Contribution of Different Elements of Inclined Trash Racks to Head Losses Modeling

Low bar spacing trash racks have been widely investigated in order to guide fish toward bypasses. In addition to this biological function, the formulae to predict head losses, for hydropower plants, are still being discussed. This paper investigates and models the global head losses generated by inclined trash racks with six different bar shapes and two different supports, in an open channel for six angles and two low bar spacings. The girders that supported the trash racks were U-shaped and different profile shapes. In addition to the previously studied rectangular and “hydrodynamic” bars, four new bar shapes, combining different leading and trailing edges, were investigated. Water depths were measured upstream and downstream of the rack for each configuration, and head loss coefficients were characterized and modeled. Three of these new bar shapes generated lower head losses than the hydrodynamic bar shape. The most efficient bar profile reduced the shape coefficient by 40% compared to the hydrodynamic profile and by 67% compared to the conventional rectangular profile. Concerning the supports, the use of a profiled girder to replace a conventional U-shaped girder also significantly reduced the head losses. The addition of the girder effect in a global formula increased its accuracy in predicting head losses of inclined trash racks upstream of hydropower plants.