SYSTRA is taking measures to secure the Arles to Tarascon line
As part of the Rhône Plan, a new dike is currently being built to protect the towns of Arles and Tarascon as well as the railway line during floods. This dike will run along the railway line’s embankment, which was built in 1845, and will include transverse structures required to evacuate any water which spills over onto the dike during major flooding. This is a first for SYSTRA.
Flood protection along the Rhône
Currently, the embankment of the Arles to Tarascon railway line acts as a protective dike against the flooding of the Rhône. Problem: it was never designed for this purpose and the most recent one-hundred year flood peak (going back 100 years), which was reached in 2003, highlighted its vulnerability. It is now recognised that the railway embankment would not be capable of withstanding another flood like that of December 2003.
To maintain the protection of the left bank of the Rhône, a spillway dike is currently being built in front of the railway’s embankment. Contrary to the current railway embankment, water will be able to spill over this dike without the risk of damage in the event of major flooding of the Rhône (going back 1,000 years). The flood peak thus spills over the dike and overflows into inland regions, to less densely populated areas, thereby helping to protect the towns of Arles and Tarascon.
It is for this reason that SYSTRA has been commissioned to implement the “hydraulic transparency” of the embankment: work undertaken will allow water to pass under the railway line, through the embankment, without affecting its solidity. Thus “removed” from the path of water, the line becomes “transparent” and the risk of a breach is reduced.
Over five linear kilometres, ten transparency structures (with 20m of hydraulic transparency) will be built perpendicular to the twin track, with each structure allowing 3.70m of water through, corresponding to a debit of up to 1,070m3 per second. This unusual challenge is a first for SYSTRA which has had employees from its local branch, and from SYSTRA Foncier for land acquisitions, to work on this project.
Five years of studies
The mission began back in 2011 as part of the Rhône Plan. SYSTRA was chosen to lead delegated operations by project owner RFF, which has since become SNCF Réseau. With five years of studies behind it, the project has now entered a four-year construction phase. The dike’s construction must be synchronised in order to ensure the safety of individuals and property.
For future work, care must also be taken not to hinder for long periods the circulation of the 200 or so trains that use the tracks each day. Only four outages will take place on weekends. During the first two, which will be the longest (55 hours), three pre-built transparency structures will be simultaneously put in place; then, during the two following, and this time shorter (44.5 hours), outages the last four structures will be installed.
Completion of work in 2020
Two “water-channelling” dikes will also be constructed at each end of the section, along with vegetation engineering: tree felling and turfing along the embankment, the installation of a geomembrane complex, ripraps and fencing each end of future structures.
Following the flooding caused by the swollen Rhône river on 3 and 4 December 2003 which affected more than 12,000 people over the entire delta and caused damage totalling more than €700 million, local authorities launched a major flood control plan called the Rhône Plan. Work on the embankment will begin in June 2018, a few months after the construction of the dike, and will be completed in December 2020.
Above, a TGV train near the Rhone river at Donzère, upstream of Arles and Tarascon.