Tidal stream turbines and sediment dynamics: the effects of array installation upon the morphology of a headland associated linear sandbank

Abstract

Tidal stream generators extract kinetic energy from tidal flows and convert it to electricity. Such hydrodynamic impacts could have implications for sediment movement and the larger-scale morphology of a site. This study investigates the likely relative impacts of multiple large-scale extraction-scenarios upon a real-world submarine sandbank, the Alderney South Banks.

The first phase of the study is an analysis of the South Banks, rooted in the literature of sandbank morphology, and performed using high-resolution bathymetry and sedimentary data. Cross-sections of large migrating sandwaves superimposed upon the sandbank were interpreted to generate a map of the sediment transport pathways. Unusually large sandwave migration rates of up to 70m across a 50 day period were observed in one part of the Bank.

The second, larger phase of the study involved the development of a 2-D numerical model to simulate the tidal flows in the vicinity of Alderney and across the wider English Channel. The model was elevation-forced using nine tidal constituents at its open boundaries and validated against tidal level and tidal-stream velocity data obtained from port tide-gauges and ADCP deployments. An existing, area-averaged roughness method for parameterising the effects of energy extraction for tidal arrays was incorporated into the model. A numerical sediment model was setup to be coupled and run in parallel with the hydrodynamic computation.

An identical 300MW tidal array parameterisation was applied to the model in 17 different locations to simulate 17 different energy extraction scenarios. The model was run for one-spring neap cycling, lasting 14.8 days. Outputs from each model run were combined with outputs from a non-extraction baseline model to assess the hydrodynamic impacts of the modelled extraction scenario in the sandbank vicinity. A set of sedimentary model-runs were also performed for a longer 90-day period with the sediment module activated. A hard-bottom method was used to limit potential erosion depths within the sediment model and to ensure realistic sediment supply.

Three difference metrics were used to systematically compare the impact of each extraction scenario upon the South Banks. These were the residual current, the residual sediment transport and the bed-level evolution. The difference metrics were used to rank the 17 extraction scenarios in terms of likely impact upon the South Banks from highest to lowest.

The results show that the impact of energy extraction decreases with the distance of array placement from the North-Eastern tip of the South Banks. A propagating eddy shed from the headland is closely related to velocities in this region with energy-extraction decreasing the rotational velocity and life-time of the eddy. The processed modelling outputs are interpreted within the context of the initial morphological analysis to help predict how the sediment transport pathways will be altered. Results suggest that the most favourable locations for maximising power output whilst minimising impacts over the South Banks are two particular regions, T63 or T49.

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Identifiers

ORCID for Abubakr Bahaj: orcid.org/0000-0002-0043-6045

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