8.4. Vessels and Other Continuous Noises (All Phases)

  1. Estimated ranges for injury to marine mammals due to the continuous noise sources (vessels) during different phases of the construction and operations are presented below.
  2. It should be borne in mind that there is a considerable degree of uncertainty and variability in the onset of disturbance and therefore any disturbance ranges should be treated as potentially over precautionary. Another important consideration is that vessels and construction noise will be temporary and transitory, as opposed to permanent and fixed. In this respect, construction noise is unlikely to differ significantly from vessel traffic already in the area.
  3. The estimated median ranges for onset of TTS or PTS for different marine mammal groups exposure to different noise characteristics of different vessel traffic are shown in Table 8.28   Open ▸ . The exposure metrics for different marine mammal and swim speeds (as detailed in section 7.6) were employed.

 

Table 8.28:
Estimated Potential PTS and TTS Ranges from Different Vessels for Marine Mammals

Table 8.28: Estimated Potential PTS and TTS Ranges from Different Vessels for Marine Mammals

 

  1. The ranges for recoverable injury and TTS for Groups 3 and 4 Fish are presented in Table 8.29   Open ▸ based on the thresholds contained in Popper et al. (2014). It should be noted that fish would need to be exposed within these potential impact ranges for a period of 48 hours continuously in the case of recoverable injury and 12 hours continuously in the case of TTS for the effect to occur. It is therefore considered that these ranges are highly precautionary, and injury is unlikely to occur.

 

Table 8.29:
Estimated Recoverable Injury and TTS Ranges from Vessels for Groups 3 and 4 Fish

Table 8.29: Estimated Recoverable Injury and TTS Ranges from Vessels for Groups 3 and 4 Fish

 

9. Particle Motion

9.1. Introduction

  1. This technical report provides an analysis of the effects of noise on marine species. However, there are uncertainties in relation to the presence of compression and interface waves at the water/ground substrate boundary during piling, and the potential effect on fish and invertebrates. Although the risk of injury to fish with and without swim bladders is addressed through the use of SEL and peak pressure thresholds (Popper et al., 2014), it is possible that some fish are only sensitive to particle motion. These fish could experience high levels of particle motion in close proximity to piling. However, the Popper et al. (2014) paper primarily addresses high amplitude noises and high dynamic pressure, rather than particle motion.
  2. The majority of measurements during piling for offshore wind farms are undertaken using hydrophones in the water column which includes contributions from both direct radiated noise from the pile into the water, as well as ground-borne radiated noise, and there are uncertainties with respect to how effectively the ground borne energy couples into the sea. If measurements were taken in an evanescent (non-propagating) field then high particle motion would not be reflected in the associated dynamic pressure measurements, particularly if those measurements were taken in shallow water and the energy is below the cut-off frequency. Consequently, it is possible that the effects on benthic fauna close to the pile could be under-estimated, particularly for species primarily sensitive to vibration of the seafloor sediment.
  3. To put this issue into perspective, under section 5.1 entitled “Death or Injury”, Popper et al. (2014) states that “extreme levels of particle motion arising from various impulsive sources may also have the potential to injure tissues, although this has yet to be demonstrated for any source”. It would therefore appear that there is currently a lack of criteria for (or detailed measurements of) particle motion during piling operations for this issue to be currently assessed. Thus, in terms of potential damage to fish, volume 2, chapter 9 has addressed the impact as far as is practicable with the existing state of knowledge, based primarily on exposure to sound pressure.
  4. The purpose of this chapter is to provide an overview of the acoustic aspects of particle motion. Potential effects on marine species are dealt with in the marine ecology topic chapters of the Array EIA Report.

9.2. Overview of Particle Motion

  1. Particle motion is defined as the motion of an infinitesimally small part of the medium relative to the rest of the medium, that is caused by a noise wave (Popper et al., 2014). Unlike the pressure variation caused by the wave, which is a scalar quantity and therefore has no direction, the particle motion is a three-dimensional (3-D) vector quantity (i.e. directional). Particle motion can be described by the velocity, acceleration, and displacement of the particle. These are related by the following equations (Nedelec et al., 2016):

where a = acceleration (m/s2), u = particle velocity (m/s), 2πf = angular frequency, and ξ = displacement (m).

  1. In the same way as the unit for sound pressure is referenced to the Pascal (Pa), likewise in the case of particle motion, the decibel (dB) unit is referenced to either the displacement, velocity or acceleration as appropriate. Therefore, units will be in the form of dB re m, m/s or m/s2.
  2. Particle motion can also be related to measured sound pressure and can be approximated from the sound pressure in simplified circumstances such as a plane wave. For a plane wave, or a wave for which a plane wave is a good approximation of its behaviour (a wave in the free field), the following relationship holds:

where P = acoustic pressure (Pa), = density of the water (kgm−3), and c = noise speed (ms−1). The quantity is also known as the characteristic acoustic impedance.

