Underwater Sound Attenuation in Construction Projects: Applying Science to Pile Driving Permits-PDF

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The paper examines how pile driving can transmit harmful underwater sound to fish and marine mammals, and how permit conditions can reduce these impacts. Sound transmission depends on factors including pile size, hammer type and energy, water depth, current, and direct contact between pile and water. Site conditions and sediment also affect how much energy enters the water.<br /><br />Seasonal restrictions—avoiding construction when sensitive species are present—are generally the most effective protection. When schedules or project conditions make restrictions impractical, regulators should assess mitigation and sound attenuation using site-specific factors such as species sensitivity, construction duration, current, energy levels, accessibility, and cost.<br /><br />Attenuation can be achieved by separating the pile from the water, as with a dewatered cofferdam, or by introducing air near the pile. Unconfined bubble curtains are relatively inexpensive but can be disrupted by currents and may require many compressors. Their potential side effects include air pollution, fuel use and spill risk, oily air entering the water, increased turbidity, and safety and maintenance concerns. Confined bubble curtains are more costly initially and require engineering, but are effective in a wider range of conditions and generally need less air.<br /><br />A case study from the San Francisco–Oakland Bay Bridge project found that a confined curtain reduced sound by 22 dB at 95 metres; the fabric enclosure alone reduced it by 13 dB. By contrast, an unconfined curtain in the project’s strong currents provided no measured reduction.<br /><br />The authors recommend retaining seasonal restrictions where feasible and using confined bubble curtains when additional protection is needed. They also call for more in-situ research, since sound propagation is complex and laboratory results have not reliably predicted real-world conditions.

Keywords

pile driving

underwater sound

marine mammals

fish protection

permit conditions

seasonal restrictions

sound attenuation

bubble curtains

confined bubble curtain

sound propagation