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Appendix E Dredge dispersion and spoil disposal modelling for the East Arm Wharf

Technical Report

Darwin Port Expansion EIS

Dredge Dispersion and Spoil Disposal Modelling

17 MARCH 2011

Prepared for

Department of Lands and Planning

5th Floor Energy House 18-20 Cavenagh Street Darwin NT 0800

42214000

URS NORTHERN TERRITORY (AUSTRALIA)

East Arm Wharf- Dredge dispersion modelling

Executive summary

As part of an Environmental Impact Statement (EIS) for the East Arm Wharf Expansion, URS/Scott Wilson

has been commissioned to assess the potential for dispersion of fine sediments during dredging operations

in Darwin Harbour and spoil disposal operations in Beagle Gulf.

The site includes for dredging within Darwin Harbour to provide new berth facilities at three locations and

subsequent dredge spoil disposal outside the harbour in Beagle Gulf. Due to the fine sediment content of

the seabed material, the dredging process has the potential to mobilise fine seabed sediments into

suspension. High levels of fine suspended sediment over long periods may have an adverse

environmental impact therefore a dredge dispersion study was commissioned to investigate these potential

impacts in detail.

The dredge method statement proposes the use of two types of cutter suction dredger (CSD) depending

on the dredge location. Local borehole or sediment grab samples were not made available therefore based

on previous studies of the Darwin Harbour, suitable sediment parameters were estimated and applied.

An existing hydrodynamic model for Darwin Harbour was used to provide a description of tidal current

flows and water level variations based upon an unstructured triangular mesh. This model was combined

with mud transport model to simulate the operational cycle of each dredger based on the dredging work

plan and sediment release rates. Model results were extracted over the model domain and at key

locations to provide instantaneous suspended sediment concentration, the unconsolidated deposition

thickness and 95th percentile suspended sediment statistics.

At the Marine Supply Base the highest levels of suspended sediment occur around the dredge location and

along the East Arm Wharf. The tidal currents also transport the plume into Hudson Creek but at a reduced

concentration. Due to suspended sediment entering the shallow inter-tidal areas of Hudson Creek and

Frances Bay, areas of reduced bed shear stress, they tend to encourage sediment deposition. In the

remainder of Darwin Harbour, unconsolidated bed thicknesses are low.

For the Defence Laydown Area, the levels of suspended sediment are lower, compared to the other

dredging operations, due to the smaller capacity dredging plant and dredge volumes. The 95th percentile

plume and area of fine sediment deposition is limited to the immediate vicinity of the dredged area. Beyond

the dredge location suspended sediment is unable to deposit due to the high bed shear stress.

At the Customs base site both a neap and a spring tide simulation were considered due to the short

duration of dredging operations. The impacts associated with the Customs Base dredging are focused

around the East Arm Wharf both in terms of suspended sediment and sediment deposition. Deposition

within Hudson Creek and Frances Bay remains low. The total area of sediment deposition is slightly larger

during spring tides, due to the larger tidal range.

The spoil disposal operation leads to small plumes of relatively high suspended sediment concentration

which are transported with the prevailing current and then disperse. The general current speeds and

resulting bed shear stresses are sufficient to prevent deposition and encourage dispersion. Small areas of

fine sediment deposition are predicted to occur in the entrance to Darwin Harbour, due to low bed shear

stress in these areas and periodic exposure to low levels of suspended sediment. Despite this, levels of

unconsolidated sediment remain very low, even when waves are excluded from the simulation.



Table of Contents

1 Introduction........................................................................................ 1

2 Dredge methodology......................................................................... 2

2.1 Introduction..................................................................................................................... 2

2.2 Dredging Plant................................................................................................................ 3

2.3 Dredging methodology.................................................................................................... 4

2.4 Summary ........................................................................................................................ 5

3 Model setup........................................................................................ 6

3.1 Hydrodynamics............................................................................................................... 6

3.2 Mud transport model....................................................................................................... 6

3.3 Simulation scenarios....................................................................................................... 6

3.4 Simulation outputs .......................................................................................................... 7

4 Impact of dredging operations......................................................... 8

4.1 General discussion of potential for siltation ..................................................................... 8

4.2 Dredge dispersion results – Marine supply base........................................................... 11

4.3 Dredge dispersion results – Barge ramp and hardstand ............................................... 14

4.4 Dredge dispersion results – Tug and small vessel berths (neap tide)............................ 16

4.5 Dredge dispersion results – Tug and small vessel berths (spring tide)......................... 19

5 Impact of spoil disposal operations.............................................. 21

5.1 Disposal methodology................................................................................................... 21

5.2 Spoil disposal results .................................................................................................... 22

6 Conclusions and Limitations ......................................................... 25

6.1 Dredge dispersion studies ............................................................................................ 25

6.2 Dredge disposal studies................................................................................................ 27

7 References ....................................................................................... 28

Appendices ................................................................................................. 29

List of figures: Figure 2-1: General location plan of Darwin Harbour, the study area and the receptor locations. ............... 2 Figure 2-2: Nearshore location plan showing the project site and outline of the proposed approach channels to be dredged............................................................................................................................. 3 Figure 4-1: Maximum bed shear stress in Darwin Harbour......................................................................... 9 Figure 4-2: Maximum bed shear stress near the study location.................................................................. 9 Figure 4-3: 95

th percentile suspended sediment concentration................................................................. 12

Figure 4-4: 95th percentile suspended sediment concentration (harbour area) ......................................... 12



Figure 4-5: Unconsolidated fine sediment deposition............................................................................... 13 Figure 4-6: 95


Figure 4-7: 95th percentile suspended sediment concentration (harbour area) ......................................... 15



Figure 4-10: 95th percentile suspended sediment concentration (harbour area)........................................ 17

Figure 4-11: Unconsolidated fine sediment deposition............................................................................. 18 Figure 4-12: 95

th percentile suspended sediment concentration............................................................... 19

Figure 4-13: 95th percentile suspended sediment concentration (harbour area)........................................ 20

Figure 4-14: Unconsolidated fine sediment deposition............................................................................. 20 Figure 5-1: 95



th percentile suspended sediment concentration. Sensitivity test without waves.................. 24

Figure 5-4: Unconsolidated fine sediment deposition. Sensitivity test without waves................................ 24

List of tables: Table 2-1: Comparison of typical fine material discharge loss rate for different types of dredging plant ...... 3 Table 2-2: Fine material discharge rate for proposed dredging plant.......................................................... 4 Table 2-3: Dredging work plan- dredging at the Tug and small vessel berths ............................................ 4 Table 2-4: Dredging work plan- dredging at the Marine supply base .......................................................... 4 Table 2-5: Dredging work plan- dredging at the Barge ramp and hardstand............................................... 5 Table 5-1: Disposal work plan ................................................................................................................. 21



Dredge Dispersion March 2011 1

1 Introduction As part of an Environmental Impact Statement (EIS) for the East Arm Wharf Expansion,

URS/Scott Wilson has been commissioned to assess the potential for dispersion of fine

sediments during dredging operations in Darwin Harbour and spoil disposal operations in

Darwin Bay. The dredge dispersion modelling was based on an existing calibrated

hydrodynamic model of Darwin.

