Effect of forest in the transit and deposition area of hillslope debris flows

In Switzerland, spontaneous shallow landslides and hillslope debris flows cause high infrastructure damage, closures of important infrastructure, evacuations, and sometimes fatalities every year. At the latest since the introduction of the cycle of integral management of natural hazard risks, the protection forest is the most important biological measure against shallow landslides and hillslope debris flows. The focus is usually on the effect of the protection forest in the landslide fracture area. However, there is often evidence in the field that the forest has a protective effect not only in the fracture area but also in the transit and deposition areas of hillslope debris flows. Nevertheless, the protective effect of forests in the transit and deposition areas of hillslope debris flows has not yet been systematically investigated. Therefore, the aim of this study was to measure the relevant forest structure parameters that determine the protective effect. In addition, we quantified this effect by means of a risk assessment in two case studies. With the help of analyses of historical hillslope debris flow events in combination with terrain surveys in the forest, it could be shown that the braking effect of the forest is mainly influenced by the number of trunks. As the density of the forest increases, the length of the runout distance decreases. In addition, it could be shown that significantly more material can be deposited behind thicker trees. This suggests that a well-structured stand with a high number of trunks, but also occasional large diameters, offers ideal protection against hillslope debris flows in the transit and deposition area. The results of the laboratory tests indicate that the braking effect of different forest structures (dense, open, with gaps) is always given compared to a situation without forest, and this at lower (16%) and higher (20%) water contents. Runout sections of historical hillslope debris flow events with and without forest in the transit section could be reproduced with simulations based on a digital elevation model (DEM) with rigid cylinders with BHD > 20 cm. This allowed a calculation and monetisation of the protective effect of forest in the transit section of hillslope debris flows. This showed that the risk-reducing effect of forest in the transit area of a hillslope debris flow can amount to CHF 6500/year for a motorway similar to the Gotthard section.