Mangrove forests can be an effective coastal defence in the Pearl River Delta, China

Coastal vegetation can reduce extreme water levels during storm events, but the controlling factors and processes in complex estuary or delta systems are still unclear. This limits an effective implementation of nature-based coastal defences in delta mega-cities in low-lying coastal areas. Here we have numerically modelled how mangroves can offer coastal protection to the large coastal cities located in the Pearl River Delta (China), such as Guangzhou and Shenzhen, during strong typhoons, like Hato (2017). Water level attenuation by mangroves is effective during extreme water level conditions and differences in mangrove forests’ properties drive their coastal protection function. The local (within-wetland) attenuation of extreme water levels is more effective with wide vegetation patches and higher vegetation drag. Narrower vegetation patches can still provide non-local (upstream) water level attenuation if located in the upper estuary channels, but their design needs to avoid amplification of water levels in other delta areas

Mangrove forests as a nature-based solution for coastal flood protection: Biophysical and ecological considerations

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

Establishment and survival of coastal mangrove trees under mechanical disturbances

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

Establishment and survival of coastal mangrove trees under mechanical disturbances

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

Identifying trait-based tolerance to sediment dynamics during seedling establishment across eight mangrove species

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

Analysis of coastal storm damage resistance in successional mangrove species

Use of mangrove ecosystems for coastal flood protection requires reliable predictions of mangrove wave attenuation capacity, especially if this capacity lessens due to storm-induced forest damage. Quantifying and understanding the variation in drag forces on and mechanical properties of mangrove vegetation can improve assessment of mangrove protective capacity. We studied five mangrove species common in the subtropical Pearl River Delta, south China. The tested species range from typically landward-occurring to more seaward-occurring and pioneer species. We sampled across seven sites in the delta to study the impact of salinity on mechanical properties. We quantified strength and flexibility of branches (branch strength and flexibility related to branch diameter, Modulus of Rupture and Modulus of Elasticity), leaf strength (leaf attachment strength related to leaf size, and Leaf Mass per Area) and drag properties (drag force related to surface area and the drag coefficient). For all tested species, larger branch diameter resulted in higher mechanical strength. Larger leaf size resulted in larger peak pulling forces and larger branch surface area resulted in stronger drag forces. Notably, species that generally occur lower in the intertidal zone, where exposure to wind and waves is higher, had relatively stronger branches but more easily detachable leaves. This may be regarded as a damage-avoiding strategy. Across the seven field sites we found no clear effect of salinity on mangrove mechanical properties. This study provides a mechanistic insight in the storm damage process for individual mangrove trees and a solid base for modelling storm (surge) damage at forest scale

A comparison and coupling of two novel sensors for intertidal bed‐level dynamics observation

Previous studies have revealed the importance of short-term bed-level change on the long-term development of intertidal ecosystems. One of the recent advancements in the short-term bed dynamics observation is the developments of laser-based surface elevation dynamics (LSED) and acoustic surface elevation dynamics (ASED) sensors. These two sensors are developed to automatically monitor tidally induced bed-level changes, and their measuring windows are during emergent and submerged periods, respectively. So far, there is no direct comparison or joint application of these two sensors. Therefore, in the current study, we first compare their observation precision in both laboratory and field conditions. Results show that both sensors' measurements are in good agreement with the manual measurements in the flume (R2 > 0.99) and field (R2 ≈ 0.70), indicating the applicability of each sensor in natural environment. Finally, a coupled system (i.e., combining LSED and ASED) is established in a mangrove wetland and both sensors show good agreement with each other (R2 = 0.69). Results also demonstrate that the coupled observation system can well capture the bed-level change during emergent and submerged conditions, which provides detailed continuous data sets to study the bed-level dynamics within a tidal cycle. Furthermore, large-scale monitoring systems can also be established via mobile network module embedded in the sensor, providing real-time monitoring for bed-level changes, which is valuable to coastal morphological research and vital for detecting deterioration of coastal ecosystems and the services they provide to be able to apply adaptive management

Mangrove forests as a nature-based solution for coastal flood protection: Biophysical and ecological considerations

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.</p

Mangrove forests as a nature-based solution for coastal flood protection:: Biophysical and ecological considerations

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