Fishways, even though concise in their structure, are expensive to build in their natural size. Fishways are hydraulically complex structures and hydraulic problems are difficult to solve with merely analytic measures. Even experimental measures with no analytic approach can only be used in solving problems connected to the very structure under consideration. Thus, model studies are profitable when studying and determining the optimal fishway structure.
The most important part in the modeling procedure is a thorough understanding of the system and the processes that take place in it. Thus, it is important to identify those parts of the system’s behavior that are relevant to the problem being considered, while other parts may be neglected. A model provides the necessary information about the response of the system to the planned activities and describes the behavior of the system under consideration in response to excitation. Physically, a model may be a set of algebraic equations that can be solved numerically. In creating a model, combining both physical (i.e. scale model studies) and analytical procedure often proves out to be profitable.
Practically all hydraulic investigations of weir flows aim at studying the relation between the discharge coefficient and the parameters influencing the flow. With dimensional analysis, the results can be extended to be valid for other geometrically, kinetically, and dynamically similar structures. If the scale models are large enough, only gravity need be taken into account, and Froude similarity laws can be applied in analysis. It should be noticed that the curves determined with dimensional analysis are only valid for similar systems.
Previously, models describing the effects of changes in water depths to fishway discharges have been examined separately for each structure. Thus, one dimensionless equation for scaling have been used for Denil and vertical slot fishways, and another for pool-and-weir fishways, even making a difference between streaming and plunging flows. Applying these equations has produced specific equations for every structure, e.g. a total of 18 separate equations for vertical slot fishways and 6 equations for Denil fishways, each of them being applicable to the special structure in concern. Each equation covers all geometrically similar structures, but work is required to apply the equations. The use of several different scaling factors causes more confusion. In solving hydraulic problems in fishways, an overall model for determining discharges and water depths would be profitable. The model should cover a large variety of structures and should be easy to use.
One of the most crucial places in fishways is the entrance. It has been noticed that there is great variety in successful fishway designs, which are used for species with similar swimming capabilities. At the Isohaara fishway, in Northern Finland, the number of large salmon entering the fishway increased after the entrance was changed from a Denil to a pool-and-weir fishway. In order to understand the differences in the hydraulic conditions below the entrance of a fishway, a study was carried out at the hydraulics laboratory of the University of Oulu, Finland. In these studies, flow decay below the entrance was studied for pool-and-weir fishways with horizontal and V-shaped weir crests and for vertical slot and Denil fishways. The designs were compared by using the mean velocity of the cross section as a scaling factor.
The main aim of this study has been to create a general dimensionless scaling equation for fishway structures. In addition, flow patterns below different fishway types have been studied and weir flows over a chain of weirs and over a single weir have been compared. The applicability of the results has been considered.