Barrages, ‘breakwaters’ and ‘bridges’: three proposals for harnessing tidal power in the Solway Firth

It’s the ‘Energy Coast’ – but where do West Cumbria’s priorities for the county, the country and, more importantly for the long-term future of the planet, lie?
We have onshore and offshore wind power-generation (Robin Rigg), an Anaerobic Digester near Silloth, a Biodigester proposed for Oldside; there are proposals for extracting undersea coking coal (West Cumbria Mining) and for undersea coal gasification (Cluff Natural Resources), a proposed new nuclear power station at Sellafield and, currently, three proposals for generating electricity from the tidal flows in the Solway Firth: a barrage, a lagoon and an ‘electric bridge’.
The UK must deliver 15% of its energy from renewable sources by 2020, under the European Union’s 2009 Renewable Energy Directive. The sea is the ultimate ‘renewable’ source of power: our island is surrounded by an unlimited store of energy carried by the tides and the waves, energy that can be captured by turbines and converted into electricity.

The Solway tides
Let’s look at that energy source with reference to the Solway. The Firth is a finger of water fed by the Irish Sea, and into which, at its inner eastern end, various Scottish and English rivers empty. Twice a day, the sea flows in and out; the volume of river-water varies with the weather.

Proposals for harnessing tidal energy stress the fact that the Firth has a very large tidal range as a selling point. Let’s examine this, too.
The difference in height between high and low tides on the Solway can be as much as 10 metres or as small as about 5 metres depending whether the moon and sun are in alignment (Spring tides) or at right angles (Neap tides); Spring tides have nothing to do with the seasons, as implied in “the chaos of the autumn and spring tides”.

Springs and Neaps happen each month on a predictable cycle – sometimes the Spring tides are especially large as during the ‘supermoon’ phase at the end of September (see below). During the flow of the Spring or ‘big tides’ as they’re known locally, a massive volume of water has to pass up the Firth (and out again) during each approximately 12-hour cycle; the rate of flow is impressive and daunting to experience if you are on the lower shore, and the bore in the Upper Solway near Bowness can be heard as well as seen.

Tidal chart for mid-September 2015 (metres and times)

Tidal chart for end of September 2015 (metres and times) (from the UK Easytide website)

As the tide reaches the innermost point of the Firth, its flow rate decreases and the difference in height between high and low water is much reduced where the sea-water meets the river flow and spreads out over the saltmarshes and mudflats. During low water in the big Spring tides, the only water flowing is from the rivers Eden and Esk (I have walked across the Firth at Bowness at this time).

Tthe end of the viaduct at Bowness on Solway

The end of the viaduct at Bowness on Solway at low Spring tide (photo: Ann Lingard)


So how do you capture ‘Tidal power’?

For clear and informative information about methods of capturing water-borne energy (and other electricity-generating and -storing information) I recommend the Mpower website.

The basic principle of harnessing tidal power is that a proportion of each incoming and outgoing tide is forced to pass across the vanes of a turbine, causing the turbine to rotate against a magnetic field within a generator and produce electricity. The turbine must be bi-directional – it must be able to work with both the ebbing (outgoing) and the flowing (incoming) tide.

Power generation will only occur when water is moving at a prescribed range of flow rates through the turbines – but tidal flow rates are not constant throughout each cycle, there are the periods of ‘slack tide’ each side of the time when the tide turns. The period of generation is thus between 6-10 hours, depending on the location.

This can be improved by trapping some of the water and releasing it, so that the available energy depends on the head of the water above a turbine and the volume of water flowing through it. If the tidal flow is constrained by walls such as a barrage or a lagoon, water-levels can be controlled via gates and overflows to provide a manageable flow rate through the turbines.

With a lagoon, some of the sea is trapped within an enclosed space while the rest of the main tidal flow passes by outside; with a barrage across an estuary, the tide must pass via the turbine spaces and also, presumably, gates built into the wall.

What does building a ‘tidal power’ generating system require, with reference to the Solway?

Turbines, obviously – which need to be set in a supporting framework, which might be a rigid wall or barrage; a generating system; transformers – which might be installed nearby (as in Robin Rigg offshore windfarm); cables to transport to the shore the electricity generated; onshore control centres and support systems; links to the National Grid with the Grid having the capacity to carry the electricity generated.

Walls – barrages, lagoons – need foundations on the sea-bed so, since large areas of the Solway’s bed are very labile and changeable, bathymetric and geological surveys are obviously necessary. Walls need cement, and boulders or baffles to dissipate the power of the sea during storms. For rocks/boulders you need a large source of stone. Also vital is the infrastructure for transport, via the sea (the Port of Workington, conveniently placed as a ‘logistics hub‘) and road and rail.

