Natural Hazards and Earth System Science www.nat-hazards-earth-syst-sci.net 1561-8633 1684-9981 9 3 2009 10.5194/nhess-9-1033-2009 http://www.nat-hazards-earth-syst-sci.net/9/1033/2009/ http://www.nat-hazards-earth-syst-sci.net/9/1033/2009/nhess-9-1033-2009.html http://www.nat-hazards-earth-syst-sci.net/9/1033/2009/nhess-9-1033-2009.pdf 1033 1046 2009-06-30 Significance of "high probability/low damage" versus "low probability/high damage" flood events B. Merz bmerz@gfz-potsdam.de F. Elmer A. H. Thieken Deutsches GeoForschungsZentrum GFZ, Telegrafenberg, 14473 Potsdam, Germany alpS – Centre for Natural Hazard and Risk Management and University of Innsbruck, Grabenweg 3, 6020 Innsbruck, Austria The need for an efficient use of limited resources fosters the application of risk-oriented design in flood mitigation. Flood defence measures reduce future damage. Traditionally, this benefit is quantified via the expected annual damage. We analyse the contribution of "high probability/low damage" floods versus the contribution of "low probability/high damage" events to the expected annual damage. For three case studies, i.e. actual flood situations in flood-prone communities in Germany, it is shown that the expected annual damage is dominated by "high probability/low damage" events. Extreme events play a minor role, even though they cause high damage. Using typical values for flood frequency behaviour, flood plain morphology, distribution of assets and vulnerability, it is shown that this also holds for the general case of river floods in Germany. This result is compared to the significance of extreme events in the public perception. "Low probability/high damage" events are more important in the societal view than it is expressed by the expected annual damage. We conclude that the expected annual damage should be used with care since it is not in agreement with societal priorities. 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