5 POTENTIAL IMPACTS AND MITIGATION – WATER QUALITY The approach adopted for this study to evaluate potential water quality impacts associated with the proposed discharge relied on the application and interpretation of calibrated far-field, two-dimensional hydrodynamic (HD) and advection dispersion (AD) modelling tool built using MIKE21. This modelling was supported, guided and informed by a range of data and other relevant information. These data, the work conducted, and key findings are presented below.
5.1 Numerical Modelling
Numerical modelling was used to simulate the transport, dilution and dispersion of the proposed discharge at Gunn Point and surrounding waters. The proposed discharge was modelled (see Appendix A) as a conservative tracer. This allows the dilution and dispersion of the effluent to be understood. The numerical modelling, and therefore the modelled tracer concentrations, can be considered conservative for the following reasons:
Biological and physical processes such as the deposition of particulate material or the take up of bioavailable nutrients or absorption by sediments and algal mats (microphytobenthos) growing on the sediments of the significant intertidal areas in and around Shoal Bay are not included in the modelling. Three-dimensional turbulent dispersion associated with wave action has not been included in the modelling. Because the model was very computationally demanding, all scenarios were undertaken in 2D. However, during the model calibration and sensitivity testing, 3D simulations were carried out to confirm the mixing processes were resolved appropriately. The strong tidal currents and shallow water mean the site is well mixed, and the 3D modelling did not provide significantly different results.
5.1.1 Simulation Scenarios
The model was run for two separate years: June 2005 – May 2006 inclusive May 2016 to April 2016 inclusive
Whilst tropical conditions are highly variable, 2005-2006 was considered a ‘typical’ wet season, and 2016- 2017 a season with higher than average rainfall. The modelling indicated less dilution of the tracer occurred in the vicinity of the intake site during high flood events. Thus the 2016-2017 simulation scenario allowed an assessment of worse than average discharge mixing conditions.
5.2 Dispersion & Dilution of Discharge
5.2.1 Discharge Characteristics
The modelled intake and discharge locations are displayed in Figure 5-1. The intake is located at -4 m AHD; the discharge -7 m AHD. The proposed discharge will have the following characteristics:
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The proposed facility will have operating conditions and discharge characteristics similar to the existing Seafarms Hatchery operations at Flying-Fish Point (FFP), Innisfail in North Queensland. As such, Seafarms propose to adopt the water quality licence conditions for that facility set by the Queensland Department of Environment and Heritage Protection (QLD DEHP). The median concentrations for the key nutrients assessed in the modelling exercise are equal to the median licence conditions and are displayed in Eight samples were collected over May and June 2017 at the existing Seafarms hatchery at Flying-Fish Point near Innisfail in North Queensland (this facility is only required to monitor twice per year and only if the ponds are in operation. The sampling was undertaken once a week for eight weeks to investigate the water quality exiting the hatchery and then leaving the settlement ponds). These results indicate that the effluent quality is typically significantly better than the median licence conditions.
Table 5-1.
Eight samples were collected over May and June 2017 at the existing Seafarms hatchery at Flying-Fish Point near Innisfail in North Queensland (this facility is only required to monitor twice per year and only if the ponds are in operation. The sampling was undertaken once a week for eight weeks to investigate the water quality exiting the hatchery and then leaving the settlement ponds). These results indicate that the effluent quality is typically significantly better than the median licence conditions.
TABLE 5-1 LICENCE DISCHARGE CONDITIONS
Median (g/L) Total Nitrogen 2000 Total Nitrogen FFP performance 1250 Total Phosphorous 400 Total Phosphorus FFP performance 110 Chlorophyll a 20 Chlorophyll a FFP performance 1 g/L
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FIGURE 5-1 MODELLED DISCHARGE AND INTAKE POINTS; FACILITY FOOTPRINT
5.2.2 Feasibility Investigation
Water Technology undertook a feasibility assessment of the intake / discharge locations. In total, this involved assessing over 20 different intake and discharge configurations. The location of the source point within the model was reviewed to determine any improvement in position of the proposed discharge. As expected, there was greater initial dilution when the discharge was placed in deeper water. Placement of the discharge in deeper waters also resulted in less “bank attached” flow where the discharge material was higher along the inshore zone.
The feasibility assessment covered four stages. The aim was to minimise the potential environmental impact, as well as the risk of recirculation through the intake pipe:
Initially, modelling of the discharge was completed over a period of four weeks for the wet season, dry season and astronomical tidal conditions. The discharge location was tested at 6 locations to review the impact at different locations within the proposed footprint.
