Fig. 1 - Cross-lying stems on the slope can stop powerful lumps of rock. Photo: Ulrich Wasem (WSL)
Fig. 2 - Left: The rock shown in the image on the left was ‘caught’ by such an impediment.Right: This mountain pine forest (right) provides sufficient rockfall protection to the Fuorn Pass road close to the Zernez (Grison) village exit. As a low-cost supplementary measure, felled trees can be arranged crossways to the slope. Photos: Urs Fitze (Grisons Office for Forest and Natural Hazards)
A cylindrical lump of rock the size of a car wheel remained hanging on a pile of felled trees. Forestry workers had piled the stems crossways to the direction of the fall. If the rock had fallen another short stretch down the steep slope, it would have crashed onto the Fuorn Pass road, not far from Zernez village exit. However, it could also have been stopped in its tracks by one of the trees in the dense forest. “That is actually highly likely,” says forestry engineer Gian Cla Feuerstein from the canton of Graubünden Office for Forest and Natural Hazards. “The stems we have position in some places as rockfall protection are just an additional safety measure.”
Up to a short time ago, Gian Cla Feuerstein could only have speculated about whether the forest alone provides effective protection against rockfall here. “Based on the topography, the dense forestation and my experience as a forestry engineer, I would have assumed so. However, I could not have vouched for it.” For this reason, it was planned to install rockfall nets here on Fuorn Pass road a few years ago – a reliable but expensive protective measure, as such nets cost up to 2,400 euro per metre.
Project "Protect Bio"
Maintaining forests is considerably cheaper. But can the forest guarantee a similar level of safety to structural measures? To be able to answer this and other questions, the FOEN carried out the project “Effectiveness of biological protection measures”, which is referred to as Protect Bio. 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.
The geologist’s eye
Fig. 3 - On such a stack of felled trees across the falling directionl, curling lumps of rock get stuck. Photo: Ulrich Wasem (WSL)
The assessment of the rockfall risk begins with a look at the past. What happened in recent years and decades? As a general rule, only spectacular events or those in which damage arose are recorded. The road maintenance crews know about everyday rockfalls – from bits of gravel to fist-sized rocks which can penetrate a car roof – as they clear the carriageway regularly. Marks on the asphalt and repaired areas also bear witness to such events.
The potential initiation area in the massive fissured rock faces over the pass road extends from around 600 m to 2,100 m asl. The slope partly exceeds 45 degrees. The geologist’s work consists primarily in taking a look and observing the rock in detail explains Andreas Huwiler, himself a geologist at the Graubünden Office for Forest and Natural Hazards. The rock does not form a homogenous mass on the surface. Instead, it can be envisaged as a mass that has been torn apart by powerful forces, like a bar of chocolate which is easy to break. If the tension is too strong, cracks form.
The interior is also permeated by crevices. To envisage it, the experts examine the rock surface for indicators which allow them to draw conclusions about what is happening inside in its invisible parts – rock faces with different orientations, for example. Scenarios relevant to the assessment of the rockfall risk can be deduced from these structural-geological surveys.
Past events provide further information: Where did the rocks that broke away from the rock face land? How did the terrain influence the trajectory of their fall? The mapping of these ‘silent witnesses’ produces a “map of phenomena” which shows the areas in which rockfall may be expected and how frequently such events may arise.
From the inspection to the simulation …
The event register, map of phenomena and the scenarios derived from the structural geological observations describe the event with “sufficient precision but on no account the accuracy of a mathematical model, for example,” says Andreas Huwiler.
The engineers, who now take the lead, must also live with the remaining uncertainty. They simulate the consequences of the rockfall using a computer model. Based on a three-dimensional terrain model the computer calculates the rockfall track and the forces released by different rock and boulder sizes. The simulation software processes these scenarios until the modelled rockfall events can be evaluated statistically – it is not unusual for this process to involve a few thousand virtual rockfalls.
… and the necessary measures
The nature of the structural measures implemented in response to such events is a question that must be answered by the client. Whether public funding is promised and assigned is a political decision.
In the case of the section of the road near Zernez this means: protection against an event that may be expected every 30 years from a long-term perspective; no structural protection against less frequent events.
Based on previous assessments – in which the insufficiently quantifiable effect of the protection forest as a natural impediment was often ignored – rockfall nets or protective barriers would have had to be built along most of the stretch of road in question.
Rockfall nets or not?
However, thanks to the Protect Bio method, it is now possible to present the effect of biological protective measures against natural hazards in a form that enables them to be taken into account in risk assessments. The site gradient, stem density and other factors are incorporated into the simulation for the determination of the forest’s retention capacity.
In the case of Fuorn Pass road near Zernez, the results are astonishing: no rockfall nets are needed on around half of the affected stretch of road. They are only needed in positions where the forest is thin. And the stems laid crossways to the slope are far better: the only cost that arises here is that incurred in the logging.
Value of the protection forest
Around half of Switzerland’s forest area, 585,000 hectares, is classified as protection forest. It was neglected for decades until a new trend based on a new assessment was introduced in the 1990s. Since then the federal authorities, cantons and communes provide annual funding of around 145 million euro for protection forest maintenance.
This represents a good investment. The economic value of the protection forest is put at 3.8 billion euro per year. It is imperative that over-aged and uniform stands be regenerated. The protective effect of the forest must sometimes be boosted through targeted structural measures. However, Protect Bio shows that such measures are not always necessary.
The method is a pioneering one developed in Switzerland. There is nothing comparable available internationally. Arthur Sandri from FOEN’s Landslides, Avalanches and Protection Forest Section estimates that the consistent use of Protect Bio throughout Switzerland could enable savings totalling several dozen million euro to be made by dispensing with technical protective structures – this increases the value of the protection forest even more.
However, this stage has not yet been reached. The data necessary for the role of protection forest services which are more difficult to quantify for natural hazard processes like avalanches, landslides and debris flows are not yet available. It is planned to use Protect Bio in other locations and, in particular, in the context of these natural hazards in the years to come and to improve its validation. The method will only be adopted as standard when this work has been completed.