Developing new methods for monitoring periglacial phenomena

Transcription

1 Developing new methods for monitoring periglacial phenomena Permafrost, Phillips, Springman & Arenson (eds) 2003 Swets & Zeitlinger, Lisse, ISBN D. Mihajlovic, D. Kölbing, I. Kunz, S. Schwab, H. Kienholz, K. Budmiger, M. Imhof Department of Geography, University of Bern, Switzerland B. Krummenacher Swiss Federal Institute for Snow and Avalanche Research, Davos, Switzerland ABSTRACT: Since 1987, the periglacial investigation area Furggentälti, located in the western part of the Bernese Alps (Switzerland), has been studied by the Department of Geography of the University of Bern. The test site extends over a range of approximately two square kilometers and is rich in periglacial forms, including solifluction lobes and several small rock glaciers. Investigations are carried out through process studies and longterm monitoring of rock glacier and solifluction activity, snow coverage and local climate. In a multi-method approach, the monitoring combines the use of known methods with the development of new techniques for studying periglacial phenomena. Methods applied include land and aerial survey as well as digital terrestrial and aerial photogrammetry, an automatic camera monitoring snow coverage, miniaturized temperature loggers and three automatic weather stations. 1 INTRODUCTION 1.1 The site The Furggentälti ( N/7 38 E), a small valley near the Gemmi pass, is located in the western part of the Bernese Alps, between Kandersteg and Leukerbad (Switzerland). It is situated well above the timber line, and is stretching over a range of approx. two square kilometers from west to east, from an altitude of approx m to approx m ASL. Most of the surface of the valley is covered by vast layers of periglacial debris, forming large talus cones on the foothills of the steep northern slopes. The periglacial forms found in the Furggentälti valley include several small rock glaciers and solifluction lobes on the southern slopes. Typical periglacial patterned ground is also present at a flat region of the valley. Historical 19th century records indicate that the valley has not been covered by a perennial snow cover during the Little Ice Age (from approx AD to 1850 AD). It has therefore probably been ice-free during the last thousands of years (Imhof 1992). The focus of interest lies on a small, tongue-shaped rock glacier in the lower western part of the valley (Fig. 1). The rock glacier is situated on a slope of about 20 of northern aspect, its front being at an altitude of 2460 m ASL. 1.2 The research project First reports on periglacial forms in the Furggentälti valley date back to 1964, when Prof. Klaus Aerni of the Figure 1. Geographic location and aerial view of the Furggentälti valley, seen in an air photo of 1992 ( Swiss Federal Office of Topography). Department of Geography of the University of Bern took first notes on its rich periglacial morphology. Since 1981, the Furggentälti site has served as an object of study during various field courses and excursions lead by Prof. Hans Kienholz of the Department of Geography. In spring 1987 a first campaign measuring the Base Temperature of the winter Snow cover (BTS) started exploring the site, providing a first picture of the permafrost distribution in the area. During the last 15 years of recurring visits to the site, a wide range of measurement techniques has been combined, developed and tested in the Furggentälti valley, and has yielded a more refined picture of its periglacial processes and environment. A short extract of the results of the research work will be presented on the following pages. 765

