Fishway attraction flow: providing evenly distributed auxiliary discharge using static mixers

Ensuring ecological connectivity in large rivers by attracting fish into fishways at barrages is a complex challenge, as it requires discharging auxiliary water to generate sufficient attraction flow. This auxiliary water is typically conveyed via a pipe from the head of the dam to a supply channel, where it enters the fishway as a high-velocity jet. For successful fish passage, this jet must be transformed into a homogeneously distributed, low-velocity channel flow. This study presents a novel supply channel design equipped with static mixers for jet homogenization, systematically investigated through computational fluid dynamics simulations and hydraulic scale model tests. Key hydraulic design parameters are identified to evaluate mixing performance and to determine flow conditions that minimize the required supply channel length. A reference homogeneity is introduced to define a threshold required to enable fish passage. The results demonstrate that static mixers achieve rapid jet mixing, producing a homogeneous flow within a short downstream distance. Their efficiency depends strongly on flow velocities within both the pipe and supply channel, but is limited by hydraulic overload, manifested as backwater rise. To address these constraints, empirical equations are developed to predict backwater rise and energy head loss, providing reliable design guidance. Finally, a comprehensive design concept is proposed that enables the calculation of all relevant hydraulic and geometric parameters required to transform high-velocity jets into homogeneous flow within compact structures. This concept offers a practical solution to enhance fishway performance and ecological connectivity in large river systems.