Forest to protect the road from rockfall : the Fuorn Pass road, Engadin Region, Switzerland

Forests provide more effective protection against natural hazards than even experts in the field believed up to now. Maintaining forests is considerably cheaper than building costly technical structures. But can the forest guarantee a similar level of safety to structural measures? The Protect Bio method enables the evaluation of this nature-based protection service. As part of this project, a method was developed which makes it possible to determine the effect of the forest and other biological protection measures and to take them into account accurately in hazard protection projects. This method aims to evaluate the forest protection functions against natural hazards or the need for implementing technical protective measures (i.e. barriers or nets) to prevent from rockfalls damages. The method was used in practice for the first time on Fuorn Pass road, which connects Zernez in the Engadin valley with Val Müstair.Fuorn Pass road, near Zernez in the Engadin region (Switzerland), is an approximately 800-metre-long stretch of road. Rockfall risk in this area was analysed with the help of the Protect Bio method. Rockfalls events registered in the past, maps of past events and scenarios derived from the structural geological observations allow describing the rockfall risk: the analysis shows the areas in which rockfall may be expected and how frequently such events may arise. A mathematical model was also used to simulate the consequences of the rockfall events. Based on a three-dimensional terrain model the computer calculates the rockfall track and the forces released by different rock and boulder sizes.The Protect Bio method also enables to take in consideration in the risk assessment the role of biological protective measures provided by forests against natural hazards (rockfalls, landslides, avalanches, etc.). The site gradient, stem density and other factors are incorporated into the simulation for the determination of the forest’s protection capacity.In the case of Fuorn Pass road near Zernez, results showed that no rockfall nets are needed on around half of the affected stretch of road. In this portion, the protection function provided by existing forests is enough to ensure protection against events with less than 1 to 30 years return period. As a low-cost complementary measure, felled trees can be arranged crossways to the slope. Technical and more expensive measures (such as nets) are only needed in stretches of the road where the forest is thin.Based on previous assessments – in which the insufficiently quantifiable effect of the protection forest as a natural impediment was often ignored – rockfall nets or other protective barriers would have had to be built along most of the stretch of road in question. With the Protect Bio method, the forest protection function has been evaluated and promoted as adaptation measure based on accurate risk assessment and cost-benefit analysis. The application of this method, thus, allowed saving millions of euros on technical protective structures that have been evaluated as not necessary.