Evaluation and operational optimization of fish passage efficiency in the Jiacha Fishway

The widespread construction of hydropower projects worldwide represents a double-edged sword, providing essential energy while simultaneously causing profound ecological disturbances by erecting impassable barriers that severely fragment riverine habitats. This fragmentation critically disrupts the longitudinal connectivity required by numerous rheophilic fish species to complete their essential life-cycle migrations—such as movements to and from spawning, feeding, and overwintering grounds—inevitably leading to the decline of native riverine fish populations. To mitigate these adverse ecological impacts, the installation of fish passage facilities has become an indispensable component of modern dam design and river restoration strategies. Among these engineering solutions, the fishway (or fish ladder) is particularly prevalent, owing to its relatively simple structure, lower construction cost, and ease of maintenance, making it a primary tool for restoring connectivity. Despite their widespread use and apparent necessity, fishways suffer from a critical deficiency: the lack of rigorous, quantitative assessment of their actual ecological effectiveness. The success of a fishway is seldom measured by the number of fish that successfully pass the barrier, making it difficult to distinguish a functional facility from a mere structural appendage. This difficulty in achieving quantitative evaluation is multifaceted, arising from the substantial variability in fish community structures across different rivers, the markedly diverse migratory behaviors and swimming capabilities of target species, the broad spectrum of existing fishway structural designs (e.g., vertical-slot and pool-and-weir types), and the highly dynamic hydraulic conditions governed by fluctuating hydropower operational regimes. Consequently, making meaningful cross-system comparisons of passage efficiency remains exceptionally challenging, firmly establishing the development of robust, standardized, and quantifiable metrics as a priority for future ecological engineering research. To address this pressing need and enhance the effectiveness of existing infrastructure, this comprehensive study conducted a detailed investigation into the hydraulic characteristics and attraction efficiency of the Jiacha Fishway during its peak migratory season. The research employed a rigorous methodology, beginning with intensive fishery surveys conducted both within the fishway structure and in the immediately downstream river reach. This effort successfully characterized the local fish community, unequivocally identifying the three dominant species of Schizothorax—namely the Schizothorax waltoni, Schizothorax oconnori, and Schizothorax macropogon—as the primary migratory targets. Collectively, these three species accounted for approximately 78.9% of the total fish encountered, underscoring their critical ecological and conservation significance. Building upon this biological foundation, the research proceeded to the critical hydraulic analysis. By synthesizing physiological data on the species’ swimming performance—including both prolonged and burst speeds—the suitable flow velocity range permitting successful and non-exhausting upstream movement within the fishway was theoretically determined to be 0.2 to 1.9 m/s. This range is essential for guiding operational limits. Furthermore, to fully characterize the flow dynamics, a detailed two-dimensional (2D) hydrodynamic model was developed specifically for the complex geometry of the fishway’s entrance pool and initial channel segment. This model was then employed to perform numerical simulations of the flow field across a comprehensive matrix of operating scenarios, encompassing varied fishway discharge rates in combination with different downstream river water levels (tailwater depths). The key findings from the simulations provided actionable insights for adaptive management. The research conclusively established that downstream river water level is the most influential factor controlling flow velocity at the fishway entrance. Notably, the simulations indicated that during periods of significantly low river levels, the maximum velocity generated at the narrow vertical slot of the entrance often exceeds the burst swimming capacity of the target Schizothorax species. This condition creates an overpowering velocity barrier, or an “impassable jet,” effectively preventing or severely deterring fish from successfully detecting and entering the facility. Based on these findings, the study proposes a refined, adaptive operational strategy essential for optimizing the fishway’s performance. To manage this velocity barrier effectively, the operational recommendation is clear: during periods of low downstream water level, the fishway discharge must be systematically reduced. This measure is intended to directly control the water head drop (energy dissipation) at the entrance, thereby ensuring that the resulting flow velocities remain safely within the ecologically determined optimal range of 0.2 to 1.9 m/s. By implementing this dynamic operational adjustment, the Jiacha Fishway can substantially minimize hydraulic interference with the natural migratory behavior of the fish. In conclusion, this research not only provides a concrete, scientifically supported optimization plan for the Jiacha Fishway—significantly enhancing its ecological effectiveness—but also establishes a robust, quantitative methodology that can be readily transferred and applied to the assessment and adaptive management of fish passage facilities across diverse river systems worldwide.