Pont-canal d'Agen

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August 23rd 2016
Published: August 28th 2016
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Pont-canal d'Agen

There are over 9,000 kilometres of navigable waterways in France. Many of these canals were built before motorised road transport and even before railway networks. They were important for transport, especially conveying agricultural produce to port. Today those canals are used mainly by leisure craft.

In the 1840’s the Pont de Garonne was built to connect two canals by aqueduct across the Garonne River.

Looking at it today, the 580-meter span looks like the infrastructure project of undergraduate engineers sitting in an academic vacuum. Imagine the professor of engineering addressing his students. ‘Your assignment is to design an aqueduct for boats to cross the river, linking the northern canal with the southern canal.’

‘Why?’ asked the dux of the class. ‘Why not let the boats float across the river?’

‘Study your fluid dynamics. The northern canal is more than 10 meters above the river. This project calls for an aqueduct.’

Bright Spark of the class looks at some charts and observes ‘On the southern side the canal is only one or two metres above the river level. A simple lock on the north side could allow boats in the river to access the canal ten meters above. A smaller lock could allow vessels to move from the river to the southern canal. Why not just build two locks?’

‘You will have to design a canal extension by aqueduct across the river, and also design a lock at the southern end of your aqueduct to allow shipping to move to and from the height of the northern canal to the southern canal water.’

Maybe it was more sensible. But as I looked at it today, I asked the question.

As we walked the 580 meters adjacent to the canal, 13 meters above the River Garonne, we wished there were boats present to complete the serenity of our vista. Luckily, as we reached the southern side Le Boat chugged up. A small crowd of like-minded observers had gathered. Well, there were six or eight people and a dog. All attention was focused on Le Boat waddling toward the lock gates. With fenders bumping against the gates, Le Boat drifted into the lock. Then the first round of shouting between Skipper d’Le Boat and Crew d’le Boat broke the shady calm of the gathering. Two mooring lines were lassoed around bollards on the shore. I say ‘lassoed’ rather than ‘secured’ because the mooring lines were passed by the bollards rather than wound around them for purchase. It was all done with such an absence of seamanship that it would have been dangerous, except that the water was dead calm. It is usual to arrange mooring lines to run fore and aft in what is known as a spring when water levels are likely to change. The problem here was that the spring was arranged back to front. Furthermore, there was no bowline – a rope from Le Boat’s bow (pointy end) that would be wound around a cleat and feeding to the hands of the crew who would then have very good control of the boat without having to exert much energy. Holding the bow gives easier control of the boat then trying to do so from the side. A properly arranged spring holds the boat against the dock.

With one hand grappling an un-cleated line (rope) while Le boat bounced around pulling the poor crew person up and down along the dock, crew person finally found a spare hand to operate the lock controls to close the gate and start the water flow into the lock.

At first, there was only a trickle. Then the inflow sped up. A wave maybe 8 or 10 inches high surged against Le Boat. With inadequate control of her bow, crew and master were unable to prevent Le Boat from breaching side on across the lock, ramming shinny gel-coated fibreglass against the coarse walled lock. The water level lifted and when Le Boat was ready, she bobbed back to the side of the lock where master and crew waited. They boarded, the gates opened and le Boat entered the top deck of Pont-canal d'Agen and continued her voyage, but now twelve meters in the air.

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