Outer Harbor dredging (Flinders Ports)
Dredging of the Outer Harbor channel was undertaken from June to September 2019, under the authorisation of an EPA licence. The EPA required Flinders Ports to undertake every possible action to reduce the impact on marine life and seagrass, and to implement the most environmentally sustainable dredging methodology.
The EPA set strict turbidity thresholds for the dredge program and required Flinders Ports to monitor seagrass condition. In addition to this, the EPA independently monitored seagrass condition before, during and after the dredging campaign.
The EPA has now completed the first of its post seagrass monitoring surveys, and published a full report on the outcome. Please see ‘Fact sheets’ below for a summary, and ‘Documents and reports’ for the full reports.
Hold determinations
Fact sheets
Media releases
Community engagement
Documents and reports
Seagrass audits and bathymetric surveys
- Outer Harbor channel widening dredging seagrass condition assessment, September 2020
- EPA pre-dredge seagrass audit, May 2019
- Flinders Ports pre-dredge seagrass audit, May 2019
- Flinders Ports bathymetric survey disposal site, May 2019
- Flinders Ports bathymetric survey site 14, April 2019
- Flinders Ports bathymetric survey site 15, April 2019
- Flinders Ports bathymetric survey site 16, April 2019
- Flinders Ports bathymetric survey site 17, April 2019
Real-time turbidity monitoring
Background
Further information
FAQs
What did the 2005–6 Outer Harbor dredge involve and what were its impacts?
In 2005–06 large capital dredging occurred within the Outer Harbor channel which removed approximately 3 million m3 of dredge spoil. This dredging campaign involved deepening the channel and disposal of dredge spoil in the same offshore area of Gulf St Vincent as the current development application (DA) proposes.
Dredging involves 2 potential activities of significant environmental risk. The first is the physical removal of sediment and turbidity generated by the dredges. In the nearshore areas, there will be turbidity (discolouration and cloudiness in the water by suspended particles) generated by the dredge(s) in operation.
The second area of risk is through the disposal of spoil at the offshore disposal site located approximately 30 km offshore. Each risk has its own management issues that are relatively independent of each other.
The 2005–06 dredging raised public concern regarding widespread and extensive turbidity generated that spread throughout the nearshore environment and impacted seagrass meadows. The EPA used aerial photography to estimate a loss of approximately 1,600 ha of seagrass near the shipping channel from the 2005–06 dredging campaign. Evidence suggests that this loss is likely to be due to the turbidity generated by the dredges at the dredging site.
Investigations conducted as part of the 2005–06 campaign indicated that there were minor short term and localised impacts at the offshore dredge spoil disposal site (approximately 30 km west of Outer Harbor in Gulf St Vincent) and the area quickly returned to pre-disposal conditions. The seafloor in this area is too deep for seagrass and has been affected by a long history of prawn trawling (Tanner 2005)[1], so is primarily bare sand.
[1] Tanner JE 2005, Three decades of habitat change in Gulf St Vincent, South Australia. Transactions of the Royal Society of South Australia 129(1): 65-73.
What did Flinders Ports propose in their original July 2017 channel dredging application?
Flinders Ports is proposing to widen the existing Outer Harbor channel and swing basin by approximately 40 m (from 130 m to 170 m). This would require the need to dredge approximately 1.55 million m³ of material along 7 km of the existing channel. The dredged material (dredge spoil) from the seafloor is proposed to be placed approximately 30 km offshore in Gulf St Vincent in an area approximately 7 km by 5 km located in deep water (>30 m) outside of major shipping routes (the same area used for Outer Harbor channel dredging spoil disposal in 2005-06).
The original July 2017 proposal from Flinders Ports was to use the same methods as the 2005–06 dredging works. Modelling undertaken for the original proposal predicted that the loss of seagrass associated with the dredging works would total between 942 ha and 1,583 ha (see figure below) depending on whether dredging was undertaken in winter or summer (respectively). It is important to note that these predicted impacts did not consider any preventative and management measures required by the EPA dredging licence.
