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Rocking Alps

Rockfall and Weathering in the Eastern Alps (Verwitterung und Steinschlag in den Ostalpen)

Fördergeber / Funding: FWF

Fördersumme / Amount of funding: 182.000 Euro

Projektstart / project start: 01-04-2012 (3 years)

Mitarbeiter / Staff:

  • Dr. Oliver Sass (principal investigator)
  • Mag. Matthias Rode (PhD student)

 

Kurzvorstellung / short presentation: Poster

Weitere Informationen / More detailed information:
Poster (from GELMON Meeting, Vienna 2011)

Kontakt / contact: Oliver Sass <oliver.sass(at)uni-graz.at>

Messtechnik am Kitzsteinhorn

 

 

 

ZUSAMMENFASSUNG
Der Prozess der Felsverwitterung und der damit verbundenen Steinschlagentstehung, insbesondere die Entstehung von Sprengdruck beim Gefrieren, ist noch immer nicht gänzlich verstanden. Ein Schlüssel zum Verständnis des Prozesses liegt in der Erfassung von Feuchteverteilungen und Feuchtebewegungen während des Frosteintritts. Diesbezügliche Daten von natürlichen Felswänden sind jedoch, wenn überhaupt vorhanden, extrem lückenhaft. Die Methode der kleinräumigen 2D-Geoelektrik erlaubt eine räumlich wie zeitlich hoch aufgelöste Beobachtung von Feuchteverteilungen im Gestein. Ein systematisches Monitoring während täglicher und jährlicher Frostwechsel ist geeignet, den Wissensstand zur Frostverwitterung und den dabei ablaufenden Prozessen entscheidend zu erweitern.

Zu diesem Zweck wird in den Nördlichen Ostalpen (Gesäuse, Dachstein, Kitzsteinhorn) ein halbautomatisches Monitoring durchgeführt, bestehend aus fest installierten 2D-Geoelektrik-Apparaturen mit Messprofilen in Nord- und Südexposition. Begleitend registriert werden die Felstemperaturen, die Gesteinsfeuchte, die zeitlich hoch aufgelösten elektrischen Leitfähigkeiten sowie der Porenwasserdruck in verschiedenen Felstiefen. Während die Messungen im Gesäuse in Talnähe (und damit in der Nähe von potenziell gefährdeten Infrastruktureinrichtungen) stattfinden, erfolgt das Messprogramm am Dachstein in hochalpinem Gelände, womit auch die Auswirkung von Permafrost und Permafrostdegradation auf die Verwitterung einbezogen wird. Die Messungen werden durch Feuchte-Simulationsrechnungen mit dem Programm WUFI komplettiert, die einen Methodenabgleich und eine Übertragung der Ergebnisse in andere Felspositionen ermöglichen.

Als Outputgröße der Frostverwitterung wird im Umfeld der Messinstrumente der Steinschlag mittels Fangnetzen und terrestrischem Laserscanning (TLS) an Testflächen gemessen. Dies ermöglicht eine Zuordnung der beobachteten Frostereignisse hinsichtlich ihrer Verwitterungswirksamkeit. Die Ergebnisse werden mit Infrarotaufnahmen verglichen, um die räumliche Verteilung der Abwitterung mit mikroklimatischen Bedingungen in Beziehung setzen zu können.

Die Ergebnisse sollen zu einem deutlich verbesserten Verständnis der Verwitterungsprozesse an Felswänden beitragen. Dies ermöglicht eine räumliche Modellierung und die Ausweisung gefährdeter Bereiche, sowie eine Prognose von Zeiten erhöhter Steinschlaghäufigkeit.

 


ABSTRACT

The process of weathering and induced rockfall, particularly the generation of pore water pressure during freezing, is still not fully understood. A key to understanding is to gather data on rock moisture distribution and pore water displacement during freeze-thaw events. However, such data from natural rockwalls is extremely sparse if altogether present. The small scale 2D-resistivity profiling technique, allows spatially and temporally high resolution observation of moisture distribution in rock. Systematic monitoring during daily and annual freeze-thaw cycles is suitable for vitally advancing the state-of-the-art knowledge on frost weathering and subsequent rock detachment.

For this purpose, a semi-automated monitoring program is implemented in the north-eastern Alps (Gesäuse, Dachstein, Kitzsteinhorn), comprised of fixedly installed 2D-resistivity instruments with survey lines in northerly and southerly orientation. Additionally, rock temperatures, rock moisture, electrical conductivity and pore water pressure are registered at different depths in high temporal resolution. While the measurements in the Gesäuse are carried out near to the valley floors (and thus, close to potentially endangered infrastructure), the test sites at Dachstein and Kitzsteinhorn are situated in high-alpine environment which allows to investigate the impact of permafrost und permafrost degradation on weathering. Moisture simulation calculations using the program WUFI complement the investigations, allowing to cross-check the results from different techniques and to extrapolate the results to other topographical positions.

