bauxite hills project - metro mining · 2018. 8. 9. · 1.4 report structure 4 2 vessel wake waves...

Metro MiningBauxite Hills Project

Environmental Impact Statement

Metro MiningChapter 21 - References

Environmental Impact Statement

Metro MiningAppendix E - Bauxite Hills Project Skardon River Vessels Assessment

REPORT

Bauxite Hills Project

Skardon River Vessel Assessment

Client: Metro Mining Ltd

Reference: M&APA1066102R001D01

Revision: 01/Draft

Date: 24 June 2016

P r o j e c t r e l a t e d

24 June 2016 SKARDON RIVER VESSEL ASSESSMENT M&APA1066102R001D01 i

HASKONING AUSTRALIA PTY LTD.

Unit 2 55-57 Township Drive

QLD 4220 Burleigh Heads Australia

Maritime & Aviation Trade register number: ACN153656252

[email protected] royalhaskoningdhv.com

E W

Document title: Bauxite Hills Project

Document short title: Skardon River Vessel Assessment Reference: M&APA1066102R001D01

Revision: 01/Draft Date: 24 June 2016

Project name: Skardon River Vessel Assessment Project number: PA1066

Author(s): Andy Symonds

Drafted by: Andy Symonds, James Lewis

Checked by: Dan Messiter

Date / initials: 23/06/2016

Approved by: Dan Messiter

Date / initials: 23/06/2016

Classification

Project related

Disclaimer No part of these specifications/printed matter may be reproduced and/or published by print, photocopy, microfilm or by

any other means, without the prior written permission of Haskoning Australia PTY Ltd.; nor may they be used, without

such permission, for any purposes other than that for which they were produced. Haskoning Australia PTY Ltd.

accepts no responsibility or liability for these specifications/printed matter to any party other than the persons by

whom it was commissioned and as concluded under that Appointment. The quality management system of Haskoning

Australia PTY Ltd. has been certified in accordance with ISO 9001, ISO 14001 and OHSAS 18001.


24 June 2016 SKARDON RIVER VESSEL ASSESSMENT M&APA1066102R001D01 ii

Table of Contents

1 Introduction 1

1.1 Introduction 1 1.2 Scope 1 1.3 Proposed Development 1 1.4 Report Structure 4

2 Vessel Wake Waves 5

2.1 Operational Vessels 5 2.2 Background 7 2.2.1 Primary Waves 7 2.2.2 Secondary Waves 8 2.3 Approach 9 2.4 Results 9 2.4.1 Primary Waves 9 2.4.2 Secondary Waves 10

3 Propeller Wash 14

3.1 Introduction 14 3.2 Natural Conditions 14 3.3 Propeller Wash Impacts 20 3.4 Plume Dispersion Modelling 24 3.4.1 Plume Dispersion Results 24

4 Summary 37

5 References 38

Table of Tables

Table 1 Metro Mining Operational Vessels .............................................................................................. 5

Table 2 Existing conditions at the five propeller wash calculation sites. ................................................ 10

Table 3 Existing conditions at sites in the Skardon River ..................................................................... 20

Table 4 Assumed vessel specification of the proposed tugs for the propeller wash assessment. ........ 21

Table 5 Model grid configuration. ........................................................................................................... 40

Table 6 Water level calibration period statistics, May 2015. .................................................................. 44

Table 7 Current speed and direction calibration period statistics, May 2015. ........................................ 46


24 June 2016 SKARDON RIVER VESSEL ASSESSMENT M&APA1066102R001D01 iii

Table of Figures

Figure 1 The barge loading facility. ........................................................................................................ 2

Figure 2 The Upstream Facilities including the Ro-Ro facility. .............................................................. 3

Figure 3 Proposed operation vessels .................................................................................................... 6

Figure 4 Primary wave components of ship induced water motions...................................................... 8

Figure 5 Secondary wave pattern. ......................................................................................................... 9

Figure 6 Critical segments of Skardon River for the transit of the barge and tugs. ............................. 10

Figure 7 Secondary wave pattern for multiple vessels with the same velocity .................................... 11

Figure 8 Secondary wave heights with distance from the vessel. ....................................................... 12

Figure 9 Upstream locations where the navigational channel is close to the channel bank. .............. 13

Figure 10 River mouth location where the navigational channel is close to the channel bank. ............ 13

Figure 11 Water depth and propeller wash calculation locations. ....................................................... 15

Figure 12 Peak flood tidal currents in Skardon River............................................................................. 16

Figure 13 Peak flood tidal currents at the ebb bar offshore of Skardon River. ...................................... 17

Figure 14 Peak ebb tidal currents in Skardon River. ............................................................................. 18

Figure 15 Peak ebb tidal currents at the ebb bar offshore of Skardon River. ........................................ 19

Figure 16 Calculated near bed current speed resulting from the tug propeller wash ............................ 21

Figure 17 Existing bed elevation along the proposed navigation channel ............................................. 22

Figure 18 Relationship between erosion rate and height above elevation threshold for sandy mud. ... 23

Figure 19 Relationship between erosion rate and height above elevation threshold for sand. ............. 23

Figure 20 Spring 1- Modelled maximum Total Suspended Solid concentration .................................... 25

Figure 21 Spring 2 - Modelled maximum Total Suspended Solid concentration ................................... 26

