hapter 17 – oastal pro esses - metro mining€¦ · skardon river bauxite project chapter 17 –...

CHAPTER 17 – COASTAL PROCESSES

GULF ALUMINA LTD – SKARDON RIVER BAUXITE PROJECT

Skardon River Bauxite Project Chapter 17 – Coastal Processes

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TABLE OF CONTENTS

17.1 Introduction ..................................................................................................... 17-1 17.2 Environmental Objectives and Performance Outcomes ..................................... 17-1 17.2.1 Environmental Objectives ........................................................................................ 17-1 17.2.2 Performance Outcomes ........................................................................................... 17-1 17.3 Legislative and Policy Context ........................................................................... 17-2 17.4 Coastal Processes and the Physical Marine Environment ................................... 17-2 17.4.1 Climate ..................................................................................................................... 17-2 17.4.2 Regional Setting........................................................................................................ 17-2 17.4.3 Skardon River Estuary .............................................................................................. 17-2 17.4.4 Tides ......................................................................................................................... 17-2 17.4.5 Storm Tide ................................................................................................................ 17-3 17.4.6 Tidal Currents ........................................................................................................... 17-3 17.4.7 Waves ....................................................................................................................... 17-4 17.4.8 River Flows ............................................................................................................... 17-4 17.4.9 Bathymetry and Morphology ................................................................................... 17-5 17.4.10 Shoreline and Bank Evolution .................................................................................. 17-9 17.4.11 Marine Water Quality ............................................................................................ 17-10 17.4.11.1 Overview ................................................................................................................ 17-10 17.4.11.2 Water Quality Sampling ......................................................................................... 17-10 17.4.11.3 Turbidity ................................................................................................................. 17-13 17.4.11.4 Salinity .................................................................................................................... 17-15 17.4.11.5 pH ........................................................................................................................... 17-15 17.4.11.6 Dissolved Oxygen ................................................................................................... 17-15 17.4.11.7 Benthic Light ........................................................................................................... 17-16 17.4.11.8 Nutrients ................................................................................................................ 17-16 17.4.11.9 Metals ..................................................................................................................... 17-16 17.4.11.10 Total suspended solids ........................................................................................... 17-17 17.4.11.11 Chlorophyll-a .......................................................................................................... 17-17 17.4.11.12 Hydrocarbons ......................................................................................................... 17-17 17.4.11.13 Summary ................................................................................................................ 17-17 17.4.12 Water Quality Objectives ....................................................................................... 17-17 17.4.13 Sediment ................................................................................................................ 17-20 17.4.13.1 Sediment Sampling ................................................................................................. 17-20 17.4.13.2 Particle Size Distribution ........................................................................................ 17-21 17.4.13.3 Metals ..................................................................................................................... 17-22 17.4.13.4 Nutrients ................................................................................................................ 17-22 17.4.14 Acid Sulphate Soils ................................................................................................. 17-22 17.4.15 Summary ................................................................................................................ 17-23 17.5 Modelling and Quantification of Potential Impacts .......................................... 17-23 17.5.1 Model Set Up .......................................................................................................... 17-24 17.5.1.1 Model Extent .......................................................................................................... 17-24 17.5.1.2 Model Bathymetry ................................................................................................. 17-24 17.5.1.3 Hydrodynamic Model ............................................................................................. 17-25 17.5.1.4 Wave Model ........................................................................................................... 17-25 17.5.1.5 Sediment Transport Model .................................................................................... 17-25 17.5.1.6 Model Calibration ................................................................................................... 17-25 17.5.2 Bed Levelling .......................................................................................................... 17-25 17.5.2.1 Bed Levelling Zones ................................................................................................ 17-27


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17.5.2.2 Bed Levelling Rates ................................................................................................. 17-28 17.5.3 Hydrodynamic Model Results ................................................................................ 17-29 17.5.3.1 Current Speeds ....................................................................................................... 17-29 17.5.4 Wave Modelling Results ......................................................................................... 17-30 17.5.5 Sediment Transport Model .................................................................................... 17-31 17.5.6 Longshore Sediment Transport .............................................................................. 17-39 17.5.7 Comparison of Bed Levelling and Dredging ........................................................... 17-41 17.5.7.1 Feasibility Comparison ........................................................................................... 17-41 17.5.7.2 Comparison of Impacts .......................................................................................... 17-42 17.5.8 Propeller Wash ....................................................................................................... 17-42 17.5.8.1 Current Speeds ....................................................................................................... 17-42 17.5.8.2 Calculation Approach ............................................................................................. 17-45 17.5.8.3 Predicted Impacts................................................................................................... 17-45 17.5.9 Vessel Wake Waves ................................................................................................ 17-49 17.5.10 Cyclone Moorings ................................................................................................... 17-51 17.6 Potential Impacts ........................................................................................... 17-52 17.6.1 Port Construction ................................................................................................... 17-52 17.6.1.1 Coastal Processes ................................................................................................... 17-53 17.6.1.2 Water Quality ......................................................................................................... 17-53 17.6.2 Bed Levelling .......................................................................................................... 17-54 17.6.3 Offshore Transhipment Area and Bulk Vessels ...................................................... 17-55 17.6.4 Barging –Vessel Wake Waves................................................................................. 17-55 17.6.5 Barging – Propeller Wash ....................................................................................... 17-56 17.6.6 Port Sediment Ponds .............................................................................................. 17-57 17.6.7 Release of Contaminants ....................................................................................... 17-59 17.6.8 Sediment ................................................................................................................ 17-59 17.6.9 Acid Sulphate Soils ................................................................................................. 17-60 17.7 Mitigation Measures ...................................................................................... 17-60 17.7.1 Port Construction ................................................................................................... 17-60 17.7.2 Vessel Wakes .......................................................................................................... 17-60 17.7.3 Bed Levelling .......................................................................................................... 17-61 17.7.4 Propeller Wash ....................................................................................................... 17-61 17.7.5 Bauxite Loading ...................................................................................................... 17-62 17.7.6 Water Quality ......................................................................................................... 17-62 17.7.7 Sediment Quality .................................................................................................... 17-63 17.7.8 Acid Sulphate Soils ................................................................................................. 17-63 17.8 Monitoring Plan ............................................................................................. 17-63 17.8.1 Site Specific Water Quality Objectives / Baseline Water Quality .......................... 17-64 17.8.1.1 Reference Sites ....................................................................................................... 17-64 17.8.1.2 Monitoring Program ............................................................................................... 17-64 17.8.2 Construction Monitoring ........................................................................................ 17-69 17.8.3 Monitoring During Operations for a REMP ............................................................ 17-73 17.8.4 Port Sediment Pond Releases ................................................................................ 17-75 17.8.4.1 Monitoring Parameters .......................................................................................... 17-75 17.8.4.2 Baseline Data .......................................................................................................... 17-75 17.8.4.3 Releases During the Operational Period ................................................................ 17-75 17.8.4.4 Wet Season Releases.............................................................................................. 17-76 17.8.4.5 Monitoring During Releases ................................................................................... 17-76 17.8.4.6 Seasonal Monitoring .............................................................................................. 17-76 17.8.4.7 Habitat Condition Assessment – Mangroves and Saltmarsh ................................. 17-77 17.8.4.8 Management Measures ......................................................................................... 17-77


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17.8.5 Sediment Quality .................................................................................................... 17-77 17.8.6 Vessel Wake Waves ................................................................................................ 17-81 17.8.7 Propeller Wash ....................................................................................................... 17-83 17.9 Risk Assessment ............................................................................................. 17-83 17.10 Cumulative Impacts ........................................................................................ 17-85 17.10.1 Skardon River ......................................................................................................... 17-85 17.10.2 Offshore Transhipment .......................................................................................... 17-86 17.10.3 Bulk Carriers ........................................................................................................... 17-86 17.11 Conclusion ..................................................................................................... 17-86

Tables

Table 17-1 Tidal Planes ............................................................................................................... 17-3 Table 17-2 Distance with Elevations above -2.2mLAT in the Bed Levelling Areas ..................... 17-6 Table 17-3 Marine Water Quality Sampling ............................................................................. 17-10 Table 17-4 Interim Water Quality Objectives .......................................................................... 17-18 Table 17-5 Sediment Sampling ................................................................................................. 17-21 Table 17-6 Existing Tidal Currents ............................................................................................ 17-44 Table 17-7 Predicted Erosion Extent and Rates ....................................................................... 17-45 Table 17-8 Predicted Mass of Sediment Eroded and Resultant SSC ........................................ 17-48 Table 17-9 Release Points – Port Area Sediment Ponds .......................................................... 17-57 Table 17-10 Release Contaminant Limits – Port Sediment Ponds ............................................. 17-58 Table 17-11 Vessel Based Water Quality Sampling Locations ................................................... 17-65 Table 17-12 Deployed Logger Location and Function ................................................................ 17-67 Table 17-13 Port Construction and Bed Levelling Monitoring Locations................................... 17-71 Table 17-14 Water Quality Monitoring - Operations ................................................................. 17-73 Table 17-15 Sediment Quality Parameters ................................................................................ 17-78 Table 17-16 Sediment Sampling Locations ................................................................................ 17-79 Table 17-17 Risk Assessment and Management Measures for Impacts to Coastal

Processes and the Physical Marine Environment .................................................. 17-83

Figures

Figure 17-1 Bed Features and Bathymetry of the Skardon River ................................................ 17-4 Figure 17-2 Bed Levelling Areas (Water depth < -2.2 mLAT) ...................................................... 17-5 Figure 17-3 Bathymetry from 2009, Long-section (Black Line), Chainage (km) .......................... 17-7 Figure 17-4 Sea Bed Elevation (m LAT) for Long-section of the Skardon River ........................... 17-8 Figure 17-5 Aerial Photograph of Skardon River from 1989 ....................................................... 17-9 Figure 17-6 Marine Water Quality and Sediment Sampling Locations ..................................... 17-12 Figure 17-7 Turbidity in the Skardon River ................................................................................ 17-13 Figure 17-8 Turbidity Upper Estuary (12 Hours December 2011) ............................................. 17-14 Figure 17-9 Turbidity, DO and Tides over 4 High Tides ............................................................. 17-14 Figure 17-10 Particle Size Distribution ........................................................................................ 17-21 Figure 17-11 Model Extent and Bathymetry ............................................................................... 17-24 Figure 17-12 Volume of Bed Levelling by Zone ........................................................................... 17-26 Figure 17-13 Depth below -2.2mLAT for Transfer of Bed Levelled Material .............................. 17-27 Figure 17-14 Volumetric Assessment of Bed Levelled Material .................................................. 17-27 Figure 17-15 Modelled Bed Levelling Zones................................................................................ 17-28


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Figure 17-16 Changes in Current Speed – Ebb Tide .................................................................... 17-29 Figure 17-17 Changes in Current Speed – Flood Tide ................................................................. 17-30 Figure 17-18 Change in Peak Significant Wave Height ................................................................ 17-31 Figure 17-19 50th Percentile Plot of SSC - Zone A ........................................................................ 17-32 Figure 17-20 95th Percentile Plot of SSC - Zone A ........................................................................ 17-33 Figure 17-21 50th Percentile Plot of SSC - Zone B1 & B2 ............................................................. 17-33 Figure 17-22 95th Percentile Plot of SSC - Zone B1 & B2 ............................................................. 17-34 Figure 17-23 50th Percentile Plot of SSC - Zone C ........................................................................ 17-34 Figure 17-24 95th Percentile Plot of SSC - Zone C ........................................................................ 17-35 Figure 17-25 Instantaneous SSC Plume for a Daily Cycle in Zone A ............................................ 17-36 Figure 17-26 Instantaneous SSC Plume for a Daily Cycle in Zone B1 .......................................... 17-37 Figure 17-27 Instantaneous SSC Plume for a Daily Cycle in Zone B2 .......................................... 17-38 Figure 17-28 Instantaneous SSC Plume for a Daily Cycle in Zone C ............................................ 17-39 Figure 17-29 Conceptual Model of Sediment Transport at the Ebb Bar of the Skardon River

................................................................................................................................ 17-40 Figure 17-30 Water Depth and Propeller Wash Calculation Locations ....................................... 17-43 Figure 17-31 Predicted Spatial Pattern of Near Bed Current Speeds.......................................... 17-47 Figure 17-32 Predicted Area of Erosion at Wharf (Red Rectangle) ............................................. 17-47 Figure 17-33 Secondary Wave Heights ........................................................................................ 17-51 Figure 17-34 Potential Cyclone Mooring Locations .................................................................... 17-52 Figure 17-35 Vessel Based Water Quality Sampling Locations ................................................... 17-66 Figure 17-36 Deployed Logger Location and Function ................................................................ 17-68 Figure 17-37 Port Construction and Bed Levelling Monitoring Locations................................... 17-72 Figure 17-38 Water Quality Monitoring - Operations ................................................................. 17-74 Figure 17-39 Sediment Sampling Locations ................................................................................ 17-80 Figure 17-40 Vessel Wake Wave and Bank Erosion Monitoring Locations ................................. 17-82


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17. COASTAL PROCESSES

17.1 Introduction

This chapter describes the coastal process and physical environment in the Skardon River and coastal areas potentially impacted by the Project, including climate, hydrodynamics, bathymetry, morphology, shoreline evolution, marine water quality and marine sediments. This chapter identifies potential Project impacts on coastal process and the physical environment describes measures to mitigate and manage impacts, and provides a risk assessment for residual impacts. The assessment of the marine environment focusses on the Skardon River, bed levelling area (in the ebb bar on the ocean side of the headlands at the mouth of the Skardon River) and offshore transhipment area.

Information in this chapter is primarily based on the information provided in Appendix 8 and Appendix 17 which provides a supplementary technical report to assess impacts on coastal processes and the physical marine environment.

Chapter 16 describes the freshwater aquatic environment and ecology of the Project area.

17.2 Environmental Objectives and Performance Outcomes

The environmental objectives and performance outcomes below are based on Schedule 5, Table 2 of the Environmental Protection Regulations 2008 (EP Regulation). The mitigation and management measures presented in this chapter are designed to achieve these environmental objectives and performance outcomes. The environmental management plan (EM Plan) presented in Appendix 13 provides a consolidated description of these mitigation and management measures.

17.2.1 Environmental Objectives

The activity will be operated in a way that protects environmental values of marine waters.

The activity is operated in a way that protects the environmental values of marine sediments.

The location of activities in the marine environment protects environmental values of adjacent

sensitive uses.

The choice of the site, at which the activity is to be carried out, minimises serious environmental harm

on areas of high conservation value and special significance in the marine environment.

17.2.2 Performance Outcomes

Storage and handling of potential contaminants will minimise release to the marine environment.

Contingency measures will prevent or minimise adverse effects on marine water quality or sediment

quality due to unplanned releases of contaminants to the marine environment.

Any discharge to marine waters will be managed so that there will be no adverse effects due to the

altering of existing flow regimes for marine waters.

Activities that disturb marine sediments will be managed in a way that prevents or minimises adverse

effects on environmental values.

Activities in the marine environment are carried out in a way that prevents or minimises adverse

effects on the use of surrounding waters and allows for effective management of the environmental

impacts of the activity.


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Critical design requirements will prevent emissions having an irreversible or widespread impact on

adjacent areas.

Shipping activities will be managed to minimise bank erosion potential and habitat impact.

Prevent or minimise the release of ship-sourced pollutants.

Implement emergency response plans for any hydrocarbon contaminant release.

The disturbance of any acid sulphate soil, or potential acid sulphate soil, will be managed to prevent

or minimise adverse effects on environmental values.

17.3 Legislative and Policy Context

The legislative and policy context for approvals for activities in marine environment is described in Chapter 2.

17.4 Coastal Processes and the Physical Marine Environment

17.4.1 Climate

Climatic factors affecting coastal processes and the physical marine environment are described in Chapter 9 and include rainfall, cyclones, wind speed and wind direction.

17.4.2 Regional Setting

The Gulf of Carpentaria is a large and relatively shallow body of water which is enclosed on three sides by the Australian mainland and bounded on the north by the Arafura Sea. It is 480 km wide and 640 km long with an area of approximately 310,000 km2 and a maximum water depth of 70m. The tidal wave enters from the north-west (the Arafura Sea) and propagates clockwise around its amphidromic point (a nodal point about which the tide rotates). The eastern shoreline current is parallel to the coast at the peak flood and ebb stages of the tide. Tidal signals around the Gulf of Carpentaria are semi-diurnal in the north, decreasing rapidly towards a diurnal signal in the south. At Skardon River the tide signals are mixed but mainly diurnal.

The Gulf of Carpentaria can be subject to seasonal fluctuations in sea level (up to 0.5m) as a result of trade winds (e.g. during the monsoon) and forcing from the Arafura Sea. These seasonal sea level fluctuations can result in large areas only being inundated by tides in the summer months (during the monsoon), as a result these areas cannot support mangrove or freshwater vegetation and therefore form salt flats. In addition, circulation within the Gulf can also be set up due to the wind stress applied by tropical cyclones at the water surface driving wind induced currents and residual water level circulations.

17.4.3 Skardon River Estuary

The Skardon River is classified as a tidal creek as it has a low freshwater input with low-gradient, seaward-sloping coastal flats. These systems are primarily influenced by tidal currents and as a result they comprise of straight, sinuous or dendritic tidal channels that taper and shoal to landward. The mudflats which surround the creeks tend to be high relative to the tidal planes, with seawater being mainly confined to the tidal channels except during high tide on spring tides. Due to the strong tidal currents which occur in tidal creeks as generated by the generally large tidal ranges, they are usually highly turbid.

17.4.4 Tides

The closest standard port to Skardon River with accurate tidal predictions and tidal plane information is Weipa. However, tidal plane information is also available at Cullen Point (near Mapoon) and Vrilya Point (60 km north of the Skardon River) but these are based on less data than for Weipa and are not considered


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as accurate. In addition, eight months of measured water level data at the Skardon River Barge Ramp has been used to predict approximate tidal planes. The tidal plane information for the four locations is shown in Table 17-1.

Table 17-1 Tidal Planes

Tidal Plane Weipa

LAT (m)

Cullen Point

LAT (m)

Vrilya Point

LAT (m)

Skardon River Barge Ramp

LAT (m)

HAT (Highest Astronomical Tide) 3.4

MHHW (Mean High High-Water) 2.9 3.0 3.6 3.6

MLHW (Mean Low High-Water) 2.2 2.0 2.4 2.6

MSL (Mean Sea Level)# 1.8 1.8 2.1 2.2

MHLW (Mean High Low-Water) 1.5 1.5 1.9 1.9

MLLW (Mean Low Low-Water) 0.7 0.5 0.6 0.7

LAT (Lowest Astronomical Tide) 0.0 0.0 0.0 0.0

# At Weipa AHD = 1.75 m above LAT which is approximately equal to MSL

The tidal signal at Skardon River is predominantly diurnal with a small semi-diurnal signal which results in a consistent small second high and low water each day. The eight month overview of the tidal signal highlights the variability in the tidal signal, with significant differences between successive lunar cycles (29.5 day cycle). In addition, it also highlights the variability in the semi-diurnal signal over time.

17.4.5 Storm Tide

There is limited storm surge data available for the Skardon River. A detailed storm tide assessment has been carried out at Weipa by Worley Parsons (2008) which can be used to provide an indication of likely storm tide conditions for the Skardon River. The assessment found that the potential for a high storm tide (combined tide and surge) to occur at Weipa was reasonably low, with a 100 year ARI of approximately 2 m above Australian height datum (AHD) (compared to a highest astronomical tide (HAT) level of 1.63 m AHD). The reasons for the predicted relatively low storm tide level was mainly a result of less intense cyclones tending to occur in the area and the likelihood that a rare severe cyclone crosses at the same time as a spring high tide is very low. Based on this analysis and combined with high water levels for the Skardon River expected to be similar as at Weipa, the storm tide levels for the Skardon River are expected to be comparable to Weipa and therefore storm tides are not considered to present a significant risk in the area.

The infrastructure, other than the wharf, within the Port infrastructure area is situated at least above 3 m AHD, and mining will occur on the bauxite plateau above the Port elevation. These areas are considered sufficiently high that sea level rise or storm tide inundation over the 10 year Project life will not present a significant risk.

