Résumé
Because of the physical context in western and southern France, ie. very low flows and high surface water consumption in summer, the hydrometric services tend to place their gauging stations upstream of non-mobile weirs, as weirs increase the reliability of rating curves and the sensitivity of gauging stations. However, improving the ecological status of surface water bodies involves a reduction of barriers to ecological continuity. Thus the DREAL in Pays-de-la-Loire has searched trade-off solutions and a methodology that could avoid studies on a case by case basis. Meanwhile, HydroSciences Montpellier was studying a re-shaping of a weir implanted near the outlet of the main tributary of the Thau lagoon, the Vene river. The key challenge is to integrate into the weir an "upstream migration structure" for the European Anguilla (Anguilla Anguilla) and sustain the sensitivity of the measurement for flood conditions while improving the sensitivity in low water period. In this study, we find in watermark, the guiding principles established by the DREAL Pays de la Loire.
As a result of the combination of hydrometric and connectivity constraints, design of a fish-friendly hydrometric weir gives priority to broad-crested weirs with a lower channel, because concentration of low-flow is favourable for both fishes and hydrometry. Examples of theoretical and prototyped designs are given.