Kangaroo Island (SA Water)
EPA dredge licence –desalination plant
The Environment Protection Authority (EPA) licenses dredging in accordance with the Environment Protection Act 1993, if a proponent has approval from the relevant planning authority. SA Water received Development Approval on 16 February 2022 to build a new seawater desalination plant on Hog Bay Road, Penneshaw (Kangaroo Island).
Dredging of approximately 1,000 m3 will be required, to install the 2 intake pipelines and the outfall pipeline which sits on the seabed. Dredging is expected to take approximately 2 months to complete.
SA Water’s construction partner, John Holland Guidera O’Connor Joint Venture, has engaged Maritime Constructions to construct the marine infrast ructure required for the new desalination plant. Maritime Constructions holds EPA licence 42842 to undertake dredging at various locations in SA.
An approved Dredge Management Plan (DMP) with conditions relating to the management of water quality, noise, and dust is also required.
Dredge management plan
- Water quality is to be managed by the licencee observing the turbid plume, and monitoring water quality to manage the risk of potential acid sulfate soils.
- Seagrass meadows must be avoided by selecting sand patches for anchoring, with dredged material placed adjacent to the excavated area.
- Pipeline installation methodology requires a causeway to be built from the shoreline to facilitate excavator access to the areas to be dredged.
- Dredged material from 2 areas will be reused for the construction of the causeway, and the material from the remaining two areas will be relocated onto sand (at the designated Dredge Material Placement Area selected, due to the seafloor only containing sand). Smothering of seagrass by the placement of spoil material is not permitted.
- Imported rocks brought into the site for causeway construction must contain less percentage of clays and fine silts than the receiving environment, the appropriate density to avoid crushing when the civil plant tracks over it.
- Short-term turbidity is expected during the installation of the causeway and throughout the dredging itself, however this is unlikely to affect the seagrass beds. Live seagrass is impacted by turbidity if the duration of the turbidity is approximately 3 months or more. Seagrass is entering into its growing and reproductive season and as such will have some resilience to cope with short-term fluxes of turbidity.
- Desalination plant operations will require ongoing monitoring of the ecological environment, including seagrass health, and this will be undertaken by SA Water.
The dredging methodology and spoil disposal has been carefully considered from a range of options (eg cutter suction, clam grabber, excavator bucket etc) to create the least long-term impacts, while minimising short-term impacts through adapting spoil management methods. The licensee used a multi-criteria assessment to assess 5 different options. The chosen method of dredging in the deeper areas will use a ‘clam grab’ situated on a barge between two dredge areas.
Dredge spoil will be relocated to bare sand areas. The rocky shoreline is not suitable to dispose of sandy spoil, so relocating the spoil from one sandy, subtidal area to another subtidal area is the best solution to manage the dredge spoil.
If works occur in the vicinity of migratory whales (May–September), the contractors operating the dredge equipment are required to undertake marine mammal monitoring.
Dolphins, seals, sea lions and whale sightings, along with their behaviour must be logged. Where a marine mammal is within the relative protective exclusion zones, management actions must be taken, with dredging to cease until any marine mammals have moved outside of the zone.
In assessing the DMP, the EPA considered the nature and scale of the dredging and what controls would be required to manage a range of potential project impacts. The EPA is confident that the implementation of the DMP, licensee reporting and EPA compliance inspections, the project will have minimal environmental impact. Risks to water quality are further minimised due to flushing and the nature of this coastline.
Construction of the causeway, including rock placement, the operation of vessels and the dredge equipment, will generate intermittent noise that may be experienced by residents living close to the construction zone. Maritime Constructions will manage noise by ensuring all equipment is maintained and working effectively, and construction only occurs Monday–Saturday within the hours of 7 am–7 pm, and on Sundays and public holidays within the hours of 9 am–7 pm. Any request to work outside of these hours is subject to EPA approval. It is unlikely that any of the equipment used in this project will cause nuisance vibration.
Impacts to air quality through dust generation is likely within the construction zone, due to the handling of materials with heavy plant, and vehicles using unsealed roads. Dust impacts may be reduced using a water truck (if and when required) and by handling practices that minimise dust and further crushing of rocks.
Site selection and soil disposal
Consideration was given to balancing acute, localised impacts from capital dredging against the potential long term impacts from brine discharge. Consideration was given to balancing acute, localised impacts from capital dredging against the potential long term impacts from brine discharge.
The location of the desalination plant was selected by SA Water because of the high flushing rates, which promote dispersion of brine. However positioning the plant on a section of exposed, rocky shoreline presents substantial challenges for the capital dredge campaign, including spoil disposal and accessibility due to weather.
Dredging is required for the purpose of laying intake and discharge systems, but will not require further dredges to maintain depth.
Spoil disposal will be managed in two ways:
- Spoil from the nearshore sections will be used to progressively construct a causeway for the excavation of a trench into deeper water
- Spoil from deeper areas will be disposed of back to un-vegetated seabeds. These spoil disposal locations are appropriate because the dredge volume is relatively low, the duration is short (approximately 2 months), and the coastline is exposed and highly rocky in nature.
Monitoring for low pH adjacent the causeway and turbidity plumes when spoil is being used for causeway construction is required. Turbidity plumes during spoil placement from deeper areas must be managed visually.
You are also welcome to contact the EPA directly:
- GPO Box 2607, Adelaide SA 5001
- Telephone: (08) 8204 2004 – if calling after hours please leave details and your call will be returned as soon as possible during business hours.