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.