Is the flood occurrence rate decreasing in Southeast Europe?

Floods are natural phenomena, which can turn into disasters and cause widespread damage, health problems and deaths. This is particularly the case where rivers have been denied from their natural floodplains and are limited by embankments, and where housing and industrial buildings have been constructed in areas that are naturally liable to flooding. However, during the last few decades, flood observations from different parts of Europe do not show a clear increase in flood occurrence rate (Blöschl et al., 2019). In the present paper we present longer-term records of winter and summer floods in one of the largest river of Southeast Europe, the Sava River for the 1926-2021 period. We analysed three group of events that were based on three flood protection levels in Republic of Serbia defined for Sremska Mitrovica station. Regular protection level is set at 4120 m³/s and emergency flood defense level which is set at 5120 m³/s, that is, minor events are up to the regular protection level, strong events are up to the emergency level and extreme events are above emergency flood defense level. For the past 95 years, we find a decrease in both summer and winter flood occurrence rates. The reduction in winter flood occurrence can partly be attributed to reduced amount of precipitation during this period of the year. Further, on the basis of these data and methods, we find that for the Sava River can be stated the following: (1) downward but not significant trends in winter flood risk during the observed period, (2) Downward trends of summer floods was also observed, with only strong events being statistically significant. This decrease can be partially due to a projected decrease in cyclone frequency in the Mediterranean region. Presented results clearly demonstrate decreasing flood occurrence rate of the Sava River, as a consequence of decreasing precipitation and increasing evaporation (due to increasing temperature). 

References

Blöschl, G., Hall, J., Parajka, J., Perdigão, A. P. R., Merz, B., Arheimer, B., Aronica, T. G., Bilibashi, A., Bonacci, O., Borga, M., Čanjevac, I., Castellarin, A., Chirico, B. G., Claps, P., Fiala, K., Frolova, N., Gorbachova, L., Gül, A., Hannaford, J., Harrigan, S., Kireeva, M., Kiss, A., Kjeldsen, R. T., Kohnová, S., Koskela, J. J., Ledvinka, O., Macdonald, N., Mavrova-Guirguinova, M., Mediero, L., Merz, R., Molnar, P., Montanari, A., Murphy, C., Osuch, M., Ovcharuk, V., Radevski, I., Rogger, M., Salinas, L. J., Sauquet, E., Šraj, M., Szolgay, J., Viglione, A., Volpi, E., Wilson, D., Zaimi, K. & Živković, N. (2019). Changing climate both increases and decreases European river floods. Nature 573 (7772), 108–111. https://doi.org/10.1038/s41586-019-1495-6.

Acknowledgements

This research was supported by ExtremeClimTwin project, which has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 952384.