Why Wales should build offshore tidal lagoons

I'm pleased to hear that the ConDem government has decided not to proceed with construction of a Severn Barrage, but I'm not at all pleased by the decision to do nothing about harnessing the renewable energy available to us from the Severn. So what I want to do in this post is try to explain from a technical point of view what the options are, what's good and bad about them ... and propose a way forward. For some, this will be an unnecessary lesson in how to suck eggs; but I hope it will be useful to others. Sometimes it's possible to be given so much technical detail that it is impossible to see the big picture. This is meant to be an overview of what matters and why it matters.
 

Types of tidal power

There are two types of tidal power. Tidal flow relies on the speed of the tide coming in and going out. It is harnessed by means of turbines, fairly similar to wind turbines, but underwater. The best tidal flow sites are around headlands or in the channel between islands, or islands and the mainland. For us in Wales, there are good sites off Ynys Môn and St Davids Head. The Severn estuary also has a good tidal flow, but this is at its best further out to sea than any location that has been proposed for a barrage.

The second type of power comes from tidal range. What matters here is the difference between low and high tides. The Severn estuary has one of the greatest tidal ranges in the world. The Bay of Fundy in Canada is first, and the Severn estuary is either second or third, vying with Baie de St Malo in Brittany. To harness this power requires building an impoundment to contain water at high tide, then to release it through turbines when the water level on the other side of the impoundment is at its lowest. In terms of operation, the turbines work in the same way as in any hydro-electric dam.
 

Types of impoundment

There are three different types of impoundment. A barrage is built across the mouth of an estuary, with the shores of the river/s which feed it forming the rest of the enclosure. An attached tidal lagoon or connected tidal lagoon is built against the shore but does not impede the flow of the main river. An offshore tidal lagoon or detached tidal lagoon is built close to the shore, but not right up against it.

Perhaps the best way of illustrating them is to look at this map showing the "long listed" schemes proposed for the Severn. Click it to open a larger version:

     

•  There are three barrages across the Severn: Lavernock-Brean, Shoots (or inner barrage) just next to the new Severn Crossing, and Beachley just next to the Severn Bridge.

•  There is one barrage across Bridgewater Bay, although this is called a lagoon.

•  There are two attached lagoons on the Welsh side: Russell between the Severn Crossing and Newport, and Peterstone between Newport and Cardiff ... and one on the English side at English Grounds.

•  There is one offshore tidal lagoon in Bridgewater Bay, plus an extension to it.

The line shown between Aberthaw and Minehead is not an impoundment, but a "tidal fence" ... a line of turbines designed to harness tidal flow.
 

Barrages

The main factor in favour of barrages is that, if built in the right place, they are relatively short compared to the area of water contained behind them. However they have to cross the deepest part of the river, and building a structure up from the bottom of the river makes them more expensive than something built closer to the shoreline. Building something twice as deep costs four times as much; building something three times as deep cost nine times as much. This is because the only practical way to build the impoundment is to pile up rocks on the sea bed, so the cross-sectional shape is always the same, and the amount of material is therefore the square of the height.

The main disadvantage of any barrage is the fact that they can only practically generate electricity at low tide. The reason for this is twofold: first because the impounded area is constantly being filled by the river; but second—and more critically—because river water contains heavy concentrations of silt. The Severn and the tributaries which feed it are particularly heavy in silt from the lower lying areas of England. We can all see this silt at low tide, and a centimetre or so is left behind inland when the river floods. If it were not for this silt, it would be possible to generate electricity both at high tide and low tide ... and this is indeed what the French thought they would be able to do when they built La Rance in Brittany in the sixties. But after a few years they realized that the only practical way of stopping silt build-up was to operate it in one direction only.
 

Attached Tidal Lagoons

The reason I have classified the Bridgewater Bay "lagoon" as a barrage is because the River Parrett carries a good deal of silt, though not nearly as much as the Severn. The secret of building a good attached tidal lagoon is to minimize the amount of river water that feeds into it. Or, second best, to make sure that the river water is relatively silt free. The shorter, faster flowing rivers of Wales fit that bill; but the less water that feeds into the lagoon from rivers, the better. To the extent that these two criteria are met, the huge advantage of a lagoon is that it is able to generate electricity both at high and at low tide. Therefore lagoons produce twice as much electricity relative to their surface area compared with an estuary barrage.

However, what is even more important than the total amount of electricity produced is the fact that it is produced more frequently. Roughly speaking, a barrage will produce electricity for maybe a two hour period every twelve-and-a-half hours. A lagoon will produce electricity for the same two hour period every six-and-a-quarter hours. In terms of the way the electricity grid works, this makes a huge difference for the better. But better than that, as grace would have it, high tide around the coast of Wales varies by about five hours, and the Severn is not the only place where we can build lagoons. The stretch of coast between Swansea and Porthcawl is also good, and so is the north Wales coast ... because even though the tidal range in Liverpool Bay isn't quite as high as the Severn's, it is still good. A series of lagoons will be able to feed a more or less continuous supply of electricity into the grid.
 