17.4.6 Tidal Currents

Skardon River is categorised as a tidal creek where tidal currents are the dominant processes, with strong tidal currents expected. It is therefore tidal action which results in the transport of sediment into the estuary, where the sheltered conditions eventually allow the coarser sediment fractions to settle out of suspension. Tidal creeks are usually highly turbid due to the strong tidal currents generated by the macro-tidal ranges allowing fine sediments to remain in suspension during spring tides.


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Figure 17-1 shows the bathymetry of the main channel in Skardon River along with areas with different bed forms. Offshore of the entrance mega ripples and sand waves occur in the main channel where current speeds remain high due to the constrained channel focusing the flow, and where the ebb tidal delta forms the tidal currents are much lower as wave action will start to dominate this area. These offshore features indicate active coastal processes and the natural sand transport of sand across the entrance to the river.

The ebb bar is the zone in which bed levelling is proposed. The mouth is the zone between the headlands.

Figure 17-1 Bed Features and Bathymetry of the Skardon River

17.4.7 Waves

Appendix 8 presents seasonal wave roses. Waves are generated by the dominant east-south-east winds during the dry season, resulting in calm conditions along the east coast of the Gulf. The largest waves which occur are from the west to west-north-west occur during the wet season (summer and part of autumn). The area offshore of the constricted entrance to the Skardon River will be influenced by both offshore generated swell waves and locally generated wind waves. Due to the narrow entrance of the Skardon River (approximately 300m) combined with the complex and relatively shallow bathymetry of the ebb tidal delta and the offshore channel, swell waves are not expected to propagate inside the Skardon River. Due to the configuration of the Skardon River locally generated wind waves in the estuary are not considered to be a significant process.

17.4.8 River Flows

A hydrological assessment, using hydrological modelling, of the Skardon River was undertaken by SRK Consulting Pty Ltd (2013). Based on this assessment the features associated with the Skardon River are as follows:


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approximately 30 km long

total catchment area of approximately 480 km2

the catchment has suffered little disturbance

the freshwater discharge is highly seasonal with significantly higher flows in the wet season

(December to April)

the mean annual discharge is 730,000ML.

The Skardon River is dominated by tidal processes and freshwater flows are relatively low.

17.4.9 Bathymetry and Morphology

Bed levelling is proposed in two areas in the ebb tidal delta area which, for the purpose of this assessment, are referred to as bed levelling area 1 and 2, with area 1 being the large area further offshore in Figure 17-2. Bed levelling is proposed at -2.2 m LAT and Figure 17-2 shows, based on the bathymetry of the proposed navigation channel (in red), where water depth is less than -2.2 m LAT (i.e. requires bed levelling).

Figure 17-2 Bed Levelling Areas (Water depth < -2.2 mLAT)

Five hydrographic surveys of the Skardon River were available from 1998, 2002, 2007, 2009 and 2015. All of the surveys extend at least from the ebb tidal delta offshore of the entrance (i.e. inclusive of the bed levelling area) up to the Port location. The change in bed elevation between the hydrographic surveys was reviewed to develop an understanding of the river morphology and its changes.

The distance where sea floor elevations are higher than -2.2 m LAT in the bed levelling areas is shown in Table 17-2, for each of the bathymetrical surveys. Table 17-2 shows how the lowering in elevation in these areas since 1998 has resulted in a reduction in the total channel length requiring bed levelling to a level of -2.2 m LAT by more than 100 m from 1,886 m in 1998 to 1,757 m in 2015.

Over the 17 year period of the hydrographic surveys, the peak elevations in Area 1 have generally reduced in elevation and the width of the area above the bed levelling elevation has also reduced. Accordingly, there has been reduction in the volume of sand above the bed levelling elevation in Area 1. In contrast,

Area 1

Area 2


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although the peak elevations in Area 2 have generally been reducing, the width of the area above the bed levelling elevation has increased. Once width of the bed levelling area is considered, the volume of material requiring bed levelling has remained relatively stable.

Table 17-2 Distance with Elevations above -2.2mLAT in the Bed Levelling Areas

Bed Levelling Area

Nov 1998 Aug 2002 Aug 2007 Sep 2009 Apr 2015

Area 1 1,454m 1,440m 1,397m 1,238m 1,267m

Area 2 432m 259m 346m 346m 490m

Total 1,886m 1699m 1,742m 1,584m 1,757m

The bed elevation along the approximate deepest section of the channel is shown in Figure 17-3 for a long-section of the Skardon River from the ebb tidal delta to the Port location. The long section with elevation in metres below lowest astronomical tide (m LAT) for each of the hydrographic surveys is shown in Figure 17-4. This shows that the bathymetry in the area between the entrance, including the entrance, and the Port has been relatively stable from 1998 to 2015 (with some small changes close to the entrance between 1998 and 2002). Conversely, the channel bathymetry offshore of the entrance has been more dynamic.

Figure 17-4 shows that between 1998 and 2015 the peak elevation for the two bed levelling areas has reduced from approximately -0.5 m LAT to -1.25 m LAT. The lowest elevations occurred in 2009, and the features have migrated in an offshore direction.

Over the 17 year period only a single tropical cyclone tracked close to Skardon River and this was a Category 1 (the lowest of the five categories) system. Strong winds and large waves during a tropical cyclone have the potential to result in significant sediment transport along the shoreline adjacent to the Skardon River mouth. Accordingly, this could result in significant bathymetric changes to the area offshore of the Skardon River mouth including the channel. As such, the bathymetric changes over the 17 years of data should be considered to represent relatively calm conditions.

Gulf Alumina, considers, that based on historical data, the risk of a cyclone resulting in changes to bathymetry is low over the 10 year operational life of the Project. Even if a cyclone does occur, historical data suggests that this may not result in changes to bathymetry. Following the wet season, hydrographic surveys will be undertaken to determine the need for maintenance bed levelling. This will be important after a wet season cyclonic event. It is possible that bed levelling may not be sufficient to re‐establish the required bed level and other methods such as dredging could be required to re-establish the channel following a cyclonic event. Gulf Alumina will seek additional approvals for these alternative methods to open the channel, in the unlikely event that this is required, at such time as the need for this is identified. Barging operations will cease until the necessary approvals are obtained. This may include an assessment under the National Assessment Guidelines for Dredging 2009.


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Figure 17-3 Bathymetry from 2009, Long-section (Black Line), Chainage (km)


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Figure 17-4 Sea Bed Elevation (m LAT) for Long-section of the Skardon River


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17.4.10 Shoreline and Bank Evolution

Historical aerial photography from 1989 (Figure 17-5) has been compared with recent aerial photography to determine changes to the Skardon River mouth, banks and adjacent shoreline over the last 26 years. The photographs show that there has been little change in the mouth or banks of the Skardon River over this period, indicating that the river has been stable. The adjacent shoreline also has not changed significantly over this period. The configuration of the channel offshore of the Skardon River mouth has also not changed significantly over this period.

The existing configuration of the shoreline to the north and south of the mouth of the Skardon River shows a depositional trend, with the shoreline showing signs of prograding and beach ridges being present. This shows that the orientation, location and width of the channel and adjacent shoals offshore of the river mouth, the river mouth and the adjacent shoreline have been stable over this period.

Sediment which is transported along the shoreline to the mouth of the Skardon River will be transported into the complex configuration of sand shoals and the ebb tidal delta and eventually bypass the river mouth. These shoals and the delta act as stores of sediment which allow sediment transported by longshore drift to bypass the river mouth during certain events.

Fringing mangroves are present along the banks of the majority of the Skardon River. The mangrove vegetation will act to stabilise the sediment along the banks by attenuating both locally generated wind waves and tidal currents. The mangroves therefore help to create a depositional environment along the river banks. The presence of fringing mangroves throughout the estuary indicates that the banks of the river are currently stable and accreting.

Figure 17-5 Aerial Photograph of Skardon River from 1989


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17.4.11 Marine Water Quality

17.4.11.1 Overview

Typical of northern Australian estuaries, both the chemical and physiochemical water properties of the Skardon River are driven by wet and dry season conditions.

Turbidity is also influenced by the seasonal rainfall, with turbidity levels lower during the dry season, particularly towards the end of the season. The dry season reduction in ambient turbidity provides increased benthic light to seagrasses and other benthic primary producers.

The seasonal drivers influence ambient water quality on a broad temporal scale (seasons). However, it is also important to recognise that shorter scale and rapid changes in water quality can occur within these highly variable coastal environments on a daily, or even hourly basis, particularly within the mangrove and estuary systems. Changes may be driven by the passage of an ebb tide, with waters empting from the surrounding mangrove systems to the primary river channels, weather conditions were calm and no rainfall occurred.

The physical processes leading to these changes in physicochemical conditions are equally as likely to alter chemical constituents within the water column such as metals and nutrients. The variable nature of water quality in these habitats results in a broad range of physical and chemical tolerances.

17.4.11.2 Water Quality Sampling

A combination of vessel based and logger based water quality physico-chemical and chemical marine water quality investigations have been undertaken within the Skardon River since 2011. These events captured a combination of dry season and wet season data sets (Table 17-3). In addition to vessel based surveys, two data loggers were deployed within the Skardon River adjacent to the existing barge ramp (Port infrastructure area) and upstream of the Port area during 2011. The objective of the program was to record time series water quality data over the 2011/2012 wet season. At least 16 locations were sampled with sampling locations shown in Figure 17-6 ranging from the ebb tidal delta beyond the mouth to upstream Skardon River South Arm. All data is presented in Appendix 8, with a summary provided below. The five vessel based surveys by PACE were at:

Site 1 within the upper estuary

Site 2 approximately 2km upstream of the existing barge ramp

Site 3 at the existing barge ramp

Site 4 within the mid estuary

Site 5 adjacent to the river entrance.

Data loggers recorded temperature, pH, dissolved oxygen, salinity, depth and turbidity. This data provides an improved understanding of the short-term variability experienced in physicochemical parameters as a result of tidal movements.

SR1 – 25/10/11 to 2/12/2011

SR2 – 25/10/2011 to 2/12/2011 and 3/12/2011 to 11/1/2011

Table 17-3 Marine Water Quality Sampling

Date Source Sites Period Chemical Physicochemical Logger

2011 PaCE 5 Oct Yes Yes Physicochemical (2 sites)

2011 PACE 5 Dec Yes No Physicochemical (1 sites)

2012 PaCE 5 Jan Yes Yes


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2012 PaCE 5 June Yes Yes

2014 PaCE 5 Nov Yes Yes Light (PAR)

2014 RPS 3 August Yes No

2015 RPS 10 March Yes Yes

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#*

#*Skardon RiverML 40082

ML 40069

Port ofSkardon River

604000 606000 608000 610000 612000 614000 616000 618000 62000086

9200

0

8692

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8694

000

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ROCKHAMPTONBRISBANE

CAIRNS

TOWNSVILLE

Queensland

No warranty is given in relation to the data (including accuracy, reliability, completeness or suitability) and accept no liability (including without limitation, liability in negligence) for any loss, damage or costs (including consequential damage) relating to any use of or reliance upon the data. Data must not be used for direct marketing or be used in breach of privacy laws.Imagery sourced from Gulf Alumina (2014). Tenures © Geos Mining (2015). State Boundaries and Towns © Geoscience Australia (2006).

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Figure 17-6 Gulf Alumina Limited

LegendMining Lease Boundaries

!( Port of Skardon River

#*Sediment and Water Quality Sampling Location (RPS)#* Sediment Sampling Location (PaCE)XW Water Quality Sampling Location (PaCE)

0 1 2 3 4 5Kilometres

G:\CLIENTS\E-TO-M\Gulf Alumina\GIS\Maps\EIS\Ch17_Coastal\FIG_17_06_MarineWQ_Sediment_Locations_LS_160316.mxd

Revision: R1

Date: 16/03/2016 Author: malcolm.nunn1:70,000Map Scale:

Coordinate System: GDA 1994 MGA Zone 54

Marine Water Quality andSediment Sampling Locations


Page 17-13

17.4.11.3 Turbidity

Turbidity (ntu) recorded during vessel based survey within the surface column ranged widely between the survey periods (range 0 to 87.6 ntu). The available dry season data reported a mean turbidity of 3.4 ntu with no significant variability between sampling locations (ANOVA, p>0.05) (Figure 17-7). Wet season turbidity increased markedly within the surface column, recording a mean of 36.1 ntu, 80th percentile of 42.5 and 90th percentile of 54.3. Minor variation between sites was recorded (ANOVA, p<0.05) with site WQ5 presenting a marginally higher mean turbidity during the wet season as shown in Figure 17-7.

The physicochemical data collected during these vessel based water quality surveys represents the upper 0.5m of the water column, and was conducted over a limited timeframe at each sampling location. The survey events did not coincide with significant wet season event based run-off periods, and as such, peak turbidity has not been captured within the available data set. Ongoing monitoring is proposed to capture turbidity logger data from key receiving habitats in detailed time series.

Figure 17-7 Turbidity in the Skardon River

Data from the deployed water quality logger located upstream of the existing barge ramp (Site SR2) depicts a different appreciation of ambient turbidity. A short extract of this data, not impacted by fouling or sensor failure, (12 hours following deployment during December, 2011) shows the occurrence of multiple short lived increases in turbidity, ranging from 50 ntu to over 350 ntu (Figure 17-8). The logger at this location was located immediately above the seabed (~0.3m) on a steel frame. This turbidity logger demonstrates the occurrence of natural turbidity fluctuations not captured by surface grab sampling. Given no significant rainfall or weather conditions, the increase in turbidity would appear to be the result of tidally induced sediment mobilisation from the bed and banks of the adjacent mangrove system. This process follows the trend in observations made within the adjacent Wenlock and Ducie Rivers.

Site

Tu

rbid

ity (

ntu

)

Mean

Mean±SE

Min-Max Season: Dry

WQ1 WQ2 WQ3 WQ4 WQ5

0

20

40

60

80

100

Season: Wet

WQ1 WQ2 WQ3 WQ4 WQ5


Page 17-14

Figure 17-8 Turbidity Upper Estuary (12 Hours December 2011)

Additional comparisons made between turbidity (ntu), water depth (m) and dissolved oxygen (DO mg/L) further demonstrate the roll of tidal exchange in elevating turbidity. During the ebb tide, waters existing the system generated an increase in turbidity and an associated decrease in available DO (Figure 17-9). The peak change in turbidity appears to occur during the mid-tidal run when velocities typically approach maximum. During the flooding tide this relationship was reversed, with DO increasing and turbidity dropping to near 0 ntu as ‘fresher’ waters from the mid and lower estuary are pushed upstream.

Figure 17-9 Turbidity, DO and Tides over 4 High Tides


Page 17-15

17.4.11.4 Salinity

Salinity (ppt) within the Skardon River appears dependent upon season (wet/dry) (ANOVA, p<0.05) and distance upstream from the entrance (ANOVA, p<0.05) (refer to figures in Appendix 8). During the wet season, increased freshwater flows from the catchment act to reduce salinity within the river (mean 25.8 ppt). Salinity reductions in the upper estuary have been recorded during January, through to June. During the dry season, increases in salinity are recorded (mean 32.6 ppt). The greatest increases in salinity were recorded during the late dry season (November), with results demonstrating an upstream increase in salinity from October (30-31 ppt), establishing a salinity maximum in November (41-42 ppt). The salinity maximum is developed from a reduction in freshwater inflows, restricted tidal flushing and an increased evaporation rate within the shallow waters and mangrove systems of the upper estuary system. The spatial variations in salinity remained significant between the upper and lower estuary reaches during all sampling events (ANOVA, p<0.05).

In addition to seasonal and spatial trends, small scale temporal trends in salinity are also observed within the Skardon River. Logger data recorded adjacent to the existing barge facility during the 2011 dry season (SR1, December 2011) demonstrates the role in which tidal exchange (flood/ebb) influences salinity. During the dry season the ebbing tide pulls higher salinity waters from the upper estuary and shallow water environments of the mangroves into the river channel. Upon the turn of the tide, the lower estuary introduces lower salinity waters. This processes recorded salinity variations between ~33 to 35 ppt over three tidal cycles. This process would occur during the salinity maximum phase (mid to end dry season). During the wet season, lower salinities dominate the upper estuary and the trend described would be reversed.

17.4.11.5 pH

Overall, the Skardon River presents a pH range of 6.9 to 8.8. pH within the Skardon River presents a strong spatial trend, from a mean of 8.4 within the lower estuary, to 7.5 within the upper estuary. During the dry season, the upper estuary (WQ1-WQ2), mid (WQ3) and the lower estuary (WQ4-WQ5), all presented distinctly different pH regimes (ANOVA, p<0.05). Although the data presents lower variation during the wet season, similar trends were observed. During the wet season the upper estuary (WQ1-WQ2) remained of lower pH than the mid and lower estuary sites (WQ3-WQ5) (ANOVA, p<0.05).

pH also varied significantly between the dry and wet seasons (ANOVA, p<0.05).

The important role of tidal flushing in defining pH within the Skardon River is demonstrated within the data logger extract taken adjacent to the existing barge ramp. Within this example, as the flooding tides push through the study area, pH increases, from ~7.3 to 8.1. As tides begin to ebb the opposite process occurs, reduced pH waters are extracted from the mangroves and upper estuary systems. The roll of increased biological activity and contact with intertidal sediments within the mangroves acts to alter a broad range of physicochemical parameters, pH being one of them. These processes keep the Skardon River in a continual process of chemical change.

17.4.11.6 Dissolved Oxygen

Dissolved oxygen (DO) within the Skardon River demonstrates a distinct spatial trend, reducing significantly as distance from the entrance increases (ANOVA, p<0.05). Within the lower reaches of the river, sites WQ4 - WQ5 records a mean DO of ~ 6 mg/L. At the existing Port facility this reduces to a mean of 5.4 mg/L (WQ3) and continues to decrease upstream, through 4.4 mg/L (WQ2), to 3.7 mg/L within the upper estuary (WQ1). Very similar spatial trends are recorded during the dry and wet season periods. The data set also describes lower overall DO during the wet season compared to the dry season (ANOVA, p<0.05).

This strong spatial trend is thought to be driven by several factors, including a reduction in flushing capacity as locations move upstream from the entrance, and an increased organic load within the upper reaches.


Page 17-16

Additional information to help further define the reduced DO observed within the upper estuary is provided from logger data extracted from site SR2 (upper estuary). The logger was deployed on a stand approximately 0.3m above the sea bed. This logger identified a high correlation between DO and tidal exchange. As the tide drops, water from the upper estuary and adjacent mangrove systems is pulled into the primary river channel. These waters have been exposed to increased organic processes, high organic loads and increased turbidity. DO reduced markedly during the ebb tide (<1 mg/L). As the tide changes, ‘freshwater’ from the lower reaches of the Skardon River is pushed into the upper estuary providing increased DO to the mangrove systems. Recovery in DO was greatest during the full tidal exchange phase, with only reduced recovery during the half tide phase.

Tidal variations appear to drive numerous water quality parameters, keeping the systems in a continual state of flux.

17.4.11.7 Benthic Light

The deployed Photosynthetically Active Radiation (PAR) loggers reported substantial variation in available benthic light between two locations (depth -1 mLAT and -2.8 mLAT). The shallower deployed PAR logger was located immediately below the observed extent of seagrass within the Skardon River. Benthic light to ~180 um/cm2/sec was recorded over a single day deployment. Over the same period, benthic light from the adjacent logger (-2.8 mLAT) remained below 10 um/cm2/sec. The horizontal distance between the loggers was estimated at approximately 10 m, with depth being the only significant variable between the two locations. Only a small tidal variation was experienced over the daytime hours; high tide of 2.99m @ 10:37 and low tide of 2.43m @ 14:50. Low tide corresponds to the second peak in the data of ~140 um/cm2/sec.

The decline in PAR from a peak of 180 um/cm2/sec to ~43 um/cm2/sec is thought to have been driven by turbidity increases during the ebbing tide, with PAR recovering to 140 um/cm2/sec as the tidal run, turbidity and overall water depth decreased. A similar process is described within the observed increases in bottom column turbidity as a result of ebbing tides.

Although the data is limited within this example, the effect of tidal movement in the reduction of PAR is a natural process which seagrasses in the area have become adapted to. This decline in PAR from 180 um/cm2/sec occurred over a period of ~2hrs, with recovery over an additional ~2hrs to 140 um/cm2/sec. Reductions in PAR during peak tidal periods may be expected to be greater.

17.4.11.8 Nutrients

Nutrient levels were compared to the Australian and New Zealand Environmental Conservation Council (ANZECC) water quality guideline trigger values for tropical systems.

