technol. 63, 1–14. Corona, Ch., Rovéra, G., Lopez, Saez J., Stoffel, M., Perfettini, P., 2010. Spatio-temporal reconstruction of snow avalanche activity using tree rings: Pierres Jean Jeanne avalanche talus, Massif de l'Oisans, France. Catena 83 (2–3), 107–118. Corona, C., Saez, J.L., Stoffel, M., Bonnefoy, M., Richard, D., Astrade, L., Berger, F., 2012. How much of the real avalanche activity can be captured with tree rings? An evaluation of classic dendrogeomorphic approaches and comparison with historical archives. Cold Reg. Sci. Technol. 74, 31–42. Eckert, N., Keylock, C.J., Castebrunet, H., Lavigne, A., Naaim, M., 2013. Temporal trends in avalanche activity in the French Alps and subregions: from occurrences and runout altitudes to unsteady return periods. J. Glaciol. 59 (213), 93–114. Falarz, M., 2002. Klimatyczne przyczyny zmian i wieloletniej zmienności występowania pokrywy śnieżnej w polskich Tatrach. Prz. Geogr. 74 (1), 83–106. Falarz, M., 2007. Snow cover variability in Poland in relation to the macro- and mesoscale atmospheric circulation in the twentieth century. Int. J. Climatol. 27, 2069–2081. Feistl, T., 2015. Vegetation Effects on Avalanche Dynamics. (PhD thesis manuscript) Technische Universitat Munchen.

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