Field Observations of Wave-Induced Streaming through a Submerged Seagrass (Posidonia Oceanica) Meadow

Abstract

This paper reports the findings of a 2 week field campaign designed to study wave-induced flows within a meadow of Posidonia oceanica at water depth 9 m. Previous laboratory experiments suggest that waves induce a mean mass drift in the direction of wave propagation ("streaming") through submerged canopies of vegetation. This paper provides the first field measurements of this wave-induced streaming. During periods of high wave activity, streaming flows with magnitudes as high as 20% of the near-bed oscillatory velocity were measured within the meadow. In addition to presenting field measurements of wave-induced streaming, this paper also considers the damping of wave-induced oscillatory flow within the seagrass meadow. Oscillatory velocities measured within the meadow were reduced by less than 30% relative to those above the meadow over the duration of the study. This is in agreement with previous laboratory and field measurements which show that oscillatory flows are damped less within submerged canopies compared to unidirectional flows. Existing analytical models underpredict the magnitude of the streaming flow and overpredict oscillatory velocity reductions. These discrepancies are thought to arise because the drag generated by flexible seagrasses moving with wave-induced flow is not well described. Key Points We present field measurements of wave-induced flow through a seagrass meadow A mean current in the direction of wave propagation is generated within meadow Wave-induced oscillatory velocities are reduced by less than 30% within meadow. Keywords: seagrass; vegetated flow; canopy flow; streaming; wave-induced current; oscillatory flow reduction