Characterization of Underwater Sounds Produced by ...

Characterization of Underwater Sounds Produced by Dredges Douglas Clarke1 , Charles Dickerson2 , and Kevin Reine1 Abstract: Potential effects of underwater noise produced by dredging operations on a variety of organisms have emerged as a concern of environmental agencies. For example, it has been hypothesized that dredging- induced noise could block or delay the migrations of fishes through navigable waterways. Few data exist that adequately characterize sounds emitted by dredge plants that would support objective decisions balancing the need to dredge against relative risk to a fishery resource. To obtain data to address this concern, field investigations were undertaken to characterize underwater sounds typical of bucket, hydraulic cutterhead, and hopper dredging operations. Bucket dredges produce a repetitive sequence of sounds generated by winches, bucket impact with the substrate, bucket closing, and bucket emptying. In contrast, cutterhead dredges generate relatively continuous sounds as the cutterhead rotates while embedded in the substrate. Hopper dredge sounds consist of a combination of sounds emitted from two relatively continuous sources: engine and propeller noise similar to that of large commercial vessels, and sounds of dragheads moving in contact with the substrate. The intensity, periodicity, and spectra of emitted sounds differ greatly among dredge types. Components of underwater sounds produced by each type are influenced by a host of factors including substrate type, geomorphology of the waterway, site-specific hydrodynamic conditions, equipment maintenance status, and skill of the dredge plant operator. Dredge sound characterization data must be integrated with knowledge of auditory thresholds and behavioral responses of those aqua tic organisms perceived to be at risk in order that potential impacts can be accurately assessed. Ultimately, should problematic effects be detected, sound characterization data can lay the groundwork for effective mitigation measures. __________________________ 1 U.S. Army Engineer Research and Development Center, 3909 Halls Ferry Road, Vicksburg, MS 39180, USA. [email protected] 2 DynCorp Corporation, Vicksburg, MS INTRODUCTION In recent years concerns have been raised regarding underwater noise of anthropogenic origin and potential impacts on aquatic organisms. Hypothetically, underwater sounds may interrupt or impair communication, foraging, migratory, and other behaviors of aquatic organisms. One prominent concern deals with disturbance of communication among marine mammals. Although underwater noise issues have arisen previously, they have primarily been associated with construction and industrial activities, particularly those associated with petroleum exploration (Richardson et al. 1995). Their consideration relevant to navigation dredging has little precedence. We are unaware of any studies that provide detailed characterizations of sounds produced by dredges engaged in either navigation maintenance or deepening operations. Studies by Greene (1985, 1987) and Miles et al. (1986, 1987) are among the very few relevant references that exist. Given the general lack of knowledge on this topic, this investigation was undertaken to characterize underwater sounds produced by common dredge types. These data should

1 Copyright ASCE 2004

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provide a better technical framework upon which to base decisions regarding management of dredging operations and protection of aquatic organisms. In this paper we present preliminary results of field studies conducted by the U.S. Army Engineer Research and Development Center, Vicksburg, Mississippi, in collaboration with the U.S. Army Corps of Engineers Alaska District, Anchorage, Alaska, and the U.S. Army Corps of Engineers Mobile District, Mobile, Alabama. This work was supported by the Dredging Operations and Environmental Research (DOER) Program under the Environmental Windows Focus Area (www.wes.army.mil/el/dots/doer). METHODS Dredge Plants Sub-surface acoustic monitoring of bucket dredging (also known as mechanical or clamshell dredging) operations occurred in Cook Inlet, Alaska. During the study, the Viking, a 1,500 hp clamshell dredge plant owned by the Manson Construction and Engineering Company, was engaged in deepening the entrance channel to the Port of Anchorage. The Viking was using a 10 yd3 bucket to remove coarse sand and gravel.

Figure 1. The bucket dredge Viking in Cook Inlet, AK



Recording of hydraulic cutterhead sounds was conducted in Mississippi Sound, Mississippi. Channel maintenance dredging was underway by the Lake Michigan Contractors Dredge James B, a 10,000 hp, 24 inch cutterhead plant.

Figure 2. Hydraulic cutterhead dredge James B operating in Mississippi Sound off Gulfport, MS Underwater sounds of a hopper dredge were recorded in upper Mobile Bay, Alabama. The Bean Stuyvesant LLC Dredge Stuyvesant, a 15,000 hp plant with an 11,140 yd 3 capacity, was performing maintenance dredging and transporting dredged material to an offshore disposal site.