Two locations were selected to assess in 3-month simulation scenarios. The preferred scenario from the 3-month simulations was run for the 2016-2017 year. The full year results provided more clarity around the behaviour of the plume. As a result, a further 13 intake / discharge configurations were assessed using the full year simulation.
The final three intake / discharge configuration options were then modelled for both the 2005-2006 and 2016-2017 years. Modelling both years ensured the prevailing conditions from a single year did not 26_R01v04_GunnPt_NOI - influence the locations. 3894
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All scenarios tested resulted in minimal increases in nutrient concentrations. The final assessment allowed the optimal configuration to be selected in terms of environmental impact, economics of construction of the intake and discharge pipes, operation of the pumps, and intake recirculation.
5.2.3 Performance Objectives
As discussed in Section 3.2, the water quality objectives applied in this study are presented in Table 5-2. This 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) that 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 concentration. Given the Darwin Harbour WQO is assigned to the full harbour system, 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.
A less geographically relevant set of tropical water quality objectives are presented in DERM 2010 for Trinity Inlet, Cairns. These set the chlorophyll a water quality guideline concentration for mid-estuary to be 3 g/L, similar to the 80th percentile value provided above. It is proposed to adopt an alternative WQO of 3.4 g/L for the purposes of this assessment. This value is included in Table 5-2 in brackets. Further investigation will be conducted going forward to refine this value. In the analysis presented in Section 5.2.5, the background concentration is added to the discharge concentration, and compared to the water quality objective for mid and outer estuary respectively.
TABLE 5-2 MEDIAN BACKGROUND NUTRIENT CONCENTRATIONS
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
5.2.4 Initial Mixing / Near-field Mixing
Upon discharge into the ocean, rapid initial dilution of the discharge water will occur at the discharge location.
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quality does not exceed local water quality objectives. Water Technology calculates that a minimum initial dilution of 1:23 at the discharge will be required.
Because the method of discharge has not yet been set, the discharge was applied in the model as a simple pipe discharge, and the tracer concentration factored up to account for any numerical dilution. Tracer was discharged from the model continuously at the average daily discharge rate for the full simulation.
5.2.5 Dispersion / Far-field Mixing
To assess the potential impact of the discharge on water quality, tracer concentration was extracted from the model at the output locations presented in Figure 5-2. These sites comprise the following:
25 points used to assess the estuary classification.
The 10 water quality monitoring sites used to set the background concentrations; a combination of study specific and Darwin Harbour Water Quality monitoring sites. Two sites included in the Darwin Harbour water quality monitoring program at the mouth of Mickett / Buffalo Creek. This is where the discharge form the Leanyer Sanderson Wastewater Treatment plant reaches Shoal Bay.
The proposed intake location.
Tracer concentration time series were converted to nutrient concentration by applying the discharge concentration value of the relevant nutrient, and adding the background value. These are presented as box and whisker plots. Locations classified as mid-estuary are presented in Table 5-3. The output location that corresponds to Figure 5-2 is shown in the left-hand column, and the corresponding site number in the box and whisker plot is shown in the right-hand column. This is presented similarly for the outer-estuary in Table 5-4.
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Mid-estuary box and whisker plots for both simulation years are presented in Figure 5-3 to Figure 5-5 for total nitrogen, total phosphorous and chlorophyll a respectively. Outer-estuary plots are presented in Figure 5-6 to Figure 5-8. The WQO are not predicted to be exceeded for any site assessed. The largest increase in concentration occurs at the Gunn Point logger site, WQM Logger. This is approximately 450m from the discharge point.
TABLE 5-3 MID ESTUARY ASSESSMENT SITES
Output Location (Figure 5-2) Figure Site Number (Figure 5-3 to Figure 5-5) P2 1 P4 2 P5 3 P6 4 P7 5 P8 6 P9 7 P10 8 P11 9 P13 10 P14 11 P18 12 WQM Howard River 13
TABLE 5-4 OUTER ESTUARY ASSESSMENT SITES
Output Location (Figure 5-2) Figure Site Number (Figure 5-6 to Figure 5-8) P1 1 P3 2 P12 3 P15 4 P16 5 P17 6 P19 7
P20 8 P21 9 P22 10 P23 11 P24 12 26_R01v04_GunnPt_NOI - P25 13 3894
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Output Location (Figure 5-2) Figure Site Number (Figure 5-6 to Figure 5-8) SLEBC08 14 SLESB02 15 Intake Point 16 G8150412 17 G8155535 18 G8155537 19 G8155595 20 G8150410 21 WQM Offshore 22 WQM Logger 23 WQM Inshore 24 G8150411 25
The median concentration over the two simulation years is presented spatially in Figure 5-9 to Figure 5-11 for total nitrogen, total phosphorous and chlorophyll a respectively.
