Why is a bridge to Portishead from the Severn Beach Line a silly idea?
I guess I have assumed it must be a silly idea because it's not something I've seen suggested anywhere, and I can spot some immediate problems. But there there are almost inevitably initial problems that need to be addressed with any project. What can I see ...
1a. It would be a low level crossing of a navigable waterway so would require some sort of swing or lifting section that allowed vessel clearance of at least 20m ... and as the bridge would be on an angle, that would mean, say, a 40m span that could be taken out. That can be done - Tower Bridge in London has 2 x 30m Bascules and is over 120 years old - see
https://www.bristol.ac.uk/civilengineering/bridges/Pages/NotableBridges/TowerBridge.html and note a credit to Ismbard Kingdom Brunel's son on in the design. To get a measure of ship / boat size, the big lock into the floating harbour takes a vessel up to 14m wide and the PS Waverley has a beam of 17.45m (can turn outside the floating harbour).
1b. A lift / swing bridge would require rail traffic to be interrupted while a boat went through and it's conceivable this might require a service to be cancelled. Probably need to build that limitation into the spec somehow / have an alternative "boat passing" timetable / contingenty plan so that the lines to Avonmouth don't get jammed with a train waiting to go to Portishead ... and so that
TOC▸ delay / repay systems don't end up paying all the fares back out when a boat pass!
1c. I would hope the bridge was around 6 metres above mean tide high water mark to minimise the need for swings - that's the same standard as the Plimsoll bridge. However, bigger vessels would typically be at the Cumberland Basin at high tide, and when they pass the Sea Mills bridge they'll be somewhat off peak tide.
2. The tidal range is enormous, the mud deep ... a pretty hostile build environment. Note that other bridges downstream from Bristol do NOT have supports in the water (probably for very good reason) but I suspect this one would probably need such supports because of the opening nature of the bridge.
3. Cost ... but then how does that compare to all the engineering works in the gorge?
4.
JFDI▸ ... we have spent so long looking at options for Portishead we just need to get on and do it and not start throwing other options into the pot (silly me!). Someone will be suggesting tram trains next!
Against those issues (and please, tell me if you can see more), you have what looks like a pretty logical route - going where people are likely to want (needs checking) and removing all sorts of other engineering and environmental issues. With a redoubled Severn Beach line to Sea Mills Junction, you're set for a 15 minutes service all stations Temple Meads (or Bedminster??) to Clifton Down, with alternate trains on for Portishead and for The Beach.
So ... I have raised six paragraphs of issues ... and a single paragraph of why it should be done.
Silly idea.Added while I as typing ...
Why is a bridge to Portishead from the Severn Beach Line a silly idea?
The Avon is part of the Port of Bristol, and large ships have a right of navigation as far as the harbour entrance. The current "air draught" under Avonmouth bridge is about 30 m at high tide, so any new bridge would need to be as high, movable, or its lower height agreed by the port company.
Yep, my item 1 of 4
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