Sustainable Marine Structures

Flume experiments were conducted to investigate how upward vegetation growth and streamwise patch expansion separately and jointly affect flow around submerged vegetation under depth‑limited conditions. Their individual effects were isolated by increasing canopy height or streamwise patch length while keeping the other parameter constant. The measurements were performed in two different flumes under closely comparable Reynolds‑number conditions. Acoustic Doppler Velocimetry (ADV) characterized velocity and turbulence on the stoss and lee sides of the patch. Non‑intrusive Particle Image Velocimetry (PIV) resolved the mean flow, canopy‑top shear layer, and vorticity above the vegetation, where shallow flow depth made ADV measurements impractical because of possible probe disturbance. Upward vegetation growth produced a stronger velocity deficit and extended the low‑velocity region over a larger part of the water column. It also strengthened the canopy‑top shear layer and increased Reynolds shear stress, turbulent kinetic energy, and vorticity near the canopy interface and within the downstream wake. In contrast, increasing patch length allowed vegetation resistance, vertical momentum exchange, and turbulent mixing to act over a longer distance before the flow reached the patch exit. Consequently, lee‑side Reynolds shear stress and turbulent kinetic energy variations became smoother and less localized. In summary, vegetation height mainly controlled the magnitude and vertical extent of the hydrodynamic response, whereas patch length controlled its streamwise development and the abruptness of adjustment at the patch exit. These complementary effects should be considered when evaluating restored seagrass meadows and nature‑based coastal protection systems.