Green light for marine renewables?

Tidal turbines

Tidal turbines

21 October 2011 by Simon Neill

Why are tidal turbines like roadworks? As a traffic engineer-turned-oceanographer, Simon Neill should know. He explains how taking energy from the tides on a large scale with farms of 'underwater windmills' could change how sand moves around our coastal seas, affecting beaches, sand banks and ultimately the risk of flooding.

Tidal currents flood and ebb, mainly due to the gravitational force of the moon, combined with the Earth's rotation. When these currents are fast enough, they pick up grains of sand from the seabed, which are then transported with the flow. This is like cars picking up passengers en route to their destination.

In the middle of the day, on a typical road, there is no particular pattern to the flow of traffic, so the number of passengers will flow equally in both directions. It's the same in the shelf seas - the shallow waters around our coastlines - where symmetrical tidal currents transport sand equally in both directions. This symmetry means that in the long term there is no significant net movement of sand.

Yet something interesting happens on the roads during the morning rush hour. There is a large net flow of passengers towards the cities, so the number of passengers transported is strongly asymmetrical. In the sea, this kind of asymmetry in the tidal currents leads to a net large-scale movement of sand in either the flood or ebb direction.

Throughout the world's shelf seas, interactions between the sweep of the tide and friction at the seabed generate a complex distribution of regions of symmetry, and regions of asymmetry. We can use computer models to make detailed maps of such processes, and these have been widely applied over the past few decades to understand large-scale sand movements.

Flooding and ebbing tidal currents, transporting sand either equally in both directions or with a net transport in one direction or the other, are responsible for the distribution of sand around our shelf seas. These large-scale sand movements feed into coastal systems like beaches and offshore sandbanks. Such systems remove the energy from storm waves, and so are vital natural forms of coastal protection.

Coastal engineers must understand these systems to manage flood risk, just as traffic engineers and town planners need to understand the volume of traffic and passenger numbers travelling through each section of the road network.

However, what would happen if we were to exploit tidal energy to generate electricity on a significant scale? How would this affect the large-scale movement of sand, and so ultimately affect this natural form of coastal protection?

Impact of energy extraction

Extracting energy from a tidal system, for example by installing a farm of tidal stream turbines or 'underwater windmills', will reduce the strength of tidal flows. This is like the impact of roadworks, which lead to a reduced flow of traffic. A reduced flow of traffic means fewer passengers can be transported. In the sea, tidal energy extraction will similarly reduce the volume of sand transported.

The amount of sand transported is proportional to the cube of the speed of the current that's carrying it, so small changes to the current speed could translate into large changes in sand transport. Of course, tidal-energy projects are only economically viable in regions of strong tidal flow, and the impact will be magnified in these regions, compared to areas with weaker tides.

Diagram showing how sand transport in the oceans is like the flow of traffic

Sand transport in the oceans is like the flow of traffic

Now, something interesting happens if we contrast the impact of extracting energy from a region of tidal symmetry (traffic in the middle of the day) with extracting energy from a region of tidal asymmetry (traffic during the rush hour).

We first assume that turbines will be designed to extract energy equally efficiently during both flood and ebb phases of the tide. This could be achieved, for example, by turning the devices when the tide changes - technology similar to that used in wind turbines.

In a region of tidal symmetry, although the dampened flow speeds will reduce the overall volume of sand transport, the transport will still be symmetrical. This is what would happen if roadworks were set up during the middle of the day on both the city-bound and country-bound carriageways.

Although slightly less traffic would flow, the impact on passenger transport would be minimal. During the morning rush hour, though, roadworks placed on both city-bound and country-bound carriageways would have a considerable impact on net passenger transport.

If the roadworks resulted in particularly long city-bound delays, they could actually lead to a situation where equal numbers of passengers are transported into and out of the city during the morning rush hour - although in reality, one would hope that the traffic management scheme would not be so inefficient! Similarly, in the shelf seas, a very large tidal energy scheme in an area of tidal asymmetry could disrupt the natural movement of sand considerably.

This could have severe consequences for coastal protection and flooding, by disrupting the supply of sand feeding into the natural systems which protect our coastlines from storms, such as beaches and offshore sandbanks.

There have been many instances where human activities like offshore dredging of sandbanks (to provide aggregate for the construction industry) have been blamed for a reduction of sand in neighbouring beaches. For example, in Pakiri Beach, just north of Auckland, New Zealand, offshore dredging has been linked to poor beach recovery following large storms.

Before developers decide on the scale and location of any large-scale tidal energy scheme, it is important that they consider tidal asymmetry and the impact of the scheme on sand transport, and how this compares to how we expect conditions to vary naturally from year to year and season to season.

Finally, we should note that artificial interventions by tidal-energy farms could actually lead to positive effects. Strategic placement of tidal-energy farms could even be used to create a natural form of coastal flood protection by artificially manipulating offshore sand deposits. However, such state-of-the-art geoengineering would have to be based on a sound understanding of the underlying oceanographic processes.

Simon Neill is a research lecturer in sediment dynamics and oceanographic modelling at the School of Ocean Sciences, Bangor University. Before returning to higher education to seek a career in oceanography, he worked for four years as a traffic and highway engineer with the Northern Ireland Roads Service.