FIGURE 5-3 PREDICTED MID-ESTUARY TOTAL NITROGEN CONCENTRATION AT MODEL OUTPUT SITES: 2005/06 (TOP) 2016/17 (BOTTOM) 26_R01v04_GunnPt_NOI - 3894
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FIGURE 5-4 PREDICTED MID-ESTUARY TOTAL PHOSPHOROUS CONCENTRATION AT MODEL OUTPUT SITES: 2005/06 (TOP) 2016/17 (BOTTOM)
26_R01v04_GunnPt_NOI - FIGURE 5-5 PREDICTED MID-ESTUARY CHLOROPHYLL A CONCENTRATION AT MODEL OUTPUT SITES:
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FIGURE 5-6 PREDICTED OUTER-ESTUARY TOTAL NITROGEN CONCENTRATION AT MODEL OUTPUT SITES: 2005/06 (TOP) 2016/17 (BOTTOM)
FIGURE 5-7 PREDICTED OUTER-ESTUARY TOTAL PHOSPHOROUS CONCENTRATION AT MODEL OUTPUT SITES: 2005/06 (TOP) 2016/17 (BOTTOM) 26_R01v04_GunnPt_NOI - 3894
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FIGURE 5-8 PREDICTED OUTER-ESTUARY CHLOROPYHLL A CONCENTRATION AT MODEL OUTPUT SITES: 2005/06 (TOP) 2016/17 (BOTTOM)
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FIGURE 5-9 MEDIAN PREDICTED TOTAL NITROGEN CONCENTRATION FOR 2005-2006 SIMULATION (LEFT); 2016-2017 SIMULATION (RIGHT)
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FIGURE 5-11 MEDIAN PREDICTED CHLOROPHYLL A CONCENTRATION FOR 2005-2006 SIMULATION LEFT); 2016-2017 SIMULATION (RIGHT)
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5.2.5.1 Cumulative Impacts The concentration at the two Leanyer Sanderson Wastewater Treatment plant monitoring sites in Shoal Bay (sites 14 and 15 in the box and whisker plots) indicate any cumulative impacts are predicted to be negligible. As presented in Section 3.4.1, the background concentration at these sites are well above the WQO. The predicted change in concentration from the proposed facility is at least a factor of 10 below the existing concentration.
5.2.5.2 Recirculation Impacts The concentration at the intake site is not predicted to have an environmental impact in terms of nutrient concentrations. However, the presence of low concentrations of discharge water at the intake poses a risk to biosecurity for the proposed facility. Seafarms propose to dose the discharge ponds with hydrogen peroxide to mitigate this risk.
Figure 5-12 presents a time series of the tracer concentration (percentage) at the intake site for the 2005-2006 and 2016-2017 simulations, together with the rainfall rate. The plots indicate that the majority of the recirculation events are due to periods of high rainfall. The corresponding freshwater flow discharging from Hope Inlet constrains the discharge to the north, which results in minor recirculation at the intake site.
The few events that occur during the dry season are a result of neap tidal cycles, combined with elevated wind speeds coming from the south-east. This results in reduced mixing, and circulation patterns that increase the concentration at the intake site.
26_R01v04_GunnPt_NOI - FIGURE 5-12 TRACER CONCENTRATION AT THE INTAKE SITE 2005-2006 SIMULATION AND CORRESPONDING RAINFALL RATE (TOP); 2016-2017 (BOTTOM) 3894
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5.3 Water Quality Control Measures The impacts of the discharge have been minimised by selecting a location for the discharge that was optimal in terms of reduction of environmental impacts (see Section 5.2.2). The modelling demonstrates that significant impacts as a result of the discharge are not expected to occur. As such no further mitigation is needed, however, as detailed engineering design of the discharge structure itself has not been completed for the project at this time, the opportunity exists to provide further reductions in nutrient concentrations within the receiving environmental system via a discharge diffuser. It is recommended that the discharge process is designed to maximise the dilution as it enters the water column.
5.4 Summary of Development Impact on Water Quality
The proposed facility is not predicted to lead to an exceedance of the Water Quality Objectives. Minor recirculation at the intake site will be mitigated by dosing of hydrogen peroxide at times of reduced mixing. Due to the nature of hydrogen peroxide, and the holding times required for its efficacy, this will generally be reduced to its breakdown products of water and oxygen prior to release to the environment.