Figure 22 Neap 1 - Modelled maximum Total Suspended Solid concentration ..................................... 27

Figure 23 Neap 2 - Modelled maximum Total Suspended Solid concentration ..................................... 28

Figure 24 Manoeuvring Simulation - Modelled maximum Total Suspended Solid concentration. ........ 29

Figure 25 Spring 1 - Time series plots of TSS. ...................................................................................... 32

Figure 26 Spring 2 - Time series plots of TSS. ...................................................................................... 33

Figure 27 Neap 1 - Time series plots of TSS during the Neap 1 simulation. ......................................... 34

Figure 28 Neap 2 - Time series plots of TSS. ........................................................................................ 35

Figure 29 Manoeuvre Simulation - Time series plots of TSS. ............................................................... 36

Appendices

Appendix A – Hydrodynamic Model Details

24 June 2016 SKARDON RIVER VESSEL ASSESSMENT M&APA1066102R001D01 1

1 Introduction

1.1 Introduction Metro Mining Ltd commissioned Haskoning Australia Pty Ltd, a company of Royal HaskoningDHV (RHDHV), to undertake a vessel induced erosion assessment for the Bauxite Hills Project (herein called the Project). This work is in addition to a previous coastal assessment included as part of an Environmental Impact Statement (EIS). As such, further information on the existing metocean conditions and coastal processes at Skardon River are provided in the relevant EIS appendix prepared by Ports and Coastal Environmental (2016).

1.2 Scope The scope of work for this assessment is as follows:

to assess the impacts of vessel wake waves from the barge and tug movements within the Skardon River;

to predict the near bed current speeds and potential erosion from the proposed barge and tug movements; and

to predict the advection and dispersion of the suspended sediment resulting from the proposed barge and tug movements within the Skardon River.

1.3 Proposed Development The Project is anticipated to produce 1 million tons per annum (Mtpa) of bauxite suitable as direct shipping ore during its first year and from year two onwards this is proposed to increase to up to 5 Mtpa. The total Project life is expected to be 12 years. The marine based components of the Project are shown in Figure 1 and Figure 2 and incorporate the following:

Barge loading facility (see Figure 1) consisting of:

o 100m long, 6m wide crest causeway along the alignment of the outloading conveyor; o piled jetty with a 6m wide concrete deck; o loading head deck of 12m by 12m to support the barge loader; and o berthing dolphins to act as a series of structures to berth the vessel against and

provide mooring points for the vessel.

Roll on/Roll off (RoRo) facility on the Skardon River (see Figure 2); and

Barging of bauxite to a proposed offshore transhipment location in deep water approximately 15 km offshore of the Skardon River mouth, and transfer from barges to bulk carriers:

o Barges with a capacity of 3,000t will be used for year one and then larger barges with a capacity of approximately 7,000t will be used from year two;


o The bauxite transportation will be via barge through the Skardon River 24 hours per day during an eight to nine month operational period (no operations during the wet season); and

o There are expected to be 6 to 7 barge movements per day during the operational period over the 12 year project duration (Metro Mining, 2016).

Figure 1 The barge loading facility.


Figure 2 The Upstream Facilities including the Ro-Ro facility.


1.4 Report Structure

The report herein is set out as follows:

in Section 2, the vessel wake wave assessment is provided; in Section 3, the propeller wash calculations and plume dispersion modelling are presented;

and in Section 4, a summary of the assessment is provided along with recommendations based

on the study findings.


2 Vessel Wake Waves This section provides details of the vessel wake assessment. There is potential for vessel wake waves to occur along the Skardon River caused by barges and associated tug boats en route to and from the offshore transhipment site.

2.1 Operational Vessels The three main vessel properties that contribute to the magnitude of vessel wake waves and hence the potential for erosion of the river banks are;

vessel size; maximum draft; and vessel speed.

To calculate the worst-case scenario vessel wake waves that may be induced as a result of operations, the larger vessels proposed to be used from year two onwards have been assessed. The barges proposed are dumb (not self-propelled) and will be pulled by a shallow draft tug. Details of the proposed vessels are shown in Figure 3 and Table 1.

Table 1 Metro Mining Operational Vessels

Vessel

Shallow Draft Tug Length O.A: 26 m Length B. P: 24 m Breadth: 11.5 m Depth: 3.5 m Draft Loaded: 2.2 m Gross Tonnage: 290T

Work Barge Length O.A: 91.5 m Breadth: 30 m beam Draft max: 3.5 m Deck Loading: 7 ton/m2: Capacity: 7,000T


Figure 3 Proposed operation vessels. Top: Shallow Draft Tug, Bottom: Barge (source: Metro Mining, 2016).


2.2 Background There are two main types of waves generated by moving vessels:

1. Primary wave (or drawdown wave); and 2. Secondary waves caused by discontinuities in the hull profile.

These are described in the following sections.

2.2.1 Primary Waves As a vessel moves, water flows past the hull in the opposite direction to the direction the vessel is travelling. This flow is known as the return current. The velocity head of the water flowing past the vessel causes the water level along the vessel’s length to fall in order to maintain the total head (energy) constant. Therefore the water level around the vessel is lowered (Figure 4). The transition between the undisturbed water level in front of the vessel and the water level depression is in the form of a sloping water surface referred to as the front wave. The transversal stern wave is the transition between the water level depression and the normal water level behind the ship. The combination of; the water level depression, front wave and transversal stern wave, act like a solitary wave with a similar wavelength to the vessel. This drawdown, or primary wave does not break at the shoreline like a normal wave, instead behaves more like a tidal pulse rising and falling as the vessel passes. Primary waves tend to be relatively minor if the channel is sufficiently wide and deep.


