When the canal was built, Pierre-Paul Riquet let other passing waterways flow into the canal. Any excess water would flow out of the canal via weirs. This system brought with it alluvium deposits which silted up the canal and disrupted navigation. In 1685, barges had difficulty navigating a great number of sections of the canal.
King Louis XIV called upon Vauban in 1686 to inspect the Canal du Midi (known back then as the Canal Royal du Languedoc).
Vauban recommended the construction of multiple engineering structures such as aqueducts to address the issue of the canal silting up due to river crossings. Due to the extremely high and sudden water levels of the Orbiel, it was decided to build a huge aqueduct with three arches.
Orbiel aqueduct was built by entrepreneurs Colin and Launay, under the management of Antoine de Niquet in 1688. It served as a model, along with the technical drawings, for hydraulic engineering structures.
This aqueduc has three arches. When water levels are low, the Orbiel only flows under the western arch. When water levels are at their highest, the river takes over the whole riverbed and sometimes the three arches are not enough to contain the flow. When there are exceptionally high levels, the structure can be completely underwater, as was the case in 2018.
Did you know?
After the decree of construction for the canal was signed in October 1666, the first part to be built was the Toulouse-Trèbes section. The route between Trèbes and the Mediterranean hadn't been completely determined. One of the proposals was to cross the Aude river in Trèbes and then head down towards Narbonne along the right banks of the river.
In the end, Riquet abandoned this idea and chose to dig his canal along the left banks of the Aude river, keeping as far away as possible from the river, to avoid the dangerous high water levels. The construction work for the section between Trèbes and Thau lagoon was entrusted to Riquet in 1669. The work on this section of the canal began in 1670 at the eastern end (Thau lagoon-Béziers) and Riquet was also entrusted the construction of the port in Sète.
The Trèbes locks are a staircase of locks with three lock chambers. This structure was built in around 1674, downstream of Trèbes port, on the outskirts of the town towards the village of Marseillette.
A booming flour milling industry was set up here on the left banks of the lock, to take advantage of the significant height difference between one side of the lock and the other.
A first upper mill was built in the late 17th century. Some of the Canal du Midi waters were diverted here to operate the mill, which was described as having four millstones, stables and a store house. This first mill was transformed in 1867 to turn it into a flour mill with 6 millstones. This new building was then extended in the 1930s and has a ground floor and 4 upper floors. It has now been converted into a restaurant.
A second lower mill was built in the 1820s.
It was very quickly turned into a flour mill and a number of outbuildings were added to the surrounding area.
In 1896, the miller built a house here for himself to live in. It was next to the mill and was key to the practical organisation of the milling business.
At the back, there was a beautiful landscaped garden.
Did you know?
After Trèbes lock, the canal flows very close to the Aude river. In the 18th century, timber from the Pyrenean forests was picked up in Trèbes and transferred to the canal via a designated passage.
This timber was then shipped by floating the logs or by boat, either to Castelnaudary which was a major supply hub for canal management, or along the lower Languedoc (Béziers, Agde, etc.). In Trèbes, the carpenters from shipyards were the first to benefit from this timber.
The further away the canal is from the main water supply at the Seuil de Naurouze, 12 kilometres upstream from Castelnaudary, it is necessary to harness other water volumes to be able to supply the canal with enough water. This is the case after Carcassonne.
When Orbiel aqueduct was built, an inlet channel was also created. This is made up of a dam in the river and a channel which flows into the canel slightly upstream from Orbiel aqueduct, on the left banks of the canal. This water inlet has been closed for several years now, but it is still in working order.
Up until 1810, Trèbes was the headquarters of one of the 7 divisions of the canal management. The building that was used for this purpose (director's housing and offices) still overlooks the port today. At the back was the inn for travellers from the 'barque de poste'. There was also a shipyard that took up a part of the left banks (today this is used for boat rentals).
After 1810, when the alternative route via Carcassonne was opened, the management offices were transferred there. Travellers no longer stopped in Trèbes. The businesses at the port were also affected by this reorganisation.
Today, Trèbes is a pleasant village on the banks of the Aude river and the Canal du Midi. It is worth a visit to come and wander around the narrow streets and admire the church. The harbour is a great place to come and enjoy a drink at one of the many outdoor seating areas on the right banks.