*********** +++++++++++++++++++++ 051496B.OAC + Source: ONR Asia + *********** +++++++++++++++++++++ Contributory Categories: BIO,ENG,ENV Country: Australia From: DSTO Aeronautical and Maritime Research Laboratory PO Box 4331 Melbourne Victoria 3001 Telephone: (03) 626 8111 Fax: (03) 626 8999 @ Commonwealth of Australia 1995 AR No. 009-213 March 1995 KEYWORDS: Australia, Spencer Bay; Acoustic survey and Range, ambient noise, biogenic acoustic noise +++++ Part III/III SITE SURVEY FOR AN OCEAN ENGINEERING PROJECT IN SPENCER GULF NOVEMBER 1993 Ian S.F. Jones, Douglas H. Cato, L.J. Hamilton, Sandra Tavener an d B.D. Scott Maritime Operatio ns Division Aeronautical and Maritime Research Laboratory DSTO-TR-0149 II. ENVIRONMENATAL IMPACT 11.1 Environmental Impact The (then proposed) Environmental Impact Statement legislation in South Australia requires developers to assess the effects of both direct and indirect factors in their proposals (Stefanson 1977). Spencer Gulf is a shallow, semi-enclosed area subject to stress by industrial activity, particularly at Whyalla, Port Pirie, and Port Augusta, a fishing industry, and increased recreational activity. The Gulf experiences virtually no ingression of fresh water, and depends on oceanographic and meteorological factors for flushing. Seagrass and mangrove communities are believed to be the major productive base for the marine ecosystem. Stefanson (1977) indicates that few environmental problems are likely to occur in southern Spencer Gulf, because of increased water movements in this area, and the large volumes of water involved, which aid dispersal and removal of pollutants. Seagrasses generally grow abundantly in subtidal waters of less than 10 m (e.g. Harris et al. 1991) and are not expected to be found in deeper waters, where light penetration is generally insufficient for photosynthesis. Right whales come into the shallow waters of the southern coastline to breed from July to September. This species is the most depleted of all in the Australian region and would be considered to be the most vulnerable. Sightings attract considerable publicity. If right whales were to enter Spencer Gulf it would be in small numbers, and in view of the size of the Gulf it seems unlikely that they would be found in the vicinity of an acoustic range and very unlikely when the range is in use. There is no reason to suppose that an acoustic range would have any more impact than any other type of mooring, and ranging operations would have less impact than that of a passing ship. 11.2Effects of environment on moored instruments Nunes and Lennon (1986) report that loose strands of seagrass in the shallow depths of the far north of Spencer Gulf occasionally bound the rotors in current meters. Rapid marine growth in the high summer temperatures of 20 to 25' C caused many problems with temperature and current measurement sensors. Significant low frequency transient noise was evident at all sites intermittently. These included thumps and dragging sounds of the kind heard when there is motion of the hydrophone on a hard bottom. There were also somewhat metallic "clicks" and "clunks" which later tests on the equipment showed could have been produced by impact of objects with the metal canister containing the hydrophone electronics. The bottom at the sites comprised compacted shell fragments including whole shells, and showed evidence of scouring. The transients were audible at all sites at much the same times for periods of several hours but not synchronised as they would be if from a common source. The times of start and finish of the periods of transients were slightly different at each site and the sounds audible at one site were not correlated with those at another. This indicates that the sources were localised to each site but mechanism was initiated by an effect that was common to all sites and is consistent with movement in the vicinity of the hydrophones resulting from a current field that influenced all sites. Examination of the current meter records does not reveal an obvious correlation of the occurrence of the transients with the speeds of the tidal currents. There is, however, some evidence of correlation with significant fluctuations in current direction. It is also possible that the generation of the transients is related to a bottom current not correlated with the tidal currents. This type of bottom mounted recording system has been used at a number of sites around Australia and usually such transients are not observed except on rare occasions. The bottom conditions are usually muddy allowing the system to sink slightly thus securina, it to the bottom. The compacted shell grit on the bottom in Spencer Gulf would not allow this and the hydrophones would be more vulnerable to movement. A system for use in Spencer Gulf needs to be designed to isolate the hydrophones from the mechanical impact noise on the bottom. The precise nature of the causes of these transients needs to be investigated further at the chosen site to provide the information required for this. 12 CONCLUSIONS Spencer Gulf appears to be suitable for a shallow water acoustic range, and to be a substantially better site than off Perth. The currents are low enough not to cause major difficulties in system design or stability. Currents are predominantly tidal and therefore predictable so that, if required, ranging times can be chosen to coincide with the very low currents that occur for periods up to five days in a fortnightly cycle, and also for other times of low currents. The currents are substantially less than off Perth. Noise levels at low wind speeds in the Gulf are significantly lower than in the open ocean. For most of the frequency range measured, the noise in Spencer Gulf would be 10 to 15 dB less than off Perth for significant periods of time. At low frequencies the noise levels are much lower because of the absence of significant traffic noise and the low frequency corn onent of wind dependent noise. At mid and high frequencies where the noise is predominantly wind dependent, noise levels are lower for substantial periods because the wind speeds in Spencer Gulf are on averagc significantly lower. For example, wind speeds are equal to or less than 10 knots for about 40% of the time compared with only 14% of the time off Perth. The effect on system performance from the substantially lower noise levels is comparable to or greater that achievable by a complex hydrophone array system over a single hydrophone. The main disadvantage of a shallow water site from the noise point of view is