  1. The following relationship holds true for the near field of a point source. The source must be far from any boundaries that could lead to the wave not propagating due to cut off frequency, or reflections that could interfere with the propagation of the wave:

where r = distance to noise source (m). All other symbols are consistent throughout the equations presented here.

  1. A plane wave is a wave that can be considered to have a flat wavefront. This generally occurs far from both the source of the wave and any sources of reflected waves. The term ’far’ is relative to the wavelength of the noise and the size of the source as both will change the distance at which the wave can be considered a plane wave. In shallow coastal and sea-shelf habitats these far-field conditions are not often met at the acoustic frequencies relevant to fish and invertebrates. This means that there is usually not a reliable way to derive particle motion from sound pressure measurement in these habitats. Technically a relationship between particle motion and sound pressure can be derived for more complicated wavefronts (e.g. by assuming that the wavefront has an idealised geometry). However, this is not necessarily reliable, and, in most cases where plane waves cannot be assumed, the only reliable solution is to measure directly (Nedelec et al., 2016).
  2. In those situations where it is appropriate to assume that waves generated by a monopole are plane waves (i.e. in the acoustic far field), it is possible to approximate the magnitude of the particle motion. It is important to understand where it is appropriate to make these assumptions. Spherical spreading occurs when noise propagates from a source without any interference and the applicability of the plane wave assumption is based on the frequency of interest and the waveguide (i.e. the duct formed by the surface and bottom of the water column), which encapsulates the water depth, distance to source, source type, and the noise speed in water and sediment. The values that are key for this assumption are the wavelength of the lowest frequency of interest (λ) and the cut off frequency (f0) based on the waveguide. These values can be calculated from the following equations (Nedelec et al., 2021):

where is the cut off frequency, D is the water depth, is the noise speed in water, and is the noise speed in sediment.

  1. If the distance to the noise source is greater than one wavelength and the lowest frequency is greater than the cut off frequency, then it is possible to estimate the magnitude of the particle motion from an SPL measurement. However, it must be noted that this only applies to a travelling plane wave and as such the signal to noise ratio must be high enough to consider other noises negligible (Nedelec et al., 2021).
  2. It should also be borne in mind that noise produced from piling is, in reality, not a monopole source. The pile acts as a line source throughout the water column and in the sediment and produces a complex Mach wavefront. Consequently, the above simplifications may not be appropriate to assess the particle motion produced by piling.

9.3. Hearing in Fish and Invertebrates

  1. All fish, and many invertebrates, detect the particle motion of a noise wave with mechanosensory organs such as the inner ear, statocyst or lateral line (Nedelec et al., 2021). The ability to hear their surroundings gives fish, and many invertebrates, an abundance of information about their environment. This ability is unaffected by light levels and is omnidirectional, allowing for the most abundant information about the environment. Of all the senses that fish, and many invertebrates, use to assess their surroundings, hearing is the most versatile in a marine environment. In particular, their hearing is able to give rapid feedback with relatively long distance 3-D information (Popper and Hawkins, 2019).
  2. The detection of noise and characterisation of the immediate noisescape is something that is key to the way that fish and many vertebrates live. This ability allows them to detect the direction of predators, and subsequently avoid them, or detect prey and move towards them. Furthermore, this ability can be used to recognise others within their own species and select a mate. Although not all fishes, or invertebrates, produce noise for communication, they are all known to use it for awareness of their surroundings. As such, any interference with this ability could impact the survival of the fish (Popper and Hawkins, 2019).
  3. There have been several studies into the hearing capabilities of fish and invertebrates. However, very few of them have used conditions that are truly representative of the environment that they would encounter in open water. This is due to tank conditions or methodologies used to observe them in an offshore environment. Furthermore, few of these studies have focussed on particle motion specifically (Popper and Hawkins, 2019).
  4. Taking this into account it is possible to establish a reasonable assumption for hearing range of various species. Most fish appear to be able to detect noise that falls between 10 Hz and 500 Hz. If the fish or invertebrates are capable of detecting sound pressure, then they may be able to detect noises at higher frequencies up to approximately 1 kHz or more. There are also a small number of fish that are capable of hearing between 3 Hz and 4 kHz due to various specialisations that they have (Popper and Hawkins, 2019). The values presented here are the upper and lower estimates of each range, there is a degree of variability in each of the values. This is in part due to the complexity of the noise field in a tank or enclosure (Popper et al., 2019). Likewise, invertebrates are also typically sensitive to lower frequencies (Nedelec et al., 2016).