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6 CLIMATE CHANGE IMPACT ASSESSMENT 6.1 Background Increased concentrations of greenhouse gases in the earth’s atmosphere are projected to cause a warming of the atmosphere and oceans which in turn are projected to drive a range of other changes to the earth’s climate and the climate variability.
To assess the potential impact of climate change on the coastal environment and the coastal infrastructure proposed as part of the Gunn Point facility, a risk assessment methodology has been adopted. The main steps of the risk assessment process are as follows:
Identification of the relevant threats associated with climate change to the coastal environment; Determination of the aspects of the development that could potentially be exposes to these threats; Assessment of the overall risk of this exposure.
For the purposes of the risk assessment process, Risk is defined as the product of the Likelihood of the occurrence of the various Threats associated with climate change multiplied by the Consequences of their occurrence.
6.2 Threat Identification Relevant climate change impacts on the physical processes operating on the coastal environment are considered the following: Sea Level Rise Tropical Cyclone Intensity and Frequency
The projected changes to the above physical processes have been gathered from the relative authoritative sources and are discussed below.
6.2.1 Sea Level Rise
Global average sea level rose by approximately 0.17 m during the 20th Century. The average global rate of sea level rise between 1950 and 2000 was 1.8 ± 0.3mm/yr. The tidal gauge in Darwin Harbour to the west of the proposed facility is one of the National Tide Centre’s array of sixteen high accuracy sea level measurement stations. The net relative sea level trend since installation in June 1992 is 7.6 mm/yr at Darwin (BoM, 2015). The Intergovernmental Panel on Climate Change (IPCC) is the authoritative source on projections of future sea-level rise due to climate change. Table 6-1 displays the sea level rise projections relative to late 20th century mean sea levels for the A1F1 high emission scenario.
TABLE 6-1 IPCC 2014 PROJECTED SEA LEVEL RISE
Scenario 2030 2070 2100 IPCC 0.18m 0.42m 0.82m
The main impacts associated with increase in mean sea level at Gunn Point are considered to be:
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6.2.2 Tropical Cyclone Intensity
There is considerable debate regarding the projected impact of climate change on tropical cyclones. The Engineers Australia ‘Guidelines for Responding to the Effects of Climate Change in Coastal and ocean Engineering’ suggests that whilst there is “no evidence that (globally) tropical cyclones are getting stronger, or are becoming more frequent or producing greater rainfall”, there is the opportunity that peak winds during a tropical cyclone may increase by 11% by the year 2100, and that rainfall associated with a tropical cyclone may increase by 20% by 2100 (Engineers Australia, 2012).
The spatial and seasonal distribution of cyclone occurrence is expected to remain approximately similar to present whilst the frequency of tropical cyclone formation may actually decline under climate change.
The main potential impacts associated with increases in tropical cyclone intensity are considered the following; Higher maximum wind speeds generating larger waves and associated wave set-up on the coastline Higher maximum wind speeds and lower central pressures generating large storm surges
The previous storm tide assessment (SEA, 2010) described in Section 2.4.2 considered the effects of sea level rise and increasing winds. The predicted storm tides for a number of return periods and horizons at Gunn Point are summarised relative to existing conditions in Table 6-2. Table 6-2 shows increases in predicted storm tide elevations at Gunn Point of 0.8 m through to 1.5 m by 2100. This increase is comprised of 0.8 m of mean sea level rise and an additional meteorologically induced component of 0.1 m for the 100-year ARI event, and 0.7 m for the 500-year event.
TABLE 6-2 PREDICTED STORM TIDE LEVELS (M AHD)
Return Period (Average Recurrence Interval, ARI) 50 100 500 1,000 10,000 Gunn Point 2010 3.5 3.7 4.2 4.7 6.3 2050 3.8 4.1 5.0 5.6 7.4 2100 4.3 4.6 5.7 6.2 7.9
6.3 Exposure to Risk
The main components of the coastal environment and the Gunn Point facility that are exposed to the climate change threats identified previously are considered to belong to the following three main categories: Intake and Discharge Infrastructure Land based facilities
Water quality and circulation
6.3.1 Intake and discharge Infrastructure
Threats
The intake and discharge infrastructure are vulnerable to potential changes to the shoreline through increased 26_R01v04_GunnPt_NOI
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energy could result with deeper water during storm events which may impact the bed more than present conditions.
