A new study on the present and projected future risk of so-called “compound flooding” – which is caused by the co-occurrence and interaction of high sea-levels (i.e. storm surges or storm tides) and heavy precipitation (resulting in large runoff) - in low-lying coastal areas of Europe, has been published in the journal Science Advances, and has been featured on several international news media outlets (including the BBC News web-site on 18 September 2019 - see below).
Estimating coastal flood risk is essential for engineering practices, disaster risk reduction, and policy-making. There are two distinct mechanisms that can lead to coastal flooding: storm surges and heavy precipitation - either by direct runoff (pluvial) or increased river discharge (fluvial). Where both occur concurrently or in close succession - as often happens - the adverse consequences can be greatly exacerbated. Despite this, compound flooding is currently not considered in coastal flood risk analyses, with sea and river flooding generally regarded as separate events. The Figure below illustrates how storm surges and heavy rain can combine to increase the risk of coastal flooding.
In a warmer climate, rising mean sea-levels pose the main threat to coastal areas, naturally increasing the likelihood of sea-level extremes in the future. Coastal planning agencies in Europe are currently becoming more aware of the rise in flood hazard and the need to take action. However, as the new study highlights, the sea-level rise will also increase the potential for compound flooding. Indeed, heavy precipitation events that today coincide with a relatively weak storm surge, may have a much stronger effect under higher sea-levels.
Furthermore, the study shows that the potential for compound flooding will not only be affected by sea-level rise, but also by changes in storminess and precipitation extremes. Future model projections show that northern Europe may experience an increase in the potential for compound flooding (in addition to that caused by sea-level rise) mainly due to increasing precipitation intensities.
In the study, climate modelling is used to show that around 3% of Europe’s coastal areas experience compound flooding events more than once every six years. At the moment, these are mainly in the Mediterranean, around the Gulf of Valencia in Spain, Algeria, the Gulf of Lyon in France, and in southern Turkey. The pan-European analysis suggests that the difficulties posed by compound events will increase in a warmer world, and will move to threaten Northern Europe far more than at present. According to the modelling, compound flooding is projected to robustly increase along the west coast of Great Britain, Northern France, and along the east and south coast of the North Sea.
The results of the study cannot directly be interpreted as projections of actual flood risk, since the latter is a complex phenomenon depending on several other factors not considered here (e.g. topography, presence of protection). The study identifies European regions potentially facing compound flooding in a warmer future climate, thereby providing a continental-scale basis for follow-up local compound flooding risk assessments and adaptation planning. In areas prone to compound flooding, assessments of future flooding risk should consider changes in sea, pluvial, and fluvial flooding simultaneously, to take account of how multiple flooding drivers might combine to exacerbate flooding impact.
The new study on compound flooding was carried out by a team of researchers lead by the University of Graz in Austria, and including the European Commission’s Joint Research Centre (JRC) in Italy.
European Commission, Joint Research Centre (JRC)
For more information:
Bevacqua, E., D. Maraun, M. I. Vousdoukas, E. Voukouvalas, M. Vrac, L. Mentaschi and M. Widmann. 2019. Higher probability of compound flooding from precipitation and storm surge in Europe under anthropogenic climate change. Science Advances 5 (9). www.doi.org/10.1126/sciadv.aaw5531