Run-out modelling of landslides

The run-out area of a landside includes all of the area that can be reached by a landslide event. The distance and area are calculated with the help of various modelling tools, for the specific landslide types.

Figure: Runout modelling of a potential rock avalanche using the FlowR software (Figure: T. Oppikofer, NGU)

Various types of modelling software are used for run-out modelling at NGU. Currently, Rockyfor3D and RocFall are used for rockfall simulation while RAMMS and FlowR are used for debris slides and debris flows. For rock avalanches, various modelling tools are used, based on the desired level of detail, as outlined in NGU’s mapping methodology for unstable rock slopes. FlowR is used for a simple assessment of rock avalanches run-out areas, while the numerical model DAN3D is used for detailed, three-dimensional analyses. Common to all modelling tools is the use of digital elevation models as input data.

The software Rockyfor3D does a three-dimensional calculation of probable run-out paths for individual rockfall blocks that fall, bounce and/or roll down a slope. The model uses a physical approach by calculating sequences that have a classic parabolic motion through the air, and whose properties change each time the block touches another surface. The model estimates a loss of kinetic energy down the slope that is affected by the block shape and the properties of the ground that the block touches.

Rocfall is a rockfall simulation program that performs statistical analyses in 2D along selected profiles. For each individual block, the energy, speed, bounce height and stopping point are calculated for the entire slope. The distribution of these parameters along the avalanche path can be graphed and includes statistics that are calculated automatically by the program. The energy returned to the block (restitution coefficient) for each of the various materials along the slope must be defined to take account of energy loss along the path.

Debris flows can be modelled using the dynamic model RAMMS debris flow. This commercial software is based on the well-proven Voellmy model. For debris-flows and rock avalanches, NGU also uses FlowR, which was originally developed for the production of debris-flow susceptibility maps in Switzerland and was also used for such maps in Norway. The run-out is calculated for each defined source area by means of a "multiple flow direction" model. The model takes into account the topography along the landslide’s path, and calculates the direction in which the material can move and how far it can get. The range is dependent on the landslide’s line-of-site, i.e. the average inclination between the source region and the outer limit of the run-out area. For debris flows, this angle is about 10-11°. For rock avalanches, this angle depends upon the landslide volume, based on an international study by Scheidegger (1973).

DAN3D simulates rock avalanche masses as a liquid, characterized by simple rheological conditions ("equivalent fluid" dynamics). The internal rheology is always dependent upon friction, while the sliding surface’s rheology can be either based on friction, viscosity or be turbulent. Friction-dependent and friction-turbulent (Voellmy) rheologies are most often used for rock avalanches. The software takes into account the shape and volume of the unstable rock slope before the slide, the materials and surface texture in the run-out area, and also assesses possible erosion of soils along the rock avalanche path. Results include the rock avalanche’s velocity, thickness of deposits and depth of erosion.