Hypothetical Severn Tunnel Replacement

[Lucan]

Upgrade the Gloucester line.

So ... the Great Way Round again. Isn't that what GWR stood for?

 

[squizzler]

If Squizzler thinks that the Severn Estuary is all silt to bury his immersed tube tunnel then can I suggest he looks at a maritime map of the area and perhaps drives over the Prince of Wales Bridge at low tide. There certainly seems to be a lot of rocks visible in both instances.

I do not think the rocks are necessarily a barrier to an immersed tube tunnel. Firstly I think the bridges (and old tunnel) are built in the shallowest bedrock part of the estuary for obvious reasons, so it is natural you see them from the bridge. Secondly, you probably do want your tube to rest on bedrock, just as long as you can bury it with loose material to protect it from damage. This might be achieved by smashing a trench through the bedrock for it to partially lie within. Could powerful machinery on barges move stone more easily from above than a tunnel boring machine can do from within?

No geologist me, but the sheer number of near terminal problems encountered by the tunnel builders suggests an immersed tube would be the easier offer. It was very touch on several occasions whether it would be completed.

 

[Bald Rick]

I do not think the rocks are necessarily a barrier to an immersed tube tunnel. Firstly I think the bridges (and old tunnel) are built in the shallowest bedrock part of the estuary for obvious reasons, so it is natural you see them from the bridge. Secondly, you probably do want your tube to rest on bedrock, just as long as you can bury it with loose material to protect it from damage. This might be achieved by smashing a trench through the bedrock for it to partially lie within. Could powerful machinery on barges move stone more easily from above than a tunnel boring machine can do from within?

No geologist me, but the sheer number of near terminal problems encountered by the tunnel builders suggests an immersed tube would be the easier offer. It was very touch on several occasions whether it would be completed.

IIRC there is a section in the Welsh side of the river (The Shoots) that is much deeper than the rest - about 20 metres deeper I think. It has sufficiently steep banks that a submerged tunnel would not be practical on that section.

Yes there were troubles when the tunnel was built - 140 odd years ago.* Fortunately tunnelling technology has moved on since then.

*EDIT: a bit of research reveals that it was exactly 140 years ago tomorrow that the Great Spring was ‘discovered’. Fortunately, no tunnellers lost their lives, indeed to quote a contemporary account ‘none of the men at work suffered any injury other than a wetting’. (It is not clear if this ‘wetting’ was from the spring, or ‘self administered’!)

 

[squizzler]

IIRC there is a section in the Welsh side of the river (The Shoots) that is much deeper than the rest - about 20 metres deeper I think. It has sufficiently steep banks that a submerged tunnel would not be practical on that section.

Yes there were troubles when the tunnel was built - 140 odd years ago.* Fortunately tunnelling technology has moved on since then.

*EDIT: a bit of research reveals that it was exactly 140 years ago tomorrow that the Great Spring was ‘discovered’. Fortunately, no tunnellers lost their lives, indeed to quote a contemporary account ‘none of the men at work suffered any injury other than a wetting’. (It is not clear if this ‘wetting’ was from the spring, or ‘self administered’!)

I think the Shoots is a feature round the area of the original tunnel and I was proposing a longer immersed tube downstream. Nonetheless it is possible to build a tunnel partially bored and partially immersed tube: the Bosporus tunnel in Constantinople uses this technique with the approaches bored and the underwater bit in tube. In this example the transition can occur where this bore emerges from this 20m rocky shelf.

 

[Llanigraham]

I really do think Sqizzler that you need to look at the maritime and geological maps of the Lower Severn valley and the Upper Bristol Channel

 

[Midnight Sun]

There is also the matter that the Bristol Channel has the second highest tidal range in the world, only exceeded by the Bay of Fundy in Canada. Having sailed a number of times between Portshead and Sharpness on the Seven going under both bridges. In each time the channel has moved, it is constantly changing hence the requiment for a pilot. The tide caused all sorts of problem when building the second bridge.

