Being prepared for storm surges on the Baltic Sea coast

The record storm surge in October 2023 caused severe damage to the German Baltic coast. Effective adaptation scenarios to rising sea levels are therefore becoming increasingly urgent. In two recent studies, researchers at Kiel University have modelled both the flooding extent along the Baltic Sea coastal areas and, for the first time, two possible upgrades for current dike lines in high resolution. They modelled various storm surge and sea level rise scenarios. Their results show that, based on the current dike line, neither an increase nor managed realignment, i.e. controlled breaching of seaward defences and relocation of primary defence line further inland, as a nature-based solution can be sufficient to protect people, infrastructure or buildings to significantly reduce the risk of flooding along the German Baltic Sea coast by the year 2100. In the model, the risk for the population compared to today's existing coastal protection was only reduced by a maximum of 26% for the scenario of managed realignment. Their results were published on 24 November in the journal Communications Earth & Environment and at the beginning of September in Natural Hazards and Earth System Sciences.

"The majority of our simulated flooding areas are located in Mecklenburg-Western Pomerania, with the hotspots in the lagoons of Fischland-Darß-Zingst, Rügen, Usedom and the Peene estuary. In Schleswig-Holstein, the Flensburg Fjord, Eckernförde Bay, Fehmarn, Travemünde and Lübeck are particularly affected", says first author Dr Joshua Kiesel, postdoctoral researcher in the Coastal Risks and Sea Level Rise working group at the Institute of Geography at Kiel University. Professor Athanasios Vafeidis, co-author of both studies and member of the research priority area Kiel Marine Science (KMS) at Kiel University, leads the working group. "Today, we can still decide for a large part of the German Baltic Sea coast what an adaptation might look like in the future. The known disadvantages of existing dikes should definitely be taken into account”, says Vafeidis. According to the researcher of Kiel University, this is also crucial for protection against future extreme events, which are very likely to become more frequent.

Process-based modelling helps to evaluate adaptation scenarios

Today, nature-based adaptation options are becoming increasingly important. One example of this is managed realignment, which aims to create a natural buffer zone between the sea and the dike. In this buffer zone, coastal wetlands can (re)establish, whose vegetation increases the surface roughness and thus makes a natural contribution to coastal protection. Rare habitats such as salt marshes and reed beds can develop in these buffer zones, which also contribute to the preservation of biodiversity.

Previous models simulate flooding according to the principle of a uniform spread of water when storm surges hit the coast. In the current, process-based modelling, the researchers now also take into account the temporal course of storm surges as well as the attenuation of the surge with its currents and peak water levels when it hits areas of varying roughness, such as wetlands, forests or paved soils. "Compared to previous supra-regional or continental studies, our coastal flooding model is the first to use high-resolution terrain data from state and regional dikes with a resolution of one metre for the entire German Baltic Sea coast. This enabled us to assess the effectiveness of existing and raised dikes on the one hand and of managed realignment on the other. However, based on current defence lines, both will probably not be sufficient to withstand the ongoing sea level rise", says coastal geographer Kiesel.

However, Kiesel and the research team estimate that the protection potential for managed realignment, including raising them, is greater than raising them without realignment. "Managed realignment means that existing dike lines are breached to reestablish regular flooding and sedimentation of low lying areas. This approach typically involves the construction of a new dike line further inland to protect adjacent people, buildings and infrastructure. There are already several examples of managed realignment along the German Baltic coast", says the scientist. During very high surges, the slightly improved coastal protection function is due to the significantly longer, landward dike line.

In order to identify physically plausible areas for managed realignment, the scientists based their study on several parameters: no direct development behind the dike and no infrastructure in the form of roads or railway networks. In order to compare the effectiveness of dike relocation with conventional coastal protection, all state and regional dikes along the German Baltic Sea coast were also raised by 1.5 metres in accordance with the climate dike concept. "There is a broad scientific debate about managed realignment and nature-based adaptation options in general. However, there has been little research into their effectiveness in terms of coastal protection. Our research fills a gap here. We wanted to know what contribution managed realignment can make to regional coastal protection if it is implemented wherever it is physically possible", says Kiesel. For this reason, socio-economic considerations and the frequent lack of acceptance among the population for such measures, which also involve interventions in the infrastructure, were not taken into account.

