Evaluating the Effects of Tidal Turbines on Water-Mass Transport with the Lagrangian Barycentric Method
Résumé
Characterising circulation pathways in tidal stream energy sites is fundamental to evaluate the effects of turbines power extraction on long-term water-mass transport. The Lagrangian residual currents are commonly used to assess the displacement of water particles over the tidal period. The associated circulation is, however, characterised by a strong dispersion as water particles may follow different trajectories depending on the release time during a tidal cycle. In order to obtain a synthetic cartography of the Lagrangian circulation, Salomon et al. (1988) proposed an original method that allocates the residual currents at the barycentre of particle trajectories. This Lagrangian barycentric method was here applied to the Fromveur Strait (western Brittany) – a region with strong potential for turbine farm implementation along the coast of France. A high-resolution depth-averaged numerical model computed the tidal circulation driven by the principal lunar semi-diurnal constituent M2. The initial particle positions were taken at the 14,026 nodes of the unstructured computational grid surrounding the area of interest with a spatial resolution below 50 m. In the strait, the residual Lagrangian circulation was characterised by a strong asymmetry between (i) a prominent north-eastern pathway with residual currents up to 0.45 ms-1 and (ii) a southward circulation. Both upstream and downstream the strait, we exhibited prominent cyclonic and anti-cyclonic recirculations. A close correlation was found between the north eddy and a prominent sand bank. We simulated the forces induced by a series of horizontal-axis turbines as an additional bed friction sink term at the scale of the tidal farm. The extraction of tidal stream energy modified the amplitude and direction of the Lagrangian circulation along the current stream emerging from the strait with (i) a tendency for surrounding eddies to get closer to the tidal stream energy site and (ii) possible effects on the evolution
of surrounding seabed features.
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