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Coastal flood risk will increase over the coming decades as sea level rise accelerates, storm patterns change and coastal populations grow. This will likely lead to a surge in costs to build and maintain reliable flood safety infrastructure. Hence, innovative nature-based solutions that use coastal ecosystems are gaining attention. Nature-based flood defence is a potentially sustainable and cost-effective solution to reduce coastal flood risk, that can be carried out with ecosystems such as mangrove forests, saltmarshes or coral reefs. Mangrove forests are increasingly studied for nature-based flood defence across the subtropical and tropical latitudes, as their sturdy vegetation can effectively attenuate flow energy from waves – surge attenuation with mangroves remains less well understood. Wider and denser forests provide more wave attenuation. As mangroves naturally fluctuate in size, so does their wave attenuation capacity. Consequently, to reliably estimate the safety of a mangrove-based flood defence, it is necessary to understand the long-term development of the mangrove forest. The studies presented in this thesis contribute important datasets and mechanistic principles that can be used to advance mangrove forest development models and estimate long-term flood protection capacity with coastal mangroves.

Nature-based coastal protection is increasingly recognised as a potentially sustainable and cost-effective solution to reduce coastal flood risk. It uses coastal ecosystems such as mangrove forests to create resilient designs for coastal flood protection. However, to use mangroves effectively as a nature-based measure for flood risk reduction, we must understand the biophysical processes that govern risk reduction capacity through mangrove ecosystem size and structure. In this perspective, we evaluate the current state of knowledge on local physical drivers and ecological processes that determine mangrove functioning as part of a nature-based flood defence. We show that the forest properties that comprise coastal flood protection are well-known, but models cannot yet pinpoint how spatial heterogeneity of the forest structure affects the capacity for wave or surge attenuation. Overall, there is relatively good understanding of the ecological processes that drive forest structure and size, but there is a lack of knowledge on how daily bed-level dynamics link to long-term biogeomorphic forest dynamics, and on the role of combined stressors influencing forest retreat. Integrating simulation models of forest structure under changing physical (e.g. due to sea-level change) and ecological drivers with hydrodynamic attenuation models will allow for better projections of long-term natural coastal protection., Coastal Engineering

Nature-based coastal protection is increasingly recognised as a potentially sustainable and cost-effective solution to reduce coastal flood risk. It uses coastal ecosystems such as mangrove forests to create resilient designs for coastal flood protection. However, to use mangroves effectively as a nature-based measure for flood risk reduction, we must understand the biophysical processes that govern risk reduction capacity through mangrove ecosystem size and structure. In this perspective, we evaluate the current state of knowledge on local physical drivers and ecological processes that determine mangrove functioning as part of a nature-based flood defence. We show that the forest properties that comprise coastal flood protection are well-known, but models cannot yet pinpoint how spatial heterogeneity of the forest structure affects the capacity for wave or surge attenuation. Overall, there is relatively good understanding of the ecological processes that drive forest structure and size, but there is a lack of knowledge on how daily bed-level dynamics link to long-term biogeomorphic forest dynamics, and on the role of combined stressors influencing forest retreat. Integrating simulation models of forest structure under changing physical (e.g. due to sea-level change) and ecological drivers with hydrodynamic attenuation models will allow for better projections of long-term natural coastal protection.

Mechanical disturbance from waves and sediment dynamics is a key bottleneck to mangrove seedling establishment. Yet, how species vary in tolerance to sediment dynamics has not been quantified. We identified how tolerance to sediment dynamics differs for three mangrove propagule traits: propagule size, successional stage, and type of embryo development. We selected eight mangrove species growing in south China that vary from small seeds to large elongated propagules, pioneer to climax species, and non-viviparous to viviparous. In a mesocosm set-up, we applied bed level treatments to establishing seedlings: erosion, control, or accretion, by removing 2 cm, 0 cm, or adding 1 cm of sediment per week over 3 weeks. We measured seedling survival, shoot, and root lengths, and the critical erosion depth that leads toppling or dislodgement. We identified five relationships between seedling morphology and accretion and erosion thresholds: (1) tall (viviparous) propagules likely had highest accretion thresholds; (2) small pioneer propagules grew relatively fast to increase accretion thresholds; (3) there was a strong correlation between the erosion threshold and root length; and (4) climax species grew longest roots overall, (5) while pioneer species grew longer roots fast in response to sediment erosion. We identify distinct strategies for successful establishment in sediment dynamics that contribute to understanding mangrove zonation and underpin the importance of restoring diverse forests containing not just robust climax species, but also adaptable pioneers. Furthermore, this study reveals maximum shoot and root length as key determinants for seedling stability across species, providing a simple proxy for modeling establishment events.