Accounting for Compound Flooding Can Prevent Maladaptation—A Baltic Sea Case Study

Abstract Estuarine areas are currently at risk of compound flooding, the frequency and intensity of which is expected to increase with climate change. Even though efforts are made to adapt against single flood drivers using hard protection, potential subsequent changes in flood risk due to compound flooding are often overlooked in flood risk assessments. This is because risk assessments mostly focus on individual flood drivers and do not account for changes in risk from adaptation measures. We address this question and use hydrodynamic modeling to simulate compound flooding for two adaptation scenarios. We consider adaptation in terms of storm surge barriers, in two locations along the Trave estuary, namely Schlutup and Trave, at Lübeck, Germany. We assess the effectiveness of both storm surge barriers in reducing flooding by simulating individual‐driver, as well as low‐ and high‐magnitude compound flood scenarios. We find that while during low‐magnitude compound flooding both barriers reduce the overall flood extent by 25%–86%, high‐magnitude compound flooding leads to an increase of up to 100%, depending on the location of the barrier. Our results suggest that the river contribution is amplified by 52%–100% by the Schlutup Barrier. The Trave Barrier, however, only amplifies flood extents in the high‐magnitude compound flood scenarios. Our findings highlight the need to consider compound flooding in adaptation planning to avoid defense failure and unexpected increases in risk. However, as this study considers only two (low probability) extreme events, a more comprehensive approach is necessary to fully understand the overall impact on risk.