Other than the occasional instance of the Garonne river breaking its banks and spilling over onto the quayside promenades, Bordeaux i...

Inside one of the detention basins that protect Bordeaux from flooding

Other than the occasional instance of the Garonne river breaking its banks and spilling over onto the quayside promenades, Bordeaux is not subject to flooding these days. But it has not always been that way. The flat, low-lying city is encircled and crisscrossed by a rich network of streams which in the past, when swollen by heavy rainfall, regularly led to flooded streets. Something had to be done!

The events that eventually triggered the deployment of a massive city-wide flood protection system can be traced back to late May and early June 1982. Violent storms struck Bordeaux and the western suburbs of Saint-Médard-en-Jalles, Le Haillan, Mérignac, Caudéran, Le Bouscat and Bruges. With rainfall exceeding 40 millimetres per hour, before too long water was flowing through city-centre streets, rising to a level of 1 metre in places; rescue services had to use boats to get around. More than 1,500 homes were affected by the floods and many families had lost virtually everything they had by the time the water subsided three days later. Bordeaux was not exactly the happiest place to be.

The aftermath of the 1982 storms on rue Chevalier in central Bordeaux, as originally reported by Sud Ouest and featured in the excellent Le Festin/a'urba book "De la ville à la métropole, 40 ans d'urbanisme à Bordeaux". And the same, drier, views today.
Over the ensuing 35 years, around 600 million euros have therefore been ploughed into an extensive system to make sure this never happens again. The setup comprises around 2,000 kilometres’ worth of tunnels and pipes, 130 pump stations, 50 rain gauges positioned in and around the city and, since 1992, a modern rainwater monitoring and control centre in central Bordeaux which goes by the name of “RAMSES” (which stands for [takes a deep breath] Régulation de l'Assainissement par Mesures et Supervision des Équipements et Stations).

But perhaps the most tangible and result of this strategy has been the development of around 80 detention basins, which can be used to store up to 2.6 million cubic metres of water. An information leaflet I recently received noted that as being the equivalent of 1,300 Olympic swimming pools. That’s a lot of liquid. Some of those storage facilities are located underground, particularly those close to the city centre. Many, though, are very much visible at ground level, like the one I’ve come to today alongside the Rocade ringroad in Eysines: “le Bassin Lamothe-Lescure”.

A handy information panel by the entrance gives some basic facts: the basin, which has been operational since 1985, covers an area of 2.3 hectares and can store up to 22,000 cubic metres of rainwater collected from the streets of Eysines. The water level here can potentially rise to a depth of 2.5 metres. The facility is managed by SGAC (which, somewhat illogically, stands for Société de Gestion de l'Assainissement de Bordeaux Métropole*), a subsidiary of Suez Environnement, the private company which handles water supply and management in the area.

The system itself is straightforward. Over to one corner of the dry basin, a concrete structure marks the point where two pipes converge, carrying the aforementioned rainwater collected from Eysines. That water then naturally flows on into a third pipe which, ultimately, will deliver that water across Bordeaux and into the Garonne river. However, when staff at the RAMSES control centre sense there is too much incoming water, the outgoing pipe is closed and the water is naturally redirected into the basin, where it will collect until it is safe for the pipe to be reopened and the water released back on its way towards the Garonne.

The inner mechanisms: bottom left and centre, incoming rainwater. To the right, a mechanical door that can be closed off, trapping the water which then spills over into the retention basin via the opening pictured top left. 
So how often do these situations occur? RAMSES teams record between 10 and 15 high-alert incidents per year. One extreme case study dates from 2013. On Friday July 26th of that year, flash storms, comparable in strength to those of 1982, hit the area and the control centre switched into crisis mode, their task hindered by the Garonne being at high tide. But Sud Ouest wrote that, other than a few waterlogged basements, no major damage was reported. Without today’s monitoring and defence system, there is every likelihood that the people of Bordeaux would have once again witnessed boats navigating the city’s streets as part of emergency rescue operations. But thanks to RAMSES and its network of detention basins, all that excess water is very much under control and the only boats you’ll see in the city centre are afloat on the river Garonne. Hooray.  

> Find it on the Invisible Bordeaux map: Bassin Lamothe-Lescure, avenue du Taillan, Eysines.
> The RAMSES visitor centre can be viewed upon reservation Monday-Friday all year round, and the control centre itself is usually open to the general public over the annual European heritage days weekend in September. 
> Ce dossier est également disponible en français !

* The "C" of SGAC probably originally referred to CUB or Communauté Urbaine de Bordeaux, the previous denomination of Bordeaux Métropole.

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