Experimental study of wave-driven sediment suspension in flexible vegetation canopies with rippled beds

Laboratory experiments have been used to investigate the impact of flexible vegetation on sediment suspension over rippled sandy beds. Flexible vegetation mimics were examined across a range of wave conditions and stem densities, with rigid vegetation cases included for comparison. Measurements of velocity and suspended sediment concentration were used to compare near-bed hydrodynamics, bedform response, and sediment resuspension within the vegetation canopies. The results show that stem flexibility does not modify the dominant mechanisms governing near-bed sediment suspension when ripples are present. Flexible vegetation did not significantly modify ripple crest height or wavelength relative to bare sediment beds (p > 0.05), whereas rigid vegetation disrupted ripple geometry in the majority of cases tested (p < 0.05). Flexible canopies produced modest, statistically non-significant reductions in near-bed velocity of 0–5% relative to bare-bed conditions, confined to the lower 25% of the stem height, compared to reductions of 14–23% under rigid canopies (p < 0.001), which was observed over the entire stem length. Near-bed TKE within flexible canopies was statistically indistinguishable from bare-bed values across all conditions tested (p > 0.74), consistent with suppression of stem-generated vortex shedding due to stem reconfiguration and the preservation of ripple-induced turbulence as the dominant near-bed TKE source. Near-bed suspended sediment concentration collapsed onto a single power-law relationship with near-bed velocity across all vegetation types, densities, and wave conditions (R² = 0.97), with good agreement obtained using bare-bed ripple formulations applied with measured near-bed velocities. The results indicate that, for wave propagation over vegetated rippled beds, sediment resuspension is governed primarily by bedform-scale processes rather than by vegetation-induced TKE. The findings contrast with flat-bed and dense-canopy studies and highlight the importance of explicitly accounting for bedforms when modelling sediment transport in vegetated coastal environments.