RockForNET: the forest as rockfall net
Luuk K.A. Dorren, Frédéric Berger · HAL (Le Centre pour la Communication Scientifique Directe) · 2005
Forests can act as rockfall nets. It is unknown, however, how effective different types of forests perform this function. Therefore, Cemagref Grenoble initiated a research program on the protective role of mountain forests against rockfall in 1983. Real-size rockfall experiments on forested and non-forested mountain slopes in the French Alps have been carried out in 1997, 2001, 2002 and in 2003. During these experiments, rockfall trajectories, including impacts against trees, were captured by high-speed video cameras. The obtained knowledge and results have been combined and formalised in a user-friendly, publicly accessible tool called RockForNET (www.rockfor.net), which allows quantifying the protective role of a forest against rockfall. RockForNET considers the forest stand as spatially distributed rockfall curtains. Here the distance between two trees in a row is 0.9 * mean rock diameter, and the distance between two tree rows along the slope is 33 m. All the trees in a row have a diameter equal to the given representative stem diameter in the stand, which in combination with the tree species determines the efficacy of a tree row regarding energy dissipation during a rockfall impact. In order to calculate the probable residual hazard at the foot of the forested slope, the model determines the number of tree rows required for optimal protection, i.e. all the falling rocks stopped by the forest stand. Thereby, the total amount of energy that has to be dissipated by the forest stand is assumed to be 2.8 times the value of the kinetic energy at the foot of the slope, as calculated with the energy line principle using an angle of 32º. This assumption is again based on the findings of the real-size rockfall experiments. Finally, RockForNET compares the required number of trees with the existing number of trees in the stand and translates the difference between the two into a probable residual hazard, ranging from 0-100%. RockForNET can be used for many different slope types and various types of mountain forests. It provides information on the expected percentage of rocks that surpasses the forested zone under a cliff face and with which amount of energy. By using this tool we are able to assess, in quantitative terms, how different mountain forests perform their protective function by acting as rockfall nets. This paper explains the developed tool, how it can be used and the underlying theory.