Flood maps for the German Baltic Sea coast

In their first study from September 2023, around a month before the severe storm surge, the research team led by Kiesel and Vafeidis had already been able to prove with two high end sea level rise scenarios that the entire German Baltic Sea coast is at high risk of flooding and that upgrades to coastal protection are necessary. To do this, they coupled a model of the western Baltic Sea with a coastal flooding model.

The spatial information on dikes and vegetation usually requires good local knowledge and cooperation with the authorities. "Without our transdisciplinary approach, we would not have been able to provide our flood maps in this resolution", adds co-author Vafeidis, whose working group works closely with the state authorities.

"Now, not later": More research into adaptation concepts

With the climate dike concept of the state of Schleswig-Holstein and the planned dike constructions along the German Baltic Sea coast, the first coastal protection measures are already underway. Kiesel, who now works as a research assistant at the Free University of Amsterdam, would like to continue working closely together in the future: "We need more research into the effectiveness of nature-based adaptation options. The dramatic events of the Baltic Sea storm surge have shown that we should start today if possible. Successive storm surges in a short period of time could also become more frequent in the future. The infrastructure already weakened by the first storm surge would be much more vulnerable to subsequent events, with even worse consequences for the people on the coast."

Developing new and effective adaptation concepts are a challenge that not only countries and people on the German Baltic coast are facing. Such risks are also increasing in Europe. Vafeidis' working group is researching European solutions in the EU Horizon 2020 project "Coastal Climate Core Services" (CoCliCo).

The research results of the two studies were produced in the project "Ecosystem-supporting Coastal Adaptation Strategies for the German Baltic Sea Coast" (ECAS-Baltic), which is funded by the German Federal Ministry of Education and Research (BMBF). The project was coordinated by the Global Climate Forum (GCF) in Berlin. In addition to Kiel University with its coastal geology and coastal geography, the following institutions were involved in the ECAS-Baltic project: the Ludwig Franzius Institute for Hydraulic, Estuarine and Coastal Engineering, Leibniz University Hannover, the Helmholtz Centre Hereon, the Leibniz Institute for Baltic Sea Research Warnemünde (IOW) and the two universities of Rostock and Greifswald.

Original publications:
Kiesel, J., Honsel, L.E., Lorenz, M., Gräwe, U. and Vafeidis, A.T.: Raising dikes and managed realignment may be insufficient for maintaining current flood risk along the German Baltic Sea coast. Commun Earth Environ 4, 433 (2023). DOI: 10.1038/s43247-023-01100-0

Kiesel, J., Lorenz, M., König, M., Gräwe, U., and Vafeidis, A.T.: Regional assessment of extreme sea levels and associated coastal flooding along the German Baltic Sea coast, Nat. Hazards Earth Syst. Sci., 23, 2961–2985 (2023). DOI: 10.5194/nhess-23-2961-2023

Photos are available for download:
www.uni-kiel.de/de/pressemitteilungen/2023/299-schilfguertel-deich.jpg
Reed belt in front of a dike section on the Holnis peninsula in Schleswig-Holstein
© Tobias Hahn, Kiel University

www.uni-kiel.de/de/pressemitteilungen/2023/299-joshua-kiesel.jpg
Dr Joshua Kiesel is the first author of both studies and was a postdoctoral researcher in the Coastal Risks and Sea Level Rise working group at the Institute of Geography at Kiel University. He is now a research associate at the Free University of Amsterdam.
© Svenja Karstens, Kiel University

www.uni-kiel.de/de/pressemitteilungen/2023/299-kontrollierte-deichrueckverlegung-england.jpg
Nature-based adaptation options in the event of rising sea levels are attracting increasing attention. One example of this is managed realignment, which aims to create a natural buffer zone between the sea and the dike. Existing dike lines are breached to reestablish regular flooding and sedimentation of low lying areas. This is a common procedure in Freiston Shore, England, for example.
© Joshua Kiesel, Kiel University

