A GIS simulation model for avalanche and debris flows

Catastrophic granular and debris flows occur in many mountain areas all over the world. Snow avalanches, rock or rock-ice avalanches, debris flows, lahars and pyroclastic flows are only some examples. An adequate management of the risk related to these phenomena requires a detailed and reliable analysis of the mechanisms involved in such processes. Even though much work has been done on this subject, and a number of physically-based models with a varying degree of complexity do exist, some problems still remain unsolved: (1) Flow over arbitrary topography, the role of viscous pore fluid or two-phase nature of flow, and particle and/or fluid entrainment have not yet been accounted for in an appropriate way. (2) Until now, no successful attempts have been made to build easy-to-handle Open Source applications of these complex models, which would be essential to make them available to a broader group of users in universities and public services. This proposal offers an effective, innovative and unified solution to these two problems. It is therefore concerned with rapid geophysical mass flows, including avalanches and real two-phase debris flows, from a known initiation zone through the flow path along natural mountain topography into the deposition zone. For a given amount of mass and its distribution in the initiation zone, we are interested in the motion and geometric deformation along the track down the arbitrary topography, including the processes of erosion and deposition of mass along the track and the ultimate distribution of the deposited mass. This will also include the effect of dynamically evolving pore fluid pressure and/or evolution of the solid and the fluid components. An equally important focus shall be put onto the development of a user-friendly and freely accessible application of the developed model. This application will build upon the GIS software GRASS, which is available as an Open Source product under the GNU General Public License. The new software will be evaluated using physical model tests and well-documented mass flow events. These tests will cover a broad range of processes and process chains including debris flows, debris avalanches and avalanches of snow or rocks.

The avaflow project has focused on the development of a novel simulation tool (r.avaflow) for the motion of debris flows, snow avalanches, rock avalanches, and process chains consisting of more than one component. r.avaflow is freely available, its code is publicly accessible (open source), and is available as a command line interface or graphical user interface (GUI). The major innovation, however, builds on the use of a so-called two-phase flow model: this means that fluid and solid parts, such as water and debris, are explicitly modelled including their interactions and different material properties. This allows the simulation of the effects of a landslide impacting a reservoir, and the associated overtopping and, possibly, erosion of the dam. Whereas the development and optimization of the underlying mathematical-physical model and respective laboratory experiments were realized by the German project partners, the focus of the Austrian part of the project was, on the one hand, put onto the translation of the system of equation into a computational code, closely coupled to a Geographic Information System (GIS) to effectively handle the corresponding terrain data. A further focus realized within the Austrian part of the project consists in the compilation of data on well-documented debris flows, snow and rock avalanches, and process chains, in order to enable the analysis of the model performance, and of the sensitivity of the simulation results against changes of the key model parameters. The observed patterns of motion and deposition could be reconstructed in a satisfactory way with r.avaflow. However, the input parameters had to be adapted (optimized) for this purpose. This means that, for the prediction of future events, it is necessary to back-calculate a certain number of well-documented events of a certain type (e.g. debris flow) in order to develop robust parameter sets for forward calculations. Innovative, automatized approaches facilitating a systematic back-calculation of past events were developed within the avaflow project. This is important insofar as good predictions represent an essential basis for the definition of hazard zones as well as for the design of protection measures (retention walls or basins, evacuation strategies), and can therefore contribute to a safer living space in mountain regions. More detailed information with regard to r.avaflow as well as the model codes and test data can be obtained from http://www.avaflow.org.