Natural Hazards and Earth System Science www.nat-hazards-earth-syst-sci.net 1561-8633 1684-9981 10 2 2010 10.5194/nhess-10-227-2010 http://www.nat-hazards-earth-syst-sci.net/10/227/2010/ http://www.nat-hazards-earth-syst-sci.net/10/227/2010/nhess-10-227-2010.html http://www.nat-hazards-earth-syst-sci.net/10/227/2010/nhess-10-227-2010.pdf 227 237 2010-02-11 Hazard assessment investigations in connection with the formation of a lake on the tongue of Unterer Grindelwaldgletscher, Bernese Alps, Switzerland M. A. Werder werder@vaw.baug.ethz.ch A. Bauder M. Funk H.-R. Keusen VAW, ETH Zurich, 8092 Zurich, Switzerland GEOTEST AG, Birkenstrasse 15, 3052 Zollikofen, Switzerland The surface of Unterer Grindelwaldgletscher glacier tongue has subsided by more than 200 m over the last 150 years. The surface lowering is not uniform over the glacier tongue but depends on the thickness of the uneven debris cover, which led to the formation of a depression on the tongue. A lake can form in this basin, which occurred for the first time in 2005. Such a glacier lake can drain rapidly leading to a so-called outburst flood. The lake basin has been increasing in size at an alarming rate and in 2008, it reached a volume which poses a significant flooding threat to the communities downstream, as was exemplified by an outburst of the lake in May 2008. The future evolution of the lake basin was extrapolated based on surface lowering rates between 2004–2008. An outburst flood model was tuned with the measured hydrograph from 2008 and then was run with the extrapolated lake bathymetries to simulate future lake outbursts and estimate their flood hydrographs. We discuss the rapidly increasing risk for Grindelwald and other communities, as well as the installation of an early warning system and possible prevention measures. Bauder, A., Funk, M., and Huss, M.: Ice volume changes of selected glaciers in the Swiss Alps since the end of the 19th century, Ann. 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