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

Abstract

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 northeastern 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.

The Tidal Stream Energy Resource of the Fromveur Strait—A Review

“…The resulting cartography revealed a complex Lagrangian Residual Circulation (LRC) highly controlled by bathymetry gradients and islands geometry (Figure 9). Partly associated with the asymmetry of tidal currents in the Fromveur Strait, the LRC exhibited especially a strong asymmetry in the central strait between (i) a prominent north-eastern pathway with residual currents up to 0.45 m s −1 and (ii) a southward circulation with residual currents restricted to 0.20 m s −1 [28]. Prominent cyclonic and anticyclonic recirculations, with diameters of around 8 km, were furthermore identified both upstream and downstream the strait.…”

“…The attention was dedicated to changes in tidal current magnitudes and associated bottom shear stresses but also to the ecological robustness of this marine area. High-spatial resolution simulations were thus exploited to characterize the tide-induced Lagrangian Residual Circulation (LRC) and evaluate changes induced by operating turbines [24,28]. Additional simulations focused on the renewal capacity of Ushant-Molène archipelago [12].…”

“…In the Fromveur Strait, these aspects were investigated by assessing modifications of the tide-induced circulation pathways for the principal semidiurnal harmonic component M 2 [24,28]. Circulation pathways were assessed in a single synoptic view by applying the original Lagrangian barycentric method proposed by Salomon et al [52].…”

“…Interpretation of major features of residual Lagrangian currents patterns without tidal stream energy extraction in the Fromveur Strait. The major modifications induced by a full energy extraction scenario are shown with pink arrows (adapted from[24,28]). …”

Turbines’ effects on water renewal within a marine tidal stream energy site

“…More recently, Guillou and Chapalain [13,14] evaluated the effects of a series of turbines arrays within the Fromveur Strait (France, western Brittany, Fig. 1) suggesting changes in (i) the magnitude and direction of the residual Lagrangian circulation emerging from the strait and (ii) the locations of surrounding eddies (Fig.…”