The objectives of this report are to provide a description of:

• Dredge methodology (Section 2);

• Modelling set-up (Section 3);

• Impact of dredging operations (Section 4);

• Impact of spoil disposal operations (Section 5);

• Conclusions (Section 6).




2 Dredge methodology

2.1 Introduction

To provide new berth facilities within Darwin Harbour the dredging operations have been

planned at three locations as shown on the location plans below (Figure 2-1 and Figure 2-2).

At each location the seabed will be dredged to provide sufficient under-keel clearance for

vessels. Figure 2-1 also shows the location of 7 arbitrarily selected receptor points which have

been considered in the modelling.

Figure 2-1: General location plan of Darwin Harbour, the study area and the receptor locations, including a zoom to sites H, I, J around South Shell Island

EAST ARM WHARF

STUDY AREA

Darwin Harbour

Frances Bay

Hudson Creek




Figure 2-2: Nearshore location plan showing the project site and outline of the proposed approach channels to be dredged

Due to the fine sediment content of the seabed material, the dredging process has the potential

to mobilise fine seabed sediments into suspension. High levels of fine suspended sediment

over long periods may have an adverse environmental impact therefore a dredge dispersion

study was commissioned to investigate these potential impacts in detail.

2.2 Dredging Plant

The dredge method statement proposes the use of two types of cutter suction dredger (CSD).

A smaller vessel with a production rate of 10,000 m3/week and a larger vessel 125,000

m3/week depending on the dredge location.

A 1% rate of sediment loss from the dredgers was agreed prior to commencing the modelling

simulations. A summary of release rates is provided in Table 2-1 below based on URS/Scott

Wilson experience and Bray et al (1997). Compared to other dredging plant, the cutter suction

dredgers to be utilised at the site generate a relatively low loss of fines.

Table 2-1: Comparison of typical fine material discharge loss rate for different types of dredging plant

Dredge method

Cutter suction

Trailing suction (no overflowing)

Bottom dump placement

Trailing suction hopper (overflowing)

Loss rate 1% 2% 5% 34%

Local borehole or sediment grab samples were not made available therefore based on previous

studies of the Darwin Harbour, a dry sediment density of 773 kg/m3 was applied for the dredge

material. Applying the 1% loss rates and the production rates specified for the dredging plant

the rate of fine sediment loss was determined (see Table 2-2) and applied at each dredge

location.

Tug and small vessel berths

Marine supply base

Barge ramp and hardstand

EAST ARM WHARF




Table 2-2: Fine material discharge rate for proposed dredging plant Fine sediment loss rate Small CSD 0.13 kg/s Larger CSD 1.60 kg/s

The dredging requirements and dredge methodology for each location are described below.

2.3 Dredging methodology

2.3.1 Site 1: Tug and small vessel berths

The design considers that the large cutter suction dredger (CSD) will be deployed to dredge an

approach channel to the new Tug and small vessel berths facility (Figure 2-2). Table 2-3 sets

out the dredger work plan based on the vessel capacity, speed and efficiency to define the

dredge duration:

Table 2-3: Dredging work plan- dredging at the Tug and small vessel berths

Vessel Type Large CSD Area of Operation approach channel Dredge depth -7 m CD Estimated dredge volume 100,000 m

3

Duration of dredging operation (days)

5.6

Duration of post dredging dispersion monitoring (days)

2

Total duration of simulation (days) 7.6

The dredger is assumed to operate continuously over the 5.6 day period. Within the model

simulation the dredge was represented by a moving point source within the dredge area to

simulate the vessel movement.

2.3.2 Site 2: Marine supply base

The design considers that the large CSD will be deployed to dredge an approach channel to

the new Marine supply base facility (Figure 2-2). Table 2-4 sets out the dredger work plan

based on the vessel capacity, speed and efficiency to define the dredge duration:

Table 2-4: Dredging work plan- dredging at the Marine supply base

Vessel Type Large CSD Area of Operation Marine supply base approach

channel Dredge depth -7.7mCD Estimated dredge volume 1,100,000 m

3


62


14

Total duration of simulation (days) 76

The dredger is assumed to operate continuously over the 62 day period. Within the model

simulation the dredge was again represented by a moving point source within the dredge area.




2.3.3 Site 3: Barge ramp and hardstand

The design considers that the small cutter suction dredger (CSD) will be deployed to dredge an

approach channel to the new Barge ramp and hardstand facility (Figure 2-2). Table 2-5 sets out

the dredger work plan based on the vessel capacity, speed and efficiency to define the dredge

duration:

Table 2-5: Dredging work plan- dredging at the Barge ramp and hardstand

Vessel Type Small CSD Area of Operation Barge ramp and hardstand Dredge depth -2.0mCD Estimated dredge volume 70,000 m

3


49


14

Total duration of simulation (days) 63

The dredger is assumed to operate continuously over the 49 day period using a moving point

source within the dredge area to simulate the vessel movement.

2.4 Summary

To set up the dispersion simulations, the operation cycle of each dredger was simulated using

the dredging work plan and sediment release rates specified above. Following completion of

the dredging operation, the simulation was continued for a further period of up to two weeks to

consider the continued dispersion of sediment.

The model outputs were further analysed to establish the unconsolidated sediment deposition

and suspended sediment concentrations that are the key output parameters required for the

interpretation of potential environmental impacts.




3 Model setup

3.1 Hydrodynamics

The existing hydrodynamic model for Darwin Harbour was used to provide a description of tidal

current flows and water level variations based upon an unstructured triangular mesh. The water

levels and flows are resolved on a flexible triangular mesh, when provided with the bathymetry,

bed resistance, wind field, and hydrographic boundary conditions. The model is provided with

additional functionality through application of a Mud Transport Module which extends the model

capabilities to consider the transport, deposition, erosion and re-suspension of fine sediments.

This module was applied to consider the potential impacts of the proposed dredging

operations.

3.2 Mud transport model

The hydrodynamic model was used to drive the mud transport model, which simulates the fate

of fine sediment associated with dredging. The model results were extracted over the model

domain for suspended sediment concentration and unconsolidated sediment accretion. Time

series at key receptor points (A to G, Figure 2-1) were also extracted to show instantaneous

model results.

Within the model setup, parameters describing the sediment characteristics and critical shear

stress thresholds must be specified. The parameters used in the simulations are as follows:

• Dry density of newly deposited unconsolidated mud: 180 kg/m3.

• Settling velocity of fine suspended sediment: 0. 5 mm/s.

• Critical shear stress for deposition: 0.1 N/m2.

