Abstract
Abstract. Flood risk assessments are typically based on scenarios which assume homogeneous return periods of flood peaks throughout the catchment. This assumption is unrealistic for real flood events and may bias risk estimates for specific return periods. We investigate how three assumptions about the spatial dependence affect risk estimates: (i) spatially homogeneous scenarios (complete dependence), (ii) spatially heterogeneous scenarios (modelled dependence) and (iii) spatially heterogeneous but uncorrelated scenarios (complete independence). To this end, the model chain RFM (regional flood model) is applied to the Elbe catchment in Germany, accounting for the spatio-temporal dynamics of all flood generation processes, from the rainfall through catchment and river system processes to damage mechanisms. Different assumptions about the spatial dependence do not influence the expected annual damage (EAD); however, they bias the risk curve, i.e. the cumulative distribution function of damage. The widespread assumption of complete dependence strongly overestimates flood damage of the order of 100 % for return periods larger than approximately 200 years. On the other hand, for small and medium floods with return periods smaller than approximately 50 years, damage is underestimated. The overestimation aggravates when risk is estimated for larger areas. This study demonstrates the importance of representing the spatial dependence of flood peaks and damage for risk assessments.
Highlights
Floods frequently occur as destructive events throughout the world
The damage is underestimated under the assumption of complete dependence for events with return periods smaller than 87 years
We show that the overestimation of risk by the complete dependence assumption that has been reported by Lamb et al (2010) and Wyncoll and Gouldby (2015) applies to large return periods only
Summary
Floods frequently occur as destructive events throughout the world. In the period 1995–2015, there were around 3100 flood events which affected 2.3 billion people worldwide with overall damages of USD 662 billion (CRED and UNISDR, 2015). In order to mitigate the destructive impacts of floods, sound flood risk assessment and management are essential. Flood risk management has gained considerable attention and has shifted from a hazard-focused approach to the broader risk-based perspective covering both physical and societal processes The EU Flood Directive (EC, 2007) was adopted in October 2007 to launch a flood risk assessment and management framework in Europe considering all aspects of flood risk, including the impacts on society
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