The EPA informed Flinders Ports that the predicted loss of extensive seagrass was unacceptable and a range of alternative dredging methods needed to be considered to reduce the predicted impacts on the marine environment.
What are the potential nearshore impacts of dredging?
In the nearshore areas, there will be turbidity (discolouration and cloudiness in the water by suspended particles) generated by the dredge(s) in operation. The movement of any plume will be dependent on the weather, with dredging during winter typically moving any plume to the south with the prevailing weather and currents. In summer plumes will typically head to the north with the dominant southerly winds. The area that may be affected is shown in the figures below. The intensity of the plume in these locations is expected to be less than the resuspension of sediment caused by a windy day.
The alternative dredging methodology is predicted to greatly reduce the area of total seagrass loss (by up to 82%) compared to the original proposal, with the loss being limited to the channel area being dredged and adjacent to the channel. Additionally, the seagrass in the area potentially affected is dominated by Heterozostera spp, which is an opportunistic species that can recover from disturbance substantially quicker than for enduring species such as Posidonia spp.
A predicted maximum 250 ha of seagrass would be lost if the dredging was unmitigated, including 130–168 ha being temporarily lost (ie seagrass recovery would be likely within 2 years in such areas). However, the EPA recommended conditions would involve extensive mitigation through the dredging licence management process under the Environment Protection Act 1993, so the loss is expected to be significantly less than 250 ha.
What are the impacts of offshore disposal of spoil?
The spoil disposal site is in the middle of Gulf St Vincent and unvegetated (see first figure below) as it is approximately 30 m deep and is similar to other comparable areas within the Gulf at this depth (see second figure) that have been impacted by prawn trawling.
Unvegetated sand and occasional shellgrit and rock
Observation of site 5 nautical miles south of the Outer Harbor offshore spoil disposal ground for comparison
During spoil disposal there will be a discolouration of the water due to turbidity. The model predicts that this turbidity will not travel outside of the boundaries of the spoil ground nor will it affect the nearshore environment. This material is likely to settle out within hours to days and have no long-term environmental impact. The average increase in turbidity will be less than 1 nephlometric turbidity unit (NTU) which is barely perceivable to the naked eye.
The spoil has been demonstrated to be not contaminated (chemicals) and as such is safe for disposal. The dredged material is characteristically very similar to the disposal area and as such will be incorporated into the bed material very quickly.
Are there risks to the fishing industry?
Tanner (2004)[1] undertook an assessment of the potential impact of the dredging on the environment and other uses (eg fishing) prior to the 2005–06 dredging.
Prawn industry
The work by Tanner (2004) found that two fishing blocks in the Gulf St Vincent Prawn Fishery overlap the disposal site (14 and 19). These blocks represented an average of 7.6% of the total Gulf St Vincent (GSV) prawn catch 1993-4 to 2002-3; however, the disposal site accounted for approximately 20% of the area of these blocks, or an area from which approximately 1.5% of the catch was taken.
The EPA assessed the annual prawn catch and the number of trawl hours were assessed for blocks 14 and 19 between 2000 and 2016. It also showed that 2008 produced the largest harvest from these blocks during this period indicating little long-term impact to these blocks.
Garfish and other fish
A decline in Southern Garfish catches throughout marine fishing area (MFA) 36 was observed in the years before and after the dredging. Garfish are largely an inshore fish species that inhabit subtidal and intertidal seagrass meadows. Catches were quite variable between years and the significant reduction in catches 2000–01 to 2002–03 was also noted. The Southern Garfish stock in northern Gulf St Vincent been heavily exploited and is currently classified as overfished (Steer et al 2016)[2]; PIRSA has implemented a range of management changes to promote recovery of the fishery. The inshore seagrass loss as a result of the 2005–06 dredging may have contributed to the pressure on the fishery.
