Tuesday, April 2, 2013

Lacus Lemanis- Tsunamis past and future?

Lac Leman, aka Lake Geneva, is one of Europe's largest post-glacial lakes. It sits in an ancient tectonic basin, eroded into a typical glacial U-shaped valley at the edge of the Alps, separating in two the Jura range. The glaciers that carved this gash in the Jurassic limestone platform (that borders the squished metamorphic rocks of the African plate's collision with Europe) advanced and retreated during the Pleistocene glaciations, widening and deepening the valley with each iteration. The lake is shared between France and Switzerland. The waters of the river Rhone, born around Mont Blanc (Europe's highest peak) flow through without mixing, taking about 7 years to do so, and depositing an underwater delta of alpine sediment as it enters. A gold mine now exists at the head of the lake, filtering precious metals out of the Rhone's water.

It is a rich and peaceful part of the world, growing light red wines and slightly sparkling whites. On its edge sits the famous spring and spa town of Evian, source of a delicious water with reputedly curative properties. The Swiss city of Geneva has existed at the lake's exit since pre-Roman times, and is now filled with the UN's European HQ, varied international organisations and NGO's, banks, oil traders and CERN (of Higgs boson fame). It is an ideal place to live: good food, comfort, Swiss efficiency and peacefulness. The lake's geological past however has not always been as placid as one might believe, as we sit enjoying the summer sun, watching the paddle steamers puffing past towards Montreux (as in the Jazz Festival), while eating lake perch and sipping a glass of wine by its blue waters. It has seen several lake tsunamis, and may see more in the future.

Lake (and fiord) tsunamis happen when a chunk of rock from the surrounding uplands fails and crashes into the lake, displacing the water. Swiss researchers from Geneva university have recently reconstructed the events of 563 CE, when a landslide fell into the Rhone upstream of its entry point into the lake, using the geological record to explain a historical mystery. Many villages were destroyed in the resulting tsunami, which overtopped Geneva's city walls and washed away its mills and bridge, resulting in extensive flooding of the city. A new examination of the sedimentary record has shown that these events happened, just as described in the old chronicles and given us the 'how' behind this devastating wave.

A tsunami should not have occurred because the landslide occurred upstream in the river, and scientists speculated that it had in fact dammed the Rhone, the wave being unleashed when the dam failed. A 250 million cubic metre nappe of turbidite sediment (measuring 10 by 5 Km) was discovered in the middle of the lake, using sonar data and sediment cores, and dated using C14 to a period bracketing the date given in the historical records. The mechanism behind this 'impossible' tsunami is simple: the landslide sparked a turbidity current in the soft sediments of the Rhone's delta at the lake head, displacing enough water to power the wave. Modelling has shown that its height reached 8 metres at Evian, and 13 at Lausanne. Despite the dissipation of the wave's energy as it travelled, the focussing effect as the lake narrows into a funnel as it nears low lying Geneva sent it back up to 8 metres, a mere 70 minutes after starting.

This was not the only such tsunami in the past, four deeper turbidite deposits were found in the sediment cores, nor will it be the last to occur. A pan of rock by Bex at the lake's head is thought to be weakening, and may spark another catastrophe in the future. Unlike in the 6th century, over a million people now live by the lakeside, a quarter of them in the Geneva funnel. Since the lake is small, warning times are much less than in the Pacific ocean, where most places get hours to evacuate the population to higher ground. Surveys of the delta are now underway, in an attempt to determine how close to collapse it might be. The older turbidites urgently need to be dated, to get a rough idea of the frequency of these events, and a hazard map for the entire lakeshore must to be compiled.

Whenever modern and past geological hazards are compared, we keep on encountering the same problem: many more people live in these at-risk areas than ever before, and expensive infrastructure has now been built there (in Leman's case, Geneva, motorways, train tracks and the villas of the hyper-rich), making the destructive potential of any future event that much greater.

Image credit: Hardo Muller.





Original paper, for those with paywall access:


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