SLHS Development Of Canal Locks

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Riding The Weir

The first canals were dug thousands of years ago to divert river water for drinking and irrigation. For example the Egyptians started watering their desert regions from about 3220BC which was significantly extended by King Scorpion in 2735BC. The Mesopotamians, then Sumerians and Akkadians, followed suit. King Sennacherib paid for a supply to Nineveh, Assyria, in 704BC; King Darius attempted to connect the Nile with the Red Sea in 510BC. However river water levels in the summer would be low and would be far higher, or even flood, with the ‘rainy season’ precipitation upstream.

Consequent destruction of installations and losses of crops meant that there had to be more organised control of water courses roundabout. Away from tidal rise and fall for inland ships to move without hazard, or to transfer cargo, the water levels were stabilised by adding weirs to the embankments and dams. Occasional flooding could still be a big problem but with enough effort it could be mitigated.

A direct consequence of this permanent difference in inland water levels was the need for cargo ships to transfer the goods from a ship on one level to another on a another. Better still, in order to save this effort, was for the very trading ships themselves to be moved from one level to another by sailing or ‘riding’ them over the weir. As can be seen from this drawing (right) Chinese ships were developed with flat bottoms, or twin keels, to accommodate this perilous task.

Planks Replaced By Gates In Locks

Whilst using inclined locks it was common practice to put planks of wood across the upper end of each lock to hold the water back. Even in summer they would be necessary to preserve precious water. Removing the planks, especially to remove the lowest submerged ones, claim lives as the water and ship lurched forward.

Inclined Lock

Just opening a gate, if one could overcome the considerable forces bearing down, and flushing a small canoe through a flash lock was not clever but closing off the water afterwards was even more dangerous.

Doing this with larger vessels was impossible and so the Chinese developed an enclosed space that contained an incline where the flow of water and the movement of the boat during ‘flushing’ could be more controlled.

(See right)

A large ship riding a weir on the Chinese Grand Canal.  Painting 1793

Flash Locks

The Chinese introduced Flash Locks in Nanyang around 70BC. However they were nearly impossible to operate and many must have lost their lives as ships surged forward and crushed the helpless slaves when they tried to comply with their master’s demands (see left).

Elsewhere in the world, where weirs were not strong enough for this, dams were constructed with simple gates that could just be opened. We even used them in this country (see right).

Flash Lock Pudding Mill Lane, London. Date unknown

The Earliest Locks

Pounds Between Chains of Inclined Locks

Using bullocks and man-power alone meant each inclined lock was limited to about 1m of height change. Obviously on hills greater than this they would chain the inclined locks one after the other. However they found that the locks further down the chain suffered rather intermittent flow depending how they were struggling with the locks above and so pounds (from ‘animal compound’ or ‘trap’) between the locks were introduced to smooth out the flow and no doubt give the haulers a rest !

A Chinese Inclined Lock

Flash Lock recently excavated near Shandong

Improving The Perilous Task

The first of many routes of the 1100mi Grand Canal started in 540BC.

However in busy areas even this loss of life was seen as an unacceptable loss and so in 983AD Commissioner Chaio Wei-Yo specified that his locks would be fitted with a gate. Unfortunately at first this didn’t go down well because the men couldn’t open the gate against the pressure of the water. Oh dear !

(Above left)

The First Gated Locks

Of course what was needed was a second gate and sluices at both ends.   (See right)

The Grand Canal

Finally a mechanism that worked. Given the burgeoning popularity of manufactured goods from the North and the need for food grown in the South it’s not surprising that the Song Dynasty set to digging out the largest canal then, and still today, from Hangzhou to Beijing -but by hand !

(See above right)

The Grand Canal was so large and busy that it had to have significantly large bridges to cross it.

A New Big Step: Gates On Locks

A Grand Solution

A Brilliant Solution

The first gated locks.

The Problem With Big Gates

You might think that there can be no improvements possible for a system that had worked so well for thousands of years ? However, Engineers say that enlargening a small working system often doesn’t scale up. Consider these problems when making a very large canal lock..

1. •Narrow gates can be slender and lightly made but wide gates need significant tree trunks to hold back the pressure of the water.
   

2. •On a small gate the force required to open it is modest but large gates become extremely difficult to move even if oxen are used.
   

3. •A small gate is easily removed for repair but shifting a huge piece of woodwork is a massive problem -especially if it weighs 3 - 15 tons !
   

4. •River silt washed against a small gate is easily swooshed away but that against a large gate will get it stuck.
   

5. •Any single gate, made of wooden, needs to be sealed well. They are made of planks that must not flex or the water will find a way through. That means a gate needs to be rigid. The gate must press and seal against the stone structure of the lock. There is more chance of variability in the matching up of the surfaces of laid stone walls the higher and longer they are. Small locks may leak a little and a tiny amount of flex in the gate will improve the seal. However enormous gates themselves may not be perfect and certainly won’t flex much. Their seal can be quite poor. In fact this is a problem even today.
   

6. •Finally the swing of a large lower gate encroaches on space that a ship should have within the lock. Thus the lock has to be bigger, more expensive and uses more water each time it is used.
   

So what can be done ?

Leonardo’s Famous Mitre Locks

In 1493 the great mind of Leonardo Da Vinci stepped in with a brilliantly simple idea (left). He proposed the use of two gates that meet in the middle. They will be approximately half the weight and more easily moved. In fact if there’s only a small boat only one would need to be opened. By being small they can jiggle more easily and the seal is significantly better. And finally they present a far smaller encroachment on the useful space in the lock so his locks use water more efficiently.

The solution is excellently demonstrated at Hatton. The canal to Warwick was first built like the Stratford Canal for single narrowboats and the locks used a narrow a single gate. However when all the small canal companies were amalgamated the canal was doubled in width (hence the name Grand Union Canal). The new 14’ wide locks were going to take time to build so they build them alongside the old ones. And, yes, Leonardo’s twin mitre gate system was used.

In fact having the choice to use wide and narrow locks increased efficient throughput so the old ones were retained until higher maintenance costs saw the narrow gates removed and the lock bricked up. (see right).

Question: Why did they always brick up the lower gate ? (answer below)

Leonardo’s gates met in the middle.

A lock in the Hatton flight

Kapeenkoski Lock, Finland

A Modern Solution

Really Large Mitre Gates Are Still Heavy

It won’t surprise you to learn that even mitre gates get significantly heavy on really big modern ship canals. Yes modern hydraulic rams can bring 100’s of tons of force to bear but that is expensive and occasions a great deal of wear. The Panama Canal suffers from this.

How about a clever Dutch idea: They do away with sluices and their required underground pipework -thus significantly reducing the maintenance burden and they are vandal proof ! To fill or empty the curved gate is just cracked open to start with to let the water through. The beauty is that the force on a curved gate is bourn against its fulcrum and is constant no matter how open the gate is.

Thus if there’s no ship in the lock one end can be opened quite widely to quickly equalise the levels in the lock for a ship to come in that end. It can all happen within moments -this is a highly reactive system.

There’s another rather important benefit: If the canal water freezes over, even to as little as 20mm thick, flat gates cannot be opened without first breaking the ice. However with curved gates, after a slightly increased detachment force, they just glide past the ice and open normally. Hence their use in Scandinavia.

Brilliant again !