The impact of seabed rock roughness on tidal stream power extraction

Abstract

Numerical assessments of environmental disturbances induced by a tidal farm project rely usually on local modifications of the friction coefficient over the area covered by a proposed array. Nevertheless, no study has investigated the sensitivity of predictions to surrounding seabed friction. The present investigation focuses on impacts of roughness parameterisation of rock outcrops, a typical seabed of tidal stream sites. A high-resolution depth-averaged circulation model is implemented in the Fromveur Strait off western Brittany, a region with strong potential for array development, integrating the heterogeneity of sediment bottom types. Rock roughness strongly influences initial predictions of tidal current and kinetic energy in the Strait with variations of available power up to 30 %. Tidal energy extraction induces noticeable reductions of tidal currents and bottom shear stresses up to 15 km from the array considered till surrounding sandbanks. Rock roughness impacts farm-induced modifications of tidal currents, bottom shear stresses and stream powers till northeastern and southward edges of the Strait with major absolute differences identified in its central part. Surrounding sandbanks are finally suggested to variations of shear stresses from 9 to 17 % over the Bank of the Four with possible implications on local sediment deposition.

Tidal Turbines’ Layout in a Stream with Asymmetry and Misalignment

“…This asymmetry is associated with a prominent flood-dominated region in the north of the strait with residual currents up to 0.8 m s −1 and an ebb-dominated area in the south. Such current asymmetry is typical of tidal strait dynamics as confirmed by numerical investigations conducted in the potential stream energy sites of Orkney [8] or the Fromveur Strait [13,20]. Moreover, the divergence area, with reduced current asymmetry in the center of the strait, presents, most of the time, the strongest tidal currents.…”

“…The northern part of the "Raz de Sein" is thus characterized by a separation of the flow between the northeastern and northwestern directions, in relation to channels formed on both parts of the islet of Tévennec (Section 2.1, Figures 1 and 4 Following the resource criteria adopted by the Carbon Trust [42], mean kinetic power density should exceed 2.5 kW m −2 for a reasonable economic viability in the setting up of first-generation devices. Along the coasts of France, such power density is found in the Alderney Race (western English Channel) and in the Fromveur Strait (western Brittany) [19,20,43]. In the "Raz de Sein", locations where this power density exceeds 2.5 kW m −2 are restricted to the vicinity of shoals and rocky islets and headlands bordering the strait (Figure 5a).…”

“…Furthermore, significant tidal recirculations with a diameter of around 2 km appear during flood and ebb peak periods, on both sides of the flow emerging from the strait. As local assessments of tidal stream power around France have primarily focused on the two prominent sites of the Alderney Race ("Raz Blanchard") in the western English Channel [18,19] and the Fromveur Strait [20,21] in northwestern Brittany, a reduced number of investigations was conducted in the "Raz de Sein". However, the values of the tidal velocities (>2.5 m s −1 ) and the water depths (>25 m) match the basic conditions required for the implementation of first-generation turbine technologies, covering the great part of existing prototype devices in pre-commercial demonstration stages [22].…”

“…The flow is considered to be turbulent over a rough bottom. Following Guillou and Thiébot [20,26] and Guillou et al [21], the associated roughness parameter z 0 is determined by matching sediment bottom types from the map established by Hamdi et al [16] with roughness observations compiled by Soulsby [27].…”

“…The roughness parameter over rock outcrops, characterized by a relative uncertainty in relation to the shape of seabed features in potential stream energy sites [20], is fixed to z 0 = 10 mm. The horizontal eddy viscosity is parametrized following Smagorinsky [28] while the vertical viscosity is computed with the mixing length model proposed by Quetin [29].…”