A coupled Nonlinear Shallow Water Equations — Exner approach for wave–sediment interplay in hybrid coastal defense

Coastal erosion, increasingly driven by human activities and climate change, poses escalating threats to shoreline stability and nearby communities. While coastal vegetation, especially mangroves, plays a crucial role in mitigating erosion, these ecosystems themselves are under threat from wave impacts and sediment loss. Grey infrastructure, such as breakwaters, is often proposed to enhance coastal resilience and protect vegetation. However, the interactions among wave dynamics, structural interventions, and sediment transport, which is crucial for vegetation survival, are frequently overlooked. This study develops a one-dimensional coupled hydrodynamic–morphodynamic model to simulate wave propagation and sediment evolution in coastal areas featuring hybrid coastal defense, which incorporates both green (mangrove) and grey (breakwater) protection strategies. The model combines the Nonlinear Shallow Water Equations (NSWE) for hydrodynamics with the Exner equation for morphodynamics, solved numerically using the Staggered Finite Volume Method. The NSWE is used due to its lower computational cost compared to other more complex models, yet it still deliver sufficient accuracy for simulating wave dynamics. Validation against diverse experimental datasets confirms the model’s accuracy in capturing wave and sediment dynamics. The validated model is then applied to investigate the effectiveness of multiple configurations of hybrid structure in reducing wave height and erosion. Additionally, sensitivity analyses explore the effects of several key parameters, offering insights to inform the development of more effective and adaptive coastal protection strategies. Further analysis is also conducted to observe how the erosion-related parameters change at field scale under storm conditions.