Grand Union Transfer Scheme

[David Mack]

25 minutes ago, IanD said:

The Project is designed to ensure that there will be no greater than 0.3M/s change in velocity of flow within the canal network.

Just to put that in context, 0.3m/s is 0.67 miles per hour. A steady current of that speed would lead to a significant difference in boat over-the-ground speed in the two directions.

27 minutes ago, IanD said:

● The adaptation of existing bridges to maintain sufficient head room for vessels

Any discussion of the heritage impact on historic bridges? In many cases the only practical adaptation would be demolition and replacement. And that ignores the impacts of raising road levels etc.

The alternative to bridge and bank raising would be to dredge the channel deeper (along with lowering weir levels). Has that been considered?

[IanD]

36 minutes ago, David Mack said:

Just to put that in context, 0.3m/s is 0.67 miles per hour. A steady current of that speed would lead to a significant difference in boat over-the-ground speed in the two directions.

Any discussion of the heritage impact on historic bridges? In many cases the only practical adaptation would be demolition and replacement. And that ignores the impacts of raising road levels etc.

The alternative to bridge and bank raising would be to dredge the channel deeper (along with lowering weir levels). Has that been considered?

 

Yes it would, but if that's the *maximum* speed in places like narrows (which is what the document implies) it'll mostly be less than that out on the open canal. What's the speed on the Llangollen, especially in the narrow bits? (quite fast from memory). And does it really matter if boats go a little bit slower one way and faster the other?

 

No discussion of historic bridges, just a handwaving "raised when necessary" -- which is why I asked whether any canal expert had been consulted.

 

Dredging wouldn't work, since they're talking about *raising* water levels to get the desired flow capacity.

Edited October 3, 2024 by IanD

[Hudds Lad]

How much wider would a bridge hole need to be?

Could they simply remove the towpath underneath, then route a path up and down each side of the bridge to maintain foot access?

[IanD]

23 minutes ago, Hudds Lad said:

How much wider would a bridge hole need to be?

Could they simply remove the towpath underneath, then route a path up and down each side of the bridge to maintain foot access?

I really don't think that this has been properly looked at, if at all -- a lot of the changes they talk about sound OK in theory but will be difficult/expensive/impossible in practice, given the historic/protected nature of large parts of the canal infrastructure.

 

Removing the towpath and routing traffic up and down over the bridges would be literally impossible in many places, and would cause problems for towpath users -- especially disabled ones, who get a lot of attention from CART... 😉 

[alias]

1 hour ago, Hudds Lad said:

Could they simply remove the towpath underneath, then route a path up and down each side of the bridge to maintain foot access?

 

I think that on one of the previous occasions that this was discussed ages ago there was a suggestion of excavating the towpath in bridge holes and replacing it with a cantilevered walkway to allow flow underneath.

[Paul C]

4 hours ago, IanD said:

 

 

Dredging wouldn't work, since they're talking about *raising* water levels to get the desired flow capacity.

 

Dredging bridgeholes would address the flowrate (in terms of water speed, not water volumetric) (but not height if the canal level were also raised).

[IanD]

Just now, Paul C said:

 

Dredging bridgeholes would address the flowrate (in terms of water speed, not water volumetric) (but not height if the canal level were also raised).

Ah, OK, understood now 🙂 

 

It's all still totally impractical though -- add up how much would have to be done to the canals, the cost would be *enormous* (always assuming the fixes were even possible), and would need multiple long closures to do the work.

[David Mack]

1 hour ago, alias said:

 

I think that on one of the previous occasions that this was discussed ages ago there was a suggestion of excavating the towpath in bridge holes and replacing it with a cantilevered walkway to allow flow underneath.

That would give an increase in width of around 1/3 (assuming 15 ft channel and 5ft towpath at present). Is that sufficient? 

It also assumes that the towpath side bridge foundations are deep enough to allow the towpath to be removed. Probably fine for more recent bridges, but I wouldn't be sure about older ones.

