Mangrove forests are important (sub)tropical intertidal ecosystems providing coastal protection and other ecosystem services like carbon storage and biodiversity. In view of the loss of these forests many rehabilitation projects have started, but unfortunately these efforts often fail. A main cause for coastal erosion is a disrupted (fine) sediment balance due to human interventions. This imbalance prevents establishment of propagules (floating mangrove seeds) at intertidal areas and the early growth of planted mangroves. Coastal solutions such as permeable dams have aimed to stop the erosion, but we need additional systematic methodologies to understand the coastal system.

The objective of this thesis is to extend the physical and ecological understanding of coastal mangrove systems, especially of currents, sediment pathways and propagule pathways, to improve rehabilitation strategies using Demak (Central Java, Indonesia) as study area. Demak's coastline suffers from erosion problems, originating from extensive land use by fish ponds and land subsidence. An idealized morphostatic 3D model was set up in Delft3D-4 to simulate the hydrodynamics of the area, influenced by the interaction between flows due to river discharge, tides and wind. The simulations are used as input for the SedTRAILS particle tracking model to compute sediment and propagule pathways.

The model results support that sediment supplied by alongshore currents and riverine sediment could serve as source for the coastline. The river plume is at the same time obstructing as well as trapping sediment and propagules, because stratification prevents mixing of the sea and river water. However, sediments suspended near the bed could be transported underneath the river plume towards the coast by estuarine circulation. Another lesson learnt is that sediment (suspended in the water column) and propagules (floating around the water surface) do not follow the same pathways, as for propagule dispersal the wind direction is of much larger influence than tidal currents. The simulations confirmed that seasonal variability in wind direction could significantly influence the establishment area and availability of propagules, and old fish pond bunds can increase the establishment area for propagules. Sensitivity analysis show that the timing of propagule release within one tidal cycle (flood-ebb) influences the probability to successfully establish on the nearby coast. A propagule survival module is developed (based on inundation-free period and bed shear stress) as stormy seasons may increase the sediment establishment, but the larger bed shear stresses may lower the survival of propagules.

Although this modelling study is focusing on Demak, there are many other similar mangrove coasts around the world where an idealized model would enable to understand where the sediment and propagules are going and the travel time from source to sink. Permeable bamboo dams would be recommended to add to the model in further research, because they could increase the survival of propagules by reducing bed shear stresses and increase the establishment of propagules by trapping them. In conclusion, this research has made a start in counteracting coastal erosion and improving the success of mangrove forest rehabilitation by studying sediment and propagule pathways, and developing a propagule survival module.

Shallow bays in the Caribbean, like Baie Orientale and Baie de L'Embouchure in Saint Martin, are often sheltered by coral reefs and covered by seagrass meadows. They provide valuable services as tourism and coastal protection. The ecosystems are linked through biological, chemical and physical processes. But they are under pressure due to sea-level rise. The response of one of the ecosystems to climate change could impact the other ecosystem.

In order to predict the impact of sea-level rise on the biogeomorphology in Baie Orientale and Baie de L'Embouchure, the hydrodynamic model Delft3D Flexible Mesh is applied. The effect of seagrass meadows and coral reefs on both flow and waves are captured with this model. In this way, the long term change in average hydrodynamic conditions due to sea-level rise is determined depending on the response of the ecosystems.

A wave-driven circulation is found in both bays with flows of 0.5 m/s over the reefs and currents of 0.2 m/s inside the bays. The hydrodynamic conditions are mainly determined by the reef height. Depending on the response of coral reefs to climate change and the amount of sea-level rise, the wave height inside the bays and the wave-induced currents increase. Under the worst-case scenario, where coral reefs degrade and seagrass meadows die, flow velocities increase by more than 100% in Baie de L'Embouchure and by 200% in Baie Orientale under a sea-level rise of 0.87 m. The significant wave height rises to 300% in Baie Orientale and doubles in Baie de L'Embouchure. But this increase of hydrodynamic stresses is not expected to lead to devastating damage to coral reefs and seagrass meadows. Instead, the response of coral reefs will be determined by changing water temperatures and ocean acidification. A shift in seagrass occurrence due to the changed hydrodynamics is expected.

The long term impact of sea-level rise on the biogeomorphology of Baie de L'Embouchure and Baie Orientale seems to be limited. The ability to mitigate the impact of sea-level rise is shown and the resilience of the ecosystems proved, which is very promising for other shallow Caribbean bays that are threatened by sea-level rise.