Figure 4 Primary wave components of ship induced water motions (source: PIANC, 1987).

2.2.2 Secondary Waves Due to the sharp rise and fall in the water surface at the bow and stern, inertia causes the water surface to lag behind its equilibrium position and produces a surface oscillation. This produces a pattern of free surface waves which propagate from the vessel, known as secondary waves (Figure 5). The pattern spreads out from the vessel with decreasing wave amplitude due to diffraction. The pattern consists of symmetrical sets of diverging waves that move obliquely out from the sailing line and a single set of transverse waves that move in the direction of the sailing line. The transverse and diverging waves meet to form cusps, the highest waves in the pattern are found along this cusp focus line. Secondary waves are generally visible in the field and on aerial photographs. These waves may also break as they approach the bank shoreline.


Figure 5 Secondary wave pattern (source: Schierech, 2001).

2.3 Approach The vessel speed is a critical parameter when calculating vessel wake waves. The barges and tug boats proposed at Skardon River will be travelling at speeds of between 4 and 6 knots along the channel. For this assessment the higher speed of 6 knots has been assumed as this is the worst case in terms of vessel wake waves.

2.4 Results

2.4.1 Primary Waves The large barge vessel is expected to produce the largest primary wave due to its large draft and length. As such, the cross-sectional area of the vessel, which is required to calculate the primary wave, was calculated assuming that the vessel was fully laden (e.g. draft of 3.5m, see Table 1). Cross-sectional area was calculated at four locations in the Skardon River as shown in Table 2 and Figure 6. 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 300m north of the proposed wharf and has a cross-sectional area of 695m2 relative to MSL (the minimum water level that the entire river can be navigated by a fully laden vessel).


Table 2 Existing conditions at the five propeller wash calculation sites.

Mouth Mid West Mid East Wharf

Depth (m MSL) mean/max -7.8/-12 -4.95/-7.45 -7.3/-9.3 -4.0/-4.5

Cross-Sectional Area m2 2,250 2,460 2,125 1,135

Figure 6 Critical segments of Skardon River for the transit of the barge and tugs.

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 0.13m has been predicted. Given the relatively small magnitude, primary waves from the proposed barges are not expected to result in any significant erosion within the Skardon River.

2.4.2 Secondary Waves 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, as well as a dimensionless multiplier, α1. This value is seen to range from 0.35 for conventional motor vessels to 1.0 for tugs, patrol boats and loaded inland motor boats. Wave heights are seen to increase with increasing speed and decrease with increasing depth. For this study α1 was taken to be 0.7 for the loaded barges and 1.0 for the tugs. Because of the larger value of the coefficient α1 for tugs, secondary waves for tugs will be larger than the barges. During escort operations, the vessels will be travelling at the same speed and the secondary waves generated by the tug and barge will not combine. This is because the sources of wave generation

Entrance

Wharf

Mid East

Mid West


remain separated by a constant distance. The overall effect from a reference point some distance away from the vessels will be a longer duration of incoming waves and not an increase in relative height (see Figure 7). As such, only the larger secondary waves induced by the tug have been investigated.

Figure 7 Secondary wave pattern for multiple vessels with the same velocity (Schierech, 2001),

The secondary wave heights for the proposed tug, 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 8. The plot shows that although wave heights of up to 0.35m can occur directly adjacent to the vessel, these reduce to; 0.1m or less at a distance of 50m away from the vessel sailing line; and less than 0.08m at a distance of 100m away from the vessel sailing line. There are three locations within the Skardon River where the proposed navigation channel is relatively close to the channel bank:

1. at the proposed wharf location the proposed navigational channel is approximately 100m away from the west bank (Figure 9). Due to the reduced vessel speed when berthing and manoeuvring away from the wharf there is not expected to be any erosion in the vicinity of the wharf due to vessel wake waves;

2. approximately 300m upstream of the proposed wharf and adjacent to the existing Skardon Port boat ramp the proposed navigational channel is approximately 100m away from the east and west banks of the river, respectively (Figure 9). These areas have a minimum depth of approximately 6m when the barge and tug could navigate the entire river, and so the anticipated wave height at the bank is estimated to be approximately 0.06m. This size wave is not expected to result in erosion of the bank, especially considering the stability of the bank in both locations due to the existing mangrove vegetation present; and


3. the mouth of the Skardon River is relatively constrained and as a result the proposed navigational channel is approximately 100m away from the south bank of the river (Figure 10). This is a deep section of the river with a minimum depth of approximately 12m when the tug and barge could navigate the entire river. As a result the anticipated wave height at the bank due to the vessels is estimated to be less than 0.04m. 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 speeds in excess of 6 knots were adopted then the resultant wave heights could be much larger. For example, an increase in tug speed to 9 knots at a depth of 4m would result in secondary waves of 1.8m adjacent to the barge and 0.42m at a location 100m away from the barge. Accordingly, it is important that 6 knots is adopted as the maximum speed within the Skardon River to minimise the risk of bank erosion due to vessel wake waves.