Total nitrogen (TN) and total phosphorous (TP) concentrations remained well over the adopted ANZECC screening criteria. TN ranged between 2000 ug/Land 50 ug/L over the survey period, recording a mean concentration of 476 ug/L (ANZECC criteria 250 ug/L). TP ranged between 30 ug/L and 92 ug/L, recording a mean of 51 ug/L (ANZECC criteria 20 ug/L).

17.4.11.9 Metals

Arsenic and zinc recorded elevated concentrations compared to cadmium, chromium, copper, lead and nickel. A natural ratio between these metals concentrations was clearly observed from all sampling locations. Although concentrations may vary, the ratios between metals at each site remain relatively consistent over the sampling period. Copper and zinc recoded metal concentrations in excess of the ANZECC criteria. Similar trends in metals ratios are reflected in analysis for aluminium and iron (no ANZECC criteria available for these metals). While concentrations may be elevated for some analytes, the broad maintenance of these ratios across all locations is indicative of natural ambient conditions.


Page 17-17

17.4.11.10 Total suspended solids

Total suspended solids ranged between 10 mg/L and 1 mg/L. While maximum TSS was reported from the upper estuary (WQ 1), no distinct spatial trend is observed. The mean from all locations ranged between ~2 mg/L to 5 mg/L.

17.4.11.11 Chlorophyll-a

Chlorophyll-a concentrations remained marginally above detection at 1-2 ug/L.

17.4.11.12 Hydrocarbons

The full suite of hydrocarbons (C6-C36) remained non detectable from all survey locations during both the wet and dry seasons.

17.4.11.13 Summary

Key findings include:

There is very high natural variability in turbidity in the Skardon estuary.

Salinity within the Skardon River is dependent upon season (wet/dry) and distance upstream from

the entrance. During the wet season, increased freshwater flows from the catchment act to reduce

salinity within the river, mixing with higher salinity waters entering through the river mouth. During

the dry, increases in salinity are recorded.

The Skardon River presents a pH range of 7-8 within the entrance and lower estuary, reducing as sites

progress up the estuary and beyond the existing barge facility (6.9-7.5). There is a strong correlation

between tides and pH.

Dissolved oxygen reduces as distance from the entrance increases. There is a high correlation

between DO and tidal exchange.

Total nitrogen and total phosphorous concentrations remained well over the adopted ANZECC

screening criteria within the Skardon River during the survey period.

The metals suite identified several elevations in metals, including copper and zinc compared to the

ANZECC criteria. Similar trends in ratios are reflected in analysis for aluminium and iron.

The full suite of hydrocarbons (C6-C36) remained non detectable from all survey locations during both

the wet and dry seasons.

Despite an absence of any substantial anthropogenic inputs, background water quality has recorded elevations in nutrients (total nitrogen and phosphorus) and metals (zinc and copper) as compared to the ANZECC criteria. These findings are considered a function of natural processes within northern tropical systems, rather than influences from contamination or catchment based affects.

Water quality conditions of the Skardon River exhibit no problematic affects associated with historical or existing land use, and the system is considered ‘near pristine’ with respect to water quality. In the absence of adjacent anthropogenic inputs, naturally occurring elevations in nutrients (nitrogen and phosphorous) and some metals are considered a feature of these biologically productive, turbid and tidally dominated tropical estuary systems. Reductions in dissolved oxygen and variability in salinity, turbidity and ORP are considered representative of the naturally occurring processes of the Skardon River.

17.4.12 Water Quality Objectives

The Skardon River Bauxite Project is situated within the Gulf Rivers province, as described within the Queensland Water Quality Guidelines (QWQG). Marine and estuarine water quality criteria have not yet been established in the QWQG for the Skardon River.


Page 17-18

The Skardon River is to be considered a high ecological value waterway (HEV). HEV assessments are based on a combination of water quality, physical and biological indicators of health. The objective of a HEV waterway is ‘no change to existing’. Where indicators of water quality health may be exceeded, then monitoring would target the assessment of other physical or biological predictors of condition to confirm ‘no change’ to existing conditions. The monitoring plan provided in Section 17.8 been developed to support the management of this HEV waterway.

Site specific water quality objectives, in accordance with the QWQG, will be developed as part of on-going baseline water quality studies proposed for the Project. The methods adopted for developing these site specific objectives will follow those outlined within the QWQG, and will require an extended data set to derive population percentiles for HEV waterways.

Interim water quality objectives are presented in Table 17-4 along with comments about their relevance to the Skardon River based on field data collected to date. Water quality objectives that are based on site data have been established based on the analysis of the 80th percentile from 5 locations in the Skardon River sampled over 5 events (total of 25 replicate samples). Where interim water quality objectives are not available, default water quality objectives in accordance with the AWQG have been proposed. In addition, where 80th percentile site data is less than AWQG default values, then AWQG default values have been used.

Revision to the interim water quality objectives outlined below will be undertaken based on the findings of an extended baseline program and the ongoing monitoring for the Project. The monitoring program will establish interim and final water quality objectives in accordance with the QWQG.

Table 17-4 Interim Water Quality Objectives

Parameter Unit Objective Basis Comment

Physicochemical

Dissolved oxygen % 80-120 AWQG – tropical estuaries

Ambient condition range widely from these standard concentrations, particularly within the upper estuary. Development of site specific criteria is recommended as part of long-term marine monitoring. Spatial consideration in criteria is required.

pH 7-8.5 AWQG – tropical estuaries

Applicable

Salinity us/cm n/a n/a No WQO proposed due to natural variability and no potential Project impacts involving releases of saline water. The presence of a salinity maximum has been identified within the upper estuary. This alters between seasons so that seasonal criteria may also be required. Spatial consideration in criteria is required.

Temperature OC n/a n/a Temperature is not considered a significant parameter regarding compliance in ambient waters.


Page 17-19


Turbidity NTU 1-200 AWQG 1-20 NTU for tropical estuaries and 2 – 200 NTU for tropical wetlands.

Based on site data, the 80th percentile wet season turbidity is 42.5 NTU and the 90th percentile is 54.3 NTU

Ambient conditions are seasonally elevated above these criteria in several of the samples, particularly from the in situ logger data. Development of site specific criteria will be undertaken as part of marine monitoring prior to operations. Spatial consideration in criteria is required.

Nutrients

Ammonium mg/L 15 ug/L AWQG – tropical estuaries

The standard ANZECC criteria are substantially lower than the conditions screened from the Skardon River at present. Development of site specific criteria will be undertaken as part of long-term marine monitoring. The 80th percentile have been provided for total nitrogen and total phosphorus. Other parameters may also require adjustment as additional data is collected prior to construction.

An extended water quality program will likely reduce nutrient water quality objectives.

Nitrite mg/L 30 ug/L AWQG – tropical estuaries

Nitrate mg/L 30 ug/L AWQG – tropical estuaries

Total Kjeldahl nitrogen

mg/L n/a No AWQG value

Total nitrogen µg/L 1000 Site data 80th percentile

Reactive phosphorous

µg/L 5 AWQG – tropical estuaries

Total phosphorus

µg/L 60 Site data 80th percentile

Metals

Aluminium µg/L 220 Site data 80th percentile

The standard ANZECC criteria are largely applicable for use in monitoring. However, long-term programs may also consider developing an improved understanding of ambient conditions to develop site specific criteria. The 80th percentile has been adopted within this table. Zinc and copper displayed elevations above the standard ANZECC criteria at the 95th percentile. Other parameters may also require adjustment

Arsenic µg/L 92 Site data 80th percentile

Cadmium µg/L 0.7 AWQG – marine 99% trigger

Chromium µg/L 4.9 Site data 80th percentile

Copper µg/L 3.9 Site data 80th percentile


Page 17-20


Iron µg/L 274 Site data 80th percentile

as additional data is collected prior to construction.

Lead µg/L 2.5 Site data 80th percentile

Manganese µg/L n/a No AWQG value

Mercury µg/L 0.1 AWQG – marine 99% trigger

Nickel µg/L 12.5 Site data 80th percentile

Vanadium µg/L 50 AWQG – marine 99% trigger

Zinc µg/L 38 Site data 80th percentile

Biological

Chlorophyll-a mg/m3 2 AWQG – tropical estuaries

The standard ANZECC criteria are largely applicable for use in monitoring. However, programs will also develop an improved understanding of ambient conditions to develop site specific criteria.

Hydrocarbons

C6-C9 µg/L 20 Model Mining Conditions

The presence of hydrocarbons within the waters of the study area is not expected. Sampling as part of operational monitoring will be undertaken.

C10-C36 µg/L 100 Model Mining Conditions

17.4.13 Sediment

The sediments of the Skardon River are largely unimpacted by development or anthropogenic processes, with very limited impact from historical kaolin operations. Sediment investigations have been undertaken in 2014 and 2015 in the Skardon River, mouth of the Skardon River, ebb bar area (bed levelling area) and offshore transshipment area.

The concentration of bauxite resources within the area presents a potential source of naturally occurring metals, particularly iron and aluminum, and minor trace metals may also be present, including arsenic, chromium, nickel, lead, tine, vanadium, zinc and zirconium.

17.4.13.1 Sediment Sampling

Several recent sediment investigations have been commissioned by Gulf Alumina and Metro Mining within the Skardon River, as summarised in Table 17-5, with information shared between these companies. These investigations provide background data regarding chemical characteristics of the system and particle size distribution. A key area for investigation was the bed levelling area where a more detailed investigation of sediments in the Skardon River entrance has been conducted in accordance with the Commonwealth’s National Assessment Guidelines for Dredging (NAGD), to provide survey, analysis and reporting pursuant to the NAGD. The objective of the sampling program was to determine the


Page 17-21

contamination status of sediments in the proposed bed levelling areas. Owing to the undisturbed nature of the study area, and lack of catchment or marine based impact from shipping activity, sediments were analysed under a program incorporating metals, nutrients and particle size.

Sediment sampling locations are shown in Figure 17-6. Further information on sediment sampling is provided in Appendix 8.

Table 17-5 Sediment Sampling

Source Survey Period Location/sites Analysis Method

RPS September, 2014 Gulf – Port area (1) River entrance (2) Offshore transshipment area (3)

Metals ASS PSD

Surface grab

PaCE November, 2014 Metro barge facility (5) Metals PSD

Surface grab

RPS March, 2015 Upstream of Port area (2) Gulf – Port area (2) Skardon River (2) River entrance (2) Inshore (1)

pH Nutrients Metals ASS

Surface grab

PaCE April, 2015 River entrance (13) Metals Nutrients PSD

Piston core 0-0.5m – 0.5-1.0m

17.4.13.2 Particle Size Distribution

Analysis for particle size (gravel, sand, silt/clay) shown in Figure 17-10 depicts a general decrease in silt and clays and increase in sands as locations progress from upstream to downstream (from left to right in Figure 17-10) locations nearer the river entrance. Entrance and offshore samples confirm a dominant sand profile. The conditions presented at the river entrance reflect active nearshore bar conditions with extensive sorting of sediment fractions by wind, waves and currents leading to a dominant coarser sediment fraction.

Figure 17-10 Particle Size Distribution


Page 17-22

17.4.13.3 Metals

A suite of metals has been analysed throughout the Skardon River and adjacent offshore sediments (refer Appendix 8). Mean concentrations of metals have been compared to the ANZECC (2000) Interim Sediment Quality Guideline (ISQG) screening criteria (ANZECC/ARMCANZ, 2000).

With the exception of arsenic, at the proposed wharf and within the upper estuary, the mean of all other analytes remained below the ANZECC screening criteria. Arsenic concentrations within Queensland’s coastal catchments may attain natural elevations due to the mineralogy of the adjacent catchments.

Iron and aluminum also both display strong spatial trends in concentration. Both iron and aluminum decrease markedly between upstream and downstream locations. Offshore sediments present the lowest recorded concentrations.

All metals analytes from the sediment investigations from the proposed bed leveling footprint reported concentrations well below the relevant NAGD criteria. Individual samples also maintained concentrations below these criteria. In accordance with the screening criteria detailed within the NAGD (2009), the sediments within the bed leveling footprint remain free of metals contamination. Metals analytes within the entrance also display significantly lower concentrations than those recorded within the upper, mid and lower estuary reaches of the Skardon River (ANOVA, p<0.05).

The sediments within the proposed bed levelling area do not present a risk of contaminant release from the bed levelling process.

17.4.13.4 Nutrients

Nutrients (total nitrogen and phosphorous) have been analysed within the Skardon River throughout the length of the estuary system. Nutrients show a general decline in concentration between upstream locations and the river entrance. The distribution and concentration of sediment nutrients often display strong relationships with sediment particle size. Silt and clay fractions within the Skardon River demonstrate a significantly increased retention of total nitrogen. The opposite relationship to nutrient retention is observed as the sand fraction increases.

Field observations made within the upper estuary reaches depict heavily tannin stained waters and an elevated organic fraction within the sediment matrix (mangrove detrital matter). Increased nutrients within the sediments have also been reported. These findings are further supported from water quality data which demonstrate distinct reductions in dissolved oxygen and elevations in nutrients within the upper estuary.

The river entrance sediments are dominated by coarse sand and gravel (shell) fractions. The sediments within the entrance present little in the way of nutrient release potential being approximately 10 to 30 times lower concentration for nitrogen and 2-3 times lower for total phosphorus compared to the lower, mid and upper estuary reaches of the Skardon River.

17.4.14 Acid Sulphate Soils

Limited acid sulfate soils potential has been screened both onshore (Metro Mining) and within the sediments of the Skardon River.

Sampling for acid sulphate soils has been undertaken as:

a screening for marine sediments at 10 locations within the Skardon River (the same locations for

marine water quality and sediment sampling)

two core samples approximately 250 m and 2 km upstream of the proposed wharf.

Available data has been screened for acid sulfate soil under the Guidelines for Sampling and Analysis of Lowland Acid Sulfate Soils (ASS) in Queensland (QASSIT Guidelines) (QASSIT, 1998).


Page 17-23

The ten (10) surface sediment samples (river bottom and banks) were submitted for acid sulphate soil assessment, extending from the upper river reaches to the entrance and immediate offshore sediments. Results are presented in Appendix 8. Based on these samples, any marine sediments disposed to land (not proposed for the Project) may present a risk of potential acid sulphate soils (PASS) impact and risk of acid leachate generation.

The site approximately 250 m upstream of the wharf approximates the conditions which may be representative of the conditions surrounding the wharf. At both coring sites, triplicate sampling was undertaken from each location to a depth of 6.0 m. Results are presented in Appendix 8. Sediments outline a low acid sulfate soil potential. Never-the-less, where soil excavation is proposed, consideration of risks from acid sulfate materials will be incorporated into the development design.

Additional information on acid sulphate soils is presented in Chapter 10.

17.4.15 Summary

The catchment feeding the Skardon System is small in comparison to the adjacent Port Musgrave and Ducie/Wenlock River systems. Freshwater inputs are limited, with the Skardon River not demonstrating a significant floodplain. Distinct physicochemical trends are observed within the available water quality data. Dissolved oxygen maintains a reduced profile within the upper estuary due to high organic loads and limited flushing. A salinity maximum is achieved within the upper estuary during the dry season driven by increased evaporation and limited tidal flushing. During the dry season the waters of the upper estuary develop deep tannin staining and remain physically different to the mid and lower estuary areas. Several metals and nutrient analytes are reported above typical ambient concentrations, perceivably driven by a heavy organic load and geochemical profiles of the surrounding landscape.

The western Cape York region experiences a diurnal tidal pattern which can exceed 3.5 m. The entrance and lower estuary system present broad intertidal zones, exposing large areas of mud and sand bank at low tide. Thinner mud bank and intertidal zones fringe the mangrove systems as the River reduces in width and becomes channelized as it progresses back into the landscape. Currents are strongest within the lower estuary and mid estuary reaches, declining as waters extend into the upper estuary. The strongest currents are identified mid channel, with distinct bed features confirming active sediment mobilisation processes. The entrance to the Skardon River is characterised by an ebb tide bar and broad area of shallow sandy shoals. The Skardon River extends approximately 8 km from the mouth before branching into two distinct systems, north and south. These branches both continue for another 8-9 km each, terminating approximately 17 km from the river mouth.

Although influenced by freshwater flooding during the monsoonal wet season, the input from these waters drops rapidly given the very limited drainage catchment.

17.5 Modelling and Quantification of Potential Impacts

Appendix 17 provides a technical assessment of the following:

Bed levelling design and sediment movement, based on bathymetrical data, to achieve a depth of -

2.2 mLAT

Numerical modelling (comprising hydrodynamic model, wave model and sediment transport model)

of the Skardon River to understand:

potential bed levelling impact on hydrodynamics and waves

dispersion of sediment plumes resulting from the bed levelling activities

longshore sediment transport, using outputs from numerical modelling

propeller wash impacts from barging, using outputs from numerical modelling


Page 17-24

wave model to understand potential impacts from vessel wake waves

conceptual cyclone mooring design and location

comparison on environmental impacts of bed levelling compared to dredging to achieve the

required channel depth.

The information in Appendix 17 supplements the information provided in Appendix 8, and is summarised below.

17.5.1 Model Set Up

The numerical modelling undertaken has utilised a professional engineering software package MIKE21 released by the Danish Hydraulic Institute (2016 release). The hydrodynamic and spectral wave modules within the MIKE21 system provide the hydrodynamic and wave basis for the assessment. Details of the model are provided in Appendix 17, with information summarised below.

17.5.1.1 Model Extent

The model extent and bathymetry is shown in Figure 17-11.

Figure 17-11 Model Extent and Bathymetry

17.5.1.2 Model Bathymetry

High resolution hydrographic survey data from April 2015 and September 2009 have been combined (with priority given to the 2015 data) to maximise coverage of the areas of the main channel of the Skardon River and ebb bar. Lower resolution bathymetric data of the remaining offshore area was obtained from digitised navigation charts provided through MIKE C-MAP. The hydrographic survey data covered the main areas of interest (i.e. bed levelling area and Skardon River) and was the primary dataset when


Page 17-25

interpolating onto the numerical model grid (i.e. the lower resolution bathymetric data was only used where there was no higher resolution data available). The bathymetry in the areas of the Skardon River where no bathymetric data was available was inferred based on any adjacent known bathymetry, aerial photography and LiDAR data. Areas of mangroves were assessed based on aerial photography and LiDAR data and included in the model domain with inferred bathymetry.

17.5.1.3 Hydrodynamic Model

As the Skardon River and ebb bar (zone in which bed levelling is proposed) are tidally dominant environments the model was run with hydrodynamics for the baseline (i.e. no Project) and scheme (i.e. with Project activities) scenarios.

17.5.1.4 Wave Model

The wave conditions over the last 15 years were analysed and the largest wave event over this period was due to TC Oswald in January 2013. Wave heights, periods and directions were specified at the offshore boundary to represent the Cyclone Oswald event of 2013 which passed in close proximity to the study site.

17.5.1.5 Sediment Transport Model

Due to the lack of previous bed levelling at Skardon River it was not possible to calibrate the sediment transport model. A number of assumptions were made to ensure representative values (with conservative assumptions also adopted to ensure impacts are not underestimated) were applied in the sediment transport model, as described in Appendix 17.

17.5.1.6 Model Calibration

Model calibration is described in Appendix 17. The hydrodynamic model was calibrated against measured water level and current data. The modelled and measured water levels agree very well at the mouth site and well at the upstream site. Given the uncertainty associated with the bathymetry in the upstream, intertidal and mangrove areas, and the resultant uncertainty in the tidal prism of the Skardon River, the overall model calibration achieved for current speed and direction is considered to be good.

17.5.2 Bed Levelling

Bed levelling has been proposed to achieve a depth of -2.2 mLAT in a channel that is 70m wide and follows the path of maximum depth within the Skardon River based on bathymetrical data. This information has been combined with the April 2015 bathymetrical data to estimate the sediment volumes required to be bed levelled. This is shown for each bed levelling zone in Figure 17-12. The total volume estimated is 46,450 m3. The shallowest bed level along the proposed navigation channel is -1.0 below LAT (maximum depth of material to be moved/dredged is therefore 1.2m). Analysis of the preceding survey from September 2009 shows a similar volume of material, indicating that limited sediment accretion has occurred in the channel over this period. The main change in the seabed between the surveys was an offshore migration of the existing bedforms.


