The impact of tidal stream turbines on large-scale sediment dynamics
Introduction
Tidal currents have been identified as a renewable resource that has a strong future potential for economic energy harvesting. Leading developers of Tidal Energy Converter (TEC) devices are at the stage of full-scale testing of pre-commercialisation examples of their respective technologies. The UK is home to many of the most advanced technologies, and has the largest proliferation of devices under development [1]. Key to the UK achieving this position is the extent of the tidal current resource in UK territorial waters. As for any renewable technology, the local availability of the resource is what drives private and public funding interest. Although resource analysis and quantification remain the subject of significant research interest (e.g. [2], [3], [4], [5]), around half of the identified ‘technically extractable’ European tidal current resource is located in UK territorial waters [6]. Harvesting of the tidal current energy resource has therefore been highlighted as a key contributor to the future UK energy mix. This is reflected in strong government support of R&D in tidal current energy in the wider context of marine renewable energy (e.g. [7], [8], [9]).
TEC devices operate by intercepting the kinetic energy in strong tidal currents (typically through a turbine unit). This intercepted energy is then converted to electrical energy through a power take-off system (e.g. an induction generator) and conditioned for dispatch to the electricity network. Theoretically, this is very similar to the operation of a typical wind energy device. What is significantly different from the wind energy analogy is the environment that the TEC device operates in [10], and the potential interaction between TEC devices and their environment [3]. Given the proliferation of at-sea demonstration devices, the environmental impacts of operation of TEC devices are a timely issue to consider. Developers such as Marine Current Turbines Ltd. (MCT) and Verdant Power have had to conduct multi-million dollar environmental monitoring programs in order to obtain environmental permission for installation of development devices. This is potentially slowing down the development of TEC technology. The major research questions in the tidal energy context have already been identified [11]. Progress to-date has, however, been limited in addressing these research issues. The impact of TEC operation on sediment dynamics has yet to be explored in the scientific literature and has motivated the present study.
The tidal stream resource is generally estimated using histograms of depth-averaged tidal current speed over time periods that include the spring/neap cycle [12]. In terms of environmental impact, no consideration has yet been given to the change in large-scale sediment dynamics and morphodynamics resulting from tidal stream energy extraction. This may have varying impacts depending upon tidal asymmetry at the point of extraction. In regions of strong tidal asymmetry, a residual transport of sediment occurs in either the flood or ebb direction. A modest amount of energy extracted from such a location could potentially have a large effect on the residual sediment transport compared with energy extracted from a region of tidal symmetry. In the latter case, since sediment transport is equal in magnitude during the flood and ebb phases of the tide, energy extracted at such a location will not significantly affect the residual sediment transport and hence the large-scale morphodynamics of the region. Tidal asymmetry is discussed in more detail in Section 2.
The importance of considering the phase relationship between the M2 and M4 constituents is demonstrated here, initially with theoretical application to energy extracted from idealised domains (Section 2). A one-dimensional (1D) morphological model is then developed (Section 3) consisting of hydrodynamic, sediment transport and bed level change components. This model is applied to the Bristol Channel (Section 4), a tidal system that has a large tidal stream resource, and exhibits a large range of phase relationships between the M2 and M4 constituents and, hence, a large range of tidal asymmetries. The model simulations also include a range of sensitivity tests in order to demonstrate the robustness of the theory.
Section snippets
Tidal asymmetry
Pingree and Griffiths [13] noted that the mean bed shear stress distributions associated with the M2 and M4 tides in the Irish Sea tend to be directed into bays and to diverge from M2 amphidromic points, reaching maximum values at M4 amphidromic points. They further suggested that the interaction between the M2 and M4 tides determines the direction of net sand transport around the British Isles. Regions of modelled bed-stress divergence compared favourably with sediment bed load partings, based
Morphological model
To test the hypothesis outlined in Section 2, a 1D morphological model was developed and applied to a case study (the Bristol Channel) where large spatial variations in tidal asymmetry occur along the length of the channel (Section 4). The morphological model consisted of three components: hydrodynamic, sediment transport and bed level change models.
Bristol Channel
The Bristol Channel (Fig. 6) separates South Wales from South West England and extends from the lower estuary of the River Severn to the Celtic Sea. For the purposes of this paper, however, the term “Bristol Channel” is taken to include the Severn Estuary. Depths in the outer region of the Bristol Channel are around 50 m, and an extensive system of narrow channels and tidal flats in the upper part of the estuary reduces the depth to less than 10 m upstream of Newport. With a mean spring range of
Discussion
Results of a model parameterised on the Bristol Channel demonstrate that tidal stream energy extraction can have a significant impact on the large-scale morphodynamics of a large-scale tidal system, the magnitude of which depends upon the tidal asymmetry at the point of extraction. This supports the hypothesis presented in Section 2: energy extracted from regions of strong tidal asymmetry will have a much greater influence on large-scale sediment dynamics than energy extracted from regions of
Conclusions
A one-dimensional numerical model has demonstrated that a small amount of energy extracted from a tidal system can lead to a significant impact on the sediment dynamics, depending on tidal asymmetry at the point of extraction. The resulting influence on the morphodynamics is not confined to the immediate vicinity of the tidal stream farm, as would occur in the case of localised scour, but affects the erosion/deposition pattern over a considerable distance from the point of energy extraction (of
Acknowledgements
Tidal data for the Bristol Channel was provided by the British Oceanographic Data Centre and the UK Tide Gauge Network. We are grateful to an anonymous reviewer for constructive comments on the manuscript. This research formed part of Emmer Litt's MSc studies at Bangor University, funded by the UK Natural Environment Research Council (NERC). The project was also supported by the Higher Education Funding Council for Wales (HEFCW) and the Welsh Assembly Government.
References (45)
- et al.
A reassessment of sand transport paths in the Bristol Channel and their regional significance
Mar Geol
(1990) - et al.
Non-linear tidal distortion in shallow well-mixed estuaries: a synthesis
Estuar Coast Shelf Sci
(1988) Hydrodynamic model predictions of tidal asymmetry and observed sediment transport paths in Morecambe Bay
Estuar Coast Shelf Sci
(1997)Sand waves in the North Sea off the coast of Holland
Mar Geol
(1971)- et al.
Morphodynamic behaviour of a nearshore sandbank system: the Great Yarmouth Sandbanks
Mar Geol
(2008) - et al.
ME1-marine energy extraction: tidal resource analysis
Renew Energy
(2006) - et al.
Intercomparison of coastal area morphodynamic models
Coast Eng
(1997) - et al.
An enhanced depth-averaged tidal model for morphological studies in the presence of rotary currents
Cont Shelf Res
(2007) - et al.
Tidal and surge modelling using differential quadrature: a case study in the Bristol Channel
Coast Eng
(2008) Hydrodynamics of the Bristol Channel
Mar Pollut Bull
(1984)