6 underwater noise monitoring

6.1 Baseline underwater noise measurements were conducted from 24^th November 2012 to 22^nd December 2012 for 28 continuous days at 100m distance from one of the first three piling sites (monitoring station R2) as shown in Figure 5.

6.2 Major components affecting the background noise level are the noise generated from sea flow and the noise from marine traffic. The noise levels could be increased significantly by strong sea flow and wave action.

6.3 Noise other than sea flow and marine traffic are also clearly audible in several sound recordings. The duration and nature of these sounds (a minute to several minutes) are generally consistent with the noise expected from passing vessels. This noise might have been due to the marine work far away. Vessels can be clearly heard in some of the recordings listened to.

6.4 Although there are some possible influencing factors affecting the baseline condition, the measured data do represent the actual environment in this region. The values reported for the baseline noise level are all well below the Action Threshold of 170dB re 1µPa RMS proposed in the underwater noise monitoring proposal. Table 6.1 summarized the hourly average RMS sound pressure level for the 28 days monitoring period.

6.5 Given the high levels of variation in received noise levels, the selection of a baseline for comparison with future data collection during construction should be done with care. Hourly RMS SPL is provided in this monitoring report that could be used to compare with the impact monitoring data on hour to hour basis.

6.6 Selection of the monitoring frequency band is also critical. Results plotted in Figure 4-4 and Figure 4-5 of the Baseline Underwater Noise Monitoring Report in Appendix E show a 15dB difference between a frequency band starting at 10Hz and one starting at 70Hz.

6.7 Power Spectral Density measurements (Figure 4-9 in Appendix E) show levels in excess of 120dB re 1 μPa²/Hz at 10Hz and around 80dB re 1 μPa²/Hz at 1kHz. These levels are similar to the higher SPL levels reported by Wenz (1962) (in Urick, 1975).

Event and Action Plan

6.8 Acoustic data will be processed in real time on the PAMBuoy, and all of the data presented in this report can be viewed in real time by connecting a PC based monitoring station with cable. This would enable real-time mitigation, with operators responding to increases in noise levels above 170 and 180 dB re1µPa in 1 second average as stipulated in the Event Action Plan (Table 6.2). Details can be referred as follows:

(a) If the 1 second RMS sound pressure level re 1µPa exceeds the Action Level, the ET will give signals to the Contractor to review the piling sequence or method in order to reduce the underwater noise levels to no higher than 170 dB. At the same time, a protection zone of 50m radius will be set if the underwater noise level measured is between 171 to 174 dB. The zone will be enlarged to 100m radius if the underwater noise level is between 175 to 179 dB. If dolphin is found within the protection zone, the construction work will stop and can only commence after confirming that no dolphin is present for 30 minutes. A scanning staff from the Contractor will scan the surrounding area near the bored piling site.

(b) If the measured underwater noise level cannot be reduced to below 170 dB within half an hour, the ET will give signals to the Contractor to stop the activities. The Contractor should not be allowed to resume the works unless the Contractor can come up with an alternative method or sequence agreeable to the Engineer that would generate noise no higher than 170 dB.

(c) If the measured noise level exceeds 180 dB, the ET will give signals to the Contractor to stop the activities immediately. Again, the Contractor should not be allowed to resume the works unless the Contractor can come up with an alternative method or sequence agreeable to the Engineer that would generate noise no higher than 170 dB.

6.9 Detailed methodology, results and discussion can be found in the Baseline Underwater Noise Monitoring Report in Appendix E.

Table 6.1 The Hourly Average Baseline Monitoring Results, dB re 1μPa_rms

Hour	SPL dB re 1μPa_rms	Hour	SPL dB re 1μPa_rms	Hour	SPL dB re 1μPa_rms	Hour	SPL dB re 1μPa_rms
00	137.0	06	131.5	12	138.5	18	143.5
01	138.0	07	134.0	13	138.2	19	144.8
02	138.5	08	136.5	14	137.5	20	144.5
03	138.2	09	137.0	15	137.1	21	142.0
04	138.0	10	136.5	16	142.0	22	138.4
05	130.5	11	138.0	17	142.8	23	138.5

Table 6.2 Event/Action Plan for Underwater Noise Monitoring

Event

Action

ET Leader

IEC

Contractor

Action level triggered

1. Inform the IEC, ER and Contractor;

2. Advise Contractor of dolphin protection zone coverage.

3. Continue to monitor underwater noise level.

1. Check monitoring data submitted by ET.

1. Inform Contractor.

1. Review the piling sequence or method;

2. Implement the mitigation measure to lower the underwater noise level to below action limit within 30 minutes;

3. Implement protection zone.

4. Closely liaise with the ET on the progress.

Limit level triggered

1. Instruct the Contractor to stop construction work;

2. Inform the IEC and ER;

3. Discuss with IEC, ER and Contractor on noise reduction proposal;

4. Assess effectiveness of Contractor’s proposal and keep IEC and ER informed.

1. Check monitoring data submitted by ET.

2. Discuss amongst ER, ET and Contractor on the potential remedial actions.

1. Review the proposal by Contractor;

2. Make agreement on the measures to be implemented.

1. Stop construction work

2. Inform the ER

3. Review the piling sequence or method in order to reduce the underwater noise levels to no higher than 170 dB.

4. Submit noise reduction proposal to the ER for endorsement.

5. Implement the agreed measures.

6. Re-submit proposals if problem still not under control;

7. Stop the relevant portion of works as determined by the ER until the exceedance is abated