A consistent set of trans-basin floods in Germany between 1952–2002
Abstract. Floods that affect many sites simultaneously can pose great challenges in the co-ordination of flood disaster management actions, as well as for the insurance and re-insurance industry, since this type of flooding leads to an accumulation of losses and the risk assessment needs to be extended to a concept representing the spatial risk of flooding. The assessment of the accumulated risk, especially over large domains, requires an analysis of the spatial and temporal coherence of flooding. For Germany the extent of spatial dependence of flooding is largely unknown and no systematic analysis has been performed so far. In this paper, we present a methodology that is capable of capturing the simultaneous occurrence of flooding using multiple series of mean daily discharge. For the first time we present a complete and consistent set of trans-basin floods in Germany for the period between 1952 and 2002. Each flood is characterised by a specific value for the timing, the location and the magnitude of discharges within the entire river network. We propose a measure for quantifying the overall event severity considering both the heterogeneous spatial extent as well as the locally varying magnitudes of a trans-basin flood. In total, we identify 80 trans-basin floods in the entire time period. The set is dominated by events that were recorded in the hydrological winter (64%); 36% occurred during the summer months. 32 events affected more than one third of the entire river network. These most severe events are predominantly winter events. Dividing the study period into two sub-periods, we find an increase in the percentage of winter events from 58% in the first to 70.5% in the second sub-period. Accordingly, we find a significant increase in the number of extreme trans-basin floods in the second sub-period. A natural extension of this study is the quantification of the spatial and temporal dependencies in a multivariate framework. This framework needs to be supported by a flood typology based on the analysis of the physical processes relevant in the genesis of trans-basin floods.