Development of the combined fish pass of Fratel’s dam using numerical and physical modelling

Abstract

In 2025, EDP, Portugal’s leading energy production and distribution company, will carry out intervention at the Fratel Hydroelectric Scheme on Tagus River, to improve the power production efficiency and to provide migratory fish access to upstream habitats. The tailrace channel will be reprofiled, and a new fish passage system was designed, which is the focus of this study. The project aims to enable fish migration past the powerhouse afterbay sill, even during low water levels, and reach the future fish lock. The fish pass system comprises three key elements: a 230-metre-long fish-dedicated channel, excavated along the tailrace, a four-pool fishway, and a bypass channel with irregular baffle walls. A compreensive study conducted at the National Civil Engineering Laboratory of Portugal (LNEC) employed computational fluid dynamics (CFD) and physical modelling to optimise the system proposed by EDP. Velocity fileds and flow depths across varying operational scenarios were evaluated. A multidisciplinary team, comprising experts in hydraulic engineering, computational and physical modelling, and ecological science from LNEC, EDP, and MARE/university of Évora, worked in close collaboration, recommending design operations. The large domain dimensions would imply an unfeasibly large phisical model. Similarly, a single CDF model to accurately reproduce all geometries would be computationally prohibitive due to the range of time and length scales incolved. Hence, the combination of CFD and physical modelling was essential for detailed testing of several design iterations, minimising physical trials, and significantly reducing costs. CFD enabled the modelling of the whole domain. The fish migration conditions between the tailrace confluence with the Tagus River and the afterbay sill were analysed in detail using a volume-of-fluid model, whereas the pool fishway was modelled indirectly. Concurrently, a 1:10 scale physical model focused on the afterbay, the pool fishway, and the bypass channel. This facilitated detailed insights regrading fish migration past the afterbay sill and the fish lock discharge attraction potential, further refining the design. The project sucess was driven by the integrated CFD and physical modelling approach and the interdisciplinary effort, demonstrating the critical role of collaboration in advancing eco-friendly hydraulic infrastructures.