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Making sure large wood does not become a danger

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Floods carry away branches and tree stems that can destroy bridges or block weirs. A research programme provides practical help to enable better forecasting of future flood-timber volumes.

Wood in stream- and riverbeds promotes biodiversity because it provides habitats and food sources for animals. But if a river transports branches, shrubs or entire tree stems with it during floods, this could be dangerous for people and infrastructure. Large wood can block weirs, damage bridge piers or result in entire residential areas being flooded by backwater.
During the extreme floods in 2005, more than 100,000 cubic metres of large wood accumulated in Switzerland – the equivalent of around 2500 truckloads. “The quantities, however, vary greatly between individual flood events,” says Nicolas Steeb, a researcher in the ‘Torrents and Mass Movements’ Research Group at WSL. That’s why it has, until now, been difficult to predict where and how much large wood will occur.


To obtain better estimates in future, the Federal Office for the Environment (FOEN) set up the ‘WoodFlow’ research programme with researchers from WSL, ETH Zurich, the Bern University of Applied Sciences and the University of Geneva. Over a period of four years, they investigated the origin, transport and deposition of large wood. “The aim was to create a knowledge base for practical applications,” says Nicolas. They developed, among other things, various tools for government and engineering offices. With these tools, they can calculate, for example, what the minimum distance between the water level and the bottom of a bridge should be to prevent the bridge being damaged by large wood.

How much large wood could accumulate?

Nicolas developed, in his part of the project, a practical GIS model for predicting the large wood potential of different catchment areas. This involved modelling processes, such as debris flows, landslides and riverbank erosion, and combining them with maps of the river network and forest areas. He also integrated into the model a map of the wood stock, which indicates the amount of wood present in an affected forest area. On the basis of this and other data, the model calculates the potential amount of large wood that could, statistically, be produced every 30, 100 or 300 years during major flood events. The local authorities can then use this information to plan appropriate protection measures in their respective regions.

In another sub-project, Nicolas investigated the wood’s size distribution. For this, he used data on around 6000 pieces of large wood that WSL measured after the floods of August 2005. Most of the pieces found were between one and three metres long.  Very large pieces were rare. Much of the wood gets considerably broken up while being transported in the water. From the length and diameter of the pieces found, it can be concluded that most of them are only a fifth of their original size. The data has been integrated into the simulation model ‘Iber-Wood’, which calculates the transport and deposition of large wood in rivers. It can be used, among other things, to predict where wood could accumulate on river- or gravel-banks. This wood must be monitored and removed if necessary as it might otherwise be carried away in the next flood and become a danger.
(Claudia Hoffmann, Diagonal 1/20)


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