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The effectiveness of plants as slope stabilizers

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As well as debris flows, large storms often also cause landslides of varying severity which can damage land and infrastructure. A sound layer of plant cover may help protect against erosion and landslides. Until now, though, it wasn’t known to what extent, or which plant combin­ations should be used. New results have been delivered by WSL’s SOSTANAH (SOil STAbility and NAtural Hazards) project, which is a part of the ‘Sustainable use of soil as a resource’ National Program (NFP 68).

To underpin the slope fortifi­cations with some solid figures, doctoral student Anil Yildiz investigated soil samples from two landslide areas in the laboratory. Using a shearing apparatus, he measured the strength required to cause a slope to start slipping – with and without alder, grass and clover. It emerged that after only six months of growth the plants had greatly strengthened the soil. A real-­life slope would therefore remain stable, even with an incline 5° steeper than can be expected from the soil material.

The effect of planting woods was verified by a study on the pre­viously unvegetated erosion and landslide zone of Hexenrübi (NW). Willow trees planted between 2009 and 2011 have created abundant biomass, both above and below ground. The next big storm will show how much the plantings have improved the slope’s stability.

The results of the various SOSTANAH sub-projects both confirm and complement existing guidelines for the management of protection forests (NaiS). A few lessons from the projects: slopes should contain as many species, age groups and root structures as possible, intensive grazing and fertilization can interfere with the protective effect, and ver­tical paths of more than 20 m down the fall line should be avoided if possible.

WSL researchers used statistical methods to investigate the effect of forest structure during landslide events. They used an example from Sachseln (OW), where heavy rainfall caused over 500 landslides in 1997. The calculations indicate that about four fifths of the more than 100 landslides taken into account would not have occurred under optimal management. It is estimated that the corresponding care and maintenance of the affected forest areas would cost only a tenth of total cost of damages. (Beate Kittl, Diagonal 2/16)