Figure 8 Secondary wave heights with distance from the vessel.

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0 50 100 150 200 250 300

Wav

e H

eigh

t (m

)

Distance from Vessel (m)

Depth = 4m

Depth = 6m

Depth = 8m

Depth = 12m


Figure 9 Upstream locations where the navigational channel is close to the channel bank.

Figure 10 River mouth location where the navigational channel is close to the channel bank.

Locations where navigational channel is approx. 100m from bank

Location where navigational channel is approx. 100m from bank


3 Propeller Wash

3.1 Introduction Vessel movements generate a current field behind the ships propeller; this is commonly referred to as the propeller wash. As part of this study it was necessary to evaluate the impact of any propeller wash resulting from the movement of the tug pulling the barge to and from the transhipment location and from the tug manoeuvring at the wharf. The study has the following aims:

predict bed current speeds and shear stresses resulting from propeller wash due to the tug operations in the Skardon River; and

estimate potential erosion rates, the associated plume dispersion and suspended sediment concentrations due to the propeller wash.

3.2 Natural Conditions A hydrodynamic model was developed as part of the Skardon River Bauxite Project and through a data sharing agreement between Gulf Alumina and Metro Mining the same model has been used for this study. The model has been used to describe the existing tidal currents in the Skardon River and to simulate the advection and dispersion of suspended sediment resulting from propeller wash erosion in the Skardon River. Further details on the model setup, configuration and calibration are provided in Appendix A. The model bathymetry is shown in Figure 11. Figure 12 to Figure 15 show the peak flood and ebb tidal currents in the Skardon River (noting that the timing of peak flood and ebb differs between the river and the ebb bar). The plots show the following:

the peak flood current speeds are typically in excess of 0.5m/s throughout the main channel up to the confluence in the river, upstream of this point the peak flood currents drop to less than 0.5m/s;

the highest peak flood current speeds occur at the mouth, with currents in excess of 0.9m/s;

peak flood currents occur at the ebb bar approximately 1 hour 20 minutes earlier than in the river. The peak current speeds in the outer part of the bar are less than 0.4m/s, while the areas of the ebb bar further inshore are typically more than 0.6m/s;

the peak ebb current speeds are generally higher than the peak flood speeds. The peak ebb current speeds are consistently more than 0.6m/s from the confluence to the mouth, with some areas in the mid channel experiencing current speeds in excess of 0.8m/s;

the highest peak ebb current speeds occur at the mouth, with speeds exceeding 1.0m/s; and

peak ebb currents occur at the ebb bar approximately 3 hours 30 minutes later than at the mouth, close to the time of low water. Current speeds of up to 0.7m/s occur in the inner and mid areas of the bar, reducing to less than 0.3m/s toward the seaward end of the ebb bar.


Figure 11 Water depth and propeller wash calculation locations. Note: the navigation channel is shown by the pink line.


Figure 12 Peak flood tidal currents in Skardon River.


Figure 13 Peak flood tidal currents at the ebb bar offshore of Skardon River.


Figure 14 Peak ebb tidal currents in Skardon River.


Figure 15 Peak ebb tidal currents at the ebb bar offshore of Skardon River.


A summary of the existing tidal currents at the locations shown in Figure 11 in the Skardon River is provided in Table 3. The table also shows:

how the existing bed sediment types vary through the river. These have been defined based on the sediment samples collected in 2014 by RPS and in 2015 by Ports and Coastal Environmental Pty Ltd;

the calculated critical erosion threshold of the bed sediment. This has been estimated based on the sediment properties and using the approach detailed by Van Rijn (1993); and

average and maximum bed shear stresses (BSS) resulting from the tidal currents over a modelled 29 day lunar cycle at the sites (derived from the hydrodynamic model).

Table 3 Existing conditions at sites in the Skardon River (for site locations see Figure 11).

Wharf Upstream Mid

Upstream Mid Mouth Ebb Bar

Depth (m MSL) 6.2 5.1 8.0 6.5 12.0 4.0

Average Speed (m/s) 0.16 0.21 0.31 0.28 0.41 0.11

Median Speed (m/s) 0.15 0.20 0.29 0.26 0.38 0.08

75th Percentile Speed (m/s) 0.21 0.29 0.42 0.38 0.57 0.14

95th Percentile Speed (m/s) 0.31 0.43 0.63 0.57 0.82 0.30

Maximum Speed (m/s) 0.40 0.58 0.84 0.74 1.06 0.52

Assumed Sediment type Sandy Mud Sandy Mud Sandy Mud Sand Coarse Sand

Sand

Critical Erosion Threshold for Suspension (N/m2)

0.5 0.5 0.5 5 13.95 5

Average BSS (N/m2) 0.03 0.06 0.12 0.10 0.18 0.02

Maximum BSS (N/m2) 0.16 0.35 0.66 0.54 0.94 0.36

The modelled bed shear stresses indicate that it is only at the Mid Upstream site where any resuspension of bed material is expected. Any resuspension would typically occur during large spring tides when the strongest tidal currents occur. 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 (in the range of 0.1 to 0.2 N/m2) and would therefore be expected to be resuspended naturally at regular intervals.

