Die Schutzfunktion in Windwurfflächen

Abstract

Protection against natural hazards on windthrow areas We give an overview on natural hazard processes on windthrow areas during the first 20 years after the winterstorm Vivian (1990). This overview is based on 1) repeated pulling experiments and a long-term analysis of stem movements in the uncleared winthrow area Cavorgia/Disentis, 2) the assessment of avalanche protection function of 26 windthrow areas, and on (3) StorMe cadastral data of natural hazard events on windthrow areas. The effective heights and stem resistance of lying logs have been reduced to ca. 40% of the original values on average in the uncleared winthrow area Cavorgia/ Disentis. In particular on very steep (>45°) and rocky slopes, some of the stems have moved several meters. Most of the 26 investigated windthrow areas did not fulfill any of the considered criteria to effectively protect against avalanches. Nevertheless, very few avalanches and rockfall events were observed on Vivian areas. The relatively large number of shallow landslides in the years after the windthrow could, however, be a sign of a certain time with increased landslide susceptibility. Our results and observations suggest that the increased terrain roughness after Vivian largely compensated for the decreased protection effects against avalanche and rockfall of former stands structures during the first years after the storm. After 20 years, the terrain roughness effect has strongly decreased and reliable protection against mass movements depends in most cases on a sufficiently advanced natural forest regeneration or on additional post-windthrow plantations. It is important to consider specific conditions on the level of single slopes when planning measures against natural hazards in windthrow areas.

Changes of forest cover and disturbance regimes in the mountain forests of the Alps

“…Other positive feedbacks can result from the removal of biomass due to windthrow, fire, or other disturbance and subsequent gravitational hazards like avalanches, rockfall, or shallow landslides. Such interactions are of particular societal importance in the densely populated Alps, and more knowledge is needed on interactions between a broad range of disturbance agents, in different forest types, and under different management regimes (Conedera et al, 2003; Bebi et al, 2015).…”

Multiscale analysis of surface roughness for the improvement of natural hazard modelling

“…In the case of old disturbed forest, a roughness time-series analysis might not distinguish between the roughness of old fallen logs, lower vegetation and tree regeneration. After years of decomposition, the fallen logs become less supportive, decrease in height, are moved and even decompose completely (Bebi et al, 2015;. A comprehensive overview of the decay process over a longer period after a disturbance (more than 20 years) would be helpful to understand the function of time and the remaining protection capacity after a disturbance such as windthrow.…”

Biological Legacies and Rockfall: The Protective Effect of a Windthrown Forest

“…However, some tensile tests showed a reduction of the breaking point of logs, mainly due to the decay of wood. It was also observed that the level of decay was related to the microsite, mainly the height above ground of logs and their vegetation cover [39]; the presence of bark on logs may also influence the speed of decaying processes [40]. Although we were able to observe and quantify the protective role played by structural legacies, it is important to underline that this role is strictly connected with the type of disturbance that hit the protection forest.…”

“…The facilitation effect provided by deadwood can be fundamental for speeding up natural regeneration processes. The presence of biological legacies, indeed, may not only provide protection, but also create the condition for shortening the protection gap, which should be the main target of management strategies for a damaged protection forest [1,12,39].…”