Why this work?
The purpose of this work, carried out by Perpignan Méditerranée Métropole (PMM), in joint ownership with the Syndicat Mixte Têt bassin Versant (SMTBV), is to restore ecological and sedimentary continuity between the Joffre bridge and the ford on the Têt in Perpignan.
These two structures in the urban crossing of the Têt are identified as obstacles to ecological continuity. Concerned about its environment, PMM wished to adopt a more global vision of all the low banks in the urban section of Perpignan, both from an ecological and a landscape point of view.
Thus, the work in progress makes it possible to meet the following objectives:
- Restore ecological continuity at the ford and the Joffre bridge and allow the migration of fish species, in particular shad and eel
- Ensure the protection of the foot of the dyke and the bank on the left bank of the Têt ;
- Designing vegetation engineering between the Joffre bridge and the ford crossing in a coherent and integrated manner as part of the Es Têt project.
The cost of the work is set at €2,592,546.78 exclusive of tax, of which €307,416 is to be paid by the SMTBV. The Water Agency is subsidising the project to the tune of €1,120,264.
PMM and the SMTBV have selected the companies VINCI Construction Maritime et Fluvial, Dasse TP, SERPE and GECO Ingénierie to carry out the work from 5 July 2021 to 31 October 2022 (the last phase of work underway).
Find out what work has been done or is underway!
The different phases :
Phase 1
Development of the ford crossing
The old structure had many weaknesses (cracks) and represented an obstacle for fish (waterfall) downstream. Hence the need to perform the following operations:
- Now, downstream of the 50 nozzles, there is now a small concrete ramp with a 20% slope that reinforces the invert of the structure and facilitates the ascent of the fish.
- In addition, two gently sloping riprap ramps were constructed at the two end flow channels. Several sizes of riprap were used.
- The smaller blocks are inserted into the ramps. The roughness created also facilitates the ascent of the fish.
- The larger blocks (of metric order) also consolidate the banks.
- Finally, the plant techniques used help maintain the islands and banks in place.
The work for this phase of the project is complete.
Phase 2
Reinforcement, ecological enhancement and refocusing of the Têt river bed
The Gloria storm at the end of January 2020 had strongly weakened the left bank causing erosion and collapse of part of the bank to the foot of the dike.
The mechanical machines have allowed to recenter the bed of the watercourse on 350 linear meters and the foot of the dike has been reinforced by riprap.
To prevent further erosion of the bank and to stabilize it, landscaping has been done:
- Shaping of a wide, gently sloping bank alternating between natural "groynes" and depressions from the displaced stream embankments.
- Stabilization of the bank with a biodegradable coco geotextile and fascines at the foot of the slope
- Maintenance of the soil in place with different types of plants to reconstitute a riparian zone: trees, shrubs and adapted grasses.
These operations contribute to the diversification of habitats for the entire ecosystem around the watercourse.
The installations are completed and maintenance on the plant engineering is planned over 3 years to ensure that it takes well.
Phase 3
Construction of a fish pass under the Joffre Bridge
At the level of the Joffre Bridge, the banks are subject to erosion. On the other hand, the invert of the Joffre Bridge constitutes an obstacle / threshold that cannot be crossed by migratory fish species (eels, shad). The objective of the fish pass is twofold: to limit the erosion of the banks by refocusing the bed of the Têt under the bridge and to allow the ascent of migratory fish.
The structure has been designed with these objectives in mind. It is built between the two central piers of the bridge. It is 47m long and 18m wide with a gentle slope of about 5%.
Several levels of operation are planned:
- At the bottom / narrower level and the most incised: it operates in low water flow in order to ensure a sufficient minimum water height even during the summer period
- Intermediate and external stages: they correspond respectively to the medium and high flows that the structure can also handle.
In this configuration, the fish pass will always be flooded. The greater the flow of the Têt, the more water will flow up the steps.
To slow down the flow of water, obstacles and roughnesses are methodically inserted in the structure:
- Menhirs are placed at regular intervals
- A gravel pack is placed along the slope to create additional roughness.
Work on the fish ladder structure began at the beginning of July 2022 for 4 months. The operations are visible from the Joffre bridge!
