Evaluating the Effects of Tidal Turbines on Water-Mass Transport with the Lagrangian Barycentric Method

Characterising circulation pathways in tidal stream energy sites is fundamental to evaluate the effects of turbines power extraction on the transport of water-mass and associated particles. The Lagrangian residual currents are commonly considered 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 Residual Circulation (LRC), Salomon et al. (1988) proposed an original method allocating 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 LRC 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 of the strait, we exhibited furthermore prominent cyclonic and anti-cyclonic recirculations. A close correlation was found between the north eddy and a prominent sand bank. We simulated finally 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 magnitude and direction of the LRC 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) potential effects on nearby sandbanks.