Hydraulic management of coastal freshwater marsh to conciliate local water needs and fish passage

Abstract 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). 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.