Wave Attenuation and Containment Dike Loading at a Marsh-Creation Site in Coastal Louisiana: SWAN-Based Assessment of Bottom-Roughness

Coastal Louisiana continues to experience rapid wetland loss, reducing the capacity of shallow nearshore areas to dissipate storm-driven waves before they reach engineered containment dikes. This study quantifies offshore-to-nearshore wave transformation and evaluates the resulting wave runup and hydrodynamic loading on a representative containment dike constructed for marsh creation in Plaquemines Parish, Louisiana. A 25-year return-period offshore wave condition was derived from long-term Wave Information Study hindcast data and propagated using the third-generation spectral wave model SWAN. Simulations were performed for two idealized foreshore conditions: a bare-bed scenario and a vegetated shallow-water scenario represented through enhanced bottom friction. Results indicate strong wave attenuation across the Louisiana inner shelf, with inclusion of vegetation-induced dissipation reducing nearshore wave heights and leading to 14–25% reductions in wave runup and 27–44% reductions in total quasi-hydrostatic loading on the containment dike across multiple still-water levels. While absolute response magnitudes depend on modeling assumptions, the relative differences between bare and vegetated foreshore scenarios are robust. The findings highlight the effectiveness of vegetation-induced dissipation in mitigating wave impacts on coastal infrastructure and support the use of nature-based features in marsh creation and coastal protection projects.