Establishment of swirling-flow fishway and analysis of its hydraulic characteristics

The structure and flow pattern of traditional fishway are relatively simple, which is usually suitable for specific fish to pass through. However, there are many types of fish to be protected in rivers. Near natural fishway simulates natural form of fish and is expected to improve fish passing efficiency. Therefore, in order to enhance the meandering and naturalness of fish passing facilities, a swirling fishway structure with curved sidewall and diversion bank was proposed in this paper. The side wall of the swirl fishway adopted circular curve, and the contraction area between the chambers was smoothly connected by the reverse circular arc, which made the water flow close to the natural sinuous channel. The swirling fishway was obtained by improving Taiji fishway. Compared to Taiji fishway, it omitted the Taiji disc and was cheaper. At the same time, the water flow in the pond could be rapid and slow, or deep and shallow, highlighting the effect of Yin and Yang and enriching the flow pattern. Through the combination of numerical simulation and physical model test, the hydraulic characteristics of swirling fishway under different working conditions were explored. First of all, through physical model test, the water depth along the wall and the flow pattern distribution of the two sides of the fishway under different working conditions were obtained, and the effect of fish passing was observed. The accuracy and rationality of the numerical calculation were verified by comparing the water depth data obtained from the physical model test and the numerical calculation results. The CFD numerical calculation method was applied to simulate distribution of surface velocity field and internal velocity field that were difficult to be measured by physical model. The free surface flow velocity, the velocity near the bottom slope and the velocity distribution in key cross section of the swirl fishway were further analyzed. According to the design guidelines of fishway, the basic points of hydraulic design and the gravity similarity criteria, the physical test model was established. After the establishment of the physical model, the water flow test under different conditions was carried out, and the water depth of the measuring point was measured by the water level probe. The results showed that the swirling flow in the fishway was zigzag and the flow pattern was rich. The maximum relative error of simulated and measured water depth was 14.2%, less than the allowable value of 15%. The variation trend of the measured and numerical water depth along the wall was approximately the same. It indicated that the simulated method was reliable. The velocity in the main flow area of the fish passage was not higher than 0.84 m/s, meeting the requirements of fish passage. The fish passage object could be traced smoothly in the fish passage. The slow flow or return area on both sides of the main flow could be used for fish rest, which showed the rationality of the fish passage shape. The depth of the shallowest water increased from 1 cm without diversion sill to 2.4 cm with diversion sill. After scarification, although the upper water flow could still overflow the diversion scarp and enter the pool chamber, the shallowest water depth on the left side of the pool chamber was about 0.5 cm higher than that on the right side, and the water depth growth was smaller. The fishway proposed in this paper enriches the form of fishway and can guide design and construction of similar fishways.