In analyzing the flow characteristics of fishways, it is crucial to consider whether the weirs act as a chain or whether each of the weirs acts independently as a local nonuniformity. Fishways are typically structures, in which the adjacent weirs act in a chain, i.e. the flow in fishway pools is affected by the flow over the adjacent weirs and pools. The resistance coefficient is also a good aid in learning more about fishway structures and their hydraulics. The formulas to calculate the resistance are for uniform flow only, but, as stated in section 2.4, the flow in fishways can be considered uniform under certain circumstances.
A working hypothesis was made that basically pool-and-weir and Denil fishways function in a similar way. This is based on the same kind of energy dissipation pattern: the energy of the flowing water is dissipated mainly because of interaction between the main stream and the circulating water mass inside the pools or pockets. Similarity of the energy dissipation patterns can be noticed also via visual observations when taking into account the water depths over the weir compared to the distance between the ‘weirs’.
An attempt was made to create a general scaling equation for dimensionless discharges that takes into account the depth of flow over the weir and the effect of pool length, i.e. distance between the weirs or vanes. The use of the depth of flow instead of other measures is advantageous for practical purposes in assessing fishway flows. The basic form of the equation for dimensionless discharges can be based on empirical equations and relations that govern open channel flows in highly rough channels. Previously, separate scaling factors and equations for dimensionless discharges have been developed by Rajaratnam et al. 1992 (vertical slot fishway), Katopodis & Rajaratnam 1984 (Denil fishways), and Rajaratnam et al. 1988 (pool-and-weir fishways). In these procedures, the depth of flow and head at the weir have been used varyingly.
A purely theoretical approach for flows in fishways is not possible, or at least it requires a tremendous effort because flow conditions in fishway pools are effected by the adjacent pools. Veijalainen (1985) conducted studies on the adjacent resistance factor. He defined the adjacent resistance factor as an arrangement in a flow channel, in which local resistance factors act together as a system. Fishways are good examples of adjacent resistance factors. Veijalainen (1985) studied a system like that in different situations by changing the location of weirs (factors causing local resistance) with different discharges. In his studies on overflow weirs, he noticed that even in a chain, the weirs act mainly as separate (individual) overflow weirs. Despite that, Veijalainen (1985) noticed that
in the case of adjacent overflow weirs, the upper weir affects the approach velocity at the lower weir causing uneven velocity distribution at the cross section of the whole pool. The velocity distribution changes with the pattern of flow decay in the lower pool. The pattern of flow decay is greatly affected by the pool geometry. This effect decreases with the bottom slope.
With some of the slopes, a standing wave is formed in the pool and it must be taken into account when considering the functionality of the fishway. The presence of the standing wave can be affected by changing the structure of the weir crest.
Water velocity in the overflowing jet affects the water depth in the pool by pushing the water mass in front of the overflowing jet. This is especially true with low water depths in the pool. This phenomenon increases the water velocity below the weir when compared with the situation that the water level in the lower pool remains horizontal.
According to Clay (1961), the flow in pool-and-weir fishways can be either plunging or streaming. 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 the weir. In the streaming mode, a surface stream appears to flow over the crest of the weirs, skimming over the surface in the pools between. Veijalainen (1985) stated that the relation between the pool length and the depth of flow determines whether the flow is plunging or streaming. The flow is plunging when the pool length is long compared to discharge i.e. the depth of flow. When the depth of flow increases the flow changes into streaming mode. For pool-and-weir fishways with notches or a triangular crest, this change occurs with lower discharges than for pool-and-weir fishways with a horizontal weir.
It appears that even though knowledge of the hydraulics of weirs is extensive (e.g. Lakshmana Rao 1975), the understanding of pool-and-weir fishways is not satisfactory. This is partly because the weirs, which are placed reasonably close to each other, act together hydraulically, at least in the range of flow that occurs in fishways. It is also not possible to predict as to when a plunging flow passes into the streaming flow regime.
The flow changes into a plunging flow even in Denil fishways for extremely low discharges, i. e when the ratio water depth to baffle spacing is low enough and when the flow plunges over the baffles. In this mode, the Denil fishway begins to act like a pool-and-weir fishway with a triangular weir crest. In Denil fishways, the flow is streaming in the normal operation range of water depths.