Environmental impacts of dredging and sediment disposal practices are a major issue, even as some of the major disputes in the dredging industry in India are on those issues. Mattijs Siebinga takes us through the risks and disposal challenges at the Port of Melbourne and how they were systematically addressed through an environment management plan.
Although the Port of Melbourne is an enormous asset to the national economy, Port Phillip Bay is also a unique social, cultural and recreational asset: almost 2,000 sq km, with 264 km of coastline and some three million people living in its vicinity. It includes two Marine National Parks and Ramsar wetlands. It is home to multiple fish species, little penÂguins, whales, dolphins and seals, extensive sponge gardens, various coldwater coral species and natural seagrass habitats and is an attractive recreational locale for swimÂming, diving and boating. The adjacent Yarra River, where the container terminals are located, has a highly contaminated silt bottom. In both areas, the environmental impacts of dredging were of clear concern to government, the Australian public and media and addressing these concerns often affected the methods and equipment used for the dredging operations.
After signing an Alliance Agreement with the Port of Melbourne Corporation (PoMC) in May 2004, Boskalis Australia became an active partner in assisting PoMC with the studies for the Supplementary Environmental Effects Statement (SEES) and the developÂment of an Environmental Management Plan (EMP). Boskalis and Hydronamic, its in-house engineering consultancy, also developed work methods and environÂmentally sound equipment for dredging of the Entrance, the removal of the contaÂminated silt in the Yarra River, the placeÂment of the contaminated material in a bunded disposal site and for placement of the sand cap. In addition, they developed and deployed a turbidity monitoring system to protect the environmental assets and to control the dredging process.
The final risk assessment of the SEES included the findings of the environmental impact assessments and formed an impoÂrtant underlying foundation for the EMP. The identified risks were docÂumented in a risk regisÂter which was reviewed periodically during the project to incorporate environmental monitoring results, to reflect operational changes or as a result of an environmental incident.
The EMP was established and approÂved both by the Victorian and Australian Federal ministers of environment. Environmental limits were set for turbidity and noise, which were monitored to prove compliance with the EMP, and work metÂhod related controls were established to manage all dredging operations. In addiÂtion, all vessels were equipped with vessel tracking systems to ensure compÂliance with dredged material management demÂands, which included limits to the quaÂntity of material to be dredged and to the vertical and horizontal tolerances, as well as to the time frame for dredging (eg, not during spaÂwning and migration periods or peak holiÂday seasons). Turbidity and vessel position were monitored automatically and transÂmitted to a database at the office. Boskalis managed the data flow and guaÂrded the data integrity, a transparent proÂcess that was audited regularly. The data could be visualised via a web page. Both the opeÂrations and PoMC staff as well as the vesÂsels had access to this web page.
Dredging performance was compared to the limits set by the EMP, which was repÂorted weekly to PoMC. However, exceÂedances, non-compliances and other inciÂdents and hazards were required to be repÂorted immediately, also to the Government. An extensive 12-audit schedule was put in place by the Government to check comÂpliance with the EMP of the vessels and the different dredging activities. Only a few minor non-conformities were found.
Furthermore, PoMC had the obligation to provide quarterly and annual reports to the Government. These contained a sumÂmary of all construction activities, proÂject progress, communications with the public and the media, notifications to the Government, training, reports, incidents and the results of all monitoring prograÂmmes, internal and external audits, risk assessments and management reviews. All notifications and reporting to the Government were publicly accessible thrÂough the websites of both PoMC and the Government.
The Entrance to Port Phillip Bay is one of the most turbulent and potentially dangerous stretches of water in the Bass Strait area, with an unusually hard seabed. Until 1986, the Entrance was deepened using explosives. Current environmental regulations prohibit this approach. Further, the risk of rock damaging deep reef habitats or leaving behind loose material after deepÂening required innovative work methods. Normally, a large stationary
cutÂter dredger would be used, but with the sea's extreme turbÂulence and the large number of shipping movements, a cutter was not the solution.
The Boskalis Research & Development, Central Technical and Dredging Departments, worÂking as an interdiÂscipÂlinary think-tank, found an innovative solÂution: A remarkable new ripper draÂgÂhead, strong enough to cut through hard rock, with excellent suction characteristics, yet able to be mounted on a trailing suction hopper dredger (TSHD). Trailers are more flexible than cutters and can work in extremely severe weather conditions.
The ripper dragheads mounted on the TSHD worked extremely well. All material was removed, even the strongly cemented edges of the rock formations. The structural strength of the ripper draghead was not compromised, indicating their excellent design and construction.