3.3 Propeller Wash Impacts A number of different approaches were considered and it was determined that the approach derived by Maynord (2000) was the most suitable for this assessment as it was derived based on tug vessels and allows the impacts of multiple propellers to be included. As the proposed tug has three propellers it is important to ensure the potential impacts of all the propellers are taken into consideration. Details of the proposed tug vessels are provided in Table 4. To represent the worst case in terms of potential erosion from propeller wash the larger barge and associated tug vessel proposed to deliver up to 5 Mtpa from year two onwards has been adopted. The proposed barges have a maximum draft of up to 3.5m depending on the payload (see Table 1), while the tug has a draft of 2.2m. For this assessment a barge draft of 3m and a total depth of 3.5m to allow safe passage of the vessels (i.e. a minimum under keel clearance of 0.5m for the barge), has been assumed. This assumption means


that the tug propellers are 0.5m closer to the seabed than when the maximum draft of 3.5m is assumed, therefore the potential impact of the propeller wash on the sea bed will be larger.

Table 4 Assumed vessel specification of the proposed tugs for the propeller wash assessment.

Aspect Details

Length Overall (LOA) 26 m

Width 11.5 m

Summer loaded draft 2.2 m

Propulsion 3 x 829 BHP @ 1900 RPM

Propeller 3 x 1800 mm diameter

Currents resulting from the tug propellers have been calculated for a range of depths representative of the depths through the river and ebb bar when the tug and barge could safely navigate the entire area. An example of the spatial distribution of the near bed current resulting from the tug’s propeller wash is shown in Figure 16.

Figure 16 Calculated near bed current speed resulting from the tug propeller wash. (water depth of 6.5m and bed elevation of -

4m CD). Note: the axes scales are not even.

The bed shear stress resulting from propeller wash bed current was calculated and based on the critical erosion thresholds for suspension (detailed in Table 3) and a potential erosion rate was calculated at the different areas of the navigational channel. The calculations showed that potential resuspension of bed sediment due to the propeller wash of the tug could occur when the bathymetry was shallower than 5m below CD for sandy mud and shallower than 2.5m below CD for sand (assuming a water level approximately equal to mean sea level = 2.2m CD). The bathymetry along the navigational channel (based on the September 2015 survey) is shown in Figure 17 along with the


elevation threshold for potential resuspension due to the tug propeller wash. The plot shows that potential resuspension of the bed is predicted along a 2km stretch of the navigation channel immediately downstream of the wharf and a stretch of approximately 2km towards the seaward end of the ebb bar. These areas are represented by the Wharf, Upstream and Ebb Bar sites in Table 3, which shows that under existing conditions there is limited natural resuspension of the bed sediment due to tidal currents.

Figure 17 Existing bed elevation along the proposed navigation channel. The red dashed line shows the elevation threshold

below which no erosion of sediment is expected (based on a water level of approximately MSL). The change in depth at

chainage 5,500m is due to a change in the bed sediment from sandy mud to sand.

The relationship between the erosion rate and height above the elevation threshold is shown for sandy mud and sand in Figure 18 and Figure 19, respectively. The plots show that the potential erosion rates increase exponentially as the bed elevation increases above the elevation threshold. These relationships were used to calculate the potential erosion rate along the navigation channel. The rates were then averaged along 500m lengths of the channel where erosion was predicted. The average rates, along with the predicted near bed currents from the propeller wash, were included in the hydrodynamic model. The duration of the suspended sediment and bed current was calculated assuming a vessel speed of 6 knots, with the vessel therefore potentially disturbing the bed for a total of 160 seconds over each 500m length.


Figure 18 Relationship between erosion rate (for resuspension) and height above the elevation threshold for sandy mud.

Figure 19 Relationship between erosion rate (for resuspension) and height above the elevation threshold for sand.


3.4 Plume Dispersion Modelling The hydrodynamic model was setup to represent the advection and dispersion of sediment predicted to be suspended by the propeller wash from the tug while en route to and from the transhipment location. Details of the sediment suspended by the tug propeller wash were included in the model, based on the results detailed in the previous section, along the navigational channel in the Skardon River. Four simulations were used to test the potential impacts and were selected to represent the worst cases for bed sediment being eroded within the river. The timings were configured so that vessels crossed the shallowest point of the river (elevation of -1.2m CD on the ebb bar) at the minimum depth (assuming the vessels required a total depth of 3.5m, resulting in a water level of 2.3m above CD) which was approximately equivalent to mean sea level (MSL = 2.2m above CD). The scenarios tested were as follows:

1. Springs 1: Tug and barge travelling from the wharf to the offshore transhipment location on a rising spring tide, assuming minimum depth for navigation over the ebb bar.

2. Springs 2: Tug and barge travelling from the transhipment location to the wharf on a dropping spring tide, assuming minimum depth for navigation over the ebb bar.

3. Neaps 1: Tug and barge travelling from the wharf to the transhipment location on rising Neap tide assuming minimum depth for navigation of ebb bar.

4. Neaps 2: Tug and barge travelling from the transhipment location to the wharf on a dropping Spring tide assuming minimum depth for navigation of ebb bar.

A vessel speed of 6 knots has been assumed in all four of these simulations, resulting in a vessel transit time from the wharf to the offshore end of the ebb bar (and therefore sediment release duration) of approximately 85 minutes.