The Flinders Ports alternative dredging method will substantially reduce inshore turbidity and the risk to the seagrass meadows compared to the 2005-06 dredge. This is expected to result in minimal seagrass loss. Offshore disposal is not expected to significantly impact on commercial or recreational fisheries.
Cockle fishery
The Section bank is a part of the northern aspect of the shipping channel between the breakwater rock and the mangroves near Pelican Point. This sand bank is covered in Heterozostera sp. seagrass and has extensive cockle populations (Katelysia spp). This area historically supported a commercial Vongole (mud cockle) fishery.
There have been concerns raised about the impact of the 2005–06 dredging on the fishery as the lowest harvest in the Port River occurred in 2006–07; this could be related to access restrictions while the dredging was occurring. Subsequently, the highest recorded harvest occurred in the Port River in 2007–08 after the dredging occurred.
The Port River Vongole fishing zone at Section Bank has been closed to recreational and commercial fishing since 2011 due to concerns around sustainability, low stock abundance and poor recruitment (Dent et al 2016)[3].
Potential sediment mounding at the spoil disposal site
There have also been claims that mounding of sediment caused by spoil disposal from the 2005–6 dredge has precluded prawn trawlers from the area. Immediately after the major works of the previous dredge campaign, Flinders Ports undertook high-resolution geoswath mapping of the entire spoil disposal location including the 1-km buffer. This soundings map shows no large mounds, and the typical undulations were less than 1 m high, similar to the terrain of this kind of environment and would be unlikely to preclude trawling.
[1] Tanner JE 2004, Environmental assessment for proposed dredging of Outer Harbor approach channel, prepared for the KBR and Flinders Ports, Adelaide, SARDI Aquatic Sciences Publication No RD04/0090. SARDI Aquatic Sciences, 72 pp.
[2] Dent, J., Mayfield, S. and Carroll, J. (2016). Harvestable biomass of Katelysia spp. In the South Australian commercial Mud Cockle fishery. Report to PIRSA Fisheries and Aquaculture. South Australian Research and Development Institute (Aquatic Sciences), Adelaide. SARDI Publication Number F2014/000191-2. SARDI Research Report Series No 898. 29pp.
[3] Steer M, R McGarvey, J Carroll, W Jackson, M Lloyd and J Feenstra 2016, Southern Garfish (Hyporhamphus melanochir) Fishery, Fishery Assessment Report to PIRSA Fisheries and Aquaculture, South Australian Research and Development Institute (Aquatic Sciences), Adelaide, SARDI Publication No, F2007/000720-4, SARDI Research Report Series.
What are the considerations around land-based disposal of spoil?
If land disposal was chosen, additional infrastructure would be required to be built within the coastal zone in order to access the slurry pipeline through its total length to the disposal location. This would be required in order to undertake routine maintenance and to clear the pipe in the event of a blockage in the pipe. Additionally, the long length of pipe requires booster pumps to maintain movement of the material in the pipe. Each booster pump would require power generators, fuel, bunding, earthworks and consideration of their proximity to sensitive habitats such as mangrove and samphire. Within each of these aspects are risks associated with slippage and pipe blockage due to the high clay content, and pump failure which may result in environmental impacts.
The EPA engaged an independent geotechnical engineering expert to undertake an independent review of the information provided by Flinders Ports. This review concluded that if an ocean-based disposal pathway did not exist, then land-based disposal options are likely to be feasible but would be subject to a wide range of risks and substantial additional costs. Where an approved ocean-based spoil disposal pathway does exist, the risk assessments undertaken by Flinders Ports regarding land-based disposal options are reasonable.
Beneficial use of spoil
Flinders Ports has undertaken geotechnical analysis on the dredge spoil which is a mix of sand, silt and clay in the spoil along with the concentration of potential toxicants such as metals. This work has shown that the area to be dredged is not contaminated with toxicants and is safe for disposal.
