Abstract The construction, rehabilitation and re-licensing of hydropower stations must nowadays fulfill strict demands for fish protection, particularly those related to enhanced turbine passage survival rates of downstream migratory species. In an analogy to the efficiency curves of a water turbine represented on a hill chart, the bio-hill chart introduced herein depicts the relationships between the operating conditions (i.e., discharge and head) and the potential survival rate of migratory fish passing through the turbine. The bio-hill chart applies over the entire operating range of the machine, which is a step forward with respect to previous works that determined hydraulic-biological evaluations for only a few operating points. The elaboration of a bio-hill chart involves two steps: (i) the calculation of turbine flows for each operating point and (ii) the corresponding evaluation of fish friendliness. The flow conditions are determined via computational fluid dynamics simulations. The calculation of fish friendliness relies on modelling approaches to determine mortality risks associated with collision on rotating blades and rapid decompression. We thoroughly describe the process to elaborate the bio-hill chart for juvenile salmonid species, as well as its implementation in a Kaplan turbine. The bio-hill chart represents a first approximation for determining those operating conditions that may mitigate fish mortality risks (i.e., maximize fish friendliness) and for quantitatively establishing the inevitable trade-off that exists between the hydraulic and environmental demands of hydropower generation.
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