The influence of vegetation on turbulence and flow velocities in European salt-marshes

Flow hindrance by salt-marsh vegetation is manifested in the structure of the tidal current; it has a significant impact on sediment transport and it has been related to increased sediment accretion. The flow characteristics in three different vegetation types (Spartina maritima, Sp. anglica and Salicornia sp./Suaeda maritima) were measured on three salt-marshes in Portugal and England. These in situ measurements differ from laboratory flume experiments with 'clean' vegetation by the complexity of natural canopies. Skimming flow develops above the Spartina canopy when the vegetation is fully submerged. In this situation, a low turbulence zone with nearly constant velocity in the denser canopy is separated from the skimming flow above by an interface characterized by high Reynolds stresses. In the low turbulence zone, a positive relationship exists between turbulence intensity and shoot density, which is due to wake turbulence generated locally in the canopy. The rate of particle settling should be increased in that zone. The lower limit of skimming flow is best predicted by the height within the canopy that includes 85% of the biomass. For emergent Spartina canopies and the short Salicornia/Suaeda marsh, the maximal velocity-gradient is shifted upwards compared to a standard boundary layer over bare sediment and the turbulence is attenuated near the bed, but to a lesser extent than for fully submerged Spartina canopies. A turbulence reduction near the bed was observed in all measured profiles; that should enhance sediment deposition and protects the bed against subsequent erosion.