Expected changes of river flood hazards across the Danube Basin

The Danube River Basin in the heart of Europe covers an area of just over 800,000 km², and fully or partly overlaps the territories of 19 countries and the living places of approximately 79 million inhabitants. Many of these people live and work in areas threatened by river floods. Despite 170 years of international coordination by various Danube river commissions and a long, sad record of disasters there are hardly any joint efforts for dealing with this hazard across borders. Barriers to better coordination arise from at least eleven different languages spoken in the basin, political tensions between some of the countries, and the heterogeneity of economic conditions. More basin-wide research about actual and future river flood hazards could play an important role in raising common problem awareness and joint action.

Our contribution – funded by the EU HORIZON project DIRECTED (grant no. 101073978) – is the further development and application of PIK’s Danube Model. Basically a combination of the eco-hydrological model SWIM (similar to SWAT) and the hydrodynamical model CaMa-Flood, it is capable of calculating river water stages and flood heights for ten-thousands of subcatchments from daily weather inputs. Using bias-adjusted ISIMIP CMIP6 climate scenario data we produced respective flood projections under SSP 370 and 585 scenarios.

We are going to present maps of the river system with each river reach coloured after the average return intervals expected for floods at the end of the century which are currently assigned 100-year return periods. Both scenario maps for SSP 370 and 585, respectively, show shifts towards longer and shorter return periods with many extremes: Current 100-year floods are often projected to occur over three times more or less frequently. Trends of shortening return periods however dominate both scenarios – river flood hazards are likely to increase under climate change. The maps also show rather different spatial patterns which indicate the high uncertainty in the projections and in the estimation of extreme value distributions despite having used 600 model years for the analysis.

Besides discussing possible systematic errors of the return interval estimations caused by single extreme climate realizations, the importance of considering levees in river flood models is practically exemplified. Especially in level areas like the Pannonian Basin the simulated flood events show more than 1 m water level differences between model runs with and without levees.