NHESS Natural Hazards and Earth System Sciences NHESS Nat. Hazards Earth Syst. Sci. 1684-9981 Copernicus Publications Göttingen, Germany 10.5194/nhess-10-227-2010 Hazard assessment investigations in connection with the formation of a lake on the tongue of Unterer Grindelwaldgletscher, Bernese Alps, Switzerland Werder M. A. 1 Bauder A. 1 Funk M. 1 Keusen H.-R. 2 VAW, ETH Zurich, 8092 Zurich, Switzerland GEOTEST AG, Birkenstrasse 15, 3052 Zollikofen, Switzerland 11 02 2010 10 2 227 237 Copyright: © 2010 M. A. Werder et al. 2010 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://nhess.copernicus.org/articles/10/227/2010/nhess-10-227-2010.html The full text article is available as a PDF file from https://nhess.copernicus.org/articles/10/227/2010/nhess-10-227-2010.pdf

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.

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