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Designing forests to protect against falling rock

25 February 2011 Irstea (formerly Cemagref)

Falling rock is a natural process in the evolution of mountain slopes. Unfortunately, it also threatens urbanised areas and the many roads below. To reduce the risks involved, Cemagref scientists have studied the protection function of mountain forests. By analysing the propagation of falling rocks, they have developed robust techniques (simulation models and management practices) to assist forest managers in optimising the capacity of forests to provide protection.

Rock falls generally occur at the end of fall and beginning of spring, when thermal variation is the greatest. But the type of terrain and its geological structure, the slope, the quantity of infiltrated water, etc., are all factors that can contribute to rock falls. In this context, how can the buildings and roads below be effectively protected? One solution is rockfall nets and other types of structures. However, field observations have revealed that in some cases, rocks are stopped by trees and that forests, if correctly managed, can constitute an ecological and economic alternative to man-made structures. In France, approximately 40% of mountain forests serve as natural rockfall barriers capable of partially or even totally blocking rock propagation. That proportion should rise in the coming years given increasing building on slopes. Cemagref scientists have developed diagnostic tools and methods to assist managers in maintaining the services provided by forests, notably protection, but also recreation, wood production and as a biodiversity reserve.

Three models for precise management

Since 2003, the Cemagref teams in Grenoble have developed mathematical simulation tools, RockforNET, RockforLINE, and participated in the development of Rockyfor3d for forest management in view of maintaining the protection function. These systems are supplied with a wide range of parameter data to take all possible cases into account (e.g. geology, rock sizes, slope, tree species and types of stand, etc.). The models, operating in one, two or three dimensions, can indicate in just seconds or hours depending on the system used, the level of protection provided by different types of forest on a given slope. They can also map the dangerous zones, determine the most suitable types of stands and provide crucial information on the trajectory heights of rocks, their speed and energy levels, and lateral deviation to ensure optimum sizing of the "protection structure".

What does an ideal forest look like?

Even though hardwood trees can absorb twice as much energy in the form of a falling rock as an equivalent-sized conifer, it is advised to mix tree species in order to maintain the forest cover over time. That improves forest capacity to dissipate the energy of falling rocks and stop them from reaching the valley. In addition, because coppice comprises a large number of stems, it offers a higher probability of collision between a rock and a tree and thus serves as an effective barrier to rock propagation.

To maintain the protection function, managers must cut certain trees to provide the light and space required for forest renewal, germination of seeds and growth of young trees[1].

Using adequate forestry techniques, it is possible to set up a strategy to replace or complement man-made structures and thus effectively counter the risk of falling rock.

The Manfred Interreg project for Alpine forests, involving 15 partners and 6 nationalities

Cemagref participates in the European Manfred project to devise management strategies to adapt Alpine forests to the risks inherent in climate change. The project addresses two main topics, first, the protection and preservation of forest ecosystems and, secondly, effective forest management based on our knowledge of adaptation strategies.

Frédéric Berger leads the work group on "Forest management targeting protection against natural hazards" with the goal of creating a European platform to facilitate the sharing of experience and knowledge.

[1] Note that the cut trees do not go to waste, they can be positioned on the slope to create obstacles and slow the progression of falling rocks.

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