In Japanese rivers, there are many river constructions, i.e., dams, weirs, drops, for the purpose of flood control. Fishways are river constructions which facilitate migration of fish past dams and weirs. There are a lot of fishway types such as pool-and-weir type, stream type, operation type and so on (see Nakamura, 1995). The pool-and-weir fishway is typical type in Japanese rivers. There are three types of flow regimes in pool-and-weir fishways such as a plunging flow, streaming flow and intermixed flow of plunging and streaming flows. Rajaratnam et al. (1988) proposed a prediction formula of these flow regimes. However, this formula has no physical meaning. Further the accuracy of formula is not high. In this study, the criterion formula, which can predict the flow regime in the pool-and-weir fishway, is suggested semi-theoretically. The experiments were conducted with changing the aspect ratio and discharge in the pool-and-weir fishway. The water surface profiles were measured with a point gauge and also two components velocities were measured with a 2-D electromagnetic current meter. A new criterion formula, which is able to predict the flow formation, is proposed.

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