This is the first in a series of features about dredging the canals, the hows and whys, and a bit of waterway history. To begin with, we've explained the issues that prevent us dredging the canal to the towpath edge.
One question we are asked all the time is why we can’t do more dredging and create more casual moorings along the towpath for boaters.
It may seem like a simple question, but it doesn’t have such a simple one-size-fits-all answer. We don’t and can’t dredge straight and deep to the towpath edge because of how the canals were originally designed and built over 200 years ago, or in some cases over 250 years ago. So, let’s do a quick recap of canal history.
The canals were built as the freight express routes of the day, the Georgian equivalent of our modern-day motorways but with horse-drawn narrowboats instead of 40-ton trucks.
Cargoes were loaded at wharves and the 'bargees' (the people who worked the boats) would transport the goods as quickly possible to their destination wharf. Long distance carriers would stop overnight at a wharf along the route where there were facilities for the boaters, with good stabling and fodder for their horses being a priority.
When first built, there was no need to provide casual moorings along the towpath. So it didn’t matter that the towpath edge was generally shallow.
Canals were built in a dish shape with the deeper water slightly to the towpath side of the middle. Boats didn’t stop on their journey or pull into the side except where they needed to tie up to work locks, open and close bridges, or wait for passage through a tunnel.
The wharves, for loading and unloading goods, were generally constructed by the carrying companies and commercial enterprises who were reliant on the delivery of goods by water. They paid the canal companies to create the deep, straight-sided wharves for the benefit of their deliveries.
The majority of these wharves were constructed on the offside of the canal, where boats being loading and unloaded would not impede other craft being towed along by horses. As a result, there are few deep areas of water on the towpath side, unless they were originally constructed as a wharf.
So, what would happen if you just decided to dig out the towpath edge to create some new casual moorings?
It's more than likely this would result in the total loss of water from the canal in that pound, depending on the method used in the original construction and the geology of the underlying ground. Oh dear, we’ve got an 'ology' but actually it’s pretty straightforward. Some types of ground are good at keeping water in, and other types, in fact most types, let it leak out.
The diagram above shows the typical dish profile of the canals in the Birmingham Canal Navigations and a proposal to deepen the canal at the towpath edge by constructing a substantial wash wall, excavating one side and installing much new clay puddling.
You might be wondering why we can’t plug the gap in the new bit we’ve dug out using the same method that the original canal builders used?
Most canals were sealed with puddling clay. This was to stop the water leaking out if they were built on soil that wasn’t good at keeping the water in. These days we still use puddling clay but also modern techniques like Bentonite clay matting. The problem comes in making a good and long-lasting seal between the original canal profile and the new deeper area.
Over the last couple of hundred years the canals have settled into the landscape and have become stable. Digging out a new trench close to the edge to create moorings could upset that stability. The foundations of the wash wall (towpath edge) could become unstable and risk the towpath collapsing or the wash wall itself falling into the canal.
The 'amended' canal is also likely to move and leak for a considerable length of time until the ground settles again. This is not something you would want, especially where the canal is built on an embankment, due to the danger of a potential breech.
It could also be that digging out a deeper area breaks into more porous or unstable ground than the rest of the canal, increasing the risk that even a minor leak will result in no water in the canal. It may come as a surprise but we've got quite a lot of canals that are built on sandy soil, chalk and limestone. All of which are not known for their water retaining properties. In these days of climate change and more extreme weather events, anything that increases the risk to navigation is not a good thing.
In the rest of the series we’ll look at how we are preparing for a dredging project in London, the limitations on where we can create new visitor moorings in areas of high demand, dredging techniques and a bit more history.
Last date edited: 8 December 2020
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