Heavy rain can suddenly trigger landslides on exposed slopes. To detect the danger before it happens, researchers at WSL are studying these hillslope debris flows. A database containing their obser vations and measurements forms the basis for numerical models and hazard maps.

Brian McArdell (l.) and Christian Rickli are investigating what happens during hillslope debris flows, using a 1:20 scale test system at the WSL.

24 July 2014: It has been raining for days and now the forecast is for heavy thunderstorms. For five hours it has been pouring down in buckets over the mountains of Emmental. The river Emme is carrying immense masses of water, stones and driftwood down-river. It washes away bridges and roads. In Eggiwil the water level rises by more than four metres in just a few hours. One has to fear the worst, but thanks to the advance warning and flood control measures no one is hurt. Not quite as spectacular as the TV images of the raging river torrent, there are photographs that show the other effects of the heavy, long downpour: One sees elongated brown areas on the green slopes – like scars on the landscape. The experts speak of shallow landslides or hillslope debris flows. In July 2014, there were about 30 of these in Emmental alone.

It is not only the floodwaters, mudflows and deep landslides that have a great destructive power, these shallow landslides on slopes can be a danger to human beings and cause damage to buildings, roads and railway tracks. For example, in August 2014 a landslide near Tiefencastel caused a train accident in which five passengers were seriously injured. In November 2014, the long period of rain in Ticino even led to four fatalities from the landslides. As is so often the case, the hillslope debris flows caught the people by surprise. “When one lives near a mountain stream with a deep channel, or next to a mountain torrent, one knows that there could be a risk of a mudflow”, says Christian Rickli, expert in shallow landslide processes at the Swiss Federal Research Institute for Forest, Snow and Landscape (WSL) in Birmensdorf. In the case of hillslope debris flows, however, the danger is not as obvious. “You don't have a warning based on the type of landscape.” Therefore the researchers at WSL are investigating this phenomenon.

“We want to understand the way it originates, and develop methods by which one can plan protective measures”, says Brian McArdell, head of the Mass Movements group within the WSL unit Mountain Hydrology and Mass Movements. “In practice the people want to know where a hillslope debris flow can get loosened and come down, how big it will be, and how far it will go.” In these shallow landslides, only the top layer of the slope, down to two metres, starts to move – a mixture of ground materials, mud and a lot of water. Typically a volume of 100 to 150 cubic metres will slide down the slope at a speed of up to 15 metres per second, as if it were on a slide. Here it is mainly the great speed of the moving mass that results in such a destructive power.

Saturated ground, young forests destroyed

Brian McArdell shows pictures of a house near Disentis. The windows are shattered, the front doors have been pushed in, tree trunks from a newly planted tract of forest have ended up in the basement. The people living in the house escaped with just a fright. On the cows' pasture about 200 metres above the building a hillslope debris flow came loose because the ground had become oversaturated, partly due to a leaking water pipe. “The pipe has now been laid in a different location, and there is now a hazards map for this municipality”, the geologist says. To produce such maps, the WSL researchers collaborated with the Swiss Federal Office of the Environment (FOEN) to develop a database in which as many events as possible are precisely documented. “The more data we have, the better we can assess the risk”, says Brian McArdell.

The specialists have now recorded over 700 hillslope debris flows so far. Many of these date from August 2005, when severe storms in the Alps led to large floods and damage to property amounting to billions of Swiss francs. 250 millimetres of rain fell in just three days. “That is an extraordinary amount”, says Brian McArdell, “it is even more than a once-in-a-century event”. All the landslides are recorded on a map of Switzerland. There are more than 5,000 of them. In the past few years Christian Rickli has measured numerous hillslope debris flows himself, on location, with other staff of WSL. At Entlebuch near Lucerne they studied almost fifty shallow landslides in an area of some five square kilometres. “We spent a lot of time and effort searching in the forest as well, in order to record all the landslides in the area”, the forestry engineer says. For in this manner comparisons and valuable conclusions about how the events happen can be derived.

The result of these and other similar studies: Shallow landslides occur on slopes that are inclined at 20 to 45 degrees. Most of the events occur where the inclination is 35 degrees. Hillslope debris flows are less common in the forests than on open ground, and they happen on the steeper slopes. “The forested areas have a better record thanks to the roots penetrating the soil”, Christian Rickli points out. Hence one of the possible measures that can be taken to protect buildings and the transport infrastructure is to plant more trees. Protection can also be provided by retention barriers and diversion barriers, or walls and nets to secure the slopes.

Tests out on the slope and in the lab

The WSL researchers collaborated with the company Geobrugg to carry out field tests to find out how effective steel nets would be in retaining hillslope debris flows. For this purpose, the experts arranged to have ten truckloads of mud and gravel come thundering down a slope in Veltheim, until the steel-wire test barriers managed to bring the mixture to a halt. Laser sensors measured the speed, and pressure sensing elements registered the impact force of the artificial landslide. “This project allowed the company to improve its nets and open up a new market”, says Brian McArdell. The test runs, each of which cost CHF 10,000, also provided the researchers important findings about the details of the landslide process. They continued with the tests in a less costly way in the laboratory at a scale of 1:20.

In a hall at the WSL site in Birmensdorf there is a three-metre high framework holding a wooden ramp. It is covered in strips of white plastic. This material is normally used to hold kitchen utensils, but it happens to have exactly the right degree of surface roughness for the slide experiments. In each test, the researchers arrange for four litres of a mixture of sand, gravel and water to flow down the ramp, while they measure the speed of the brown concoction using laser sensors. During the many trials they vary the water content, for example, or increase the volume. “In this way we can systematically study how these changes affect the process”, explains Brian McArdell. “The trials are very realistic.”

Adjoining this installation there is a so-called shear-strength apparatus – a blue metal frame in which there is a 50 centimetre wide yellow container. The container consists of two frames, one on top of the other, and is filled with a sample of soil. Fixed to the apparatus, the pressure on the container is increased until the two frames slide apart; the sample shears across. “We developed the device ourselves”, says Christian Rickli. “The special feature is that we can apply pressure to the sample container not only horizontally but also up to an inclination of 45 degrees.” This allows them to simulate conditions similar to those on a slope outside, in the laboratory. The researchers use the shear-strength apparatus to test how well plants can stabilize the soil. Because the containers are larger than those used in earlier laboratory tests, the plants are able to grow in them in a similar way to when planted outside in nature, and their roots penetrate the soil in the same way. The shear-strength experiments should be able to make the protective function of the plants measurable for the first time.

The shear-strength apparatus measures how much force a soil sample can bear.

Computer model shows the hazards

Using the data from the events, and the field tests and laboratory tests, the researchers have developed a model which allows one to simulate hillslope debris flows on the computer. There are already similar models for avalanches, debris flows and rockfalls. “The equations are the same, even if the types of material are very different”, says Brian McArdell. The model has been tested by various engineering firms and is available commercially. It is intended to be used in combination with the new database to help in producing hazard maps, and thus enable an effective protection against hillslope debris flows.

The best measure to take would be to not build in the endangered zones at all, from the start, says Brian McArdell – a fact that one is well aware of here in Switzerland, he adds. “The land use planning in Switzerland is brilliant”, says the specialist who originally hails from America. Therefore the WSL researchers are often visited by colleagues from abroad who want to learn more here about how to deal with natural hazards. Christian Rickli is confident: “Switzerland is leading the way internationally in the use of hazard maps in land use planning.”