Leaving a lock gate open can drain a pound

Several times recently you have stridently rejected the assertion that leaving a lock gate open can drain a canal pound, writes Mike Todd.

I am afraid that, for once, you are quite incorrect—it can. Let me explain.

Why locks were devised that way

If all gates and lock walls were watertight then your claim might well be true, for the reasons you quote. Indeed, this is why the pound locks were devised the way they are.

The design of gates is deliberately such that when there is a difference in water level either side, then they are kept tightly closed, sometimes helped by a mitre design, often at bottom gates on narrow canals and both ends of a wide canal. In addition, many locks are also provided with a bywash, designed such that as soon as a lock is full, then any further water coming down will go around the lock to the next lower pound.

Some locks were designed differently without bywashes, using either the upper gate culverts or gaps over the top of the gates to perform the same function. In any event, whichever method is used, then when working as intended leaving (for example) a bottom gate open will not cause extra water loss. As soon as the upper water level reduces to its design level then the lock is watertight once more.

Not working as designed

But we all know that engineering is rarely about things working as designed! Almost all locks will leak water at both ends when there is a level difference. In some cases this is deliberate but mostly it is the result of wear and tear. The classic and most visible instance is when the leading edges of mitred bottom gates are worn, probably through boats rubbing against them (there are other explanations) and in bad cases a considerable cascade can be seen.

Other leakages are not always as visible but nevertheless can still be substantial. Rubbish collected on an underwater gate cill can prevent a gate from closing fully.

In many cases the gates at either end have different amounts of wear and tear and thus one set will leak faster than the other. Let us consider a case where the top gates leak a lot but the bottom ones are watertight, or more so than the top ones. In this case, water will continually leak from the top pound and even a modest leak and a short pound can result, overnight especially, in an empty navigation. However, if the bottom gates are properly closed then the lock will catch the leaked water and when the lock is full the flow will cease. Leave the gate open and it will continue until the pound is empty.

Other scenarios

There are other scenarios (such as when lock walls leak (in some cases since the day they were built!) but the one detailed above is perhaps the easiest to visualise. Indeed, in some cases, the dynamics of the leakage are not readily spotted when passing through which is why the longer term view of canal staff can be helpful—Okay, so the old lengthsmen were especially good at this but in some cases technology (SCADA monitoring) can also detect them.

Just recently on the Llangollen canal, we passed a boat that just left the next lock a few minutes earlier. By the time we arrived, hoping just to open the top gate, the chamber was half empty and we had to raise paddles before it could be opened.

Quite often one encounters locks which stabilise at about half full/empty as the pressure from the upper level reaches a point at which it helps the gate to form a better seal.

Not a popular view

It may not be a popular view, but in general it is wisest to assume that the experts on the ground do know what they are talking about—after all, these conclusions date back a long time when the 'it was much better in the olden days' lock keepers imposed the same rules. They are not new.

Leaving gates and or paddles open can and regularly does lead to excessive water loss especially overnight. I have never gone very long on a cruise without encountering such experiences—fortunately the staff are generally on the ball much earlier in the day than we are and so able to correct it before we are delayed—but it does happen. It is surprising how quickly even a modest leak can empty a pound and overnight is plenty long enough.

There is little to be gained by encouraging boaters to ignore (the rules)—the extra time taken is little enough and allows us to better enjoy our surroundings.