Fish management: the new wave30 August 2000
Intake screens come under huge pressure from fish runs. At Doel, sound may manage the problem.
Fish protection at water intakes has traditionally been achieved by mechanical screens, but these are expensive and block easily, and the fish that get stuck against the screen usually perish. Acoustic fish deterrent systems are growing in popularity because they can be retrofitted quite simply to existing cooling water intakes and are a cost-effective addition to new stations.
How do they work?
Acoustic fish deterrents are based on the way that fish hear, the fish frequency range, the sorts of sounds that are repellent but harmless to fish, the sound field that fish react to best, and the level of sound that will cause fish to react.
Many species of fish have well-developed hearing characteristics. Fish detect sound as a vibration of the water (particle velocity) or a change in pressure (sound pressure). They sense low frequency vibration using their inner ear and lateral line organ. Fish that have an inner ear connected to the swim bladder (used to maintain buoyancy) are highly sensitive to sound pressure and acoustic fish deterrents. It is important that the plant operator is aware of its main problem species, as the effectiveness of an acoustic fish deterrent system will vary with the type of species present.
Fish are typically able to detect sound in the 0-3000Hz range, but deterrent
signals need to be pitched at the more sensitive parts of their hearing range
(20-500Hz). The level of sound generated needs to be set above any background sounds (eg pumps) and at a level that will create an avoidance reaction.
An acoustic deterrent needs to create
a sound field that extends smoothly across the width of the cooling water intake, and reaches a sufficient distance from the intake so that fish will not be drawn into the flow. This can be designed with hydraulic and acoustic modelling.
Hydraulic modelling predicts the streamlines entering the intakes and enables the necessary extent of the sound field to be determined, taking into account fish swimming speeds in relation to intake water velocities.
Once the size of the sound field has been determined, this can be incorporated
into an acoustic model to determine the most appropriate type, number and configuration of sound projectors required. Modelling underwater sound around
cooling water intakes is complicated by sound waves reflecting off the water
surface, sea or river bed and concrete walls. Only by taking into account all of these parameters is it possible to design a smooth and effective sound field with a maximum level in the areas where an avoidance reaction is required. Rapid changes or areas of low sound may
Types of acoustic deterrents
There are several types of underwater acoustic systems used to block and guide fish movements. The type used to create a repellent sound field ahead of a cooling water intake is called a sound projector array (SPA). This comprises:
•A single or multiple signal generator to produce the required signal or signals. Multiple signals are used to avoid resident fish becoming accustomed to any one
•Power amplifiers to boost signal levels.
•Underwater sound projectors to create the required sound field.
•A diagnostic/alarm unit to monitor the output of the amplifiers and sound projectors and send an alarm to the station control room if a fault is detected.
The system operates similarly to a public address or domestic high fi. Signals are recorded onto EPROM chips, and can be manually selected or played at random or in rotation. The sound projectors are electromagentic devices comprising a piston and rubber diaphragm. The projectors must be pre-pressurised appropriately for the operating depth or have a pressure compensation device. Each amplifier and sound project will require around 1kVA.
Best results are obtained when the projectors are close to the intake opening. Ideally, it should form a steep acoustic gradient close to the entrance, but one which extends far enough for fish to be able to swim against the intake flow. Acoustic nulls caused by intereference must be avoided as they may cause fish to be guided into the intake.
Other underwater acoustic
systems, such as the bio-acoustic fish fence (BAFF) have different applications. The BAFF uses an air bubble curtain to contain a sound signal that is generated pneumatically. Effectively, this creates a wall of sound that can be used to guide fish round river structures (eg hydroelectric stations or other water intakes) by deflecting them into fish passes.
A case study
Under pressure from environmental regulators and fishermen, Belgian power utility Electrabel was looking to reduce the 140t of fish that was being drawn into the cooling water of the Doel nuclear power station each year. The main species under discussion were herring and sprat.
In 1997, a SPA fish deterrent system was designed and installed on the offshore intake. In total, 20 large FGS Mk II 30-600 sound projectors were installed to create a repellent sound field close to the water intake openings, warning fish of impending danger and causing them to veer away. A multiple signal generator was used to avoid resident species becoming habituated to any single signal.
Dr Jeremy Nedwell, engineering
director of the project says: “The challenge has been to be able to create a sound
field that is effective, but which is confined to the immediate area of the intake to avoid driving fish away in a way that would impact upon legitimate fishing activity.” One part of the project it was important to get right was the deployment system that places the sound projectors
in an optimum configuration around the intake openings. The system allows the projectors to be raised for maintenance while the cooling water system is in use. Without this system, maintenenace would have to be
carried out during outages and divers would be required.
The acoustic installation has now undergone a number of evaluation trials by researchers from Belgium’s Leuven University. These show a reduction in the target species of 98%. In addition, the catch of other non-target species has been reduced. The overall reduction reaches 81%.
So that the equipment can be serviced while the system is in operation, a frame has been installed to lower sound projectors into position and to raise them for routine inspections and maintenance.
The new technology is now being adopted at new and existing nuclear and thermal power stations. Trials at other nuclear stations, such as British Energy’s 1200MW Hartlepool AGR, have also shown significant reductions in fish entrainment. Acoustiic systems at such plants typically cost £50,000-200,00.