How can we improve protection against rockfall risks?

Marion Bost, Chief Engineer for the French State Public Works Department and Director of the Rockfall Risk and Geotechnical Structures Laboratory (RRO/GERS) and Doctor Anne-Sophie Colas, Divisional Public Works Engineer and a member of the Rockfall Risk and Geotechnical Structures Laboratory (RRO/GERS) at Gustave Eiffel University.

The risk posed by rockfalls is a major public safety issue, affecting not only mountainous areas, but also coastlines and valleys. Although this is a nationwide risk, the techniques used to prevent it and protect habitable areas are not uniform. Two experts from Gustave Eiffel University – Marion Bost and Anne-Sophie Colas – explain rockfall risks and how to prevent them more effectively.

Rockfall incidents occur nationwide, from the mountains to the coast and in certain valleys. The size of the boulders, their speed and the areas exposed vary greatly and determine the risk from place to place. Although rockfalls are occasional, their unpredictability increases the risk to people and the built environment. "We find it very difficult to establish the cause, and to predict and design preventive measures to counter rockfalls", explains Doctor Marion Bost, Chief Engineer for the French State Public Works Department and Director of the Rockfall Risk and Geotechnical Structures Laboratory (RRO/GERS) at Gustave Eiffel University.

"It's a question of dynamics, which makes it a phenomenon that we don't fully control", adds Doctor Anne-Sophie Colas, Divisional Public Works Engineer and a member of the Rockfall Risk and Geotechnical Structures Laboratory (RRO/GERS) at Gustave Eiffel University: "But a significant combined effect of boulder mass and speed is observed at speeds of less than 100 km/hour". These observations are facilitated by tools that enable researchers to carry out large-scale experiments. The Montagnole rockfall station, for example, enables the release of boulders at more than 13,000 kilojoules, and is equipped with a catapult that can carry out tests at up to one megajoule.

This natural risk phenomenon, which is difficult to prevent, makes the implementation of a natural risk prevention plan all the more necessary. By first defining a regulatory zoning scheme, the Natural Risk Prevention Plan (Plan de prévention des risques naturels – PPRN) delimits the vulnerable areas and determines the land-use planning conditions, particularly in terms of building regulations.

In populated areas, local authorities can protect these areas by various means: by causing rocks to fall in a safe manner, by installing retaining walls or by securing boulders with anchor bolts. Other protective techniques can be used to mitigate the consequences of the phenomenon by halting or deflecting rockfalls.

Such protection can be very costly, and not all regions can afford these costs. "Public players need to study the most likely scenarios for the initial hazards and define a protection strategy by making choices based on their finances", stresses Marion Bost, before concluding: "The challenge for our research is to improve the reliability of protection systems and the qualification of the initial hazard".

Research is being conducted and prevention plans are being implemented in the light of climate disruption. Erosion and the melting of permafrost caused by global warming are exacerbating the rockfall phenomenon. Rockfall occurs in populated areas. "No increase has been observed in populated parts of mainland France, but rockfall now occurs throughout the year", notes Marion Bost.

The erosion of cliff bases, for example, is intensifying the problem of coastal recession. More frequent fires are also damaging the protective structures and vegetation that may afford protection by supporting boulders.

The rockfall phenomenon is an occasional risk that is affecting increasingly developed areas. Closely observed and studied by researchers, rockfall risk is becoming a greater concern in certain urban policies.

Rockfall risk: When the hazard is linked to a potential impact (damage to infrastructure, danger to people, etc.).

Joule: Unit of energy. One joule corresponds to the amount of work done by a force of one newton over a distance of one metre.

Regulatory zoning: Combination of the hazard map and the map of key issues for a given area.

The interview was conducted by support team for public policies.