K&A locks left empty?

[dmr]

3 hours ago, David Mack said:

Locks on the Stratford (and probably elsewhere where the bottom end has ground paddles) had this arrangement at top and bottom ends. So any overflowing water passed through the lock, and locks left empty (with the gates closed) would tend to refill. Which may be annoying for someone travelling uphill, but does save water and keep both sets of gates wet.

 

BW added conventional bywashes, some of which are on the towpath side, presumably for reasons of access and land availability.

 

Some of the K&A bywashes were added quite recently, mostly in the Bedwyn to Froxfield area. These were all on the towpath side and not very well done as sometimes they exit right in the middle of the locklanding.

I believe they were done, or at least financed, by the EA, something to do with improving water quality on the Kennet though I don't quite understand the connection.

The overflows above the top paddles are sometimes known locally as"letter boxes". I believe there is a similar scheme on some Rochdale????

[Pluto]

On 28/08/2021 at 10:31, Loddon said:

Going back to the locks:   

GU and K&A locks were built with an "internal" bywash  this worked by using the top paddle chambers to pass water into the lock and over the bottom gates which were slightly lower than the top of the chambers this set the weir level on the pound above.

However

Due to some incompetence by BW bottom gates were built higher than the originals, this meant that it raised the level in the pound above. One way round this was to leave the bottom paddles up when exiting the lock.

 

It used to be a common sight on the lower GU to see water cascading over the bottom gates as the bywash was working as intended. Also you could pass a boat that was going downhill only to find that the lock has refilled itself by the time you arrived😟

 

Those canals built under Rennie, such as the Rochdale, Lancaster and Royal in Ireland, tend to have the air hole system, where excess water passes over a weir built above the upper ground paddle. The top of the upper gate needs to be marginally lower than the top of the bottom gate. Excess water flows into the paddle culvert and thus into the chamber, which it fills and then pours over the lower gate so that it does not dry out. If the levels of the top of the gates are incorrect, they are very difficult to open, as happens in Manchester. Only the early locks on the Rochdale have this system as the air hole is easily blocked, so the later locks have by-washes. The photos show one of the Tewitfield locks, a Rochdale lock, and an 1820 drawing of a Royal Canal ground paddle.

[David Schweizer]

Back in the 1990's many of the gates on the K&A were made at Devizes, and I was told by one of their workers that the reason that the locks needed to be left empty was that the bottom gates were a bit lightweight, and risked failing under water pressure if the lock was left full and the top gates were left open. 

[David Mack]

11 minutes ago, David Schweizer said:

Back in the 1990's many of the gates on the K&A were made at Devizes, and I was told by one of their workers that the reason that the locks needed to be left empty was that the bottom gates were a bit lightweight, and risked failing under water pressure if the lock was left full and the top gates were left open. 

Surely they are no more at risk of failing when the lock is full but not being used that when it is in use. Indeed the worst case must be when a descending boat fails to stop before reaching the bottom gates - full water pressure and impact loading.

[David Schweizer]

19 minutes ago, David Mack said:

Surely they are no more at risk of failing when the lock is full but not being used that when it is in use. Indeed the worst case must be when a descending boat fails to stop before reaching the bottom gates - full water pressure and impact loading.

 

Many of the gates on the K&A had a tendancy to swing open on their own account, which I remember as a pain when working single handed. When that happened, the pressure on the bottom gates came from several miles head of water rather than a mere 75 feet. hence the requirement to leave the lock empty.

[GUMPY]

10 minutes ago, David Schweizer said:

 

Many of the gates on the K&A had a tendancy to swing open on their own account, which I remember as a pain when working single handed. When that happened, the pressure on the bottom gates came from several miles head of water rather than a mere 75 feet. hence the requirement to leave the lock empty.

I may be wrong but the pressure on  the  gate is due to the depth of the water not the length of the pound so 7 ft, 70ft or 700ft length makes no difference.

Edited August 30, 2021 by Loddon

[Marbling]

I recall having a lot of trouble getting through some of the K&A locks at the eastern end following heavy rainfall. As it was 20 years ago I don't remember whether it was water going over the bottom gates making the lock impossible to fill sufficiently to get the top gates open, or the other way round - water flowing over the top gates making it difficult to empty the lock.

