EAW Expansion Project DEIS
E
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.
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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
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Figure 4-5: Unconsolidated fine sediment deposition............................................................................... 13 Figure 4-6: 95
th percentile suspended sediment concentration................................................................. 14
Figure 4-7: 95th percentile suspended sediment concentration (harbour area) ......................................... 15
Figure 4-8: Unconsolidated fine sediment deposition............................................................................... 15 Figure 4-9: 95
th percentile suspended sediment concentration................................................................. 17
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................................................................. 23
Figure 5-2: Unconsolidated fine sediment deposition............................................................................... 23 Figure 5-3: 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
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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).
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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
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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
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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
Duration of dredging operation (days)
62
Duration of post dredging dispersion monitoring (days)
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.
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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
Duration of dredging operation (days)
49
Duration of post dredging dispersion monitoring (days)
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.
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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
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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.
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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.
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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
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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.
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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.
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Figure 4-3: 95th
percentile suspended sediment concentration
Figure 4-4: 95th
percentile suspended sediment concentration (harbour area)
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Figure 4-5: Unconsolidated fine sediment deposition
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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
percentile suspended sediment concentration
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Figure 4-7: 95th
percentile suspended sediment concentration (harbour area)
Figure 4-8: Unconsolidated fine sediment deposition
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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.
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Figure 4-9: 95th
percentile suspended sediment concentration
Figure 4-10: 95th
percentile suspended sediment concentration (harbour area)
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Figure 4-11: Unconsolidated fine sediment deposition
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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
percentile suspended sediment concentration
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Dredge Dispersion March 2011 20
Figure 4-13: 95th
percentile suspended sediment concentration (harbour area)
Figure 4-14: Unconsolidated fine sediment deposition
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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
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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.
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Figure 5-1: 95
th percentile suspended sediment concentration
Figure 5-2: Unconsolidated fine sediment deposition
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.
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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
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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.
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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.
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• 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.
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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.
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Appendices
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Appendix A Instantaneous Model Results
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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.
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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.
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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.
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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.
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Dredge Dispersion March 2011 35
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.
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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.
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Figure A1-13: Instantaneous suspended sediment concentration - spring ebb tide, disposal site.
Figure A1-14: Instantaneous suspended sediment concentration - spring flood tide, disposal site.
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Figure A1-15: Instantaneous suspended sediment concentration - neap ebb tide, disposal site.
Figure A1-16: Instantaneous suspended sediment concentration - neap flood tide, disposal site.
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Appendix B : Time Series Model Results
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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)
Site A Site B Site C
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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)
