John McNown’s “Canals in America”

By Jeremy Lane

In the 17th and 18th centuries, Britain experienced a boom in the creation of canals, going from around 650 miles of navigable waterways to about 4 times that in just two centuries. The primary means of transportation of goods on the waterways was by horse-drawn barges.  A single horse pulling a barge could pull as much as 50 tons, while the same horse on paved roads could only pull 2 tons; on unpaved roads, they could only pull up to 1200 pounds. The Americans, lacking even paved roads in most places, saw the clear advantage of the creation of canals and began to mirror this boom in the 1800s.

By the time the US started to create their canals, the engineering behind their creation was widely known. The cross-section of the canal is trapezoidal, where the height is relatively shallow: only 3 or 4 feet in most cases. The slope along the banks would be very shallow, greatly reducing the effects

Image from: History of the Canal System of the State of New York … / by Noble E. Whitford (Albany : Brandow Printing Co., 1906) — vol. 2, p. 1039

of erosion. The bottom and banks of the canal would be coated with a loam and sand mixture to prevent the water from seeping into the ground. Finally, overflow weirs would be placed at regular intervals to prevent flooding in the surrounding areas in the case of heavy rain.

 

In order to solve issues with connecting canals to other waterways at different levels, the Americans made use of the V-shaped lock. The V-shaped lock operates similar to a normal linear lock* but also makes use of two separate chambers that each have their own opening on the downstream side but sharing an opening on the upstream side. V-shaped locks have two main advantages over a linear lock.

First, the V-shaped lock could reduce the amount of water that was discharged downstream. When lowering the water on one side of the lock, they could discharge it into the other chamber, which would allow other boats to rise until they hit equilibrium at the halfway point. When this happened, they would be forced to discharge the rest downstream while continuing to fill the upwards moving side from upstream. This halves the amount of water lost downstream.

The second major advantage was that the stress on the lock would operate similarly to that of an arch. Because the edges of the lock joined together at a point, the pressure from upstream would push the walls of the lock towards the center, where they could then support each other. This made the whole structure much more sound than other formations would.

*In case you don't know how a lock works, here is a
video of a linear lock being used. Basically, there is
a small chamber that can hold a variable amount of water
connecting the two waterways. On one end of the chamber is
a gate leading upstream (towards the higher river) and the
other gate leads downstream. Whenever someone wants to go
upstream, they open the bottom gate while the water is at
the lower level and allow the boat to enter. Then they
close the lower gate and allow a small amount of water to
enter from the higher side, which will cause the chamber to
fill at a controlled rate. Then, when it fills to the
level of the other waterway, the upper gate can open.
Whenever someone wants to go down, they can discharge
the water slowly downstream to lower the level in the lock
and then open the lower gate.

In case you are interested in some of the canals in the US, here is a link describing their creation and some of the difficulties they faced.

616 words.

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