Project Sea Dragon Stage 1 Hatchery Coastal Environment and Impact Assessment Seafarms Group Limited October 2017 Document Status Version Doc type Reviewed by Approved by Date issued v01 Draft Report Christine Arrowsmith Christine Arrowsmith 08/09/2017 V02 Draft Report Christine Arrowsmith Christine Arrowsmith 9/10/2017 V03 FINAL Christine Arrowsmith Christine Arrowsmith 24/10/2017 V04 FINAL Christine Arrowsmith Christine Arrowsmith 26/10/2017 Project Details Project Name Stage 1 Hatchery Coastal Environment and Impact Assessment Client Seafarms Group Limited Client Project Manager Ivor Gutmanis Water Technology Project Manager Elise Lawry, Joanna Garcia-Webb Water Technology Project Director Christine Lauchlan-Arrowsmith Authors EAL, PXZ, JGW Document Number 3894-26_R01v03_GunnPt_NOI.docx COPYRIGHT Water Technology Pty Ltd has produced this document in accordance with instructions from Seafarms Group Limited for their use only. The concepts and information contained in this document are the copyright of Water Technology Pty Ltd. Use or copying of this document in whole or in part without written permission of Water Technology Pty Ltd constitutes an infringement of copyright. Water Technology Pty Ltd does not warrant this document is definitive nor free from error and does not accept liability for any loss caused, or arising from, reliance upon the information provided herein. 15 Business Park Drive Notting Hill VIC 3168 Telephone (03) 8526 0800 Fax (03) 9558 9365 ACN 093 377 283 ABN 60 093 377 283 04_GunnPt_NOI 26_R01v - 3894 Seafarms Group Limited | October 2017 Stage 1 Hatchery Coastal Environment and Impact Assessment Page 2 EXECUTIVE SUMMARY Project Sea Dragon is a proposed large scale, integrated, land based prawn aquaculture venture operating across northern Australia. At full production, the proponent, Seafarms Group Limited (Seafarms), intends to provide 100,000 tonnes of prawns to the Australian and Asian markets annually. Water Technology Pty Ltd has been commissioned by Seafarms to undertake an assessment of the existing physical processes, environmental values and water quality of the coastal environment and the impact upon these processes and environment of the proposed Stage 1 Hatchery, to be located at Gunn Point. The proposed Gunn Point Stage 1 Hatchery is to be located on Murrumujuk Drive between the Tree Point Conservation area and the former Gunn Point Prison Farm, as shown in Figure 1-1. The current proposal includes construction of land based facilities, an intake pipe and pump facility to draw water into the facility and a discharge pipe and flow control to cycle water from the facility back to the open waters of Shoal Bay. The discharge will be equivalent to the volume of water drawn from the ocean to circulate through the facility. This will be on average 954 kL/day. The waters around the site comprise of Shoal Bay to the west and Hope Inlet to the south. A number of freshwater waterways flow into Shoal Bay and Hope Inlet, with Howard Springs Creek the most notable waterway. Minor creeks drain across Murrumujuk to the north of the facility. The freshwater waterways are presented in Figure 1-2. The study involved a desktop literature review, physical data collection and numerical modelling. The existing coastal environment is described in Section 2. The potential impacts of the proposed facility and their control measures are summarised below. A full description can be found in Sections 4 to 6. Potential Impacts & Mitigation - Coastal Values The main coastal features at Gunn Point that could be potentially impacted by the proposed facility are in the vicinity of the intake and discharge pipe location. Both pipes will be buried under the dune and out as far as possible through the intertidal zone via directional drilling. Offshore, once directional drilling is no longer possible, they will be placed on the seabed. There is potential that the pipe could interrupt sediment transport within the intertidal zone. This could lead to a change in the configuration of the intertidal flats as material accumulates on either side of the pipe, and a net loss of material is experienced away from the pipe. There may be some visual disturbance due to the construction of the pipe, in an area with a low level of development. The presence of the pipes in this area could impede navigation of recreational vessels in the area. Control Measures The following actions are recommended to ensure there is minimal impact of the project on the coastal values around Gunn Point: Ensuring adequate scour protection is provided in the design of the pipe bedding Minimise the visual impact of the pipe across the intertidal bank by extending the buried section of pipeline as far offshore as possible. 