Anyhow Weir Lock

[Peter Thornton]

Does anyone know why Aynho Weir Lock, on the Oxford Canal, is shaped like a lozenge rather than the normal narrow canal coffin shape?

We've come through it today and couldn't think of any reason for it's strange configuration. I'm sure there's a simple explanation.

[Gillie]

An excerpt from a Wiki article about the RIver Cherwell...

 

Shortly after Nell Bridge, the River Cherwell crosses the Oxford Canal at a right-angle, flowing in on the east side and out over a weir on the west side. Such level river crossings are fairly uncommon on English canals. A few yards below this crossing is Aynho Weir Lock. This lock is unusual in that instead of a rectangular chamber, it has a wide lozenge-shaped chamber. This is because the lock lowers the canal by only 12 inches (300 mm) and the extra width of the lock chamber compensates for the smaller amount of water which would otherwise be passed from the River Cherwell to feed the lower level of the canal.

[nbfiresprite]

The extra width of the lock chamber compensates for the smaller amount of water, which would otherwise be passed through the lock, Due to the small change in level.

Edited May 13, 2013 by nbfiresprite

[MtB]

Quite. Its so a whole full sized lockful of water passes with each boat passage.

 

I'd have said it was more like 18" to 24" fall

 

 

MtB

[the grinch]

simple the next lock down is Somerton Deep Lock, which would drain the pound above it, if the weir lock was not able to pass down the same amount of water.

Edited May 13, 2013 by hamsterfan

[Fu Manchu]

The extra width of the lock chamber compensates for the smaller amount of water, which would otherwise be passed through the lock, Due to the small change in level.

Yes, that seems to be the accepted reason, but I'm not totally convinced.

My 1966 BWB Inland Cruising Booklet gives the fall as eight and a half inches, while Nicholson's says one foot. The lozenge shape probably makes it equivalent to three times the width of a normal lock.

But the next lock downstream is Somerton Deep Lock with a fall of twelve feet.

So even if the fall at Ayno Weir is more like 18" to 24" that would still only be the equivalent of less than half the water needed at Somerton.

What am I missing?

[alan_fincher]

 

 

So even if the fall at Ayno Weir is more like 18" to 24" that would still only be the equivalent of less than half the water needed at Somerton.

What am I missing?

Nothing that I can see, and I have always thought the same.

 

Clearly a passage of Aynho weir lock lets through far less water than is required at Somerton.

 

That said, it would be much worse still if it wasn't the way it is!

[Scholar Gypsy]

I agree the sums don't really add up - i.e. comparing the drops at Aynho and Somerton. And there is of course no bywash weir at Anyho. There might be a small sluice to pass water from the river into the canal, separate from the lock, but I've never been able to find it!

 

That pound is also topped up by a couple of small streams at Souldern, which may make up the difference?

[magpie patrick]

I suppose we need to hear from someone responsible for operations to see if it works! If the pound to Somerton sometimes needs to be topped up then Aynho Weir Lock isn't doing it's job as stated.

 

Crossing rivers on the level breaks the hydraulic continuity of a canal, the water from the next lock up at Nell's Bridge just disappears down the Cherwell, as does anything coming over the bywash there. A bywash at Aynho wouldn't work as, if the level of the river were six inches up, the volume of water would be such that the canal downstream wouldn't be able to cope, so in terms of water supply it is a case of starting again at Aynho.

 

It could be that streams do compensate: Bath Deep Lock needs no extra supply so long as it's only worked every couple of hours, despite being twice the depth of the lock above with a much shorter pound. The Top lock at Cefn on the Crumlin Branch can be worked four times a day with no obvious supply, the water levels then recover over night. However, I doubt the engineers got their sums so accurate at any of these locations, and Aynho Weir Lock, if it was built like this to pass enough water, was probably just a "make do" effort.

[Proper Job]

The perminently leaking gates at Aynho weir make up the difference.

 

It's the same at Shipton weir

[DaveC]

Of course there is no point in getting two or three boats into Aynho Weir lock only for them to need to go through Somerton Deep lock singly - but it does happen!

 

 

 

Dave

[Proper Job]

Of course there is no point in getting two or three boats into Aynho Weir lock only for them to need to go through Somerton Deep lock singly - but it does happen!

 

 

 

Dave

There in also lies a conumdrum, because the displacement of additional boats reduces the volume of water let down.

 

Obviously the original plan was that only full length 70' boats used the canal and these could not fit alongside a bot in the centre section

[alan_fincher]

There in also lies a conumdrum, because the displacement of additional boats reduces the volume of water let down.

