Major breach at Whitchurch on Llangollen

[magnetman]

11 minutes ago, David Mack said:

You are reading far too much into a single word, spoken by an engineer, not a PR person.

In your opinion. 

 

She is an engineer so she will use precise language. One of the essentials.

 

In that sentence she has told us what happened because she knows what happened. 

 

Anyway it's a bit pointless going on an on about it so I shall stop now.

 

 

Edited January 16 by magnetman

[ditchcrawler]

3 hours ago, magpie patrick said:

Just aeen the @CourtAboveTheCut video of the two boats being dragged out - very impressive! And kudos to Steve for getting agreement  for access. 

 

And getting a few words from the boss lady 

[Wafi]

30 minutes ago, magnetman said:

It's like an interesting circumstance because the water under usual conditions does flow down that canal but I think the canal bed is flat and the flow relies on water going over by washes.

 

So it would be difficult to capture the movement of the water without using a pump at all. 

 

If the canal bed is at exactly the same datum level on both sides then presumably a syphon would not work. The way to do it would be to maintain a higher water level on the "upstream" end and cut out an overspill then put large bore pipes in. 

In normal operation the surface of this canal (like a river) will have a continuous (if imperceptible) gradient. The height of the canal bed at different points is irrelevant, what matters is the water surface. A siphon set up between two bodies of water will act to equalise the surface level in those two bodies. You can set up the syphon by setting up pipes over the embankments; because it's a syphon you don't have to keep everything below surface water level. All you need to ensure is:

• The ends stay submerged
• The pipes are sufficiently incompressible to hold their shape wherever they're above the level of the water in the pounds (ie where they pass over the embankments) because the water in the pipe is at a lower pressure than the air outside the pipe.
• The pipes don't have any leaks above water level (which would let air in and stop the syphon working). Minor leaks below water level are ok (you just lose water)
• No part of the system is more than about 10m above surface water level (because you'll get a vacuum)
• You have an easy method of repriming the syphon if required

One way to visualise this is to imagine water pouring over the bywash at Grindley Brook. This will reduce the water level in that pound, making it lower than the pound upstream of the breach. The syphon then "sees" a gradient, and acts to equalise the levels, causing water to flow into the lower pound.

 

One caveat is that you may need quite large pipes to achieve the required flow rate. I don't know what the required flow rate is, and it's a while since I used the relevant equations; that's another factor that might make it more practical to stick with the pumps.

[magnetman]

The bloke in the video says the piling is 'fine' so why did the rest of the Shropshire Union have sloped stones at the edge on embankments? Was this just for fun. Would Telford have used 8ft piling if he had it? No. 

 

It's obvious why the stones are better. They are not vertical. They absorb wash. A vertical surface gets a slap effect (yes horse drawn vessels do move water)., Sloped stones do not provide a vertical surface for water to creep behind and go under.

 

 

They don't separate the canal from the bank thereby allowing the bank to dry out

 

Short piling breaks the structure and adds weight to the top. Gravity. 

 

 

An embankment is a live structure it needs to be able to move as the seasons change. Water weeping through the stones is not a problem. Differential drying on one side of a vertical metal blade is a serious problem.

 

Sloping stones also prevent vessels from mooring which is a bonus, 

 

When this embankment is rebuilt it will not be piled up earth with 8ft long sheet piling. It will be with 30ft Larssen piles because that is what is used. 

 

The 8ft piles are cosmetic and harmful. 

 

 

 

 

 

 

3 minutes ago, Wafi said:

 

One way to visualise this is to imagine water pouring over the bywash at Grindley Brook. This will reduce the water level in that pound, making it lower than the pound upstream of the breach. The syphon then "sees" a gradient, and acts to equalise the levels, causing water to flow into the lower pound.

 

 

This is what I said but in a slightly different way. 

54 minutes ago, magnetman said:

 

 

So it would be difficult to capture the movement of the water.

 

If the canal bed is at exactly the same datum level on both sides then presumably a syphon would not work. The way to do it would be to maintain a higher water level on the "upstream" end and cut out an overspill then put large bore pipes in. 

 

Pumps probably easier. 

 

It would be nice to have a little aquaduct. 

 

We are actually staying exactly the same thing it's just I said it in a different way. 

 

 

I was visualising it rather than using maths.

[BoatingLifeUpNorth2]

Just watched his Vlog, have a watch at about 11:10 into it where he mentions Seftons History, He says he’s been contacted and it’s only 1 of 2 boats built by the builder, the other is in a museum. Looking at CaRT’s boat listing it was built by Mike Hayward with a 22hp engine. Is he getting his wires crossed and talking about the engine fitted, as painted on the side is Ruston & Hornsby?

 

 

 

Edited January 16 by BoatingLifeUpNorth2

[magnetman]

Of course the syphon would work but you also think about practicality and where you actually want the pipes. It makes more sense to have an overspill constructed for the purpose so the flow would just be differential height rather than at any point needing to get the pipes over a bank. Buried pipes and a cutout.

