3. Tidal Stream

3.6. Environmental Interactions

The greatest advantage of tidal (marine) current devices is that during their operational life they emit no greenhouse gas emissions and appear unlikely to produce pollutants. Tidal stream devices can be deployed in a range of sites and at a range of scales, from a single device to a farm of 30 or more devices or a number of such farms within an area. It is likely that the future will see a number of such farms deployed within a relatively small area in order to maximise the use of the available tidal resource. This means that the level of environmental effects resulting from tidal stream energy is likely to increase gradually over time, and the cumulative effects of a number of farms operating in one area may become significant. In addition, the types of environment affected will change over time, as technological improvements and economic requirements result in the movement of tidal stream farms from shallower water to deeper water and back again.

Currently, although much can be predicted, little is actually known about the environmental effects of tidal stream devices (no Environmental Impact Assessment of tidal stream devices has been made available). It is therefore difficult to predict the magnitude of the cumulative environmental effects resulting from the deployment of tidal stream devices and farms throughout the available tidal stream resource areas. The level of impact would be determined by, among other things, the quantity of units installed and the packing density.

Some specific issues related to the tidal stream devices interactions with the environment are the following:

  • Technology: The technology used to secure the turbine to the seabed may also have different impacts. For example, the proposed first-generation of marine current turbines uses mono-piles which could disturb seabed habitats during installation. A second-generation design could use mooring systems, which may have less of an effect on seabed wildlife whilst allowing the turbines to operate in deeper waters.
  • Packing density: The density in which units are installed may also influence any environmental effects. For example, if a number of turbines with diameters of 16-metres are about to be placed across the tidal flow, these would be spaced out some 60-metres apart. This would leave a minimum gap of 44 metres from blade tip-to-tip. The turbines would be positioned 1000-metres downstream from each other in order to reduce the negative effects on performance caused by turbulence and allow for the tidal streams to restore themselves. The horizontal and axial spacing would result a packing density of 18 units per km2, which could allow for tidal stream, the various wildlife and larger mammals to pass through unaffected.
  • Layout: The layout of arrays of marine turbines could also influence the impact. For example, rows of turbines located besides each other may act as a larger barrier to oncoming tidal stream compared to farms where the turbines are installed in scattered arrays.
  • Depth: Waters of depths from 20 to 80 metres chart datum would be used; therefore any wildlife or marine activities using water in depths outside this range would not be directly affected.
 
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