Lessons learned from the Ascension floods 25 years ago still have an impact today
Published: Wednesday, May 8th 2024, 10:11
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The Swiss Ascension and Whitsun floods 25 years ago triggered measures that are still helping to protect the population today. These include the development of warning and alerting systems for natural hazards by the federal government and cantons.
In addition, the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL) announced on Wednesday that improvements in lake regulation and the construction of the relief tunnel on Lake Thun were initiated as a result of the devastating floods. At the time, the research institute was in charge of the analysis of the events in 1999 commissioned by the federal government.
The Ascension and Whitsun floods caused damage amounting to CHF 580 million in 1999. The entire Central Plateau was affected: in the Bernese Matte, people were driving around in boats. Large areas of the canton of Zurich were under water. The left bank of Lake Thun was 400 meters further inland, and in some communities on Lake Constance, houses were under water for weeks.
Melting snow and lots of rain
The WSL researchers identified a combination of several factors as the reason for this: Waterlogged soils, melting snow and heavy rainfall. At the beginning of the year, it had snowed a lot several times, even down to low altitudes. April then saw above-average rainfall. Days of rainfall shortly before Ascension caused the rivers and lakes, which were already full to bursting, to overflow for the first time. After brief but heavy rainfall at Pentecost, flooding occurred for a second time.
"This has shown that we have to pay attention to combined events. Above all, however, it has made us aware that the attenuation function of lakes has its limits," Christoph Hegg, Acting Director of WSL, was quoted as saying in the press release.
Melt water is calculated
In addition, the event was a great motivation to systematically calculate the amount of snow water over the whole of Switzerland, according to the WSL. This is now being done. Since 2009, the WSL Institute for Snow and Avalanche Research SLF has been providing regular forecasts of how much meltwater can be expected.
"They might not have prevented the flooding, as the weather conditions would have occurred anyway, but they might have given us more time to prepare for the event," said Tobias Jonas, head of the snow hydrology service.
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