Abstract: Accelerated contracting flow around the intake generated by a hydropower plant in the generation or discharge operation has certain attractive and entraining effects on the Four Major Chinese Carp juveniles in the near-dam water, and it plays an important role in helping the fish in the near-dam water enter the intake and pass the discharge channel downstream. This study conducts a laboratory experiment of the contracting flow to examine its fish influencing factors and certain downstream migration strategies. The results show that increasing the flow rate at the intake inlet raises significantly the probability that the fish upstream of the dam is carried downstream. The fish-carrying flow upstream of the dam has a velocity close to but slightly lower than the threshold of the juvenile’s swimming speed, achieving the optimal transport effect. Under the same accelerated converging flow at the intake, bighead carps (Aristichthys nobilis) of all the fish species, are most prone to follow the flow actively downstream, black carps (Mylopharyngodon piceus) are most likely to escape, and silver carps (Hypophthalmichthys molitrix) exhibit the greatest ability to regulate their body condition in the transport. Our findings reveal the juveniles prefer to move along the walls in the low-velocity marginal areas, as being carried downstream by the accelerated converging flow.

Key words: hydropower station intakes, accelerated contracting flow, juvenile Four Major Chinese Carps, downstream migration strategies, experimental study