Page 17-26

Figure 17-12 Volume of Bed Levelling by Zone

In order for bed levelling of the Skardon River ebb bar to be feasible there must be sufficient capacity below -2.2m LAT in the area for the required volume of moved sediment to be transported into. The bed features in this area have historically been moving in an offshore direction and so it would be preferred to move material in this direction to prevent movement of the same material in subsequent bed levelling campaigns.

Figure 17-13 shows the depths and volumes of the areas below -2.2m LAT adjacent to the bed levelling zones. The total volume of the two areas adjacent to the bed levelling zones available for material to be moved into is 28,400m3. This leaves just over 18,000m3 of material which will be moved west to the edge of the ebb bar, as shown in Figure 17-14.


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Figure 17-13 Depth below -2.2mLAT for Transfer of Bed Levelled Material

Figure 17-14 Volumetric Assessment of Bed Levelled Material

17.5.2.1 Bed Levelling Zones

For the purpose of modelling bed levelling impacts, the zones in Figure 17-15 have been considered, based on different sediment sample data.


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Figure 17-15 Modelled Bed Levelling Zones

17.5.2.2 Bed Levelling Rates

Bed levelling rates used in the numerical model are based on information provided by bed levelling contractors with experience in the Gulf of Carpentaria, with assumptions as follows:

A bed levelling rate of 2,750 m3/week has been assumed. This rate assumes that the activity will be

undertaken for 7 days a week and 12 hours a day. Activities are not limited to ebb tide only but to all

periods when water depth allows a bed levelling vessel access to the bed level area.

Bed levelling will take 118 days of continuous bed levelling to complete.

All of the fine grained sediment (D50 < 63µm) in the material being moved by the bed levelling is

resuspended during the activity. This results in a rate of 0.3 kg/s at bed levelling Zones A and B to

the east (with an average composition of 1% silt) and 1.6 kg/s at Zone C (with an average

composition of 6% silt) (refer to Figure 17-15).

20% of the sand sized sediment in the material being moved by the bed levelling is resuspended

during the activity. This results in a rate of 4.2kg/s at all the sites (with an average composition of

80% sand).

The model has been run to represent the bed levelling activity for a one week period at each of the bed levelling zones. A single source point for the sediment discharge has been included for each area. This is considered to represent a worst case scenario (highest possible suspended sediment concentrations (SSC)).

The results from the discrete week long simulations have been used to produce a representation of how the plume will behave throughout the entire bed levelling activity.


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17.5.3 Hydrodynamic Model Results

17.5.3.1 Current Speeds

Due to the proposed bed levelling, modelled peak ebb currents at the time series point increase from 0.34 to 0.38m/s on the spring tide (a 12% increase in tidal current speed). Due to the proposed levelling scheme, maximum flood currents can be seen to increase from 0.19 to 0.20m/s for the spring tide modelled.

The maximum change in current speed for ebb tides is shown in Figure 17-16, which shows that:

the change in bathymetry from the bed levelling results in localised increases and decreases in

current speed in the order of 0.05m/s (10 – 15% change in speed)

increases in current speed occur to the west and east of the bed levelling zones, while the decreases

in current speed occur to the north and south of the zones and within the eastern zone (Zone A);

and

changes are very localised to the area with a change in bathymetry and do not result in any changes

outside of the ebb bar.

The maximum change in current speed for flood tides is shown in Figure 17-17. The spatial pattern of changes in tidal current speed shows that very little change to the flood currents occur due to the change in bathymetry from the bed levelling exercise.

Figure 17-16 Changes in Current Speed – Ebb Tide


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Figure 17-17 Changes in Current Speed – Flood Tide

17.5.4 Wave Modelling Results

The peak modelled estimate change in wave heights was based on wave heights during Tropical Cyclone Oswald (2013).

Figure 17-18 shows the impact that the change in bathymetry due to the bed levelling has on wave heights (as inferred for Tropical Cyclone Oswald). The results show an increase in wave height to the north of the bed levelling area of up to 0.12m and a reduction in wave height to the south of the bed levelling area of up to 0.2m. The area to the north has an increase in wave height as a direct result of refraction away from the deeper sections of the channel causing increased wave focussing. Similarly, the decrease in wave heights to the south is due to more wave energy being focussed towards the shallower banks to the north due to the greater gradient in bathymetry caused by the bed levelling. A predicted small reduction (0.02m) in wave height along the shoreline in the lee of the bed levelling areas is shown.

The wave conditions modelled should be considered as representative of a worst-case scenario for this location and, as the predicted impacts of the proposed bed levelling are small and localised for these conditions, any impacts for smaller wave events (i.e. typical conditions) are expected to be negligible.


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Figure 17-18 Change in Peak Significant Wave Height

17.5.5 Sediment Transport Model

A MIKE21 Mud Transport (MT) model was used to investigate potential impacts of the bed levelling activity to water quality. To represent the release of sediment from the seabed due to the bed levelling activity, sediment sources were included in the model at varying temporal and spatial frequencies. The locations of the sediment sources adopted in the model are shown in Figure 17-15.

Sand and silt fractions were included in the model with varying release rates for each source location based on the variable sediment composition as discussed in Section 17.4.13. Sediment was released in the model at each zone for a 1 week period (except Zone B2 where the release continued for 10 days).

Suspended sediment concentrations (SSC) at the sediment release location of the model (i.e. the area with the highest concentration) was typically less than 20mg/l at Zones A and B1, up to 40mg/l at Zone B2 and up to 300mg/l at Zone C. Zone C has a significantly higher SSC as the sediment sampling showed that this area has a higher percentage of silt in the sediment and it is the silt which results in the highest SSC. The peaks in SSC closely follow the 12 hour period when bed levelling was assumed to occur, with concentrations returning to zero in the 12 hours between bed levelling periods.

Figure 17-19 to Figure 17-24 show the 50th and 95th percentile plots for SSC (combined sand and silt concentrations) for each of the source release periods. The plots do not show an actual plume at any point in time but are duration-based plots which show statistical summaries of the plume dispersion over the calculation period. The percentile plots represent the concentration in each model grid cell for which SSC were below for either 50 or 95 percent of the simulation period.

The plots show that although local advection and diffusion of the fine grained sediment occurs at the ebb bar and in the area directly offshore a plume is not transported into the entrance of the Skardon River, or along the coastline to the north or south. The concentrations resulting from bed levelling in Zones A and B are low (less than 4mg/l) away from the source location. Higher concentrations and larger percentile extents occur due to the bed levelling at Zone C; this is due to the higher composition of fine grained material in the bed sediment in this area.


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The 50th percentile plume for Zone C does show that SSC was more than 10mg/l for 50% of the time for a 1km diameter zone directly offshore of the source location. It is important to note that the increased composition of silt sized sediment at Zone C (from 1% at Zones A and B to 6% at Zone C) is based on four sediment samples, with fine grained sediment percentages ranging from 0% to 21%. Due to the significant range of values in the limited sample set, it is possible that an erroneous sample has biased the estimated percentage of silt in this area. Accordingly, the plume modelling may be overestimating the amount of fine grained material which will be resuspended by the bed levelling activity.

The tendency for the plumes resulting from the bed levelling activity to remain outside the entrance of the Skardon River can be attributed to the dominance of the ebb flow of the river; meaning any plume that is generated through the bed levelling activity will have a tendency to move offshore with the residual flow.

The modelling has been undertaken for the initial bed levelling campaign. Maintenance bed levelling is expected to result in lower volumes of sediment movement over a shorter duration and hence impacts from maintenance bed levelling will be less than initial bed levelling.

Figure 17-19 50th Percentile Plot of SSC - Zone A


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Figure 17-20 95th Percentile Plot of SSC - Zone A

Figure 17-21 50th Percentile Plot of SSC - Zone B1 & B2


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Figure 17-22 95th Percentile Plot of SSC - Zone B1 & B2

Figure 17-23 50th Percentile Plot of SSC - Zone C


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Figure 17-24 95th Percentile Plot of SSC - Zone C

Map plots showing the spatial extent of the instantaneous SSC plume during a single daily bed levelling cycle at Zones A, B1, B2 and C are shown in Figure 17-25 to Figure 17-28. The figures show:

plume extent was limited to the area of the bed levelling activity and adjacent areas directly

offshore and inshore

for Zones A, B1 and B2 the SSC was less than 10mg/l except for the immediate area where the bed

levelling activity was being undertaken

for Zone C, a plume with SSC of up to 50mg/l occurred directly offshore of the bed levelling area,

however, the extent of the plume where concentrations reached 50mg/l was relatively small

the plume where SSC was in excess of 10mg/l extended approximately 1250 m by 1250 m.


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a) Start of bed levelling daily cycle b) 3 hours after start

c) 6 hours after start d) 9 hours after start

e) 3 hours after end of daily bed levelling f) 9 hours after end of daily bed levelling

Figure 17-25 Instantaneous SSC Plume for a Daily Cycle in Zone A



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Figure 17-26 Instantaneous SSC Plume for a Daily Cycle in Zone B1



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Figure 17-27 Instantaneous SSC Plume for a Daily Cycle in Zone B2



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Figure 17-28 Instantaneous SSC Plume for a Daily Cycle in Zone C

17.5.6 Longshore Sediment Transport

Based on the results of the numerical modelling along with the coastal processes understanding (Section 17.4), a conceptual model of the sediment transport processes at the Skardon River ebb bar has been developed. The conceptual model is shown in Figure 17-29. The conceptual model demonstrates that sediment transport on the Skardon River ebb bar is driven by two primary processes, tidal currents and waves.

Tidal currents dominate the sediment transport in the channel through the ebb bar, with the ebb dominance in the tidal current speed in this area resulting in a net transport in an offshore direction. Sand will therefore be transported as suspended load and bedload along the channel in the ebb bar until it reaches the offshore edge of the bar. Some of the sediment will then be deposited in deeper water on the offshore slope of the bar, promoting offshore growth of the bar, while some sediment will be transported in a net southerly direction due to wave action (as described below).

Waves dominate the sediment transport along the offshore edge of the ebb bar as this is where the wave energy is highest. The waves drive a net southerly longshore transport along the shoreline (estimated to be approximately 10,000m3 (Worley Parsons, 2010)) due to the dominant wave direction (west-north-west) and the shoreline orientation. This longshore drift predominantly occurs in the summer (some also occurs in the autumn) when the waves are largest. During the winter and spring very little wave action occurs as the dominant wind direction is from the east and there is no offshore swell. The wave action allows sediment to be transported along the edge of the bar (it can be over the entire bar during extreme events) allowing sediment to be transported around the river mouth by longshore transport.


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Figure 17-29 Conceptual Model of Sediment Transport at the Ebb Bar of the Skardon River

The ebb bar has built up over time in a dynamic equilibrium between the two dominant driving sediment transport mechanisms (waves and ebb dominated tidal currents). The ebb bar allows the natural net southerly longshore sediment transport of the region to bypass the Skardon River mouth. The ebb bar acts as a very large sediment sink, containing in excess of 10 Mm3 of sediment, periodically releasing sediment during wave events as part of the natural longshore drift process.

The process of bed levelling on the ebb bar will move some sediment to the offshore edge of the bar. As such, the activity is replicating a natural process which already occurs on the bar, namely the movement of the bed sediment in the channel in an offshore direction to the offshore edge of the bar due to the ebb dominance in the tidal currents. The wave modelling demonstrated that bed levelling will not influence the wave conditions offshore of the bed levelling areas and as such once the sediment has been moved to the offshore edge of the ebb bar by bed levelling it is expected to behave in the same way as sediment which has been naturally transported to this area. That is some of the material will be transported in a southerly direction by wave action, while the remainder of the material will be deposited and promote the continued offshore growth of the ebb bar (until a sufficiently large wave event occurs which can transport the material). Effectively, the bed levelling activity will be replicating the natural processes which occur on the ebb bar. Based on the modelled wave conditions offshore of the ebb bar during TC Oswald, the depth of closure for the area can be estimated to be approximately 6m below LAT. This elevation is reached 400 m offshore of the end of the bed levelling area, which is considerably further offshore than the bed levelling activity will transport any sediment. Therefore, the sediment moved by bed levelling will remain within the active zone and will not be lost from the longshore transport system.

The activity is to some extent speeding up the natural processes by moving some sediment to the edge of the ebb bar at a higher rate than it is naturally transported. However, this is not expected to impact


Page 17-41

the transport of sediment around the ebb bar or the longshore drift along the shoreline to the north or south of the ebb bar as it will not change the amount of sediment available for transport on the edge of the bar. This is because there is already a large volume of sediment on the ebb bar and the wave conditions along the edge of the ebb bar will not be impacted by the change in bathymetry due to the bed levelling.

The assessment shows that the proposed bed levelling activity is in line with the existing natural morphological evolution of the features. The local changes in bathymetry from bed levelling are not expected to result in any regional changes to the morphology of the complex bathymetry offshore of the Skardon River mouth as sediment is not being removed from the system. In addition, a rapid recovery of the natural system and morphology would be anticipated following cessation of the bed levelling activity at the close of the Project with no ongoing impacts.

17.5.7 Comparison of Bed Levelling and Dredging

17.5.7.1 Feasibility Comparison

Bed levelling of the Skardon River ebb bar is feasible as:

sufficient water depth is available for long enough periods to operate

material can be moved to areas that limit risk of refilling the channel

suitable production rates can be achieved.

Based on the typical drafts of bed levelling vessels (2 - 2.5 m) they are expected to be able to operate throughout the tide except for at low water during spring tides. Based on the measured water level data at the river mouth the duration when no bed levelling could occur is likely to be between three and four hours per day during spring tides. This would equate to approximately 140 hours over the120 day duration of the works based on the vessel working for 12 hours per day.

Typical bed levelling rates can range between 2,000 and 5,000 m3/week depending on the site conditions and the vessel. An average rate of 2,750m3 has been estimated based on information provided by a bed levelling contractor. This rate assumes the vessel would be working for 12 hours per day, 7 days a week and would mean that the total activity would take approximately 120 days to complete.

Based on this assessment bed levelling is thought to be a feasible approach to achieve the required design depths on the ebb bar of the Skardon River.

Dredging options for a trailer suction hopper dredger (TSHD) and a cutter suction dredger (CSD) have been considered to achieve the required depth of -2.2 mLAT.

Dredging of the Skardon River mouth using a TSHD would be possible if enough water depth is available for long enough periods to operate (alternatively the vessel could partner with a bed levelling operation), and if the material can be placed close to shore in the active sediment transport environment. The vessel specifications for a suitable TSHD are a minimum draft of 3m and a fully laden draft of 6.25m. Based on the measured water level data at the river mouth the duration when the vessel could access the areas requiring dredging would only occur during the upper half of the tide, while the vessel would not be able to become fully loaded as there is insufficient water depth (assuming no positive or negative surges). As such, the only way a TSHD with these specifications would be feasible would be if it was working in conjunction with a bed leveller which could act to help move the sediment to deeper areas where the TSHD could safely operate. Due to the tidal constrictions of operating a TSHD it is difficult to accurately determine the rate and associated duration of the activity, but it is likely to be three to six weeks depending on the draft and capacity of the TSHD.

The common form of a CSD is that of a rectangular pontoon, although larger cutter suction dredgers can be ships. A cutter suction dredger may be self-propelled but is more commonly dumb (non-self-


Page 17-42

propelled). Dredging only takes place with the dredger moored in some way and it involves an initial powerful cutting action with suction and pump discharge via a pipeline to barges, below water or an onshore area for land reclamation or disposal. Dredging of the Skardon River ebb bar using a CSD would be possible if the sea-state conditions can be accommodated (wind, waves, current). Typical rates for a CSD vessel would be 25,000 to 40,000m3/week and as such the dredging would be expected to take up to two weeks. This approach would allow the material to be pumped to the south of the ebb bar to replicate the natural longshore drift process.

Other options for dredging the Skardon River ebb bar could include backhoe dredging, agitation dredging or side castings. These options have not been explored but are not expected to achieve the task as well as a bed-leveller, TSHD or CSD.

17.5.7.2 Comparison of Impacts

The potential environmental impacts of bed levelling and dredging are compared below:

Potential impacts of the change in bathymetry will be similar regardless of whether bed levelling or

dredging has been adopted. This is quantified through numerical modelling above.

There is some variability between the bed levelling and dredging in terms of potential impacts to

water quality. Dredging will have a higher production rate, higher suspended sediment

concentrations (SSC) and larger plume extents during operation than bed levelling. However, the

bed levelling will take longer to complete the work causing an impact of longer duration to water

quality.

Despite the longer duration bed levelling will likely result in less impact to any marine flora or fauna

as the resultant plume would be of lower concentration, and less likely to extend within the Skardon

River or to sensitive receptors.

Neither bed levelling or dredging are expected to significantly impact the longshore drift due to the

relocation of sediment from the ebb bar. The bed levelling could replicate the natural process

whereby sediment is moved to the offshore edge of the ebb bar by tidal currents (refer to Section

17.5.6), while dredgers could relocate material to the south, thereby replicating the natural

longshore drift.

Neither bed levelling or dredging are expected to not have significant impact on the environment. Of the two options, bed levelling potentially has the lesser impact due to lower SSC and smaller plume extent during the activity.

17.5.8 Propeller Wash

Modelling and calculations have been undertaken to:

predict bed current speeds and shear stresses resulting from propeller wash due to barge

operations in the Skardon River and Panamax class cargo vessels at the transhipment location

estimate potential erosion rates and associated suspended sediment concentrations due to the

propeller wash.

17.5.8.1 Current Speeds

Currents resulting from the barge propellers have been calculated at four sites through the river, these locations are shown along with the water depth at high water in Figure 17-30.The hydrodynamic model was used to estimate peak flood and ebb tidal currents in the Skardon River.


Page 17-43

Figure 17-30 Water Depth and Propeller Wash Calculation Locations


Page 17-44

A summary of the existing tidal currents at the four sites is provided in Table 17-6, which shows

existing bed sediment types at the propeller wash sites, based on the sediment sample locations

and the modelled tidal currents.

calculated critical erosion threshold of the bed sediment, estimated based on Van Rijn (1993).

average and maximum bed shear stress (BSS) resulting from the tidal currents over a modelled 29

day lunar cycle at the sites derived from the hydrodynamic model).

The model extent is not sufficient to provide current speeds at the transhipment site.

The modelled bed shear stresses indicate that it is only at the Mid site where any resuspension of bed material is expected to occur, and this would only occur during large spring tides with the highest tidal currents. However, it is expected that a thin mobile layer of loosely consolidated sediment will be present at some locations within the river (especially in the lower energy upstream areas); this material will have a relatively low critical erosion threshold and would therefore be expected to be regularly resuspended. Such materials will mobilise through the system in a continual flux, losing some material from the system where deposition occurs and recovering material to the system where erosion occurs. Turbidity loggers deployed within the upper estuary have recorded this process where ambient tidal forces mobilise these materials during the tidal period, with concentrations exceeding 50 ntu.

Table 17-6 Existing Tidal Currents

Parameter / Location

Barge Mid Mouth Ebb Bar Offshore

Depth (m MSL) 7.0 6.5 12.0 4.5 14.0

Average Speed (m/s)

0.18 0.28 0.41 0.11 -

Median Speed (m/s)

0.17 0.26 0.38 0.08 -

75th Percentile Speed (m/s)

0.24 0.38 0.57 0.14 -

95th Percentile Speed (m/s)

0.36 0.57 0.82 0.30 -

Maximum Speed (m/s)

0.48 0.74 1.06 0.52 -

Assumed Sediment type

Sandy Mud Sandy Mud Coarse Sand Medium Sand Medium Sand

Critical Erosion Threshold for Suspension (N/m2)

0.5 0.5 13.95 5 5

Average BSS (N/m2)

0.04 0.10 0.18 0.02 -

Maximum BSS (N/m2)

0.23 0.54 0.94 0.36 -


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17.5.8.2 Calculation Approach

As described in Appendix 17, the approach derived by Maynord (2000) was the most suitable for the propeller wash assessment as it allows the impacts of multiple propellers to be included. As the proposed barges have three propellers it is therefore important to ensure the potential impacts of all the propellers are taken into consideration. In addition, this approach is also widely regarded as the most reliable.

17.5.8.2.1 Barges

The near bed current speeds resulting from the barge propellers have been calculated at two different water levels which represent:

the minimum water level that fully laden vessels can safely navigate the River and ebb bar

the mean high high water (MHHW) level relative to the Skardon Barge Ramp.

