It is above all the rise in sea levels that is responsible for the increasing risk of flooding along Europe’s coasts. However, the danger is also heightened if storm surges occur together with heavy precipitation. This is the finding of an international research project that looked at the compounding of natural events from the aspect of climate change.

Previously, these phenomena have been examined separately in risk analyses. According to the head of the study, Douglas Maraun, the danger of co-occurring incidents have been underestimated. Storm surges and torrential rain are often triggered by the same weather conditions. Maraun researches climate change at the Wegener Center for Climate and Global Change, part of the University of Graz. He is interested in man-made climate change and particularly in extreme precipitation.

When torrential rains and storm surges interact …

… the resulting impact may be greater than that of a single incident. The resulting impact is determined by several mechanisms. In most cases, this is the extreme runoff rainwater (precipitation) that builds up on the coast – and especially in river estuaries – as a result of a storm surge. Another mechanism is precipitation on soil that has become saturated by a previous storm surge.

Hochwassergefahr (c) Universität Graz                           Image: (c) University of Graz

The simultaneous occurrence of storm surges and heavy rainfall is not a phenomenon of the future. There have already been numerous examples. One of the most notable cases was the flood warning issued by the Norderzijlves Water Board in the Netherlands, which led to a precautionary evacuation in 2012. During 2014, Ravenna witnessed a major flood and Bristol experienced one in 2015.

Heightened risk due to global warming

The international research team around Maraun wanted to find out whether the risk of flooding from heavy precipitation and storm surges is changing because of climate change. Risk analysis is important for stipulating regulatory policy and disaster management for vulnerable coastal regions. Building dikes is a classic measure. Nevertheless, Maraun also envisages more sustainable solutions. To quote: “In some regions, less waterproofing and more natural vegetation could also be used to prevent rainwater from running off too quickly.”

Project partners from the Joint Research Center (JRC) of the European Commission in Ispra, Italy provided the database. They used a mathematical model of the sea surface in order to calculate climate model simulations that predicted the occurrence of future storm surges. They did this by generating storm surge and wave models.

The team around Maraun then analyzed the results together with other simulations of precipitation in a complex statistical model. In this way, the European coastal regions were identified which are likely to be confronted with the simultaneous occurrence of storm surges and heavy precipitation stemming from global warming.

Redefining the problem zones

This showed that “precipitation in Northern Europe is becoming more intense, so that the risk of simultaneous heavy precipitation and storm surges is likely to increase,” Maraun reports. In the case of river estuaries, flooding is also expected to increase. In Southern Europe, on the other hand, the number of storm surges is likely to drop – “which should reduce the overall risk of extreme precipitation occurring at the same time,” according to the climate researcher. The researcher cites the following reasons for this increase:

“In Southern Europe, the risk of storm surges is on the decline, which is consistent with our belief that there will be fewer storms there in the future. In Northern Europe, precipitation is becoming more intense. Especially as there is more moisture in the air at higher temperatures. And storms do have a tendency to shift northwards.” Douglas Maraun.

A markedly high probability of an increase in the simultaneous occurrence of storm surges and heavy precipitation – which is triggered by global warming – was shown most notably in the following regions:

  • on the western coasts of Great Britain and Northern France;
  • on the Baltic and southern coasts of the North Sea;
  • on the eastern half of the Black Sea (see Figure 3A).

 

Hochwasser, Sturmflut, Starkniederschlag

(c) University of Graz

Raising rate of recurrence

The proportion of coasts experiencing a recurrence cycle of less than six years is expected to rise from 3% at present, to 11% by the end of the 21st century. Hot spot regions where the recurrence rates will fall below this figure are: the Bristol Channel and the Devon and Cornwall coasts in the United Kingdom, as well as the Dutch and German North Sea coasts (Fig. 3B). Meanwhile, around the Noorderzijlvest Water Board area, which is also facing the largest rise in sea levels, the average probability for a simultaneous occurrence of storm surges and heavy precipitation will triple. And the risk for the nearby Norwegian west coast area of Bergen is expected to increase by a factor of five.

Factors not taken into account

The study focused on the space-time dynamics of the meteorological forces driving floods. However, the actual flood risk is dependent on several factors, such as topography and the presence of preventive measures. As an example, for sites with cliffs the risk may be negligible even though the potential flood risk is still high. Therefore, the results of this study are to be seen as groundwork for further detailed research which also takes local conditions into consideration, such as the shape of the coastline, dikes, port facilities or flood barriers.

Other team members

The research analysis was carried out by doctoral student Emanuele Bevacqua, who also studied the Ravenna floods and is now a postdoctoral fellow at the University of Reading (near Bristol) in Great Britain.

The LSCE in Gif-sur-Yvette (France) and the University of Birmingham (United Kingdom) were also involved in this study.

Publication

“Higher probability of compound flooding from precipitation and storm surge in Europe under anthropogenic climate change”
E. Bevacqua, D. Maraun, M. I. Vousdoukas, E. Voukouvalas, M. Vrac, L. Mentaschi, M. Widmann
Science Advances, Vol. 5, no. 9, eaaw5531, 18 September 2019

 

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