The Vilaine estuary is located in South Brittany, along the French West coast. The river has a flow discharge ranging between 2 and 1500 m3/s. Arzal dam is located eight kilometers upstream from the mouth of the Vilaine in the Atlantic Ocean. It is primarily intended to regulate the flow of the Vilaine and provide drinking water during the tourist season. This is one of the rare estuarine dams in the world.
The “Institution d’Aménagement de la Vilaine” (public organization created to carry out the necessary structural adjustments) has initiated design studies for the construction of a second lock at the Arzal dam. This lock must fulfil different and sometimes even contradictory set of functional objectives (protection against floods, drinking water reservoir, river navigation, road crossing, fish crossing, hunting ...). Among these, maintaining and preserving a water resource for the production of drinking water is the major objective of the project. Currently, desalination is carried out by a siphon system which pumps the brackish water in a pit located upstream the dam and evacuates water further downstream. This system is highly water consuming and poses problems especially during the summer.
The solution proposed by Artelia and its partners consists essentially of reducing very considerably the quantities of sea salt that enter the reservoir at each lock operation. The tests on the 3D model showed that this abatement can be obtained at the price of a substitution of the brackish waters contained in the lock by fresh water taken from the reservoir before the opening of the upstream doors. The effectiveness of this substitution was tested on a physical model of the new lock, which confirms this and refine the design and operating rules of the lock. A 3D numerical model has also ensured that the impact of the device will be large enough so that salinity problems in the reservoir are fully resolved.
The substitution of brackish water by fresh water in the lock is fundamental for the proper functioning of the new lock. A withdrawal of the brackish waters combined with an introduction of fresh water in the upper part makes it possible, by minimizing the mixtures between the two waters thanks to the difference of density, to rapidly lower the salinity present in the lock. The physical model at scale 1/12 allowed assessing the lock water supply system, the duration of the lock chamber emptying, the crest level setting of the outlet ports, the curves of decay of the salinity in the lock as a function of time. The model also assessed the impact of door opening on freshwater and brackish water trapped below the upstream lock’s threshold.
The numerical model of the whole reservoir upstream of the dam allowed evaluating the impact of the new lock on the saline intrusion. The 3D model in place allows to analyze all the processes and to test different configurations of operation of the structures.