Consequences
The consequences of changes to the shoreline are likely to be minimal for the intake and discharge pipe locations as the pipe will be buried. Consideration should be given to bury them deep enough so as to not be exposed during an erosion event.
Increased wave energy on the bed could lead to increased scour and movement of the pipelines, potentially causing damage to the pipe and loss of production.
Mitigation
The risk posed by climate change to intake and discharge infrastructure can be accommodated by designing intake and discharge facilities setback from the present-day coastline to allow for any changes as a result of increased inundation to 2100.
Design of the intake pipe and bedding should consider the potential for increased wave conditions into the future.
Where the pipes are buried, they should be deep enough such that they do not become exposed during erosion events.
6.3.2 Land based facilities
Threats
Facilities associated with the project adjacent to or near the existing shoreline could potentially be exposed to threats associated with shoreline recession. The proposed location of the facility is approximately 150 m landward of the existing vegetation line. The proposal is at concept stage only and further refinement of the footprint is to be completed.
The elevation of the development is above the predicted storm tide levels thus the inundation threat is low.
Consequences
Areas of the facility located adjacent to the existing shoreline could potentially be impinged upon by shoreline recession hazards by 2100. The consequences of exposure to this risk include potential exposure to more significant inundation by storm tides, exposure of the buried intake / discharge pipelines, and foundation instability risking building integrity.
Mitigation
The likely extent of erosion by 2100 should be included in the detailed design process, and the facility adapted accordingly.
6.3.3 Water Quality and Circulation
Threats
As sea levels rise, there will be greater water exchange occurring and a net effect of more flushing, which should see lower concentrations of the discharge waters within the Gunn Point region.
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There are unlikely to be any adverse consequences.
Mitigation
Not required.
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7 CONCLUSIONS To assess the risk posed to the marine physical environment by activities undertaken as part of the proposed project a risk assessment has been undertaken. This risk assessment addresses the potential impacts and consequences of the construction and operational phases of the project described in the above sections along with residual risk following implementation of the proposed mitigation measures. A standard risk assessment matrix as presented in Table 7-1 below. The risk assessment of the marine physical environment impacts of the project are provided in Table 7-2.
TABLE 7-1 RISK ASSESSMENT MATRIX
Likelihood Consequences 1 – Insignificant 2 – Minor 3 – Moderate 4 – Major 5 - Extreme 5 – Almost Certain Medium Medium High Extreme Extreme 4 - Likely Medium Medium Medium High Extreme 3 – Possible Low Medium Medium Medium High 2 – Unlikely Low Low Medium Medium Medium 1 – Rare Low Low Low Medium Medium
As indicated by Table 7-2, the primary risk to the coastal environment is the impact on water quality from the facility discharge.
However, these preliminary studies indicate that the location of the discharge point, the discharge regime and the physical structure for discharging can be optimised and detrimental impacts on water quality minimised.
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TABLE 7-2 GUNN POINT RISK ASSESSMENT
Environmental Potential Impacts of Preliminary Risk Control Strategy Residual Risk with Consideration Development and their Assessment (C,L) Score Control Strategies Consequences Adopted Coastal Values . Visual impact of intake and . (2,4) Medium . Pipeline buried as far offshore as . (2,2) Low discharge structures . (3,3) Medium possible . (3,1) Low . Potential change in coastal . (3,3) Medium . Sensitive design and vegetation . (3,1) Low alignment management strategies . Navigational risk to boaters . Navigational markers to flag pipeline presence Bathymetry / Sediment . Scour around intake & discharge . (3,3) Medium . Design of scour protection and . (3,1) Low Transport structures bedding for intake pipe . Design of discharge structure to prevent high flow scour Oceanographic . Negligible . (3,1) Low . The impacts on the oceanographic . (3,1) Low conditions are considered negligible Water quality . Increased nutrient concentration . (3,2) Medium . Design of the discharge to ensure . (3,1) Low
from the discharge waters initial mixing is adequate
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8 REFERENCES ANZECC, 2000. The Australian Water Quality Guidelines for Fresh and Marine Waters, prepared by the Australian and New Zealand Environment Conservation Council
Aurecon, 2013. Buffalo Creek Water Quality Improvement Plan. Prepared for Defence Housing Australia.
