Measurements of underwater noise in the Arun River during piling at County Wharf, Littlehampton

Transcription

1 Submitted to: Submitted by: Mr Chris Moore Dr J Nedwell David Wilson Homes Ltd Subacoustech Ltd 15 Horsham Court Long Barn City Business Centre Mandalay Farm Brighton Road Forester Road Horsham Soberton Heath West Sussex RH14 5BB Hants SO32 3QG Tel: +44 (0) Fax: +44 (0) subacoustech@subacoustech.com website: Measurements of underwater noise in the Arun River during piling at County Wharf, Littlehampton Report Reference: 513 R 0108 by Dr Jeremy Nedwell, Mr Bryan Edwards 1 August 2002 Approved for release:......

2 Contents 1. Introduction Aims of the measurements Piling methods The Measurements Location of pile driving and measurement locations Instrumentation Measurement procedure Analysis of measurements and results Impact driver Vibro driver Effects of operations on selected species Conclusions...10 Figures...11 Appendix 1: Hydrophone calibration certificates...25

3 1. Introduction Aims of the measurements During 2002 piling was undertaken along the margin of the River Arun as part of a project undertaken by David Wilson Homes Ltd. to construct dwellings at County Wharf, Littlehampton. The piling took place between April and May, and was performed on their behalf by Dew Pitchmastic Ltd. The Environment Agency (EA) has a long term remit to maintain fish stocks. Piling at or near the banks of a river has the potential to generate high levels of noise, and there is consequently concern over the possible effects this noise may have on fish migration. While there was no suggestion that the piling at County Wharf would have significant effects on the local fish population, there was little information on waterborne noise from piling. David Wilson Homes therefore agreed to fund measurements of the underwater noise created during the piling with the intention of providing high-quality information to the EA. Measurements were taken in the river both adjacent to the piling, and elsewhere in the river at distances of up to 650 metres from the piling. High levels of airborne noise from impact piling can cause annoyance to local communities, and hence over recent years many attempts have been made to reduce the airborne noise by developing alternative piling techniques and equipment. These alternative techniques include hydraulic, vibratory and bored piling. At County Wharf, in addition to impact piling, vibratory equipment was used. Measurements were therefore taken of the pressure resulting from two sorts of piling. The monitoring work was carried out by Subacoustech Ltd under contract to David Wilson Homes Ltd, and this report documents the measurements that were taken and the results that were obtained. 1.2 Piling methods The two sorts of piling undertaken on the site were impact piling and vibropiling. Impact piling is performed using hammers which drive the pile by first inducing downward velocity in a metal ram, as shown in the Fig. 1. Upon impact with th e pile accessory, the ram creates a force far larger than its weight, which moves the pile an increment into the ground. Most impact hammers have some kind of cushion under the end of the ram which receives the striking energy of the hammer. This cushion is necessary to protect the striking parts from damage; it also modulates the force-time curve of the striking impulse and can be used to match the impedance of the hammer to the pile, increasing the efficiency of the blow. Vibratory pile drivers are machines that install piling into the ground by applying a rapidly alternating force to the pile. This is generally accomplished by rotating eccentric weights about shafts. Each rotating eccentric produces forces acting in a single plane and directed toward the centreline of the shaft. Fig. 2 shows the basic setup for the rotating eccentric weights used in most current vibratory pile driving/extracting equipment. The weights are set off-centre of the axis of rotation by the eccentric arm. If only one eccentric is used, in one revolution a force will be exerted in all directions, giving the system a good deal of lateral whip. To avoid this problem the eccentrics are paired so the lateral forces cancel each other, leaving only axial force for the pile. Machines can also have several pairs of smaller, identical eccentrics synchronised and obtain the same effect as with one larger pair. 1

