Characterization of Underwater Sounds Produced by Trailing Suction Hopper Dredges During Sand Mining and Pump-out Operations

Abstract

Underwater sounds were characterized for three trailing suction hopper dredges (TSHD) during the removal of 3.1 million yd 3 of sand from an offshore borrow area and during offloading of the escavated sediment at the pump-out stations in support of the Wallops Island, Virginia Beach Stabilization Project. Sounds were recorded simutaneoulsy at two depths, 3 and 9.1 m from the surface. Sound sources included sediment removal, pump-out of material, pump-out of clear water during pipe flushing, and transit to the borrow site (hopper empty) and to the pump-out stations fully loaded. Received and 1/3-octave Sound Pressure Levels (dB re 1 µPa, rms) are reported for each sound source. Source Levels (dB re 1µPa-1m, rms) were back-caculated using fitted regression (15.788LogR). Source Levels (SL) ranged from 161.3 dB to 176.7 dB re 1µPa-1m rms. Highest SL were obtained for the dredge Liberty, which is nearly twice the size (e.g. hopper volume, displacement) of the dredges Padre and Dodge Islands. Sounds emitted during transit produced the highest SL, whether the hopper was empty or full. Attenuation to ambient was dependent on the sound source, and ranged from 0.85 km (flushing pipes) to 2.65 km during transit with the hopper at maximum capacity.

A critical analysis of the direct effects of dredging on fish

“…Several published studies exist that have quantified dredging sounds from hydraulic and mechanical dredging (e.g. Reine, Clarke, & Dickerson, ; Reine, Clarke, Dickerson, & Wikel, ; Thomsen, McCully, Wood, White, & Page, ). The available evidence indicates that dredging scenarios do not produce intense sounds comparable to pile driving and other in‐water construction activities, but rather lower levels of continuous sound at frequencies generally below 1 kHz.…”

“…Despite the necessity of dredging for industrial development, its potential impacts on the environment are of particular concern as multiple potential stressors associated with dredging activities have been well documented. Chief among these are sediment stress (suspended and deposited), release of toxic contaminants, hydraulic entrainment and noise pollution (Figure ; McCook et al ., ; Reine & Clarke, ; Reine, Clarke, & Dickerson, ; Reine, Clarke, Dickerson, & Wikel, ; Wilber & Clarke, ). Although there are significant dredging operations undertaken across a range of aquatic environments, and an increasing body of literature documenting dredging‐related effects on fish is available (e.g.…”

“…If both the exposures and responses are accurately assessed, appropriate risk management measures can be identified to balance the need to construct and maintain coastal infrastructure with adequate protection of vulnerable species and valuable finfish fishery resources. This review and meta‐analysis synthesizes and characterizes the known direct effects on fish from exposures to the most commonly cited potential stressors associated with dredging: sediment, release of toxic contaminants, hydraulic entrainment and noise (McCook et al ., ; Reine & Clarke, ; Reine, Clarke, & Dickerson, ; Reine, Clarke, & Dickerson, ; Reine, Clarke, Dickerson, & Wikel, ; Wilber & Clarke, ), with an emphasis on exposures relevant to dredging processes.…”

Characterization of underwater sounds produced by hydraulic and mechanical dredging operations

“…SLs from commercial shipping exceeded the SLs for both the cutterhead dredge Florida and the backhoe dredge New York by 10-15 dB and by as much as 33 dB for the CSD monitored by Greene (1987). SLs were 9-17 dB lower for three medium capacity TSHDs monitored by Reine et al (2013) but only 1-3 dB lower for the larger capacity TSHDs monitored by Greene (1987). The larger size class hopper dredges were similar to a container ship traveling at a modest speed.…”

“…SLs ranged from 179 to 187 dB re 1 lPa-1 m, considerably higher than measurements for the cutterhead and backhoe dredges working in the present study. Reine et al (2013) also reported SPLs (1/3 octave) for three medium size class (hopper capacity ¼ 835-1521 m 3 ) TSHDs, the Padre Island, Dodge Island, and Liberty Island, during offshore sand mining where SLs ranged from 161 to 178 dB re 1 lPa-1 m rms. SPLs for the smaller size class TSHDs during sand mining operations fell within the range of SPLs measured for the dredge Florida and New York during rock fracturing and excavation.…”