BoM, 2015.Tropical cyclones frequency data: http://www.bom.gov.au/cyclone/about/northern.shtml#history
BoM, 2017. Tr opical cyclones frequency Data: http://www.bom.gov.au/cyclone/about/northern.shtml#history
Calnan. T, 2006. Rainforest to Reef just 40km from Darwin. An assessment of the conservation values of the Gunn Peninsula/Vernon Islands area and the impacts of the propose Glyde Point heavy industry and residential estate. Environment Centre of the Northern Territory and Australian Marine Conservation Society Australia.
CSIRO. (2015). BlueLink: Scientific and Technical Information. Retrieved from: http://wp.csiro.au/bluelink/global/oceanmaps/
DENR, 2009.Towards the Development of a Water Quality Protection Plan for the Darwin Harbour Region, Phase 1 Report. Department of Environment and Natural Resources.
DENR, 2016. Darwin Harbour Region 2016 Report Card, Water Quality Supplement. Department of Environment and Natural Resources.
Department of Environment, Water, Heritage and the Arts, 2007. Characterisation of the Marine Environment of the North Marine Region. Outcomes of an Expert Workshop, Darwin, Northern Territory.
DERM, 2010. Trinity Inlet Environmental Values and Water Quality Objectives, prepared by the Queensland Department of Environment and Resource Management
Doyle. N, 2001. Extractive Minerals Within the Outer Darwin Area. Department of Business, Industry and Resource Development.
Engineers Australia, 2012. Guidelines for Responding to the Effects of Climate Change in Coastal and ocean Engineering
Fortune, Julia 2015. Effect of Tide on Water Quality of Jones Creek, Darwin Harbour. Aquatic Health Unit, Department of Land Resource Management.
Geoscience Australia, 2015. Digital Elevation Model (DEM) of Australia derived from LiDAR 5 Metre Grid. Geoscience Australia, Canberra. http://dx.doi.org/10.4225/25/5652419862E23
Harris et al, 2003. Geomorphic Features of the Continental Margin of Australia, Report to the National Oceans Office on the production of a consistent, high-quality bathymetric data grid and definition and description of
geomorphic units for part of Australia’s marine jurisdiction. Geoscience Australia Record 2003/30
Harrison. S, 2014. Nearshore Environmental Monitoring Plan. Ichthys Gas Field Development. Prepared for nPt_NOI INPEX by Cardno.
Metcalfe. K, 2007. The Biological Diversity, Recovery from Disturbance and Rehabilitation of Mangroves in Darwin Harbour, Northern Territory. PhD Thesis. Faculty of Education, Health & Science. Charles Darwin University. 26_R01v04_Gun - 3894
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Northern Territory Government, 2016. Tree Point Conservation Area Fact Sheet. Accessed on 23 August 2017 at: https://nt.gov.au/__data/assets/pdf_file/0010/200080/tree-point-conservation-area.pdf.
NTEPA, 2012. Marine Water. EAW Expansion Project DEIS.
NRETAS, 2009. The Impact of Urban Land-Use on Total Pollutant Loads Entering Darwin Harbour. Prepared by the Aquatic Health Unit, Department of Natural Resources, Environment, the Arts and Sports.
Oosterzee. P, Morris. I, Lucas. D, Preece. N, 2014. A Natural History and Field Guide to Australia’s Top End.
SEA, 2006. Darwin Storm Tide Mapping Study 2006 – numerical modelling and risk assessments, J0606- PR001C prepared for Northern Territory Emergency Services.
SEA, 2010. High Resolution Storm Tide and Climate Change Impacts Study – 2010, J0911-PR001B prepared for Northern Territory Department of Lands and Planning.
Siwabessy. P.J.W, 2015. Outer Darwin Harbour Marine Survey. Pot Survey Report. Geoscience Australia.
Smith. Jodie, 2009. The Role of Sediments in Nutrient Cycling in the Tidal Creeks of Darwin Harbour. AUSGEO news, Issue95, September 2009.
Smith. N, 2000. Beagle Gulf Benthic Survey: Characterisation of soft substrates. Technical Report NO.66. Parks and Wildlife Commission of the Northern Territory.
Williams. David, 2006. Hydrodynamics of Darwin Harbour. The Environment in Asia Pacific Harbour, pp 461- 476. July 2006.
Woodroffe. C.D, Chappell. J.M.A, Wallensky. B.G.T and E, 1986. Geomorphological and Evolution of The South Alligator Tidal River And Plains, Northern Territory. Australian National University, North Australia Research Unit.
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APPENDIX A DESCRIPTION OF NUMERICAL MODELS
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Model Domain
The extent of the model used to review the discharge into waters offshore of Gunn Point is shown in Figure 8-1. This model domain has been adapted from the earlier work undertaken by Water Technology to assess proposed facilities at Point Ceylon within Bynoe Harbour and on the northern shore of the Cox Peninsula.