 

[ashkeba]

There is also the matter that the Bristol Channel has the second highest tidal range in the world, only exceeded by the Bay of Fundy in Canada. Having sailed a number of times between Portshead and Sharpness on the Seven going under both bridges. In each time the channel has moved, it is constantly changing hence the requiment for a pilot. The tide caused all sorts of problem when building the second bridge.

The tide was a factor in causing the collisions that destroyed the first bridge.

 

[Midnight Sun]

The tide was a factor in causing the collisions that destroyed the first bridge.

More a case of thick fog and the failing to use a pilot therefore missing the lock to Sharpness Docks

 

[HSTEd]

Well modern bridges are much more resistant to barge strikes, by using heavier construction and tending to have much longer spans.

 

[Peter Kelford]

I would rather upgrade the Gloucester line.

 

[squizzler]

I really do think Sqizzler that you need to look at the maritime and geological maps of the Lower Severn valley and the Upper Bristol Channel

Here is a geological map of the Bristol Channel with a handy cross section of the bedrock on the proposed route between Cardiff and Weston Super-mare. Here is one that conveniently has a cross section of where the original tunnel goes. It looks like the rock is in lots of awkward layers with faults running through it, but I am not trained in digging rail tunnels. I defer to your superior knowledge of such matters and trust you can now enlighten me as to why an immersed tube tunnel (or combination of such with bored tunnel) would be deemed impractical at these locations!

 

[Llanigraham]

Here is a geological map of the Bristol Channel with a handy cross section of the bedrock on the proposed route between Cardiff and Weston Super-mare. Here is one that conveniently has a cross section of where the original tunnel goes. It looks like the rock is in lots of awkward layers with faults running through it, but I am not trained in digging rail tunnels. I defer to your superior knowledge of such matters and trust you can now enlighten me as to why an immersed tube tunnel (or combination of such with bored tunnel) would be deemed impractical at these locations!

See post #36!
And your idea of a tunnel route that far west has been criticised by others previously.

 

[Mountain Man]

In the 80s, consideration was given to an electricity-generating barrage across the Severn. This would have run from Brean Down just south of Weston to Lavernock Point near Cardiff. Obviously it hasn’t been built, but one of the selling points was a road and a railway on top of the barrage.

Every few years the barrage idea gets raised, and every time its gets dismissed because its an utterly stupid idea and would lead to an ecological disaster on an almost unprecedented scale.

 

[krus_aragon]

I would rather upgrade the Gloucester line.

It would have to be a pretty good upgrade to replace the current tunnel's travel time for Cardiff-Bristol commuters.

 

[Meerkat]

Every few years the barrage idea gets raised, and every time its gets dismissed because its an utterly stupid idea and would lead to an ecological disaster on an almost unprecedented scale.

Please expand. Green energy in exchange for some mud. You can’t have everything

 

[JamesT]

Please expand. Green energy in exchange for some mud. You can’t have everything

That mud is currently a Special Area of Conservation as it provides an ecology for many birds. Putting a barrage in is likely to completely change that environment.

 

[Meerkat]

There is a lot of mud around the country. If the energy generation potential stacked up then refusing to do it to save some mud would be crazy.

 

[Mountain Man]

Please expand. Green energy in exchange for some mud. You can’t have everything

The wetlands of the Severn Estuary are some of the most critical in the world. They are protected at a international level in particular due to their impact on wildlife, and in respect of migratory birds.

Furthermore you are interfering with vast areas of flood plains. Which would cause billions and billions of pounds in additional cost above and beyond the basic cost of the barrage.

The reality is a barrage will never ever get built, it makes no sense financially, no sense environmentally and you have more chance of getting planning permission to build a multi storey car park in Trafalgar Square than you do a Severn Barrage

The world has moved on from large scale barrages to other more effective and less damaging forms of harnessing tidal power

 

[Mountain Man]

There is a lot of mud around the country. If the energy generation potential stacked up.

