A comparison between fluid shear stress reduction by halophytic plants in Venice Lagoon, Italy and Rustico Bay, Canada analyses of in situ measurements

A series of in situ experiments on bed stability were carried out at three sites across Venice Lagoon using the benthic annular flume - Sea Carousel. Turbulence measurements were made at a range of flow speeds over different vegetated beds as well as "smooth" muddy beds. The drag induced by the various bed types was estimated using flow deceleration. Bed shear stress was also estimated using three methods, and the results were compared with the bed shear stress as determined over a smooth bed in a laboratory equivalent of Sea Carousel-Lab Carousel. The stress was found to increase with increasing bed roughness and with the addition of vegetation in the form of the sea grasses Cymldocea nodosa and Zostera noltii. the stress was also found to be affected by a the bending of the sea grass blades under flow veclocites exceeding 0.4 m s-1, the sea grasses became flattened and the shear stress was found to decrease to produce skimming flow. It was concluded that thte presence of sea grasses decreases erosion due to (1) stress reduction and (2) stabilization of the bed erosiona and hence habitat destruction. Stress was also reduced by an increase in levels of turbidity level in the water column.