Recommendations for construction in fishways

The effectiveness of the systems (optical and acoustic) used to monitor fish passes depends mainly on the hydrological conditions prevailing in the water. The work of optical systems is strongly dependent on the degree of water transparency. Transparency of water in practice does not have a major impact on the correct operation of acoustic systems.
Often, however, the design of monitoring systems is based on the use of both optics and acoustics. In these cases, the degree of aeration and water transparency is important.
The basic requirement for both types of monitoring systems is to keep as little water as possible where the monitoring equipment is installed. Air bubbles in the water cause significant disturbances in the operation of the systems. Especially in the case of fast currents, larger air bubbles take the form of a spindle-like shape, from which the echo can be incorrectly recognised as an echo from a fish.

Such a shape of air bubbles in conditions of low water transparency may cause misidentification of the image from the camera and the qualification of larger bubbles as quickly moving fish.

Causes of reflections in water
The basis of all acoustic systems is the directional transmission of sound at different frequencies through the water and the recording of its reflections by appropriate receivers. The reflection (echo) occurs on the boundary layers between the water and the water bed or fish. Similarly, any air bubbles in the water cause very pronounced boundary layers. In conditions where the water is not aerated, the sound wave penetrates through it without hindrance, enabling correct transmission and correct readings of data about objects in the water. But if air appears in the way of a sound wave in the form of small or large bubbles (with sizes from less than 1 mm to more than a few cm), the sound wave is suppressed and reflected. As a result, either it is impossible to correct the echo recording or the fish are difficult to distinguish against the background noise.

Causes of aeration in water
The first source is surface aeration of the water caused by wind, which induces water wave formation, as well as rainfall. As wind speed increases, the resulting waves mix the water with air. By observing the water, one can notice a white surface containing air bubbles.
The second source of aeration is the movement of water in confined spaces. Due to the higher water velocity, the concrete surfaces of the channel boundaries create vortices, which contribute to the formation of air bubbles that cause acoustic wave reflections. In cases of irregular channel wall surfaces in fish passes, high-velocity currents capture surface bubbles and draw them deeper into the water. When the current is slow, the surface bubbles rise to the top and dissipate on their own.
More hidden are the bubbles formed as a result of cavitation processes, known as "cavitation bubbles." The causes of cavitation formation are not fully explained or predictable. Often, scientific explanations do not hold up, and one has to rely more on established facts. From practical experience, it is known that cavitation bubbles form when there are small or even very small vortices, especially near sharp edges. Water in vortices, even on such a small scale, undergoes significant pressure variations. At sharp edges, the pressure changes suddenly, which can lead to the formation of cavitation bubbles. Cavitation bubbles are not, in essence, air bubbles, but bubbles of temporary vacuum. Under the pressure of the water, these vacuum bubbles undergo a rapid implosion, causing the generation of strong acoustic disturbances in the water.

Based on the phenomena and causes of acoustic disturbances in water described above, several key conclusions can be drawn, which may be helpful in the process of designing fish monitoring locations in fish pass channels.

1. The best location for a monitoring station is the so-called "upper water." 2. It is not recommended to place flow-reducing elements upstream of the monitoring station, as they may cause harmful disturbances in the water. 3. Smooth surfaces of the fish pass channel walls should be maintained. 4. It is advisable to design the fish pass so that the section of the channel upstream of the monitoring station is as straight as possible, without bends. Bends can cause water disturbances. 5. Forcing a change in the direction of a fast-flowing current always leads to significant aeration of the water. 6. If there is a strong current on the upstream side, it is recommended that the edges of any elements, such as the guides of sluice gates closing the fish pass channel, be rounded. Sharp edges should be avoided as much as possible. Sharp edges or shapes lead to the formation of cavitation bubbles.