 

Just keep the top pound at a higher level and cut out an overspill then connect sensible sized pies to it. 

 

Which is what I suggested

2 minutes ago, BoatingLifeUpNorth2 said:

Just watched his Vlog, have a watch at about 11:10 into it where he mentions Seftons History, He says he’s been contacted and it’s only 1 of 2 boats built by the builder, the other is in a museum. Looking at CaRT’s boat listing it was built by Mike Hayward with a 22hp engine. Is he getting his wires crossed and talking about the engine fitted, as painted on the side is Ruston & Hornsby?

 

 

 

Mike Heywood, Evans built a lot of narrowboats.

[MtB]

1 hour ago, Wafi said:

An inverted syphon, for instance,

 

What's one of them please?

 

From the way you write about later, I think you mean a plain "syphon".

 

 

[BoatingLifeUpNorth2]

4 minutes ago, magnetman said:

 

Which is what I suggested

Mike Heywood, Evans built a lot of narrowboats.

I know but watch the Video and you’ll see what I mean. Is the engine rare?

[IanD]

2 minutes ago, MtB said:

 

What's one of them please?

 

From the way you write about later, I think you mean a plain "syphon".

 

A syphon goes up from the liquid level and then back down again, an inverted syphon -- this one, for example -- goes down and then back up again.

[Chris Lowe]

15 hours ago, Momac said:

Consultation with the relevant local authority planning department should be part of this process. 

don't let the Council anywhere near it, some pillocks would turn it down

[magnetman]

1 hour ago, magnetman said:

 

It would be nice to have a little aquaduct. 

 

There will be a surplus of water which needs dealing with if navigation at New Marton locks is to be allowed during the closure.

[Wafi]

4 minutes ago, MtB said:

What's one of them please?

 

From the way you write about later, I think you mean a plain "syphon".

I guess I really meant a combination of the two: A regular syphon (per @IanD's succinct description) to get it over either embankment, and an inverted syphon to cross the valley between. An inverted syphon on its own would do the job, and be immune from priming issues, but would require the embankments to be breached; the thinking behind my "up/down/up" impementation is that it could be non-invasive.

 

I was going to apologise for mis-spelling "syphon" but it looks like "siphon" is also valid, so I got away with it!

[magnetman]

So basically you lower the weir level at the next lock down and you put in a bywash immediately above the breach with a slightly higher weir level.

 

But it needs to be off good capacity so an aqueduct would be nice.

 

Unlikely though.

2 minutes ago, Wafi said:

... embankments to be breached; the thinking behind my "up/down/up" impementation is that it could be non-invasive.

 

Good point. It is not an embankment where the spider 360 went into the canal. Is it an embankment where the lift bridge is? 

[David Mack]

14 minutes ago, magnetman said:

The bloke in the video says the piling is 'fine' so why did the rest of the Shropshire Union have sloped stones at the edge on embankments? Was this just for fun. Would Telford have used 8ft piling if he had it? No. 

Probably yes. Lots of canals had banks protected by wooden piling, the older equivalent of today's piling. But because of timber's limited life, not much remains today.

 

17 minutes ago, magnetman said:

Sloped stones do not provide a vertical surface for water to creep behind and go under

No. The water can go straight through between the stones.

 

19 minutes ago, magnetman said:

Sloping stones also prevent vessels from mooring which is a bonus, 

Except this is an area which is designated for mooring - the towpath side is the mooring for the lift bridge operation and the offside is signed as a visitor mooring.

 

Since the canals exist as a leisure facility, places for boats to moor are an intrinsic requirement. If not here and similar locations within walking distance of canalside towns and villages, where should such moorings be provided? And how would you provide an adequate depth for boats to moor adjacent to the bank and a reasonable towpath surface alongside, at reasonable cost, without using light gauge steel piling?

[magnetman]

Top end

 

[David Mack]

22 minutes ago, BoatingLifeUpNorth2 said:

Just watched his Vlog, have a watch at about 11:10 into it where he mentions Seftons History, He says he’s been contacted and it’s only 1 of 2 boats built by the builder, the other is in a museum. Looking at CaRT’s boat listing it was built by Mike Hayward with a 22hp engine. Is he getting his wires crossed and talking about the engine fitted, as painted on the side is Ruston & Hornsby?

Somebody is confusing working boats built at Sephton's boatyard at Hawkesbury junction with a modern leisure boat built by Mike Hayward called Sefton!

[Wafi]

2 minutes ago, magnetman said:

So basically you lower the weir level at the next lock down and you put in a bywash immediately above the breach with a slightly higher weir level. 

You don't need to do any of that. Before the breach happened, the water level was already higher just upstream of the breach site than it was just downstream of the breach site; that needed to be the case for the water to flow. The height difference may have been imperceptible, but it was there. That's all that's required for a syphon to work; the syphon just replaces the navigable channel with a slightly more convoluted route, but the mechanics driving the flow are the same.