The large model domain allows for regional tidal and wind driven processes to be resolved. The model structure is a flexible mesh, which allows offshore areas, and areas more distant from the area of interest to be represented by a larger model cell. Within the flexible mesh, model cells are made up of triangles. Along the Gunn Point coastline and in the area of the discharge, the side length of each model cell is between 50-100 m. Sensitivity testing of the mesh resolution around the discharge has established that reducing the resolution further in this area does not result in significant changes to model results.
The original model has been improved around the study site with bathymetry data of the outer Darwin Harbour and the area along the Gunn Point, as described in Section 2.5.5. Resolution of Hope Inlet and the tidal waterways along the northern shore of Darwin have also been incorporated. Bathymetry data within Hope Inlet is not available and assumptions of depth have been made based on observed tidal flow paths and inundation.
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FIGURE 8-1 MODEL DOMAIN (TOP) AND MESH RESOLUTION AROUND DISCHARGE (BOTTOM). BATHYMETRIC VALUES ARE TO AHD 26_R01v04_GunnPt_NOI -
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Model Calibration & Validation Water Levels
The model has been calibrated using astronomical tidal water levels around Darwin and measured water levels at Gunn Point and the AIMS buoys located offshore of Darwin and within the Beagle Gulf. As shown in Figure 8-2, the model provides an excellent representation of tidal water level fluctuations across the model domain. The model also shows excellent correlation with measured water levels, which include the effects of meteorological drivers of winds and pressure at the Gunn Point and AIMS measured data points.
FIGURE 8-2 WATER LEVEL CALIBRATION: ASTRONOMICAL TIDES AT NIGHTCLIFF (TOP), MEASURED 26_R01v04_GunnPt_NOI - WATER LEVELS AT AIMS BEAGLE GULF A AND OUTER DARWIN (MIDDLE) AND GUNN POINT (LOWER) 3894
Seafarms Group Limited | October 2017 Stage 1 Hatchery Coastal Environment and Impact Assessment
Salinity
Salinity within the model is driven by salinity change on the model boundaries, freshwater inflows and rainfall onto the ocean. Measured salinity data at the AIMS Beagle Gulf Buoy was considered representative of the potential variation of salinity offshore and was used to drive model boundaries.
Gauged data has been used where available to provide freshwater inflows, and scaled to be representative of the potential conditions at the model freshwater boundaries. Catchments which drain into the Hope Inlet and along the north coast of Darwin are ungauged and have been scaled from the Howard River gauging station (upstream of the Inlet) based on the catchment size. Flows are thus an estimate only.
Rainfall data collected at the offshore buoys has been compared to rainfall measurements on land. Whilst rain across the top end, especially during the wet season, is often in very heavy, isolated and fast-moving storms, the offshore and land based measured rainfall data showed a reasonable correlation. For modelling simplicity, a consistent rain map was used across the model domain.
The model has also been calibrated to measured salinity during the 2016-17 wet season at Gunn Point and at the AIMS buoys offshore of Darwin Harbour and the Beagle Gulf. Time series of measured and modelled salinity is presented in Figure 8-3.
FIGURE 8-3 MEASURED VERSUS MODELLED SALINITY
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Tidal Currents
As discussed in Section 2.3.2, the Australian Institute of Marine Science (AIMS) has developed a hydrodynamic model of Darwin Harbour which has been undergoing refinements and updates since 2006 as new field data is collected (Williams, 2006). Water Technology has used the results of this calibrated, validated model to validate our hydrodynamic modelling, in terms of modelled currents, offshore from Gunn Point. The modelled current maps are compared to the AIMS current maps for flood currents during a spring tide (Figure 8-4) and ebb tide (Figure 8-5). These indicate the Water Technology model resolves the circulatory pattern appropriately.
FIGURE 8-4 WATER TECHNOLOGY PREDICTED CURRENTS (TOP) AIMS CURRENTS (BOTTOM) FOR FLOOD TIDE 26_R01v04_GunnPt_NOI - 3894
Seafarms Group Limited | October 2017 Stage 1 Hatchery Coastal Environment and Impact Assessment
FIGURE 8-5 WATER TECHNOLOGY PREDICTED CURRENTS (TOP) AIMS CURRENTS (BOTTOM) FOR EBB TIDE
Tidal currents around the discharge point have been analysed and are presented in Figure 8-6. The time series demonstrates varying range of current speeds through the neap-spring tidal cycle. During spring tides, currents are generally in excess of 0.1m/s and up to 0.25m/s. The current speed roses, pictured below the time series, also show the current direction for the month, spring tide period and neap tide period from left to right respectively. Strong currents during the spring tides are from the southeast and south southeast and to a lower proportion from the north through northwest. Neap tide currents are within the range of 0.05-0.1m/s with a more north-south directional pattern evident in the current rose.
