Assessing the Impact of Breakwater Spatial Design on Hydrodynamics for Mangrove Restoration

Abstract

Mangrove ecosystems play an important role in (sub)tropical coastal zones by providing services such as carbon storage, biodiversity support, and natural protection against erosion and flooding. Their dense root systems trap sediment and reduce wave energy, contributing to shoreline stability. Despite these benefits, global mangrove areas have declined rapidly in recent decades, mainly due to deforestation and coastal retreat. In response, various restoration measures have been implemented, including the construction of permeable structures along eroding coastlines. These structures aim to reduce hydrodynamic energy and promote sediment deposition, creating conditions suitable for mangrove re-establishment. However, they do not always succeed in providing the required sediment accumulation.

This study focuses on a breakwater in the study area, located along the coast of Bạc Liêu, Vietnam. The area has a concave bed profile with limited wave energy dissipation and short inundation-free periods, which, together with net erosion, hinder both mature mangrove stability and seedling establishment. Hydrodynamic forces such as longshore currents, tidal flows, and waves generate bed shear stresses that resuspend sediment and limit sediment deposition near the shore. The existing permeable breakwater fails to provide the sheltered conditions needed for mangrove survival and recovery. As part of the Mangrove Living Lab project, this study investigates how the spatial design of the existing Pile-Rock Breakwater (PRBW) influences hydrodynamic processes relevant to sediment transport and deposition, focusing on minimising the maximum bed shear stress near the mangrove fringe. The considered spatial design parameters of the permeable breakwater are the gap width and the distance to shore. Field measurements and numerical modelling using Delft3D are combined to assess current conditions, evaluate the effectiveness of the existing design, and explore potential improvements.

Results show that narrower gaps reduce wave energy in the sheltered area but concentrate flow through the gaps, locally increasing velocities. Placing the breakwater further offshore allows more space for dissipation and reduces bed shear stress at more exposed areas behind the gaps, but also increases the incoming energy near the mangroves in more sheltered zones. The recommended spatial design requires a balance of these effects, with the breakwater placed approximately 70 metres further offshore and featuring narrower gaps to enhance shelter and reduce resuspension. Recommendations for future work include more detailed modelling including diffraction, long waves, and morphodynamics, as well as gathering more data from the area to improve understanding. Further research should also investigate simultaneous adjustments of the spatial design parameters and explore alternative breakwater types.

Overall, this research shows the complexity and importance of a site-specific breakwater design. Optimising the spatial layout offers potential to improve the breakwater’s effectiveness, but further research is needed to improve the design and develop a more thorough understanding of the local conditions and ongoing coastal processes. These improvements are essential to support sedimentation and establish stable conditions for mangrove survival and long-term restoration along the coast of Bạc Liêu.