• Critical shear stress for erosion: 0.2 N/m2.

• Erosion constant: 1.0 x10-5 kg/m

2/s

The modelling considered a conservative ambient background suspended sediment

concentration of zero, and therefore all results are presented relative to this level. The

interpretation of these results should consider the significance of this assumption, relative to

any measurements of background suspended sediment obtained at the site. During each

simulation the corresponding wind speed and direction was included over the model domain,

based on time-series data obtained for Darwin Airport. Average wind speeds during the

simulations were 2.6 m/s (~5 knots). The action of waves on sediment transport was not

considered in the model, as wave height is likely to be small given the short fetch lengths. The

sediment transport results are therefore conservative with regard to sediment re-suspension.

3.3 Simulation scenarios

URS/Scott Wilson set out the proposed modelling scope and approach to be adopted in the

dredge dispersion modelling, based on the requirements for the EIA and the proposed dredging

methodology. The scope only considered the baseline conditions. Based on the agreed scope

and methodology the three dredge simulations were completed, each simulation was modelled

independently, therefore the combined impact of all the proposed dredging operations has not

been considered. Due to the small dredge volumes required for the Tug and small vessel

berths and the large capacity dredging plant, the dredge duration was limited to just under 6




days, therefore both spring and neap tides were simulated and analysed independently as the

precise timing of the dredging operation has yet to be determined.

3.4 Simulation outputs

The simulation outputs include the flow speeds and direction which were used to derive peak

bed shear stresses, which provide a preliminary indication of regions of potential deposition

and erosion. Instantaneous and the 95th percentile statistics for suspended sediment

concentration and the net unconsolidated seabed deposition were output to assist with the

interpretation of potential impacts. The 95th percentile and unconsolidated deposition results

are presented below. Instantaneous results are presented in Appendix A. The time series

results for the key parameters at sites A-G (as shown in Figure 2-1) are included in Appendix

B.




4 Impact of dredging operations

4.1 General discussion of potential for siltation

4.1.1 Current speed and sediment transport

Before describing the modelling results, this section discusses the potential for siltation as a

function of the general hydrodynamics around the development site. Suspended sediment will

settle if current speeds are low enough. For sediment transport studies, the potential for

settlement or mobilisation of sediment is usually quantified in terms of the bed shear stress.

When tidal currents are slack and the shear stress is lower than the deposition threshold,

material will begin to settle and siltation will occur. As the current speed increases the shear

stress may exceed the threshold for settlement, preventing sediment from settling but not

necessarily causing sediment erosion. A further increase in the current speed may exceed the

erosion threshold at which point the sediment on the seabed will be re-suspended into the

water column and erosion will occur.

To delineate the potential siltation areas, the maximum bed shear stress distribution for the

existing situation (no development) has been investigated and is plotted in Figure 4-1 below.

The shear stress threshold for erosion is based on assumed values for typical conditions, as no

calibration data were available. The dark blue shading represents the areas for which the bed

shear stress is always less than the deposition threshold assumed to be 0.1 N/m2. Any fine

sediment spread to these areas will potentially settle and have no chance to be re-suspended,

leading to deposition. In the light blue coloured areas, the maximum bed shear stress exceeds

the deposition threshold but is less than the assumed erosion threshold (0.2 N/m2). Fine

suspended sediment that is transported to these areas will only settle at certain times, such

that deposition will occur at a reduced rate. Once material has settled, there is no chance of re-

suspension. The yellow areas are where the bed shear stress exceeds the erosion threshold,

leading to re-suspension and erosion of deposited sediment.

The model results show that throughout most of Darwin Harbour, under typical conditions, bed

shear stresses exceed 0.3 N/m2 (Figure 4-1) and can potentially mobilise fine sediments and

erode seabed sediments. Further into the harbour and within individual creeks and inlets

sheltered from the stronger tidal currents in the main channels, the bed shear stresses are

reduced. Of particular significance to this study are the sheltered inlets to the north of the

proposed dredge sites, were bed shear stresses fall below 0.1 N/m2 (Figure 4-2) and accretion

of fine sediment would be anticipated. These regions appear to coincide with existing mud flat

areas.




Figure 4-1: Maximum bed shear stress in Darwin Harbour

Figure 4-2: Maximum bed shear stress near the study location

4.1.2 Suspended sediment concentration

For each scenario, the spatial distribution of the instantaneous suspended sediment

concentration during spring and neap, ebb and flood tidal conditions was extracted. These

demonstrate the path of the dredge plume and plume extent. The 95th percentile suspended




sediment concentration was also calculated throughout the simulation. It is important to note

that these values are statistics and do not therefore occur at the same time and at an instant in

time the area covered by the plume may be smaller than that shown.

Note that the simulated suspended sediment concentrations are excess values, i.e. are values

above the assumed ambient background level of zero concentration.

4.1.3 Deposition rates

The deposition rates provided in the following sections relate to accumulation of fresh

unconsolidated mud with density of 180 kg/m3. If current speeds remain below the threshold for

re-suspension, the deposited sediment would further consolidate. Once consolidated, the dry

bulk density of the fine sediment would be expected to increase up to 500 kg/m3, reducing the

layer thickness by approximately 20-30%, and consequently the permanent (long-term) siltation

thickness will be reduced accordingly. Note that the consolidation process has not been

considered in the model.

The model considered dredging characteristics for the plants, dredge rates and estimated

durations provided in previous sections; any unspecified increase in the dredge volume, rate or

duration may increase the rate of deposition.

Dredging has been modelled and provides an understanding of suspended fine sediment

transport, its fate and deposition rate over the estimated duration of the dredging works. Key

results from the modelling are discussed below and presented in a series of figures for each

dredge location.




4.2 Dredge dispersion results – Marine supply base

The instantaneous suspended sediment concentration during dredging of the access channel

to the Marine supply base results in a plume of fine sediment entering Hudson Creek. This

plume is dispersed over a significant distance by the strong tidal currents at the point of

dredging, which also leads to increased sediment dispersion. During the ebb tide the plume is

transported to the east and passes the East Arm Wharf, along the main navigation channel into

Darwin Harbour. During periods of slack tide when the tidal currents change direction, an area

of the dredge plume passes to the north of the East Arm Wharf and enters Frances Bay. During

neap tides, suspended sediment concentrations are slightly increased locally due to the

reduced current speed, however the plumes follow a similar path and direction, but of reduced

extent.

The 95th percentile results (Figure 4-3, Figure 4-4) show that in the vicinity of the Marine supply

base, suspended sediment concentrations remain below 5 mg/l. Further to the west into

Hudson Creek suspended sediment concentration fall to 2 mg/l or less. The elevated

suspended sediment concentrations are generally confined to the dredge area, the southern

face of the East Arm Wharf and the outer edge of Frances Bay.

