POWER October 2011 - 52

PLANT DESIGN
bulk of the impinged fish. These smaller,
weaker swimming fish are unable to escape
the intake current and are drawn in to the intake
screen.
Technologies growing in favor are those
that use behavioral modification, a system
that uses stimuli such as electricity, sound,
light, and air bubbles. The results obtained at
several power stations and other water intakes
over the past 10 years have proven such technologies
to be effective in protecting many of
the juvenile or mature fish species.
Multi-Purpose Fence
The bio-acoustic fish fence (BAFF) system
is a novel approach to blocking fish from impinging
on intake structures. The pneumatic
nonphysical barrier system introduces sound
and, in some cases, light into a bubble curtain.
This wall of sound, light, and bubbles is
very effective in guiding and deflecting fish.
Sound Fence. The BAFF system consists
of series of sound projector arrays
(SPA) connected to a source signal generator
via a series of amplifiers by special underwater
cables. The sound projectors are
designed to transmit sound into water for
varying water depths.
The difference in effectiveness of the
BAFF is attributed to differences in specie
sensitivity, principally the anatomy of the
hearing mechanisms. Sound is detected in all
species by the otolith organs of the inner ears.
The hearing range of most fish falls within
the audible range to humans, maximum sensitivity
lying in the sub-3-kHz band down to
infrasound (less than 20 Hz).
An acoustic fish deterrent (AFD) system
exploits fish hearing sensitivity in the 20 Hz
to 500 Hz range. Low-frequency sound (10
Hz to 3 kHz) is used for all species other
than clupeids (small river fish like herring);
for clupeids, either low-frequency sound or
ultrasound (a frequency above the limit of
human hearing, about 20 kHz) has been used
with good results.
The sensitivity of fish to sound frequency
can be depicted on an audiogram that describes
the detectable sound pressure threshold
to different frequencies (Figure 1). A
well-designed BAFF is a deterrent for up
to about 80% for many teleost species (rayfinned
bony fish possessing a developed
swim bladder) and for up to 90% to 100% for
the most sensitive species, such as herring.
The AFD has been extensively tested
in various power plant applications, usually
with good results. For example, at the
Hartlepool Nuclear Power Station, located
in northeast England, an AFD was 79% effective
in deflecting herring but only 55%
effective with whiting. Scotland's Blantyre
1. Fish hearing test results. The reference for the figure is a relative " loudness " value
of 1.0, which translates into ±0 dB as the baseline. Because the scale is logarithmic, at -10 dB,
the relative loudness is reduced to 0.5 of the baseline; at -20 dB, 0.25 and so on. A sound level
measurement of 1 pascal is equivalent to a sound pressure level (SPL) of 94 dB, the volume
level we actually hear. This graph allows us to estimate the SPL hearing threshold of various fish
species for different frequencies. For example, cod can detect very low sound levels in the 100
Hz to 250 Hz frequency range. Source: A.D. Hawkins, " The Hearing Abilities of Fish, " Hearing
and Sound Communication in Fishes, 109-33 (Springer-Verlag, 1981).
+20
+10
-10
-20
-30
-40
-50
-60
-70
Salmon
Hydroelectric Plant tests were effective on
74% of the salmon and 92% effective on
mixed cyprinid species (soft-finned freshwater
fish). And testing at Electrabel's
Doel Nuclear Power Station, Units 3 and
4, located in Belgium, found the following
diversion effectiveness: herring (95%),
sprat (88%), bass (76%), smelt (64%), and
gobies (46%).
Light Fence. High-intensity flashing light
has been found to be effective as a fish deterrent.
The BAFF can include a narrow line of
high-intensity flashing lights that are located
near the SPA. A special signal generator and
accumulator powers the light bars. Operating
results at several stations have proven the
effectiveness of light stimuli on various species,
especially juvenile American shad.
Air Bubble Fence. At the base of the SPA
and the high-intensity light bar, a bubble curtain
is created by using specially designed diffuser
tubes to create a dense and continuous
air curtain. The number of SPAs, light bars,
and the length of the curtain of air bubbles
are selected based on specific site conditions.
An air bubble curtain is the most basic stimulus
successfully used as a fish deterrent, with
deflect efficiencies up to 98% reported, but
fish quickly adapt to bubble curtains alone,
so they become less effective over time.
When using SPA or high-intensity light
bars alone, neither the sound nor the light
is concentrated. Instead, the bubble curtain
creates an intense and largely contained
field. The result is an electromagnetic or
pneumatic sound transducer coupled to a
bubble curtain, causing the sound waves to
propagate within the rising curtain of bubbles.
Water, which is more transparent in the
bubble sheet, allows light to reach the surface
even in turbid water (Figure 2).
The novel method of entrapping sound
and light inside the specially developed in2.
Virtual fence. A typical sound projector
array with a high-intensity light bar with a curtain
of bubbles forms an effective fish fence.
Source: Ovivo USA LLC
Catfish
Cod
Dab
30 50 100 200
52
500 1,000 2,000
Frequency (Hz)
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POWER | October 2011
5,000 10,000
Sound pressure threshold (dB re.1 Pa)

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POWER October 2011

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