The Tidal Reef: a potentially ‘greener’ tidal power project for the Severn Estuary
The RSPB is concerned that an exciting and environmentally benign engineering option for harnessing tidal
power in the Severn Estuary could be dropped by the UK Government. This option, known as the Tidal Reef,
offers an alternative to the familiar Severn Barrage, at less cost and with higher electricity output, while largely
maintaining the habitats upon which birds depend and causing less harm to fish species. The RSPB therefore
believes it to be imperative that this option should not be dismissed at this stage - and without the detailed
consideration that it clearly warrants.
The threat posed by climate change demands nothing short of a revolution in the way we generate and
use energy. The RSPB believes that this revolution should take place in harmony with the natural
The Department of Energy and Climate Change (DECC) is currently leading a Feasibility Study into
whether the UK Government can support a tidal power project in the Severn Estuary and if so under
Ten tidal power projects have been proposed and ministers will meet in mid December 2008 to approve
a short-list of these options worthy of further consideration.
One such option, the so-called Tidal Reef, offers a less environmentally damaging way of harnessing
the estuary’s tidal power. The RSPB is excited by this proposal as it starts from the premise that
environmental considerations can and should be addressed from the outset of major infrastructure
projects. However, we are concerned that, as the Reef is presented as a concept rather than a more
developed proposal, it might be overlooked in favour of other more traditional ideas.
The RSPB therefore commissioned Atkins to critique the feasibility, energy generation potential and
cost of the Tidal Reef. Atkins is the largest firm of consulting engineers in Europe, and have unique
experience of marine renewable energy and offshore oil platforms. The Atkins report concludes that a
Tidal Reef scheme could make use of conventional barrage technology. It also suggested that:
? it would be feasible for this concept to generate more energy than the Cardiff-Weston barrage
? it is probably economic and would cost ?2bn less than the Cardiff-Weston barrage proposal
? it could be developed to a project design with ?0.5 m within the two-year timescales of the
Government Feasibility Study.
In light of the report findings, the RSPB urges DECC to examine the Evans Engineering Tidal Reef
proposal fully. We believe that short-listing options at this early stage in the Feasibility Study is not the
right approach to devising environmentally acceptable engineering solutions for harnessing the tidal
power of the Severn Estuary.
Climate change and energy policy context
The RSPB believes that climate change is the greatest threat we face and that unless action is taken to reduce greenhouse gas emissions, one third of all land based species may be committed towards extinction by 2050. We have welcomed the UK Government’s plans to cut emissions by 80% by 2050
and we support the Government’s pledge to deliver the UK’s share of the EU renewable energy target for 2020. The UK Government’s Renewable Energy Strategy has proposed that, to contribute its fair
share to the target, it will seek to generate 15% of its energy (and up to 40% of electricity) from renewable sources. This will require a revolution in the way that we generate and use energy. The RSPB argues that this revolution should take place in a way that minimises damage to the natural environment.
The Feasibility Study and SEA
The UK Government has, with the support of the Welsh Assembly Government and SW England Regional Development Agency, established a two year Sever Tidal Power Feasibility Study. The Study will determine whether, in the context of its energy and climate change goals and the alternative options for achieving these, the Government could support a tidal power project in the Severn Estuary and on what terms. A Strategic Environmental Assessment (SEA) of the Feasibility Study is also being carried out to assess the likely environmental effects of the ten long-listed options, which include the Cardiff-Weston barrage - the focus of the Sustainable Development Commission report on tidal power (published in 2007). Ministers are to approve a short-list of proposals in mid December that will undergo further assessment in 2009.
The Severn’s thirteen metre tidal range – the difference between low and high tide – is the second
highest in the world. The RSPB agrees that we should investigate the potential to generate renewable electricity from this unique natural resource. Clearly, any tidal power project will have some impact on the Severn Estuary, which is internationally important for wildlife, and has been designated as a Special Protection Area (SPA), Special Area of Conservation (SAC) and Ramsar site. We argue that tidal power projects that seek to minimise adverse environmental impacts should not be prematurely excluded from the Feasibility Study.
The Tidal Reef: a ‘greener’ tidal power project?