4 2. The Measurements Location of pile driving and measurement locations. The site where the piling was undertaken, County Wharf, is located near to the mouth of the River Arun, on the eastern bank of the river and on the western edge of Littlehampton. The location of the site is illustrated in Fig 3,which is a sketch map of Littlehampton in the region where the piling operation was being carried out. In total, 41 piles were to be driven at the site. Fig. 4 is a sketch giving the location of the individual piles on the County Wharf site. The piling can be seen in the centre of the photo montage of Fig 5, which is compiled from photographs taken from the Arun Yacht Club, on the bank of the Arun directly across from the work site. Fig. 6 is a photograph taken from the Yacht Club looking towards the site, and shows the vibro driver (the red coloured item suspended by the crane) positioned on top of the pile it has nearly finished driving. To the left of it, standing on the quayside, is the impact driver (the yellow coloured item). Fig. 7 is a close-up view of the impact driver, which is a Menck Hydraulic Impact Hammer. Fig. 8 is a close-up of the vibro driver, a PTC 60HD, on top of a pile. Sound measurements were initially taken on the County Wharf site, adjacent to the pile driving. The levels recorded were well above background, so a second position on a pontoon at the Yacht Club, on the opposite bank of the river directly facing the site was added. The pontoon was at a distance of about 80 metres from the position of the piling. The levels were also found to be high at this position, so a third set of measurements was taken at a position on a footbridge over the River Arun, about 650 metres upstream from the position of the piling. The position of each measurement and the pile which was being driven when the measurements were taken are indicated in Fig. 4, with PX denoting the pile and MX the associated measurement location. The choice of measurement location was dictated by accessibility to the river and depended on which pile was being driven. Several methods of instrumenting the underwater noise were considered. The authors have used hydrophones deployed from a workboat to take measurements. While initially this approach appeared beneficial, it was dismissed because of the difficulty of ensuring that the position of the boat remained constant during the relatively long periods of the measurements. This was a result of the fact that an engine could not be used during the period of measurement, the strong tidal flow in the river, its narrow width, and the fact that it was used for navigation. Consequently, measurements were taken from the banks of the river. When working on the County Wharf site the hydrophone was deployed by being thrown out into the river on a heaving line; the line had a large anchor weight attached to it with a buoy to locate the hydrophone at a constant distance below the surface. The anchor weight was dragged back after deployment such that the hydrophone was located 2 m away from the existing quayside wall (the piles were being placed in front of this wall). At the Yacht Club pontoon the hydrophone was hung in the water from the pontoon; similarly for the measurements taken from the upstream bridge the hydrophone was hung from the bridge into the water. The distance between the piling and measurement locations was measured using a hand-held GPS receiver and display. 2

5 Piling was not conducted on a continuous basis at the site. In practice, piling was interrupted by the supply and positioning of new piles, by the malfunction of piling equipment, by the piles striking subterranean debris from previous piling which had to be removed, and by other problems including the unwanted rotation of piles. Consequently, the driving of piles occurred intermittently and in total only for perhaps 20% of the time. It was therefore difficult to ensure that measurement equipment was on site during active piling periods. Nevertheless, a total of 13 sets of measurements was taken, each comprising typically 5 minutes of recorded data, and several sets of data directly digitised onto a computer. Table 1 below lists the details of the various measurements that were made. Measurement i.d. Identifying number of the pile that was being driven Distance between pile and measurement location (m) Method of driving P1/M Impact P2/M Impact P3/M Impact P4/M Impact P5/M Impact P6/M Vibro P7/M Vibro P8/M Vibro P9/M Vibro P10/M Impact P11/M Impact P12/M Impact P13/M Impact Table 1. Details of piles driven and the measurement locations 2.2. Instrumentation. Two hydrophones were used. The first was a Brüel & Kjær Type 8103 miniature hydrophone, serial number , and the second was a Brüel & Kjær Type 8105 hydrophone, serial number The latter is larger, but also more sensitive, than the former, and also has a longer cable. Details of the calibration of these hydrophones and their traceability to International Standards are given in Appendix 1. The hydrophone being used was connected to a Brüel & Kjær Type 2635 Charge Amplifier to condition the signal. For the measurements on the site and on the pontoon the output signal from the charge amplifier was fed simultaneously to a Sony TCD-D8 DAT recorder and to an analogue-to-digital converter card which was inserted in a PCMCIA slot in a Sony laptop computer. The computer was running a data acquisition and analysis program written using the National Instruments LabVIEW application. Fig. 9 is a block diagram of the instrumentation setup Measurement procedure. When taking measurements at the County Wharf site the hydrophone was located at its measurement position by being taped to a rope which had a weight fixed at its end and which 3

6 was placed in the river such that the weight was resting on the river bed. At the Yacht Club site the weighted rope was hung from the edge of the pontoon, again such that the weight rested on the river bed. At the footbridge the weighted rope with attached hydrophone was hung over the side of the bridge such that the buoy attached to the rope 1.5 m above the hydrophone was floating on the river surface. For measurements on the site and at the Yacht Club the hydrophone was attached to the rope such that it was 1 m below the water surface. In one case, however, (driving of P8 and measuring at M8, at the Yacht Club), the tide was going out while the recording was being made and the water level had fallen to a level which necessitated the hydrophone being suspended about 0.2 m below the water surface to be at approximately mid depth. When the pile tube had been readied and it was clear that the driving operation was about to commence, the DAT recorder was started. Once the driving had been established the charge amplifier s gain was adjusted to give a satisfactory output, all the while the recording continuing. During the driving operation some data (a number of captures each of 20 seconds duration) were directly acquired on the computer, except at the footbridge, where only a DAT recording was made. In most cases the recorder was left running for the duration of the driving operation, but sometimes difficulties were encountered with a pile tube and the driving was halted for adjustments to be made to it. In these instances the tape recorder was stopped for the duration, and recording recommenced when the driving was continued. 4

