Turning Back The Tide

The Severn Estuary has one of the largest tidal ranges in the world, and there has been talk of building a barrage to harness this energy for more than 100 years.  But despite the massive energy potential nothing has been built, although there have been some ambitious plans.

There are only a few operational tidal power schemes around the world, including the first (and for many years the largest) tidal barrage at Rance in Brittany which has a capacity of 240 MW (Mega Watts). This has recently been exceeded by the Sihwa Lake barrage in South Korea but with only a slightly larger capacity of 254 MW. These barrages are dwarfed by the various proposals for the Severn Estuary.

There have been a number of proposals made over the years with the following schemes being most developed:

Tidal Barrages

There have been around 10 different proposals for Severn Barrages, ranging from the smallest near the Seven Bridge to the largest joining Gower to Devon.  But the two following schemes are the best developed and appear to be the most viable:

The Cardiff-Weston Barrage

This huge barrage would close the Severn Estuary between Bream Point in Somerset and Lavernock Point in Glamorgan and would be around 16km long.  This would have a generating capacity of 8,640 MW – or 36 times the size of the Rance barrage.   This would not be cheap, with an estimated cost of £12 billion in 2006, equivalent to £15 billion today.

This scheme would have a huge environmental impact, and would need a major lock system to accommodate shipping, especially to Avonmouth Docks. There is a lot of risk and uncertainty associated with this project, and some estimates have been costed as greater than £22 billion.

The Shoots Barrage

This more modest proposal would close the estuary a little downstream of the Second Severn Crossing and would be around 5km long. It would have a generating capacity of 1,050 MW and cost ‘only’ £2.3 Billion at today’s value.  This would have a lesser environmental impact than the Cardiff-Weston Barrage and would not need such an extensive lock system (being upstream of Avonmouth). One particular advantage of this barrage is that it would be ideally placed to carry transport links, including a replacement for the Severn Tunnel railway.  The Shoots Barrage could also be built in conjunction with alternative tidal lagoon schemes which would be precluded by the Cardiff Weston Barrage.

Tidal Lagoons

As an alternative to tidal barrages, it is proposed to develop tidal lagoons, which do not close the entire estuary, but only enclose a limited area although the technology is similar. There have been a number of schemes proposed:

The Swansea Bay Lagoon

This proposal involves building a 9.5km embankment between the river Tawe and river Neath enclosing 11.5 km2 of water. This would generate only 320 MW which is still 50% larger than the Rance barrage and would cost an estimated £1.3 billion.  This ‘small’ lagoon is intended to be a prototype – a world-first to prove the tidal lagoon concept before developing larger lagoons.

The Cardiff Tidal Lagoon

This full-sized proposal involves building a 18km barrage between the river Taff and river Usk enclosing 70 km2 of water. This would generate 3,000 MW and cost around £8 billion. It would also provide flood protection to the low-lying coast.

The Newport Tidal Lagoon

Also known as the ‘Fleming Lagoon’ this is similar in size to the Cardiff Lagoon and would be built between the river Usk and river Wye. This scheme could be integrated with the Shoots Barrage and provide flood protection.

The Economic Case

Looking at the development costs of each of the schemes compared to their potential generating capacity, there are clear advantages of some schemes compared to others, and for comparison purposes I have included the Hinckley Point 3 Nuclear Power Station.




(£ Billion)

Unit Cost

(£ Million/MW

Cardiff-Weston Barrage 8,640 15 1.75
Shoots Barrage 1,050 2.3 2.1
Cardiff Tidal Lagoon 3,000 8 2.7
Swansea Tidal Lagoon 320 1.3 4.0
Hinckley Point 3 Nuclear 3,200 22.9 7.6


The Cardiff-Weston barrage appears to provide the best return, but apart from the environmental impact of this scheme there have been serious concerns raised regarding the risks associated with this project, and the costs could significantly exceed this making the project unviable.

The Swansea Tidal Lagoon was rejected by UK Government as being too expensive, and you can see why looking at the figures above. But this project should have been considered as a prototype with costs shared over future projects (both lagoons and barrages), which would then de-risk the later projects reducing their costs. It would also be an opportunity to establish Swansea Bay as a global centre of excellence for tidal power.

The Shoots Barrage appears to be very viable, especially when considering that it could be combined with a new Severn Rail Crossing. Electrified rail services could be run across the top of the barrage, with the Severn Tunnel retained as a freight only line.

The Cardiff Tidal Lagoon would produce as much power as a nuclear power station but at one third of the cost. The ongoing operating and maintenance costs would also be significantly lower than nuclear power.

At first glance nuclear power appears to be far more expensive at almost 4 times the cost per MW of the Shoots Barrage. But the advantage of Nuclear power is that it generates consistent power 24 hours/day compared to Tidal power which can only generate power for a limited time each day. But against this, nuclear power has much higher operating and maintenance costs, together with the un-priced cost of decommissioning and unknown long term environmental impact.

Power Generating Profile

The problem with tidal power is that it is dependent on the time of the tides, with power generated in 4 windows a day – twice a day on incoming tides and twice a day on outgoing tides, and in practice its average power generation is only around 25-30% of its full capacity. When it is not generating the ‘missing’ power would need to be generated by gas-fired stations.

However, the uneven power output of a Cardiff Tidal Lagoon can be balanced up by building a similar lagoon at Colwyn Bay – where tides are around 4 hrs earlier than in Cardiff. It would be generating power when Cardiff was idle and vice versa. The costs and generating capacity of the Colwyn Bay Lagoon have already been assessed and are similar to the Cardiff Lagoon.  There would still need to be some additional power supply needed by using gas-fired stations, but this would be significantly reduced by building the two lagoons.

‘Time and Tide Wait For No Man’

We need to consider tidal power as our ‘North Sea Oil’.  We can generate far more power than is needed in Wales, and our surplus energy would be exported and be a valuable addition to our GDP. We also have a unique opportunity to establish Wales as a world leader in tidal power, establishing related fabrication and manufacturing industries. But we can not wait or the opportunity will be taken by others.


One thought on “Turning Back The Tide

  1. Interesting and compelling . We on the south cf the Isle of Wight are concerned and fighting the local PTEC propass,mainly on viability,but also on environmental impact and sustainability.

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