# An Experimental Study of Turbulent Structures in a Flat-Crested Weir-Type Fishway
## 1 Introduction  
A fishway is a low-head hydraulic structure, which can assist fish species to overcome obstacles to assist in spawning, feeding, and overwintering migrations. A lot of fishways such as Doulonggang tidal shuice, Taiping sluice, and Yuxi sluice were built in shuice complexes in China during the 1960s and 1970s [1; 2; 3]. Some fishways such as Qilililong fishway at Fuchunjiang hydropower station, Yangtang fishway at Hengdong pumped storage power station were built in hydropower stations [4; 5]. A fishway effect is not only related to hydraulic characteristics of the pool, but also depends on swimming behavior of fish species. The investigated data [6; 7] showed that fishways that fish can smoothly pass through were no more than half in the built fishways in the world. According to the UN World Water Development Report [8], freshwater fish has been reduced by 30% worldwide since the 1970s. The British fish monitoring data indicated that the regression rate of Atlantic salmon apparently lowered in the past decades [9], and the effectiveness of fishway was doubtful. So it is of important practical significance to further study the hydraulic characteristics of fishway based on the swimming behavior of fish species, especially in the effect of turbulent structures in fishway on fish swimming behavior. Fishway can be divided into pool-type [10; 11; 12], Denil-type [13], and nature-like bypass-type [14; 15; 16; 17] though other types exist. Designs of almost all types of fishways are based on the dissipation principle so as to lower flow velocity for facilitating anadromous fish species. The basic principle of pool-type fishway consists of a series of pools separated by a certain number of baffles, which breaks up the total head into a series of small heads to lower the velocity of fish passage and create a waterway that fish can swim through. The baffles can be used to control the flow pattern in the pool and provide an environment that can be of both swimming and rest for fish. In general, according to the shape of baffle, pool-type fishway can be divided into vertical slot, crested weir, orifice, and their combined fishways. The crested weir fishway is suitable for fish behavior that has a preference for swimming and jumping in the surface layer. Up to now, hydraulic design of most of fishways focuses only on mean velocity, neglecting turbulent characteristics in fishways. Fish can detect turbulence by their inner ear, lateral line, and nerve hillock [18]. However, how do fish cope with external stimulation? It has been not reported yet. Some studies on the effect of turbulence on fish behavior, energy, and distribution showed that fish can benefit from some turbulent regimes, for example, the experimental results of [PERSON], [PERSON], and [PERSON] et al. [19] showed that adult rainbow salmon could benefit from the vortex energy when swimming in steady vortices behind an obstacle. The salmon swam in the form of ski-jump among the vortices, which could reduce the axial muscle activity. It means that the energy cost of a fish swimming in turbulent flow could be reduced if the fish swam appropriately. The similar observed results showed that adult sockey salmon utilized vortex zones to minimize its energy cost on the way of anadromous migration [20; 21]. [PERSON] and [PERSON]'s [22] experiment revealed that fish could benefit from a vortex due to the leading fish in the fish school, for example, the beating frequency of their tail fin could lower 9-14%, and the energy cost could reduce 9-23% in the rear of a sea bass school than that in the front. If flow in a fishway would be of such ecology-friendly flow structures, it could greatly contribute to fish swimming. It can be said that the experimental study of flow characteristics in a fishway pool focused mainly on a one-dimensional aspect because of the limit of test means in the past. Actually, fish species are in three-dimensional flow fields, and experience effects of three-dimensional turbulent structures.