Arrangement of fishway entrance in power station tailrace channel

Abstract: In large and middle rivers, the scale of the fish way entrance is relatively small, and the design discharge is really low, which often leads to insufficient momentum of the attraction flow, and induce poor attraction effect. In practice, tailrace channel of power station is an ideal gathering place for the migration fish. Therefore, how to reasonable arrange the fishway entrance in tailrace channel is one of the key problems in fishway construction in nowadays. Although, there are some basic principles for the fishway entrance layout, but they are not enough to apply because of the complexity of flow field structure in the tailrace channel. The present research results are still not effectively to guide the specific layout of the fishway entrance in the tailrace channel. In this paper, numerical simulations are used to study the flow field for the arrangement of fishway entrance in the tailrace channel, and to evaluate the connectivity of the migration corridor to downstream river. First of all, a physical model experiment is carried out to verify the numerical model, the maximum error of the flow velocity is 0.03 m/s and the maximum error of the water surface line is less than 5 mm. Then, the numerical simulation is used to analyze the fishway entrance arrangement in the tailrace of a power station, based on the typical three-dimensional flow structure and the swimming ability of the fish. The results showed that the water flow produced by the power station unit has great momentum, and a passage which is suitable for fish migration is formed between the tail water channel and the river. The inducing fish flow is greatly influenced by the operation mode of the unit, the location of the fish inlet and the outlet direction. The operation mode of the power station not only determines the location of the main stream in the tailrace, but also forms several recirculation zones on both sides of the main stream. The flow rotation direction and position in recirculation zones have great influence on the inducing fish flow. The transition region between the mainstream and the recirculation zones is the key area for the layout of the fishway entrance. The selection of the specific location should consider the swimming ability of fish, and the connectivity with the migratory passage downstream. Because the momentum of attraction flow is less than flow of the recirculation zones, the former is enhanced and the coverage range is increased when the attraction flow has the same flow direction with the recirculation zones. When the attraction flow is opposite to the flow direction of the recirculation zones, it is restrained and the cover range will be reduced. The layout design of fish entrance should be designed through the operation mode of the power unit reasonable position and outflow direction. In practice the momentum of attraction flow is usually relatively small, the direction of the attraction flow should be in accordance with the direction of the flow in recirculation zones, and the fish entrances can be arranged in multiple locations according to the operation mode of the unit. The research results will provide important reference for the layout optimization of the fishway entrance design in the large and medium-sized river.