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Forest as a landslide breaker

WHFF project 2019.16

Project leader: Prof. Dr. Luuk Dorren, Bern University of Applied Sciences, School of Agricultural, Forest and Food Sciences (HAFL)

The short video about the project on Youtube can be watched under the following link:

The most important facts in brief

• Research into the influence of forests on shallow landslides and hillslope debris flows in Switzerland, which regularly pose considerable damage and risks to infrastructure.

• By analyzing historical landslide events and field surveys, it was determined that the density of the forest, in particular the number of trees, influences the braking effect. Laboratory tests with simulated hillslope debris flows and 3D simulations showed that well-structured forest areas with a high number of trunks and isolated large-diameter trees offer ideal protection.

• Simulations showed that the risk-reducing effect of the forest along landslide transit areas, such as highways, can be estimated at CHF 6,500 per year.

• The maintenance of protection forests is crucial to ensure effective protection against natural hazards in the long term. The existing guidelines in Switzerland should be improved based on the results of this project in order to promote forest management adapted to natural hazards.

Project description

In Switzerland, spontaneous shallow landslides and hillslope debris flows regularly lead to considerable damage to infrastructure, closures, evacuations and even fatalities. Since the introduction of holistic natural hazard risk management, protective forests have been considered a decisive biological measure against shallow landslides and hillslope debris flows.

So far, the influence of forests on the runoff of hillslope debris flows has not been systematically investigated, and this was the main objective of this project. Using databases on hillslope debris flows in Switzerland, the affected sites were visited and characterized. In addition, laboratory tests were carried out. By using a constructed slide and small wooden sticks simulating trees, different fluid thicknesses could be used to mimic the course of hillslope debris flows. Finally, the protective effect of the forest against landslides was investigated in a 3D simulation model with various settings.


As expected, the evaluation of historical hillslope debris flow events in conjunction with field surveys showed that it is primarily the number of trees (i.e. the density of the forest) that influences the braking effect of the forest. As the density of the forest increases, the distance over which the hillslope debris flow runs decreases. In addition, the measurements carried out on site revealed a clear relationship between the diameter of the trees and the amount of material deposited behind the trees.

The results of the laboratory tests indicate that different forest structures (dense, open, with gaps) slow down landslides more than non-forested terrain.

In conclusion, it can be said that a well-structured tree population with a high number of trunks and occasional large diameters offers ideal protection against landslides in the runoff route. Simulations of the course of historical hillslope debris flows with and without forest enabled a monetary evaluation of the protective effect of the forest in the runoff route. Accordingly, the risk-reducing effect of the forest in the transit area of a hillslope debris flow, for example along a highway such as the Gotthard route, could amount to CHF 6,500 per year.

However, it is important to manage protection forests specifically and in a way that is adapted to natural hazards in order to ensure effective protection against natural hazards in the long term. In Switzerland, there are corresponding guidelines for this, and the results of our project are directly incorporated into the improvement of these guidelines.

The full report to download (DE)::

SB 2019.16 Dorren_Wald_als_MurenBrecher
Download PDF • 32.28MB

You can find more information about the project on ARAMIS.

The project was supported by the Swiss Forest and Wood Research Funding Switzerland (FOWO-CH) of the Federal Office for the Environment (FOEN) and the Conference for Forests, Wildlife and Landscape of the cantons. 


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