A new Severn tunnel - or other crossing?

[whole diary]

[Celestial]

Talking to someone who regularly travels to Bridgend from Bristol Parkway they said that in conversation with staff at BPW^» (Bristol Parkway - next trains) the staff are commenting that in the long run it might be cheaper to bore a second Severn Tunnel than to keep spending millions each year TRYING to maintain the present tunnel.

Going back to the comment that started it, I do wonder how much "in the know" staff at Bristol Parkway are as to the engineering challenges of the Severn Tunnel and possible alternatives, and as those comments have come third hand via a commuter to a member of the forum, (so we can't know the context in which they were made), I'm not sure I would pay much attention to them.

[Oxonhutch]

Just looking up the geology of the Severn Tunnel which I am finding a bit scarce at the moment.  The big problem was/is the Great Spring and the huge volumes of fresh (not estuarine) water that it produces.  Off to the library tomorrow to consult some old tomes but if Stuving et al. has access to the Proceedings of the ICE, Haswell (1973), v. 54 (3), pp. 451-486 - https://doi.org/10.1680/iicep.1973.4855 might make an interesting read. A relatively modern HVAC power tunnel that might have been properly mapped. It had water problems too.

[stuving]

Just looking up the geology of the Severn Tunnel which I am finding a bit scarce at the moment.  The big problem was/is the Great Spring and the huge volumes of fresh (not estuarine) water that it produces.  Off to the library tomorrow to consult some old tomes but if Stuving et al. has access to the Proceedings of the ICE, Haswell (1973), v. 54 (3), pp. 451-486 - https://doi.org/10.1680/iicep.1973.4855 might make an interesting read. A relatively modern HVAC power tunnel that might have been properly mapped. It had water problems too.

While the ICE are helpful only to students, it seems, I've found other things in the cyber-stuffheap. Like a short IEEE piece written in 1970 during the work - which strangely says nothing about springs. It does, however, lament that they (CEGB) were refused an overhead link at Sharpness on amenity grounds - "a somewhat surprising decision decision in view of the fact that a 275kV overhead crossing has existef for some years, and will continue to exist" (which of course it still does). That meant a four-year delay and a cost of £3.5M rather than £1.5M (1970 prices).

The two 400kV circuits in the tunnel were designed for a 2.6GVA rating but to be operated at half power initially. The big issue was cooling - they were to be put in PVC pipes and water-cooled via heat exchangers at the surface. Ironic that they are pumping 63 m³/hr of water out the tunnel, while struggling to get enough water down the pipes to cool the cables - apparently.

National Grid would like a new link along that route anyway, and in planning for Hinckley Point C will need several more new and uprated links locally. A (now rather old) report on that says a new cross-Severn link is needed just to reach 2.2GVA per circuit, and if new Hinckley feeds in via Aberthaw (one of the options, though discounted) even more. There are plans for new grid stations at Aust and Iron Acton, and possible Tockngton.

I wonder if they'd get approval for a new high-level Severn crossing if the cables hung off the pylons of a new railway bridge...

[Oxonhutch]

Back from a little trip down the library to pull some papers on the geology of the tunnel with a view to looking at a second bore under the estuary. The original report by Charles Richarson is available online for you to read: the University of Michigan’s long out of copyright reproduction of the Proceedings of the Bristol Naturalists’ Society of 1887 followed by Lloyd Morgan’s description of the geological section: pp. 82ff. Click on the page to turn over.

The coloured sections to which both articles importantly refer are missing from the archive scan, but not from the Bod’s copy from which I have reproduced below for your reference. Richardson’s paper is a fascinating to read. Raise your cap to the courage of diver Lambert who sealed off two devastating breaches – one with an experimental and novel diving suit – nerves of Victorian steel!

From the Haswell 1973 ICE paper on the HVAC tunnel, very similar problems with excessive water intake despite pre-treating the ground ahead of the face: in excess of 1000 gallons per minute continuously over the majority of the excavation (Figs 7 & 12 referenced and reproduced under private research fair use).