It is expected that the tug vessels could be required to manoeuvre at the wharf between journeys to the transhipment location. To assess the potential impacts the predicted erosion rate due to a tug manoeuvring at the wharf was calculated assuming a mean low water spring water level (1m CD). It was assumed that vessel manoeuvring would last for 15 minutes. A fifth model scenario was undertaken to simulate this operation.

3.4.1 Plume Dispersion Results Initial analysis of the plume dispersion modelling results showed that the plume was of a relatively low concentration (peak concentrations away from the source of approximately 5mg/l) and that it dispersed relatively quickly (peak concentrations last for less than 1 hour). As such, the mean and median Total Suspended Solid (TSS) concentrations over the duration of the vessel transit through the Skardon River are less than 1mg/l throughout and therefore have not been presented. To show the full extent of the areas potentially impacted by the plume, the maximum TSS which occurred over the model simulation period is shown for the five scenarios in Figure 20 to Figure 24. It is important to note that these plots do not show an instantaneous representation of the plume, rather the maximum concentration at each discrete grid cell in the Skardon River model representation which occurred over the duration of the model simulation.


Figure 20 Spring 1- Modelled maximum Total Suspended Solid concentration (mg/l).


Figure 21 Spring 2 - Modelled maximum Total Suspended Solid concentration (mg/l).


Figure 22 Neap 1 - Modelled maximum Total Suspended Solid concentration (mg/l).


Figure 23 Neap 2 - Modelled maximum Total Suspended Solid concentration (mg/l).


Figure 24 Manoeuvring Simulation - Modelled maximum Total Suspended Solid concentration (mg/l).

14 March 2016 SKARDON RIVER BAUXITE PROJECT M&APA1066R001F01 30

It is important to consider the following when interpreting the propeller wash erosion results:

erosion resulting from the propeller wash of the barge is expected to be predominantly 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 tugs manoeuvre in this area;

it is expected that in the upstream area 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; and

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 50N/m2.

The plots show that the highest TSS concentrations simulated occur due to the vessel movements between the model output locations, Wharf and Upstream (this is approximately the location of the Skardon Port). The sediment resuspended by the propeller wash over this area is then either advected up or down stream depending on the tidal state (upstream on the flooding tide, downstream on the ebbing tide). The TSS concentrations reduce rapidly as the suspended sediment is transported away from the source, with maximum TSS concentrations of generally less than 5mg/l upstream of the Wharf and downstream of the Upstream output location. There is less advection of the suspended sediment during the neap tides compared to spring tides and as such the TSS concentrations are higher but the total plume extent is smaller. During spring tides maximum TSS concentrations greater than 1mg/l can occur 2km downstream of the Mid Upstream output location (Figure 21) and up to 1.5km upstream of the Wharf Upstream output location (Figure 20).

The highest maximum TSS concentrations simulated occurred during the vessel manoeuvring simulation, with concentrations at the Wharf output location reaching 25mg/l during the 15 minutes the vessel manoeuvring was assumed to occur. Suspended sediment from this source location was subsequently transported upstream (as the simulation assumed low water, the following tidal state was a flooding tide), with TSS concentrations in excess of 1mg/l extending almost 4km upstream of the Wharf Upstream output location, although the maximum concentrations in this area remained less than 5mg/l.

Time series plots showing how the TSS concentrations vary with time at the four fixed locations noted on the map plots are shown in Figure 25 to Figure 29. The plots highlight how the simulated propeller wash erosion from the tug, results in a short duration increase in TSS concentrations during the time the vessels operate in the area. Following cessation of the propeller wash, either due to the vessel having travelled through the area or the manoeuvring finishing, the TSS concentrations quickly reduce. At the output locations where peaks in TSS concentration of more than 5mg/l were simulated the concentration typically reduced below measureable levels (less than 1mg/l) by the subsequent tide. The results therefore indicate that the plume is advected and dispersed quickly due to the natural tidal currents in the river. As such, any increase in suspended sediment concentration due to propeller wash from the tug vessels is expected to be of a short duration, with concentrations above 5mg/l only influencing the upstream areas of the river where direct erosion from propeller wash is anticipated.


Over the duration of the project there is expected to be up to seven vessel movements per day. Based on this it is possible that two vessels could be transiting the river simultaneously and as such the TSS concentrations could be higher than shown in the modelling which assumed a single vessel. The worst case for combined impacts from vessels movements is expected to be vessels travelling from the wharf to the transhipment area in a convoy (i.e. one after the other). If we assume that this scenario could result in double the TSS concentration relative to a single vessel (it is expected to be less) then short duration peak TSS concentrations of up to 50mg/l could occur. However, these would be rapidly (within one hour) advected and dispersed to concentrations of less than 10mg/l and on the following tide the concentrations would be expected to have reduced to less than 2mg/l.


Figure 25 Spring 1 - Time series plots of TSS.


Figure 26 Spring 2 - Time series plots of TSS.


Figure 27 Neap 1 - Time series plots of TSS during the Neap 1 simulation.


Figure 28 Neap 2 - Time series plots of TSS.


Figure 29 Manoeuvre Simulation - Time series plots of TSS.