7 3. Analysis of measurements and results Impact driver A typical pressure~time history, in this instance taken on the County Wharf site, for the impact driver case is shown in Fig. 10. The time histories obtained from the pontoon are similar to this figure. Part of the time history obtained from the footbridge is shown in Fig. 11(a). The impacts could be distinctly heard on the recording, but are buried in the general noise at this location. The tide was going out when the recording was made, and was running quite strongly. The signal has been high-pass filtered at 200 Hz and the resulting signal is shown in Fig. 11(b). The parts due to the impacts can be identified in this signal, as can be seen in Fig. 11(c). The data captured has been displayed in the LabVIEW program and the peak pressures, both positive and negative, determined by inspection of the graph. For each location the average values so determined have been expressed as peak levels, defined as average peak pressure peak level = 20 log db re 1µ Pa where the peak pressure is in Pa. The peak levels are plotted in Fig. 12. It may be seen that three of the measurements, made on the Wilson Homes site, are well below the general trend. It is thought possible that this was a result of the shielding of the measurements by existing piles on the site. In order to generalise measurements to provide an objective assessment of degree of any environmental effect and the range within which it will occur, it is normal to represent the sound in terms of two parameters. These are:- 1. The Source Level (i.e. level of sound) generated by the source, and the 2. Transmission Loss, that is, the rate at which sound from the source is attenuated as it propagates If a given sound can be represented in terms of these two parameters it allows the sound level at all distances to be specified. Usually, the sound pressure level (SPL) is modelled as being due to geometric losses, that is, where the sound mainly reduces as a result of being spread over an increasing area. Under these circumstances, the sound pressure level (SPL) is modelled as SPL = SL N g log(r) where SL is the source level of the noise source, N g is an geometric attenuation constant and R is the range in metres from the source. However, the measurements presented in this report do not fit well to a model of this sort. It appears that the losses are mainly due to absorption; consequently a reasonable fit is given by SPL = SL N a (R) where the Source Level is about 192 db re 1 µpa, and the Transmission Loss rate N a is about 0.07 db per metre. 5

8 These levels are very much lower than others obtained by the authors for underwater piling in deep water, where an effective Source Level of 246 db re 1 1 metre was recorded, associated with propagation to large distances Vibro driver. A typical pressure~time history obtained on the County Wharf site for the vibro driven pile case is shown in Fig. 13(a), and it can be seen that it has a triangular appearance to it. A time history obtained from the yacht club pontoon is shown in Fig. 14(a), and in this case the signal appears random since the low frequencies are masked by higher frequency background noise. For the vibro drive cases sections of the captured time histories have been frequency analysed using the LabVIEW program to obtain the spectra of the sound signal. Figs. 13(b), (c) and (d), which respectively have top frequencies of 100kHz, 10kHz and 500 Hz, are the spectra obtained from the data which was used in part to give Fig. 13(a). From Fig. 13(d) it can be seen that the dominant frequency, that at which the driving was taking place, was about 27 Hz. Figs. 14(b), (c) and (d) are the spectra obtained from the data which is presented in Fig. 14(a). Again there is a strong component a t around 27 Hz, but the levels are generally higher in the mid frequencies. For each measurement location and capture, selected parts of the pressure~time histories of the vibro-drive measurements have been displayed in the LabVIEW program, and the peak pressures noted. The average of these pressures has then been calculated and expressed as a level, defined as 0.71 average peak pressure ' RMS ' pressure level = 20 log db re 1µ Pa where the pressure is in Pa. The results are given in Table 2 below. Measurement ident. RMS level of vibro drive (db re 1µPa) P6/M6 152 P7/M7 144 P8/M8 151 P9/M9 132 Table 2. Pressure levels due to vibro driving. The pressure levels obtained are plotted in Fig. 15. It may be seen that there is a considerable degree of scatter, with the levels at 80 metres nominal distance varying by 20 db or so. Consequently, it is not possible to approximate the level of sound as being due to a given source level, minus a transmission loss dependent on distance. It is apparent that the level of sound generated by the source is varying; initially it 6