It doesn't. There is no financial case at all

 

[AndrewE]

The world has moved on from large scale barrages to other more effective and less damaging forms of harnessing tidal power

It has, and I liked this proposal: https://www.kcet.org/redefine/belgium-may-build-hole-in-the-ocean-to-store-energy

A Belgian politician is proposing an unusual way to store intermittent renewable power from the nation's offshore wind turbines: build a hole in the ocean, then let it fill up to generate electricity. Johan Vande Lanotte, Belgium's Minister of Economy, Consumer Affairs and the North Sea, is proposing his government build a nearly two-mile-wide island off the coast of Flanders with a seawater reservoir to store surplus energy from Belgian offshore wind turbines for later use.

Despite the grand scale, the concept is fairly simple. The island would be atoll-shaped, with a 100-foot-deep lined reservoir in the middle, and a seagate with generating turbines connecting the reservoir to the open sea. When the wind is blowing, excess power from wind turbines would be used to pump water out of the reservoir into the ocean. When the wind dies down and extra power is needed, the seagate would be opened to allow seawater to flow back into the reservoir, generating electricity as it flows.

I don't know if it went anywhere though.

 

[squizzler]

See post #36!

To quote the relevant post:

There is also the matter that the Bristol Channel has the second highest tidal range in the world, only exceeded by the Bay of Fundy in Canada. Having sailed a number of times between Portshead and Sharpness on the Seven going under both bridges. In each time the channel has moved, it is constantly changing hence the requiment for a pilot. The tide caused all sorts of problem when building the second bridge.

For a supposed moderator, you seem very keen to shut down what I feel is legitimate on-topic discussion. Firstly you tell me that if only I look at a map of the rocks it would be clear why immersed tube tunnel technology is impractical. I found maps and invited you to point out the show-stoppers. Rather than do such, you now claim another poster's mention of tidal ranges is proof that such a building method is impractical. Again, I welcome constructive speculation as to why this should be outside the bounds of modern engineering. With respect to the Marmaray tunnel it might interest you that the Bosporus has currents that reach 7 - 8 knots. As I understand from this page, the currents are dependent on weather conditions rather than a nice predictable tide table.

This is the speculative part of the forum where speculation (and constructive rebuttal thereof) should be encouraged.

And your idea of a tunnel route that far west has been criticised by others previously.

As a moderator whose role is to facilitate discussion, might I suggest a link to this criticism be provided so others can follow the argument? As I have floated this idea in previous discussions on High Speed Rail you might be thinking of a line of argument those who follow this thread might be unaware.

 

[Peter Kelford]

Why not use the existing pillars of the M4 or M48 bridge and strengthen them to take a severn double-decker bridge like the Britannia in Wales?

 

[krus_aragon]

have more chance of getting planning permission to build a multi storey car park in Trafalgar Square than you do a Severn Barrage

That'd be fun: you could drive your car to the top and declare "I see no spaces"!*

*Granted, he said that at Copenhagen rather than Trafalgar.

 

[Llanigraham]

To quote the relevant post:
For a supposed moderator, you seem very keen to shut down what I feel is legitimate on-topic discussion. Firstly you tell me that if only I look at a map of the rocks it would be clear why immersed tube tunnel technology is impractical. I found maps and invited you to point out the show-stoppers. Rather than do such, you now claim another poster's mention of tidal ranges is proof that such a building method is impractical. Again, I welcome constructive speculation as to why this should be outside the bounds of modern engineering. With respect to the Marmaray tunnel it might interest you that the Bosporus has currents that reach 7 - 8 knots. As I understand from this page, the currents are dependent on weather conditions rather than a nice predictable tide table.

This is the speculative part of the forum where speculation (and constructive rebuttal thereof) should be encouraged.

As a moderator whose role is to facilitate discussion, might I suggest a link to this criticism be provided so others can follow the argument? As I have floated this idea in previous discussions on High Speed Rail you might be thinking of a line of argument those who follow this thread might be unaware.

Squizzler,
I do not know where you get the idea that I am a moderator from, but you are totally wrong on that matter.

The height of the tides and the geology of the Severn Valley and the Bristol Channel is completely relevant to your "idea" of a sunken tube tunnel, as has been pointed out, and I fail to see what the speed of currents has to do with it. However I can assure you that tidal speeds in the Severn Estuary can easily exceed 8 knots!

