Physically based modelling of coastal wetlands for climate change

Climate change and anthropogenic pressures pose a serious threat to coastal wetlands. Recent research has shown that sediment and biological accretion can attenuate sea-level rise (SLR) effects. However, sediment research on inland catchments draining to coastal ecosystems has often been detached from research addressing sediment impacts on coastal ecosystems. This thesis aims to integrate the hydro-sedimentological behaviour of catchments with the eco-geomorphological (EGM) behaviour of coastal wetlands under current conditions and future scenarios, considering the feedback between vegetation, sediment, and hydrodynamics in Pacific Island catchments. The EGM assessment of the Dreketi mangrove wetland (Fiji), a rural context, and the Moata'a mangrove wetland, an urban context in Samoa, is presented. The amount of water and sediments generated in the catchments is estimated using a hydro-sedimentological model, for current and future conditions. Then, the response of the mangrove’s wetlands to sea-level rise, land use and climate change is analysed using an EGM model. The hydro-sedimentological model proved to be suitable to represent the sediment concentration in the contributor catchments for both wetlands, showing that even though the sediment export is considerable, there is a high deposition rate in the lower plains of the catchments. The EGM represented accurately the spatial distribution of suitable areas for mangrove habitats, given the current conditions for both wetlands. The results indicate that under SLR events, after 100 years, a significant amount of the suitable area could disappear. The model shows the profound effect that SLR and sediment accretion have on the wetland's future evolution, being more sensitive to reductions in sediment supply, which are linked to human activity and more likely in urban contexts. Nevertheless, for all the scenarios analysed, the loss of Moata'a and Dreketi mangrove wetlands could be between 25% and 30% in the best-case scenario. Understanding mangrove functionality is vital for determining appropriate design alternatives to enhance mangrove resilience. Engineering solutions not considering mangrove functionality may threaten mangrove survival by limiting sediment supply or restricting mangrove migration areas. Hence, ridge-to-reef management must be a priority for decision-makers in the Pacific Islands Countries for mangroves to remain resilient to climate change.