Hydraulic Design of a Tilting Weir Allowing for Periodic Fish Migration

The hydraulic design of a tilting weir is presented, which allows for periodic exchange of potadromous fish between freshwater ecosystems. The application domain includes inland waters that need to be isolated hydraulically, preserving the existing ecological connection with the surrounding areas as much as possible. In the absence of a hydraulic gradient, the weir is opened in its neutral position and fish can bypass through passages sideways of the weir. When a hydraulic gradient develops in either of two directions, the weir rotates until a new balance of moments of force is reached, while fish can still bypass. Above a threshold value of the hydraulic gradient, the weir falls shut. When the hydraulic gradient returns to zero, the weir reopens, restoring the ecological connection. Four variants of the weir were investigated in the Hydraulics Laboratory at Wageningen University. The versatility of the design is demonstrated by showing that the hydraulic gradients required for closure and reopening can be manipulated largely independently. Patterns of flow velocity and turbulence kinetic energy were analyzed, which suggest that relevant fish species can bypass the weir unimpeded. The effectiveness of the weir will mainly depend on hydrostatic aspects, which determine when the weir is opened and closed, and on the absence of large hydraulic gradients in the migration season.