A new framework for flood adaptation: introducing the Flood Adaptation Hierarchy

Traditional flood risk paradigms and associated strategies are no longer sufficient to address global flood adaptation challenges due to climate change and continued development in floodplains. The current flood adaptation approach is failing to take advantage of the benefits provided by intact ecosystems and perpetuates social and economic inequities, leaving those who are most vulnerable at highest risk. Rooted in the experiences of the United States, we propose a new framework, the Flood Adaptation Hierarchy, which prioritizes outcomes into six tiers. Overall, the tiers distinguish between nature and nature-based solutions, with preference given to natural ecosystems. The most important outcome in our hierarchy is to avoid risk by protecting and restoring natural floodplains; next, eliminate risk by moving communities away from danger; and then to accommodate water with passive measures and active risk reduction measures. We include, but deprioritize, a defense of community assets using nature-based engineering and hardened engineering. Throughout the hierarchy, we provide guidance on the equity considerations of flood adaptation decision making and highlight "impacts," "resources," and "voices" as important equity dimensions. Implementing the framework through an iterative process, using justification criteria to manage movement among tiers, alongside equity considerations, will support adaptation to changing environmental and social conditions and contribute to risk reduction at scale. Though this approach is focused on U.S. flood management and adaptation, prioritizing risk reduction, elimination of risk, and accommodation of hazards over the defense against threats not only has global applicability to flood adaptation, but should also be evaluated for applicability to other climate-driven challenges.