04_GunnPt_NOI Employ directional drilling as a method of pipeline construction to minimise impacts to vegetation Install navigational markers to notify boaters of the potential hazard of the pipeline 26_R01v - 3894 Seafarms Group Limited | October 2017 Stage 1 Hatchery Coastal Environment and Impact Assessment Page 3 Potential Impacts & Mitigation – Bathymetry, Sediment Transport & Coastal Processes Placement of the intake pipe on the bed will result in minor and localised changes to the bathymetry in deeper water due to scour and deposition. More significant changes could be observed in the intertidal region where sediments are more mobile, however this is still expected to be relatively minor. Discharge flows have the potential to cause scour of the bed, however it is unlikely due to the low flow rate. Control Measures The intake and discharge pipes will be buried underneath the dune and across the intertidal zone. To minimise impacts to the coastal processes, these should be buried as far offshore as possible. Ensure adequate scour protection is provided in the design of the pipe bedding and construction. Mitigate the potential for scour across the discharge point by ensuring appropriate scour protection is considered during the detailed design phase. Potential Impacts & Mitigation – Oceanographic Conditions The macro tidal environment within Shoal Bay means that the potential changes to tidal water levels and currents associated with the intake or discharge are extremely low. The tidal prism (i.e. the volume of water which is exchanged during each tide) in the vicinity of Gunn Point north of Hope Inlet is of the order of 4 x 107 m3 during a spring tide and 4 x 106 m3 during a neap tide. During a single tide, the intake pipe will remove approximately 500 m3 of water from Gunn Point (over approximately 12 hours) at the ultimate development state. This represents 0.001% of the tidal prism during a spring tide and 0.01% during a neap tide and is thus unlikely to have any impact on tidal water levels or currents. High tidal currents will necessitate the armouring of the intake pipe to the bed to prevent damage through movement. Rock armouring along the pipeline, or concrete braces could be considered. Control Measures Removal and discharge of seawater through the intake / discharge pipes is unlikely to result in any changes to the hydrodynamic conditions. Design of adequate anchoring of the pipes will be required to ensure they are stable. Potential Impacts & Mitigation – Water Quality The Water Quality Objectives for Darwin Harbour recommend the water quality that supports the maintenance of the ecosystem, and are designated under Part 7 of the N.T. Water Act as a local guideline level in accordance with the National Water Quality Management Strategy (NWSMS) and Australia and New Zealand Environment and Conservation Council (ANZECC) guidelines (DENR, 2016). The estuary classifications of upper, mid and outer are defined by flushing times (DENR, 2009): Upper estuary zones are considered poorly flushed, with residence times over 32 days. Mid estuary zones have greater mixing, with residences times between 14 and 32 days. Outer estuary areas have considerable mixing with the ocean; residence times are less than 14 days. Outer Estuary objectives can be applied to areas offshore from Gunn Point and Shoal Bay, and in the outer 04_GunnPt_NOI areas of Hope Inlet. The flushing times at various locations within Hope Inlet were assessed to categorise that area (Section 3.2.1); the assessment revealed that mid-estuary objectives apply further upstream within Hope Inlet. Background concentrations and water quality objectives for upper and mid-estuary are presented in the 26_R01v - table below. 3894 Seafarms Group Limited | October 2017 Stage 1 Hatchery Coastal Environment and Impact Assessment Page 4 Parameter Background Concentration (g/L) Darwin Harbour WQO (g/L) Mid Estuary TN 230 270 TP 15 20 Chlorophyll a 2.1 2 (3.4) Outer Estuary TN 160 440 TP 8 20 Chlorophyll a 0.5 1 The above table indicates the background mid-estuary chlorophyll a concentration is above the corresponding water quality objective. When defining performance objectives, in accordance with National Guidelines (ANZECC 2000), there is an accepted hierarchy of documentation in this regard. This hierarchy requires that where there are no locally specific guidelines (which would require comprehensive local water quality data collection typically spanning at least a 1 to 2-year period), management decisions should default to relevant State based guidelines, and in their absence to National guidelines. In this instance, local monitoring has been undertaken to obtain the background concentrations. Given the Darwin Harbour WQO is assigned to the full harbour system, including Shoal Bay and Hope Inlet, it is proposed that the WQO objective be adjusted at this location for the purposes of this assessment. The upper-estuary Darwin Harbour WQO is 4 g/L, and the mid-estuary WQO 2g/L. Examination of the available water quality data indicates an 80th percentile value of 3.4g/L. Using the 80th percentile as an objective is in line with the recommendations in the National Guidelines. This value sits between the mid and upper WQOs, which appears to be a reasonable interim trigger value to adopt on this basis.