Tidal farms interfere with sea-floor sediments
24 July 2009, by Sara Coelho
Tidal energy farms may disrupt the natural balance of marine sediments. This can have positive or negative consequences for coastal environments and should be taken into account when deciding where to build such farms.
The Bristol channel.
Tidal farms harness the energy of tides in a similar way to wind turbines, and are a possible source of renewable energy for a sustainable, carbon-free economy. But since tidal turbines operate underwater, they have the potential to interfere with marine dynamics and ecosystems.
Dr Simon Neill and Professor Alan Davies from Bangor University, and colleagues from Edinburgh University and Plymouth Marine Laboratory, designed a mathematical model to investigate how tidal turbines may affect the Bristol Channel estuary, which is being considered as a possible location for tidal generation.
They found that tidal turbines influence the natural transport of sands near the bottom.
Neill explains how: 'tidal farms remove energy from the environment. But if you take energy out, then there will be less energy available for moving sediments around. So sediment transport is weakened near the tidal farms.'
The findings, published last week in the Renewable Energy journal, also show that the effect is widespread. 'Transport is disturbed over a distance of 50km away from the turbines,' says Neill.
In practice, less transport means changes in the natural equilibrium of the coast, with unusual sediment accumulation near the tidal farm. At first, the changes will barely be visible, but after the 30-year life-cycle of a tidal farm they can add up to become significant. 'This can have positive or negative effects on the estuary,' says Neill.
More sand accumulating in the right spots may provide additional protection against coastal erosion and an extra resource of sand, gravel and other inert building materials.
But if sediments accumulate in the wrong places, waves might start breaking closer to the shore, which could increase coastal erosion and flooding. Neill adds that the consequences for bottom-dwelling marine animals are also unclear and 'only site-specific modelling studies could determine possible effects.'
The model also shows that the possible effects of the turbines on sediment transport depends on the pattern of local sediment transport pathways, not just the strength of the current at the point of energy extraction. This is especially true in the Bristol Channel, where shallow water and the interference of incoming and reflected tides creates complex sediment transport patterns.
'Our results suggest that, to minimise the effects of the tidal farms, the turbines should be placed in areas that lack local mobile sediment sources and have symmetric flood/ebb transport patterns,' says Neill.
S.P. Neill, E.J. Litt, S.J. Couch and A.G. Davies. The impact of tidal streamnext term turbines on large-scale sediment dynamics. Renewable Energy, available online 15 July, doi:10.1016/j.renene.2009.06.015
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