Calculations have been made assuming the barge is fully laden. For an empty vessel the currents at the bed from the propellers would be lower due to the reduced draft. Assuming a fully laden vessel is therefore considered the worst case scenario.

17.5.8.2.2 Bulk Vessels

The near bed current speeds resulting from the Panamax cargo vessel propeller have been calculated for two different propeller depths which represent:

a fully laden vessel moving away from the transhipment area, assuming a minimum underkeel

clearance of 2m; and

an unladen vessel arriving at the transhipment area, assuming the same water depth as for the fully

laden vessel.

The currents from the propellers have been calculated at a single site to represent the transhipment area.

17.5.8.3 Predicted Impacts

17.5.8.3.1 Barges

The predicted maximum velocity which occurs adjacent to each of the three propellers of the proposed barges is 3.75m/s. As the jet travels away from the propeller its diameter increases, while its velocity decreases. As such, the jet has the potential to reach the sea bed (depending on the water depth) and therefore to result in increased bed shear stresses and potential erosion.

The predicted near bed current speed, bed shear stress and erosion rate resulting from the barge propellers are shown for the four sites in Appendix 17.

The predicted distance behind the propeller where erosion is likely to occur as well as the predicted peak erosion rates are shown in Table 17-7.

Table 17-7 Predicted Erosion Extent and Rates

Site Erosion Extent (distance from propeller (m)) Peak Erosion Rate (kg/s/m2) Assumed Critical Erosion Threshold for Suspension (N/m2)

MSL MHHW MSL MHHW

Barge 150 0 0.0001 0 0.5

Mid 170 0 0.0004 0 0.5


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Site Erosion Extent (distance from propeller (m)) Peak Erosion Rate (kg/s/m2) Assumed Critical Erosion Threshold for Suspension (N/m2)

MSL MHHW MSL MHHW

Mouth 0 0 0 0 13.95

Ebb Bar 180 0 0.005 0 5

Table 17-7 shows that at the Barge, Mid and Ebb Bar sites erosion is expected for between 150 and 180m behind the propeller at MSL, while no erosion is predicted at MHHW due to the additional 1.3m water depth. The erosion rates are variable, with the rates at the ebb bar being more than an order of magnitude higher than at the other two sites. The propeller jet does not result in any erosion at the Mouth site due to the existing depth at the site.

The barge is assumed to be travelling at 4 to 6 knots (2.1 to 3.1m/s), therefore it will travel between 120 and 180m in 60 seconds. As such, at the Barge, Mid and Ebb Bar sites, erosion will occur at a single point on the bed for approximately 60 seconds as the vessel passes. The barges will sail down the centre line of the channel and therefore the erosion will be limited to the middle of the channel.

A schematic prediction of the spatial distribution of bed current speeds resulting from the barge propeller wash at the barge site (i.e. the wharf area) is shown in Figure 17-31. Based on model outputs, erosion of the bed occurs when the current speed exceeds approximately 0.5m/s at this location, indicating that erosion would be expected to occur for a distance of approximately 125m behind the vessel over a width equal to the vessel width (20m). As such, the predicted area of erosion outside of the proposed navigational channel and berth area at the wharf, which occurs when a fully laden barge moves away from the wharf at mean sea level, is shown in Figure 17-32. The area is mainly focused around the old barge ramp where the bathymetric data from 2015 shows a number of scour holes are already present. As the predicted erosion zone is adjacent to the old barge ramp the scour holes are thought to be from previous propeller wash erosion during operation of the ramp.


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Figure 17-31 Predicted Spatial Pattern of Near Bed Current Speeds

Figure 17-32 Predicted Area of Erosion at Wharf (Red Rectangle)


Page 17-48

Based on the spatial extent of the erosion it is possible to predict the mass of sediment eroded per metre of the vessel path, this is shown in Table 17-8. In addition, it is assumed that because of the high currents and associated turbulence caused by the propeller jet any sediment suspended would be rapidly mixed with the surrounding water and therefore the SSC resulting from the erosion can be predicted. It is assumed that any suspended sediment will be entrained into the entire water column extending to the full width of the barge. Based on this, the predicted SSC resulting from the erosion is also detailed in Table 17-8.

Table 17-8 Predicted Mass of Sediment Eroded and Resultant SSC

Site Average Erosion Rate (kg/s/m2)

Mass of Sediment Eroded per metre of vessel track (kg)

Predicted SSC resulting from the erosion per metre of vessel track (mg/L)

MSL MHHW MSL MHHW MSL MHHW

Barge 0.0001 0 0.15 0 1.5 0

Mid 0.0003 0 0.45 0 2.8 0

Mouth 0 0 0 0 0 0

Ebb Bar 0.003 0 4.5 0 40 0

The results show that SSCs of less than 3 mg/l are expected at the Barge and Mid sites due to the propeller wash erosion, while at the Ebb Bar site they are 40mg/l. The higher SSCs at the Ebb Bar site will not occur for long as the sediment is predominantly made up of sand and as such will quickly settle out (medium sized sand settles at 0.03m/s (1.8m/minute)) once the barge has passed.

When interpreting the propeller wash results it is important to consider the following:

Erosion resulting from the propeller wash of the barge is expected to predominantly be within the

centre of the navigation channel (predicted maximum width of approximately 20m). The only area

where an impact outside of the navigation channel could occur is adjacent to the wharf when the

barges manoeuvre onto and off the berths;

It is expected that in the upstream areas there will be highly consolidated sediment under the softer

surface layer assumed in this assessment. The consolidated sediment would be expected to have

critical erosion thresholds in excess of 1N/m2 and as such any erosion resulting from the propeller

jet would be significantly reduced once the softer surface sediment has been eroded.

Along the ebb bar where the highest bed shear stresses are predicted due to the propeller wash it is

likely that as the sand sized sediment is eroded by the propeller wash the bed would become

armoured as coarser gravel sized sediment is left (material on the ebb bar is up to 25% gravel). This

would protect the bed from future erosion as the critical erosion threshold for resuspension of

granule sized gravel is approximately 50 N/m2.

Resuspension of existing bed material due to propeller wash at MHHW is not predicted.

Bathymetric surveys indicate that the channel in the ebb bar has remained relatively stable with

limited accretion occurring. Based on this and the predicted propeller wash resulting from the

barges over the ebb bar, ongoing maintenance of the -2.2m LAT design depth over the ebb bar is

expected to be low. When the barges are operating it is expected that no maintenance will be

required. However, accretion could occur during the summer months when no operations are

planned.


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Vessels will navigate along the centreline of the proposed navigation channel wherever possible to

restrict any erosion of the bed due to propeller wash to this area.

17.5.8.3.2 Bulk Vessels

The predicted maximum velocity which occurs adjacent to the propeller of an example Panamax vessel is 6.5m/s. As the jet travels away from the propeller its diameter increases, while its velocity decreases. As such, the jet has the potential to reach the sea bed in the transhipment area and therefore to result in increased bed shear stresses and potential erosion.

The predicted near bed current speed, bed shear stress and erosion rate resulting from the Panamax vessel at the transhipment area are shown in Appendix 17. This show that bed current speeds of up to 1.5m/s occur when the vessel is fully laden, while when the vessel is unladen bed current speeds are reduced to a maximum of 1.07m/s. The bed shear stresses resulting from the propeller induced currents have the potential to result in erosion for a distance of up to 250m from the propeller when the vessel is laden, while no erosion is predicted when the vessel is unladen.

The sediment composition at the transhipment site is made up of approximately 15% gravels, 25% coarse sand, 50% fine to medium sand and 10% silt and clay. The silt, clay and fine to medium sand are all predicted to be resuspended by the maximum bed current speeds from the propeller wash, while sediment coarser than this (40% of the bed sediment) will only be transported short distances as bedload. As such, it is expected that the fine grained sediment will be eroded from the bed in the area impacted by the propeller wash, while the coarser grained sediment will remain with local bedload transport occurring. It is therefore expected that over time the sea bed will become armoured with coarser sediment protecting the sediment below from further erosion.

The process of erosion from propeller wash will lead to periodic short term increases in turbidity (every 4-5 days). Benthic communities would also be disturbed to some extent either by deposition or winnowing of finer sediments. This is further assessed in Chapter 18.

17.5.9 Vessel Wake Waves

Appendix 17 describes the types of waves generated by vessel movement – primary waves and secondary waves.

The vessel speed is a critical parameter when calculating vessel wake waves. The barges are modelled as travelling at speeds of up to 6 knots along the channel.

The cross-sectional area of the vessel, which is required to calculate the primary wave, was calculated assuming that the vessel was fully laden (i.e. maximum draft). To represent the worst case scenario when calculating the primary wave the section of the navigation channel which lies within the smallest river channel cross-sectional area was used. This area is located approximately 300 m north of the proposed wharf and has a cross-sectional area of 1,100m2 relative to mean sea level.

It has previously been demonstrated that the theoretical effects of a ship transiting a channel can be derived from the Bernoulli equation (Schiereck, 2001). Calculated wave conditions based on this approach have previously been shown to correlate well with field-measurements (Moffatt & Nichol, 2003). Based on this approach a primary wave height of 0.12m has been predicted. Given the relatively small magnitude of this value, primary waves from the proposed barges are not expected to result in any significant erosion within the Skardon River.

To calculate the secondary waves (heights of the cusps) an approach detailed in a PIANC working group report on canal design was adopted (PIANC, 1987). The wave heights are closely linked to the vessel speed and the water depth, with wave heights increasing with increasing speed and decreasing with increasing depth.


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The secondary wave heights for the proposed barge, travelling at a speed of 6 knots, at a range of water depths which represent the possible depths when barges will be navigating the Skardon River are shown in Figure 17-33. It is important to note that the shallowest depth of 4.5m is only present at the ebb bar, where offshore waves will also occur. Depths of 6m and deeper are representative of the area within the Skardon River, where wave heights adjacent to the vessel are less than 0.2m. The plot shows that although wave heights of up to 0.25m can occur directly adjacent to the vessel, the heights are less than 0.07m at 50m away from the vessel sailing line and less than 0.05m, 150m away from the vessel sailing line.

There are two locations within the Skardon River where the navigation channel is relatively close to the channel bank:

Upstream close to the proposed wharf the centreline of the channel is 80m away from the west

bank of the river. This area would have a minimum depth of approximately 6m when the barge

could navigate the entire river, and so the wave height at the bank is predicted to be less than

0.05m. This size wave is not expected to result in erosion of the bank, especially considering the

stability and vegetated nature of the bank at present. In addition, due to the reduced vessel speed

when berthing there is not expected to be any erosion in the vicinity of the wharf due to the vessel

wake waves during this activity.

The mouth of the Skardon River is relatively constrained and as a result the centreline of the

channel is 100m away from the south bank of the river. This is a deep section of the river with a

minimum depth of close to 12m when the barge could navigate the entire river, and so the wave

height at the bank is predicted to be less than 0.03m. This size wave is not expected to result in

erosion of the bank.

The small wave height resulting from the vessel wakes is primarily a result of the relatively low vessel speeds adopted for navigation of the Skardon River. If a theoretical maximum barge speed of 9 knots was adopted then the resultant waves for a depth of 4.5m would be 1.2m adjacent to the barge and 0.33m at 50m away from the barge. Accordingly, 6 knots has been adopted as the maximum speed within the Skardon River to ensure no impacts from vessel wake waves, unless it can be demonstrated that increased speeds to not result in vessel wake wave erosion of the shoreline.


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Figure 17-33 Secondary Wave Heights

17.5.10 Cyclone Moorings

As Skardon River is prone to tropical cyclones it is necessary for cyclone moorings to be available for the vessels operating in the river as part of the Bauxite Project. As such, concept designs for cyclone moorings for two barges and one supply vessel are provided in Appendix 17. A sketch drawing of possible storm moorings for the maritime fleet to be sheltered in the Skardon River during tropical cyclones is shown Appendix 17.

The moorings are assumed to be located in a long narrow waterway; possible locations for the moorings in the Skardon River are shown in Figure 17-34. Additional site specific investigations, including hydrographic survey and benthic assessment, will be undertaken prior to the location being confirmed. The moorings are configured as fore-and-aft so as to reduce movement of vessels and hence the heavy ground chains, minimising damage to the seabed and any benthic flora and fauna.

Gulf Alumina will design, construct and locate cyclone moorings such that impacts to the marine environment are minimised, without compromising the safety function of the cyclone moorings. Proposed cyclone mooring locations will avoid offshore reef habitats or any other sensitive marine areas, marine species and/or MNES. Cyclone moorings will be operated such that impacts on reefs and/or seagrass due to chain drag are avoided.

Cyclone moorings, irrespective of their location, are unlikely to create a significant impact on the marine environment, with safety of mooring vessels being the primary consideration in their design and location.

The Code for self-assessable development Minor impact works involving the removal, destruction or damage of marine plants MP06 is relevant to the installation of navigation aids (e.g. channel markers) and cyclone moorings outside of the mining lease, which are listed as activities subject to the Code. For activities outside the mining lease such as navigation aids and cyclone moorings, the proponent will comply with the requirements of the Code for self-assessable development Minor impact works involving the removal, destruction or damage of marine plants MP06.


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Figure 17-34 Potential Cyclone Mooring Locations

17.6 Potential Impacts

The potential impacts on coastal processes and the physical marine environment are a function of:

Port construction

bed levelling

barging / shipping operations within the Skardon River and near shore

offshore transhipment of bauxite

bulk vessel movements

changes to water quality as a result of mining and Port activities

17.6.1 Port Construction

The selection of piling construction over infill construction or causeway methods for wharf infrastructure presents a significant impact minimisation measure as:

short term and long term impacts associated with permanent habitat loss are significantly reduced

hydrological regimes are not impacted to any significant extent

interaction with acid sulphate soils and potential acid sulphate soils is minimised

the open design structure will also allow the passage of tidal waters and seasonal flood flows

passage of fauna residing in and adjacent to the mangrove habitats will not be substantially

constrained

the fisheries resource values of the Project area are maintained.

No dredging is proposed in the Port area for loading export barges or handling supply vessels. Depths at the proposed wharf facility are sufficient, and deepening of the barge berthing pocket is not required. This design measure results in significantly less disturbance to the marine environment at the Port.


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17.6.1.1 Coastal Processes

Minor changes in current flow will result from a reduction in channel cross-sectional area due to pile construction. The reduction in cross-sectional area of this reach of the river has been estimated in the order of ~1.0%, and as such the change in current flow due to this facility is unlikely to be significant. This estimate has been based on a river width of ~270 m, average depth of ~4 m and reduction in cross-sectional area by piles of approximately 10 m2.

The wharf facility may also generate water turbulence. In addition to turbulence in the water column, the near bed turbulence may result in a localised scour around the base of the piles, the extent of which will be determined based on particle size distribution and underlying geology. Sediment samples from the adjacent area indicate a dominance in sand and gravel fractions of around 70 - 80%. Given this distribution, a minor quantity of silts/clays and finer grained sands may be subject to mobilisation following construction. Over time, sediments surrounding the base of the piles will establish an equilibrium dominated by the fractions which are less prone to dispersion. Alternatively, an underlying consolidated layer may be exposed and prevent scour.

Based on side scan imagery of existing mooring blocks in the Skardon River (refer Appendix 8), scour zones from the piles are expected to extend in the order of 2m surrounding the base of each pile.

When utilising the wharf the barges themselves may also induce altered current speed and turbulence (depending on their loaded draft) and may lead to localised erosion of the finer grained sediments within the berth pocket. Increased current velocities surrounding the barge hull are possible. Given a channel width of ~270 m and average depth of ~4 m the channel presents a cross-sectional area of ~800 m2. The proposed barge would present a reduction of approximately 100 m2 given a beam of 25 m and draft of 4 m. This represents a reduction in cross-sectional area of ~9%. The increased currents surrounding the hull will generate a turbulence similar to that experienced for the pile construction, though the presence will not be continuous or static given the range in barge draft between empty and fully loaded, and tidal height variations. The berth will also be empty for periods between barge loading. Overtime these processes would establish a new equilibrium state for sediments directly beneath and adjacent to the barge loading berth pockets.

Impacts from Port development on coastal processes will be small and restricted to the areas directly adjacent to the structures and, as the area already has marine port facilities present, the impacts on coastal processes from wharf construction are not considered to be significant.

17.6.1.2 Water Quality

Water quality impacts are likely to be encountered during construction processes over a limited construction period (2-4 months). The proposed wharf construction, including conveyor alignment across mangrove communities and intertidal areas, will impact water quality over the short term during piling and construction vessel support. Supply, construction barges and support vessel movements will also induce propeller wash in shallow waters. Propeller wash has the potential to induce localised sediment erosion, entrainment of these materials within the water column and propagation of turbid water plumes.

Development of the wharf structure, dolphins for mooring and conveyor alignment will be based on pile construction methods. Some limited shoreline revetment, above the tidal zone, is proposed in completing the abutments joining the wharf and conveyor to the shoreline. The adjacent barge ramp will also be used to provide access for material and equipment supply. No dredging or excavation of sediments is proposed within the Port area.

Given the proposed construction methodologies, turbid water generation will be limited as far as practicable. Impact will be localised to the immediate construction area. While available water quality data (mid-stream) indicates a relatively low surface turbidity regime during the dry season (mean 5 ntu), preliminary PAR and deployed logger data have described regular natural processes of sediment mobilisation driven by tidal forces exceeding these concentrations. The deployed logger locations have


Page 17-54

identified regular turbidity increases within the mid and upper estuary beyond 50 to 300 ntu (within the bottom column).

Adjacent mangrove and disturbed shorelines of the existing development footprint within the Skardon River are unlikely to be sensitive to increases in turbidity, reduction in light and minor increased deposition which may accompany these construction processes. The most sensitive receiving habitats are seagrasses located on the opposite river bank (~230m) and upstream of the proposed works along the southern bank (~500m). Should turbid waters be released during construction, currents would disperse suspended sediments predominately along the southern shoreline. Given ambient currents of 0.2 to 0.3m/sec sediment plumes may intersect seagrass habitats within 30-40 minutes of the flood tide. Given the strong directional currents within the Skardon River, water quality along the northern shoreline is not considered at risk during construction processes.

Propeller wash from construction and support vessels would also present the potential for water quality impacts. However, unlike the proposed bauxite barges, these vessels would present substantially smaller drafts and lower engine horsepower. The effects of propeller wash erosion and dispersion of turbidity plumes would be of lower scale. The small scale, periodic disturbances due to propeller wash is not considered a significant risk to water quality during the construction phase.


The bed levelling activity and modelled impacts from bed levelling are described in Section 17.5.1 to Section 17.5.7, which demonstrates:

Bed levelling is a feasible option for achieving the required design depths, for a predicted duration

of 120 days. Dredging options for a TSHD or CSD are feasible although there are potential operability

issues as a TSHD will likely have to combine with a bed leveller and a CSD requires stable sea

conditions. Neither bed levelling or dredging are expected to have significant impact on the

environment. Bed levelling is expected to result in similar, or lesser, environmental impacts than

dredging due to lower SSC and smaller plume extent during the activity.

The change in bathymetry of the ebb bar is predicted to result in relatively small and localised

changes to tidal currents and waves. The changes are restricted to the ebb bar and do not influence

the Skardon River.

The bed levelling activity is predicted to result in a relatively small, localised plume with peak SSCs of

up to 40mg/l during the majority of the bed levelling activity (14 weeks).

Increased plume extents and SSCs are predicted during bed levelling of the area adjacent to the

offshore edge of the ebb bar (furthest west) due to the increased amount of fine grained material

(silt and clay) in the sediment in this area (6% compared to 1% at the other areas). Bed levelling of

this area is expected to continue for approximately three weeks.

The plume extent during bed levelling is limited to the area directly offshore of the ebb bar. The

plume does not extend within the Skardon River or along the shoreline to the north or south of the

ebb bar.

Peak SSC at the location of the bed levelling activity can reach up to 300mg/l, although the median

SSC offshore of the ebb bar is less than 25mg/l over the duration of the bed levelling.

The bed levelling activity is not expected to result in any impacts to the longshore sediment

transport at the shoreline to the south or around the ebb bar adjacent to the Skardon River.

Following the initial bed levelling campaign (assessed in this report) any subsequent bed levelling

maintenance activity would be expected to have similar or lesser impacts to those for the initial bed

levelling campaign.