Figure 3. Hopper dredge Stuyvesant dredging in Mobile Bay, AL 3 Copyright ASCE 2004


Field Recording Sound source audio of the various dredging activities was recorded with a RESON TC-4032 low noise hydrophone with a built- in 10dB preamplifier, which was connected to a RESON EC-6070 hydrophone audio amplifier via a 50- meter deck cable. The EC-6070 was used to amplify the source levels an additional 18dB (total source amplification of 28dB) for both bucket dredging and cutterhead dredging sessions before the audio data were recorded on a TASCAM DA-P1 Digital Audio Tape (DAT) recorder. Due to the higher sound intensities associated with the hopper dredging operation, the EC-6070 gains were set to 0 dB (total source amplification 10 dB). All hydrophone source audio data were recorded on the left audio track of the DAT recorder, while simultaneous field notes were narrated and recorded on the right audio track. During recording sessions the DAT gain was set to 5 on the left (hydrophone) channel and 10 on the right (notes) channel. Simultaneously, the hydrophone audio data were input into a Sound Technologies analog-to-digital converter where the audio data were digitized and fed to a laptop computer running Sound Technologies Spectra Lab v4.32 audio analysis software. The audio analysis software was used to display a real time audio spectrum. The system was powered by two 12VDC deep-cycle marine batteries connected to a StatPower pure sine-wave inverter, which provided a 120V AC power source to an APC Smart-UPS 1000 uninterruptible power supply. The Smart-UPS 1000 powered the DAT recorder, the laptop computer, and a ProTek model 3033 variable voltage DC power supply, which was used to provide 24VDC power to the EC-6070 hydrophone audio amplifier. By using the two 12VDC batteries as the only power source, the entire system could be operated with the listening vessel completely shut down to a “quiet” mode. This eliminated any noise effect that would be introduced by the engine or generator operating on the vessel used as the listening platform. All bucket dredging sessions were recorded aboard the Manson Dredging Company’s steel hull launch Margaret M. Due to extremely high tidal amplitudes and flow velocities in Cook Inlet, it was determined that monitoring the acoustic levels from a fixed or anchored position was not possible. Instead, a “drift” approach was used in which the Margaret M was maneuvered to a predetermined distance away from the sound source and was then completely shutdown and allowed to drift with the current during the recording session. This approach minimized the flow conditions present at the hydrophone, thereby reducing drag imposed on the hydrophone. Small vessels were used as listening platforms for both the cutterhead and hopper dredge studies. A similar sampling protocol was employed to record sounds of the cutterhead dredge James B, involving multiple recording sessions at increasing distances from the dredge plant. To record sounds of hopper dredging operations, the listening vessel was held in a stationary position a short distance to the side of the track of the advancing Stuyvesant, and the position was maintained as the hopper dredge continued past that location. At the start and end of each recording session the distance to the dredge sound source was measured. At close ranges the distance was determined with a laser range finder and the distance recorded in meters. When the distance was further than the maximum usable distance for the laser range finder (approximately 500 meters), ranges were estimated using radar units and recorded in nautical miles. In addition to the distance notes narrated on the DAT tape, positional information including latitude and longitude was recorded at the beginning and end of each drift session with differential GPS.



Three major categories of dredge plants (bucket, hydraulic cutterhead, hopper) remove sediment from waterways in very different manners. Hopper dredges hydraulically remove sediment from the seafloor through dragheads, which are in contact with the substrate as the hopper dredge is underway. Thus hopper dredges are similar to large vessels. Much of the sound produced during filling of the hopper is associated with propeller and engine noise with additional sounds emitted by pumps and generators. These sounds are continuous in nature. In contrast, hydraulic cutterhead dredges are relatively stationary as the cutterhead turns at 1 to 10 rpm while imbedded in the substrate. Hydraulic cutterhead dredge sounds are therefore largely continuous as the cutterhead swings laterally across an arc. The dredge progresses forward by alternately swiveling on posts called “spuds.” Winch and generator sounds transmitted through the hull of the dredge are additional sound sources associated with this type of dredging operation. Bucket dredges, exemplified by the Viking (Figure 1), are also relatively stationary. However, much of the sound produced by bucket dredges is repetitive rather than continuous. Bucket dredging entails lowering the open bucket through the water column, closing the bucket after impact on the bottom, lifting the bucket up through the water column, and emptying the bucket into a barge. Sound Data Analysis Procedures Each dredge sound recording session was digitized from DAT tape using a Sound Technologies analog-to-digital converter and SpectraLab v4.32 software into MS Windows compatible 16 bit stereo WAV files with a sampling rate of 44.1 kHz. Each WAV file was reviewed and the contents of each file summarized. Initial sessions were used only to determine appropriate gain settings for the hydrophone audio channel (left) on the DAT recorder and not for audio analysis purposes. Twelve additional sessions were conducted to monitor the Viking dredging operations for repeated bucket deployment/retrieval cycles at a number of distance ranges from the dredge. Because cutterhead sounds are continuous, several minute intervals were measured at selected distances. Durations of the hopper dredge sessions were determined as the dredge began and ended an individual pass within the navigation channel. Each pass involved turning on the draghead pumps, lowering the dragheads to the bottom while underway, dredging along a zigzag track along the channel, raising the dragheads, and turning the vessel to repeat the process in the opposite direction. An important consideration in assessing the probability of detrimental effects of dredging sounds is ambient underwater sound levels. Numerous factors contribute to ambient sounds at a given location, including tidal hydrodynamics, meteorological conditions and sea state, the presence or absence of ice, and sounds of biological origin. Cook Inlet ambient sound data were collected in two ways. On one occasion, ambient sounds were recorded at the dredging site with the Viking shut down into as quiet a mode as possible. The listening vessel was tied off to the port side of the Viking and the major sound sources sequentially eliminated, i.e., first the bucket hoist engine was shut down, followed by the power generator. On a second occasion, data were recorded at a site approximately 20 km from the dredging location. Data from the latter effort were used to plot ambient conditions (e.g., Figure 5). Alternatively, an approximation of ambient sound levels was obtained by examining time intervals between dredging cycles at various distances from the dredge. A comparison of WAV files for various sound events and for ambient conditions revealed that ambient sound levels occasionally were more intense than dredge- induced sound levels. We attribute this to variation in ambient sounds influenced by tidal flows. Although