The total unconsolidated sediment was determined for the duration of the simulation (Figure

4-5). Fine sediment accretion is limited to areas exposed to modest levels of suspended

sediment concentration, which also have low bed shear stress. Therefore in the vicinity of the

dredge location and to the south of the East Arm Wharf there is negligible sediment deposition

due to the high bed shear stress. Further inshore, on the inter-tidal area around the perimeter

of Hudson Creek and Frances Bay, the bed shear stress is sufficiently low throughout the

simulation to allow sediment to accrete with an unconsolidated thickness of 0.1-1.0 mm.

Sediment deposition is most significant over the central shallow inter-tidal zone in these areas

and to the north of the East Arm Wharf with unconsolidated sediment in the range 1.0-5.0 mm.

Reviewing the deposition results for the wider area, unconsolidated deposition within Darwin

Harbour is generally less than 0.5 mm and typically occurs in the shallow creek system to the

south east of the East Arm Wharf, with small isolated areas up to 1.0 mm thick.




Figure 4-3: 95th

percentile suspended sediment concentration

Figure 4-4: 95th

percentile suspended sediment concentration (harbour area)




Figure 4-5: Unconsolidated fine sediment deposition




4.3 Dredge dispersion results – Barge ramp and hardstand

Relative to the dredging requirements for the Marine supply base, the Barge ramp and

hardstand facility requires a significantly smaller dredged volume (approximately 7%), therefore

a small dredger is proposed, with a smaller rate of release of fine sediment. Furthermore the

point of dredging is located just outside the main navigation channel, where current speeds and

bed shear stress are reduced. The simulation results reflect this with instantaneous suspended

sediment concentrations during dredging, generally remaining below 1.0 mg/l and within 1 km

of the dredge location. Beyond this range the concentration of suspended fine sediment is

below 0.2 mg/l.

The 95th percentile results (Figure 4-6) show that only in the immediate vicinity of the Barge

ramp and hardstand facility, suspended sediment concentrations exceed 1.0 mg/l. Beyond 100

m from the dredge location suspended sediment concentrations fall to 0.5 to 1.0 mg/l. The 95th

percentile suspended sediment concentration is below 0.2 mg/l throughout the remainder of the

model domain.

The results for the unconsolidated sediment deposition show more widespread deposition than

the suspended sediment concentration would suggest. This is due to low levels of suspended

sediment (less than 0.2 mg/l) entering into Frances Bay and Hudson Creek during each tide

and depositing small volumes of sediment over the shallow inter-tidal areas. These depositions

are small in magnitude but the cumulative effect is sufficient to lead to a small accumulation of

order 0.1-0.5 mm (unconsolidated). The highest levels of deposition occur immediately to the

west of the dredge point in the lee of the East Arm Wharf where unconsolidated thicknesses of

between 1.0-5.0 mm occur over an area about 200 m wide, however, beyond these the

sediment reduces to 1.0 mm or less. Sediment deposition is negligible throughout the

remainder of the model domain and is less than 0.1 mm.

Figure 4-6: 95th





Figure 4-7: 95th






4.4 Dredge dispersion results – Tug and small vessel berths (neap tide)

In terms of dredge volume the dredging required at the Tug and small vessel berths is

comparable to the Barge ramp and hardstand. However, due to the higher water depth at this

location, a larger capacity dredger will be utilised. Therefore the dredge duration is limited to

just under 6 days and could occur during either spring or neap tides, thus both tidal conditions

have been considered. This section considers the neap tide results.

The instantaneous suspended sediment concentration during dredging generates a plume

which during the flood tides is entrained along the southern edge of the East Arm Wharf. At the

eastern end of the wharf the plume disperses covering a wider area into the lee of the wharf.

During the ebb tide, the plume moves clear of the wharf and enters the main navigation

channel, with part of the plume also entering Frances Bay.

The 95th percentile results (Figure 4-9, Figure 4-10) are consistent with the instantaneous

suspended sediment concentration results and show that the suspended sediment is generally

concentrated within the main navigation channel at the East Arm Wharf, and in the lee of the

Wharf to the east and west. The area of highest suspended sediment concentration occurs at

the dredge location (5.0-10.0 mg/l). Beyond this area and generally along the perimeter of the

Wharf, suspended sediment concentrations are reduced to 2.0-5.0 mg/l. Beyond this region

the suspended sediment concentration reduces further and is largely confined to the navigation

channel. Over the model domain the 95th percentile suspended sediment concentrations are

less than 0.5 mg/l.

Despite the elevated levels of suspended sediment in the navigation channel at the East Arm

Wharf, the high bed shear stress levels in these areas prevent the deposition of the fine

sediment. Beyond these areas where the channel width increases, the values of current speed

and bed shear stress reduce allowing deposition to occur.

The most significant sediment deposition occurs immediately adjacent to the dredge location

(10.0-50.0 mm) and to the north of the eastern tip of the East Arm Wharf (5.0-10.0 mm) due to

the reduced shear stress at these locations, which are sheltered from the tidal currents by the

Wharf. Sediment accretion also occurs on the inter-tidal areas within Frances Bay and Hudson

Creek, with unconsolidated depths of 0.1-0.5 mm and a small isolated pocket of sediment to

the east of the East Arm Wharf (1.0-5.0 mm). Over the wider area sediment deposition is

limited to the area around the East Arm Wharf.




Figure 4-9: 95th


Figure 4-10: 95th









4.5 Dredge dispersion results – Tug and small vessel berths (spring tide)

Compared to the neap tide results, spring tides lead to an increase in current speeds and

resulting bed shear stresses. This in turn leaded to a reduction of 95th percentile suspended

sediment concentrations from 2.0-5.0 mg/l (neap) to 1.0-2.0 mg/l to the south of the East Arm

Wharf during dredging. The overall length of the 95th percentile plume extents was also

reduced.

The unconsolidated sediment deposition results are similar for both spring and neap tides with

increased tide range associated with increased tidal excursion distances leading to deposition

of material further into the inter-tidal area in both Hudson Creek and Frances Bay with a

thickness of 0.1 to 0.5 mm. At the point of dredging, sediment tends to deposit further to the

east, due to the increased bed shear stress, therefore sediment deposition is reduced. As a

result deposition around the East Arm Wharf is reduced to the east of the Wharf compared to

the neap simulation, but enhanced in an area further to the north and east of the Wharf with a

similar unconsolidated thickness ranging from 0.5 to 5.0 mm.

Figure 4-12: 95th





Figure 4-13: 95th






5 Impact of spoil disposal operations Following the assessment of the potential for dispersion of dredged fine sediments in Darwin

Harbour during the dredging operations, the modelling considered impacts of fine sediment

spoil disposal from a selected site within Darwin Bay. The objective of the study was to

consider the fate of fine sediment released during the spoil dumping operation. The potential

short term and long term transport of spoil material and fines on the seabed in the disposal

area are beyond the scope of this study.