www.uni-kiel.de/de/pressemitteilungen/2023/299-fischlandwiesen.jpeg
Coastal wetlands increase the surface roughness with their vegetation and thus make a natural contribution to coastal protection. As a buffer zone, salt marshes are also effective carbon reservoirs and a nursery for fish and birds.
© Joshua Kiesel, Kiel University

www.uni-kiel.de/de/pressemitteilungen/2023/299-potentielle-deichrueckverlegungsflaechen-studie.png
Study area and potential managed realignment sites across the German Baltic Sea coast: The detection of managed realignment sites excludes all areas in which major roads, railways or any kind of development occur and which are higher than ten metres above sea level.
© Kiesel, J. et al. (2023), Commun Earth Environ

Background information on dike construction measures on the Baltic Sea coast:
Dikes can create a sense of security among the population and enable or even encourage human development directly behind the dike. This can paradoxically increase the risk of flooding, which is described in science as the Levee effect. At the same time, dikes reduce the adaptability of ecologically valuable coastal wetlands, which increase the surface roughness with their vegetation and thus make a natural contribution to coastal protection.

Low-lying land behind dikes is often drained for further utilisation and the regular supply of sediment through flooding does not occur, which prevents the land behind the dikes from growing at the same rate as the sea level. There is therefore a lack of sediment and at the same time the land sinks as a result of drainage, in some areas to below mean sea level. The loss of coastal wetlands and sinking land further increase the risk of flooding. For this reason, nature-based adaptation options are now receiving increasing attention. One example of this is controlled dike relocation, which aims to create a natural buffer zone between the sea and the dike. Rare habitats such as salt marshes and reed beds can develop in these buffer zones, particularly on the German Baltic Sea coast, thus contributing to the preservation of biodiversity.

About the Coastal Risks and Sea-Level Rise (CRSLR) research group:
The research group "Coastal Risks and Sea-level Rise" (CRSLR) at Kiel University currently has a total of ten scientists working and is headed by Professor Athanasios Vafeidis. The team assesses the extent of climate change-induced impacts on coastal regions worldwide and conducts research on how coastal systems respond to the combined pressures of natural and anthropogenic factors. Using data from observations and modeling as well as long-range, scientific analyses, the researchers focus on coastal adaptation, coastal population trends, and coastal influence and vulnerability analysis. Their research activities also include the EU Horizon 2020 Coastal Climate Core Services (CoCliCo) project, which runs through September 2025. The project aims to better inform decision makers about coastal protection measures and adaptations to sea level rise and associated, future hazards to the population.

About Kiel Marine Science (KMS):
Kiel Marine Science (KMS), the Center for interdisciplinary marine science at Kiel University, is devoted to excellent and responsible ocean research at the interface between humans and the ocean. The researchers combine their expertise from various natural and social science disciplines to investigate the risks and opportunities that the sea provides for humans. The success of Kiel Marine Science is based on close interdisciplinary cooperation in research and teaching between researchers from seven faculties at Kiel University. Together with actors from outside the scientific community, they work globally and transdisciplinarily on solutions for sustainable use and protection of the ocean.

More information:
To the webseite of the CRSLR-research group
About the BMBF-Project ECAS-Baltic
About the EU-Project CoCliCo

Scientific Contacts:
Dr Joshua Kiesel
Research group Coastal Risks and Sea-Level Rise
Kiel University
Institute for Environmental Studies (IVM)
Vrije Universiteit Amsterdam (VU)
E-mail (CAU): kiesel@geographie.uni-kiel.de
E-mail (VU): j.kiesel@vu.nl

Prof. Athanasios Vafeidis
Research group Coastal Risks and Sea-Level Rise
Institute of Geography
Kiel University
E-mail: vafeidis@geographie.uni-kiel.de

Press Contact:
Tobias Hahn
Public Outreach / Science Communication
Press, Digital and Science Communication Services and Kiel Marine Science (KMS)
Kiel University
E-mail: thahn@kms.uni-kiel.de
Telephone: +49/431/880-7185

Kiel University
Press, Communication and Marketing, Eva Sittig, Text/editing: Tobias Hahn
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