Flume experiments were carried out to investigate the suitability of the off-bottom seaweed canopy in the channel between the two polders in the ‘Double-dike’ system for sediment retention. To the best of our knowledge, no studies have been conducted specifically to the role of seaweed canopies in sediment dynamics. For practical reasons, artificial seaweeds were used in most experiments after confirming the similarities between the real Saccharina latissima and artificial seaweeds. The bending angle of seaweed as a function of flow velocity, the variations of vertical flow velocity, turbulent strength and suspended sediment concentration distribution, and the variation of bed shear stress along the seaweed canopy are used to demonstrate if the off-bottom seaweed canopy is a suitable system for sediment retention.Results show that: (1) Seaweeds can flow upward easily at a flow velocity around 20 cm/s, and occupy only a small part of the water column during most flow conditions. This is a first indication that off-bottom seaweed canopy has little influence on sediment transport; (2) A velocity reduction area occurs around the seaweed canopy, while an increase in flow velocity occurs beneath the seaweed canopy. Stronger sediment transport is expected in the lower water column; (3) Total turbulent kinetic energy (TKE) variations along the seaweed canopy indicate that there is a strong mixing effect around the seaweed canopy that prevents sediment retention; (4) Bed shear stress slightly decreases in the front section of the seaweed canopy, while it increases more quickly farther along the seaweed canopy. It is likely that the bed shear stress exceeds the critical shear stress for erosion, leading to resuspension of bed sediments. We conclude that an off-bottom seaweed canopy seems ineffective as a sediment retention system. On the contrary, such a system stimulates a larger sediment flux behind the canopy.
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