Abstract

This chapter provides an overview of sea-level rise (SLR) risks and adaptation in estuaries, with a focus on coastal flooding as one of the key coastal risks in estuaries, together with its interaction with wetlands and the emerging ideas around Nature-based solutions (NbS) and estuarine and habitat restoration. Climate change–induced SLR threatens estuaries with a range of interconnected and cascading coastal hazards and risks, including enhanced coastal flooding due to higher extreme sea-levels with consequent damages to people, their livelihoods, physical assets and resources, as well as changes or losses of intertidal wetlands with adverse implications for biodiversity, ecosystem services and associated human livelihoods. In many estuaries, mean and extreme sea-levels, as well as coastal wetlands, have, however, also been significantly modified by local human activities, as estuaries have been a focus of human habitation for millennia, reflecting agriculture and trade and, since the industrial revolution, industry. Adapting to SLR thus requires the adoption of a systemic perspective that not only focuses on climate change drivers, but also on past and ongoing human modifications of estuaries. In principle, a wealth of different measures to respond exists, with all of these having strengths and weaknesses and thus having complementary roles to play in an integrated adaptation strategy. On a basic level, adaptation responses include: (1) protection of the shoreline; (2) advancement (or shortening) of the line of defense through land reclamation or storm surge barriers; (3) retreat from the shoreline; or (4) accommodation of the effects of SLR. Adaptation can thereby be delivered by combinations of hard engineering structures (e.g., dikes, seawalls, bulkheads, breakwaters, and storm surge barriers) and Nature-based solutions, which include nourishment of degraded sediment systems, as well as restoring, creating, and maintaining naturally dissipative intertidal ecosystems, such as salt marshes to mitigate erosion and flooding. Importantly, there is no one-size-fits-all measure, but choosing a particular combination of measures needs to carefully consider the estuarine natural and social context, including uncertainties. This means recognizing that, while estuaries have many common factors, in detail they are all distinct, in their geological context and history of human intervention. Furthermore, there is the need to consider multiple policy objectives and multiple interests of diverse stakeholders. For example, the need to protect both human and natural capital in estuaries can often give rise to conflicts, such that significant trade-offs will be required to ensure human safety whilst also sustaining healthy ecosystems. These kinds of challenges can be addressed by using multiple-criteria frameworks and embedding these into inclusive participatory processes. Finally, uncertainties can be addressed by implementing low-regret measures and keeping future options open by either delaying decisions that do not necessarily need to be made today or building flexible measures that can later be adapted as one learns more about how much SLR to expect. Furthermore, decisions that need to be made today can be improved by factoring in uncertain SLR through employing robust and adaptive decision-making frameworks.