Oliver Sass

Temporal Variability of Rockfall in the Bavarian Alps, Germany

Abstract This paper presents data derived from extensive rockfall investigations in the Karwendel Mountains and in the Ammer Mountains near the German-Austrian border. The rock faces in the areas investigated consist of Mesozoic limestone and dolostone. The debris was quantified by means of more than 60 rockfall collectors, which were emptied regularly. Air and rock temperature data were measured by a meteorological station and by several dataloggers equipped with pT100 temperature probes. The temporal distribution of rockfall was highly variable, which is due to the combination of several trigger parameters. A statistical analysis of the dataset revealed that various meteorological conditions enhance the probability of rockfall occurrence. Deep frost in winter, freezing and thawing during wet weather conditions, and wetting/drying contribute to the weathering of the rock faces investigated. The actual removal, leading to rockfall, is triggered by a complex combination of frost shattering, temperature fluctuations, and downwash by precipitation. Despite the irregular patterns, it is evident that the mean removal in the summer months is much higher than in winter, which is probably due to the lack of water in the cold season. Even a large number of temperature oscillations across zero do not seem to affect weathering as long as there is an insufficient moisture supply.

Impact of forest fires on geomorphic processes in the Tyrolean Limestone Alps

Sass, O., Haas, F., Schimmer, C., Heel, M., Bremer, M., Stöger, F. and Wetzel, K.‐F. 2012. Impact of forest fires on geomorphic processes in the Tyrolean Limestone Alps. Geografiska Annaler: Series A, Physical Geography, 94, 117–133. doi:10.1111/j.1468‐0459.2012.00452.x ABSTRACT We investigated geomorphic processes on two slopes (Arnspitze and Bettelwurf) burned by wildfire in the region north of Innsbruck. Both burned in 1946 and both are still characterized by severe vegetation destruction. Sparsely grass‐covered rock and debris slopes have developed replacing the former dwarf pine (Pinus mugo) shrub stands. Our aim was to establish disturbed and undisturbed erosion rates and to decide whether recent debris flows can be assigned to these historical wildfires or not. We measured fluvial erosion by means of collectors, estimated the amount of post‐fire erosion from stratigraphic exposures in the adjacent talus, modelled bedload discharge with a statistical model developed in a nearby study area and quantified recent debris flow activity by combining airborne and terrestrial laser scans. We measured erosion rates of 3–30 gm−2a−1, which is roughly ten times higher than the undisturbed sediment yields. Slopewash was higher than linear fluvial transport in the four years of our investigation. Surplus material was removed from the channels by avalanches and debris flows; both being more important for the sediment budget than fluvial action. The modelling approach allowed measured sediment yields to be transferred to larger slope parts and to calculate scenarios of pre‐fire conditions. The concordance of measured and modelled yields was reasonably good; deviations may be explained by differing amounts of precipitation. Our results support the impression that current debris flow activity at the Bettelwurf was enhanced by the aftermath of the 1946 wildfire.

SPATIAL AND TEMPORAL PATTERNS OF TALUS ACTIVITY – A LICHENOMETRIC APPROACH IN THE STUBAIER ALPS, AUSTRIA

Abstract. Lichenometric measurements using Rhizocarpon ssp. were carried out on 20 talus slopes in the cirques of the Finstertal valley (Austria) at an elevation of 2300–3000 m a.s.l. The aim was to assess activity patterns on selected slopes and between the slopes of the study area, to find evidence of rockfall pulses in the last centuries and to calculate rockwall retreat rates. A calibration curve was derived from five sites of known age and adapted to the prevailing size of talus boulders. We measured the five largest lichens on more than 300 boulders and the percentage coverage of Rhizocarpon‐free clasts on more than 1000 test fields. Most of the investigated talus cones are characterized by moderate rockfall supply, with the apex being more active than the talus foot and moderate redistribution by avalanche and debris flows. Considerably enhanced activity was found under rockwalls influenced by permafrost, particularly on the north faces at an elevation of >2600 m a.s.l. At currently moderately active sites, boulder falls seem to have been slightly more frequent in the late nineteenth and first half of the twentieth century. In positions where permafrost is expected in the rockwalls, a weak maximum in the late nineteenth century and highly active present‐day conditions were found, the latter being assigned to current permafrost melt. Rockwall retreat rates derived from lichen coverage are between 400 and 1500 mm/ka which is in good concurrence with talus volume assessments, but higher than the rates derived from direct rockfall measurements. The rates derived from lichen coverage have to be taken with caution as the effects of debris redistribution are hard to quantify.

