A model for landslides involving water and rock

Extreme natural phenomena are becoming more frequent, and at the same time our planet is becoming more and more densely populated. This increases the risk of disasters and the need to prevent them. Therefore it is important to set up dams, avalanche barriers and other protective measures in the right places. While computer programmes have been used for many years to simulate landslides, they represent moving material in debris flows or avalanches in a very simplistic way. This can be a problem as the geographer Martin Mergili from the University of Natural Resources and Life Sciences in Vienna (BOKU) explains. Models which are too simple often fail to reflect the complex reality. In a project funded by the Austrian Science Fund FWF, a research group led by Mergili has now developed a simulation environment that is intended to provide a more realistic picture of flowing material.

Complex processes

Debris flows or avalanches are generally difficult to simulate, explains Mergili: “These phenomena involve complex physical processes. They contain material of different grain sizes - from very fine mud to large pieces of rock - plus water, and, in extreme cases, ice and snow. That means there are many materials that interact, and this mixture moves down the slope.” He goes on to explain that there are some models on the market for modelling processes from avalanches to debris flows and rockfalls, but “most of them use a so-called single-phase model. The water and the rubble is all aggregated and calculated as a homogeneous mass.” Mergili explains that this is inadequate for simulating many natural events and quotes the example of the Bondo landslide in Switzerland a year ago: several million cubic metres of rock mass started to slip and eventually ended up as a huge mudslide, devastating the whole place. Rock and water coincided in this instance, and this is too complex for previous calculation methods.

Liquid and solid components in debris flows

Another particularly complex incident took place in Peru, in the Santa Cruz valley of the Cordillera Blanca mountain range. A slope slid into a lake and triggered a tidal wave. “The lake overflowed and the tidal wave reached a lower-lying lake”, explains Mergili. “The lake below overflowed as well and eroded a great deal of loose material. The result was a mudflow with great amounts of water and loose material, which moved a few kilometres downhill until it reached a third lake, where it came to a standstill.” According to Mergili, previous single-phase models would not be sufficient for such phenomena. “It’s exactly for processes of that level of complexity that you require a two-phase model so that water and debris can be considered separately”, explains the researcher. Such a model was developed in the context of international cooperation between a number of German and Austrian universities and research institutes, including the BOKU under the leadership of Martin Mergili. While the physical principles were developed in Germany, Mergili’s group was responsible for the integration of real terrain data. The Austrian part of the joint project was financed by the FWF.