Improving effect of hydraulic characteristics of nature-like fishway with pools and cobblestone weirs

Abstract: The nature-like fish way is one of the most promising layouts of fish pass structures, its remarkable characteristics are that the individual cobblestone weirs from local river bed substrates are adopted in the fishway construction to imitate as closely as possible the flow pattern of Natural River. Therefore it has higher running efficiency for fish migration than traditional technical fishways. In order to identify the differences of the hydraulic characteristics between the nature-like fishway and the traditional fishway, we conducted comparative studies. The three-dimensional mathematical models of the fishways with cobblestone weirs and water proof weirs were developed respectively. The cobblestone weirs had porosity about 12%. It was defined as the ratio of pervious area to total area of the vertical section along with the longitudinal direction. Each mathematical cobblestone feature was defined as obstacles with special shapes as the natural one. The gaps between the cobblestones allowed water to pass through existing calculation elements. A hydraulic model of the fishway with cobblestone weirs was designed based on the law of gravity similarity at a scale of 1:5. The fishway structure of the model test was consistent with the mathematical model. The water depth and velocity at typical measurement points were recorded. The results were used to verify the numerical model. In the scenario that the fish way upstream and downstream water depth was 1.4 m, the hydraulic characteristics in the fish ways, such as flow pattern, flow velocity, turbulence kinetic energy, and flow rate were compared. The results showed that the currents coming from the cobblestone gaps improved the flow dynamics conditions in the pools. The velocity distributions of the gaps in the staggered weirs appeared as the velocities of the surface were smaller than the bottom; the velocities of the sides were smaller than that of the middle. Obviously, using permeable weir fishway, the abundance of velocity distribution for fish was quite obvious, and the velocity distribution of impervious weir fishway was close to uniform. Turbulence in the mainstream can attract fish back to migration, the turbulence kinetic energy (TKE) was considered as a potential index to assess the suitability of the fishway structure. The turbulent intensity preferred by fish may depend on their swimming capabilities. In the fishways with cobblestone weirs, the TKE had greatly increased; the TKE on the surface was higher than that near the bottom. As the water surface of each of the pool was kept at the same level, the water consumption flow is very different between the impervious weir and cobblestone weir fishway. The consumption flow of the impervious weir fishway was Q=1.39 m3/s, while the consumption flow of the cobblestone weir fishway was Q=2.86 m3/s, which was increased to about twice. Thus, the nature-like fishways with cobblestone weirs needed more water flow in operating. The flow pattern of the nature-like fishway was rich and varied, and it was closer to the natural river; environment. As such it provided abundant hydrodynamic conditions for different types of migration fish. The effecting factors on the hydraulic characteristics of the nature-like fishway were analyzed, such as partition type, bottom slope, and different operation depth and so on. In general, the simulation technique of nature-like fishway was still at the exploratory stage, and the study should be further developed with a consideration with special life habits of migratory fish. The approach used in this paper can provide reference for the hydraulic design and optimization of nature-like fishway.