Rhine flood stories: Spatio-temporal analysis of historic and projected flood formation in the Rhine River basin

The genesis of riverine floods in large river basins often is complex. Streamflow originating from precipitation and snowmelt and different tributaries can superimpose and cause high water levels threatening cities and communities along the river banks. In this study, we develop an analytical framework that captures and shares the story behind major historic and projected streamflow peaks in the large and complex basin of the Rhine River. Our analysis is based on hydrological simulations with the mesoscale Hydrolgical Model (mHM) forced with an ensemble of climate projections. The spatio-temporal analysis of the flood formation includes the assessment and mapping of antecedent liquid precipitation, snow cover changes, generated and routed runoff, flood extent and the excess runoff from major sub-basins up to ten days before a streamflow peak. An interactive web-based viewer provides easy access to result figures of major historic and projected streamflow peaks at four locations along the Rhine River. Our results indicate that each streamflow peak is driven by a specific sequence of precipitation and snowmelt from different areas in the Rhine River basin. Furthermore, we map how rising temperatures increase liquid precipitation in the Alps, in turn, increasing streamflow peaks along the Rhine River. The highest streamflow peak simulated at Cologne using climate projections exceeds the historic record by almost 50 % and was driven by excessive rainfall over several days over large parts of the Rhine River basin. Such an event taking place today would have catastrophic consequences. Further research is required to assess the impacts of changes in the persistence of circulation patterns on flood extent and hazard.