The currents cycle in an anticlockwise direction, with currents at slack low tide generally from the north to north-northeast, rotating around to north and northwest with the peak flood tides coinciding with currents from the northwest as the water level approaches mid-tide. The current speed drops as the tide continues to rise and the current direction becomes more onshore and then low northerly currents drift alongshore at the peak of the high tide. Ebb current speeds peak with water levels above mid tide, strong currents running parallel to
26_R01v04_GunnPt_NOI shore or directly north. As the water level drops current speeds gradually reduce and become offshore as the - water drains from the intertidal area. 3894
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FIGURE 8-6 ASTRONOMICAL TIDAL CURRENTS OFFSHORE OF THE DISCHARGE OVER ONE MONTH (TOP), CURRENT SPEED-DIRECTION ROSES FOR ONE MONTH (BOTTOM LEFT), SPRING TIDES (BOTTOM CENTRE) AND NEAP TIDES (BOTTOM RIGHT)
Typical Modelled Wet Season Conditions
It should be noted there is no real “average” wet season condition – the climate in the Top End is characterised as much as by the differences from year to year as the wet-dry climate, as observed in the stark difference between wet season conditions from 2015-16 and 2016-17.
However, the wet season of 2005-2006 has previously been identified as being a “representative” wet season and used in modelling of conditions around Darwin. Rainfall and freshwater flows are of an average range across the catchment and the wind climate is generally representative of long term average conditions.
The month of January 2006 was used in model simulation. The wind speed and direction averaged across a number of gauges around Darwin is presented in Figure 8-7, along with a comparison of the January wind rose and the long-term average wet season months wind conditions. The wind climate indicates an initial period of low wind speeds varying in direction from east through south and west before stronger west and north- westerly wind conditions from the middle of the month begin to dominate across a week. The wind speeds then drop and the direction is offshore (i.e. south-easterly and south-westerly) before strong winds from the
west and northwest are consistent for the final week of January.
Rainfall and freshwater flow has been added to the model to provide additional flushing and dilution in line with the wet season conditions.
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FIGURE 8-7 WIND CONDITIONS, JANUARY 2006 AND LONG TERM WET SEASON AVERAGE
Typical Dry Season Conditions
Dry season conditions across the Top End and around Darwin are characterised by low rainfall and the predominance of weak south-easterly winds blowing across the continent. Previous works have identified June of 2005 as having a wind climate and temperature in the ranges of long term averages. The wind climate for the June of 2005 is presented in Figure 8-8. A repetitive daily pattern of varying wind speeds is a feature of dry season conditions, with light winds peaking during the middle of the day and rotating in direction from south to north through east across the course of the day before becoming generally calm overnight. The light winds during the middle of the day correspond to winds generally from the southeast through east.
Freshwater flows and rainfall are minimal during the dry season conditions.
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FIGURE 8-8 WIND CONDITIONS, JUNE 2005 AND LONG-TERM AVERAGE DRY SEASON
Wind Driven Currents
Given the high tidal range and low wind speeds during typical dry season conditions, there is little impact on tidal currents from winds during the dry season. Gunn Point is largely sheltered from wind impacts from the southeast and current speeds and circulation around Shoal Bay is very similar to that modelled without the impact of winds.
Wet season conditions are dominated by onshore west and north-westerly winds and high flow events. Modelling indicates wet season currents have more variation from astronomical than the dry season conditions. Current speeds around the discharge can be both higher and lower than astronomical conditions with the impacts of the wet season winds and flows added to the modelling. Strong westerly winds result in a stronger current towards the shore during flood tide and can set up an inshore northward current along the coast. At times, this northerly current can result in a lower zone in the deeper section of the discharge waters as currents shift from onshore to northward alongshore. High flows into Shoal Bay can result in weak eddies forming towards the entrance to Hope Inlet on the flood tide as water flows out of the estuary and away from the coastline. Despite this, changes to current speeds are minimal, in the order of 0.1m/s.
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Seafarms Group Limited | October 2017 Stage 1 Hatchery Coastal Environment and Impact Assessment