Environmental Engineering Reference
In-Depth Information
the Kaplan-style hydroturbines (an estimated 143 dB re 1 µPa at 1 m), as well as
unknown levels and frequencies of sound from wave interactions with the body of
the device, hydraulic pumps, and the mooring system.
In 2008, the Ocean Renewable Power Company (ORPC) made limited measure-
ments of underwater noise associated with operation of their 1/3-scale working proto-
type instream tidal energy conversion device, its turbine generation unit (TGU). The
TGU is a single horizontal-axis device with two advanced-design cross-flow turbines
that drive a permanent magnet generator. An omnidirectional hydrophone, calibrated
for a frequency range of 20 to 250 kHz, was used to take near-field measurements
adjacent to the barge from which the turbine was suspended and at approximately 15 m
from the turbine. Multiple far-field measurements were also made at distances out to
2.0 km from the barge. Noise measurements were made over one full tidal cycle, with
supplemental measurements being taken later (USDOE, 2009). Sound pressure levels
at 1/3-octoave frequency bands were used to calculate rms levels and SELs. During
times when the turbine generator unit was not operating, background noise ranged
from 112 to 138 dB re 1 µPa rms, and SELs ranged from 120 to 140 dB re 1 µPa. A
single measurement made when the turbine blades were rotating (at 52 rpm) resulted
in an estimate of 132 dB re 1 µPa (rms) and an SEL of 126 dB re 1 µPa at a horizontal
distance of 15 m and a water depth of 10 m. These very limited readings suggest that
the single 1/3-scale turbine generator unit did not increase noise above ambient levels.
In addition to the sound intensity and frequency spectrum produced by the opera-
tion of individual machines, impacts of noise will depend on the geographic location
of the project (water depth, type of substrate), the number of units, and the arrange-
ment of multiple-unit arrays. For example, due to noise from surf and surface waves,
noise levels in shallow, nearshore areas (≤100 m deep and within 5 km of the shore)
are typically somewhat higher for low frequencies (≤1 kHz) and much higher for
frequencies above 1 kHz.
Potential Effects of Noise on Aquatic Animals
Because of the complexity of describing underwater sounds, investigators have often
used different units to express the effects of sound on aquatic animals and have not
always reported precisely the experimental conditions. For example, acoustic signal
characteristics that might be relevant to biological effects include frequency content,
rise time, pressure and particle velocity time series, zero-to-peak and peak-to-peak
amplitude, mean-square amplitude, duration, integral of mean-square amplitude
over duration, sound exposure level, and repetition rate (National Research Council,
2003; Thomsen et al., 2006). Each of these sound characteristics may differentially
impact different species of aquatic animals, but the relationships are not sufficiently
understood to specify which are the most important. Many studies of the effects of
noise report the frequency spectrum and some measure of sound intensity (SPL, rms,
and /or SEL).
Underwater noise can be detected by fish and marine mammals if the frequency
and intensity fall within the range of hearing for the particular species. An organ-
ism's hearing ability can be displayed as an audiogram, which plots sound pressure
level (dB) against frequency (Hz). Nedwell et al. (2004) compiled audiograms for a
number of aquatic organisms. If the pressure level of a generated sound is transmitted
