Water control structures, used to regulate water levels and flow exchange in coastal marshes, act as barriers during fish migration between the ocean and brackish or freshwater ecosystems. Usual fish pass solutions may be unsuitable for obstacles subject to significant water level variations such as tidal range. This study proposes new solutions that were developed, implemented and evaluated on a marsh controlled by a series of hydraulic structures. These solutions were based on soft physical modifications (passive management) of the control gates, and on adaptations of their operation rules (active management). To evaluate the impacts of these adaptations, a hydraulic model of the marsh was built. It solves the one-dimensional Saint-Venant equations and appropriate gate equations. The model was used to identify management rules of control structures in a way to improve fish migration without significantly affecting the initial hydraulic management of the marsh (i.e. targeted seasonal water levels). It was also showed that fish passability of upstream structures could be improved by managing downstream ones. It was concluded that the combination of active and passive management of water control structures could largely increase the passability of these obstacles during glass eel migration, while limiting seawater intrusion in the marsh and maintaining water levels into a range compatible with marsh management needs.

To promote water control in coastal areas developed for agriculture, tidal hydraulic structures (doors and flap gates) are used for the hydraulic management of irrigation and drainage networks. By closing when the water level increases due to the tide, they prevent sea water inflow. However, fish using the tidal wave to move forward (e.g., glass eels) are also blocked from accessing the hydrologic network. There are hydromechanical solutions to mitigate these problems, such as adding a wooden block to leave an opening at high tide or adding a spring to delay the closing of the structure. The latter solution allows the structure to remain open only during a part of high tide. Designed empirically, these solutions can either alter the hydraulic control performance or be insufficiently effective for fish passage. This study proposes an operating model approach for hydromechanical structures that takes these passage solutions into account. This approach makes it possible to evaluate the performance of the passage solutions for fish passage and hydraulic regulation. With the help of these models, it is possible to design solutions that ensure a compromise between the passability of fish species and agricultural uses.