Degrading mangrove forests and coinciding erosive coastlines is an urgent global concern. Mangroves are essential for their interaction with hydrodynamic and sediment processes at mangrove-mud coasts. Earlier on, solutions were sought in building ’hard’ structures, such as concrete sea walls, to lower incoming wave energy and enhance sedimentation to restabilize erosive coastlines and bring back mangroves. As these approaches proved to be counterproductive and even accelerate destabilization of the coastline, nature-based solutions might provide a new path to a durable solution. A pilot project was initiated in Demak, Indonesia, where several permeable brushwood and bamboo dams were constructed near the receding coastline. The open structure of the dams allows sediment to flow through, while they decrease wave heights by dissipation. This creates calmer conditions behind the dams and allows mud to settle. The deposition of fine sediments enhances a rise in nearshore bedlevel and provides nursery ground for mangroves to reestablish, realizing a first step towards rehabilitation of the mangrove ecosystem and restabilizing the coastline. A conceptual 2DH process-based model, using Delft3D FM, was created to analyse the effect of permeable structures under changing boundary conditions at various shorelines. Three boundary conditions were varied to see the influence on dam performance: wave height, wave direction and suspended sediment concentration. A decrease or increase in wave height resulted in similar rise in bedlevel elevation behind the dams. Without the dams, an increase in wave height increased the tidal flat elevation and steepened the foreshore, inducing a concave profile shape. Placing the dams on the tidal flat induces extra rise in elevation, but won’t mitigate the erosion of the foreshore. Smaller wave heights tend to reshape the profile towards a more convex profile. When dams are implemented on, an increase in bedlevel behind the dams occurs. The waves stir up sediment at the tidal flat boundary which are redistributed by the tidal currents forming a more convex profile. The dams could be placed best close to the shoreline to allow for the natural redistribution of the sediment when at the same time induce a rise in bedlevel for mangroves to reestablish. Altering the nearshore sediment supply affects the dam performance significantly. The dam performance increases for lower sediment supplies. Dams reduce erosion due to decreased sediment input, but it is likely a short-term solution. For higher sediment supplies, the coastline can more effectively restore by itself and the dams reduce in efficacy. The model results show an increase in dam performance for increasing wave angles (shore-normal to shore-parallel). The bed shear stress is decreased more effectively for smaller wave angles, but also reduce the sediment concentration behind the dams. With the many distinctive boundary conditions at mangrove-mud coasts in mind, the simplified model setup is made easily adaptable to enable future use of the model. Based on the model setup for Demak and the model results in this research, an improvement on the computational grid is suggested before applying the model to other coastal sites.

Coastal bays in the Caribbean accommodate different marine ecosystems, including seagrass meadows and coral reefs, which provide important ecosystem services. However, these marine ecosystems are endangered. Seagrass ecosystems have a key role in coastal bays, but despite their alarming rates of loss, they receive little attention compared to other marine ecosystems. This study aims to provide a better understanding of the role of seagrass ecosystems in coastal bays by assessing their impact on the morphodynamic behavior. In order to achieve this, an existing hydrodynamic model has been extended to include morphodynamics. Two different bays have been investigated: Baie Orientale and Baie de l’Embouchure (St. Martin). These bays represent a partially exposed and fully sheltered bay, respectively. Both regular wave conditions, as well as extreme storm conditions, have been investigated to understand the consequences of seagrass loss on coastal erosion for different environmental climates. The coastal erosion has been found to be more prominent in the exposed region, whereas the more sheltered areas are more resilient. The erosion takes place in shallow waters but remains limited under regular swell conditions. The increased wave energy, during the extreme storm event, increases the erosion rates considerably. However, the regular swell conditions are normative in shaping the morphological development of these coastal bays when longer timescales are considered. The seagrass counters erosion most effectively in the foreshore and much less in deeper regions. Removal of seagrass in the foreshore (between 1-3m) halves the sediment stabilization. In contrast, the sediment stabilization increases if the meadows are able to spread especially towards the shore. In addition, the wave energy and waveform have a significant impact on the sediment stabilization. The stabilization typically decreases for higher and shorter (storm) waves, whereas it increases for smaller and longer (swell) waves. The long-distance interactions between seagrass meadows and coral reefs have led to their mutual coexistence, which is also found to be beneficial for the erosion control of the coastal bays. The dissipation of wave energy on top of the reefs fosters the sediment stabilization by seagrass. Moreover, the impact of the reefs on sediment stabilization also increases in the presence of seagrass meadows due to the additional drag exerted by the seagrass. The seagrass meadows are typically more effective under swell waves, whereas the reefs are more dominant in the dissipation of storm waves. Seagrass meadows and coral reefs form thus a synergy, in which the stabilization of sediments provided by the individual ecosystems is facilitated, reinforced, and complemented by the proximate presence of the other ecosystem. Finally, the role of seagrass on sediment stabilization has been explored under climate change. The coastal erosion increases for the considered sea-level rise scenarios. The impact of the seagrass on sediment stabilization decreases when the bays, seagrass and reef ecosystems are not able to keep up with sea-level rise. The synergy between seagrass and reefs emphasizes that the responses of both ecosystems are of importance for the future of tropical sheltered bays and should therefore not be managed in isolation.