The Hertford Union Locks Problem

The Hertford Union Canal loses water. The problems boaters have when they encounter its emptied second pound have become a running joke on the London Boaters Facebook page.

But what causes this problem, and what can be done about it?

Four years ago I first noticed the recurring problem and set about investigating it over the May bank holiday as narrowboaters moved to and from the IWA Cavalcade in Little Venice. It rapidly became clear that each locking through drained the pound between the top and middle locks by a few inches, leading to the pound becoming unnavigable after only a few lockings.

I wanted to know why. Was it leaking gates, permeable lock walls, boaters leaving paddles up?

Looking into the construction of the three lock flight the answer turned out to be remarkably simple – and rather stupid.

The bottom lock falls 3’9 feet (0.84m.) The middle lock falls 8’1 feet (2.46m.) The top lock falls 6’3 feet (1.905m.)

So, let’s do some maths. I’ll use metric measures, and assume that the locks are 23m long and 4m wide. The pound between the top and middle locks is about 135m long and 10m wide with an average depth of 1m (more towards the middle, less to the sides.)

The top lock empties 23 x 4 x 1.905, or 175.26 thousand litres into the pound. The middle lock empties 23 x 4 x 2.46, or 226.32 thousand litres from the pound. The pound holds 1350 thousand litres. With a full pound the first locking through loses 226 thousand litres, dropping the level of the pound by 17cm (any extra water has already passed through the spillway.) Each subsequent locking loses the difference between the volume of the top and middle locks or 51.06 thousand litres, and the level falls 4cm.

After 7 lockings the average depth of the pound falls to 59cm a little below the 60cm of depth needed for navigation. That’s in the best case assuming the gates are perfect and no water is wasted. In practice, after only five lockings through the pound has the canal bed showing at the edges.

So why was the canal built this way? The engineer hired by George Duckett was Francis Giles, who worked on a number of canals, and despite some misjudgements appears to have been fairly competent. It’s possible Giles was expecting the Hackney Brook (which flows into the lower pound) would actually flow into the upper pound and replenish it. The Regents Canal eventually featured some back-pumping through Victoria Park, but my understanding is that this was added after the Hertford Union was built.

The Regents Canal Company objected mightily to the amount of water lost to the Hertford Union, so much so, that only a few years after it was complete, they built a dam at the junction of the canals to prevent any of its water being taken. That lead to the Hertford Union effectively being emptied until ten years later when the Regents Canal Company bought the Hertford Union Canal and removed the dam.

Even later, in the 1870’s, the reengineering of the Hackney Cut raised the level of the Lee Navigation below the locks by a few feet reducing the fall of the final lock of the flight.

From then on, until the demise of British Waterways, the lock keeper at the Hertford Union top lock would have the responsibility of running extra water down to maintain the level of the troublesome pound.

And so today the pound continues to run dry, and questions are asked about why the upper pound runs dry and the lower pound overflows.

What can be done?

There are two interconnected issues to address here: the problem of the pound between the top and middle locks running dry, and problem of the loss of water from the Hertford Union Canal and Regents Canal pound above the top lock.

The first problem needs (at the minimum) a change in how the locks are used to encourage extra lock-fulls of water to go from the top lock into the pound below. The first thing I’d suggest is that the top lock should always be left full. Refilling the top lock would require a few extra minutes work on the part of boaters descending the locks. Also,I’d suggest that the top lock would also be for one boat at a time, while the locks below should be shared when possible. Both those solutions would probably need a volunteer lock keeper to ensure that boaters did the right thing, and who could run extra water down (by filling and emptying the top lock) when necessary.

The second problem is more intractable, as the Hertford Union Canal and Regents Canal pound above the locks will always lose the amount of water in the deepest lock of the flight, 226 thousand litres, each time the locks are used. There is an engineering solution to this problem though: the second lock could be extended and reconfigured as a two-chamber staircase lock.

By splitting the second lock into a staircase, the amount of water used by the lock could be reduced – leaving the deepest lock as the top lock and saving 51 thousand litres of water on each locking though.

It would be quite a task, reengineering the middle lock, but it wouldn’t be impossible and would likely cost less than the recent rebuilding of the first lock on the Hurlestone flight at Llangollen.

An alternative, with less engineering required, would be to dredge out the second chamber and reinstate the cross chamber sluice at the doubled Mile End Lock at the other end of the Mile End pound. Using this as a side pound would save about 73.6 thousand of the 220.8 thousand litres of water lost every time that lock is used.

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