The Baltic Sea storm surge in October 2023: Damages to coastal flood defense and protection structures and consequences for coastal flood and erosion management in Schleswig-Holstein

From October 19 to October 21, 2023, a severe to very severe storm surge hit the Baltic Sea coast of Schleswig-Holstein. The extreme water levels, combined with two days of stormy seas, caused serious damage to public and private infrastructures. The coastal flood defense structures were subject to extreme hydraulic loads. While the state's protective dikes were able to withstand the storm surge, eight of 35 regional dikes failed as a result of dike breaches, overflow and bypassing. Ten regional dikes suffered moderate to severe damage and restoration measures were required on 20 regional dikes.

Based on an analysis of the consequences of the October 2023 flood, the following conclusions relevant to future coastal protection on the Baltic Sea coast of Schleswig-Holstein can be drawn. The storm surge caused severe damages to and failure of regional dikes on the mainland coast. With a few exceptions, they are not designed to withstand such a storm surge. For the small-scale water and soil associations that are responsible for the regional dikes on the mainland coast, ensuring long-term, adequate coastal flood protection, particularly in the more densely populated coastal lowlands, represents an enormous financial and personnel challenge. Finally, the October 2023 flood led to flooding and significant damages in several unprotected coastal towns. Without proper flood defense measures, the damage expectations in these coastal towns due to stronger sea-level rise will increase disproportionately compared to rural areas. As a main consequence of the October 2023 flood, the state has offered to take over responsibility for coastal flood protection in densely populated coastal lowlands (guideline value > 50 inhabitants) that are currently protected by regional dikes. The state's focus is on protecting settlements in the overriding public interest.