There is also local shipping to consider – ranging from tankers and other merchant vessels visiting the ports of Workington and Silloth, to trawlers, windfarm support vessels and pleasure yachts visiting or working out of Whitehaven and Maryport – which needs access via sea-locks.

Another requisite which is rarely mentioned, is an understanding and modelling of the potential effects of altering the currents, and of the building process itself, on the erosion and deposition of sediment.

And a final requisite is serious, dispassionate thought about what is valuable about the Solway Firth and what we, as the human species, ‘want’ from it.

The three proposals
1. NorthWest Energy Squared (NWE2) model

Map of the NWE2 scheme (from their website)

Map of the NWE2 scheme (from their website)

The aim of the scheme, supported by the North West Business Leadership Team (NWBLT) is to build a series of ‘tidal gateways’ across the estuaries on the west coast linked by a dual carriageway. These tidal gateways – actually barrages – would be built across the Dee, Mersey and Ribble estuaries, Morecambe Bay and the Solway Firth.

The themes are power-generation and ‘connectivity’.

The latter involves improvements – dual-carriageways – for the road system from the North Wales coast road to Stranraer, with additional roads across the tops of the barrages. This would reduce road transport times along the system and beyond: for example, from Workington to Stranraer would be reduced by about 70 minutes. Improvements to the A595 road up the west coast of Cumbria could ‘add a further 7 million tourists to the 35 million who already visit the region, providing a much-needed boost to the local economies of places such Whitehaven and Maryport’. It is also suggested that the scheme would help in flood control, ‘as the gateway could help mitigate flood risk suffered by Carlisle’.

The Kirkcudbright-Workington ‘gateway’ would be 30 kms long and create around 3600 construction jobs per year for 10 years. As for power-generation, it’s estimated that the Solway scheme would generate 8.44TWh (8440 million kWh) per year, power for nearly 2 million homes.

NWE2 model at The Beacon, Whitehaven

NWE2 model at The Beacon, Whitehaven (photo: Ann Lingard)

NWE2  is based in Manchester and its chairman, Alan Torevell, approached Arup with the idea of creating a computer-generated 3D model which, with a video, was on show at The Beacon, Whitehaven, in May.

The Solway Firth 'gateway' on NWE2's model

The Solway Firth ‘gateway’ on NWE2’s model

How Arup came to design the model is explained in detail on page 8 of The Times’ Business Insight North, June 2014. Unfortunately only the 3D-printed, physical model of the North-West was available at The Beacon. This was a shame though understandable – but with the complementary digital and interactive model, images can apparently be projected and overlaid on the physical model, and a touchscreen should ‘allow visitors to gain further information and answers to questions such as “What will be the impact on the mudflats of a specific estuary due to a tidal gateway?” ’

What indeed? According to Alan Torevell, in an interview on pages 6 and 7 of the same issue of Business Insight North, potential financers such as pension and insurance companies ‘ “won’t make any investment until they know the Government is going to have the conditions in place, that they are not going to be held up for 10 years by birdwatchers…”.

alan torevell the times cover

Alan Torevell on the cover of The Times’ Business Insight North

The article quotes his ‘simple message for the environmentalists: tidal gateways won’t harm birds… the reality is it won’t disturb the birds. “There will be environmental problems during construction, but not a lot and they will still keep going there. The main argument is about high and low tide levels and the way it impinges upon the mudflats… But the gateways aren’t going to change the fact that the tide will still come in and go out as normal. So why should we worry about the birds if we are not doing them any harm?” ’

As for shipping, there was no mention of that on the video but Deb Wheeler, who was on hand to explain the physical model at The Beacon, told me that sea-locks would be included.

2. Tidal Lagoon Power

(Update, January 18th 2016: questions raised about bi-directional efficiency of the turbines, and the effect of turbulence.)

Tidal Lagoon Power, under its ‘fast-talking and ebullient’ CEO Mark Shorrock (as Terry Macalister describes him in a Guardian report)  has plans to build 6 tidal lagoons, one of which would be in the Solway Firth, from Workington to Dubmill Point at the north end of Allonby Bay.