A Tidal Reef from Minehead to Aberthaw, proposed by Evans Engineering, is one of the Feasibility Study’s ten long-listed options. Its objective is to be as environmentally benign as possible, while
generating the greatest output of renewable energy. Unlike other barrage proposals, the Tidal Reef takes into account the needs of wildlife, migratory fish and navigation from the outset and identifies engineering solutions that could potentially prevent damage to entire ecosystems.
A Tidal Reef is a method of capturing tidal energy that is halfway between conventional barrages, that delay the tide by many hours in order to hold back large volumes of water, and a tidal stream turbine that extracts some of the energy from a moving current of water.
The proposed Minehead to Aberthaw Reef would enclose a larger area of estuary and have more, much slower running, turbines than a conventional barrage. Crucially, the Reef would hold back only two metres of water, rather than the ten metres required by the Cardiff-Weston barrage. As a result, the natural tidal cycle that is key to maintaining the Estuary’s important intertidal habitats would only delayed by an hour (compared to six hours with a Cardiff-Weston barrage). The small change in natural tidal cycle and water levels should result in less impact on the valuable inter-tidal feeding grounds on which almost 70,0000 birds feed in winter. Evans Engineering also argue that the project’s
impact on migrating salmon and eel is likely to be much less as the pressure difference across the Tidal Reef is very small and the low-head turbines are large and slow running.
The details of the Tidal Reef proposed by Evans Engineering are novel (see http://www.evans-
engineering.co.uk/). Above fully immersed concrete caissons, there are steel modules, each of which
acts as a siphon, and contains four low-head vertical-axis turbines. The modules can rotate through 180
degrees about a vertical axis, so that sections of the Tidal Reef could open at certain times to maintain
required water levels when it is important for particular estuary users allow and free passage of
shipping. It also incorporates vast navigation gates to allow large container ships to pass through on
route to Avonmouth docks. Evans Engineering propose that a Tidal Reef could be built in sections
within a shorter time frame than a traditional barrage. Therefore, the scheme could be pilot-tested and
begin generating power within a few years of construction.
The Atkins report on the feasibility, power and cost of the Tidal Reef
The RSPB is excited about the Tidal Reef proposal. We recognise that any tidal power project in the
Severn Estuary will have some impacts on wildlife, but agree that the likely environmental implications
of this proposal are likely to be less than most of the other options currently being considered. We are
concerned that the Tidal Reef, and other potentially less environmentally damaging tidal power
proposals, are perceived as ‘nascent technology’ and may not be short-listed for further examination by
the Feasibility Study and parallel SEA.
To establish that the Tidal Reef is feasible, and ensure that the Government considers potentially less
damaging alternatives, the RSPB commissioned Atkins to determine the Reef’s feasibility, energy generation potential and cost. Atkins are the largest firm of consulting engineers in Europe, and have
unique experience of marine renewable energy and offshore oil platforms. As there are a number of
ways in which a Tidal Reef could be designed and still retain its potential environmental benefits,
Atkins considers a conventionally engineered fixed-concrete version of the Tidal Reef in their Report.
However, the essential features of the Tidal Reef concept is retained, which is a large water volume, a
low water level difference across the structure and large slow-moving low-head turbines.
The Atkins report concluded that the Tidal Reef scheme:
? Is feasible and could generate 20TWh/yr, which is 3TWh/yr more than the Cardiff-Weston
? Is probably economic and would cost ?2bn less than the Cardiff-Weston barrage proposal.
? Could be developed to a project design with ?0.5 m within the two-year timescales of the
Government’s Feasibility Study.
The Atkins Report did not consider the likely environmental impacts of a Tidal Reef in detail. More
research on this proposal’s likely environmental impacts is needed as many uncertainties remain.
Implications for the Severn Tidal Power Feasibility Study
In light of the report findings, the RSPB urges DECC to examine the Evans Engineering Tidal Reef
proposal fully. We believe that short-listing options at his early stage in the Feasibility Study is not the
right approach to devising environmentally acceptable engineering solutions for tidal power projects in
the Severn Estuary.
For more information please contact Martin Harper, Head of Sustainable Development,
email@example.com and 01767 693314, or Mariam Ali, Planning Policy Office,
firstname.lastname@example.org and 01767 693486.