Along the canal towpath you will sometimes see a small mechanism standing in the ground. This is part of a sluice and, like an iceberg, most of it is hidden away under the surface.
All you can usually see of a sluice is the mechanical device, which uses a handle called a windlass to wind it open or shut. The sluice itself is a sliding vertical gate below the surface of the ground or the water. There will also be a drain or culvert down there.
A sluice controls water levels in sections of the canal, by allowing or stopping water to flow in or out. This can help to prevent flooding, or to empty sections of canal for maintenance.
Sluices, weirs, culverts and overflows are examples of engineering design that people have used for thousands of years but have changed very little in that time.
Llangollen Canal needs water not only for the hundreds of boats that use it but because it also supplies drinking water to people in north-west England.
Sluices and other forms of water engineering help to keep the water flowing, from the River Dee at Horseshoe Falls, through the World Heritage Site and into Shropshire.
Sluice gates are an important part of locks, which lift boats up and down to different levels. See how they operate in this film. This film was made for the Canal and River Trust’s Science, technology, engineering, and mathematics (STEM) educational programme.
More Information About Windlass
A sluice is a small dam that stops water from flowing through a channel but can be raised or opened when the flow is needed. The sluice gate is held in place in a slot, either side of the channel. When the panel is raised, the water flows underneath it.
This is what you can usually see of a sluice beside the canal. It is a rack and pinion mechanism, made from cast iron and bolted onto a stone slab. It is attached to a gate, below the ground. The gate stops water flowing out from the canal.
A rack and pinion mechanism converts circular motion into vertical or horizontal linear motion. The pinion is a wheel with teeth that is rotated. It pulls or pushes corresponding teeth on a bar, called a rack. In a sluice, the rack moves the gate up or down to control waterflow.
Most sluices on the Llangollen Canal are small, hand-operated mechanisms. At the Horseshoe Falls a much larger set of sluices is operated by computer. This measures the water from the River Dee into the canal and ensures that the sluices are opened or closed to always keep the level the same.
A weir is a dam that allows the water to flow over its top edge. It is not designed to hold back all the water but to maintain a constant level in the river above it. At Horseshoe Falls, this enables a consistent flow of water into the canal.
To measure that the water level in the canal stays the same, simple height markers are placed against the canalside. You can see the depth at a glance. Originally this would have been checked regularly by canal employees called Lengthsmen but today electronic devices report the level to a central computer.
At Llangollen, water was taken from the canal via a sluice gate to drive a water turbine, which powered the looms in a woollen mill. Canals don’t usually have enough water for such operations but the Llangollen Canal is unusual for the large amount of water that continuously flows through it.
Water allowed through a sluice enters a narrow underground channel called a culvert. The only part you usually see is the outlet, releasing the water into a ditch, stream or river. Culverts can also carry existing streams underneath the canal. This keeps the stream from damaging or flooding the canal.
Afon Bradley is a large stream near Chirk that flows into the canal. A nearby overflow weir, built by Jessop and Telford, reduces the risk of flooding. Excess water in the canal drains over the weir into a culvert that runs under the canal bed and towpath, into the old stream bed.
An overflow weir at Trevor Basin is hidden under the towpath, just before the Pontcysyllte Aqueduct. The culvert opens into a brick-lined channel, down to the river. The engineers designed this to take the excess water away from the piers of the aqueduct, so that they would not be eroded.
Locks lift boats up and down different levels on canals. The sluices, called paddles, are used to fill or empty the lock. At the bottom end, the paddle is built into the lock gate, so a long spindle along the lock arm allows the mechanism to be operated from the canal bank.
Around 12 million gallons (55 million litres) of water passes through Llangollen Canal every day. Much of it is used as drinking water for people in Cheshire, England. Where the canal ends, at a junction with the Shropshire Union Canal, a sluice allows some of its water into Hurleston Reservoir.