Bridges are Falling Down

In the past few months I’ve become increasingly aware of a phenomenon that I believe is going to cost the Canal & River Trust a lot of money. It’s not going to be a problem that’s easily solved.

An animation showing how moving water erodes a canal wall and causes a void.

Canal wall erosion occurs whenever a boat moves close to a canal wall. The massive amounts of water displaced cause water to flow and erode the mortar between bricks and stones, so that with each passing boat more material is washed away from the structure of the canal wall.

This erosion is particularly evident at lock landings, where boats use their engines more, and at narrow bridges, or bridges by lock gates where the channel is narrow and the distance between the wall and the displaced water is decreased.

In time this leads to a few effects that most boaters will have encountered:

  • Voids under the towpath, these start as small hollows in the towpath, but can open into leg-trapping holes filled with canal water as the void gets too big to support the path above.
  • Cracks in bridges. This happens when the bridge footings move, because they have voids beneath them which are not supported.
  • “Pissers” in locks, where the void behind the brickwork traps canal water which is released as the lock empties.

It’s that second effect that’s going to get more and more costly.

This is Deepdale Bridge #17 on the Trent & Mersey canal in October 2022 (from this tweet.)

A photo of bridge 17 on the Trent & Mersey canal with an immense crack on the offside (from 2022.)
A photo of bridge 17 on the Trent & Mersey canal without the crack (from 2017.)

And this is the same bridge in 2017, just five years ago (from Canalplan).

Last year the Canal & River Trust had to repair a number of towpath voids alongside this bridge, but I guess they didn’t consider how the bridge might be affected by the same forces that were eroding the towpath.

The footing for the bridge abutment has evidently shifted cracking the entire wall where it has been previously repaired.

Like Hazelhurst Bridge, which is being entirely rebuilt having collapsed in 2020, this is a grade II listed structure, as are many bridges along the canals. That rebuild will cost the Canal & River Trust two million pounds.

During my travels on the system this year, I’ve seen damage to a great number of bridges caused by canal wall erosion, and I’ve reported a number of towpath voids near bridges. I expect we will see more and more cracked, and collapsed bridges, as canal boating becomes more and more popular.

So what can be done?

Well, in the ‘good old days’ canal companies had teams of brickies that would repair stonework and brickwork on a rotating, never-ending basis. These days we don’t have a commercial enterprise supporting the waterways, only a heritage-and-wellbeing based charity. It’s possible that we may simply have to lose a number of ‘accomodation’ bridges that will be too expensive to keep standing. It’s possible that repairs to listed structures could bankrupt the Canal & River Trust.

Reducing the maximum speed on canals (say from 4mph to 3mph) may help a little, but not at lock landings and lock bridges, where it is the acceleration of boats from a standstill that causes erosion. Or we may have to limit the number of boats on the system. Neither of those options seems desirable to me.

Or we could fund repairs to our canal heritage directly from central government. They are, afterall, a public good, worth maintaining from the public purse.

What do you think?