Abstract
Concern has been expressed over the possible effects of man made underwater noise caused by windfarms. This has been cited as having the capacity at high levels to cause death, physical injury (such as deafness) and behaviour changes in marine mammals and fish. Since the impacts caused by waterborne noise are not yet fully understood, Subacoustech Ltd have been contracted by The Crown Estate on behalf of Collaborative for Offshore Wind Research Into the Environment (COWRIE) to measure and interpret the underwater noise generated by offshore windfarms and their construction. The purposes of the measurements are to evaluate the pre-existing background noise environment, to rate noise from construction and operation of windfarms in terms of its potential for environmental effect, and to provide information which will aid estimation and minimisation of the impact of noise during the lifecycle (construction, operation and decommissioning) of windfarms.
The report presents a significant body of underwater noise measurements taken in the period 4/2003 to 1/2004 at operational and construction stage windfarm sites in the UK. A detailed analysis of the measurements has been made which indicates the spatial, temporal and statistical properties of the noise. An estimation of the likely behavioural and physical effects on a selection of the most common species of fish and marine mammals is also presented using both conventional analysis and the dBht (species) scale.
The measurements of ambient noise in shoals indicates that in general, the levels are towards the upper bound of typical deep water ambient noise levels. The overall sound pressure level varies significantly more during the daytime than at other times of day, due to the higher number of short local ship movements. The noise levels are higher at low wind speeds, contrary to the normal assumption that they will rise with increasing wind speed.
Measurements of background noise at North Hoyle indicated that the Douglas Platform is probably a significant pre-existing contributor to the background noise level. Its Source Level may be estimated to be about 206 dB re 1 Pa @ 1 metre. Measurements of piling indicated a Source Level of 260 dB re 1 Pa @ 1 metre for 5 metres depth, and 262 dB re 1 Pa @ 1 metre at 10 metres depth, associated with a Transmission Loss given by 22 log (R) where R is the range. Calculations using the dBht scale levels indicate that strong avoidance reaction by a range of species would be likely at the ranges of up to several kilometres. The levels of sound recorded during piling are such that within perhaps a hundred metres they could cause injury. Measurements of cable trenching at North Hoyle indicate a Source Level of 178 dB re 1 Pa @ 1 metre if a Transmission Loss of 22 log(R) is assumed. Measurements of rock socket drilling were made, which showed strong fundamental component at 125Hz, and harmonics up to 1Khz, but it was not possible to establish the Source Level and Transmission Loss. Components of the drilling could however be identified at ranges of up to 7 km.
Measurements of piling at Scroby Sands were similar in level to those at North Hoyle, and similar conclusions pertain in respect of possible environmental effects.
On the basis of the measurements, piling should in particular be regarded as capable of causing significant environmental effect, and planning of piling operations should take account of the effects of its noise on sensitive species. If the environmental consequences of the piling operation are unacceptable, then use must be made of suitable mitigation measures to reduce the impact to an acceptable level.