Abstract
Megafloods that far exceed previously observed records often take citizens and experts by surprise, resulting in extremely severe damage and loss of life. Existing methods based on local and regional information rarely go beyond national borders and cannot predict these floods well because of limited data on megafloods, and because flood generation processes of extremes differ from those of smaller, more frequently observed events. Here we analyse river discharge observations from over 8,000 gauging stations across Europe and show that recent megafloods could have been anticipated from those previously observed in other places in Europe. Almost all observed megafloods (95.5%) fall within the envelope values estimated from previous floods in other similar places on the continent, implying that local surprises are not surprising at the continental scale. This holds also for older events, indicating that megafloods have not changed much in time relative to their spatial variability. The underlying concept of the study is that catchments with similar flood generation processes produce similar outliers. It is thus essential to transcend national boundaries and learn from other places across the continent to avoid surprises and save lives.
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Data availability
The flood discharge data from the data holders/sources listed in Extended Data Table 1 that were used in this paper are available at https://github.com/tuwhydro/megafloods.
Code availability
The data analysis was performed in R using the supporting packages circular, lubridate, plotrix, quantreg, raster, RColorBrewer, rgdal, rworldmap and scales. The code used can be downloaded from https://github.com/tuwhydro/megafloods.
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Acknowledgements
We acknowledge all flood data providers listed in Extended Data Table 1. G.B. and M. Bertola were supported by the FWF projects ‘SPATE’ (I 3174, I 4776) and W1219-N22. B.M. and B.G. were supported by the DFG ‘SPATE’ project (FOR 2416). A.V., P.C., D.G., M. Borga and E.D. were supported by the European Union NextGenerationEU ‘RETURN’ Extended Partnership (National Recovery and Resilience Plan—NRRP, Mission 4, Component 2, Investment 1.3—D.D. 1243 2/8/2022, PE0000005). S.K. and J.S. were supported by the Slovak Research and Development Agency (number APVV-20-0374) and the VEGA Grant Agency (number 1/0782/21). J.H. and S.T. were supported by the ROBIN (Reference Observatory of Basins for INternational hydrological climate change detection) initiative, with funding from the Natural Environment Research Council (grant number NE/W004038/1). The authors acknowledge the involvement in the data screening process of M. Haas.
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G.B. and M. Bertola initiated the study and wrote the first draft of the paper. M. Bertola, G.B., M. Borga, A.C., P.C., E.D., D.G., A.M., A.V. and E.V. designed the study. M. Bertola collated the updated version of the database with the help of most of the co-authors and conducted the analyses. G.B. and M. Bertola interpreted the results in the context of underlying geophysical mechanisms. B.G., M. Boháč, A.C., S.K., O.L., S.L. M.M.-G., K.G., Z.G., B.G., J.K., B.M., P.M., J.P., L.P., I.R., K.S., J.S., P.V., P. Ward, P. Willems. and N.Ž. interpreted the results in Central Europe. G.T.A., O.B., M. Borga, A.C., I.Č., G.B.C., P.C., E.D., D.G., A.G., A.M., L.M., D.O., M.Š., A.V. and E.V. interpreted the results in Southern Europe. B.A., B.K., D.L. and N.V. interpreted the results in Northern Europe. J.H., S.H., C.M., S.T. and E.S. interpreted the results in Western Europe. I.D., N.F., L.G., M.K., J.K., M.O. and V.O. interpreted the results in Eastern Europe. All authors contributed to framing and revising the paper.
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Extended data
Extended Data Fig. 1 Map of the European study area.
Background colours indicate five European hydroclimatic regions. Open circles indicate the location of the 8,023 hydrometric stations analysed.
Extended Data Fig. 2 Sensitivity of results to the threshold distance Dmax.
(a) Number and (b) fraction of megafloods larger than envelope as a function of threshold distance; (c) average number of donor catchments as a function of threshold distance. Colours indicate the five hydroclimatic regions in Europe.
Extended Data Fig. 3 Sensitivity of results to the weights used for estimating distance D.
(a) Number and (b) fraction of megafloods larger than envelope for four weight combinations: equal weights (‘equal’, α = β = γ = 1), double weight for Area (‘wA’, α = 2), double weight for mean annual flood (‘wQm’, β = 2), double weight for CV (‘wCV’, γ = 2). (c) Average number of donor catchments for the four weight combinations. Colours indicate the five hydroclimatic regions in Europe.
Extended Data Fig. 4 Megafloods in Europe for alternative hydroclimatic regions.
(a) Five alternative hydroclimatic regions17. (b–f) Maximum observed specific flood discharges (points) and mean of annual specific flood discharges (squares) over the entire observation period at each stream gauge as a function of catchment area. Regional envelope curves (thick lines) and median regional annual specific flood discharges (thin lines) are shown for each hydroclimatic region.
Extended Data Fig. 5 Sensitivity of results to the threshold distance Dmax.
(a) Number and (b) fraction of megafloods larger than envelope as a function of threshold distance; (c) average number of donor catchments as a function of threshold distance. Colours indicate the five alternative regions of Extended Data Fig. 4a.
Extended Data Fig. 6 Sensitivity of results to the weights used for estimating distance D.
(a) Number and (b) fraction of megafloods larger than envelope for four weight combinations: equal weights (‘equal’, α = β = γ = 1), double weight for Area (‘wA’, α = 2), double weight for mean annual flood (‘wQm’, β = 2), double weight for CV (‘wCV’, γ = 2). (c) Average number of donor catchments for the four weight combinations. Colours indicate the five alternative regions of Extended Data Fig. 4a.
Extended Data Fig. 7 Predicted versus observed megafloods.
Similar to Fig. 4 but for the five alternative regions of Extended Data Fig. 4a. a, Predicted specific envelope discharge for 498 target catchments versus observed specific discharge of the megafloods in the same catchments. Predicted envelope discharges are estimated using discharge observations from a pool of donor catchments up to the year before the target megaflood. Colours indicate the ratio of observed and predicted discharge. b, Location of target catchments. Megafloods occur all over Europe and are less surprising than commonly assumed. c, Circular distribution of the timing of the megafloods observed in the target catchments (black lines), and mean timing of the ten largest floods in the donor catchments (coloured points) and their distribution (brown lines). The distance of the points to the centre is inversely proportional to the standard deviation of the flood timing. N-E, North-Eastern; C-E, Central-Eastern; MED, Mediterranean; ALP, Alpine; ATL, Atlantic.
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Bertola, M., Blöschl, G., Bohac, M. et al. Megafloods in Europe can be anticipated from observations in hydrologically similar catchments. Nat. Geosci. 16, 982–988 (2023). https://doi.org/10.1038/s41561-023-01300-5
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DOI: https://doi.org/10.1038/s41561-023-01300-5
