Review of Cabling Techniques and Environmental Effects Applicable

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Review of Cabling Techniques and Environmental Effects Applicable to the Offshore Wind Farm Industry – Technical Report Noise associated with cable laying will therefore impact upon hearing specialists to a greater extent. The effect of underwater noise on fish can be categorised as (Nedwell et al., 2003): ● Primary effects: These include immediate or delayed fatal injury of animals near to powerful sources, such as the blast from underwater explosives; ● Secondary effects: These include injuries and deafness which may have long term implications for survival; and ● 116 Tertiary effects: These are most likely to be associated with cable laying and include avoidance of the area which could have significant effects in the vicinity of breeding grounds, migratory routes or schooling areas. Nedwell et al. (2003, 2005) proposed a measure of sound that takes into account the differences between species in terms of hearing ability. This measurement is referred to as dB ht (Species). The derivation of dB ht (species) involves the measurement of sound passing through a filter that mimics the hearing ability of individual species and provides a species specific measurement of the likely level of perception of sound by the species. For a full description of the dB ht (Species) measurement and the uncertainties involved in its application, the reader is directed to Nedwell et al. (2003, 2005). While the dB ht (Species) approach is still being validated and reviewed, available information suggests that species of fish and marine mammals (see Section 5.6.1) will show a strong avoidance reaction to sound levels of 90 dB ht and above. Strong avoidance by most individuals is likely to occur at 100 dB ht with a mild avoidance reaction occurring in a minority of individuals at levels above 75 dB ht . The effect of such impacts will be dependant upon a combination of factors including the type and magnitude of the noise along with the proximity of an organism to the source of the noise. In the context of offshore wind farm construction, much of the work relating to the impact of noise upon species has focussed on the effect of pile driving, as this is by far the ‘worst case scenario’ in terms of the production of high intensity impulsive sound. Sound levels associated with cable installation have received considerably less attention and very little monitoring data is available. Nedwell et al. (2003) have reported that cable trenching in sandy gravel at North Hoyle offshore wind farm produced noise at a source level of 178 dB re 1μPa @ 1m. However, when illustrating the dB ht levels of the noise as a function of range, the following figure was produced. Although based on some uncertainty, due to the high levels of variability in the noise produced, Figure 5.2 clearly shows that, for each species the dB ht level is below 90. In this situation, significant avoidance reactions amongst fish would not be expected to occur.
Potential impacts and mitigration measures Figure 5.2: Cable burial noise (North Hoyle Offshore Wind Farm) Source: After Nedwell et al., (2003) The example in Nedwell et al. (2003) only covers one installation method in one type of substrate. It is possible that certain burial tools in certain seabed sediments may produce noise at a higher level than recorded at North Hoyle. Further information is required on the noise levels associated with other forms of cable installation before any clear guidance on the expected levels of associated disturbance to fish and/or mammals can be made. However, the early indications are that there is no significant impact from cable burial noise on fish species. In addition, no harmful events have been reported from the well established subsea telecommunications industry. For a more detailed assessment of the effects of man made noise on fish see Nedwell et al. (2003). For further information on noise effects on marine mammals see Section 5.6.1. Turbidity A reduction in light levels within the water column can create a number of adverse effects on fish and shellfish resources. Long term increases in turbidity will reduce the extent of the photic zone (the depth to which light can penetrate the water column) resulting in changes to flora and fauna. Cable laying operations result in temporary increases in suspended sediment which, while generally not sufficient to alter biotopes, will impact upon sensitive species or those reliant on certain levels of visibility. Decreased visibility through increased concentrations of suspended sediments can affect predatory fish such as mackerel (Scomber scombrus) and turbot (Psetta maxima), which rely on vision to detect and locate prey, leading to decreased 117
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REVIEW OF CABLING TECHNIQUES AND EN
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Review of Cabling Techniques and En
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Abbreviations AIS Automatic Identif
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TTS Temporary Threshold Shift TWA T
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3 Cable types and 18 installation t
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Page 82 and 83: 4 Physical change 4.1 Introduction
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Page 144 and 145: References Introduction Sinden, G.
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Page 164: Printed in UK on recycled paper. De
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