4 Summary A summary of the main findings of this assessment are as follows:

based on the proposed vessel speed limit of 6 knots within the Skardon River the wake waves estimated to be generated by vessels are small when they reach the shoreline. These waves are not anticipated to significantly impact the banks of the Skardon River;

propeller wash from the tugs as they pull the barges to and from the transhipment location has been shown to have the potential to result in erosion of the bed in the proposed navigation channel in the upstream and ebb bar areas of the Skardon River. The highest estimated erosion rates occur in the upstream areas where the sediment type is sandy mud;

erosion from propeller wash in both the upstream and ebb bar areas is expected to be limited by the sediment properties as consolidated layers are exposed under the softer surface layers in the upstream area and bed armouring by coarser sand and gravel occurs on the ebb bar;

the increase in TSS resulting from the propeller wash erosion is estimated to be of short duration (peaks of less than one hour) and relatively low concentration (peaks of less than 25mg/l). The plume is indicated by modelling to be rapidly advected and dispersed with concentrations typically falling below measurable values (approximately 1 – 2mg/l) within one tidal cycle.

Based on the findings of this assessment the following recommendations can be made:

as proposed, the vessel speed should be limited to 6 knots within the Skardon River to ensure that vessel wake waves do not significantly impact the shoreline; and

vessels should navigate along the centreline of the proposed navigation channel wherever possible to limit any erosion of the bed due to propeller wash.


5 References Maynord, S. T., 2000. Physical Forces Near Commercial Tows, Upper Mississippi River-Illinois Waterway System Navigatin Study, ENV Report 19, Vicksburg, MS, 73 pp., Appendices. Metro Mining, 2016. Bauxite Hills Project – Environmental Impact Statement. Chapter 2: Description of the Project. Moffatt & Nichol, 2003. Arthur Kill Ship Wave Study. Final Report prepared for the Port Authority of New York and New Jersey. PIANC, 1987. Guidelines for the Design and Construction of Flexible Revetments Incorporating Geotextiles for Inland Waterways. Report of Working Group 4 of the Permanent Technical Committee. Ports and Coastal Environmental, 2016. Bauxite Hills Project – Environmental Impact Statement. Appendix B3 – Marine Ecology and Coastal Processed. Report No. 2015001-001. Report prepared for Metro Mining. Schiereck, G.J., 2001. Introduction to bed, bank and shore protection. Delft University Press. 397p. Van Rijn, L.C., 1993. Principles of sediment transport in rivers, estuaries and coastal seas. Aqua Publications, Amsterdam.


Appendix A – Hydrodynamic Model Details


A1 Description of Model The numerical modelling undertaken has utilised a professional engineering software package MIKE21 released by the Danish Hydraulic Institute (2016 release). The software is designed to simulate flows, waves, sediments and ecology in rivers, lakes, estuaries, bays, coastal areas and seas. The modelling system is designed in an integrated modular framework with a variety of add-on modules, which allows the user to customise the software package to suit project requirements.

The Hydrodynamic (HD) module within the MIKE21 system provide the hydrodynamic basis for other modules such as Sand and Mud Transport. The HD module simulates the water level variations and flows in response to a variety of forcing functions on flood plains, in lakes, estuaries and coastal environments.

A1.1 Model Setup The extent of the model grid along with the interpolated model bathymetry of the Skardon River area is shown in Figure 30. Details of the variable model resolution throughout the domain are provided in Table 5. The numerical modelling domain consists of over 8600 mesh nodes.

Table 5 Model grid configuration.

Description Cell arc length range (m)

Offshore 600

Ebb Bar 150

Bed Levelling Areas 60 x 20

Skardon River 60


Figure 30 Model mesh and interpolated bathymetry.

A1.2 Bathymetry High resolution hydrographic survey data from April 2015 and September 2009 was provided by Gulf Alumina, these surveys were 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 (Figure 31) and was the primary dataset when interpolating onto the numerical model grid (i.e. the lower resolution bathymetric data was only used where there was no higher resolution data available).


Figure 31 Model mesh and extent of bathymetric data (2015 and 2009 hydrographic survey) and offshore C-Map data.

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. The different bathymetric data utilised were corrected to Australian Height Datum (AHD), assuming the correction of -2.52m relative to LAT at the Skardon River Barge Ramp, and then interpolated onto the model grid. The final model bathymetry is shown in Figure 30 on the previous page.

A1.3 Hydrodynamic Model The model boundaries were setup using predicted water levels derived for the Skardon River mouth based on measured data. The boundaries were driven by water levels along the west, north and south boundaries. A spatially uniform bed roughness with a Manning’s M roughness coefficient of 35 was applied. This value represents a medium bed roughness and was found to result in the best model calibration (see Section A1.4).

The hydrodynamic model was calibrated using water level and tidal current data collected at two locations within the Skardon River, at the mouth and upstream of the proposed wharf facility. The calibration process is detailed in the following section and demonstrated that the hydrodynamic model provides a good representation (within the bounds of the desired accuracy) of the tidal currents in the Skardon River.

A1.4 Model Calibration Model calibration is the process of setting physically realistic values for model parameters so that the model reproduces observed values to the desired level of accuracy. A calibration exercise is required to demonstrate that the performance of the hydrodynamic model is considered to be of an appropriate accuracy to satisfy the objectives of the investigation.


Hydrodynamic models are typically calibrated against measured water level and current data at a number of locations throughout the model domain. An assessment of the differences between the measured and modelled values is undertaken to enable the level of calibration achieved to be quantified. The calibration of a hydrodynamic model in which tidal forcing dominates requires a minimum of one spring neap tidal cycle (approximately 14 days) or preferably a full lunar cycle (approximately 29 days).