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Siltation in the bed levelled areas above the required -2.2m LAT level is expected to be relatively slow during calm conditions in the dry season, but could be rapid during extreme cyclone and monsoon storm events in the wet season. Therefore, proposed annual maintenance bed levelling after the wet season is likely to be required.

It is anticipated that the natural tidal flows and seasonal processes of flood flows and episodic cyclone/monsoon storm activity will act to restore pre-development conditions once mining operations have ceased.

The nearest potential sensitive habitat location (nearshore coral reef) is situated approximately 6000 -7000 m south of the outer ebb bar (refer to Chapter 18). The maximum extent of dispersion (>1mg/L) during bed levelling within Zone C (finest grained sediment) does not surpass 2000 – 2500 m at the plotted 95th percentile (Figure 17-24) (i.e. 95% of the time the extent will be less than this). The 50th percentile plot extends to 1,500 m marginally above 1mg/L. At its predicted maximum extent (95th percentile) the bed levelling plume will remain 3,000 to 4,000 m north of the known nearshore reef habitats.

17.6.3 Offshore Transhipment Area and Bulk Vessels

The offshore transhipment area location was selected on the basis of benthic habitat and sediment surveys which identified very low density benthic communities and sediments that are sand dominant. This location will minimise impacts to marine ecology from offshore anchoring and bauxite transhipment.

Offshore transhipment of bauxite from barges to bulk vessels will not involve any permanent structures in the marine environment and therefore there are expected to be no impacts on coastal processes and the physical marine environment from structures. It is expected that the fine grained sediment in the offshore transhipment area will be eroded from the bed in the area impacted by the bulk vessel propeller wash, while the coarser grained sediment will remain with local bedload transport occurring. It is therefore expected that over time the sea bed will become armoured with coarser sediment protecting the sediment below from further erosion.

Further information about the potential bulk vessel impacts on benthic habitat, marine ecology and Commonwealth marine waters is provided in Chapter 18..

17.6.4 Barging –Vessel Wake Waves

Modelled and calculated impacts from vessel wake waves are described in Section 17.5.9.

Based on the proposed vessel speed within the Skardon River of 6 knots the wake waves predicted to be generated by vessels are small when they reach the shoreline. These waves are therefore not predicted to impact the banks of the Skardon River.

Appendix describes various studies which provide a range of various management criteria related to vessel generated wake waves. The criteria adopted in these studies is that no management action is required where wave height within 25 m of the sailing line is < 0.2m. Potential maximum wave heights reaching the nearest shores (80 m to 100 m) from the proposed navigation line are estimated between 0.07 and 0.05m, well below the adopted threshold of 0.2m.

In addition to defining potential wake wave heights, the frequency of their occurrence also requires consideration. Given the proposed barging program approximately 8 barge passages may be expected per day (4 out laden and 4 return unladen). This would equate to a passage every 3 – 4 hours. The predicted wake waves are considered both small, and infrequent.

As mangroves are present along the majority of the banks of the Skardon River any vessel wake waves (not predicted) are expected to be attenuated by the established mangrove vegetation.


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17.6.5 Barging – Propeller Wash

Modelled and calculated impacts from propeller wash are described in Section 17.5.8.

Resuspension of existing bed material due to propeller wash at mean high high water (MHHW), when barges will be operating, is not predicted. Resuspension of existing bed material in the mouth at all tidal levels when barging occurs (i.e. mean seal level (MSL) is not predicted.

Barging at MSL (when vessels fully laden) is has been shown to have the potential to result in erosion of the bed in the proposed navigation channel in the mid, upstream and ebb bar areas of the Skardon River. The results show that SSCs of less than 3 mg/l are expected at the barge (i.e. near the wharf) and mid (i.e. mid Skardon River) sites due to the propeller wash erosion, while at the ebb bar (bed levelling zone) site they are 40 mg/l. The higher SSCs at the ebb bar site will not occur for long as the sediment is predominantly made up of sand and as such will quickly settle out (medium sized sand settles at 0.03 m/s (1.8 m/minute)) once the barge has passed.

Measured mean dry season surface column turbidity was recorded at ~ 4 ntu, and total suspended solids concentration between 10 mg/L to 1 mg/L (overall site means recorded during vessel survey ranged between ~2 mg/L to 5 mg/L). These dry season TSS concentration and associated turbidity measures, appear to be at or above the predicted modelled propeller wash outputs of 0 to 2.8mg/L (within the River). Given these findings, it is considered unlikely that the barge propeller wash impact would substantially elevate turbidity to levels significantly above ambient concentrations within the Skardon River itself.

At MSL (when vessels fully laden), at the barge, mid and ebb bar sites, erosion will occur at a single point on the bed for approximately 60 seconds as the vessel passes. The barges will sail down the centre line of the channel and therefore the erosion will be limited to the middle of the channel. Due to the proposed propeller layout of the barges (central propeller with additional propellers 10m either side) erosion resulting from the propeller wash of the barge is expected to predominantly be within the centre of the navigation channel (predicted maximum width of approximately 20m).

The passage of barges every 3-4 hours, and an erosion influence in any given area of approximately 60 seconds, can be best described as an episodic short term impact. Whilst expanded spatial data surrounding time series turbidity within the Skardon River has yet to be undertaken (proposed for pre -construction baseline monitoring), it is inferred that existing near bed and mid column turbidity would naturally exceed the recorded ambient surface concentrations. Further, the predicted through column SSC generated during barge passage reflects minimum operating depth conditions, and as such, a conservative over estimate of potential suspension processes. Based on the modelled predictions, increasing water depth to high water alleviates the impact of sediment mobilisation within the River entirely.

it is expected that in the upstream areas (barge and mid sites) there will be highly consolidated sediment under the softer surface layer assumed in this assessment. The consolidated sediment would be expected to have critical erosion thresholds in excess of 1N/m2 and as such any erosion resulting from the propeller jet would be significantly reduced once the softer surface sediment has been eroded.

Along the ebb bar where the highest bed shear stresses are predicted due to the propeller wash it is likely that as the sand sized sediment is eroded by the propeller wash the bed would become armoured as coarser gravel sized sediment is left (material on the ebb bar is up to 25% gravel). This would protect the bed from future erosion as the critical erosion threshold for resuspension of granule sized gravel is approximately 50 N/m2.

It is recognised that for fine fraction sediments, the velocities required to retain material in suspension is lower than that required to initially suspend them. The mobilised materials will disperse within the Skardon River generally along the lines of peak current flow. Given then channelized nature of the river, these materials are predicted, in the main, to remain central to the channel alignment. Due to the dominant ebb tide flow, suspended materials are likely to continue mobilisation downstream. Depending


Page 17-57

upon the depth of unconsolidated sediments, the continued process of vessel operation within the channel may act to preferentially sort coarse fractions along the barge path. This may lead to armouring. Alternatively, sediments may remain open to mobilisation, or consolidated substrates (clays, rock, gravel etc) may be encountered.

The screening of sediments against the NAGD (2009) guideline criteria indicates a limited potential for the release of contaminants (metals) to the water column during resuspension processes.

17.6.6 Port Sediment Ponds

The design and management of Port sediment ponds is described in Chapter 6.

This section describes the ‘end of pipe contaminant’ release limits for the Port sediment ponds from the overflow weirs.

Water quality and ecological monitoring within the Skardon River (the receiving environment) is described in Section 17.8 and Chapter 18 respectively.

Release points from Port area sediment ponds will be located at the downslope end of these structures, for which coordinates are provided in Table 17-9, based on the conceptual Port area design.

The existing EA nominates release point W2 at the Port as the ‘discharge point from stormwater drains on the bank of the Skardon River’. This EIS replaces this monitoring location with monitoring of the release point from the existing sediment pond, which more accurately reflects existing and proposed water management at the Port area.

Release points have been situated as far away from Skardon River as possible given the constraints of collecting runoff downstream of Port infrastructure, above the Skardon River flood zone, and with 100 m to 200 m separation distance to the mangroves and Skardon River.

Table 17-9 Release Points – Port Area Sediment Ponds

Release Point Reference per existing EA

Easting Northing Monitoring Frequency

S13 – Release point from the existing Port sediment pond

W2 - Discharge point from stormwater drains on the bank of the Skardon River

616718 8699703 Within 24 hours of any discharge from S13 (formerly W2) and thereafter daily whilst discharging for physico-chemical parameters in Table 17-10.

Monthly for nutrients, metals and hydrocarbons as per Table 17-10.

S14 - Proposed sediment pond – Option 2

n/a 616520 8700248 Within 24 hours of discharge and thereafter daily whilst discharging for physico-chemical parameters in Table 17-10.

Monthly for nutrients, metals and hydrocarbons as per Table 17-10.

The monitoring parameters and proposed release limits for Port sediment ponds are provided in Table 17-10. These contaminant release limits will be updated as additional baseline monitoring data is collected in order to finalise site water quality objectives. The release characteristics of sediment ponds will be reviewed once operational to ensure water quality data is collected during periods of high risk.


Page 17-58

Table 17-10 Release Contaminant Limits – Port Sediment Ponds

Parameter Units Release Limit

Basis Frequency

Turbidity NTU 54.3 90th percentile wet season site data

With 24 hours of release, then daily

Electrical Conductivity µS/cm n/a n/a With 24 hours of release, then daily

pH ph units

7 - 8.5 AWQG – tropical estuaries


Dissolved oxygen % 80-120 AWQG – tropical estuaries


Total nitrogen µg/L 1000 Site data – 80th percentile

Monthly

Total phosphorus µg/L 60 Site data – 80th percentile

Monthly

Total suspended solids mg/L 50 Engineering design standard

Monthly

Aluminium µg/L 220 Site data 80th percentile Monthly

Arsenic µg/L 92 Site data 80th percentile Monthly

Cadmium µg/L 0.7 AWQG – marine 99% trigger

Monthly

Chromium µg/L 4.9 Site data 80th percentile Monthly

Copper µg/L 3.9 Site data 80th percentile Monthly

Iron µg/L 274 Site data 80th percentile Monthly

Lead µg/L 2.5 Site data 80th percentile Monthly

Manganese µg/L n/a No AWQG value Monthly

Mercury µg/L 0.1 AWQG – marine 99% trigger

Monthly

Nickel µg/L 12.5 Site data 80th percentile Monthly

Vanadium µg/L 50 AWQG – marine 99% trigger

Monthly

Zinc µg/L 38 Site data 80th percentile Monthly

Petroleum hydrocarbons (C6-C9)

µg/L 20 Model Mining Conditions

Monthly

Petroleum hydrocarbons (C10-C36)

µg/L 100 Model Mining Conditions

Monthly

Oil or grease n/a No visible film or detectable odour to Skardon River


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17.6.7 Release of Contaminants

Impacts to water quality from Port construction, bed levelling, propeller wash and vessel wake waves are described above.

Works undertaken on, over or adjacent to the Skardon River have the potential to release chemicals to the marine environment. This may occur during handling and transport or during operational and maintenance use. Chemicals used at or transited through the Port area may include hydrocarbon fuels, paints, herbicides/pesticides, coolants, oils, waste oils, greases, hydraulic fluids, acids, bases, degreasers and detergents.

Although the potential impact for chemical release and contamination is significant (particularly bulk material release) the likelihood of any substantive impact is considered very low with the implementation of design and controls to Australian standards. A program of monitoring and environmental auditing will be undertaken to ensure water quality and the systems designed to protect it are maintained throughout operations.

All vessel based sewage will be transferred to dedicated on-shore facilities. No disposal of sewage or bilge will be undertaken within the marine environment.

17.6.8 Sediment

Although sediments sampled vary, the sand and gravel fraction appears to dominate the channel areas throughout the Skardon River. Mangrove and bank environments may possess greater silt and clay fractions as materials deposit along these shorelines. However, these environments will not be open to significant perturbations beyond the initial construction period at the wharf facility.

Currents within the river channel are relatively high as can be witnessed by sand wave ripples present throughout the river system.

Sediment sampling central to the proposed navigation channel was limited to the bed levelling area. Samples adopted within the Skardon River (mouth to barge loading facility) for assessment reflect shoreline sediment conditions, not exposed to the ambient current velocities predicted by hydrodynamic modelling. Consequently, estimates of particle size distribution, metals and nutrient concentrations are considered conservative, with these sediments not having been exposed to strong tidal currents and ambient shear stresses. Review of bathymetric survey data (multibeam sonar) has identified physical evidence of active sediment sorting and mobilisation along the proposed channel alignment. Features such as ripples, sand waves, isolated rock and broad rocky patches are indicative of active sediment migration processes.

Modelling indicates that sediments within the channel (barge area, mid estuary and ebb bar crossing) will be exposed to short lived episodic erosive forces during the minimum barge operating depth periods. As water levels increase, these forces are reduced, and erosion processes are not predicted. Scour is also predicted at the proposed transhipment area for laden vessels leaving the area.

Over time, regular sediment disturbance via propeller wash may:

act to sort sediments and potentially armour the channel alignment (increase coarse fraction)

continue erosive processes until bed shear stresses reduce sufficiently (in deep unconsolidated

sediments) or

encounter consolidated underlying strata such as clay or rock or gravels.

Spillage of bauxite during loading or transhipment may impact upon the physical nature of the adjacent sediments, introducing additional gravel fractions to the sediment matrix. Spillages of bauxite are not considered a contamination risk.


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The sediments of the Skardon River represent ambient conditions and display little evidence of historical contamination. Concentrations do not represent a substantial risk to water quality following mobilisation, and contaminant effects as a result of sediment mobilisation are not anticipated.

Port activities have the potential to impact marine sediment quality through the accidental release of hydrocarbons and fuels. However, the risk of release is low with the implementation of Australian standards for hydrocarbon storage, handling and transfer.


Sampling for acid sulfate soils indicates some minor potential of ASS and slightly greater PASS generation from sediments if disturbed and exposed to oxidation. The river based sampling confirms an absence of ASS though these sediments presented elevated PASS concentration within the upper estuary and adjacent to the existing Port.

Sediments within the proposed wharf areas have the potential to create acid drainage problems should these be exposed to oxygenated conditions or oxygenating processes. However, given the proposed piling construction methods the risk is considered low. The Queensland Acid Sulfate Soil Technical Manual: Soil Management Guidelines identify piles as a low impact construction method for ASS impacted areas. Australian Standard AS2159-1995 − Piling Design and Installation (Standards Australia, 1995) provides guidance on the use of piles in soils that contain pyrite or are saline.

17.7 Mitigation Measures

17.7.1 Port Construction

Given the proximity of the proposed wharf facility to adjacent seagrass habitats (seagrasses located on the opposite river bank (~230m) and upstream of the proposed works along the southern bank (~500m)), monitoring is proposed during the construction process to ensure conditions at these habitats are not impacted. Monitoring of marine water quality will be undertaken throughout the estuary during construction. This will incorporate locations within the adjacent seagrass habitats and locations up and down stream of the construction works.

Where monitoring identifies potential impacting processes due to construction (increased turbidity and deposition and benthic light reductions) outside baseline thresholds (to be defined) additional mitigation measures may be identified. These may include the application of silt curtains (deployed parallel to the prevailing current) or implementation of respite periods during the construction process.

17.7.2 Vessel Wakes

Although vessel waves are not predicted to impact the banks of the Skardon River, wherever possible existing native riparian vegetation around the Port will be maintained to minimise any impact to bank stability, water quality and habitat loss at the Port location. This is especially important for mangrove vegetation as it will help to prevent bank erosion due to locally generated vessel wake waves from the barges.

River bank position and bank vegetation monitoring will be undertaken should there be any indication of potential changes resulting from the vessel wake waves.

Barge speeds will be limited to 6 knots to prevent wake wave impacts along the shoreline of the Skardon River. Barge speed will only be increased if it can be demonstrated that increased speeds do not result in vessel wake wave impacts to the shoreline. Barge speed will also consider Port safety considerations, and on the barge vessel size and capacity as well as the transport frequency. The marine vessel operations plan, including vessel speed and access plan is described in Chapter 22.


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Vessel access and navigation channels will be defined so that vessels remain within the deep water navigation channels during transit.


Bed levelling depth has been selected to minimise direct impacts on the sea bed sediments but allow for efficient operation of barges. Sediment with a maximum depth of 2.2 m below LAT is proposed to be relocated for the initial bed leveling campaign. The bathymetric surveys show that the natural bed level of the Skardon River is at least 1.1 m below LAT. Hence to achieve a bed levelling depth of 2.2 m below LAT, a maximum depth of bed levelling of approximately 1.1 m is required with an average bed levelling depth of approximately 0.5 m.

Bed levelling will be relatively short in duration (120 days for initial bed levelling and 60 days for maintenance bed levelling) and will occur annually.

The bed levelling will aim to relocate material in a westerly direction wherever practical to replicate the natural bedform migration and to help minimise the requirement for repeat bed levelling.

The channel offshore of the mouth of the Skardon River will be hydrographically surveyed, as required for safe navigation, and typically annually at the end of the wet season. Proposed barge activities will not occur during the wet season (mining shut down period) due to the potential for extreme weather events over this period. In addition, the barge activities will not recommence until after the hydrographic survey has been undertaken and any maintenance bed levelling completed at the start of the dry season to ensure the offshore channel is navigable.

The marine vessel operations plan, including vessel speed and access plan is described in Chapter 22.

The modelled assessment of bed levelling demonstrates that there are no significant impacts predicted from sediment plumes or to longshore sediment transport.

Given the active nature of the ebb bar, no significant receiving habitats have been identified within the impact footprint. Sediments are dominated by fine to medium grained sands and gravels fractions. The chemical nature of the sediments does not present a significant risk to water quality according to the adopted NAGD (2009) guideline criteria.

During bed levelling water quality monitoring will be undertaken during the first 2-3 weeks of activity. This will include moderate-resolution imaging spectroradiometer (MODIS) imagery acquisition and analysis to track the dispersion of the bed levelling plume in real-time. Deployed water quality loggers will be established within the identified dispersion pathway. The nearest sensitive receiving habitat (inshore coral reef) located approximately 6,000 m to the south of the ebb bar will also be included within the monitoring program.

Monitoring will be compared to modelling predictions to ensure processes remain as anticipated and sensitive down current habitats are protected. Habitat condition thresholds will be developed for the nearshore reef systems during baseline data collection programs and a reactive approach to bed levelling management would be developed. Should turbidity, deposition or benthic light parameters exceed habitat condition thresholds at sensitive habitats works would be suspended until conditions recover.


Water quality impacts from propeller wash from barges are predicted to represent minor episodic increases in background turbidity during the passage of barges. Erosional processes are expected to occur over a 60 second period 6-8 times each day. Water quality concentrations are expected to be within or below ambient surface concentrations and below estimated mid and bottom column concentrations.


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Monitoring programs will be established to define the extent of any water quality impact, with a baseline data collection processes to be implemented prior to the construction development process to gauge the full range of baseline conditions.

Vessels will navigate along the centreline of the proposed navigation channel wherever possible to restrict any erosion of the bed due to propeller wash to this area.

Vessel movements will be controlled to minimise propeller wash (refer Chapter 22). Vessel passage over or immediately adjacent to seagrass habitats will be limited. Defined shipping routes, following the vessel access plan will be used.

Bulk vessels will operate in the offshore transhipment area where benthic cover is approximately 0.5%. The process of erosion from propeller wash will lead to periodic short term increases in turbidity (every 4-5 days) for laden vessels, but not unladen vessels. Bulk vessel loading zones will be controlled so that, over time, the sea bed will become armoured with coarser sediment protecting the sediment below from further erosion.

17.7.5 Bauxite Loading

The conveyor and chute barge loading systems will be designed to control dust (e.g. sprayers along conveyor and spill catch trays beneath conveyor) and minimise spillage of bauxite by directing bauxite directly to barges. Losses during transhipment will be recorded and reported. Monitoring of sediments is described in Section 17.6.8. Dust management measures at the Port (e.g. haul road and stockpile) are described in Chapter 19.

17.7.6 Water Quality

Management measures to prevent or minimise the release of contaminants from the Port infrastructure, including hydrocarbons, fuel, chemicals and wastes are described in Chapter 11.

The following plans have been produced for the management of the Port of Skardon River:

Oil Spill Contingency Plan (Ports Corporation Queensland, 2003)

First-Strike Oil Spill Response Plan - A supplement to the Queensland Coastal Contingency Action

Plan (MSQ, 2005)

Port Rules (Ports North, 2015)

These plans will be reviewed in conjunction with Port North and updated as required to meet Project requirements, with Gulf Alumina as first responder. Marine transport and operations management, including pollution controls and oil spill response plan are further described in Chapter 22.

