The canal aqueducts are ingenious creations and constituted a major development in the world of inland waterway navigation, improving traffic flow. Those that were built on the Canal du Midi are remarkable works of art.
An aqueduct is very similar in architecture to a bridge. The difference is that instead of a road passing over the bridge, it is a canal. A large-scale structure of this kind is called a 'canal aqueduct', or 'water bridge'.
An aqueduct isolates the waters needed for a canal to function, from the free-flowing waters of a river. In isolating the canal from natural hazards such as high water levels in rivers, it is protected from alluvium and uncontrolled water volumes. These factors can be catastrophic for the canal and hamper navigation.
The most iconic canal aqueduct is the Répudre aqueduct. The creation of the Répudre aqueduct was a historical milestone in inland waterway architecture. It was the very first bridge designed especially for boats!
This technical feat was built by architect Emmanuel de l’Estang between 1677 and 1680 and he was celebrated for his audacity. This engineering structure is actually fairly modest compared to the vast canal aqueducts that came later.
The construction of Répudre aqueduct was costly. That is why Riquet decided to take a different approach for the rest of the canal. He focused on the river crossing points with embankments so that the canal would cross the rivers at the same level, and so avoid some of the water.
But there was a problem with this approach, in that when water levels were high, navigation would need to be suspended. Then, after these episodes, the canal would systematically need dredging!
After his 1686 inspection, Vauban modified Riquet's designs.
He recommended building 60 aqueducts. In the end, 49 of them were built, in the construction work carried out between 1687 and 1693.
The aqueducts were built with barrel vaults (semi-circular) with an apron (foundations) that the stream would flow over. The slope of the apron was not regular as it was steeper for the first third of the structure departing from the entrance, to then gain in depth quickly and increase the output water flow in the tunnel.
This technique helped prevent siltation at the bed of the aqueduct!
Two catch basins on either side ensure that only the surface waters flow inside the tunnel. Other innovations followed on from this, such as the concave aprons to keep water away from the foundations and restrict the amount of deposits. Back then, the Cesse and Orbiel canal aqueducts were among the biggest ever built in Europe!
Did you know?
Streams with low water flows cannot be directed over a canal aqueduct. They are either barricaded or left to flow directly into the canal, after the waters have passed over either a slipway or a settling basin. To prevent any issues arising due to the accumulation of run-off water on the canal banks (which means the surrounding land cannot be cultivated), a network of irrigation canals was dug out to drain this water and direct it to a slipway or aqueduct.
There is another benefit to creating an aqueduct. It means the canal route can be simplified!
To be able to plan out crossing points, you have to firstly find the perfect crossing location, often up the inside of the small valleys. But thanks to aqueducts, these detours are not needed and can be replaced by simply using an embankment.
Navigation was therefore made much easier, because hairpin bends are not ideal for boats, and skippers would choose to avoid them.
The last canal aqueduct to be built was in Béziers over the Orb. It was designed in 1854 by Urbain Maguès and Achille Simonneau, to provide a crossing over the river at a time when the construction of the railroad between Toulouse and Sète was forging ahead.
Meanwhile, the same engineers were also designing a one-of-a-kind system over the Libron river.
There wasn't a steep enough slope here for the river to pass under the canal. This incredible system of mobile aqueducts opens up over the canal in the event of flash floods, and is then closed.
The 'Works of Libron' is an amazing achievement. Here the roles are reversed and it is the river that passes over the canal when water levels are high! The Ouvrages du Libron were added to the great family of aqueducts.
The main sources of excess waters are streams and run-off water.
To evacuate this excess water, a great number of spillways and weirs were built in the 17th century! These water evacuation systems were mostly built near crossing points over rivers or aqueducts once Vauban's modernisation plans had been completed.
Spillways and weirs are very different water evacuation systems
Canal weirs are structures on the surface of the water that allow excess water to flow down to a river's natural bed, without human intervention. They are also known as 'passelis' in French (at Agde canal pound for example), and these small regulation structures are essential during periods of heavy rain. They involve lowering the towpath to the same level as the water. Pedestrians can cross over these structures using footbridges or a series of archways.
Spillways differ from weirs because they feature floodgates to control the flow of water. Human intervention is required to operate these engineering structures, such as the Patiasses spillway. Excess water can flow over the crest of these structures or the floodgates can be opened. They are very useful when parts of the canal need to be drained in preparation for maintenance work.
Spillways and weirs can be combined in one single engineering structure, as is the case of the Argendouble spillways in La Redorte for example.
The engineering structures used today mostly date from the 18th century. There are various types of structures, such as weirs, 'passelis', straight drop spillways or siphon spillways. The latter were designed by engineer Garipuy who came up with this system in the 1770s. When the water reaches the crest, it flows downstream, and the water flow is then stopped by an air inlet built into the structure.