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Extreme rainfall event led to Swiss landslides

Heavy rainfall over the weekend in Switzerland triggered severe landslides that blocked both roads and rail lines and led to two fatalities but the national rail operator SBB believes that hazard mapping currently underway will enable risks to be better managed in the future.

Heavy rainfall in the cantons of Fribourg, Vaud and Lucerne resulted in failures on a number of slopes with a shallow layer of silty gravel and clayey silts with a slight plasticity.

According to Switzerland’s section head of landslides, avalanches and forest protection for the federal department of environment, transport, energy and communications Arthur Sandri, when combined with a slope angle between 10° and 40° the geology in these regions is very susceptible to shallow landslides or hillside mudflows.

Sandri added that rapid erosion adversely affects these slopes and that any artificial slope which has been built in solid rock can weather to become unconsolidated material within a few decades of construction. “Many artificial slopes of the Swiss railway network are in this exact situation and SBB has made an evaluation of its problem slopes and is now starting a remediation programme,” he said.

According to Sandri, the mapping that the government carried out is helping to identify problem slopes and predict where failures might occur. Specific rainfall rates that increase risk were also identified using analysis of a flooding event in 2005. Nonetheless, rainfall over the weekend in the Fribourg, Vaud and Lucerne cantons was classified as an extreme event which prevented the hazard mapping from predicting these failures but rainfall at these levels is a rare event.

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