Port area sediment management, including design, construction and operation of Port sediment ponds is described in Chapter 6. Proposed sediment pond water quality release criteria are described in Section 17.6.6.

Management measures to prevent sedimentation of the marine environment from mining activities are described in Chapter 6 and Chapter 12, involving capture and retention of runoff within pits during mining operations, thereby preventing release to the environment.

Wetland buffers zones (including buffers to marine wetland areas) are described in Chapter 16. A wetland buffer zone is proposed along the Skardon River South Arm supratidal wetland, which will provide at least 100 m separation distance between mining and wetland areas. This buffer zone will also act to contain any sediment runoff.

Commercial vessels involved in the site construction and operational phases will be subject to international, national and state policies and guidelines to restrict environmental impacts as a result of spillages, anticorrosion products, wastewater products, and solid wastes. Marine transport and


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operations management, including pollution controls and oil spill response plan are described in Chapter 22.

17.7.7 Sediment Quality

The measures described to manage impacts to water quality will also minimise impacts to marine sediment quality.

The adoption of management processes, operational standards and guidelines, site auditing and a regular program of sediment monitoring is considered to be sufficient to identify, mitigate and control potential impacts to the sediments of the Skardon River.

Ongoing sediment monitoring will ensure operations remain compliant to management processes. Any evidence of sediment impact would be cause to review operating standards and procedures to ensure sediment quality does not degrade over the longer term.

Monitoring of sediments within the proposed wharf footprint, barge route and offshore transhipment area will be undertaken as part of operational management processes. This screening process will be undertaken as a baseline program (pre-construction) and repeated with sufficient regularity to confirm operational processes are not leading to significant impacts of the surrounding sediments. Where contaminants are identified, management measures will be implemented to mitigate potential release pathways and prevent reoccurrence.

While not a chemical contamination process, spillages of bauxite to the environment will be minimised, and amendments to loading equipment or processes made if such events are occurring on a repetitive basis. Where sediment monitoring identifies the occurrence of material spillages management processes or engineering mitigation measures would be implemented to prevent reoccurrence.


Once the footprints for wharf facilities are defined and construction methodologies finalised, an additional field investigation will be undertaken to identify ASS or PASS and if required, support the preparation of an ASS Management Plan. Should a detailed ASS Management Plan be required, then additional analysis within the construction footprint would be undertaken in accordance with the QASSIT Guideline. Any ASS will be managed (test and treat the soils) in accordance with the detailed methods outlined in the Queensland Acid Sulphate Soil Technical Manual (Dear et al 2002). No construction work will be undertaken in the wharf area prior to field investigations for ASS.

Construction activities at the site will be undertaken outside of the wet season period, which will reduce the potential for erosion, off site transport of sediments and generation of acidic leachate. Clearing of mangrove vegetation and disturbance of marine muds during construction of the conveyor alignment may disturb sediments recognized to contain PASS. Care will be taken to ensure disturbed sediments remain below the tidal inundation zone so exposed materials do not become reactive.

17.8 Monitoring Plan

Appendix 8 provides a detailed marine monitoring plan for the Skardon River and offshore area, which describes the following:

water quality monitoring to establish baseline water quality and set site specific water quality

objectives in accordance with QWQG for HEV waters for different estuary zones in the Skardon

River, open coastal waters and offshore waters

water quality monitoring during construction of the wharf

water quality monitoring during bed levelling


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water quality during operations along the navigation channel / barge route for changes in water

quality resulting from propeller wash

water quality monitoring in the River near Port sediment pond release zones

sediment quality monitoring in the Skardon River and offshore

vessel wake wave monitoring comprising wave monitoring and river bank monitoring

monitoring of propeller wash impacts

seagrass monitoring, primarily near the Port infrastructure area

mangrove monitoring near the Port infrastructure area

marine pest monitoring.

Monitoring of the marine habitats (i.e. seagrasses, mangroves, benthic habitat, marine pests) is described in Chapter 18.

17.8.1 Site Specific Water Quality Objectives / Baseline Water Quality

17.8.1.1 Reference Sites

To establish site specific water quality objectives in accordance with QWQG baseline, the QWQG recommend the number of sampling events and period of monitoring from reference sites. The QWQG states “Based on these analyses it is recommended that, for one to two reference sites, estimates of 20th or 80th percentiles at a reference site should be based on a minimum of 18 samples collected at each site over at least 12 and preferably 24 months (in order to capture two complete annual cycles)”.

Percentile estimates based on eight or more samples will be used to derive interim water quality objectives, as described in Section 17.4.12. Additional data will be collected over 12 – 18 months to set site specific water quality objectives. Construction activities would consider interim water quality objectives using all data collected prior to construction commencing). Operations (are scheduled for April 2017 and would initially be monitored against updated interim water quality objectives (using all data collected prior to operations commencing), and then subsequently monitored against site specific water quality objectives (expected mid to end 2017).

The Skardon River and offshore marine environment area suitable as reference site as there have been minimal anthropogenic influences or impacts on the River in the past.

Reference sites will be established in all defined estuarine / marine zones as per the QWQG, namely:

Upper estuary (tidal limits to physicochemical/chemical indicators)

Mid estuary (from upper estuary lower limit to the lower estuary upper limit)

Lower estuary (10% of the estuary Length ~1.7km)

Open coastal (shoreline to 3 nautical miles/5.55km)

Offshore marine (> 3 nautical miles)

Sampling locations are proposed in each zone in order collect sufficient water data to set site water quality objectives.

17.8.1.2 Monitoring Program

The monitoring program comprises:

Vessel based logger monitoring

Deployed water quality loggers


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The vessel based program involves monthly samples with insitu measurements obtained from surface, mid and bottom water depths with a composite sample (surface/mid/bottom) obtained for laboratory analysis.

Appendix 8 describes the monitoring methodology for the vessel based loggers in order to collect and analyse accurate, reliable and quality assured data.

The parameters proposed for monitoring are listed in Table 17-4.

The proposed sampling locations for monthly vessel based monitoring are presented in Table 17-11 and shown in Figure 17-35.

Table 17-11 Vessel Based Water Quality Sampling Locations

Site Justification

SRUE1 to SRUE2

Upper estuary. To identify water quality above the Port infrastructure area and capture catchment based inputs.

SRME1 to SRME3

Mid estuary – At the proposed Port infrastructure area and at locations up and down stream.

SPRZ1 Sediment pond release zone. To be applied to the development of receiving water quality criteria for mangrove systems which will experience wet season release from the proposed sediment pond.

SRECLE1 to SRECLE2

Enclosed coastal/Lower estuary – Located within the entrance channel centerline and includes the ebb bar crossing.

SROC1 Open Coastal – Water quality defined within the nearshore open coastal areas. Coral habitats located south of the entrance within the open coastal zone.

SRO1 Offshore – To define water quality within the offshore area at the transhipment zone.


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Figure 17-35 Vessel Based Water Quality Sampling Locations


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The deployed logger program is designed to provide improved temporal data for a reduced number of parameters which may be most likely influenced by marine operations (turbidity, deposition, and benthic light). Locations will be situated to provide data from key waterway types, receiving habitats and operational locations. Loggers provide a vastly improved temporal assessment, targeting key variables such as spring/neap tides, flood and ebb tides, and episodic natural events such as flooding or tropical storms and cyclones. Turbidity, deposition and benthic light availability are key drivers in the predicted impacting processes within the marine environment (propeller wash and release of turbidity plumes). The temporal scale of natural perturbations (frequency, duration and concentration) are critical factors to understand when it comes to defining the potential assimilative capacity of the marine environment and the bounds of ‘natural conditions’ within which the Project will strive to achieve.

Deployed loggers will measure turbidity, PAR, deposition, depth and temperature. Logger instrumentation and deployment methods are described in Appendix 8.

Deployed loggers will be situated to assess a broad spatial distribution in turbidity, deposition and benthic light irradiance within a combination of key receiving environments.

Proposed logger locations and function are described in Table 17-12 and shown in Figure 17-36.

Table 17-12 Deployed Logger Location and Function

Site Justification

SRML1 Upper estuary

SRML2 Seagrass habitat adjacent to the wharf facility. Upstream of the proposed sediment pond discharge area.

Natural turbidity, deposition and light regimes will be identified. These conditions will be used to establish monitoring criteria to be applied to construction and operational monitoring.

SRML3 Channel alignment

SRML4 Channel alignment

SRML5 Seagrass - lower estuary

SRML6 Located within the nearest inshore coral reef system approximately 6km south of the proposed bar entrance.

Conditions supporting the establishment of nearshore coral reef systems will be identified. The reef is situated south of the proposed bar entrance. Baseline conditions can be reviewed against initial bed levelling and potential plumes released during propeller wash during operations.

SRML7 Offshore transhipment area


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Figure 17-36 Deployed Logger Location and Function


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In addition to site observations Gulf Alumina will obtain monthly MODIS. Images will be captured to provide a broad spatial assessment of water quality. Each MODIS image will be assessed and reported in an estimated TSS (mg/L) plot.

In addition to deriving site water quality objectives for the different marine / estuarine zones, specific analysis will also be undertaken to define ambient conditions for potential receiving habitats including:

river substrates

seagrass communities

offshore coral communities.

Water quality objectives will be used to assess whether the Project is resulting in change in water quality during construction and operation. The assessment of change will consider the QWQG methodology for assessing confidence intervals around the 20th, 50th and 80th percentile sample data in comparison to population (baseline) percentiles.

Data will be analysed to determine key characteristics including:

frequency of natural turbidity events

duration of events

concentrations experienced during these events

analysis with regards to season

analysis with regards to flood and ebb tides

analysis with regards to neap and spring tides

consideration of depth data variability (wave event periods) and how they relate to recorded values

consideration of wind speed (local) and how it relates to recorded values

general statistics, ranges and percentiles.

17.8.2 Construction Monitoring

Construction monitoring will target areas potentially impacted by construction of the wharf and the initial bed levelling campaign.

Baseline monitoring will inform:

ambient water quality at seagrass beds near the wharf and at the nearshore coral reef located 6km

south of the proposed bed levelling area

ambient water quality during flood and ebb tides; spring and neap tides; and elevated wind or sea

conditions.

Comparison of water quality during construction at the Port and during bed levelling area to ambient conditions will inform management measures.

The parameters proposed for monitoring are provided in Table 17-4.

Sampling during construction will utilise handheld multiparameter instruments. Analysis will be conducted for:

depth

turbidity

salinity

pH


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dissolved oxygen

temperature.

Deployed loggers will be installed over seagrass beds near the Port area and the inshore coral reef area during bed leveling operations. Loggers will include analysis for:

turbidity

deposition

par

temperature

depth.

Port Construction

Two fixed locations have been identified for combined physicochemical and chemical analytes adjacent to the construction works at the wharf. Three locations have been identified for deployed loggers over adjacent seagrass habitats. The locations will be refined during construction to best target impacted water quality. Where plumes are known to extend away from operations, loggers will be deployed to best encounter these dispersion areas. MODIS imagery may assist in refining these locations.

Physico-chemical and chemical analysis will be conducted weekly to screen for potential contamination impacts. Methods and results will be compared to water quality objectives. Deployed loggers over seagrass beds will be downloaded weekly and reported against water quality objectives. Where water quality demonstrates ‘changed’ in accordance with QWQG methodology, Gulf Alumina would conduct infield inspection of seagrass condition and health.

Bed Levelling

An additional logger location has been identified for deployment at the near shore reef habitat, 6000 m south of the proposed bed levelling program. This area will also include vessel based physicochemical survey tracking the plume generated during bed levelling. These surveys will be supported by daily MODIS imagery, reverting to weekly after an initial survey period.

The proposed bed levelling program would extend for a period of 120 days. Modelling has predicted a localised impact to water quality regarding suspended solids. Predictions indicate plumes will not extend to the nearest sensitive receiving habitat (nearshore coral reef).

A deployed logger will be established at the coral reef to monitor for changes in turbidity, deposition and PAR. Water quality objectives established from the baseline deployment of loggers will be compared to water quality during bed levelling and used to derive management response criteria for these habitats. Loggers will be downloaded weekly for the first 2 weeks and screened for impact. Should impact potential be identified, then deployed loggers would remain for the remainder of the program. If conditions confirm no incidence of bed levelling impact, then the logger will be removed.

During the initial 2 weeks, vessel based sampling would be undertaken using a multiparameter probe to assess the full physicochemical suite. MODIS satellite imagery would also be used to confirm the plume trajectory and concentration, and help identify the plume centreline for vessel based survey. Measures would be captured from the plume centreline and from locations surrounding the bed levelling process.

Should the deployed logger confirm no impact at the reef location and the vessel based logger records conditions less or equivalent to the predicted scenarios using plume dispersion modelling, the monitoring program would be discontinued after a period of 2 weeks. Should loggers identify impact at the reef habitat and local criteria are exceeded, field inspection of coral habitat would be undertaken.


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MODIS imagery acquisition and analysis would continue to be conducted weekly for the duration of the bed levelling program.

The proposed locations monitoring of Port construction and bed levelling is provided in Table 17-13 and shown in Figure 17-37.

Table 17-13 Port Construction and Bed Levelling Monitoring Locations

Site Justification

SRCWQ1 – SRCWQ2

Adjacent to barge loading facility construction.

SRCL1-SRCL3 Deployed loggers at seagrass beds

SRCL4 Deployed logger at inshore coral reef habitat.

SRCWQ3 Vessel based logger zone surrounding the bed levelling operations. This will include transects surrounding the bed levelling activities.

Management measures during wharf construction and bed levelling involve:

comparison of baseline water quality objectives to water quality during activities

where there is a ‘change’ in water quality an investigation will be undertaken identify the source

where Project activities have resulted in a change, field inspections of seagrass or near shore reef

habitat will be conducted

implement management measures such silt curtains where possible, shutdown or respite periods,

selection of tidal windows for works or change work locations.


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Figure 17-37 Port Construction and Bed Levelling Monitoring Locations


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17.8.3 Monitoring During Operations for a REMP

A routine receiving environment monitoring program (REMP) will be implemented during operations. This will include a subset of locations sampled during baseline monitoring program.

Locations have been selected to encompass key operational areas (wharf, navigation channel, ebb tide bar and the transhipment area). Deployment of a permanent logger is also proposed at the seagrass bed where there is potential impact from barge propeller wash.

Monthly monitoring and reporting will provide early warning of potential long-term impacts and facilitate development of management response.

The parameters proposed for monitoring are provided in Table 17-4.

Sampling during operations will utilise handheld multiparameter instruments. Analysis will be conducted for depth, turbidity, salinity, pH, dissolved oxygen and temperature.

Deployed loggers will include analysis for turbidity, deposition, PAR, temperature and depth.

Monthly MODIS imagery will be captured to review the spatial distribution of operational turbidity/TSS regimes.

Routine water quality monitoring would be undertaken monthly from 6 locations, in addition to a deployed logger within the predicted propeller wash zone adjacent to the wharf within known seagrass habitat. The proposed locations are provided in Table 17-14 and shown in Figure 17-38.

Table 17-14 Water Quality Monitoring - Operations

Site Justification

OWQ-1 Upstream of wharf to define upstream or catchment derived conditions.

OWQ-2 Wharf and navigation channel.

OWQ-3 – OWQ4 Mid and lower estuary within navigation channel.

OWQ-5 Ebb tide bar crossing within navigation channel.

OWQ-6 Transhipment location

OWQL-1 Seagrass beds potentially impacted by propeller wash.

For annual maintenance bed levelling activities, where water quality conditions did not exceed predicted concentrations or dispersion profiles during the initial bed levelling campaign, then monitoring during operations will be limited to MODIS imagery acquisition each week to track the dispersion plume. If plumes impact or potentially impact the near shore reef habitats, a deployed logger program will be undertaken as for the initial bed levelling period.

Management measures during operations involve:

comparison of baseline water quality objectives to water quality during operations

where there is a ‘change’ in water quality an investigation will be undertaken identify the source

where Project activities have resulted in a change, field inspections of potentially impacted habitat

will be undertaken

implement management measures such amendments to vessel speed and access plans.


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Figure 17-38 Water Quality Monitoring - Operations


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17.8.4 Port Sediment Pond Releases

The monitoring program for the Port sediment ponds comprises:

Monitoring of sediment pond water quality prior to any controlled releases during the operational

period

Monitoring of sediment pond water quality during the wet season

Monitoring of Skardon River water quality during releases

Determination of whether the Project is resulting in a ‘change’ in water quality

Monitoring of marine ecology should water quality monitoring indicate a change in water quality

Management measures if a change in water quality and ecology is identified.

17.8.4.1 Monitoring Parameters

The monitoring parameters for daily monitoring during release events will be:

pH – used to measure compliance

Turbidity (NTU) – used to measure compliance

DO – used to measure compliance

EC – for information only, as salinity is not a parameter of concern in sediment ponds or estuarine

waters

Temperature – for information only.

Monthly monitoring will also be taken for the parameters in Table 17-4 over the wet season:

On a daily basis, during release events, visual inspections will also be made for any visible films or detectable odours of oil or grease.

17.8.4.2 Baseline Data

Baseline data collection to set site specific water quality objectives is described in Section 17.8.1. Because the estuarine environment can differ with proximity to shore, baseline data will be collected from the:

rear of the mangrove communities and saltmarsh, where waters enter the receiving environment

mid estuary of the Skardon River for the comparison to water quality within the primary channel.

The 20th, 50th and 80th percentiles will be calculated for this data, with a 75% confidence interval around each percentile.

Baseline data may also be differentiated between wet season and dry season for each WQO. Interim water quality objectives will be used until site specific water quality objectives are defined.

17.8.4.3 Releases During the Operational Period

During the operational period, controlled releases will only occur when water quality in the sediment ponds achieve local water quality objectives (i.e. achieves ‘no change’ criteria in comparison to population (baseline) percentile distributions) for the physico-chemical parameters against which compliance will be assessed (refer to Section 17.8.4.1).

Release locations are nominated as:

S13 – existing sediment pond

S14 – proposed sediment pond.


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These release locations are shown Chapter 12, Figure 12-13. in If any uncontrolled releases occur during this period, monitoring will occur as per the monitoring program for wet season releases described below.

17.8.4.4 Wet Season Releases

The QWQG relate to water quality in the receiving environment (i.e. the Skardon River), which is not necessarily the same as water quality released from the Port sediment ponds. Any releases from Port sediment ponds do not necessarily need to achieve the 20th, 50th and 80th population percentiles, so long as water quality in the receiving environment is unchanged (i.e. sampled water quality in the Skardon River in unchanged from population percentiles). As population percentiles have not been defined for the Skardon River, use of interim water quality objectives based on the 80th percentile of available data or based on default WQOs from the AWQG is considered reasonably conservative in setting release criteria for the Port sediment ponds (refer to Section 17.6.6).

17.8.4.5 Monitoring During Releases

Physico-chemical water quality parameters (Section 17.8.4.1) will be monitored with 24 hours of a release and then daily during releases at the overflow weirs.

If the physico-chemical parameters for release water quality achieve local water quality objectives (i.e. achieve ‘no change’ criteria in comparison to population (baseline) percentile distributions), then no further daily monitoring will be undertaken.

If the physico-chemical parameters for release water quality do not achieve local water quality objectives, then receiving environment monitoring will be undertaken.

Field surveys will be undertaken to compare areas impacted from releases to prevailing ambient conditions up-current of the location. If plumes can be seen these will be surveyed for water quality concentrations and spatial distribution.

Sampling for up-current ambient conditions will be conducted on or after the mid tide (flood or ebb) to allow any residual impact from discharge to be mobilised by the changing tide (this will best ensure that true ambient conditions are encountered). Where prevailing impact concentration distributions remain below the receiving waters baseline distributions then monitoring will continue each day for the duration of release and results reassessed. Where the baseline distributions are exceeded over a period of one week, then Gulf will progress to habitat impact sampling (saltmarsh and mangroves).

17.8.4.6 Seasonal Monitoring

Over the course of a wet season, data will be collected for physico-chemical parameters against which compliance will be assessed. The 20th, 50th and 80th percentiles for this data will be compared to the population (baseline) percentiles to determine if there has been a change over the course of a season (i.e. as per the REMP described in Section 17.8.3).