Site D Site E Site F Site G
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
lid
ate
d t
hic
kn
ess (
mm
)
Site A Site B Site C
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
lid
ate
d t
hic
kn
ess (
mm
)
Site D Site E Site F Site G
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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
)
Site D Site E Site F Site G
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)
Site A Site B Site C
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)
Site A Site B Site C
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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)
Site D Site E Site F Site G
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 14/07/2008 00:00
Tim e
Un
co
ns
olid
ate
d t
hic
kn
ess
(m
m)
Site A Site B Site C
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 14/07/2008 00:00
Time
Un
co
nso
lid
ate
d t
hic
kn
ess
(m
m)
Site D Site E Site F Site G
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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
Time
Ma
ss
(kg
/m3
)
Site H Site I Site J
0
1
2
3
4
5
6
7
8
9
10
5/05/2008 0:00 19/05/2008 0:00 2/06/2008 0:00
Time
SS
C (
mg
/l)
Site H Site I Site J
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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)
Site H Site I Site J
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
Time
Un
co
nso
lid
ate
d t
hic
kn
ess (
mm
)
Site H Site I Site J
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
2/06/2008 0:00 16/06/2008 0:00 30/06/2008 0:00 14/07/2008 0:00
Time
Un
co
ns
olid
ate
d t
hic
kn
ess
(m
m)
Site H Site I Site J
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Barge ramp and hardstand
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
Ma
ss (
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
Mas
s (
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)
Site A Site B Site C
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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)
Site D Site E Site F Site G
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
Time
Un
co
ns
olid
ate
d t
hic
kn
ess (
mm
)
Site A Site B Site C
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
Time
Un
co
nso
lid
ate
d t
hic
kn
ess
(m
m)
Site D Site E Site F Site G
URS NORTHERN TERRITORY (AUSTRALIA)
East Arm Wharf- Dredge dispersion modelling
Dredge Dispersion March 2011 48
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
/m3
)
Site D Site E Site F Site G
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)
Site A Site B Site C
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)
Site A Site B Site C
URS NORTHERN TERRITORY (AUSTRALIA)
East Arm Wharf- Dredge dispersion modelling
Dredge Dispersion March 2011 49
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)
Site D Site E Site F Site G
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
Time
Un
co
nso
lida
ted
thic
kn
es
s (m
m)
Site A Site B Site C
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
Time
Un
co
nso
lida
ted
thic
kn
es
s (
mm
)
Site D Site E Site F Site G
URS NORTHERN TERRITORY (AUSTRALIA)
East Arm Wharf- Dredge dispersion modelling
Dredge Dispersion March 2011 50
Barge ramp and hardstand - 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
Time
Ma
ss
(kg
/m3
)
Site H Site I Site J
0
1
2
3
4
5
6
7
8
9
10
5/05/2008 0:00 19/05/2008 0:00 2/06/2008 0:00
Time
SS
C (
mg
/l)
Site H Site I Site J
URS NORTHERN TERRITORY (AUSTRALIA)
East Arm Wharf- Dredge dispersion modelling
Dredge Dispersion March 2011 51
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
Time
SS
C (
mg
/l)
Site H Site I Site J
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
Time
Un
co
nso
lid
ate
d t
hic
kn
ess (
mm
)
Site H Site I Site J
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
2/06/2008 0:00 16/06/2008 0:00 30/06/2008 0:00
Time
Un
co
nso
lid
ate
d t
hic
kn
ess
(m
m)
Site H Site I Site J
URS NORTHERN TERRITORY (AUSTRALIA)
East Arm Wharf- Dredge dispersion modelling
Dredge Dispersion March 2011 52
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)
Site A Site B Site C
0
0.001
0.002
0.003
0.004
0.005
0.006
0.007
0.008
0.009
0.01
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
)
Site D Site E Site F Site G
0
1
2
3
4
5
6
7
8
9
10
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)
Site A Site B Site C
URS NORTHERN TERRITORY (AUSTRALIA)
East Arm Wharf- Dredge dispersion modelling
Dredge Dispersion March 2011 53
0
1
2
3
4
5
6
7
8
9
10
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)
Site D Site E Site F Site G
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
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
Un
co
ns
olid
ate
d t
hic
kn
ess
(m
m)
Site A Site B Site C
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
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
Un
co
nso
lid
ate
d t
hic
kn
ess
(m
m)
Site D Site E Site F Site G
URS NORTHERN TERRITORY (AUSTRALIA)
East Arm Wharf- Dredge dispersion modelling
Dredge Dispersion March 2011 54
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)
Site H Site I Site J
0
1
2
3
4
5
6
7
8
9
10
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
C (
mg
/l)
Site H Site I Site J
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
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
Un
co
nso
lid
ate
d t
hic
kn
ess (
mm
)
Site H Site I Site J
URS NORTHERN TERRITORY (AUSTRALIA)
East Arm Wharf- Dredge dispersion modelling
Dredge Dispersion March 2011 55
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
)
Site D Site E Site F Site G
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)
Site A Site B Site C
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)
Site A Site B Site C
URS NORTHERN TERRITORY (AUSTRALIA)
East Arm Wharf- Dredge dispersion modelling
Dredge Dispersion March 2011 56
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)
Site D Site E Site F Site G
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)
Site A Site B Site C
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
)
Site D Site E Site F Site G
URS NORTHERN TERRITORY (AUSTRALIA)
East Arm Wharf- Dredge dispersion modelling
Dredge Dispersion March 2011 57
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)
Site H Site I Site J
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)
Site H Site I Site J
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
)
Site H Site I Site J
URS NORTHERN TERRITORY (AUSTRALIA)
East Arm Wharf- Dredge dispersion modelling
Dredge Dispersion March 2011 58
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
URS NORTHERN TERRITORY (AUSTRALIA)
East Arm Wharf- Dredge dispersion modelling
Dredge Dispersion March 2011 59
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