weather conditions were relatively calm and stable throughout the study period, tidal flow velocities changed substantially during and between recording sessions. Some amount of background noise attributable to high flows over the hydrophone and attached cable and fairing may account for some of the observed variation in ambient underwater sound levels. Ambient conditions at the study locations in Mississippi Sound and Mobile Bay included other anthropogenic sound sources, including commercial and recreational vessel traffic and shoreline development activities. RESULTS Bucket Dredge Sounds For the purposes of sound characterization we identified six distinct “events” comprising a single cycle of bucket deployment and retrieval. The first event is winch noise as the dredge derrick and bucket swing outward and the bucket is lowered. Although not classified as an “event” herein, a “splash” sound caused by the bucket hitting the surface of the water could be detected at relatively short distances from the source. This sound was variable depending on the speed and angle of the bucket as it entered the water. In the second event a sudden and often very intense impact noise is produced as the bucket contacts the bottom. It should be noted that the sound characterizing this event is influenced by the granulometry of the sediments being dredged, i.e., a bucket impacting coarse sands and gravels, as was the case in this study, would produce very different sounds than a bucket striking unconsolidated muds. In the third event, a “grinding” type noise is produced as the bucket is closed and the dredge material (coarse sand and gravel) is removed. In the fourth event, a “snap” is sometimes audible as the jaws of the bucket close against each other. In the fifth event, more winch noise similar to the initial “event” winch noise is audible as the bucket is raised to the surface and the derrick swings over the barge. Finally, in the sixth event, material being dumped into the barge may or may not produce an audible sound depending on the level of material existing in the barge. For example, material striking metal in an empty or partially full barge generally produced an audible sound, whereas material falling upon previously dumped material frequently did not. A time series plot of a typical bucket dredge cycle is given in Figure 4. The six sound events are repeated on approximately a one- minute cycle with minor (e.g., 10-15 sec) variations due to different derrick operators and prevailing current velocities. Occasional interruptions in the cycles occurred to accommodate barge maneuvering, dumping and washing activities, and equipment maintenance. Segments of audio sessions containing dredge cycle event sequences representative of as wide a selection of ranges and distances as possible were ordered by increasing range and distance and analyzed. Individual cycles were selected based on the nearest match to the representative range. Each cycle was then edited from the respective session WAV file and a new 16 bit mono WAV file with a 44.1 kHz sampling rate was created for each of the individual cycles for the representative range. The WAV files for these individual cycles were reviewed and the contents relative to the 6-event sequence summarized. Bucket dredging cycle WAV files were partitioned into “event” WAV files. A spectral analysis of each event WAV file was conducted for the “bottom contact”, “grab”, “snap closed”, and “winch in/up” events categories using Sound Technologies SpectraLab v4.32 audio spectral



analysis/Fast Fourier Transform (FFT) software. An example for the bottom contact event is given in Figure 5. A FFT block size of 32,768 was used whic h produced a frequency resolution of 1.346 Hz. A Hanning window was used to reduce “leakage” during the FFT analysis and a 50% overlap was used to increase the time resolution for the FFT analysis. A peak and infinite average FFT analysis was conducted for each of the “event” WAV files and a spectral plot produced

Figure 4. Pressure waveform of a complete bucket dredge cycle. Peak frequency, peak amplitude (referenced to dB re 1 microPa), and total root mean-squared (RMS) power were calculated for each of the 4 peak and infinite average plots. A plot of peak amplitude and total RMS power vs. range was then produced. An example plot is given in Figure 5, representing the bottom impact event. All amplitudes are reported in relative dB units. The SpectraLab v4.32 software is calibrated to display the relative power levels in dB where 0dB is the strongest possible signal that can be represented by a 16 bit WAV file. A summary of relative sound intensities for the major sound event components of the bucket dredging cycle is given in Table 1. Table values represent estimated relative dB levels, in terms of both peak amplitude and total RMS power, in close proximity to the source and at maximum detection distances. Clearly, the bottom impact sound event produces the most intense, i.e. loudest, noise



150 140 130 120 110

Sound Pressure Level (dB RMS)

100 90 80 70 60 50 40 30 20 10 0 10

100

1000

10000

100000

Frequency (Hz) Bottom Strike

Ambient

Figure 5. Sound pressure level produced by a bucket striking coarse channel bottom sediments as measured over ambient conditions. SOUND EVENT Bottom Impact Grab Bucket Close Winch In/Out

Peak Infinite Average Peak Infinite Average Peak Infinite Average Peak Infinite Average

Peak Amplitude (dB)