Chapter 2. Procedures to model fishway flows
- Table of Contents
- 2.1. Pool-and-weir fishways
- 2.2. Denil fishways
- 2.3. Vertical slot fishways
- 2.4. Basic concepts for modeling fishway flows
From the biological point of view, fishways are structures that enable fish to continue their migration past obstructions. In terms of hydraulics, fishways are energy dissipating structures. The flow inside fishways should be concise (clear) and guiding, and not too demanding for the variety of desired species – total passage is not always required or even desirable.
Flow control in fishways is achieved by employing devices that dissipate the energy of the flowing water and maintain velocities within the biokinetic capabilities of migrating fish. A variety of energy dissipating schemes has been invented which give rise to a diversity of fishway types. According to energy dissipation scheme, three basic types are usually recognized: the pool-and-weir, the Denil, and the vertical slot fishway (Katopodis & Rajaratnam 1983).
2.1. Pool-and-weir fishways
The pool-and-weir fishway consists of a channel with regularly spaced weirs, each slightly higher than the one immediately downstream; thus a series of step-like pools are created (Fig. 1). In this fishway, the weirs may incorporate orifices or short surface chutes. Water cascades over the weirs or flows through orifices and chutes into the pools, setting up a circulation pattern around an axis perpendicular to the channel walls. Through this mechanism, water energy is dissipated and velocities are controlled. Fish ascend from pool to pool by jumping or swimming over the weirs, or by passing through the orifices or chutes (Katopodis & Rajaratnam 1983).
Clay (1961) stated that the flow in pool-and-weir fishways can be either in plunging or streaming modes (Fig. 2). When the flow is in the plunging mode, the water level in the pool immediately below the weir (producing the plunging flow) is generally below the crest of this weir. In the streaming flow mode, a surface stream appears to flow over the crest of the weirs, skimming over the water surface of the pools in between (Rajaratnam et al. 1988).
Pool-and-weir fishways have been studied at the University of Alberta, Canada, in the early 1980’s (App. 2). For scaling the discharges of pool-and-weir fishways with plunging flow regimes, Rajaratnam et al. (1988) used equation
where b is width of the fishway (‘free opening’), and h is head over the weir (Fig. 3a). For streaming flow regimes the scaling equation was
where b is width of the fishway, and d is depth of surface flow (Fig. 3b). For mild slopes, the depth of the surface flow is about the same as the head over the weir. Using these equations for scaling, they received for the dimensionless discharge of the plunging flow the equation
and for the streaming flow the equation
where L is pool length and d is depth of surface flow.
