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A bypass on the Cher river

In the middle of the city of Tours, a bypass around the Rochepinard weirs on the Cher river was put into operation in April 2011. This reopening is a key step in restoring the potential for migration in the Cher, for eels, Allis shad and sea lampreys.

Following the removal of the Maisons Rouges dam on the Vienne river and the Blois dam on the Loire river, the Rochepinard weirs on the Cher had become the primary obstacle on the main migration channels in the Loire basin. The weirs, located near the mouth of the Cher, were impossible for migratory fish to overcome and particularly difficult to modify. In the beginning of the 1970s, the sections of the Cher in the city of Tours were rectified and then “urbanised”. Two branches were created around a manmade island and the former Rochepinard needle weir was replaced with two large flap weirs on either side of the island. In this context, the creation of standard fish passes was impossible.

the city of Tours to lower the flaps during migratory periods, a solution was found, i.e. an artificial river, 200 metres long and 12 wide, bypassing the weirs. The upper section comprises two branches, one of which is a “sports branch”. Water is supplied by a gate equipped with three complementary types of fish pass. Onema contributed actively to this project by providing technical advice to the installation owners (Touraine regional semi-public company, Tours urban area, city of Tours) and by supervising the engineering office working on the ecohydraulic part of the project, from the design stages through to completion. This supervision will continue with the design of the optimum hydraulic configuration to ensure both the free passage of fish and canoe/kayaking in the sports branch.

In the spring of 2011 and for the first time since the construction of the Rochepinard weirs in the early 1970s, sea lampreys succeeded in passing the obstacle and in reproducing upstream, near Chenonceau.

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Promoting restoration of aquatic environments on the European level

In the lead up to the World water forum, Onema prepared a symposium on the preservation and restoration of aquatic environments. Co-organised by the European centre for river restoration (ECRR) and Onema, among others, the symposium brought together some 20 organisations in Ljubljana (Slovenia) in November 2011. The discussions dealt with using and sharing project feedback and with the best practices for the preservation and restoration of rivers throughout Europe. The symposium also produced recommendations on how to raise the awareness of the general public. Onema presented ten examples of large projects targeting hydromorphological restoration of rivers in France under implementation conditions likely to interest the European colleagues.

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