Trends in flood risk of the River Werra (Germany) over the past 500 years / Tendances du risque d'inondation dans la vallee de la riviere Werra (Allemagne) durant les 500 dernieres annees

A record of floods from 1500 to 2003 of the River Werra (Germany) is presented. The reconstruction is based on combining documentary and instrumental data. Because both data types have overlapping time intervals, it was possible to apply similar thresholds for flood definition and obtain a rather homogenous flood series. The kernel method yielded estimates of time-dependent flood risk. Bootstrap confidence bands helped to assess the significance of trends. The following was found: (a) the overall risk of floods in winter (November—April) is approximately 3.5 times higher than the summer flood risk; (b) winter flood risk peaked at around 1760 and 1860—it increases again during the past decades; and (c) summer flood risk peaked at around 1760—it shows a long-term decrease from then on. These trends for the Werra contrast with those of nearby River Elbe, reflecting the high spatial variability of orographic rainfall.