[MtB]

42 minutes ago, Loddon said:

I may be wrong but the pressure on  the  gate is due to the depth of the water not the length of the pound so 7 ft, 70ft or 700ft length makes no difference.

 

This is correct.

 

But in extremis, I find it difficult intuitively to accept the pressure on the gate would still be the same if the length of the pound was reduced to say, 0.7mm. But fluid dynamics tells us yes it would. 

[David Schweizer]

15 hours ago, Loddon said:

I may be wrong but the pressure on  the  gate is due to the depth of the water not the length of the pound so 7 ft, 70ft or 700ft length makes no difference.

 Both actually, although the length of the body of water has a decreasing impact depending upon the depth.

[IanD]

9 minutes ago, David Schweizer said:

 Both actually, although the length of the body of water has a decreasing impact depending upon the depth.

Not according to my mechanics textbooks...

[furnessvale]

On 27/08/2021 at 17:20, blackrose said:

I don't think it matters whether the lock is left full or empty at that location, but the gates shouldn't have been left open.

 

Leaving them open might help someone coming in the opposite direction, but not someone following.

On the other hand, leaving them closed hinders someone coming from either direction! 😃

 

I'll get my popcorn!

[magnetman]

On 28/08/2021 at 10:31, Loddon said:

Going back to the locks:   

GU and K&A locks were built with an "internal" bywash  this worked by using the top paddle chambers to pass water into the lock and over the bottom gates which were slightly lower than the top of the chambers this set the weir level on the pound above.

However

Due to some incompetence by BW bottom gates were built higher than the originals, this meant that it raised the level in the pound above. One way round this was to leave the bottom paddles up when exiting the lock.

 

It used to be a common sight on the lower GU to see water cascading over the bottom gates as the bywash was working as intended. Also you could pass a boat that was going downhill only to find that the lock has refilled itself by the time you arrived😟

 

Some of the GU locks also have windows in the paddles. Or at least they used to when the paddles were made of wood. 8 inch square hole in the board. 

 

Stockers lock has windows because Springwell is a deeper lock and there would be a net loss of water if nothing was moving through the lock. 

 

A lock that definitely should have windows is the top of Sarah's 2 in Watford. That short pound used to be terrible for going low. Windows would sort it out. 

 

Not been on the cut there for years now so it may have been sorted out or perhaps a paddle clipped slightly open. 

 

I'm pretty sure Aynho Weir lock has windows and quite big ones at that.  

 

 

 

 

Edited August 31, 2021 by magnetman

[GUMPY]

3 minutes ago, magnetman said:

Stockers lock has windows because Springwell is a deeper lock and there would be a net loss of water. 

Stockers is river fed, Springwell is not. 

[magnetman]

26 minutes ago, furnessvale said:

On the other hand, leaving them closed hinders someone coming from either direction! 😃

 

I'll get my popcorn!

 

Closing all gates is a great way to hide lack of maintenance. 

 

Locks should be maintained properly. Leave gates open = problems directly related to basic maintenance. People will start complaining about dry pounds. Maybe if people complain something gets done? 

 

Close the gates = less problems but there is still a lack of maintenance. Nobody complains then the whole thing falls apart. 

 

Asking people to close gates after use was a BW scam to attempt to cover up the fact they were reducing the maintenance and also a way to get some conflict going with users who conveniently don't notice the maintenance issues because they are too busy having a go at each other. 

 

Anyone who can't deal with arranging for their boat to go through the lock if the gates at the other end are left open needs to consider another activity !

 

 

 

6 minutes ago, Loddon said:

Stockers is river fed, Springwell is not. 

There is a river section above stockers lock yes but the level of the waterway in normal conditions is quite low compared with a normal canal section and there is no bywash. 

 

Springwell is a deeper lock therefore uses more water. There would be a net loss of water in the pound below Stockers without a feed of some sort. 

 

So you cut windows in one top and one bottom paddle and provide a constant unchanging flow of water through Stockers lock in any conditions when lock is not in use. Gates open or closed either end same feed as long as the window is not blocked with debris. It will clean itself when the paddle is operated. A nice simple arrangement no drama no fuss and very easy to implement. 