Artist's impression of the Swansea tidal lagoon

TLP’s proposed Swansea lagoon

At present, all the emphasis is on the first lagoon, a 9.6 km (~6 miles) ‘breakwater’ with 16 turbines in Swansea Bay. It is expected to generate 500GWh of power by 2019, enough to power 155,000 homes for 120 years. It would also, according to an article by the Landscape Institute, provide ‘a major sports, tourism and leisure destination, contributing to local regeneration,’  with landscaping and a visitor centre.

Artist's impression of the Swansea lagoon

Artist’s impression of the Swansea lagoon

Amongst the appointed contractors  are Laing O’Rourke and Arup for the turbine housing and sluice gates,  Atkins for design engineering, General Electric UK for the turbines, and the China Harbour Engineering Company for building the wall.

TLP had hoped that all the permissions and the price for the electricity would have been finalised by this autumn, so that work could begin, but now this is unlikely to happen until August 2016 for two main reasons. One problem is the negotiation with the Department for Energy and Climate Change (DECC) over the Contract for Difference, which sets out the price over a fixed period that will be paid to a power-generating company for its electricity. The ‘strike price’ that TLP is seeking, of £168/MWh for 35 years is very much higher than the £92.50 strike price for the proposed nuclear reactor at Hinkley Point. In mitigation, TLP says its five future lagoons will be larger, apparently benefitting from economies of scale (although the major part of the cost, the cost of building each kilometre of wall, will presumably be roughly the same) so that a strike prices of less than £100 might be possible by the third lagoon – in other words, TLP is treating its six-lagoon scheme as a package.

There was also allegation that the tendering process for building the wall had ‘been awarded improperly’ to the Chinese company.
But the most recent news is that the Chinese company state that the cost of building the Swansea wall has been greatly underestimated – and thus TLP would require an even higher strike price from the government.

In the light of these setbacks, TLP have recently stated (see for example, the ‘Events‘ section and their Twitter account, @TidalLagoon) that the whole process has been deferred for a year until autumn 2016, with building to start in 2017.

The engineering challenges
Ignoring all that for a moment, let’s consider what building a lagoon and retaining wall entails, in a place where there are 4 tidal flows a day. If you look at the construction in terms of its engineering, it presents interesting challenges.

Cross-section through the wall, from Constrcution magazine

Cross-section through the lagoon wall, from Construction-manager website

The Swansea wall – ‘breakwater’ is a euphemism, the lagoon is essentially a U-shaped dam – will be 9.6km long, 5 m high at the onshore end and 20 m high further out, of which up to 12 m will show at low tide. According to the article in Construction Manager, ‘First, two  metre-high barrier walls of “quarry run” – randomly shaped stone – are laid parallel under water with the space between them filled with sand. Further progressively narrower layers of rock and sand are placed on top of the construction until a barrier with a triangle-shaped section is formed. Larger “rock armour” is then positioned on top of the structure to protect it from the sea.’ And the whole is topped-off with a concrete road.

Where the rock will come from is currently a matter of controversy, especially amongst Cornish residents along the Lizard Pensinsula (Cornwall Against Dean Superquarry), where Shire Oak Quarries, a company headed by TLP’s CEO Mark Shorrock, has plans to turn an existing coastal quarry into a ‘super-quarry’ complete with jetties.

It is estimated that 7 million tonnes of sand will be dredged from Swansea Bay, probably from within the lagoon area, to infill the rock walls.

There is a detailed treatment of concerns and possible remedies for the impact on the marine life within the lagoon and during construction in the Planning Inspectorate’s report and recommendation (from p136, section 4.13.55, for example). For example, research is being carried out at the University of Swansea on the feasibility of translocating the reefs built by the honeycomb worm, Sabellaria (similar reefs are also found in the Solway’s Allonby Bay).

How this type of lagoon would affect the Solway Firth will very much depend on the final site and length. The current proposal, for a 31km wall from Workington to Dubmill Point (and thus across the proposed Marine Conservation Zone in Allonby Bay) would contain 90 turbines.

Regarding TLP’s plans for the Solway Firth, Lisa Jenkins, part of the TLP communications team, told me in May that the West Cumbria project was at too early a stage to give me any details, the company was working on ‘very initial feasibility studies’, there were no formal consultations as yet, but they ‘were really encouraged by the enthusiasm shown within the area’. Roger Woods, TLP’s Development Director for West Cumbria, reiterated this in an email. The website is uninformative.

Might there be a Plan B for a smaller lagoon based around Workington, should DEFRA approve Allonby Bay as a Marine Conservation Zone? (Might DEFRA be persuaded that Allonby Bay is an unsuitable candidate? We must await the outcome of their consultation process, due January 2016.)

