Characteristics of historical extreme floods in the Danube and Main catchments in Austria and Germany compared to modern ones

Historical flood events occurred before the begin of systematic flow records, represent valuable information that should be considered in flood frequency analyses. However, in most cases historical data is still stored in printed volumes and therefore not easily accessible and ready-to-use for hydrological analyses. The aim of this study is to compare the largest historical floods in the Danube and Main catchments to modern (i.e. between 1950 and 2022) floods in terms of their spatial, temporal and causal carachteristics. Here we collect, digitize, and compile a dataset of the largest historical flood events in the Danube and Main catchment between 1845 and 1950. The newly developed dataset contains daily and peak discharge and water level measurements observed at several locations for 13 and 9 flood events in the two catchments. Flood hydrographs were also recovered at several locations in the Danube catchment. Using the developed dataset, we compare the characteristics of the historical flood events to the characterisitcs of modern large flood events. The findings show that the historical flood discharges are among the largest ever measured in the two catchments and that large floods occur more frequently in summer than in the past. In summary, this work reviews the spatial, temporal and causal characteristics of these very large historical events in comparison with recent events and discusses the implications for flood hazard assessment.