In any event bridges may need underpinning or otherwise protecting to prevent any scour from the increased flow washing out material below the foundations.

[Mike Todd]

On 03/10/2024 at 18:11, David Mack said:

That would give an increase in width of around 1/3 (assuming 15 ft channel and 5ft towpath at present). Is that sufficient? 

It also assumes that the towpath side bridge foundations are deep enough to allow the towpath to be removed. Probably fine for more recent bridges, but I wouldn't be sure about older ones.

In any event bridges may need underpinning or otherwise protecting to prevent any scour from the increased flow washing out material below the foundations.

Seems to work on Pontcysllte and elsewhere.

[IanD]

21 minutes ago, Mike Todd said:

Seems to work on Pontcysllte and elsewhere.

The Llangollen -- which I believe was designed as a water feeder -- transfers about 55M litres per day, which is about what this scheme says could be transferred down the GU (50M litres/day) without all the "improvements" needed to push capacity up to the 115M litres per day and make the scheme viable...

Edited October 6, 2024 by IanD

[hughc]

IT would almost be cheaper to build the Grand Contour canal.

[Mike Todd]

2 hours ago, IanD said:

The Llangollen -- which I believe was designed as a water feeder -- transfers about 55M litres per day, which is about what this scheme says could be transferred down the GU (50M litres/day) without all the "improvements" needed to push capacity up to the 115M litres per day and make the scheme viable...

I was referring to the use of a towpath suspended above the water to allow boats to move in a channel otherwise little more than the width of a boat, which is why we, at least, come to an almost standstill every time we pass a standard bridge when navigating up stream. It happens most places but is more pronounced on the Llangollen because of the stream flow, which would otherwise only be encountered on such a scale on a river.

[IanD]

10 minutes ago, Mike Todd said:

I was referring to the use of a towpath suspended above the water to allow boats to move in a channel otherwise little more than the width of a boat, which is why we, at least, come to an almost standstill every time we pass a standard bridge when navigating up stream. It happens most places but is more pronounced on the Llangollen because of the stream flow, which would otherwise only be encountered on such a scale on a river.

I know, the narrows section running up to Llangollen itself is bad for this too, if anything worse than the aqueduct (which I suspect is deeper).

 

My point was that what works there is not going to be sufficient for what is proposed for the GU with over double the flow.

 

The things they're proposing to increase flow -- widening narrow bits, raising bridges, deeper dredging, raising water levels/banks -- are just never going to happen without an enormous amount of money and manpower and closures stretching over many months all along the GU, there are just so many places that would need "fixing", and some may be pretty much impossible -- how are you going to raise a 200-year-old stone bridge without completely dismantling and rebuilding it, closing both canal and road for months?

 

I can't believe that this proposal has been vetted by anyone who understands the canals and what would really be needed to make it work... 😞

Edited October 6, 2024 by IanD

[NB Alnwick]

Our boat just loves to have a bit of a current flowing against it.

[David Mack]

3 hours ago, IanD said:

I know, the narrows section running up to Llangollen itself is bad for this too, if anything worse than the aqueduct (which I suspect is deeper).

 

My point was that what works there is not going to be sufficient for what is proposed for the GU with over double the flow.

Except the GU is (nominally) a wide canal, and the bridges are twice the width of those on the Llangollen.

[Lady M]

Some of the boats on the GU are a lot wider than those on the Llangollen.

[IanD]

16 hours ago, David Mack said:

Except the GU is (nominally) a wide canal, and the bridges are twice the width of those on the Llangollen.

Yes, but the Llangollen was intended for use as a water feeder and the GU wasn't, so has more restrictions -- possibly a lot more locks being the biggest one, but also a lot more bridges since the Llangollen is far more rural.

 

Which is why the Lllangollen and the (unimproved) GU would have similar flow rates, and all the "improvements" in the proposal are needed to get the GU flow rate up to something to make the project viable.