This report illustrates the influence of the storm event Lothar on hazardous processes in torrential catchments in Switzerland. Within four sub-projects, results were acquired in two investigation areas as well as under laboratory conditions
In the context of investigations on the runoff formation carried out on three scales (nested approach), forest influence was studied in the Sperbelgraben catchment. By means of irrigation experiments on soil profiles (1 m²), as well as runoff measurements on plots of 50 to 110 m² size, two main processes producing surface runoff could be identified: Soil saturation on Gleysols leads to large amounts of saturated overland flow whereas hydrophobic reactions in the organic litter layer on well drained Cambisols sporadically cause temporary Hortonian overland flow. Analysis of runoff data from sub-catchments (approx. 20'000 m²) suggest that geomorphologic characteristics and the spatial distribution of wet areas do have a large impact compared to the fraction of forest cover. Effects of storm damage elements as well as locally occurring soil compaction due to forest clearing are of minor importance with respect to the runoff behaviour on all considered scales.
Investigations regarding the effect of storm damage on the sediment budget were carried out on the heavily damaged slope in the Spissibach catchment as well as in the Sperbelgraben. The emphasis of the study focused on sediment delivery from the hill slope to the stream channel, as well as further transport within the channel itself. The developed characterisation is based on detailed inventory mapping, as well as on area-wide photomonitoring. On the investigated slopes, the post-storm slope dynamics are dependent largely upon the overall character of the test area, the type of disturbance (damage) and the occurrence of forest clearing activities. The results indicate that the slope dynamics were intensely influenced by Lothar only in some local areas within the test site. Effects on the catchment scale or increased torrential hazards to the alluvial fan were not observed.
In a new approach to quantify the influence of vegetation on soil stability, vegetation effects are considered as a virtual increase of soil density, enhancing the shear strength of the soil. Correlations between water stability of soil aggregates, dry unit weight and shearing resistance were investigated. First test results indicate that planted soil specimens have an improved aggregate stability and slightly increased angles of shearing resistance compared to unplanted specimens.