 

Altogether now..........

 

Oh no it doesn't!

[magpie patrick]

Altogether now..........

 

Oh no it doesn't!

 

Trouble is, a pantomime response of "Oh Yes it does but this is only material in the unlikely circumstances that a boat's displacement changes between one lock and the next" doesn't really work does does it

[David Mack]

Of course there is no point in getting two or three boats into Aynho Weir lock only for them to need to go through Somerton Deep lock singly - but it does happen!

 

 

 

Dave

 

There is when you are on a seventy footer and behind two very slow 50 footers. They went in first and took the two side positions, and we followed taking the central slot. And so we were first out, and arrived at the next lock well before them!

[Proper Job]

Altogether now..........

 

Oh no it doesn't!

It does if they're craned out at Aynho wharf

 

Trouble is, a pantomime response of "Oh Yes it does but this is only material in the unlikely circumstances that a boat's displacement changes between one lock and the next" doesn't really work does does it

The volume of displacement of the additional boats is also only available to that boat as well. The volume water cannot be used by other boats as the original boat will selfishly hang on to it

[MaryP]

I agree the sums don't really add up - i.e. comparing the drops at Aynho and Somerton. And there is of course no bywash weir at Anyho. There might be a small sluice to pass water from the river into the canal, separate from the lock, but I've never been able to find it!

 

That pound is also topped up by a couple of small streams at Souldern, which may make up the difference?

I asked a BW person about this lock once. It's the only lock which has beaten me when single handing. Have you ever noticed the water sort of "boils"? There is a permanent paddle open under the water apparently which why the lock is so hard to empty, and so easy to fill!

Edited May 14, 2013 by Mary P

[Dominic M]

I asked a BW person about this lock once. It's the only lock which has beaten me when single handing. Have you ever noticed the water sort of "boils"? There is a permanent paddle open under the water apparently which why the lock is so hard to empty, and so easy to fill!

April 1st of which year was this?

[tony p]

Don't know anything about the mythical extra paddle Mary p, but you're right about it being a bugger to open the gates, I nearly gave myself a hernia on Saturday! We should campaign for bw to mechanise it....

[DaveC]

There in also lies a conumdrum, because the displacement of additional boats reduces the volume of water let down.

 

 

The same amount of water goes through the lock even if the lock is operated with no boats in at all. If CaRT were to repair the gates and or paddles there would be no problem operating the lock - and I don't believe the yarn about a "permanantly open" paddle!

 

 

 

Dave

[ditchcrawler]

Altogether now..........

 

Oh no it doesn't!

Prove it,( popcorn please)

[magpie patrick]

Prove it,( popcorn please)

 

Given you live in Lowestoft, you are ideally placed to research whether Mutford Lock uses any water at all!

[ditchcrawler]

Given you live in Lowestoft, you are ideally placed to research whether Mutford Lock uses any water at all!

That depends on the state of the tide, It could use a lock full at low tide, or none at all when Lake Lothing and Oulton Broad are on the level. Then again at high tide it could even give you some extra water.

I am not sure that it is back in operation again as someone crashed into the roadside barrier and put the bridge out of action.

[Scholar Gypsy]

Dear Ditchcrawler

 

How much water to fill a lock (or how much is released into the lower pound to empty it)?

 

Consider a lock as a rectangular box of volume V1 + V2, where V1 is the volume of water in the lock when it is empty, with no boat, and the gates are shut, and V2 is the volume of water added to fill the lock to the upper level, when there is no boat in the lock - so V1+V2 is the total water in the lock when filled.

 

Now imagine the lock is empty, and a boat of displacement V3 cubic meters (so of mass roughly V3 tonnes) goes into the lock, When the lock is empty and the gates shut there will be water of volume V1-V3. When the lock is filled the amount of water in it is V1+V2-V3. Therefore the amount of water let in by the top paddles etc is (V1+V2-V3) - (V1-V3) = V2. QED.

 

This does of course ignore the fact that the water level in a pound will be affected by filling and emptying locks, or by a boat being craned into or out of the pound - both second order effects, unless the locks are very close and wih no side ponds etc.

[magpie patrick]

Imagine the upper pound has room for 100 boats, and is 4 feet deep, and a single lock with a fall of ten feet leads out of it.

 

All 100 boats then loaded to a draught of 3 feet 11 inches, as water is displaced due to loading, it goes round the lock bywash.

 

Now try filling the lock...