Installing large structures in shallow marine environments requires the insertion into the seabed of support elements, called piles, made of wood, steel, or reinforced concrete. These piles, once installed, extend from above the water surface to as much as several hundred feet below the seabed. Installing the piles may require impact pile driving, which uses a hydraulic hammer to repeatedly strike the top of the pile, pushing it into the seafloor. The blows are delivered approximately once per second. Depending on the size of the hammer, bottom properties, and the required penetration depth, it may take several hours and as many as 5,000 strikes to drive one pile into the seabed.
Impact pile driving produces intense, broadband impulsive sounds that can propagate many kilometers. Near source (within 10 m of the pile driving activities) sound pressure levels (SPLs) range up to 220 dB peak, but vary substantially according to the size of the hammer, power of the hammer, pile material, diameter of the pile, as well as properties of the seafloor.
Two criteria for assessing the potential impacts of pile driving sounds are: 1) SPL peak and 2) cumulative sound exposure level (SELcum). For more information, please see the Advanced topic on Sound Pressure Levels and Sound Exposure Levels.
To investigate the potential effects of pile driving on fishes, several species of freshwater fish were exposed in a chamber to simulated pile driving sounds. The scientists reported that injuries varied according to the peak pressure, number of strikes, the species of fish, and whether a fish had a swim bladder. The scientists found that at the lowest sound pressure levels and fewest number of pile strikes there were no detectable injuries. Injuries reported included damage to hair cells, swim bladders, and soft tissues, according to the exposure level.
These studies were conducted in a laboratory setting and research is still needed on free-swimming animals to determine the potential for injuries on fishes in the wild from pile driving sounds. It is not known how the sounds of a pile driving operation will affect fish behavior or mask hearing of biologically relevant sounds.
Most research on potential effects of pile driving on marine mammals has been conducted in European waters in association with the development of offshore wind farms. Potential effects have been investigated on several cetaceanand pinniped species.
Research results have shown that pile driving sounds may cause temporary threshold shift (TTS) or permanent threshold shift (PTS). Scientists measured pile driving sounds during construction of a wind farm off the northeast coast of Scotland. Although the levels to cause PTS have not been directly measured, based on the regulatory criteria currently in use, broadband peak-to-peak sound levels consistent with TTS onset would have been exceeded for cetaceans within 10 m of the pile driving operations and within 40 m for pinnipeds. The level estimated to cause PTS onset would occur within 5 m for cetaceans and 20 m for pinnipeds. Scientists determined that no form of injury or hearing impairment would occur at ranges greater than 100 m.
Observed behavioral responses in response to pile driving include changes in direction, swimming speed, dive profiles, group movements, vocalizations, and respiration rates. There are concerns that avoidance responses to pile driving can impact foraging, nursing, and/or mating activities, and therefore overall fitness, of a marine mammal.
Various mitigation measures, such as bubble curtains, screens, or cofferdams (an insulating sleeve around the pile), have been developed to diminish the potential impacts of pile driving on marine life. Acoustic deterrent devices have also been deployed prior to pile driving activities at many European wind farm sites in order to deter marine mammals from entering the construction zone. Pile driving activities may also begin with a “ramp-up” or “soft start” where lower hammer energy levels are used to start the pile driving process, and then the force of pile driving is gradually increased.
Listen to the sound below to hear pile driving activity before and after a bubble curtain system is activated. Please note, this sound recording demonstrates the effectiveness of this mitigation option and is not meant to demonstrate how it is actually done (e.g., the bubble curtain would already be up and operational before any piling started in a real-world scenario)
- Behavioral Changes in Mammals
- Behavioral Changes in Fishes
- Bubble Curtain
- How is sound used to protect marine mammals?
- Moderate or eliminate the effects of human activities
- Sound Pressure Levels and Sound Exposure Levels
- Wind Turbines
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