Revealing the secrets of the lake floor

23 February 2008

Chrüztrichter and Vitznau basins of Lake Lucerne. In the northeast (northwest of Vitznau) and along the southern margin of the Vitznau basin, below the steep northern flank of Mount Bürgenstock, various rockfall deposits can be seen. Visible on the northern edge of the basin are scars of underwater landslides, with slide deposits on the lake floor. The scar of the landslide triggered by an earthquake that struck Central Switzerland in 1601 can be traced over a distance of around 6 km, from Meggen to an area east of Weggis.

Flavio Anselmetti, a limnogeologist, is highly enthusiastic: topographical surveys of the bottom of Swiss lakes have never been performed in such detail before. On a 15-day excursion, a team of Eawag scientists aboard the research vessel Thalassa travelled back and forth on Lake Lucerne, using special sonar equipment to scan the lake floor. Although this method has been employed for some time by coastal states to produce bathymetric charts of the seabed, it has never previously been used in Switzerland. The use of two transmitters and four receivers for the acoustic signal results in overlapping scan sectors. This means that depths can be measured accurate to a few centimetres. The data is then processed to produce three-dimensional images. The features that can be detected include shipwrecks or – as in the Reuss delta of the Urnersee – excavation pits from underwater gravel extraction.

Insights into the past

Particularly fascinating for the researchers are the traces of historical landslides or rockfalls that have now become much more readily visible. Southwest of Weggis, for example, the scar of a major landslide can be traced over a distance of several kilometres, with slide deposits detectable at a greater depth. This landslide, which is known to have been triggered by an earthquake in 1601, generated a tsunami – with a wave 4 metres high rushing across Lake Lucerne. Between Vitznau and Weggis, the new maps show debris from an immense rockfall that plunged into the lake from the Rigi mountain some 3000 years ago. Southwest of Vitznau, the survey reveals a terminal moraine, consisting of debris and boulders left behind by the retreating Reuss glacier at the end of the last ice age 15,000 years ago.

Risk management and monitoring

As well as providing answers to historical questions, however, the new images are valuable to seismologists and sedimentologists seeking to predict future events: in areas where deposits are now visible on steep slopes, underwater landslides could be triggered when a future earthquake occurs. The images could also be used to monitor sediment transport into lakes: changes in stream flows associated with climate change will lead to altered patterns of sediment transport and deposition. The excavation pits visible in the Reuss delta indicate that precise bathymetric images could also be used for monitoring gravel extraction. Archaeologists hoping to uncover evidence of settlements from earlier periods (when lake water levels were lower) have also expressed an interest in the images. Enquiries have also been received from the army, as to whether the new method could possibly be used to pinpoint dumped munitions.

Swisstopo among the sponsors

While Eawag researchers have been exploring Lake Lucerne aboard the Thalassa, a team from Geneva University has also taken part in the project, mapping the bottom of parts of Lake Geneva. As yet, this new survey project is only at the pilot stage. It was made possible with technical support from the Geological Survey of Norway (interferometric sonar on Lake Lucerne) and from the University of Ghent in Belgium (multibeam echo sounder on Lake Geneva). Financial support was provided by swisstopo (Federal Office of Topography), the Federal Office for the Environment, and the Federal Department of Defence, Civil Protection and Sport, among other sponsors. The reasons for swisstopo’s interest are clear: to date, the depth contours shown for lakes on official maps have generally been based on soundings taken in some cases a hundred years ago. If the example of the moraine below Vitznau is used to compare the conventional and bathymetric relief maps, the difference in the level of detail is obvious.

Morainic ridge separating the Gersau basin from the Vitznau basin. Right: representation of the terrain in the 1:25,000 Map of Switzerland with a contour interval of 20 m (10 m for lighter intermediate contour lines).

Morainic ridge extending across the lake in the Gersau basin

Southern part of the Urnersee, including the Reuss delta. The most striking feature is the impact of sand and gravel extraction and sediment deposition on the morphology of the delta. In the eastern part, channels and fan-shaped structures can be seen on the delta front; surprisingly, however, underwater canyons are lacking. At greater depths, dune-type structures predominate. Lying along the western shore are the characteristic alluvial cone at Isleten and the small Bolzbach delta (at the southwestern edge of the image).

• For permission to reproduce images please contact medien@eawag.ch
• Tages-Anzeiger article published on 23 February 2008 (pdf, 1 MB)
  
• 24Heures article published on 7 February 2008 (pdf, 1.6 MB)