ASSESSMENT OF EFFECTS ON THE
PHYSICAL ENVIRONMENT FROM THE TRANSTASMAN RESOURCES MARINE CONSENT
APPLICATION: SEDIMENT PLUME MODEL
Date: March 2014
Review by: Dr Chenonn Chin
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Executive Summary
1. Trans-Tasman Resources Limited (TTR) is proposing to conduct iron ore extraction and
processing operations in the South Taranaki Bight, involving the excavation of up to 50 million
tonnes (27 million cubic metres) per year of seabed material containing iron sand. TTR has
carried out a sediment plume model study and prepared technical reports related to the sediment
model to support the application for marine consent lodged under the Exclusive Economic Zone
and Continental Shelf (Environmental Effects) Act 2012 (EEZ Act). SKM has been engaged by
the Environmental Protection Authority (EPA) to carry out an independent technical review of the
sediment model study to ascertain the validity and accuracy of the model, highlight any
shortcomings, and recommend measures which are required to satisfy the EEZ Act.
2. A set of nested model grids has been set up for the Regional Ocean Modelling System (ROMS)
ocean model, an outer grid covering Cook Strait at a resolution of 2 km and a pair of alternative
inner grids, one covering the South Taranaki Bight and the other a smaller area on Patea Shoals,
both at 1 km resolution with the option of refining them to 500 m if necessary.
3. In general the sediment plume model is done appropriately, however we have two areas of
concern:
a. Number of sediment samples taken to calibrate the model, and
b. Grid size
4. The ROMS ocean model or the hydrodynamic part of the model was well calibrated with
measured tidal data which indicate its validity and accuracy.
2
5. The 20-year excavation covers an area of 65.76 km . Only two core samples that provide
sediment particle size distribution data were presented to characterise the sediment in the
working area. This is grossly inadequate for a mining operation of the scale and size proposed by
TTR and does not allow the effects of Total Suspended Solid (TSS) to be adequately predicted by
the model.
6. The results of the two core samples indicate there is a great variation of sediment characteristics
with sediment depth. Mud contents of more than 50% and up to 82% are found in the samples. It
is highly probable that the sediment in the large excavation area will also be variable.
7. It is unclear how the variation of sediment layers is accounted for in the sediment plume model.
8. TTR has proposed to not carry out the mining operation where the operation encounters a mud
layer with high mud contents. This will reduce the uncertainty in the interpretation of the modelling
results and the assessment of the environmental impacts. Based on this understanding the
shortcomings of the sediment samples will not change the general assessment of low
environmental impact.
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9. The accuracy of the model will affect the working site and the surrounding areas but is unlikely to
affect the coast near shore due to the strong wave environment where mud contents are unlikely
to stay.
Glossary
10. Relevant terms explained
Sand
Sediment with grain sizes between 63 μm and 2 mm.
Silt
Sediment with grain sizes between 2 and 63 μm.
Clay
Sediment with grain sizes < 2 μm
Mud content
Silt and clay components
11. List of acronyms
EEZ
Exclusive Economic Zone
EPA
Environment Protection Authority
PSD
Particle Size Distribution
ROMS
Regional Ocean Modelling System
SKM
Sinclair Knight Merz Ltd
TTR
Trans-Tasman Resources Ltd
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Table of Contents
Executive Summary ............................................................................................................................... 2
Glossary .................................................................................................................................................. 3
Table of Contents ................................................................................................................................... 4
Introduction ............................................................................................................................................ 5
Description of proposal ......................................................................................................................... 5
Description of sediment plume model............................................................................................ 5
Further information ................................................................................................................................ 6
Assessment of validity of sediment plume model ............................................................................. 7
Evaluation ............................................................................................................................................... 8
References .............................................................................................................................................. 9
Appendices ........................................................................................................................................... 10
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Introduction
12. Trans-Tasman Resources Ltd (TTR) has submitted an application to the EPA for a marine
consent to mine iron sand from an area in the South Taranaki Bight. The project involves
excavation of up to 50 million tonnes (27 million cubic metres) per year of seabed material
containing iron sand for processing on an offshore floating vessel. Approximately 10% of the
material will be processed offshore into iron ore for export, with the remaining material returned to
the seabed.
13. The mining operation will result in suspended sediment plumes and sediment deposition on the
seabed that may cause environmental effects on the ocean environment. Dispersed sediment
plumes could also reach the near-shore coastal environment.
14. The Environmental Protection Authority (EPA) has engaged Sinclair Knight Merz Ltd (SKM) to
review the sediment plume model prepared by TTR to assess the effects of suspended sediment
plumes produced by the mining operation. The review broadly includes the following:

Critical appraisal of the application information in terms of assessment of effects of the
activities on the environment,