Rockfall, being the output parameter of frost weathering, is determined at test plots in the vicinity of the measuring instruments by means of wire mesh collectors and terrestrial laser scanning (TLS), allowing to rate the monitored frost events according to their effectiveness for weathering. The results are compared to infrared photography to find relations between microclimatic parameters and weathering patterns.

The results are thought to contribute to a better understanding of weathering processes at rockwalls, thus allowing to model and delimitate particularly weathering-prone areas and to predict times of heightened rockfall activity.

 

 

INTRODUCTION

Rockfalls are ubiquitous natural hazards in alpine regions, affecting individuals like hikers and tourists, but also infrastructure like roads and buildings. The detachment of rock fragments from alpine rockwalls is mainly assigned to frost weathering. However, the actual process of frost weathering as well as the contribution of further weathering processes (e.g. hydration, thermal fatigue) are poorly understood. Consequently, the impact of ongoing climate change on hazardous rockfall events cannot be assessed without further knowledge on the subject.

In the framework of the ROCKING ALPS project investigations are carried out in three areas of the Eastern Alps of different elevation and lithology. The first study area is the Gesäuse as part of the north-eastern Limestone Alps. The prevailing rock types are the Dachstein limestone and the Wetterstein dolomite. The study sites are at an elevation of 800-1200 m. The Dachstein area is situated west of the Gesäuse and reaches a summit height of up to 2.995 m. The rockwalls are also built up of Dachstein limestone. The existing permafrost in the north-facing rockwalls is a highly interesting comparison area to the geologically similar Gesäuse. The third area is the Kitzsteinhorn (3203 m) in the Hohe Tauern range consisting of calcareous mica-schist with permafrost.

 

 

 

 The aims for ROCKING ALPS are (1) to get information of the rock moisture in high temporal and spatial resolution, (2) to see the pore water movement during freeze-thaw events and (3) to connect the moisture and rockfall data (TLS). Therefor the planned 2D-resistivity measurements will be combined with rockfall monitoring.

Monitoring of water content, water displacement and freeze-thaw processes in rock using permanently installed geoelectric survey lines (ERT). Very short-term moisture fluctuations (e.g. those induced by freeze-thaw events) cannot be accurately recorded by geoelectric profiling. Thus, additional temperature and moisture measurements at higher temporal resolution have to be carried out. Two techniques of moisture measurement will be applied - transitional resistivity measurements and heat capacity measurements. For investigating the impact of observed moisture fluctuations on weathering, regular laser scan measurements (TLS) are carried out at several monitoring sites in the vicinity of the geoelectric instruments. The achieved datasets allow assessing the dominant controlling factors of rockfall. The datalogger and geoelectrical measurements are performed at fixed, comparatively small sites at high temporal resolution, while TLS offers the detailed spatial distribution within the test sites. To tie these two approaches closer together, georeferenced infrared photos will be taken at regular intervals. This enables to visualize temperature patterns and amplitudes. The cross-check with TLS data will offer interesting insights into the relevant drivers of weathering.

 

 

 

 

As freeze-thaw cycles of different duration and intensity can contribute to rock shattering, these events can only be adequately investigated by means of a continuous monitoring program.  The design of the survey lines enables detailed observation of small-scale water movement during wetting, drying and freeze-thaw events (50 electrodes, spacing 0.06 m, Wenner-array) while the additional longer profiles at the Dachstein are thought to record the presence of frozen rock to a depth of c. 3 m (25 electrodes spacing 0.5 m, Wenner-Array).

The survey lines will be maintained over a period of at least one year each. Freeze-thaw cycles of different duration and intensity can contribute to rock shattering:

 

  1.  Night frost in spring, summer and autumn, or daily thawing during sunshine in winter (both lasting some hours);
  2.  freeze-thaw events during the passage of cold fronts (lasting some days);
  3.  winter frost of several weeks or months with consequently deeper penetration of subzero temperatures.

 

Considerably different freezing behavior between north- and south-facing sites, as well as between permafrost and non-permafrost sites is to be expected.

 

 

 

The acquisition of temperature mainly point-based measuring methods have been used and accordingly, two-dimensional temperature data is rare. To overcome this limitation, an infrared camera is used to collect and analyse data on the spatial temperature distribution on 10 x 10 m sections of rock faces.

 

 

 

 

 

 

 

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