9 was thought possible that higher levels might correspond with difficulty in driving the pile, and hence increased forces superimposed on the substrate. It was noted that: 1. When the P6/M6 measurements were taken the tide was coming in, but the river level was quite low. There did not appear to be any great difficulty with the driving operation, and the pile appeared to go in smoothly. 2. When the P7/M7 measurements were taken the tide was going out, the river level was fairly low and the water speed fairly high. Difficulties were encountered with the pile driving it appeared to hit a buried object, and the piling was eventually stopped with the pile being only part driven. 3. When the P8/M8 measurements were taken the tide was going out fast, and it is estimated that the water depth at the pontoon was about 0.5 m, and that the hydrophone was about 0.2 m below the water surface. The driving operation seemed to go fairly easily after the initial setting up. 4. When the P9/M9 measurements were taken the tide was just after high tide and the river was flowing out slowly. The pile went in quite easily after the initial setting up, going down to depth in about 4 minutes. It appears that there is no obvious relationship between the level of sound recorded and either the ease of driving the pile or the level of water. It is possible that the variations in level are a result of differing propagation conditions caused by variations in soil density, etc. near to the pile, but without further measurements it is impossible to demonstrate this Effects of operations on selected species The measurements for selected cases have been processed in a procedure which assesses the likely impact of the noise on selected species. The response of a living organism to a given sound is dependent on the particular species, since each species has its own range of frequencies over which it can hear and its own hearing thresholds. In man a commonly used measure of the effect of a sound is the Sound Level measured in db(a). In that scheme a frequency dependent weighting, derived from man s hearing threshold curves, is applied to the sound signal before presenting it to the measuring network, which gives its output as a number in db(a). The human ear is most sensitive to sound at frequencies of the order of 1 to 4 khz, and hence these frequencies are of greatest importance in determining the physical and psychological effects of sound for humans. At lower or higher frequencies the ear is much less sensitive, and humans are hence more tolerant of these frequencies. To reflect the importance of this effect a scale of sound, the db(a), has been developed, which allows for the frequency response of the human ear. Measurements of sound level in db(a) thus relate well to the degree of both physical and behavioural effects of sound on humans. This approach has also been extended to underwater human exposure to sound (where hearing ability differs greatly from that in air), yielding the db(uw), which allows the effects of sound on submerged humans to be estimated. In the db ht (Species), the authors have used a similar approach to arrive at a number for the level of a given sound which is indicative of how much that species will be affected by that sound. A frequency dependent filter is used to weight the sound; the suffix ht relates to the 7

10 fact that the sound is weighted by the hearing threshold of the species. The level expressed in this scale is different for each species and corresponds to the likely perception of the sound by the species. The benefit of this approach is that it enables a single number (the db ht (Species)) to describe the effects of the sound on that species. It may be noted that the effective noise levels of sources measured in db ht (Species) are usually much lower than the unweighted levels, both because the sound will contain frequency components that the species cannot detect, and also because most marine species have high thresholds of perception of (are insensitive to) sound. The results of this calculation are given in Table 3 below. It should be noted that the fact that numbers are quoted for a wide range of species is not intended to imply that those species will be present at Littlehampton. It is rather a means of generalising the results so that they can be applied to estimating the effect of other piling operations. The results for the dab may be considered to be typical of flatfish which may be present in estuarine environments. It may be seen that in general the levels are relatively low and are a maximum of 42 db above the hearing threshold, that is, 42 db ht (Limanda limanda). The results for the salmon are slightly lower at a maximum of 40 db ht (Salmo salar). Neither of these figures is greatly above the threshold of hearing of the species, and it may be concluded that, in this case, the risk of the sound inducing behavioral responses is correspondingly small. It is probable that this is largely due to the sound energy from the piling being at frequencies at which the fish are relatively insensitive. However, it should be noted that recent measurements during piling in the USA have generated much higher levels and caused significantly greater environmental effects, and hence it cannot be concluded at the present state of knowledge that piling is, in all cases, unlikely to cause environmental effects. It may be seen that the results for marine mammals are much higher, partly as a result of much of the energy of the piling being at the high frequencies at which the mammals are sensitive. In general the levels are in excess of 70 db ht, and in two cases the levels exceed 100 db ht. These levels are probably at the upper end of the dynamic range of hearing of all three species for which they have been calculated, and hence it may be concluded that there is a significant likelihood of the noise inducing a behavioural response, such as fleeing the area. 8