[magnetman]

3 minutes ago, David Mack said:

Probably yes. Lots of canals had banks protected by wooden piling, the older equivalent of today's piling. But because of timber's limited life, not much remains today.

 

No. The water can go straight through between the stones.

 

Except this is an area which is designated for mooring - the towpath side is the mooring for the lift bridge operation and the offside is signed as a visitor mooring.

 

Since the canals exist as a leisure facility, places for boats to moor are an intrinsic requirement. If not here and similar locations within walking distance of canalside towns and villages, where should such moorings be provided? And how would you provide an adequate depth for boats to moor adjacent to the bank and a reasonable towpath surface alongside, at reasonable cost, without using light gauge steel piling?

 

Fair point about practicality for leisure use. 

 

So how come the other embankments on the Shroppie are still up? The ones with the stones. Water seepage doesn't matter if it doesn't have an 8ft vertical metal blade to travel down. If it has this path it will follow it. This is what destroyed this embankment. 

 

There should be no mooring on embankments unless constructed with piling which reaches the natural ground level.

 

This kind of thing will happen again. 

 

 

2 minutes ago, Wafi said:

You don't need to do any of that. Before the breach happened, the water level was already higher just upstream of the breach site than it was just downstream of the breach site; that needed to be the case for the water to flow. The height difference may have been imperceptible, but it was there. That's all that's required for a syphon to work; the syphon just replaces the navigable channel with a slightly more convoluted route, but the mechanics driving the flow are the same.

Yes. Add to that the excess water building up anyway and the level difference would be even greater so you are right none of that is needed. 

[David Mack]

1 hour ago, Wafi said:

You can set up the syphon by setting up pipes over the embankments; because it's a syphon you don't have to keep everything below surface water level.

Indeed you can. But the flow through a syphon is dependent on the difference in head (water level) at each end, and the size of the pipe(s) used. At this location the water levels at each end are nominally equal, although in practice the isolated Whitchurch - Grindley Brook section is probably a little lower. That means the pressure head to drive water through the pipes will be minimal. And since the combined cross sectional area of the pipes is far smaller than the canal channel, the flow rate is much much less. Hence the need for pumps to push water through the pipes.

Edited January 16 by David Mack

[Wafi]

6 minutes ago, David Mack said:

And how would you provide an adequate depth for boats to moor adjacent to the bank and a reasonable towpath surface alongside, at reasonable cost, without using light gauge steel piling?

I suppose you could build a concrete channel as used on the Mon & Brec? If they do gown that route, it hope it's a bit deeper and squarer: I'm not entirely sure I could get my boat down some bits of the Mon & Brec; it would certainly be a challenge to pass another similarly-drafted boat coming the other way...

[magnetman]

10 minutes ago, David Mack said:

Indeed you can. But the flow through a syphon is dependent on the difference in head (water level) at each end, and the size of the pipe(s) used. At this location the water levels at each end are nominally equal, although in practice the isolated Whitchurch - Grindley Brook section is probably a little lower. That means the pressure head to druve water through the pioes will be minimal. And since the combined cross sectional area of the pipes is far smaller than the canal channel, the flow rate is much much less. Hence the need for pumps to push water through the pipes.

Or an aqueduct?

[David Mack]

5 minutes ago, magnetman said:

So how come the other embankments on the Shroppie are still up?

Because there is puddle clay behind the stone pitching, which is only there to prevent wash from passing boats (yes, horse-drawn boats would produce wash) washing out the puddle.

 

Few earth dams or other water retaining structures are completely water tight. What is needed is a water seepage pattern that doesn't carry material away - that is what leads to piping failures. Modern earth dams are designed for this, with low permeability zones of puddle clay or similar material, supported by carefully graded layers of courser more permeable material to provide support to the puddle and weight to resist the pressure of the retained water. They also have instrumentation installed to measure movement and water pressures within the dam, to detect any deviations from expected values that might be indicative of problems forming. 18th century engineers had neither the knowledge nor the equipment to do that, hence such structures were designed and built on the basis of experience. And sometimes they failed.

[magnetman]

10 minutes ago, Wafi said:

I suppose you could build a concrete channel as used on the Mon & Brec? If they do gown that route, it hope it's a bit deeper and squarer: I'm not entirely sure I could get my boat down some bits of the Mon & Brec; it would certainly be a challenge to pass another similarly-drafted boat coming the other way...

It seems most likely it's going to be 30ft thick wall deep web interlocked piles. Although that is expensive.

 

 

[Graham Davis]

20 minutes ago, magnetman said:

Top end

 

Those are not the pumps being used for the transfer of water around the site.
As stated in the video they are there to provide pumping for the possible "docking" situation,

[David Mack]

12 minutes ago, Wafi said:

I suppose you could build a concrete channel as used on the Mon & Brec?

Exactly what was done on sections of the Llangollen above Trevor, following a breach in the early 80s. The difference there (and I suspect on the Mon and Brec) is that the canal clings to the side of a fairly steep sided valley, and the design constraints were different from Whitchurch where the embankment is crossing a valley.