Locations of the calibration sites for this investigation along with the extent of the model grid and the interpolated bathymetry are shown in Figure 32. The hydrodynamic model was calibrated against measured water level and current data. The model calibration has been undertaken at sites at the mouth and directly upstream of the proposed wharf for the period 01/05/15 to 30/05/15.

Figure 32 Model calibration sites.

A1.4.1 Water Levels Time series plots of the modelled and measured water levels at the mouth and upstream sites are shown in Figure 33 and Figure 34, respectively. The measured water levels have been processed to remove any residual water levels to make the comparison with the output water levels from the model which is forced by predicted tidal levels (i.e. no residual water level influences). The modelled and measured water levels agree very well at the mouth site and well at the upstream site.

It order to further assess the level of calibration achieved, statistical analysis was undertaken to quantify the difference in elevation and timing between the modelled and measured high and low water values. The results of the analysis for the difference in water level at high and low water are presented as absolute values in Table 6.


Table 6 Water level calibration period statistics, May 2015.

Statistical Description Mouth Upstream

Mean HW Difference (m) -0.01 -0.01

Mean HW Difference relative to Tidal Range (%) -0.4 -0.9

Mean LW Difference (m) -0.02 -0.02

Mean LW Difference relative to Tidal Range (%) -1.4 -1.1

RMSD for HW (m) 0.01 0.07

RMSD for LW (m) 0.02 0.06

Mean HW Phase Lag (mins) 2.3 -14.6

Mean LW Phase Lag (mins) 10.2 9.8

Note: The differences in phase of the high and low waters were derived by subtracting the time of the measured value from the time of the model value. A

negative value therefore indicates that the model is early compared to the measured data.

Mean water level differences were less than 0.05m for both sites over the calibration period, indicating only very small differences (<2%) relative to the tidal range. The difference in phasing was less than 15 minutes at both sites at high and low waters. The Root Mean Square Deviation (RMSD) values show that there is some variation between the modelled and measured water levels at the upstream site and very little variation at the mouth site. The differences are comparable for high and low water.

Figure 33 Measured (residual component removed) and modelled water levels at the mouth site.


Figure 34 Measured (residual component removed) and modelled water levels at the upstream site.

A1.4.2 Tidal Currents

The measured tidal currents were analysed as part of a harmonic analysis to remove the influence of any residual events and ensure the currents used for the calibration were due to solely astronomical forcing (the tide). Time series plots for the calibration of current speed and direction are shown in Figure 35 to Figure 38 on the following pages. The plots show that the model is capable of consistently predicting current speed at the two sites. The model consistently replicates the direction of the peak flood and ebb tidal currents at the upstream site. The differences in current direction shown in Figure 37 and Figure 38, when the measured directions are between 200° and 360°, occur around slack water when the current speed is low and so the differences do not influence the models capability of accurately representing the flood and ebb tidal currents at the upstream site. There are some differences between the modelled and measured current directions at the mouth site; this is discussed in more detail below.

A statistical summary of the calibration is provided in Table 7. The mean differences between the modelled and measured peak current speeds and directions, shown for the mouth and upstream sites, are less than 6% of the measured peak speeds over a lunar cycle. This percentage difference shows that the model is well calibrated based on typical guideline ranges for estuaries and coastal areas (e.g. speeds to within 10-20% of measured speeds). The RMSD values are relatively low showing that there is some variability in the model performance but that the modelled currents reasonably consistently replicate the measured currents.

The average difference of 22° between the modelled and measured tidal current direction at the mouth site during the flood tide is related to the complex local bathymetry in this area. The measurement location is to the south-east of a deep channel through the mouth of the river and to the north of a shallow intertidal area. The current directions will be controlled by both of these features


and, as the bathymetry in the model for the intertidal area was inferred (Section A1.2), it is expected that this resulted in the directional difference between the model predictions and the measurements.

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.

Table 7 Current speed and direction calibration period statistics, May 2015.

Statistical Description Mouth Upstream

Mean Difference in Speed of Flood (m/s) -0.03 0.01

Mean Difference in Flood Speed Relative to

Maximum Observed Speed (%) -5.2 1.6

Mean Difference in Speed of Ebb (m/s) 0.02 0.01

Mean Difference in Ebb Speed Relative to

Maximum Observed Speed (%) 4.8 3.2

RMSD for Flood Speed (m/s) 0.05 0.06

RMSD for Ebb Speed (m/s) 0.06 0.06

Mean Difference in Direction of Flood (º) 22 -6

Mean Difference in Direction of Ebb (º) 12 2

Note: The differences were derived by subtracting measured values from model values. A negative value therefore indicates that the model is under-

predicting measured values.


Figure 35 Measured (residual component removed) and modelled tidal current speed and direction at the Mouth site over a lunar

cycle.


Figure 36 Measured (residual component removed) and modelled tidal current speed and direction at the Mouth site over a 10 day

period.


Figure 37 Measured (residual component removed) and modelled tidal current speed and direction at the Upstream site over a lunar

cycle.


Figure 38 Measured (residual component removed) and modelled tidal current speed and direction at the Upstream site over a 10

day period.

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