In setting up the disposal simulation, the model boundaries of the existing dispersion

simulations were extended to the north and to the west, the mud transport model was then re-

run using the same sediment characteristics and transport thresholds specified above for the

following input scenarios.

5.1 Disposal methodology

The disposal method statement proposes the use of bottom dumping hopper barges with a

capacity of 3,000 m3 and an efficiency of 80%. A 5% rate of sediment loss from the dumping

operation was agreed, based URS/Scott Wilson experience and Bray et al (1997). Based on

the dry sediment density of 773 kg/m3 considered above, less than 1% loss rate from the

dredging operation, an average fine sediment loss rate of 41.4 kg/s was considered over the 10

minute dumping operation.

The design considers that dredging at the three locations within the harbour would be

conducted in sequence and that a sufficient number of hopper barges would be available to

ensure continuous operation of the dredging plant. Table 5-1 sets out the spoil disposal work

plan based on the vessel capacity, speed and volume of material for disposal ():

Table 5-1: Disposal work plan

Vessel Type Hopper Barge Area of Operation Spoil Disposal Area Water depth (m) -20 m CD Estimated total dredge volume 843,000 m

3

Duration of dredging operation (weeks)

6.8

Single barge operation cycle 10 hours Number of barges 3 Disposal frequency 3.3 hours Duration of post dredging dispersion monitoring (weeks)

2

Total duration of simulation (weeks) 8.8

The spoil disposal scenario was completed over an 8.8 week period. The disposal operation

was considered to operate continuously over the operation period, with the barge considered

as a moving point source within the disposal area. The 95th percentile suspended sediment

concentration was calculated from the model results along with the unconsolidated sediment

deposition thickness.

Due to the increased fetch length in Beagle Gulf compared to Darwin Harbour, the disposal

modelling considered the impact of the local wave climate on fine sediment deposition. Wave

induced bed shear stress acting in isolation, or combination with tidal bed shear stress, has the

potential to re-suspend deposited material and keep sediment suspended. To determine wave

conditions over the simulation period, the local wind conditions at Darwin Airport were

combined with local hindcast wave conditions in a fully spectral wave model and applied to




simulate the nearshore wave conditions. The wave conditions during this period were typically

short period wind waves from the south-east, due to the prevailing wind direction.

5.2 Spoil disposal results

The instantaneous suspended sediment concentration results show that each spoil dumping

operation leads to an instantaneous plume of the order 50-100 m in diameter, with a peak

concentration of 10.0-20.0 mg/l. These plumes then follow the prevailing current direction and

disperse, increasing in diameter but reducing in concentration to generally less than 5.0 mg/l.

The fate of the plume is dependent on the tidal and wave conditions. During neap tide periods

and low wave energy the plumes can persist for over 6 hours and occur in combination with

more recent disposal plumes, however at a much reduced suspended sediment concentration

due to dispersion.

The 95th percentile results (Figure 5-1) show that in the Beagle Gulf suspended sediment

concentrations remain below 3.0 mg/l. Analysis of the data below 3.0 mg/l shows that the fine

sediment has been dispersed widely throughout the model domain. These concentrations

remain very low due to sediment disposal only occurring every three hours and waves and

currents encouraging dispersion.

The total unconsolidated sediment thickness was determined for the duration of the simulation

(Figure 5-2). Fine sediment accretion only occurs at the entrance to Darwin Harbour. This is

due to low levels of suspended sediment frequently entering this area of low bed shears stress

(see Figure 4-1) during the simulation. During periods of slack tide the sediment can fall out of

suspension and deposit on the bed. However, the thickness of sediment deposition remains

smaller than 0.5 mm.

Unconsolidated sediment deposition at the spoil disposal site remains less than 0.1 mm at the

end of the simulation period (Figure 5-2). Analysis of the time series results shows that during

neap tide periods, when current speeds are at their lowest, sediment deposition can reach an

unconsolidated depth of 0.5 mm, however a combination of spring tides and wave action leads

to re-suspension of this material and further dispersal.




Figure 5-1: 95

th percentile suspended sediment concentration


The spoil disposal simulation included wave action, which can increase fine sediment dispersal

and in combination with tidal currents, can mobilise sediments deposited on the seabed. To

test the sensitivity of the simulation to these processes the model was re-run without waves, to

provide a more conservative estimate of suspended sediment concentration and bed thickness.




The sensitivity tests show that without waves the 95th percentile concentrations (Figure 5-3) are

increased within the disposal area with a concentration of 2.0-4.0 mg/l due to the reduction in

dispersion processes, leading to a higher concentration compared to the baseline (with waves)

case. The results for unconsolidated deposition remain consistent with the baseline with a

small area of accretion in the entrance to Darwin Harbour (Figure 5-4). The unconsolidated

thickness at this location is less than 0.5 mm, but slightly smaller in extent then the baseline

due to the generally lower levels of suspended sediment in this simulation compared to the

baseline, where waves and currents re-suspended and disperse the sediment more widely.

Throughout the remainder of the model, unconsolidated deposition thickness remains

negligible at less than 0.1 mm.

Figure 5-3: 95

th percentile suspended sediment concentration. Sensitivity test without

waves

Figure 5-4: Unconsolidated fine sediment deposition. Sensitivity test without waves




6 Conclusions and Limitations

6.1 Dredge dispersion studies

The potential impacts of dredging activities associated with the East Arm Wharf developments

for the Marine Supply Base, the Defence Laydown Area and the Customs Base have been

evaluated in terms of elevated suspended sediment concentration and deposition of

unconsolidated silt. Interpretation of this information within the EIA will determine any potential

adverse impact on the local sensitive receptors based on the predicted changes to the physical

environment as described in this report.

A mud transport model was setup based on an existing hydrodynamic model of Darwin

Harbour. The model was setup to simulate typical hydrodynamic conditions in the harbour and

then applied to simulate potential dredging scenarios, to provide an understanding of

suspended fine sediment transport, its fate and deposition rate over a the anticipated duration

of dredging operations.

6.1.1 Study limitations

The modelling methodology is based upon the concept dredge design and layout, and

assumes that dredging is undertaken by cutter suction dredgers. The modelling approach is

conservative in terms of fine sediment release rates and is therefore likely to over-predict levels

of accumulation, as the model neglects the influence of wave induced shear stresses that,

particularly in the un-vegetated inter-tidal shallows, could potentially exceed the threshold for

fine sediment re-suspension. On the upper inter-tidal zone, which includes large areas of

vegetation (for example Mangroves), the effect of the waves is likely to be less significant as

the vegetation will encourage sediment deposition and provide shelter.

The model scenarios assume a continuous programme of dredging and release of fine

sediment. Significant downtime or alterations to the dredge methodology, plant or area of

operations may influence the cumulative impact of the dredging activities. The modelling has

assumed that each dredge scenario occurs independently, the mobilisation of multiple dredgers

operating in unison has not been considered.