Offshore Tidal Lagoons

A detached tidal lagoon has exactly the same advantages as an attached tidal lagoon with no rivers feeding into it, and therefore no problem with silting. In technical terms the differences are in the length and depth of the impounding structure.

The length of the impounding structure that needs to be built is shorter for an attached lagoon because the shoreline itself does part of that job. However the depth of water gets less towards the shore, so the electricity generating potential is not simply a matter of surface area, but of the tidal range across that area. The ideal place to build an offshore lagoon is on a relatively shallow area of sea bed just—but only just—beyond the line of the lowest tide. On the map above, it is beyond the beige of the exposed mud, in the lightest blue area. If we look in more detail at the map, this should explain why the offshore lagoon in Bridgewater Bay is where it is.

In my opinion an equally good area to build an offshore lagoon would be more or less where the Peterstone lagoon between Newport and Cardiff is, but with the inner wall on the line between beige and light blue. This would allow the Rhymni to continue to flow directly into the Severn estuary. The Russell lagoon east of Newport is in a less ideal place, because it does not harness the full benefit of the tidal range.

The second big advantage of offshore tidal lagoons is that is it relatively easy to link them together. By building the impoundment only slightly higher (say a metre or two) than the highest high tide, there is the potential to use any electricity produced from one lagoon in the middle of the night (when demand is low) to pump water into the other lagoon, to then be released back to produce electricity a few hours later when demand is higher.

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But the main advantage of offshore tidal lagoons is ecological. There is a wealth of wildlife in the area between low and high tide, adapted to survive with that tidal cycle. Offshore lagoons do not interfere with this, leaving wildlife untouched. These environmental considerations may not rank highly with some people, and it is possible to argue that the damage might not be too bad. But why risk it ... after all, isn't the whole point of switching to renewable energy in order to save the environment? Yes, offshore tidal lagoons will cost more to build because the length of the impounding structure is longer, but this will be partly offset by being able to harness the full tidal range across the whole area of the lagoon.
 

Roads and Railways

I want to address this matter separately, because one of the reasons often cited for building a barrage across the Severn is that it can then also be used as a road or rail link. This is one of the reasons most often given by those who favour the Shoots barrage.

The impounding structure for a barrage used only for the purpose of generating electricity is a relatively simple to construct. It involves building up a pile of material from the sea bed up to the highest high tide. It does not need to be any higher than say half a metre above that, because it simply won't matter if waves break over the top of it in stormy conditions.

But once we start thinking about putting a road or railway on top of it, things become much more complicated. First, we have to build it higher so that waves don't break over the top of it, probably four or five metres higher. We also have to make the structure much more stable, because huge damage will be done to the road/rail surface if there is any substantial structural movement. This will increase the cost substantially. In contrast, if a simple impounding structure is damaged by a storm, it can be fixed relatively easily from a floating barge. The second factor is what to do about shipping. If any shipping is to pass, there needs either to be a swing or lift bridge section of the road/railway ... and I find it hard to imagine this being acceptable on, say, a new high speed rail line. We would therefore have to build the line higher than the shipping that would pass below it, and because the line would have to have minimal gradients we would in effect be building a bridge. The two structures simply do not mix.

If we want a barrage, we should build a simple barrage. If we want a crossing for transport, I think it will always be better to build a different structure rather than try to combine the two.
 

Conclusions

In my opinion it is insane not to make use of the best form of dependable renewable energy we have available to us in Wales. For that reason I would not have been entirely unhappy if the decision had been made to go ahead with the Lavernock-Brean barrage. Clean renewable energy for at least a hundred years, if not more, is not something that should be spurned. In fact I would prefer us to build the Lavernock-Brean barrage in preference to one of the other barrages.

But it is definitely one of what the French would call les grands projets. It is an all-your-eggs-in-one-basket solution that has the potential to go horribly wrong and cost many times more than anyone envisaged. Once started, there is no way of scaling it back if things go wrong. To the credit of the ConDem government, they seem to have recognized this and are not prepared to take the risk.

If I were English, the scheme I would build is the smaller Bridgewater Bay offshore lagoon – though I would design it in such a way as to allow for the expansion at a later date if the inevitable unexpected problems are not as bad as they might otherwise be. The lessons we learn from building the first will reduce the risks we take when building the second ... and so on.

But I'm not English. I want this energy for us in Wales. For that reason, of the sites shown on the map above, I would build an offshore tidal lagoon, roughly sharing the same footprint as the Peterstone lagoon. On the map below it is called "Cardiff". The locations shown are from the Tidal Electric website, and their preferred first option is Swansea Bay, for which WS Atkins have produced a technical study. The idea is to start small, and build up from there.

     

The politics of the matter is something else. Obviously I have my views on it, but I wanted in this post to concentrate on the technical reasons why I think a series of offshore tidal lagoons is a much better solution to harnessing renewable energy from the tidal range of the Severn estuary than any barrage could be.