Figure 8. Sound pressure level produced from dumping material into hopper barge measured over ambient conditions and its attenuation with distance from the point source in Cook Inlet, Alaska

ERDC TN-DOER-E14

August 2001

a. Ambient conditions

b. Attenuation with distance from the point source

Figure 8. Sound pressure level produced from dumping material into hopper barge measured over

ambient conditions and its attenuation with distance from the point source in Cook Inlet, Alaska

peak SPL measurements for the Viking dredging in coarse sediments (119.4 dB re 1 Pa-m). Sound

attenuation for the Crystal Gayle events occurred very rapidly, falling to 90.6 dB re 1 Pa-m at the

2,900-m range, or approximately 17 dB above peak ambient levels (Figure 9). In comparison,

bottom strike events in soft sediment ranged from 11.6 dB re 1 Pa-m quieter at 555 m to 27 dB re

1 Pa-m quieter at 2,900 m than peak SPL measurements recorded for bottom strike events at similar

distances in coarse substrate.

DISCUSSION: The majority of underwater sounds produced by bucket dredging operations

monitored in this study were in relatively low frequency ranges, primarily 20 to 1,000 Hz. Noise

levels decreased with increasing distance from the source. SPL (dB rms) diminished from 15 to 30 dB

re 1 Pa-m at 150-m and 5,500-m distances, respectively. In this study dredge sounds were audible

at 5,500 m, whereas at 7,000 m only the most intense event, that of the bucket striking the bottom,

remained faintly audible. The apparent maximum detection distance of 7 km observed in this study

is probably influenced by a number of factors. Much greater detection distances for dredge noise

above ambient were reported by Greene (1987, 1985), who measured broad-band (20-1,000 Hz)

noise emitted by a hydraulic cutterhead-pipeline (cutter-suction transfer) dredge at ranges extending

to 25 km in the Beaufort Sea. Miles, Malme, and Richardson (1987) and Miles et al. (1986) recorded

sounds produced by a bucket dredge, noting most intense sounds in the 1/3 octave at 250 Hz, ranging

from 150 to 162 dB re 1 Pa-m. Cook Inlet waters throughout the study area were extremely turbid.

High prevailing suspended sediment concentrations may have a pronounced sound-scattering effect,

thereby reducing sound detection distances rapidly compared with sounds emitted from sources in