11 Table 3. dbht for a number of species. 9

12 4. Conclusions 1. During 2002 piling was undertaken along the margin of the River Arun as part of a project undertaken by David Wilson Homes Ltd. to construct dwellings at County Wharf, Littlehampton. 2. Sound measurements were taken on the County Wharf site, adjacent to the pile driving, and from a pontoon at the Yacht Club, on the opposite bank of the river directly facing the site. Some measurements were also taken at a position on a footbridge over the River Arun, about 650 metres upstream from the position of the piling. 3. In respect of the noise from impact piling, the losses were mainly due to absorption; consequently a reasonable fit is given by SPL = SL N a (R) where the Source Level is about 192 db re 1 µpa, and the Transmission Loss rate N a is about 0.07 db per metre. The levels are very much lower than others obtained by the authors for underwater piling in deep water, where an effective Source Level of 246 db re 1 1 metre was recorded, associated with propagation to large distances. 4. Levels of 132 to 152 db re 1 µpa were recorded for the vibrodriving, although there was a considerable degree of scatter in the results, with the levels at 80 metres nominal distance varying by 20 db or so. It is possible that that the variations in level are a result of differing propagation conditions caused by variations in soil density, etc. near to the pile. 5. The measurements for selected cases have been processed into db ht (Species). The level expressed in this scale is different for each species and corresponds to the likely perception of the sound by the species. For the dab, the levels were relatively low and were a maximum of 42 db above the hearing th reshold, i.e. 42 db ht (Limanda limanda). The results for the salmon were slightly lower at a maximum of 40 db ht (Salmo salar). ). Neither of these figures is greatly above the threshold of hearing of the species, and it may be concluded that, in this case, the risk of the sound inducing behavioral responses is correspondingly small. However, it cannot be concluded at the present state of knowledge that piling is, in all cases, unlikely to cause environmental effects. 6. It was found that the results for marine mammals were much higher; in general the levels were in excess of 70 db ht, and in two cases the levels exceed 100 db ht. These levels are probably at the upper end of the dynamic range of hearing of the three species for which the db ht (Species) have been calculated, and hence it may be concluded that there is a significant likelihood of the noise inducing a behavioural response, such as fleeing the area. 10

13 Figures Fig. 1. Sketch to illustrate principle of impact piling. Fig. 2. Sketch to illustrate principle of vibro pile driving. 11

14 Fig. 3. Sketch map showing location of pile driving site in Littlehampton 12

15 Fig. 4. Sketch showing general layout of piles, and measurement locations for particular driving operations. 13

16 Fig. 5. A photo montage showing County Wharf, taken from the Arun Yacht Club on the west bank of the river. The wharf is seen looking to the east, behind the River Arun, which is at low tide. The river mouth is at the extreme right of the picture. 14

17 Fig. 6. View of Wharf site from Yacht Club, showing the vibro driver on top of a pile, and the impact driver standing nearby on the quayside. Fig. 7. MENCK Hydraulic Impact Hammer. 15

18 Fig. 8. PTC vibro driver on top of pile. 16

19 Fig. 9. Block diagram of instrumentation. Fig. 10. Typical pressure~time history for the impact driver. Measurement taken on County Wharf site. 17

20 Fig. 11(a). A typical pressure~time history at the footbridge. Unfiltered signal. Fig. 11(b). A typical pressure~time history at the footbridge. Signal has been high-pass filtered at 200Hz. 18

21 Fig. 11(c). A typical pressure~time history at the footbridge. Detail of part of Fig. 11(b), to show section due to impact driving. Impact driving, pk pressures pk level (db re 1microPa) Wilson Homes site Arun Y C Bridge distance (m) Fig. 12. Variation of peak pressure level with distance for impact driving. 19

22 Fig. 13(a). Pressure~time history for vibro driving. This is for measurements taken on the County Wharf site. Fig. 13(b). Sound spectrum for the above Count Wharf site vibro driving case. Top frequency of plot 100kHz. 20

23 Fig. 13(c). Sound spectrum for the above Count Wharf site vibro driving case. Top frequency of plot 10kHz. Fig. 13(d). Sound spectrum for the above Count Wharf site vibro driving case. Top frequency of plot 500Hz. 21

24 Fig. 14(a). Pressure~time history for vibro driving. This is for measurements taken from the yacht club pontoon. Fig. 14(b). Sound spectrum for the above vibro drive case, measured from the pontoon. Top frequency of plot 100kHz. 22

25 Fig. 14(c). Sound spectrum for the above vibro drive case, measured from the pontoon. Top frequency of plot 10kHz. Fig. 14(d). Sound spectrum for the above vibro drive case, measured from the pontoon. Top frequency of plot 500Hz. 23

26 'RMS' level (db re 1 micropa) P7/M7 P6/M6 P8/M8 P9/M distance (m) Fig. 15. Variation of sound level with distance for vibro driving cases. 24

27 Measurements of underwater noise in the Arun River during piling at County Wharf, Littlehampton Appendix 1: Hydrophone calibration certificates 25

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