Tsunamis in Lake Geneva

I came across this article from the November 2012 Economist:

IN 563AD a tsunami devastated Geneva. ...

The tsunami of 563 started at the opposite end of the lake from Geneva, at the point where it is fed by glacial meltwater carried into it by the Rhône. Both accounts say the wave began with a massive rockfall on what was then called Mount Tauredunum (this has led to the tsunami becoming known as the Tauredunum event). Tauredunum is thought to be a mountain now called the Grammont, which is located near the river mouth.
In the past, one favoured theory was that this rockfall created a natural dam across the Rhône, which held the waters back until it could no longer sustain the pressure. When the dam burst, the resulting wave swept the length of the lake. A one-off event, in other words. But a paper just published in Nature Geoscience by Dr Kremer and her colleagues offers a different and more worrying explanation.

... Dr Kremer thinks that the rocks crashed down onto soft sediments which had accumulated at the river mouth because of the slowing of the river’s flow when it enters the lake. These sediments form an underwater delta that has several canyon-like channels. When the falling rocks hit the delta they destabilised the sediments and caused the canyons to collapse. It was this collapse that created the tsunami.

It is a plausible theory. What suggests it is true is that the sediment from such a collapse would have been propelled towards the lake’s centre, forming a large tongue of material on the lake bed. And, using sediment cores and an instrument called a pinger, which analyses the reflections of sound waves that can penetrate the material of the lake bed, Dr Kremer thinks she has found this tongue.

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The crucial element in this explanation is the accumulation of sediment in the underwater delta. That is a continuous process. Once enough sediment is there, it just needs a trigger to set it going. That could be a rockfall, an earthquake (though this part of the world is not particularly seismically active) or even a violent storm. Moreover, Dr Kremer’s pinger shows evidence of four layers deeper in the lake bed which also look like turbidite. The formation of these might or might not have triggered tsunamis. But they are a worrying sign.

 The article also describes what probably happened in 563 AD:

Within 15 minutes of the collapse [of the delta], a wave 13 metres high would have reached Lausanne, a city on the northern shore of the lake. But Lausanne is built on steep slopes, so most of it would have been spared. The damage would have been much greater when, 55 minutes after that, an 8-metre wave reached Geneva, at the other end of the lake. Geneva is a lower-lying city than Lausanne and, to make matters worse, the lake narrows here, funnelling water to the point where the Rhône becomes a recognisable river again. It would similarly have funnelled the wave.

... The wave, they believe, would have passed over the city walls, and wiped out watermills and a bridge across the Rhône, just as the two accounts say it did. En route, as Marius wrote, it would have destroyed many lakeshore villages, “with humans and cattle and even churches”.