2 2 MONITORING ROCK GLACIER ACTIVITY 2.1 Aerial survey First aerial photographs of the Furggentälti valley date back to 1960 and were taken by the Swiss Federal Office of Topography. Further high-altitude air photos of that series, at a scale of approx. 1:30,000, date from 1974, 1980, 1985, 1992 (Fig. 1) and Low-altitude air photos of 1990 and 1995 resulted from a rock glacier monitoring program, initiated by the Laboratory of Hydraulics, Hydrology and Glaciology of the Swiss Federal Institute of Technology in Zurich (VAW/ETHZ), in collaboration with the Federal Flight Service and Coordination Center for Aerial Survey and Aerial Photos in Dübendorf (KSL). These air photos show much more details, as they were taken at a scale of approx. 1:5000. Two more sets of air photos of similar quality have been taken in 2000 (sponsored by Photogrammetrie Perrinjaquet, Gümligen, Switzerland) and in 2001, by the Department of Geography. Thanks to the fact, that the entire series of air photos covers a time span of more than 40 years, the results of analytical and digital photogrammetrical survey revealed an interesting image of a rock glacier with a rapidly changing activity. 2.3 Rock glacier activity Both aerial and terrestrial survey contributed to unexpected findings about the activity of the largest rock glacier at the Furggentälti site. The findings show high and accelerating annual displacement velocities of a rock glacier with a rapidly changing activity pattern Volumetric changes Photogrammetrical surveys in the Furggentälti by Budmiger (in Krummenacher et al. 1998) led to multi-temporal Digital Elevation Models (DEM). With the aid of these DEMs, volumetric balances between different years can easily detect volumetric changes at the rock glacier surface. An example of volumetric balance of the Furggentälti rock glacier surface is shown in Figure 2. The volumetric balance measured by Budmiger indicate an increasing subsidence movement of the rock glacier surface, from an average annual volume loss of approx. 380 m 3 /yr for the period of compared to an average annual loss of approx m 3 /yr over the entire period Terrestrial survey First terrestrial survey campaigns started in Over a period of several years, displacement rates of boulders on a solifluction lobe with an inclination of approx. 8 were determined. A special network of control points had to be designed, allowing precision deformation measurements with an accuracy of approx. 1 mm in position and approx. 3 mm in height (Blank, in Krummenacher et al. 1998). Since 1994, further terrestrial survey campaigns provided ground control points for the photogrammetrical survey. Several of these ground control points have been placed on boulders located on the rock glacier surface, and have since been measured at an annual base, tracking changing annual displacement rates (Fig. 4). From August 1998 to October 1999, seasonal displacement patterns of a total number of 32 boulders on the rock glacier surface have been measured in repeated survey campaigns at a monthly interval. Although many of the boulders disappeared under the enormous masses of snow during the record breaking winter , these measurements have revealed an interesting seasonal pattern in the activity of the rock glacier in the Furggentälti (Kölbing 2001). Figure 2. Volumetric balance of the Furggentälti rock glacier surface between 1960 and Regions mapped as cut have shown a subsidence movement within the measurement period, fill regions have shown a rise of the surface and mark the deposits of the ongoing rock glacier activity (Budmiger in Krummenacher et al. 1998; altered). 766

3 These ongoing changes become clearly visible, looking at the cross-section along the rock glacier (Fig. 3) Annual displacement velocities Since 1994, repeated terrestrial survey of the photogrammetry ground control points has confirmed increasing annual displacement velocities of both rock glacier front and surface (Kölbing 2001). This behavior has also been observed in photogrammetrical survey (Budmiger, in Krummenacher et al. 1998). Tables 1 and 2 show details of the survey. Compared with other rock glaciers in the Swiss Alps, these displacement velocities are very high: the peak displacement rate on the Furggentälti rock glacier amounts to approx. 2.5 m/yr in , measured at boulder 418 in the center of the rock glacier surface (Figs 5 6). A comparably high activity of a rock glacier in the Swiss Alps has up to now only been witnessed at the Val Sassa rock glacier, where a maximum displacement distance Figure 3. Cross-section showing the advancing rock glacier front and the subsidence of the rock glacier surface (Budmiger, in Krummenacher et al. 1998; altered). velocity of approx. 1.6 m/yr has been reported (Chaix, in Barsch 1996). In contrast to the accelerating drift of the Furggentälti rock glacier, the high annual displacement values at Val Sassa had continuously slowed down over a time span of 50 years. This finally led to a standstill in 1972, with the Val Sassa rock glacier since remaining in an inactive state (Barsch 1996). Another interesting fact about the displacement velocities of the Furggentälti rock glacier emerges from the comparison of mean annual displacement velocities of several boulders (302, 304, 305 and 316, Fig. 5) with the mean air temperature of the summer months of June September of the respective year. Figures of recent years clearly indicate a link between the two parameters (Fig. 4). This clearly indicates that the activity of the Furggentälti rock glacier shows a short term reaction to changes in atmospheric conditions Seasonal displacement velocities In a series of survey campaigns designed to assess a possible seasonal pattern of rock glacier activity, the displacement of a total number of 32 boulders has been measured at monthly intervals between August 1998 and October 1999 (Fig. 5). All of the boulders clearly show a seasonal change in displacement velocities, with summer velocities being higher than winter velocities (Fig. 6) (Kölbing 2001). There are several possible explanations for this behavior. The most likely is an increased activity due to the presence of water during summer months and the resulting solifluction process of settlement and displacement within the active layer of the rock glacier. Changes in the plasticity of the rock glacier material due to warmer ice temperatures during summer are Table 1. Increasing average annual displacement velocity of the rock glacier front; derived from photogrammetrical survey (Budmiger, in Krummenacher et al. 1998). Period Avg. velocity 0.3 m/yr 0.4 m/yr 0.5 m/yr Table 2. Increasing average annual displacement velocities of boulders (302, 304, 305 and 316, compare Fig. 5), located in the center of the rock glacier surface; derived from photogrammetrical ( ) and terrestrial ( ) survey (Kölbing 2001). Period Avg. velocity 0.75 m/yr 1.3 m/yr 1.4 m/yr 1.9 m/yr % change Figure 4. Relation between the average annual displacement velocities of boulders (302, 304, 305 and 316) on the rock glacier surface and the mean air temperature of the summer months (June September) of the respective year (Kölbing 2001; altered). 767