Preliminary meetings have been held with Allerdale councillors and with Britain’s Energy CoastTM. One councillor told me he was ‘quietly optimistic’.

But since the future of the Swansea lagoon is looking uncertain, the prospect of a Solway lagoon is looking more uncertain still.

It would be nice to know what is happening.

3. Solway Energy Gateway, SEG
In 2009 Halcrow, Mott McDonald and RSK published an interesting and detailed study of the bathymetry, tidal flows and many environmental facets of the Solway with the aim of assessing the feasibility of various methods for generating tidal power; the methods included barrages, lagoons and ‘fences’ at different locations within the Firth (see Slide 11 of SEG’s presentation).

Out of this grew the idea for a power-generating system at position B3/R3 on the above-mentioned slide, and Solway Energy Gateway Ltd, a ‘local company committed to a sustainable and ethical business model’, was founded and chaired by Nigel Catterson (who is the current Chairman of Britain’s Energy Coast and has strong involvements with other ‘green’ projects including ‘Utropia’ at Broughton Moor near Cockermouth).

Neither a barrage nor a lagoon, the ‘electric bridge’ would also provide a route for pedestrians and cyclists between England and Scotland, at the site of the vanished Annan-Bowness viaduct. Rather than standard turbines, the bridge would incorporate VerdErg’s Venturi Enhanced Turbine Technology (VETT) and would be able to generate ‘about 160MW (megawatts), with a consistent output of about 29MW or 245GWh … enough to power about 76,000 homes’.

The basic priciples of the VETT (formerly known as SMEC) turbine

The basic priciples of the VETT (formerly known as SMEC) turbine

The VETT, which can work in low water levels by increasing the ‘head’ of pressure that drives the turbine, and which uses only 20% of the water passing through – the other 80% by-passes the turbine – has been used in small field trials. The only moving part in the structure is the actual turbine, so the system is cheap to install and run. Another trial using 900 fish showed that none were damaged by passing through the VETT; this would be especially important at the Annan-Bowness site because salmon pass through this narrow neck of the Firth to breed in the Esk and Eden.

At present, the VETT system works only in one direction; VerdErg are currently working with Arup to design a bi-directional turbine and Nigel Catterson tells me that the testing will finish at the end of March 2016 and that the evidence that the bi-directional device will be as efficient as the unidirectional device is ‘very encouraging’.

Arup have also recently used a drone to make useful aerial surveys of the Firth at the site of the ‘bridge’ (the images will soon be on the SEG website – they will probably be much better quality than my own, taken from the gyroplane!).

The length of the proposed ‘bridge’ would be very much less than the walls required in the barrage and lagoon proposals, and much cheaper to build – in the region of £300-400 million. Although the design has not yet been finalised, Catterson says it can be thought of as a ‘movable weir’ that can be raised or lowered by bladders of water or air according to the state of the tide; on top of the row of VETTs will be a plate that can be angled so as to focus and regulate the tidal flow through the orifices that contain the turbines.

It will be necessary to excavate the sea-bottom to reach the bedrock, so that the systems can be firmly anchored; slabs will be installed in the base of the estuary and above them ‘will rise the pillars of an elegant-looking bridge’ for cyclists and pedestrians.

From the Solway Energy Gateway's workshop, Dumfries

From the Solway Energy Gateway’s workshop, Dumfries

SEG are committed to involving the communities on each side of the Firth, and Catterson held a Dumfries workshop in May under the auspices of the Solway Firth Partnership to gather opinions and find out what people wanted from the project: the responses are summarised in this presentation.

The system would apparently qualify for DECC’s Tidal Stream Payments (similar to that offered for the MeyGen underwater turbine scheme in the Pentland Firth) which offers a high strike price of £305/MW; but ‘even at half that strike price, the scheme would be paid for in 10 years’. SEG has also received interest from potential investor companies including pension funds. And Nigel Catterson tells me he is still aiming on 2020 for the electric bridge to start generating power.


I have deliberately avoided discussion of the effects, on the people and the life and functioning of the Solway Firth, of harnessing the power and energy of its tides.

All I have tried to do here is present the background and some of the facts about the three proposals. It’s early days – untried technologies, questions about construction, about prices, and in some cases a lack of ‘transparency’ – but if we want to get away from our dependency on fossil fuel, the power of the tides is (eventually) there for the taking. It’s how we do ‘the taking’ that must concern us too.

This entry was posted in renewable energy, tide & wind, The 'Energy Coast' and tagged , , , , , , , . Bookmark the permalink.