If land-based disposal is chosen, there are two major considerations; one is the time it will take for the material to dry out. The second is the end use of the material which may affect future use of the land at the disposal site.
Drying the spoil out (called dewatering) is required as the dredging process collects the spoil in a slurry mix approximately 80% water and 20% spoil. Typically, large areas of land are required to allow the material to settle out of the water, so that water can be drained off. Alternatively, high walls would need to be built to allow the spoil to be disposed into smaller land areas. The high clay content in the spoil means that the water takes a long time to dry out, potentially many years, and this will be exacerbated by deeper spoil or water depth. Other considerations to be taken into account include the ownership of the land during this period, and the potential for dust and nuisance pests (eg midges, mosquitoes).
The geotechnical assessment indicates that the spoil is not suitable for either beach replenishment or use as engineered fill, a common beneficial use of waste material. This means that the land holding this material would be very difficult to build upon, having long-term implications for the land use. There is the possibility of blending with additional material to make it suitable, but with additional costs and machinery.
What factors was the EPA legally required to consider when assessing the Outer Harbor channel dredging application?
Objects of the EP Act and General Environmental Duty considerations
When assessing development applications, section 57 of the Environment Protection Act 1993 (EP Act) requires the EPA to have regard to, and seek to further, the objects of the EP Act, have regard to the general environmental duty as specified in section 25 of the EP Act (ie that all reasonable and practicable measures have been taken by the proponent to prevent environmental harm for both the dredging activity and disposal of dredge spoil, and have regard to any relevant environment protection policies).
In relation to the objects of the EP Act, the EPA must consider social and economic impacts in addition to short and long term environmental impacts when assessing development applications. The general environmental duty also requires the EPA to make a judgement about a number of considerations, including:
- the need for a person to take all reasonable and practicable measures to prevent or minimise any resulting environmental harm; and
- the need to have regard to:
- the nature of the pollution or potential pollution and sensitivity of the receiving environment
- the financial implications of the various measures that might be taken, and
- the current state of technical knowledge and likelihood of successful application of the various measures that might be taken.
In relation to the Flinders Ports channel dredging development application, the EPA made a judgement that the alternative method of dredging proposed met the general environmental duty and the requirements of relevant Environment Protection Policies.
In summary, the EPA needed to consider the following technical issues:
- The dredge spoil meets national guidelines for disposal of dredge spoil to sea (including in regard to contamination levels).
- The sea based receiving environment is non vegetated sand (no macro algae is present, nor seagrass, which does not grow at this depth), which supports a sparse marine invertebrate community that recovers relatively quickly from dredge spoil placement.
- The sea based receiving environment is an established spoil ground.
- The land based disposal sites will require supportive infrastructure to be placed in sensitive coastal and estuarine environments that contain mangroves, samphire and seagrass.
- The risks of environmental harm to the sensitive estuarine environment containing seagrass and mangrove associated with potential clogging/unclogging of the pipe transporting spoil from the dredge sites to the dredge spoil disposal site.
- The land based disposal sites will require water decanting from the settlement ponds to be directed back into coastal and estuarine environments that contain mangroves, samphire and seagrass.
- The time taken for the dredge spoil to dry, and implications for opportunity cost associated with use of this land for an extended period of time (years).
- The limited quality of the dried dredge spoil for use (classified for non-engineered fill only) and not suitable for beach replenishment.
- The cost of land based disposal which is significantly greater than the costs of sea based disposal.
Given the above, the EPA considered that sea based disposal meets the general environmental duty, the objects of the EP Act, and the relevant environment protection policies.
What are the biosecurity considerations?
PIRSA has been advising Flinders Ports and Government on known biosecurity risks associated with the Outer Harbor channel dredging development application. Key issues include the need to mitigate risks of spreading aquatic pest species, including Caulerpa taxifolia, and aquatic diseases, specifically Pacific Oyster Mortality Syndrome (POMS).
For further information see:
