Fish passage using broad-crested labyrinth weirs for low-head dams

Abstract

The majority of dams can be classified as low-head dams with a headwater/tailwater difference less than 2–3 m and often span the river width. The importance of alternate fish passage designs especially for weaker fish has been established by their unsuccessful passage rates through traditional fishways. A model study was conducted on an alternate design allowing a comparison between a straight weir and a modified labyrinth weir, in a pool and weir fishway. The model was conducted at a 1:8 scale Froude model of a low-head dam. The dam was notched with a pool and weir fishway constructed in the notch and extending downstream on a ramp. Velocities and water surface profiles were recorded for both weir designs at 1:10 and 1:7 slopes using three different flow rates. Results show that the labyrinth weirs produce lateral cyclical velocities compared to the relative uniform velocities over the straight weirs with slower velocities over the downstream apexes and faster velocities over upstream points. A comparison with straight weirs shows that the broad-crested labyrinth weirs create regions potentially more conducive to fish passage. This provides an effective alternative design for fish passage at low-head dams.