Luuk Dorren

Hillslope processes: Mass wasting, slope stability and erosion

This chapter describes the dominant types of processes present on hillslopes where both gravity and running water are active. The impact of natural hillslope processes is important and is currently strongly influenced by human activity due to land use change and vegetation removal, and is becoming even greater due to climate change. Both the fundamentals of erosion and slope stability are discussed in this chapter with respect to processes, causes and impacts. To fully appreciate the role of vegetation in the remediation of adverse slope processes, the fundamentals of these slope processes are addressed. In the first part, the role of mass movements is discussed. The definitions used and physical principles underlying mass movements are explained and keys and diagnostic parameters are given to explain how to recognize certain types of mass movements in the field. The causes of mass movement are described, amongst which deforestation, adverse hydrological conditions or slope undercutting, are summarized. The main types of mass movements i.e. falls, slides and flows are then separately discussed, giving full details with regard to their causes, processes and consequences, as well as a first glimpse to the solutions to slope failure problems, which will be addressed in more detail elsewhere in the book. The second part addresses erosion processes. Accelerated erosion is considered as one of the greatest problems of land degradation as it removes the fertile topsoil at high rates. Mankind, who is removing the original vegetation for agricultural purposes, is causing this problem. Again the general principles behind soil erosion are illustrated, giving attention to the causes and the different soil erosion processes such as sheet erosion, rill and gully erosion, piping and tunnel erosion as well as tillage erosion.

Quantifying the Stabilizing Effect of Forests on Shallow Landslide-Prone Slopes

Shallow landslides are natural hazards that can affect human life and infrastructure both directly and indirectly. Such landslides usually involve low-cohesion soil mantles less than a few meters deep. As shown by evidence worldwide, the presence of forests can lead to increased slope stability, due to mechanical and hydrological mechanisms, and therefore significantly reduce the landslide risk in many locations. Therefore, the nationwide project SilvaProtect-CH, which provided data and defined uniform criteria for protection forest delimitation in Switzerland, has also included shallow landslide protection forests. According to the modelling results of SilvaProtect-CH, approximately 27 % of the Swiss protection forests provide a protective function against shallow landslides. To facilitate a quick quantitative evaluation of the slope stabilizing effect of such forests, we developed the tool SlideforNET, which is described in this chapter.

A Novel DEM Approach to Simulate Block Propagation on Forested Slopes

In order to model rockfall on forested slopes, we developed a trajectory rockfall model based on the discrete element method (DEM). This model is able to take the complex mechanical processes at work during an impact into account (large deformations, complex contact conditions) and can explicitly simulate block/soil, block/tree contacts as well as contacts between neighbouring trees. In this paper, we describe the DEM model developed and we use it to assess the protective effect of different types of forest. In addition, we compared it with a more classical rockfall simulation model. The results highlight that forests can significantly reduce rockfall hazard and that the spatial structure of coppice forests has to be taken into account in rockfall simulations in order to avoid overestimating the protective role of these forest structures against rockfall hazard. In addition, the protective role of the forests is mainly influenced by the basal area. Finally, the advantages and limitations of the DEM model were compared with classical rockfall modelling approaches.

The Importance of Rockfall and Landslide Risks on Swiss National Roads

To obtain standardized information on the type, frequency, intensity and location of natural hazards that threaten national roads, the federal roads office FEDRO initiate a Swiss-wide project in 2008. This paper presents the methodology used in this project and presents a summary of the monetarised risks of the evaluated road sections. The natural hazards that need to be assessed are snow avalanches, rock- and icefall, flooding, debris flows, landslides (permanent, spontaneous and slope type debris flows) and collapse dolines. Risk hot spots mainly occur due to road closure related to rockfall or bank erosion. Damage to infrastructure represents generally only up to 20 % of the total calculated risk; person risks (casualties) add up to 8 % of the total risk. Rockfall is responsible for 35 % of the total calculated risk, rock avalanches for 8 %, permanent landslides for 5 %, spontaneous landslides for 3 % and slope-type debris flows for 1 %.

Quantifying the Relevance of Rebound Modelling Approaches Using Field Experimental Results

The relevance of two rebound modeling approaches classically used in rockfall simulation codes was assessed using field experiments of single rebounds. A lumped mass model, modeling the rock as a single material point, and a rigid body one, explicitly accounting for the rock shape, were used. Both of them are efficient with only a few calibration parameters. The main limitations of each approach are the calibration of a parameter accounting for both the roughness of the soil and the rock shape, for the lumped mass approach, and the estimation of the rock length and height, for the rigid body approach. Finally, both rebound models require being improved to better predict the rotational velocities distribution.

Approche phytosanitaire de l’impact des blocs rocheux sur le tronc des arbres en forêts de montagne

As part of its research programme on interaction between upland forests and rockfall, Cemagref Grenoble carried out a series of life size simulations by causing rockfalls on a forested slope in the French Alps. The phytosanitary impact of injury to tree stems caused by rockfall is not known, motivating the Cemagref Grenoble mountain forest team to initiate a research project to explore this area. This article presents initial findings.