Sensor fish data characterize physical forces during downstream passage at conventional and innovative low-head Kaplan turbines

The global expansion and modernization of hydropower are controversial due to known adverse effects on aquatic habitats and biological communities, particularly fish. Recent hydropower expansion and retrofitting aim not only to increase capacity, but also to reduce environmental impacts through optimized turbine design and operation. This study used sensor fish to characterize and compare the physical and hydraulic forces experienced by fish during downstream passage of a conventional vertical (VKT) and an innovative horizontal Kaplan turbine (HKT) under two common operating conditions. Measured accelerations (1.3–289.7 g -force), rotational velocities (106–3948 deg/s), and maximum pressure change ratios (0.21–0.59) suggest that hydropower plant passage can cause serious injury and mortality to fish in both turbine types. While the risk for severe strikes (>95 g -force) was similar (25% of runs at HKT, 21% at VKT), decompressions at the HKT (mean nadir 82 kPa) were potentially less harmful to fish than at the VKT (51 kPa). Surprisingly, ‘normal load’ operation (runner opening: 96–97%) at the HKT was potentially more harmful than ‘partial load’ operation (59–61%), whereas only minor differences between operating conditions were found at the VKT. Despite optimized design, innovative turbines are not necessarily less harmful to fish than conventional turbines, as site-specific conditions such as head drop, runner speed, intake depth, or operation mode can influence ecological performance. Sensor fish data can help identify critical aspects related to turbine design and operation contributing to less harmful and thus cleaner hydropower production in the future. • Physical and hydraulic assessment of conventional and innovative Kaplan turbines. • Strike events, shear forces and pressure changes were measured by sensor fish. • Injuries and mortality for passing fish can be expected at both turbine types. • The risk of barotrauma-related injuries was lower at the innovative turbine. • Negative impacts for passing fish can be reduced by optimized turbine operation.