Numerical modelling of submerged vegetation as a Nature-based Solution for coastal protection.

Nature-based Solutions are increasingly considered for protecting eroding coasts, yet the conversion of wave attenuation by submerged vegetation into a measurable reduction of nearshore erosion remains only partly quantified. This work presents a phase-resolving Boussinesq-type hydro-morphodynamic model, with a canopy-flow vegetation submodule, that simulates wave attenuation, sediment transport and cross-shore morphology evolution. The model is validated against two independent laboratory experiments spanning two vegetation types, two scales and two morphological regimes: a surrogate Posidonia oceanica meadow on a mild slope where breaker bars form, and an artificial Laminaria hyperborea kelp field fronting a steep, retreating dune. In both cases the model reproduces the canopy-induced wave attenuation and morphological response using common sediment-transport coefficients and published, vegetation-specific drag relations, none calibrated against the present validation profiles; bed-profile Brier Skill Scores are 0.30–0.97, excellent in seven of eight. The meadow reduces and displaces the breaker bar, with shoreline retreat only slightly reduced, while for the kelp, dune protection is governed by the still-water level through its combined effects on canopy submergence, wave breaking and dune exposure. Wave attenuation is shown not to translate one-to-one into protection of the profile, a dependence best quantified with the process-based model presented in this work.