4 Figure 7. Seasonally changing displacement velocities at boulder number 416, compared with measured ground surface temperature. A similar pattern has been observed at several other boulders (Kölbing 2001). Figure 5. Location and numbering of 32 monitored boulders on the rock glacier surface. Boulder 316 identical with boulder 416 (Kölbing 2001; altered) Figure 6. Seasonal change of displacement velocities of the 32 boulders with summer velocities being higher than winter velocities (Kölbing 2001; altered). also plausible. Even a reaction to the additional weight loaded onto the rock glacier by the winter snow cover, measurable after a certain latency time, is thinkable and cannot explicitly be excluded prior to closer examination. Comparing the seasonal displacement pattern of several boulders with continuously measured surface temperatures (compare 3.1), measured at a distance of a few meters from the respective boulder, the influence of water in the process becomes obvious (Fig. 7). The rapid increase of rock glacier activity begins after the complete saturation of snow cover with water and the following zero curtain (marked by a ground surface temperature of 0 C), but before the complete meltdown of the snow cover (marked by fluctuating ground surface temperatures). This shows, that the impact of warm spring temperatures is already noticeable at the ground surface, when the rock glacier itself is yet packed under a thick layer of wet snow. This happens through intrusion of water into the rock glacier system Outlook The present results of the monitoring of the Furggentälti rock glacier activity show, that a continued effort is necessary to be able to analyze and understand the complex system behind the process of rock glacier drift. Responding to the presented facts and the technological progress of digital photogrammetry in recent years, it was almost inevitable to have a closer look at the activity of the neighboring rock glaciers at the Furggentälti site. For that reason, the existing network of ground control points had to be extended to the upper eastern parts of the Furggentälti valley in summer A renewed air photo flight in October 2001 then freed the way for setting up a full aero-triangulation for the entire air photo series since This allows enlarging the perimeter of the aerial survey and will provide a higher accuracy. First results of the neighboring rock glaciers indicate a similar rapid increase of activity over the last 30 years on at least one of the rock glaciers (Mihajlovic, in prep.). These new findings will yet need to be confirmed by control measurements. Monitoring the periglacial environment 2.4 Monitoring the temperature regime of the ground surface under the winter snow cover One of the great achievements of the research work was the development of a miniaturized, water- and shock-resistant temperature logger by Bernhard Krummenacher (Krummenacher et al. 1998). The Universal Temperature Logger (UTL) allows continuous measurements of ground surface temperatures, even below the winter snow cover (compare Fig. 7). 768