the presence of a continual background from snapping shrimps. Above about 5 kHz, noise levels can be expected to be higher than at low wind speeds in the open ocean. The Thistle and Wedge sites are significantly more suitable than the Berry site in having lower noise levels for a significant proportion of the time, and less variable currents. Boats were observed to cluster around Corny Point causing higher low frequency noise levels at Berry. Wind speeds, and thus wind noise level, are significantly higher at Berry for significant periods of time, usually in the afternoon. The Thistle site did not show evidence of the choruses observed at Wedge, and is more sheltered from swell, but otherwise the two sites are comparable. The differences are not major. The fish choruses are likely to occur at any site, and the evening chorus is predictable and significant only for about one hour per day. Swell at all sites is small compared to the open ocean. The Wedge and Thistle sites are thus recommended in preference to the Berry site off'Corny Point. The margin of Thistle over Wedge is sufficiently small that both sites should be considered acceptable in terms of the considerations of this survey. The noise recordings in Spencer Gulf showed evidence of movement in the vicinity of the hydrophones, causing significant transient noise at times. This is considered to be due to water flow over a relatively hard bottom resulting in motion of debris and possibly the hydrophone. An acoustic range would need to be designed with moorings and hydrophone suspension suitable to isolate the hydrophone. This will need to be based on more detailed information about the nature of the mechanisms causing the transients than is presently available. Further investigation at the chosen site is therefore recommended. ACKNOWLEDGMENTS The successful execution of the complex series of field measurements owes much to the efforts of the trials team: Doug Bellgrove (trials officer), Brian Jones, Mark Savage and Tony White (with a little assistance from two of the authors). Brian Jones developed the noise recording system and Doug Bellgrove assisted by Tony White developed the anemometer buoy. Gert Johansson of Flinders University made current measurements at the three sites and we are grateful for his general assistance in the other measurements. We much appreciate the willing efforts of the masters and crew of the S.A. Fisheries vessels TUCANA and NGERIN in deployment and recovery of the equipment and moorings. Some of the recovery was made from fishing boats KIOARA III and SANTA ROSA. Steedman Science and Engineering made current profile and wave rider buoy measurements at the Wedge site. The survey flights were piloted by John Rogers. Anthony Wong of the University of Sydney analysed the white cap photographs. We are grateful to the port managers: Captains Buchanan, Mansfield, Marshall, Myles for assisting with the statistics of vessels movements, and to the ship captains who returned the survey forms. REFERENCES Andrew C. (1993). Tidal currents of lower Spencer Gulf. Flinders Institute for Atmospheric and Marine Sciences report, December 1993, University of South Australia, Adelaide. Australia Pilot, Volume 1 (1973). South coast of Australia from Cape Leeuwin to Green Point. Sixth edition, N.P.13., Hydrographer of the Navy, Ministry of Defence, Taunton, United Kingdom. Banner M.L. and Cato D.H. (1988). Physical mechanisms of noise generation by breaking waves - a laboratory study. In Sea Sur ace Sound, Kluwer, Dordrecht. edited by B.R. Kerman. Bannister J.L. (1985). Southern Right (Etibalaena australis) and Humpback (Megaptera novaeangliae) whales off Western Australia: Some recent work. 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Numerical Simulation of Fluid Motion, Nortli-Holland, Amsterdam, edited by J. Noye, 285-356. Nunes R.A. and Lennon G.W. (1986). Physical property distributions and seasonal trends in Spencer Gulf, South Australia: an inverse estuary. Australian Journal of Marine and Freshwater Research, 37, 39-53. Nunes R.A. and Lennon G.W. (1987). Episodic stratification and gravity currents in a marine environment of modulated turbulence. Journal of Geophysical Research, 92, C5, 5465-5480. Petrusevics P.M. (1993). SST fronts in inverse estuaries, South Australia indicators of reduced gulf-shelf exchange. Australian Journal of Marine and Freshwater Research, 44, 305- 323. Prenc S. (1993). Manual for the programme B&KNOISE. Materials Research Laboratory Technical Note (unpublished). Provis D.G. (1993). Currents at Underwater Radiated Noise Range sites in Spencer Gulf. Steedman Science and Engineering Report No. R663. Radok R. and Raupach M. (1977). Sea level and transport phenomena in St. Vincent Gulf. 3rct Australian Conference on Coastal and Ocean Engineering, Melbourne, 18-21 April 1977, 103-@ Stefanson R. (1977). Spencer Gulf - a review of the oceanography, marine biology and the implications for development. 3rd Australian Conference on Coastal and Ocean Engineering, Melbourne, 18-21 April 1977, 13-16. Steinberg C.R. (1983). Tidal exchanges at the mouth of Spencer Gutf, South Australia. Honours BSc Thesis, School of Earth Sciences, Flinders University of South Australia. Tate P.M. (I 988). A guide for Aanderaa current meter processing. Ocean Sciences Institute Report No. 18, University of Sydney. Toba Y. and Chaen M. (1973). Quantitative expression of the breaking of wind waves on the sea surface. Rec. Oceanogr. Works Japan, 12, 1 -1 1. Tronson K. (1974). The hydraulics of the South Australian Gulf System. I. Circulation. Australian Journal of Marine and Freshwater Research, 25, 413-426. Wenz G.M. (1962). Acoustic ambient noise in the ocean: spectra and sources. Journal of the Acoustics Society of America, 34, 1936-1956. Wong A. (1993). Whitecaps. Unpublished document, University of Sydney. Wu J. (1979). Oceanic whitecaps and sea state. J. Phys. Oceanogr., 7, 1064-1068. +++++ End Part III/III +++++ CMR Disclaimer================================================== This document could contain information all or part of which is or may be copyrighted in a number of countries. Therefore, commercial copying and/or further dissemination of this text is expressly prohibited without obtaining the permission of the copyright owner(s) except in the United States and other countries for certain personal and educational uses as prescribed by the "fair copy" provisions of that countries Copyright Statues. ================================================================ ************** END Msg. B.OAC **************