The model has considered a typical seabed sediment grading based on the available

information. From this an appropriate estimate of the sediment erosion and deposition

threshold and settling rate has been made. However the seabed material grading is likely to

vary spatially, therefore the type and rate of fine sediment release will also vary in time as the

dredger moves location. There is insufficient data to determine these regions therefore the

modelling represents a typical average fine sediment release.

The deposition rates provided relate to accumulation of fresh unconsolidated mud, if current

speeds remain below the threshold for re-suspension the deposited sediment would further

consolidate, reducing the layer thickness to approximately 20-30%. The consolidation process

has not been considered in the model, and consequently the permanent (long-term) siltation

thickness will be reduced significantly.

In conducting the dredge dispersion study, results for the instantaneous suspended sediment

concentration, the unconsolidated deposition thickness and 95th percentile suspended

sediment statistics were obtained and presented in a series of figures and as time-series data.

The EIA should consider the potential impacts for the sensitive receptors, in terms of deposited

fine sediments suspended sediment levels relative to the ambient background levels.




6.1.2 Dredging Marine Supply Base

• The highest levels of suspended sediment occur around the dredge location and along

the East Arm Wharf, with a 95th percentile concentration of 2.0-5.0 mg/l. Tidal current

transport the plume into Hudson Creek, however the 95th percentile concentration

remains between 1.0-2.0 mg/l.

• Due to suspended sediment entering the shallow inter-tidal areas of Hudson Creek and

Frances Bay, areas of reduced bed shear stress, they tend to encourage sediment

deposition, in contrast to the higher concentration areas where bed shear stress is

sufficient to prevent deposition. Deposition is also shown to the north of the East Arm

Wharf, but the unconsolidated thickness is limited to 1.0-5.0 mm. Elsewhere it is less

than 1.0 mm.

• In the remainder of Darwin Harbour, unconsolidated bed thicknesses occur of between

0.1 and 1.0 mm, though most of the more significant areas are at the lower end of this

range.

• It is noted that within Frances Bay around the Darwin Waterfront there are numerous

marinas, and at the East Arm Wharf there are other berths and jetties. These features

are too small to be represented within the model domain, but are likely to lead to

include a berth pocket and area of reduced bed shear stress. It seems reasonable to

assume that given the proximity of these developments to the areas of deposition

identified in Figure 4-5, these will be subject to increased levels of fine sediment

deposition following dredging of the Marine Supply Base.

6.1.3 Dredging Defence Laydown Area

• Relative to the other dredging operations, the smaller dredging plant and volumes at

this location lead to low levels of suspended sediment. The 95th percentile plume is

limited to the immediate are of the dredged area (1.0-2.0 mg/l) but reduces within

100m to less than 0.2 mg/l.

• As a result unconsolidated deposition is focused at the point of dredging, and is of the

order 0.5-1.0 mm. Beyond the dredge location suspended sediment is unable to

deposit due to the high bed shear stress, except in the sheltered area in the lee of the

East Harbour Wharf. However this sediment enters Frances Bay and Hudson Creek

where current speed reduce and low levels of deposition occur over the inter-tidal area

(0.1-0.5 mm unconsolidated).

• The impact of the suspended sediment is generally limited to the immediate vicinity of

the Defence Laydown Area. Beyond this, sediment deposition and suspended

sediment concentrations are low.

6.1.4 Dredging Customs Base

• Due to the short duration of dredging operations, the simulation considered a spring

tide and a neap tide period.

• The two simulations indicate broadly similar results, with the spring tide simulation

tending to reduce suspended sediment concentration south of the East Arm Wharf

from 2.0-5.0 mg/l in the neap simulation to 1.0-2.0 mg/l in the spring tide simulation due

to increase dispersion from as a result of the increased current speeds.




• The total area of sediment deposition is similar between simulations, however the

larger spring tidal range leads to deposition further into Hudson Creek and Frances

Bay, with an unconsolidated thickness of 0.1-0.5 mm. Near the dredge location,

deposition around the dredge point is enhanced in the neap tide simulation compared

to the spring, with sediment falling out of suspension more quickly. Whereas during

spring tides this material deposits further to the east.

• The impact associated with the Customs Base dredging is generally focused around

the East Arm Wharf both in terms of suspended sediment and sediment deposition.

Deposition within Hudson Creek and Frances Bay are low (<0.5 mm).

6.2 Dredge disposal studies

The potential impacts of spoil disposal operations in Beagle Gulf associated with the dredging

for the East Arm Wharf have been evaluated in terms of elevated suspended sediment

concentration and deposition of unconsolidated silt. The mud transport model setup for the

dredge dispersion modelling was extended and combined with a spectral wave model to

simulate a potential spoil disposal scenario by bottom dumping barges.

6.2.1 Study limitations

The modelling methodology is based upon the concept dredge disposal plan, and is consistent

with the dredge programme, plant and volumes applied in the dredge dispersion modelling

scenarios. The model has considered the influence of wind wave induced shear stresses that

can potentially exceed the threshold for fine sediment re-suspension. The model has not

considered longer period ocean swell waves or storms.

The model scenarios assume a continuous programme of spoil disposal within the nominated

disposal site using a typical sediment grading based on the available information. The potential

short term and long term transport of the spoil material and fines on the seabed in the disposal

area by waves and tidal currents is beyond the scope of this study.

The environmental impact assessment should consider the likely duration and cumulative

impact of the dumping activities, including the potential impact on benthic flora and fauna within

the disposal site.

6.2.2 Disposal in Beagle Gulf

• The spoil disposal operation leads to small plumes of relatively high suspended

concentration which are transported with the prevailing current, and then disperse. The

general current speeds and resulting bed shear stress are sufficient to prevent

deposition and encourage dispersion. Therefore sediment is dispersed widely but at

low levels with the 95th percentile value less than 3.0 mg/l, throughout the model

domain.

• Small areas of fine sediment deposition are predicted to occur in the entrance to

Darwin Harbour, due to low bed shear stress in these areas and low levels of

suspended sediment entering this area during period of slack water. The deposition

area has been shown to be sensitive to wave conditions during the dumping

operations. The waves tend to encourage sediment re-suspension and thus increase

local suspended sediment concentrations slightly. Despite this, levels of

unconsolidated sediment remain less than 0.1 mm though the remainder of the model

domain, even when waves are excluded from the simulation.




7 References Bray, R.N., Bates, A.D., Land, J.M. (1997) Dredging a handbook for engineers- 2

nd edition.

Butterworth Heinemann, Oxford UK.

HR Wallingford (2010) Ichthys Gas Field Development Project – Dredging and Spoil Disposal

Modelling. Report EX6219 R.5.0. Report prepared for INPEX Browse Ltd., Perth, Western

Australia.