The Severn Valley is extensively faulted within variable but often hard Carboniferous and Triassic rocks – a nightmare for tunnelling even with modern methods. Compare that to the clay rich and impervious, but soft, chalk of the Lower Cretaceous in which the Channel Tunnel was bored and the equally workable Eocene London Clay of Crossrail.

All the geological information to hand points to a bridge or barrage if a second rail crossing is required.  Much easier to engineer and much cheaper too if I were to hazard a guess.

[broadgage]

A Severn barrage has long been proposed in order to generate electric power from tidal flows. If this could be combined with a new rail crossing, then the idea has considerable merit.

With pedestrian and cycle access it could become an attraction in its own right, for sightseeing, exercise and angling.

[Rhydgaled]

Two tidal barrages with railway atop. Railway on rising inclines from each shore, with a bridge in the centre of the channel.

A bridge in the centre? So not actually a barrage spanning the whole river? Interesting, would that generate much power and would it avoid the loss of mudflat habitats that a full barrage would cause?

[ellendune]

Two tidal barrages with railway atop. Railway on rising inclines from each shore, with a bridge in the centre of the channel.

A bridge in the centre? So not actually a barrage spanning the whole river? Interesting, would that generate much power and would it avoid the loss of mudflat habitats that a full barrage would cause?

There was always intended to be a navigation channel through the Severn Barrage, IIRC^▸ (if I recall/remember/read correctly) it would have lock gates.  There would need to be a bridge over it.  That said a small gap in the barrage would still allow it to function as there is a very high tidal flow. 

[broadgage]

I doubt that a barrage with an opening in the middle would work.
The tidal flow through the opening would be vast and liable to scour away the riverbed under or near the foundations.
An opening closed by lock gates should be fine.

[grahame]

Following discussions on the previous Severn Rail Bridge, I was interested to come across a BBC^» (British Broadcasting Corporation - home page) article at https://www.bbc.co.uk/news/uk-scotland-south-scotland-49675863 describing the "Solway Junction Railway: The ill-fated Scotland to England rail route" which included a long viaduct that proved the line's downfall.

[johnneyw]

From Gloucestershire Live.  It seems a bit fanciful to me.

https://www.gloucestershirelive.co.uk/news/gloucester-news/another-bridge-over-river-severn-3389047

The article implies this can be looked at as a regional project although it is admitted that no one has any idea where the money would come from.

[broadgage]

I again met a Chartered engineer whom has previously spoken in favour of a new combined Severn rail crossing and tidal barrage.

They favour a very substantial structure that is largely a solid causeway with a double track high speed railway atop this. Steadily rising towards the middle which will be a bridge of sufficient height to permit passage of ocean going ships under the bridge and through the locks installed for this purpose.

A hugely expensive project but with two major benefits, a new rail crossing AND a large tidal power generating plant.
A road would provide opportunities for walking, cycling, sightseeing, and angling. Suitable for ambulances and fire engines in case of emergency, but not open to road traffic ordinarily.

This and similar schemes have been previously considered, but are worthy of considering again for several reasons.
Public opinion is moving way from fossil fuels for both electricity generation and road transport.
The current much increased cost of natural gas has tilted the economics towards renewable generation of electric power and away from gas.

[CyclingSid]

A tunnel normally finds favour with our most esteemed leader.

[Surrey 455]

A tunnel normally finds favour with our most esteemed leader.

Oh, I thought he preferred bridges. 

• Garden bridge over the Thames
• Bridge between Scotland and Northern Ireland

[Western Pathfinder]

Better yet a bridge between Shirehampton and Pill for the PortZ railway.

[broadgage]

Our leader may favour tunnels, but I suspect that both engineers and health and safety consultants would favour a bridge, causeway, viaduct or similar crossing.

Long tunnels are inherently vulnerable to fire.
Tunnels have ongoing energy use.
A bridge or causeway can be combined with tidal or wind power equipment.

Ans finally I suspect that most passengers would prefer a sea view to a tunnel.