31. 10. 12. - 10:32
Call to assess risks of Alpine lake tsunami in Austria
Lakeside towns and villages should reassess the risk from tsunami as global warming leads to a melting of the permafrost "glue" that holds many of the peaks together.
Large parts of the Alpine region that lie higher than 2,500 meters are held together by the soil which is permanently frozen. Until recently only the surface layer has been known to thaw during summer.
But new research from the University of Innsbruck shows that this permafrost is now disappearing at the rate of between 5 and 10cm a year, and has been for the last decade when between 50 and 80 cm of Alpine permafrost has vanished.
The warning that this could cause a so-called "mass movement" leading to a lakeside tsunami follows a study in Switzerland that found how a massive rock face that had sheared off 1,500 years ago caused a tidal wave 13 meters high that swept away villages, people and cattle.
Lakes have seen a huge rise in the populations that inhabit their shores, and the risk of a lake tsunami is heightened by the fact it is associated with a near field source region, which means a decrease in warning times to minutes rather than hours.
Alpine Global Warming expert Professor Johann Stötter from the University of Innsbruck said: "Mountain areas are especially sensitive to global warming not just in the visible melting of glaciers, but also through degradation of permafrost which is much harder to estimate."
It was his team that recorded the fact the ice is disappearing at a rate of 5-10cm a year. Permafrost is like glue that holds steep mountain slopes together, and it’s degradation leads to natural hazards such as rock falls and land slides.
Professor Stötter said: "A good example of where there would be a risk is at reservoirs. We have an example from Italy in 1963 when a massive landslide of about 260 million m3 of forest, earth, and rock fell into the reservoir."
Although the tsunami this caused did not burst the damn, the massive wave sent 50 million m3 of water over the top of the dam which then washed down into the Piave valley, destroying the villages of Longarone, Pirago, Rivalta, Villanova and Fae and killing around 2,000 people.
He said: "Since then, hydropower companies do careful studies of the local geography to prevent a repeat of this – and we also have a lot of general studies on potential mass movements.
"But I am not aware of any studies that have been done specifically with regard to the impact on natural lakes.
"I heard about the research at Lake Geneva today and obviously it is something we should look at again with regards to the risks at certain lakes. Of course not every lake is at risk but there are certainly some where we should consider the risk."
Asked to name a lake with a geography that should at least be considered he named Lake Achen at Jenbach in Tyrol, but said there were others as well that should be considered.
He said: "I believe we should reconsider the possibility of whether a massive rock fall into a lake could happen and what steps we might take to minimise risk."
The Swiss researchers started looking into the incident on lake Geneva after finding an account by a French bishop, Gregory of Tours, who described how a "giant wave had crashed across the lake, destroying villages and herds of animals, and then passed over the city walls of Geneva, on the western tip, where it drowned a great many people".
The devastation of the Lake Geneva tsunami shows the risks that a significant rockfall could pose.
Katrina Kremer, an Earth scientist at the University of Geneva, said: "The risk is underestimated because most of the people just do not know that tsunamis can happen in lakes."
In a letter to the journal Nature Geoscience she said that after reading the Bishop's account they had delved into the "Tauredunum Event," an episode that occurred in AD 563, and found evidence that part of a mountain slipped into the River Rhone about five kilometres from where it flows into Lake Geneva at the lake's eastern point.
Kremer's team then swept the deepest part of the lake with high-resolution radar and uncovered a huge, oval-shaped pile of sediment, more than ten kilometres long, five kilometres wide and five metres thick.
The team then took four cores of the sediment and carbon-dated tiny vegetal remains embedded in it. The estimated age of this debris is between AD 381 and AD 612.
"Since the AD 563 event is the only significant natural event recorded in historical accounts within our calculated age interval, we consider our dating results to be a strong indication that the deposit is linked to the AD 563 rockfall and tsunami," she said.
"Our numerical simulations with a shallow water model show that delta collapse in the lake generates a large tsunami at various locations along the shore, where a wave of 13 metres is observed after only 15 minutes, and at Geneva where a wave of eight metres arrives 70 minutes after the mass movement is initiated," she added.