 

Springwell is in effect river fed because of the windows. There is a sluice on the towpath side half way between the locks to deal with high level problems. 

Edited August 31, 2021 by magnetman

[GUMPY]

What I meant by stating river fed us that there is a decent water supply..........to enable the pound below to be kept "on weir" 

The "normal canal section" is to prevent flooding 😱

[MtB]

1 hour ago, David Schweizer said:

 Both actually, although the length of the body of water has a decreasing impact depending upon the depth.

 

Can you cite any technical explanation for that please David? 

 

I don't think there is one, because water pressure doesn't work like that. 

[David Schweizer]

1 hour ago, MtB said:

 

Can you cite any technical explanation for that please David? 

 

I don't think there is one, because water pressure doesn't work like that. 

 

There is one, but it is difficult to explain, and there is a very complicated formula to calculate the pressure of water against a wall (or lock gate), but basicly, as I understand it,  the pressure increases from the top to the bottom in a triangular form. The length of the body of water will determine the maximum pressure when measured against the height, and if the base of the triangle is truncated by another wall (or lock gate) at the other end it will increase pressure from the other end (I think). This video explains it, but I challenge anyone to give a precis of the theory in layman's terms!

 

Having looked at the video, I accept  that a long pound will not have any significant (if any) impact upon the pressure against the bottom gates, but leaving the lock empty or full will.

 

 

 

 

Edited August 31, 2021 by David Schweizer

[Momac]

Locks should be left full , or filling when left .

The reasons are 

1. If someone eg drunk person, falls in it is less far to fall and easier to get out.

2. The water pressure from a full lock is greater than the retained earth pressure . Therefore for part of the time   countering the long term creep deflection of the lock walls due to the retained earth pressure. Therefore prolonging the service life of the lock walls.

3.Gates are best kept wet . If they dry out they will shrink just like wooden boats need to be kept wet.  Constant shrinking  and swelling will reduce the service life of the gates.

[magnetman]

2 hours ago, Loddon said:

What I meant by stating river fed us that there is a decent water supply..........to enable the pound below to be kept "on weir" 

The "normal canal section" is to prevent flooding 😱

Yes and what I was referring to was that the pound below Stockers lock will go low without the windows in the paddles because there is no other feed of water into that pound when the lock is not being used. 

 

And Springwell lock is deeper therefore uses more water. 

 

Above stockers lock the towpath is raised and extra reinforcement for flood protection this means that water does not overflow the gates and for obvious reasons there can not be a bywash there so we have to have another way to get the water through the lock, continuously and in a controlled manner.  

If you look into Stockers lock when empty or below the lock when full you can see the water emerging from the windows. 

 

Sorry to derail the thread !

 

 

Edited August 31, 2021 by magnetman

[dmr]

2 hours ago, MtB said:

 

Can you cite any technical explanation for that please David? 

 

I don't think there is one, because water pressure doesn't work like that. 

 

This is really one to thrash out over a beer.

Yes. (and obviously) the pressure depends only upon the depth, and the length of the pound does not matter, but taking things to extremes is always a good way to understand things and tax the brain and I reckon with your 0.7mm example a lot of other effects kick in. Its a bit like static electricity, it might be 20,000 volts but it won't hurt because its got no balls (new unit of electricity related to amps per hour 😀)

Statically the pressure is probably the same though surface tension/capillary effects might even come into play, but if you drill a hole the water will not come shooting out as much as expected because its like flow in a restricted channel and the flow "friction" will subtract from the pressure.

 

We need to experiment with bottles of cider and drinking straws.

[Scholar Gypsy]

16 minutes ago, dmr said:

 

This is really one to thrash out over a beer.

Yes. (and obviously) the pressure depends only upon the depth, and the length of the pound does not matter, but taking things to extremes is always a good way to understand things and tax the brain and I reckon with your 0.7mm example a lot of other effects kick in. Its a bit like static electricity, it might be 20,000 volts but it won't hurt because its got no balls (new unit of electricity related to amps per hour 😀)

Statically the pressure is probably the same though surface tension/capillary effects might even come into play, but if you drill a hole the water will not come shooting out as much as expected because its like flow in a restricted channel and the flow "friction" will subtract from the pressure.

