Dredging application to widen the Outer Harbor channel
Flinders Ports Pty Ltd has applied to the State Commission Assessment Panel (SCAP) to widen the existing Outer Harbor shipping channel to accommodate larger vessels. This proposal involves dredging 1.55 million m3 of material along 7 km of channel.
In September 2017 the EPA provided advice to Flinders Ports that they needed to ensure their dredging methodology would not cause large scale loss of seagrass near the shipping channel due to turbidity (which occurred after the 2005 dredging works).
The EPA is required to assess proposals against the general environmental duty as described in section 25 of the Environment Protection Act 1993. The EPA considers that the alternative methodology achieves this subject to Flinders Ports meeting conditions relating to:
- The proposed alternative dredging methodology being implemented.
- An environmental monitoring program being prepared and implemented.
- The preparation of a dredge management plan.
- Restrictions on the timing and methods employed for pile driving.
Maps of affected zones* – modelling of original and alternative dredge methodologies for summer and winter
*Maps attached model the potential impact of both original and alternate dredging scenarios without any mitigation. However, both Development Approval conditions (if approved) and subsequent EPA licencing will require further actions be undertaken to minimise turbidity.
In the previous dredge (2005–06), the turbidity arising from the dredging activity itself (in the vicinity of the Outer Harbor channel) contributed to the loss of about 1,600 hectares of seagrass.
There is no seagrass at the spoil ground which is in deep water. Seagrass in Gulf St Vincent does not naturally occur in waters at the depth of the spoil ground (over 30 metres).
The ecological issue surrounding the dredge proposal arises from the dredging activity itself, not the disposal of the spoil, 38 km off the metropolitan coast.
Alternate dredging methodology
The EPA had requested Flinders Ports to investigate alternate dredging methodologies from the method originally proposed and model the potential water quality impact on seagrass.
The EPA considered that an alternate dredging methodology – involving no ‘side casting’ from the cutter suction dredging process – should be further investigated and the likely turbidity and associated seagrass impacts modelled. Flinders Ports undertook such modelling and subsequently amended their development application to use this alternative method.
The original dredging proposal was predicted to cause a seagrass loss of approximately 1,000 hectares in winter to approximately 1,500 hectares in summer. This compares with the alternative dredging method which has a significantly reduced predicted seagrass loss of approximately 250 hectares for both the summer and winter dredge scenarios.
However the EPA has advised SCAP that action be taken to further reduce seagrass impact as outlined in the EPA recommended conditions of development approval. This should result in less seagrass loss than predicted by the models.
The seagrass loss is expected to be confined to an area dominated by Heterzostera seagrass which does tend to grow back rather than the original dredge proposal areas of impact, dominated by Posidonia seagrass, which do not tend to grow back.
Although impact on beaches is not anticipated, additional turbidity control over the summer months is required as an additional safeguard. This includes stopping dredging for periods of time.
Disposal of dredge spoil – land and sea considerations
The mangroves are a nursery for the fish in Spencer Gulf and provides highly valuable remnant habitat for other native fauna. The sea-based disposal is in a degraded area largely devoid of life, compared to the alternative site which may impact on a highly valuable coastal habitat.
The EPA has not required disposal of dredge spoil to be land based, for reasons including:
- The risks associated with transporting dredge spoil by pipe to suitable land-based locations across sensitive mangrove habitat.
- The designated disposal site is in deep water (therefore naturally devoid of seagrass) and as it has been used as a spoil ground previously is degraded with minimal sea life present.
- The unlikely suitability of the dredge spoil material to be used as fill.
The EPA requires analysis of the sediments to ensure characteristics, including contamination of pollutants, are understood.
Results from analysis of sediment to be dredged in the Outer Harbor channel indicated that the dredge spoil quality meets the National Assessment Guidelines for Dredging 2009 and suitable for ocean disposal.
Samples taken for this assessment were collected in 2016 and 2017. These results are also consistent with samples taken in 2001 and 2004 for the previous 2005–06 Outer Harbor dredging project.
Turbidity and sedimentation associated with the disposal of the spoil have been investigated using hydrodynamic modelling. This modelling predicts minimal environmental impact at the designated spoil disposal site 38 km west of Outer Harbor in the middle of the Gulf.
Surveys of the dredge spoil site before the 2005 dredge spoil placement and recently have shown that the site has recovered to its pre-dredge spoil historic state, which is degraded. The volume of spoil to be deposited at the site for this current proposal is half that disposed in 2005, so it is expected to have negligible if any ecological consequence.
The alternative site that was considered on land would necessitate the placement of an 8-km pipe to the coastal site, through the Adelaide Dolphin and International Bird Sanctuaries containing significant sensitive mangrove habitat and construction and operation of a wharf also in sensitive mangrove habitat. The risk of pipe breakage and flow of spoil into the environment is considered to an unacceptable alternative.
The EPA briefed non-government organisations (NGOs) during December 2017 and January 2018, including on the dredging impact modelling it had received from Flinders Ports’ consultants.
The EPA has provided advice and presented this to SCAP at their meeting on 18 January 2018.
If development approval is provided to Flinders Ports to undertake the dredging, Flinders Ports will be required to apply for a dredging licence from the EPA. If this occurs, the EPA will place further detailed conditions on the licence including relating to minimising impact from dredging operations, real time monitoring including turbidity levels and availability of that data to the public.
The EPA licence application and assessment process provides an opportunity for community comment and submissions.