In addition to using an innovative ripÂper draghead at the Entrance, a work metÂhod was applied to minimise the likelihood that any residual rock might tip over the edge into the 80-m deep canyon and damÂage the precious sessile fauna like sponges and soft corals. This entailed the TSHD Queen of the Netherlands dredging away from the canyon edge and cleaning up for 18 hours after every 24,000 m3 of dredging, 460,000 m3 in total. A special data analysis tool applied to the vessel traÂcking system was used to prove that the operations proÂceeded in accordance with the EMP. After 22 clean-up sessions 15,500 m3 of loose material was collected and video surveys after coÂmpletion of the Entrance verified their effectiveness. Video recÂordings along 165 tranÂsects with a total lenÂgth of 25 km were used for the retriÂeval of 1,350 stills for the classification of the spill in the dredged areas. Later surÂveys have shown re-growth of the original kelp vegetation.
At the Yarra River and Williamstown Channel, toxicity tests conducted as part of the SEES determined that all silt in these areas was highly contaminated. Therefore, an underwater disposal facility in the Port of Melbourne Dredge Material Ground (DMG) was created for the safe disposal of 1.4 mm3 of contaminated silt in 20-m deep water. A 5-m high clay bund with 40-m crest width to enclose the 2,000 x 600 m area was built by the Queen of the Netherlands with clay from Port Melbourne Channel and Williamstown Channel. The TSHD Cornelis Zanen then dredged the contaminated silt. Extensive survey efforts were required to prove that 100 per cent of the silt had been removed. The Grab Dredger Goomai removed the silt inacÂcessible to the TSHD. The dredged silt was then discharged through a Boskalis-pateÂnted diffuser into the disposal area at planned positions. No mixing with the ambient water occurred, no dredged silt was found outside the disposal area and water quality was not affected. A 12 m wide spreader was developed for the Queen of the Netherlands to place a 0.5 m sand cap in subsequent thin layers over the deposited contaminated silt. The deposited silt stayed in place and the individual sand layers did not cause the silt to become unstable. Tube samples taken after compÂleting the cap revealed a sharp interface betwÂeen the silt and capping sand.
During the SEES, a turbidity disÂpersion model was calibrated based on measureÂments deterÂmined during the earlier eight-week dreÂdging trial and provided realistic environmental turbidity limits. Boskalis managed the turbidity meaÂsÂurements using a system of 22 buoys, which tested the seawater every 12 seconds. These were used to check compliance at 11 locations as well as to support further repÂorting purÂposes such as backÂground values. Turbidity was moniÂtored for up to 585 days. Weekly maintenance of the buoys was reqÂuired to cope with the bio-fouling of the sensors. Verification of the sensors in stanÂdard solutions was appÂlied monthly. On average, two buoy replÂacements were carÂried out on average every week. A high level of redÂundancy was created by using two indeÂpendent sensors per buoy and by doubling the computer system at the office. Over 99.98 per cent of data capture was achieved durÂing the approximately 300,000 service hours of the 22 buoys. No mechÂanical or electrical breakdown of the moniÂtoring system occurred during the last nine monÂths of the operations. Data was retriÂeved by two-hourly downloads through the local GSM network. Compliance was baÂsed on six-hourly exponentially weighted moving averages, based on which the TSHD's work method could be adjusted. Indeed, adjustÂments were only necessary during strong winds with elevated levels of turbidity and it was decided to move the TSHD to a different dredging area.
PoMC conducted several bay-wide moÂnitoring programmes to provide broader information on the status of key species, habitats and ecological processes in the Bay. As part of this, Boskalis conducted the Plume Intensity and Extent monitoring programme. Turbidity was measured by sailing tracks in each dredging area during at least 14 consecutive days. This meant that plume monitoring was conducted durÂing 117 days, covering a distance of 4,400 km. A streamer with two or three turbidity sensors was towed along straight tracks across the dredging area. The streamer could be automatically raised and lowered in the water column by means of a winch. The monitoring results were reviewed every six months by comparing the field data with predictions from the modelling. None of the environmental limits had to be adjusted after these reviews.
FULL COMPLIANCE AND SUCCESSFUL COMPLETION
The combined efforts of the client PoMC and the contractor Boskalis Australia in an Alliance Contract led to nearly full compliance and successful comÂpletion of the project. Significantly, stakeÂholder acceptance of the project was closely related to the accurate and transÂparent communications made by the Alliance Team. These included public conÂsÂulÂtations, a dedicated website, a 24-hour toll-free telephone number, weekly press conferences, media releases, mailing lists, signage around the Bay and notices to mariners.
The author is Managing Director, Boskalis Australia Pty Ltd. Royal Boskalis Westminster NV is a service provider operating in the dredging, maritime infrastructure and maritime services sectors.
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