Creative Solutions for Marine Construction: Managing Underwater Noise from Pile Driving

Harbor Seal

Balancing Ecological and Industrial Needs

The Snohomish River mouth in Puget Sound is a busy place. Bald eagles, seabirds, salmon, seals, and the occasional whale frequent this area year-round. The area is also a hub of industry with vessels large and small coming and going from the Port of Everett’s International Seaport and recreational marinas, and adjacent US Navy facilities. This convergence of ecological and industrial uses can sometimes lead to conflicts that require careful management.

When the Port of Everett needed to upgrade its South Terminal Wharf to accommodate bigger cranes, it turned to Hart Crowser to assist with Endangered Species Act (ESA) permitting. The project would require partial demolition of the existing wharf, driving hundreds of new steel piles, and reconstructing the wharf deck over water. Pile driving in particular is known to generate high levels of underwater noise that can be a nuisance and cause physical harm to fish, diving birds, and marine mammals. Therefore, the project would need to demonstrate that sufficient precautions would be taken to avoid detrimental effects.

The biological assessment (BA) prepared by Hart Crowser analyzed the potential impacts of the project on southern resident killer whales, marbled murrelet, Chinook salmon, steelhead and bull trout – species that are listed under the ESA and known to occur in the area. Using the best available scientific information, Hart Crowser calculated the noise disturbance and injury zones for these species. In some cases, this zone extended over 9 kilometers from the site!

The US Fish and Wildlife Service and the National Marine Fisheries Service (collectively referred to as the Services) reviewed the BA and concurred with our analysis. The Services issued an “incidental take” statement that allowed the port to continue with the project subject to several terms and conditions. These included working within the “in-water work window” to avoid the salmon outmigration period, a limit of 2,000 blows per day for impact pile driving, acoustic monitoring to verify noise levels, and monitoring to immediately identify any fish kills and shut down if ESA-listed species entered the affected area.

Subsurface Conditions Require a Creative Solution

Port of Everett dock

Given these constraints, the construction contractor, Advanced American Construction, commenced work in September but soon realized that the subsurface conditions in several areas required significantly more blows with the impact hammer to install each pile. On many days, they were required to stop work early to avoid exceeding the daily blow count limit. This led to concern that an extended construction timeline might push past the in-water work window. If this were to occur, it would likely require shutting down for several months and returning for a second construction season, which would add considerable cost to the project.

In the meantime, The Greenbusch Group conducted the acoustic monitoring and recorded site-specific noise levels for the type of hammer, pile, and substrate. While they confirmed that the underwater noise from pile driving of 24-inch diameter piles matched the assumed levels, they also noted that in certain situations – such as when 18-inch diameter piles were driven, or when piles were driven in areas above the waterline – the noise was several decibels less.

Computer on Table at Sunrise
At the Port of Everett’s request, and in partnership with Greenbusch, Hart Crowser reviewed the acoustic results and dissected the Services’ analyses of incidental take due to noise. The daily blow count limit was driven by the threshold for injury to ESA-listed fish such as Chinook salmon, steelhead and bull trout as defined by the cumulative sound exposure level, or cSEL. The calculation of cSEL incorporates the source noise level and a logarithmic function of the number of blows. Therefore, Hart Crowser biologists determined that by substituting a lower source level into the equation based on pile size, the blow count could be increased and still maintain the same authorized cSEL limit. Hart Crowser and Greenbusch developed a spreadsheet calculator to allow the contractor real-time management of the cSEL based on driving different pile sizes , and prepared a technical memorandum explaining this proposed approach to the Services and US Army Corps of Engineers.

The negotiation was successful, and the contractor was allowed to proceed with a higher blow count if the calculator was used to carefully track the number of blows per pile-type . The creative approach, availability of site-specific data, detailed technical justification, and demonstration of how the new tool would be used resulted in approval without the need for time-consuming formal reinitiation of consultation. Hart Crowser marine mammal monitors on site kept a running tally of the blow counts for different pile diameters and advised the contractor when approaching the cSEL limit. The project is currently on track for completion in 2020.

Orca

Photo: Tim Cole from Unsplash

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