Wasko, C.D., Williams, D., Miller, B.M., Mehrabi, S. (2010) Hydrodynamic and sediment

modelling for the East Arm Port Expansion, Darwin Harbour. The University of New South

Wales water research laboratory. Manly Vale Australia.




Appendices




Appendix A Instantaneous Model Results




Figure A1-1: Instantaneous suspended sediment concentration - spring flood tide, Marine supply base.

Figure A1-2: Instantaneous suspended sediment concentration - spring ebb tide, Marine supply base.




Figure A1-3: Instantaneous suspended sediment concentration - neap flood tide, Marine supply base.

Figure A1-4: Instantaneous suspended sediment concentration - neap ebb tide, Marine supply base.




Figure A1-5: Instantaneous suspended sediment concentration - spring flood tide, Barge ramp and hardstand.

Figure A1-6: Instantaneous suspended sediment concentration - spring ebb tide, Barge ramp and hardstand.




Figure A1-7: Instantaneous suspended sediment concentration - neap flood tide, Barge ramp and hardstand.

Figure A1-8: Instantaneous suspended sediment concentration - neap ebb tide, Barge ramp and hardstand.




Figure A1-9: Instantaneous suspended sediment concentration - neap flood tide, Tug and small vessel berths.

Figure A1-10: Instantaneous suspended sediment concentration - neap ebb tide, Tug and small vessel berths.




Figure A1-11: Instantaneous suspended sediment concentration - spring flood tide, Tug and small vessel berths.

Figure A1-12: Instantaneous suspended sediment concentration - spring ebb tide, Tug and small vessel berths.




Figure A1-13: Instantaneous suspended sediment concentration - spring ebb tide, disposal site.

Figure A1-14: Instantaneous suspended sediment concentration - spring flood tide, disposal site.




Figure A1-15: Instantaneous suspended sediment concentration - neap ebb tide, disposal site.

Figure A1-16: Instantaneous suspended sediment concentration - neap flood tide, disposal site.




Appendix B : Time Series Model Results




Marine supply base

0

0.05

0.1

0.15

0.2

0.25

0.3

05/05/2008 00:00 19/05/2008 00:00 02/06/2008 00:00

Time

Mass (

kg

/m3)

Site A Site B Site C

0

0.05

0.1

0.15

0.2

0.25

0.3

05/05/2008 00:00 19/05/2008 00:00 02/06/2008 00:00

Time

Mass (

kg

/m3)

Site D Site E Site F Site G

0

1

2

3

4

5

6

7

8

9

10

05/05/2008 00:00 19/05/2008 00:00 02/06/2008 00:00

Time

SS

C (

mg

/l)





0

1

2

3

4

5

6

7

8

9

10

05/05/2008 00:00 19/05/2008 00:00 02/06/2008 00:00

Tim e

SS

C (

mg

/l)


0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

05/05/2008 00:00 19/05/2008 00:00 02/06/2008 00:00

Tim e

Un

co

nso

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ate

d t

hic

kn

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mm

)


0

0.1

0.2

0.3

0.4

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1

05/05/2008 00:00 19/05/2008 00:00 02/06/2008 00:00

Tim e

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co

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lid

ate

d t

hic

kn

ess (

mm

)





0

0.05

0.1

0.15

0.2

0.25

0.3

02/06/2008 00:00 16/06/2008 00:00 30/06/2008 00:00 14/07/2008 00:00

Tim e

Ma

ss

(kg

/m3

)


0

1

2

3

4

5

6

7

8

9

10

02/06/2008 00:00 16/06/2008 00:00 30/06/2008 00:00 14/07/2008 00:00

Tim e

SS

C (

mg

/l)


0

0.05

0.1

0.15

0.2

0.25

0.3

02/06/2008 00:00 16/06/2008 00:00 30/06/2008 00:00 14/07/2008 00:00

Time

Ma

ss

(k

g/m

3)





0

1

2

3

4

5

6

7

8

9

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02/06/2008 00:00 16/06/2008 00:00 30/06/2008 00:00 14/07/2008 00:00

Tim e

SS

C (

mg

/l)


0

0.1

0.2

0.3

0.4

0.5

0.6

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1

02/06/2008 00:00 16/06/2008 00:00 30/06/2008 00:00 14/07/2008 00:00

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Un

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olid

ate

d t

hic

kn

ess

(m

m)


0

0.1

0.2

0.3

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1

02/06/2008 00:00 16/06/2008 00:00 30/06/2008 00:00 14/07/2008 00:00

Time

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ate

d t

hic

kn

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(m

m)





Marine supply base - sites (H, I, J) around South Shell Island

0

0.05

0.1

0.15

0.2

0.25

0.3

5/05/2008 0:00 19/05/2008 0:00 2/06/2008 0:00

Time

Mass (

kg

/m3)

Site H Site I Site J

0

0.05

0.1

0.15

0.2

0.25

0.3

2/06/2008 0:00 16/06/2008 0:00 30/06/2008 0:00 14/07/2008 0:00

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Ma

ss

(kg

/m3

)


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1

2

3

4

5

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7

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9

10

5/05/2008 0:00 19/05/2008 0:00 2/06/2008 0:00

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SS

C (

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/l)





0

1

2

3

4

5

6

7

8

9

10

2/06/2008 0:00 16/06/2008 0:00 30/06/2008 0:00 14/07/2008 0:00

Time

SS

C (

mg

/l)


0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

5/05/2008 0:00 19/05/2008 0:00 2/06/2008 0:00

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d t

hic

kn

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mm

)


0

0.1

0.2

0.3

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2/06/2008 0:00 16/06/2008 0:00 30/06/2008 0:00 14/07/2008 0:00

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ate

d t

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kn

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(m

m)





Barge ramp and hardstand

0

0.05

0.1

0.15

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0.3

05/05/2008 00:00 19/05/2008 00:00 02/06/2008 00:00

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Ma

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/m3)


0

0.05

0.1

0.15

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05/05/2008 00:00 19/05/2008 00:00 02/06/2008 00:00

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1

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3

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5

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8

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05/05/2008 00:00 19/05/2008 00:00 02/06/2008 00:00

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SS

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0

1

2

3

4

5

6

7

8

9

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05/05/2008 00:00 19/05/2008 00:00 02/06/2008 00:00

Time

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0

0.1

0.2

0.3

0.4

0.5

0.6

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1

05/05/2008 00:00 19/05/2008 00:00 02/06/2008 00:00

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d t

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mm

)


0

0.1

0.2

0.3

0.4

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1

05/05/2008 00:00 19/05/2008 00:00 02/06/2008 00:00

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kn

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(m

m)





0

0.05

0.1

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0.3

02/06/2008 00:00 16/06/2008 00:00 30/06/2008 00:00

Time

Ma

ss

(kg

/m3

)


0

1

2

3

4

5

6

7

8

9

10

02/06/2008 00:00 16/06/2008 00:00 30/06/2008 00:00

Time

SS

C (

mg

/l)


