Forests can foil rockfalls
Rockfalls are part of the natural evolution of cliffs. Due to their unpredictable force, they can often overcome manmade defences and pose imminent danger. Researchers at Cemagref, the French land management research institute based in Grenoble, have been exploring ways to ‘design’ forests to act as natural barriers to falling rocks.
Often the result of erosion processes and climatic factors, such as rainfall and temperature variations, rockfalls are something of a geological certainty, says Frédéric Berger, a research engineer at Cemagref. ‘What is upslope will eventually finish downslope.’
Berger thinks that well-thought out forest management can offer an effective defence mechanism, with trees intercepting and dissipating the kinetic energy of rocks. He asserts that well-structured stands of trees and a variety of forest developmental phases can beat traditional methods, such as man-made net structures. ‘It is not possible, for economical reasons, to put nets everywhere, [and] in mountain areas, the slopes are very often covered by forests,’ notes Berger.
He adds that, as well as fixing nets on living trees, or using logs for rockfall barriers, other natural methods include designing the forest so that the trees are positioned to divert the rocks’ trajectories. ‘It costs less to manage forests than to replace them with civil engineering works – the main question is to determine where the forest offers a sustainable and efficient solution. Our tools are dedicated to answering this question.’
Berger’s team is developing and trialling three simulation models which they hope will help predict how and where rockfalls will occur. Rockfor.NET is a rapid assessment tool, freely and publicly available on the internet. It is a 1D model that quantifies the protective capacity of a forest stand, and predicts the number of ‘curtains’ that would be required to form a protective area.
The other models are RockforLIN, a statistical 2D tool used to build up a geographic information system, and RockyFor, which simulates trajectories of falling, bouncing and rolling rocks in 3D. ‘RockyFor allows a more thorough analysis of the rockfall hazard because slope irregularities are taken into account, and it provides spatially explicit output. Moreover, it explicitly simulates the impact of falling rocks against trees,’ says Berger.
The team has already used the models to make the Risk Prevention Plan for a commune in Veyrier du Lac in France. ‘Two days after the presentation of our results, an event occurred and its run-out zone was exactly the same as the ones obtained with our models,’ says Berger.
The models can be applied to forested areas across the globe, with 25 private companies currently using Rockyfor3D, and Rockfornet in the USA, Japan, Australia, Turkey and the EU. Berger adds that although the models have not been trialled in all possible situations because of logistical reasons, ‘we are developing a community of users and a web exchange platform in order to communicate their experiences, knowledge and success and non-success stories. The goal of this is to improve our models with the information coming from the users’.
According to Berger, there are many measures that forest managers can take to optimise the protective capacity of forests, such as guaranteeing the sustainability of an efficient forest, avoiding clear cutting and oversized regeneration gaps.
Although man likes to think he can master nature, Berger thinks that it is healthy to be a little scared of it and not over-estimate man-made protection. ‘A 100% protection doesn’t exist.’ As with town planning schemes that cut unnecessary street clutter, such as fences, signs and crossings to encourage drivers and pedestrians to be more aware of each other, he hopes that ‘working with the forest will help to develop a real risk culture so people will develop more intelligent and responsible behaviour in risk prevention’.
The Forestry Commission’s Barry Gardiner, based in Midlothian, UK, an expert in forest modelling, says that ‘this is absolutely brilliant work that is at the cutting-edge of the subject’. He is particularly impressed with how Cemagref’s research validated its computer models, predicting the trajectory of rocks falling down slopes and crashing into and bouncing off trees. ‘The models have the physics fully calculated and incorporated and are very realistic. The result is that they are able to determine the best arrangement of trees to dissipate the energy of rockfalls and to plan strategies for protecting slopes, roads and the public,’ he adds.