 

We need to experiment with bottles of cider and drinking straws.

Cider is no good. You need to use canal water as it will have the correct surface tension.  

 

Another of way of thinking about how the force does not depend on the length of the pound is to imagine inserting a very light plastic gate G  across the canal, at any arbitrary position.  This divides the canal into two, a section one side of the gate (A) and a section the other side (B). In a static situation, the net force on the gate is zero, which means that the sideways pressure from the body A on the gate is exactly equal and opposite to the sideways pressure of the body of water B on the gate. Both are obviously independent of the shapes of A and B (as both A and B can be varied independently, and F(A) = -F(B) always). Now if you remove the water in A you have the sideways force on the gate (if the plastic is strong enough...)      

 

PS the centre of action of the force is 2/3 of the way down the gate.

Edited August 31, 2021 by Scholar Gypsy

[MtB]

1 hour ago, dmr said:

Its a bit like static electricity, it might be 20,000 volts but it won't hurt because its got no balls (new unit of electricity related to amps per hour 😀)

 

The reason it doesn't hurt is, as you say, no balls and yes, this is related to amps per hour given I suspect 1 ball (coincidentally) equals approximately 20,000 volts per second. 

 

[Steilsteven]

2 hours ago, dmr said:

 

This is really one to thrash out over a beer.

Yes. (and obviously) the pressure depends only upon the depth, and the length of the pound does not matter, but taking things to extremes is always a good way to understand things and tax the brain and I reckon with your 0.7mm example a lot of other effects kick in. Its a bit like static electricity, it might be 20,000 volts but it won't hurt because its got no balls (new unit of electricity related to amps per hour 😀)

Statically the pressure is probably the same though surface tension/capillary effects might even come into play, but if you drill a hole the water will not come shooting out as much as expected because its like flow in a restricted channel and the flow "friction" will subtract from the pressure.

 

We need to experiment with bottles of cider and drinking straws.

All I know is, if I installed a long water tank in someone's loft to replace a shorter one, even though the volume of water had been increased due to the extra length there would be no increase in pressure. But if I raised the existing tank there would be. 

 

Keith

[dixi188]

1 hour ago, Steilsteven said:

All I know is, if I installed a long water tank in someone's loft to replace a shorter one, even though the volume of water had been increased due to the extra length there would be no increase in pressure. But if I raised the existing tank there would be. 

 

Keith

Done that in my bungalow.

Raised the tank about 3 ft as the shower was not very good. It's better now but not as good as the one on my boat. I may have to fit a pump, but that involves a new hot water cylinder.

Dixi.

[dmr]

3 hours ago, Scholar Gypsy said:

Cider is no good. You need to use canal water as it will have the correct surface tension.  

 

Another of way of thinking about how the force does not depend on the length of the pound is to imagine inserting a very light plastic gate G  across the canal, at any arbitrary position.  This divides the canal into two, a section one side of the gate (A) and a section the other side (B). In a static situation, the net force on the gate is zero, which means that the sideways pressure from the body A on the gate is exactly equal and opposite to the sideways pressure of the body of water B on the gate. Both are obviously independent of the shapes of A and B (as both A and B can be varied independently, and F(A) = -F(B) always). Now if you remove the water in A you have the sideways force on the gate (if the plastic is strong enough...)      

 

PS the centre of action of the force is 2/3 of the way down the gate.

 

You experiment with canal water, me and mtb will experiment with cider, and then we can compare results.

 

I think a more intuitive version of your "gate" thinking is to place this thin gate (made from clingfilm) 1/4 along the pound, splitting it into a short and long pound, if there is a pressure difference (relating to distance from the locks), then the clingfilm will deform. However you are not thinking this through; if there is a pressure difference (without the gate) then the water must flow so as to equalise that pressure and so the gate will never see any pressure difference, what will happen is the pound will become a little deeper towards the locks and shallower at the middle. Now we do actually know this to be true, for example on the Oxford summit I can get into Napton and Claydon top locks without trouble but always run aground towards the centre of the pound

 

That 2/3 centre of action is not immediately intuitive, I have a horrible feeling it needs calculus to work that one out?????