0

0.05

0.1

0.15

0.2

0.25

0.3

02/06/2008 00:00 16/06/2008 00:00 30/06/2008 00:00

Time

Ma

ss (

kg/m

3)





0

1

2

3

4

5

6

7

8

9

10

02/06/2008 00:00 16/06/2008 00:00 30/06/2008 00:00

Time

SS

C (m

g/l)


0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

02/06/2008 00:00 16/06/2008 00:00 30/06/2008 00:00

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ted

thic

kn

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s (m

m)


0

0.1

0.2

0.3

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02/06/2008 00:00 16/06/2008 00:00 30/06/2008 00:00

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thic

kn

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s (

mm

)





Barge ramp and hardstand - sites (H, I, J) around South Shell Island

0

0.05

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0.15

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5/05/2008 0:00 19/05/2008 0:00 2/06/2008 0:00

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kg

/m3)


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2/06/2008 0:00 16/06/2008 0:00 30/06/2008 0:00

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/m3

)


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1

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5/05/2008 0:00 19/05/2008 0:00 2/06/2008 0:00

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SS

C (

mg

/l)





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1

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3

4

5

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2/06/2008 0:00 16/06/2008 0:00 30/06/2008 0:00

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0.2

0.3

0.4

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5/05/2008 0:00 19/05/2008 0:00 2/06/2008 0:00

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mm

)


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d t

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(m

m)





Tug and small vessel berths, spring tide

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

03/07/2008 00:00 05/07/2008 00:00 07/07/2008 00:00 09/07/2008 00:00 11/07/2008 00:00

Time

Ma

ss

(k

g/m

3)


0

0.001

0.002

0.003

0.004

0.005

0.006

0.007

0.008

0.009

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03/07/2008 00:00 05/07/2008 00:00 07/07/2008 00:00 09/07/2008 00:00 11/07/2008 00:00

Tim e

Ma

ss

(kg

/m3

)


0

1

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03/07/2008 00:00 05/07/2008 00:00 07/07/2008 00:00 09/07/2008 00:00 11/07/2008 00:00

Tim e

SS

C (

mg

/l)





0

1

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3

4

5

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03/07/2008 00:00 05/07/2008 00:00 07/07/2008 00:00 09/07/2008 00:00 11/07/2008 00:00

Tim e

SS

C (

mg

/l)


0

0.1

0.2

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0.6

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03/07/2008 00:00 05/07/2008 00:00 07/07/2008 00:00 09/07/2008 00:00 11/07/2008 00:00

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(m

m)


0

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d t

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kn

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(m

m)





Tug and small vessel berths, spring tide - sites (H, I, J) around South Shell Island

0

0.05

0.1

0.15

0.2

0.25

0.3

3/07/2008 0:00 5/07/2008 0:00 7/07/2008 0:00 9/07/2008 0:00 11/07/2008 0:00

Time

Mass (

kg

/m3)


0

1

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4

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3/07/2008 0:00 5/07/2008 0:00 7/07/2008 0:00 9/07/2008 0:00 11/07/2008 0:00

Time

SS

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Time

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ate

d t

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kn

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mm

)





Tug and small vessel berths, neap tide

0

0.001

0.002

0.003

0.004

0.005

0.006

0.007

0.008

0.009

0.01

12/07/2008 00:00 14/07/2008 00:00 16/07/2008 00:00 18/07/2008 00:00

Time

Ma

ss

(kg

/m3

)


0

1

2

3

4

5

6

7

8

9

10

12/07/2008 00:00 14/07/2008 00:00 16/07/2008 00:00 18/07/2008 00:00

Time

SS

C (

mg

/l)


0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

12/07/2008 00:00 14/07/2008 00:00 16/07/2008 00:00 18/07/2008 00:00

Time

Ma

ss (

kg/m

3)





0

1

2

3

4

5

6

7

8

9

10

12/07/2008 00:00 14/07/2008 00:00 16/07/2008 00:00 18/07/2008 00:00

Time

SS

C (m

g/l)


0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

12/07/2008 00:00 14/07/2008 00:00 16/07/2008 00:00 18/07/2008 00:00

Time

Un

co

nso

lida

ted

thic

kn

es

s (m

m)


0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

12/07/2008 00:00 14/07/2008 00:00 16/07/2008 00:00 18/07/2008 00:00

Time

Un

co

nso

lida

ted

thic

kn

es

s (

mm

)





Tug and small vessel berths, neap tide - sites (H, I, J) around South Shell Island

0

0.05

0.1

0.15

0.2

0.25

0.3

12/07/2008 0:00 14/07/2008 0:00 16/07/2008 0:00 18/07/2008 0:00

Time

Mass (

kg

/m3)


0

1

2

3

4

5

6

7

8

9

10

12/07/2008 0:00 14/07/2008 0:00 16/07/2008 0:00 18/07/2008 0:00

Time

SS

C (

mg

/l)


0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

12/07/2008 0:00 14/07/2008 0:00 16/07/2008 0:00 18/07/2008 0:00

Time

Un

co

nso

lid

ate

d t

hic

kn

ess (

mm

)





Disposal Site

0

0.05

0.1

0.15

0.2

0.25

0.3

05/05/2008 00:00 12/05/2008 00:00 19/05/2008 00:00 26/05/2008 00:00 02/06/2008 00:00

Time

Mass (

kg

/m3)

Disposal Site

0.00

2.00

4.00

6.00

8.00

10.00

12.00

14.00

05/05/2008 00:00 12/05/2008 00:00 19/05/2008 00:00 26/05/2008 00:00 02/06/2008 00:00

Time

SS

C (

mg

/l)

Disposal Site

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

05/05/2008 00:00 12/05/2008 00:00 19/05/2008 00:00 26/05/2008 00:00 02/06/2008 00:00

Time

Un

co

nso

lid

ate

d t

hic

kn

ess (

mm

)

Disposal Site




0.00

2.00

4.00

6.00

8.00

10.00

12.00

14.00

02/06/2008 00:00 09/06/2008 00:00 16/06/2008 00:00 23/06/2008 00:00 30/06/2008 00:00

Time

SS

C (

mg

/l)

Disposal Site

0

0.05

0.1

0.15

0.2

0.25

0.3

02/06/2008 00:00 09/06/2008 00:00 16/06/2008 00:00 23/06/2008 00:00 30/06/2008 00:00

Time

Ma

ss

(k

g/m

3)

Disposal Site

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

02/06/2008 00:00 09/06/2008 00:00 16/06/2008 00:00 23/06/2008 00:00 30/06/2008 00:00

Time

Un

co

ns

oli

da

ted

th

ick

ne

ss

(m

m) Disposal Site

EAW Expansion Project DEIS E - Home - NTEPA Expansion Project DEIS E ... The dredge method statement proposes the use of two types of cutter suction dredger ... dredging plant Dredge

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