During the period of releases monthly samples will be taken from the sediment ponds and the receiving environment for monthly monitoring parameters in Section 17.8.4.1. Monthly sampling results will be compared to WQOs to identify if there has been a release of hydrocarbons or changes in metal concentrations. Monthly monitoring will also compare release site and upcurrent monitoring of hydrocarbons and metals to determine if there are other sources of hydrocarbon or metals releases within the Skardon River.

In the event of a known or detected hydrocarbon release (e.g. visual inspection or accidental spill) monitoring frequency will be increased to understand potential impacts and inform management measures (including emergency spill management).


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Over the course of the wet season, data collected will enable the calculation of the 20th, 50th and 80th percentile for hydrocarbons in the receiving waters. These will be compared to WQOs to determine if there is a ‘change’ in water quality over a season.

17.8.4.7 Habitat Condition Assessment – Mangroves and Saltmarsh

Where receiving water ambient turbidity concentration distributions are exceeded within the impacted waters, then Gulf Alumina will conduct rapid ecological assessment within the receiving marine environments (mangrove and saltmarsh habitats). To effect this, routine saltmarsh and mangrove community monitoring locations will be established within the proposed spillway release zone (refer to Chapter 18). Gulf Alumina will have developed condition data for these locations from fixed survey transects and quadrat areas. Video and photographic evidence will also be collected. Any residual sediment deposits originating from the sediment ponds within these habitats will be sampled and analysed for contaminants. These sample findings will be compared against sediment quality guidelines (ANZECC, 2000 or NAGD 2009) and predevelopment sediment condition data (20th percentile, 50th percentile and 80th percentile).

Should laboratory findings exceed identified ANZECC (2000) or NAGD (2009) criteria, or ambient locally developed HEV distribution criteria, then additional biological investigation would be undertaken. Where sediments remain compliant then sediments are not considered of substantive chemical threat. Receiving water quality monitoring will continue for the duration of the discharge period and still image and video survey of the impacted habitats will be undertaken from fixed photo station quadrats and transects on a monthly basis for 3 months. This program will focus on defining any residual physical impacts from fines deposition.

17.8.4.8 Management Measures

Should monitoring detect a change in marine habitat as a result of sediment pond releases then the following management measures will be implemented:

All practical management measures (e.g. operational controls) will be implemented during the

period of releases.

A post wet season assessment on the design and operation of sediment controls, including ponds

will be undertaken and design changes made as required (i.e. engineering controls). Operational

controls for the next wet season will be reviewed and amended as required.

If hydrocarbon contamination is detected, the hydrocarbons will be analysed to identify the

potential source and controls implemented to prevent or contain any future releases.

Gulf Alumina will implement the Oil Spill Emergency Response Plan in the event of accidental

release of hydrocarbons.

17.8.5 Sediment Quality

Gulf Alumina conduct sediment investigation in accordance with the requirements of AS/NZS 5667.1:1998. Australian/New Zealand Standard Water quality—Sampling (Part 1: Guidance on the design of sampling programs, sampling techniques and the preservation and handling of samples. Reference will also be made to ANZECC (2000), the National Assessment Guidelines for Dredging (NAGD, 2009) and the recently released Sediment Quality Assessment - A Practical Guide (Simpson and Batley, 2016).

Historical impacts on sediment quality are very limited. Sediment sampling conducted to date is described in Section 17.4.13.

Based upon available data, historical practices and the proposed operational processes, the parameters proposed for sediment quality monitoring are provided in Table 17-15. Detailed suites will be conducted prior to project construction as a baseline characterisation of sediments. Detailed suites will be


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undertaken less frequently during operations (every 3 years). The basic suite will be undertaken as part of routine monitoring every year.

Table 17-15 Sediment Quality Parameters

Contaminants NAGD PQL NAGD Screening Detailed Suite Basic Suite

Copper 1mg/kg 65mg/kg X X

Lead 1mg/kg 50mg/kg X X

Zinc 1mg/kg 200mg/kg X X

Chromium 1mg/kg 80mg/kg X X

Nickel 1mg/kg 21mg/kg X X

Cadmium 0.1mg/kg 1.5mg/kg X X

Mercury 0.01mg/kg 0.15mg/kg X X

Arsenic 1 mg/kg 20mg/kg X X

Silver 0.1 mg/kg 1mg/kg X X

Manganese 10mg/kg na X X

Aluminium 200mg/kg na X X

Cobalt 0.5mg/kg na X X

Barium 0.5mg/kg na X X

Iron 100mg/kg na X X

Tin mg/kg X X

Vanadium 2mg/kg na X X

Selenium 0.1mg/kg na X X

Antimony 0.5mg/kg 2 X X

Total Nitrogen 0.1 mg/kg na X X

Total Phosphorus 0.1 mg/kg na X X

Ammonia 0.1 mg/kg X X

Total Petroleum Hydrocarbons (C6-C36) mg/kg 550 mg/kg X X

Polyaromatic Hydrocarbons (PAHs) ug/kg 10,000 ug/kg X X

OC/OP Pesticides ug/kg Various X

Radionuclides Bq/g 35 Bq/g X

Particle Size Description mm na X X

Total Organic Carbon (TOC) % X X

Settling velocity X

Shear velocity X


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The sediment quality monitoring locations and function are described in Table 17-16 and shown in Figure 17-39.

Table 17-16 Sediment Sampling Locations

Period Location Comments

Baseline SRS1-SRS15 Baseline sediment quality assessment

Operations SRS2 – SRS7 Wharf area

Operations SRS8, SRS10, SRS12 Navigation channel

Operations SRS14 Transhipment area

Operations SRS15 Mangrove sediments at the sediment pond discharge area.

Sediment sampling will inform management of Project activities:

Assess whether sediments at and surrounding the wharf remains free of contamination. Bauxite

spillages within the berths will be identified.

Assess whether the receiving environment experiences and contamination from the periodic

releases from the Port sediment ponds.

Assess whether sediments within the navigation channel remain free of contaminants from the

passage of vessels.

Determine potential chemical and physical changes of sediments within the channel alignment and

seagrass beds adjacent to the wharf over time.

Assess whether sediments at the transhipment area remain free of contamination. The extent of

material spillage would be defined if applicable.

Results would be compared to the NAGD (2009) sediment quality criteria and the findings of baseline characteristics, whereby local guidelines will be developed. Where changes in sediment quality are identified Gulf Alumina will review operational procedures to remove impacting processes as far as practicable.

Should contamination be identified outside guideline criteria and local conditions, additional detailed investigation will be undertaken and site clean-up and recovery implemented.


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Figure 17-39 Sediment Sampling Locations


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17.8.6 Vessel Wake Waves

Monitoring of vessel wake waves is proposed to confirm modelled predictions and provide feedback confirmation or amendments to the proposed vessel access and speed management plan.

An acoustic Doppler current profiler (ADCP) wave logger will be deployed for the first 6 months of barge operation to confirm modelled predictions of wave height generated from vessel wake. Infield measurements will be made to determine the height of vessel wake waves during the passage of bauxite barges through the Skardon River. Heights will be reviewed in relation to predicted values adopted within the EIS to confirm predictions are accurate and impact is not anticipated. Field observations would also be made to assess the impact if any of these waves by measuring additional field parameters such as turbidity and total suspended solids. Locations will be selected so as to provide a representative assessment of the shorelines at varying distances from the proposed navigation channel.

Graduated stakes and high definition video will be used to measure the wave heights. Sampling can then be effectively undertaken either from settable shorelines or from small vessels to access intertidal environments of the river. Infield water quality instruments will be adopted for turbidity measurements. Laboratory samples would be collected for TSS (mg/L) and sent to analytical laboratory. Multiple replicate samples will be obtained over a 3 day period. Periods will encompass a combination of vessel passage and non-vessel passage periods and combination of high and low tidal periods.

Bank erosion survey will be established at 3 locations in the Skardon River. These will include photographic reference sites and infield reference devices. Locations will be selected to represent the shoreline conditions at typical ranges from the navigation channel. Deployed stakes will provide reference to quantitatively measuring erosion or bank recession and photographic reference location will provide comparative assessment between periods. Surveys will encompass a range of tidal stages between high tide, and low tide.

Wave height and bank erosion monitoring will be conducted during the initial 6 months of operations. Should wave height monitoring confirm model findings and bank erosion monitoring demonstrate no change as a result of barge movements, the requirement for monitoring will be reviewed.

Proposed wake wave (WW) monitoring locations and bank erosion (BE) monitoring locations are shown in Figure 17-40.

Monitoring will inform the following management measures:

Ensure predicted wake waves height are represented in the field.

The vessel speed management plan may be revised pending field findings on wake waves.

Bank erosion will provide an appreciation of vessel wake processes and operations to confirm

impacts remain acceptable. Should erosion be identified and attributable to the Project, actions to

further limit wake waves, or protect elements of the shoreline will be proposed.


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Figure 17-40 Vessel Wake Wave and Bank Erosion Monitoring Locations


Page 17-83


Water quality sampling and sediment sampling (described above) in the channel alignment, transshipment area and adjacent seagrass beds will improve the knowledge of the sediments in potential impact zones. Analysis will include a combination of chemical and physical parameters.

The following monitoring will be undertaken to further understand potential impacts from propeller wash.

A baseline side scan sonar survey will provide a pre-development imagery of the seabed features in the channel alignment, transhipment area and adjacent seagrass beds. Imagery from physical features such as sand waves, ripples, gravel, smooth sands or rock will be identified. Surveys every 6 months are proposed for the first year and then every year after that. Regular surveys will identify higher risk areas for propeller wash. Bed features will be presented in detail and key features of note would be mapped using GIS tools.

By incorporating propeller wash survey Gulf Alumina will:

define the physical location of bed disturbance/erosion using side scan

identify potential management options to reduce propeller wash impact (i.e. adjust speeds).

define the potential distribution of any water quality impacts using MODIS imagery and adjust the

water quality program as required.

17.9 Risk Assessment

A risk assessment assessing the likelihood and significance of impacts to coastal processes and the physical marine environment from the Project is provided in Table 17-17. The risk assessment considers mitigated risk of significant impacts; that is, the impact from the Project with the implementation of management measures. The risks to coastal processes and the physical marine environment are low to medium.

Table 17-17 Risk Assessment and Management Measures for Impacts to Coastal Processes and the Physical Marine Environment

Environmental Value

Impacts / Emissions / Releases

Proposed Management Practices

Likelihood Consequence (Magnitude)

Risk Rating

Hydrodynamics Port construction. Refer Section 17.6.1

Refer Section 17.7.1

Unlikely Minor Low

Bed levelling. Refer Section 17.6.2


Possible Minor Medium

Shoreline and bank evolution

Wake waves from barge movements. Refer Section 17.6.4


Unlikely Negligible Low

Bed levelling Refer Section 17.6.2

None proposed

Rare Minor Low


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Environmental Value

Impacts / Emissions / Releases

Proposed Management Practices

Likelihood Consequence (Magnitude)

Risk Rating

Morphology / Longshore sediment transport


None proposed

Unlikely Minor Low

Sediment transport



Likely Minor Medium

Marine water quality

Onshore mining. Refer Section 17.6.7


Unlikely Minor Low

Port area operations. Refer Section 17.6.6 and 17.6.7



Wharf construction. Refer Section 17.6.1

Refer Section 17.7.1 and 17.7.6





Sediment quality

Onshore mining. Refer Section 17.6.8


Rare Minor Low

Port area operations. Refer Section 17.6.8


Unlikely Minor Low

Barge operations / prop wash. Refer Section 17.6.5

Refer Sections 17.7.4 and 17.7.7

Likely Minor Medium

Bulk vessel prop wash. Refer Section 17.6.5

Refer Sections 17.7.4 and 17.7.7

Likely Insignificant Medium

Bauxite loading. Refer Section 17.6.8

Refer Section 17.7.5 and 17.7.7

Possible Insignificant Low

Acid sulphate soils



Possible Insignificant Low


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17.10 Cumulative Impacts

The only project considered to have a cumulative impact to coastal processes and the physical marine environment with the Skardon River Bauxite Project is Metro Mining’s Bauxite Hills project. This project will have similar impacts to the Skardon River Bauxite Project as it will involve Port construction (approximately 1.5 km upstream of the existing Port), mining of bauxite from areas surrounding the Skardon River, barging of bauxite (or barge and tug) and offshore transhipment of bauxite to bulk vessels. The Bauxite Hills project does not propose bed levelling (or other means of increasing the depth of the channel across the ebb bar) and therefore there are no cumulative impacts from bed levelling.

The construction process for both projects is very similar with regards to barge infrastructure. A short construction period during the dry season is proposed for both projects. This would include pile based construction and an increased vessel traffic for construction and operation. Due to separation distance between ports (approximately 1.5 km) and low potential for simultaneous construction periods, cumulative impacts during construction are likely to be low.

The operational scenario would present a substantial increase in vessel traffic should both projects overlap. To meet the basic annual tonnages and weekly bulk carrier loading targets, up to 100 barge movements would be required within the Skardon River each week (3600 - 4000 movements annually) (in comparison, the Port of Weipa experiences approximately 1000 movements along the channel (in and out) annually. These movements would be accompanied by additional movements associated with fuel and materials supply.

17.10.1 Skardon River

Should both projects occur over the same period, or overlap to some extent the Skardon River would be exposed to significant vessel traffic. The impacts to coastal processes and the physical marine environment associated with such traffic volumes include vessel wake wave impacts, propeller wash turbidity, water quality impacts and navigation safety.

It is assumed that both projects would utilise the same navigation channel (other than the section between Gulf’s and Metro Mining’s wharves) which would double the incidence of propeller wash within the navigation channel. Both projects would also present impacts to adjacent seagrass beds with respect to propeller wash. Metro Mining’s proposed tug and barge operation is expected to present a smaller footprint for propeller wash, though its capability to operate on a wider tidal range may counter the potential reduced impact, due to increased vessel movements. Propeller wash has been described as a localized impact within the navigation channel given the identified conditions and the results of modelling with regards to impacted water quality. Alteration to mobilisation and deposition processes in specific areas, such as the wharf facilities has the potential for longer-term impacts to seagrass beds.

Neither project presents a significant impost on water quality outside the process of propeller wash. The risks associated with leaks, spillages, both small and large, of hydrocarbons and general chemicals will increase with two projects. However, both projects will implement design and management measures to prevent contaminant releases and both projects will have spill responses. Both projects are expected to implement sediment management control at Port infrastructure areas, which will minimise potential for cumulative impacts.

The barges exporting bauxite will provide the bulk of vessel movements for both projects. These vessels are relatively large and slow, with both projects expected to operate at approximately 6 knots in the River, with neither project resulting in significant vessel wake wave heights at the shoreline

As bed levelling is only proposed for the Skardon River Bauxite Project, there will not be cumulative impacts from multiple bed levelling operations.


Page 17-86

17.10.2 Offshore Transhipment

The proposed offshore transshipment locations for both projects are located several kilometers apart, though the passage for vessels exiting the Skardon River for the transshipment area will be relatively similar over most of its length. Bulk carriers will anchor within the transshipment areas and load bauxite from the barges. Gulf Alumina propose the use of self-unloading barges, Metro propose the use of deck cranes.

Potential impacts associated with the transshipment operation include propeller wash from bulk vessels and any incidence of water quality or sediment impact associated with spills and releases (minor chemical spills, hydrocarbons and bauxite).

The operation of transshipment zones will be duplicated, as will the potential for propeller wash during departure of the bulk carriers. Given an absence in significant benthic habitat and a broad expanse of immediate alternative habitat for use and colonization, cumulative impacts are unlikely to be significant.

17.10.3 Bulk Carriers

Approximately 140 bulk carriers would be required to service both projects ach year. The nearby port of Weipa processes approximately 450-500 bulk carriers annually, exporting some 30 million tons of bauxite. The additional carriers required for the Skardon River Bauxite Project would represent a 10% increase in bulk carrier movements for the local area. A further 10% would be attributable to the proposed Bauxite Hills project.

Cumulative impacts on marine water quality, through ship-sourced pollution are possible. However it is expected that both operations will implement ship-source pollution prevention plans, including management of ballast water.

17.11 Conclusion

Surveys and monitoring have been undertaken for marine water quality, marine sediments and bathymetry of the Skardon River. Desktop reviews have been undertaken for the area potentially impacted by the Project, including published literature by third parties, other environmental studies for the EIS, environmental studies for other projects in the region, and historical data and reports from the Project area.

An assessment of coastal processes and the physical marine environment was undertaken for the Skardon River estuary, including the bed levelling area in the ebb bar beyond the mouth of the Skardon River. The assessment includes tides, storm tide, tidal currents, river flows, bathymetry and morphology, shoreline and bank evolution, marine water quality, sediment and acid sulphate soils. Bathymetry and morphology was found to be relatively stable over the course of bathymetry surveys dating back to 1998. The channel alignment was constant with variation in depth of the channel at the river mouth attributable to localised sediment movements. Sediments in the bed levelling area are generally sandy. Water quality of the estuary, including turbidity is highly variable over tides and seasons.

Potential Project impacts on the marine environment include wharf construction at the Port, Port operations, bed levelling, vessel operations, offshore transhipment and bulk vessel movements. Modelling and calculations were undertaken to predict impacts from bed levelling on sediment transport and longshore sediment movement, vessel wake wave impacts and propeller wash impacts.

Piling construction will be limited to approximately 2 months, with this method minimising disturbance in comparison to other wharf construction methods. Impacts from Port development will be small and restricted to the areas directly adjacent to the structures and as the area already has marine port facilities present, the impacts on coastal processes are not considered to be significant.


Page 17-87

The change in bathymetry of the ebb bar is predicted to result in relatively small and localised changes to tidal currents and waves. The changes are restricted to the ebb bar and do not influence the Skardon River. The bed levelling activity is predicted to result in a relatively small, localised sediment plume. The plume extent during bed levelling is limited to the area directly offshore of the ebb bar. The plume does not extend within the Skardon River or along the shoreline to the north or south of the ebb bar

The bed levelling activity is not expected to result in any impacts to the longshore sediment transport at the shoreline to the south or around the ebb bar adjacent to the Skardon River.

Vessel wake waves predicted to be generated by vessels are small when they reach the shoreline. These waves are therefore not predicted to impact the banks of the Skardon River.

Resuspension of existing bed material due to propeller wash at mean high high water (MHHW), when barges will be operating, is not predicted. Resuspension of existing bed material in the mouth at all tidal levels when barging occurs (i.e. mean seal level (MSL) is not predicted. Barging at MSL (when vessels fully laden) is has been shown to have the potential to result in erosion of the bed in the proposed navigation channel in the mid, upstream and ebb bar areas of the Skardon River. Erosion will occur at a single point on the bed for approximately 60 seconds as the vessel passes. Erosion resulting from the propeller wash of the barge is expected to predominantly be within the centre of the navigation channel (predicted maximum width of approximately 20m). In the upstream areas any erosion resulting from the propeller jet would be significantly reduced once the softer surface sediment has been eroded. Along the ebb bar, where the highest bed shear stresses are predicted due to the propeller wash, it is likely that as the sand sized sediment is eroded by the propeller wash the bed would become armoured as coarser gravel sized sediment is left (material on the ebb bar is up to 25% gravel), thereby protecting the bed from future erosion.

Release criteria have been proposed for the Port sediment ponds based on water quality of the Skardon River or on Australian Water Quality Guidelines.

The chemical nature of the sediments from the entrance does not present a risk of contamination to marine biota or impact to water quality. Concentrations of metals contaminants within the bed levelling areas do not represent problematic levels and impacts outside turbidity elevations are not anticipated. ASS investigations will be undertaken as required, although risk of exposure of ASS is low due to wharf construction methodology, and an ASS Management Plan prepared if required.

The primary management and mitigation measures for these impacts are:

vessel management, including speed and areas of operations

speed controls on vessel to minimise vessel wake waves and sedimentation from propeller wash

barges using a defined navigation channel to minimise the extent of disturbance from propeller

wash

wharf design, using piles, which minimises hydrodynamic impacts and potential ASS impacts

release criteria for the Port sediment ponds

design features to prevent spills of contaminants and response plans for spills of contaminants

compliance with ballast water management regulations

ongoing marine water quality and marine sediment monitoring

With the implementation of the proposed marine infrastructure design and construction, operational plans and mitigation measures, the risk of impacts to coastal processes and the physical marine environment is predicted to be low to medium.

hapter 17 – oastal pro esses - metro mining€¦ · skardon river bauxite project chapter 17 –...

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