Hydraulic characteristics for the optimal structure of long-distance fishways with high head

Fishway flow is one of the most important influence factors on the efficiency of the fish migration over the dam. High-head, long-distance fishways are often constrained by the terrains, multiple bends, and turns, thereby resulting in complex and variable flow patterns. This study aims to investigate the hydraulic characteristics of a high-head, long-distance fishway, according to practical engineering applications. Experiments and numerical simulations were conducted to determine the flow patterns in the key sections, including the chamber, typical rest chamber, turning rest chamber, and entrance. The key hydraulic parameters were measured, such as the water depth and flow velocity. Results showed that the flow depth of the chamber was basically maintained at about 1 m. Among them, the vertical slot section was narrowed, and the flow depth increased to a maximum of 1.06 m. Flow velocities were ranged from 0.91 to 1.17 m/s in the vertical slot. While the mainstream flow velocity was averaged around 0.50 m/s in the typical rest chamber. Flow trajectories followed an S-shape in the chamber and a typical rest chamber. There were distinct main flow zones and variable recirculation zones. The distribution of the flow velocity was generally favorable for the fish migration. In the upper half of the 180° turning rest chamber, the main flow velocity ranged from 0.40 to 0.70 m/s and then flanked by two recirculation zones. In contrast, the latter half exhibited the expanded flow with the lower velocities of 0.20 to 0.30 m/s and the minimal velocity gradients. Thereby the migration of the fish was disoriented potentially. Similarly, the flow velocity of the mainstream flow ranged from 0.50 to 0.90 m/s in the large-angle long-distance turning section. While the recirculation zone remained below 0.20 m/s. The flow velocity was stabilized at 0.20-0.30 m/s in the latter part of this section. It was difficult for the fish to maintain their migration direction. Additionally, the flow velocity was low and nearly stagnant upstream of the horn-shaped fishway entrance. The rheotactic threshold was required for the fish to locate the entrance. The effective length of the entrance attraction was only 50%, thus hindering the fish entry. Various strategies were proposed to optimize these flow conditions: (1) Four sets of the conventional partitions were added at the large-angle, long-distance turning sections; (2) The 0.30 or 0.50 m deflectors were installed at 45° intervals in 180° turn sections; And (3) the fishway entrance was modified from a horn shape to a circular arc connection. Numerical simulations verified that the best flow was achieved after optimization, where the 0.30 m deflectors at 45° and 135° positions on the inner wall, 0.50 m deflectors at 90° and 135° positions on the outer wall, and a 0.30 m deflector at 180° on the outer wall of left turns. There was a stable flow velocity of 0.50 m/s in the rest chamber. A favorable flow field was also formed with a central mainstream and low-velocity zones on both sides, allowing for the fish to rest, detect flow, and continue upstream migration. Additionally, the horn-shaped entrance was replaced with a circular arc connection. The overall flow velocity increased to above 0.10 m/s, in order to improve the fish detection and entry with the attraction range. The findings can provide valuable insights to optimize the fishway flow for the fish passage efficiency in the high-head and long-distance fishways.