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Sliding slopes: hillslope debris flows

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During heavy rain, soil can become waterlogged and lose its stability. On steep slopes, this often leads to shallow landslides and hillslope debris flows (referred to simply as landslides below). We conduct research in the laboratory, at field sites and using computer models on how and where these types of landslides occur and how the public can be protected.


Landslides and hillslope debris flows consist of a mixture of water, soil material and mud. A spontaneous landslide is a loose mass of rock that slides rapidly along a sliding plane. It is different from a permanent landslide, which moves continuously and evenly downhill over long periods of time (centuries, millennia). Hillslope debris flows develop from spontaneous, usually shallow landslides which lose their structure once they have broken away and descend as «mudslides». A hillslope debris flow is therefore more fluid than a landslide.

Since they most often occur without any warning and given that the soil matter can flow down the slope very rapidly, landslides pose a serious threat to people, buildings and infrastructure such as roads and railway lines.


Documenting landslides

Following storms that result in hillslope debris flows and shallow landslides, there is the opportunity to document as many of these types of events as possible. We gather information on the dimensions of the landslides, the topography, the site characteristics, the vegetation, the land use and the sliding mechanism. This information is fed into a database that we manage together with the Federal Office for the Environment (FOEN). The database is also set to be used in future to create hazard maps and hazard assessments for hillslope debris flows.


Plants stabilise slopes

Plants reinforce steep and unstable slopes with their root systems, thereby helping to reduce the risk of landslides. We analyse this effect in field and laboratory experiments. Using a new, specially developed device called a "shear apparatus", we can, for the first time, analyse soil in the laboratory under similar conditions as outside on the slopes.

We use this to examine how stable slopes are with and without plants. To do so, we load planted and unplanted soil samples with precisely defined forces in the shear apparatus and record when the soil starts to slide. The device can simulate slope gradients of up to 45°. The initial findings show that in comparison to bare soil, planted samples can withstand up to 65% higher loads.

The benefits of planting steep slopes have been shown by further findings from our field and laboratory studies: Actively planted soils develop a stable protective vegetation cover more quickly. Using a computer model developed by us, we can also calculate the force with which roots anchor the soil. This takes into account root growth, interactions between the soil and roots, different groundwater levels and other important soil characteristics.