2.3. Vertical slot fishways
Basically, a vertical slot fishway consists of a rectangular channel with a sloping or stepped floor, which is divided into a number of pools of regular lengths. Narrow slots, adjacent to either one or both walls, extend vertically over the full height of the baffle. Water flows through the slots creating small drops and disperses in the pools, dissipating energy through circulation around the axis perpendicular to the flume floor (Katopodis & Rajaratnam 1983). Fish ascend from pool to pool by swimming though the vertical slot at a depth they prefer.
Vertical slot fishways tolerate reasonably large upper and lower water level fluctuations. One reason for this lies in their hydraulic function: flow patterns inside the pools and water velocities in the slots are almost independent on the water depth in the fishway. Velocity distribution in the slots is even, and the same velocity prevails from bottom of the slot to the water surface.
Rajaratnam et al. (1992) carried out a large study on the hydraulics of vertical slot fishways (App 4). For scaling the discharges of different structures, they used the same equation (Eq. 5) as in their studies on Denil fishways. This equation gives a dimensionless discharge for vertical slot fishways that can be expressed as
where α and γ are constants dependent on the structure geometry, yo is water depth in the pool, and bo is slot width.
The dimensionless discharge equations for the simplest and yet effective vertical slot design (Fig. 55) can be defined by equations (9) or (10), depending on the depth of flow:
