Acoustic fish barriers are designed primarily either to deflect or to guide fish movement.
Typically water intakes are located on rivers or estuaries where there is a natural flow of water past the intake, and so when designing the system the aim is to create a sound field that deflects the fish into the flow streamlines that will carry the fish past the intake. It is important to ensure the fish are not entrained into the flow entering the intake, and so the sound field must extent to sufficient distance that the fish are deflected before the intake velocity exceeds the fish’s maximum swimming speed. This is achieved using a Sound Projector Array (SPA) system, and is the most common form of installation (see figure 1).
When a system is installed on a lake, canal or other area where there is not a significant flow past the intake the fish are more likely to be resident in the vicinity of the intake and so the risk of the fish habituating to the sound signal is increased. Habituation to the signal can be overcome by changing the acoustic signal at intervals, so typically multi-signal systems are installed for these applications.
There are occasions where it is necessary to block fish movement and prevent fish swimming along a waterway or canal, and the same principle is used, apart from a fish barrier is designed to traverse across the entire width of the waterway. This application is typically used when a barrier is required to prevent the movement of invasive species, such as Asian carp.
For certain applications, such as hydro-electric power stations, the vast majority of the flow will enter the intake or intake canal, and only a small proportion may be used as a compensation flow. For a fish pass or fishway at a hydro-electric power station this is typically less than 5%. Without any screening only 5% of the fish approaching the intake would be expected to enter the fish pass, and even when an intake is screened, the fish still need to find the fish pass before they are entrained into the flow entering the intake (see figure 2).
For this application a BAFF system is used, as the sound field is concentrated in close proximity to the bubble curtain, and the ‘wall of sound’ can be used to accurately guide the fish towards the fish pass.
For some applications it may be considered the fish need additional stimulus to help guide the fish pass an intake, for example when an intake is particularly wide in proportion to the width of the main river, or when the positioning of the intake on the outside of a bend lends itself to fish naturally swimming into the intake. In these instances a BAFF based system may be more appropriate, to help ensure the fish remain in the main river.
Historically, many attempts to use sound for fish repulsion have been unsuccessful. These have generally resulted from one of the following causes:
Failures can lead to high costs for remedial action and lack of fish protection. FGS can offer the professional advice, support, measurement and modelling services to minimise and avoid the risk of failures.
In order to design an effective system both hydraulic and acoustic modelling may be required.
For further information on specifying a fish deflection system please contact FGS.