5 Figure 8. BTS in winters 1997 to 2000, monitored by Universal Temperature Loggers (UTL) both on (11, 16 and 17) and next to (29 and 32) the rock glacier (Mihajlovic, in prep). Figure 9. Left image: snow depth on the rock glacier, recorded during one of the survey campaigns. Right: map of snow coverage, derived from the photo monitoring (Kunz 2000; altered). A total of 50 of such devices have been in use at the Furggentälti test site, some of them for more than 8 years, and keep logging the temperature regime at the base of the wintry snow cover (Fig. 8). The UTL has proven to be a reliable and safe instrument for monitoring the BTS, even when the test site is not accessible due to difficult weather and avalanche conditions. 2.5 Monitoring snow coverage and snow depth Since 1993, the spatial and temporal development of the snow cover in the investigation area has been assessed by a terrestrial photo-monitoring system using digital orthophotos. The camera, installed at the site, has over the past years supplied more than 1700 photographs, observing the snow coverage. After orthorectification of the photos, the snow coverage on the rock glacier and adjacent regions can be mapped and then statistically analyzed (Fig. 9). The orthophoto sequence shows, that the typical spatial pattern of snow coverage disappearance is similar in each year, defined by the topography of the area. Unlike the spatial pattern, the temporal variations of the duration of snow coverage are high. They range from approx. 240 to more than 300 days per year. Combined with the results of UTL-measurements, the photo monitoring of the snow coverage has proven that the duration of snow coverage is one of the key parameters modulating the energy balance of the periglacial environment at the Furggentälti test site. After the record-breaking winter of , both the depth and the meltdown of the snow cover have been monitored during several terrestrial survey campaigns. Results show a maximum of approx. 8 m of snow, with the convex and concave forms of the topography defining the pattern of both depth and duration of snow cover (Fig. 9). Figure 10. Modeled and measured meltdown of the snow cover at a location in a hollow next to the rock glacier front; based on a simple degree-day model (Kunz 2001; altered). These data, combined with snow depths measured at the neighboring IMIS-Stations of the Swiss Federal Institute for Snow and Avalanche Research, allow modeling both snow coverage and snow depth during the meltdown period from May to September of each year (Fig. 10). 2.6 Monitoring the local climate In order to improve knowledge about the local climate of the Furggentälti valley, a first automatic weather station, measuring air and ground temperatures, has been installed at the site in 1988 (Krummenacher et al. 1998). In autumn 1999, two new weather stations, designed by Schwab (2000), have been set up. Parameters measured include air temperature, air humidity, wind and radiation parameters. One of the stations is located next to the 1988 station, at a local radiation maximum (Fig. 11). It is continuing the measurement series that now extends over a period of more than 13 years. 769

6 Figure 12. The special tripod design of the weather station allows compensating the slow tilt movement of the boulder it stands on (Schwab 2000; altered). Figure 11. One of the automatic weather stations installed in 1999 (Schwab 2000; altered). Another station has been installed on a large boulder on the rock glacier surface. The idea behind choosing this location was to prevent the station from being completely covered or overrun by the enormous masses of snow during winter. The difficulty in the task was that the boulder is slowly tilting, while drifting downhill at the speed of the rock glacier surface of currently approx. 2.5 m/yr. This problem was solved by developing a special tripod, which allows the entire weather station to be re-adjusted to an upright position when it becomes necessary (Fig. 12). In addition to air temperature, humidity, wind direction and wind speed, the station on the rock glacier is logging all components of the short- and long-wave radiation balance on the surface of the rock glacier. 2.7 Outlook The data of the newly installed automatic weather stations, together with the existing knowledge about the spatial and temporal distribution of the snow cover, offer new possibilities in understanding and modeling the periglacial environment of the Furggentälti valley. Efforts are on the way to implement existing data in a model that will allow monitoring the energy balance of the rock glacier and its surroundings with an unprecedented spatial and temporal resolution. As a first step, a radiation balance monitoring model has been developed by Mihajlovic & Schwab (2000), which will use current data of short- and long-wave radiation as well as the actual situation of snow coverage. It is designed to provide blanket coverage results across the rock glacier and neighboring solifluction lobes. 3 CONCLUSIONS One of the main problems in permafrost and periglacial science is the complexity of the systems and processes that make up the periglacial environment. The search for new methods of monitoring processes and parameters is one approach to meet this challenge, allowing further insights and increasing knowledge about periglacial phenomena. REFERENCES Barsch, D Rockglaciers Indicators for the Present and Former Geoecology in High Mountain Environments. Springer-Verlag. Berlin. Imhof, M Permafrostkartierung am Blockgletscher im Furggentälti (Gemmi, VS). University of Berne, Department of Geography. Unpublished. Kölbing, D Saisonale Bewegungen des Blockgletschers im Furggentälti, Gemmi/VS. University of Berne, Department of Geography. Unpublished. Krummenacher, B., Budmiger, K., Mihajlovic, D. & Blank, B Periglaziale Formen und Prozesse im Furggentälti, Gemmipass. Mitt. Eidg. Institut für Schnee- Lawinenforschung. Davos. Nr. 56: 245 S. Kunz, I Die räumliche und zeitliche Variabilität der Schneehöhe im periglazialen Untersuchungsgebiet Furggentälti/Gemmi (VS). University of Berne, Department of Geography. Unpublished. Schwab, S Untersuchungen zur Lokalklimatologie und zur raum-zeitlichen Dynamik der Permafrostverteilung im periglazialen Testgebiet Furggentälti (Gemmipass/VS). University of Berne, Department of Geography. Unpublished. 770