Effective Adaptation Options to Alleviate Nuisance Flooding in Coastal Megacities—Learning From Ho Chi Minh City, Vietnam

The economies and livelihoods of many coastal megacities are at serious risk from flooding, despite investments in flood defenses. For instance, in Ho Chi Minh City, the construction of a large‐scale ring‐dike has mitigated negative effects from storm surges, yet damage is still frequently caused by high‐intensity rainfalls leading to nuisance flooding, which is responsible for the highest proportion of flood losses in the city today. Because sustainable flood risk management requires detailed spatial information, we analyze the local risk and its components based on a chain of novel models previously calibrated and validated for Ho Chi Minh City. Furthermore, we assess the effectiveness of two decentralized adaptation options, namely private precautionary measures and rainwater retention, for mitigating pluvial flooding. Our integrated risk assessment reveals that the approaches are complementary, which is a major advantage for their implementation. Implementation of both approaches has the potential to reduce the expected annual damage and the number of annually affected households by 16% and 56%, respectively. This is also reflected in a significant reduction of annual losses per household, which we propose as an additional, people‐centered indicator of flood risk. Moreover, these measures are well‐suited to strengthen citizen participation in risk reduction beyond top‐down protection schemes. Complementing the ring‐dike with decentralized adaptation options can therefore be seen as an effective and generic strategy to alleviate the impacts of nuisance flooding in coastal megacities, such as Ho Chi Minh City, and should be incentivized by decision‐makers. Aside from hydrological and metocean site conditions, both the methodology and findings of this study are transferrable to any coastal megacity facing similar challenges. Plain Language Summary In many delta cities like Ho Chi Minh City in Vietnam, flooding continues to be a serious threat, despite investments in flood defenses. Even with a large‐scale ring‐dike in place, high‐intensity rainfalls that lead to so‐called nuisance flooding continue to cause considerable damage. To support effective risk management, we analyze flood risk and its components by combining various innovative models that were calibrated and validated for Ho Chi Minh City. Moreover, we assess two decentralized adaptation measures against flooding, namely private precautionary measures and rainwater retention. To evaluate the eff