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Coherent Flow Structures in Gaps Within Meadows of Posidonia Oceanica

Author(s): A. M. Folkard; A. Maltese; G. Lombardo; G. Ciraolo; G. La Loggia

Linked Author(s): Andrew Folkard

Keywords: Seagrass; Turbulence; Coherent Flow Structures; Heterogeneity; Coastal DefenceBed Roughness; Soft Engineering

Abstract: Meadows of the seagrass Posidonia oceanica are found in numerous coastal locations with the Mediterranean Sea. These dense macrophyte stands support highly biodiverse ecological communities that comprise epiphytic micro-flora and-fauna as well as larger organisms which use the meadows as nursery and sanctuary habitats. Often, gaps appear in these meadows as a result of anthropogenic disturbance, storm events or bioturbation. The recovery of these gaps, and their influence on ecological and sedimentary processes are important factors in understanding of the nature of these meadows. The work reported here elucidates the dynamics of the flow within the meadow-with-gap configuration described. This is achieved via laboratory experiments which meadows of Posidonia oceanica are simulated by collections of strips of plastic matched with samples of the seagrass for flexibility, shape, size, density and surface roughness. These are placed in a flume and a flow is passed over them. Three-dimensional velocity measurements are taken at numerous locations within the flow using an Acoustic Doppler Velocimeter. From these, coherent structures in both the mean and turbulent flow components are derived, using analytical techniques taken largely from fluvial geomorphological studies of flow structures over alluvial bedforms. Both the flow discharge rate and the gap extent are varied between experimental runs. The results show that where the gap is wide enough, a large, persistent eddy with cross-flow vorticity develops, above which a turbulent shear zone extends to the water surface. Where the gap is too small for this eddy to form, the flow appears to be dominated by weak upwelling, which orders itself into coherent cells if the gap size is large enough to allow this. Implications for sediment and nutrient dispersion within the meadow and gap are discussed.


Year: 2003

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