UAV and TLS for monitoring a creek in an alpine environment, Styria, Austria

ABSTRACT Flood events often endangered and damaged the small town of Oberwölz (Styria, Austria) and its nearby surroundings. In 2011 the last bigger flood occurred. After that, the main watercourse, the Schöttlbach creek, became a centre of a national funded research project. A wide variety of methods were applied together with intra- and extra-universitarian project partners to investigate that creek from a fluvial-geomorphological point of view. Apart from hydrological, meteorological, and geophysical methods, both, terrestrial and airborne remote-sensing techniques were applied. In particular, terrestrial laser scanning data as well as terrestrial and aerial photographs were frequently recorded since 2012. On the one hand the terrestrial monitoring focused on some representative erosion sites to quantify detailed erosion rates. On the other hand recording the sediment catchment output at the flood detention basin was intended. The aerial-based monitoring included three flight campaigns using mainly a fixed-wing unmanned aerial vehicle to cover approximately an area of about 170 ha that corresponds roughly to the major parts of the Schöttlbach creek. The main focus of the study lies on the adequate processing, georeferencing, and qualitative and quantitative comparison of the recorded elevation data. This was primarily done by using structure from motion photogrammetry and geodetic measurements. The resulting point clouds, digital elevation models, and orthophotos allow a visualization and quantification of morphodynamic processes. As a main outcome of this study, the combined usage of the mentioned methods can be seen. In this way, data recording from diverse perspectives is possible and allows a consideration of the varying terrain inclination. In addition, the significance of the resulting data for a geomorphological interpretation was assessed by describing the uncertainties. Beyond that, this methodology meets the need for observation at different scales.

Long-term vegetation development on a wildfire slope in Innerzwain （Styria, Austria）

Forest fires in mountainous areas can cause severe deforestation which can potentially trigger secondary natural hazards like debris falls and avalanches. We documented an extreme case study for the range of possible post-fire land cover (LC) dynamics. We investigated a 15-ha, steep (10°–65°) burnt slope in Styria (Austria) at elevation of 760°-113 m, which burned in 1946 and has not fully recovered to date. Seven 8-class legend LC maps were produced (1954, 1966, 1973, 1982, 1998, 2004, 2009) and integrated in a vector-based GIS, mainly by on-screen interpretation of aerial photos. Our aim was to clarify how post-wildfire LC dynamics take place on a severely damaged, steep slope and to give a basic projection of the future vegetation recovery process. The pre-fire Pinus sylvestris stands have been mainly replaced by Picea abies and Larix decidua. Regeneration proceeded mainly from the base of the slope upwards. All tree species together still cover no more than 40% of the slope after more than 60 years of recovery, while grassland communities and rock/debris areas have expanded. Multitemporal analysis showed a slow but steady increase in woodland cover. Degraded rock/debris areas, however, expanded as well because soil erosion and related debris flows remained active. Slope angle (with a threshold value of approx. 35-40°) seemed to control whether erosion or regeneration prevailed. According to a simple extrapolation, the slope will not reach its former condition before 2070. This extreme disturbance window of more than 120 years is owed to the steepness of the slope and to the shallow soils on dolomitic bedrock that were severely damaged by the fire. The neglect of any game fencing is a further factor slowing regeneration.

Influence of Salt and Moisture on Weathering of Historic Stonework in a Continental-Humid, Urban Region

On the long term, the presented project deals with the processes involved in salt weathering at monuments and natural rock outcrops in climatically different areas. As a pilot study we investigated an example of a cultural heritage structure in the continental-humid, urban region of Graz, Austria using a range of methods for moisture and salt monitoring. The aim was to assess the influence and the interaction between moisture and salts in the formation of the observed weathering structures of the involved limestones. In the spirit of an integrative, multi-method approach we use a range of techniques: (1) Mapping of weathering phenomena; (2) ERT (Electrical Resistivity Tomography); (3) Handheld Moisture Meter; (4) Paper pulp poultices; (5) Laboratory investigation (IC—Ion Chromatography). The various methods were applied to investigate the causes of weathering phenomena at the columns of the city gate ‘Paulustor’ in Graz.

Patterns of long-term regeneration of forest fire slopes in the Northern European Alps – a logistic regression approach

ABSTRACT We report on vegetation recovery after stand-replacing wildfires on steep slopes on limestone and dolostone bedrock. The sites burnt in the nineteenth and twentieth century and some have remained almost bare of vegetation. We mapped and analysed 26 burned slopes to determine site-specific factors influencing regeneration. We followed a multinomial-logistic regression approach to explain the patterns of grassland, shrub and tree vegetation in 10 × 10 m cells. We considered eight potential controlling parameters (time since fire, elevation, aspect, slope, topographical index, distance to forest edge, land cover of neighbouring pixels and lithology). The pseudo R² values for the multinomial-logistic regression point to a reasonably good model fit, even if some potentially important parameters (e.g. post-fire management) could not be incorporated. Tree regeneration is impeded by higher elevation and by steep slope angles while shrubs are unaffected by inclination. Although the probability of pixels with grasslands is higher in proximity to neighbouring grassland pixels, we found no such effect for trees or shrubs. There is no statistical correlation between the type of vegetation and time elapsed since the fire. This means that under adverse conditions, the time required for forest recovery is longer than the time period captured by our investigation (200–300 yr).