Vegetation effects on dune erosion under wave collision: Influence of planting density, biomass distribution and arrangement in scaled experiments

Coastal dunes serve as vital natural defenses against storms, with vegetation playing a key role in sediment stabilization and erosion mitigation. This study examines the effects of planting density, planting strategy, and biomass distribution on dune erosion resistance, using Ammophila arenaria in 1:7 scale flume experiments exposed to wave collision regimes. Tests with whole plants (uncut) and belowground-only biomass (cut) at varying planting densities resulted in erosion volume reductions of up to 31.2 % compared to bare dunes. Intermediate densities with well-developed root systems and buried shoots showed the most consistent reductions, while variability indicated the importance of root development and plant health. Belowground biomass alone provided nearly equivalent resistance compared to whole plants. Vegetation also influenced failure mechanisms, promoting notching and slumping with block detachment and deposition at the dune toe. Time-resolved 3D surface data from laser scanning revealed dynamic erosion patterns, while Structure-from-Motion photogrammetry supported detailed end-state analyses.