If you wish to make speculative suggestions then surely you also have to expect responses that are contrary to your ideas?

 

[PartyOperator]

There's currently a project to bore three tunnels under the Bristol Channel from Hinkley Point, two of them to 3.5km from the shore (6m diameter). Progress here may give some indication of the challenges (they started boring about a month ago so not much news yet).

I note that the last nuclear power station built in the UK used immersed tube tunnels - presumably they considered this option before deciding on bored tunnels.

https://www.edfenergy.com/energy/nuclear-new-build-projects/hinkley-point-c/about/tunnel-boring

 

[ashkeba]

[...] you have more chance of getting planning permission to build a multi storey car park in Trafalgar Square than you do a Severn Barrage

Plans are ready!

From https://londonist.com/2011/11/unbuilt-london-the-transport-schemes-that-never-were

 

[HSTEd]

Every few years the barrage idea gets raised, and every time its gets dismissed because its an utterly stupid idea and would lead to an ecological disaster on an almost unprecedented scale.

It's not really a stupid idea.
The economics depend on what interest rate you assume on the capital cost, and if the Government was to actually use it's own cost of borrowing as the yardstick, it would become extremely economic given its extremely long lifespan and the negative real interest rate of Government debt.
But thanks to the madness of Thatcherism......

As to the ecological argument, I'm afraid all the intertidal wetlands that might be affected - and that is less than you might think with a modern dual generation barrage - are already doomed.
Those intertidal wetlands will largely be drowned within the next century or two anyway by rising sea levels.
The net primary production of most of the Severn Estuary is nearly zero - which means its largely dead, due to the huge amount of silt that is moved around by tidal forces. A barrage would alter the environment behind it, but as experience from the Rance project has demonstrated, it would be a different environment but not really a worse one by most definitions.

The world has moved on from large scale barrages to other more effective and less damaging forms of harnessing tidal power

More effective at throwing banknotes on a fire perhaps.
Tidal stream is never going to go anywhere, it costs hundreds of pounds per megawatt hour, and has huge maintenance costs that it can't escape.

 

[Mountain Man]

It's not really a stupid idea.
The economics depend on what interest rate you assume on the capital cost, and if the Government was to actually use it's own cost of borrowing as the yardstick, it would become extremely economic given its extremely long lifespan and the negative real interest rate of Government debt.
But thanks to the madness of Thatcherism......

As to the ecological argument, I'm afraid all the intertidal wetlands that might be affected - and that is less than you might think with a modern dual generation barrage - are already doomed.
Those intertidal wetlands will largely be drowned within the next century or two anyway by rising sea levels.
The net primary production of most of the Severn Estuary is nearly zero - which means its largely dead, due to the huge amount of silt that is moved around by tidal forces. A barrage would alter the environment behind it, but as experience from the Rance project has demonstrated, it would be a different environment but not really a worse one by most definitions.



More effective at throwing banknotes on a fire perhaps.
Tidal stream is never going to go anywhere, it costs hundreds of pounds per megawatt hour, and has huge maintenance costs that it can't escape.

The economics don't stack up when you consider the additional costs beyond the barrage itself. Building the barrage itself is the cheap bit and used in headline grabbing statements. The total cost is far far higher and would take into account the damage to the flood plain and through life costs

The business case never stacked up, and certainly doesn't now given costs of alternatives are falling

 

[Bald Rick]

The economics don't stack up when you consider the additional costs beyond the barrage itself. Building the barrage itself is the cheap bit and used in headline grabbing statements. The total cost is far far higher and would take into account the damage to the flood plain and through life costs

Not least the cost of getting the electricity in to the grid. At peak output of 8MW, that is a lot of transmission. Then the issue of what to do with the power when demand is low - something not unique to tidal of course (it is becoming increasingly an issue with wind power), however with all that power being generated in one place, some of the potential storage needs to be nearby. And it can’t all go to Dinorwic!

 

[The Ham]

Those intertidal wetlands will largely be drowned within the next century or two anyway by rising sea levels.

There's a big difference between significantly reducing the amount of mud over the course of a few years and a couple of centuries