Assessment and management of the residual effects, and

Critical appraisal of the application information in terms of the validity of the sediment
plume model used.
15. TTR has prepared an overall impact assessment and 23 technical reports and a statement of
evidence (see Appendix A) related to sediment plume modelling, oceanographic processes and
the physical environment to support the application for a marine consent lodged under the
Exclusive Economic Zone and Continental Shelf (Environmental Effects) Act 2012 (EEZ Act). The
main documents reviewed are listed in Appendix A.
16. A table listing the reports previously reviewed by SKM is provided in Appendix B, with a summary
of the reports’ key findings and issues identified in the review.
Description of proposal
Description of sediment plume model
17. TTR carried out the sediment plume modelling using a software package known as Regional
Ocean Modelling System (ROMS) which can be a standalone model or coupled to atmospheric
and /or wave models. The software appears to be widely used by the scientific community for a
diverse range of applications including sediment plume modelling. ROMS is well suited for the
sediment plume modelling for this project.
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18. A set of nested model grids has been set up for the ROMS ocean model, an outer grid covering
Cook Strait at a resolution of 2 km and a pair of alternative inner grids, one covering South
Taranaki Bight and the other a smaller area on Patea Shoals, both at 1 km resolution with the
option of refining them to 500 m if necessary.
19. The outer grid covering the Cook Strait is particularly important due to the dominant tidal
movements across the strait which in turn has a significant influence on the tidal movement at the
project site. The smaller inner grids provide more detailed input and output results at critical
regions where the largest sediment movements and deposition are likely to occur.
20. One of the fundamental factors that affects the accuracy of a sediment plume model is the correct
input of Particle Size Distribution (PSD) data spatially for the work site. The seabed is initially
assumed to be populated with a combination of coarse sand (500–1000 μm, 20%), fine–medium
sand (128–500 μm, 72%), very fine sand (63–128 μm, 6%), coarse silt (16–63 μm, 1.5%) and fine
silt (4–16 μm, 0.5%). The proportions were initially based on seabed PSD data from the extraction
area, and the fine sediment fractions (presumably silt size and below – it is not made clear in the
reviewed reports) were then adjusted so that the model produced surface Suspended Sediment
Concentrations (SSCs) of approximately the correct magnitude in the near-shore area.
21. TTR intends to mine iron sand to a sediment depth of 10 m. According to Reference 4 in
Appendix A, two core samples; STH012RC and STH010RC, which contain information that
relates the variation of PSD with sediment depth up to 15 m were supplied by TTR. The core
sample STH012RC (close to Measurement Site 7) contains a high proportion of mud content
ranging from 58% to 82% between sediment depths of 6m to 8m and from 7% to 27% of mud
content between sediment depths of 9 m to 10 m. For the core sample STH010RC (close to
Measurement Site 10), the mud content ranges from 2% to 4% from the seabed surface to 10 m
sediment depth. From 10 m to 13 m, the mud content increases from 37% to 84%. From the
results of these two samples, they indicate that there is a PSD variation with sediment depth. It is
highly probable that sediment layers with high mud content at variable depths are likely to be
found at the work site.
Further information required
22. An accurate prediction in the sediment plume modelling normally requires a correct
representation of the PSD inputs especially for the silt and clay components. A good
representation or characterisation of the sediment over the relatively large working site in order to
account for PSD spatial variations requires further bore samples to be taken from the mining
areas and further information on how the variation in sediment layers is mitigated in the model. As
it currently stands the report does not provide sufficient details for an accurate assessment of
PSD effects. However, risks to the physical environment at the work site and surrounding area
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are generally low as the seabed consists of mainly sandy material. The shortcomings of the
sediment samples will not change the general assessment of the low environmental impact
provided that TTR does not intend to carry out the mining operation when a mud layer is
encountered.
23. The EPA provided SKM with 10 Statements of Evidence prepared on behalf of TTR, in response
to a further information request and issues raised by the EPA, its independent reviewers and
public submissions. The Statements of Evidence were specifically focussed on the physical
environment, although some of the information presented was relevant to the assessment of the
sediment plume modelling (Brown 2014; Orpin 2014; Hadfield 2014). Collectively, the information
provided within these Statements of Evidence has been considered and incorporated into this
review, where relevant.
Assessment of validity of sediment plume model
24. In general the sediment plume model is done appropriately, however we have two areas of
concern:
a. Number of sediment samples taken to calibrate the model, and
b. Grid size
25. The number of sediment core samples with detailed PSD (two) collected by TTR is not adequate
enough to delineate the variable sediment layers or pockets over the many square kilometres of
the working site and as such could significantly affect the modelling results. The annual
3
2
excavation of 50 million tonnes (27 million m ) roughly amounts to 3 km of excavation area per
2
annum. For 20 years of the operation, the total area involved is 65.76 km . To give a perspective
2
of the area involved, the area of Auckland central business district is about 4.3 km and that of
Wellington is about half of that. The sediment PSD and spatial distribution in this large area
certainly requires a number of field core samples to provide an accurate characterisation. More
sediment core samples are required to delineate this issue. The number of core samples required
depends on the spatial variability of PSD. Further information from Brown (2014) and Orpin
(2014) has provided clarification on the PSD inputs to the model. While this additional information
shows that the PSD inputs to the model are representative of the surficial sediment layers where
a small number of cores were taken, further replication of cores would have been desirable to
provide greater certainty in the level of variation.
26. It is unclear how the variation of sediment layers is accounted for in the sediment plume model.
27. The choice of grid size used in the model determines the accuracy of the results (Roberts, 2004).
A smaller grid size means that more information or grid cells about the hydrodynamics (such as
current velocity and direction) and sediment movements (e.g. transport or deposition) per unit
8
area are available. This is analogous to having more pixels (grid cells) in an image. The more
pixels an image has the more definition the image has.
28. Over an area of interests such as at a dredging site and its surrounding area, grid sizes of ≤ 50 m
x 50 m or 100 m x 100 m are normally used for a sediment simulation (e.g. APASA, 2012 and
eCoast, 2013, respectively). The former project involved the dredging of pipeline trench crossing
3
the Narrows in Curtis lsland, Queensland. The dredged sediment volume was about 430,000 m .
The later project involved the dredging of a harbour and the associated navigation channel and
3
spoil disposal in Vietnam. The dredged volume was about 4 million m .The choice depends on
the model area covered, the computing resources available and the accuracy required. The
minimum grid size of 500 m used by TTR appears to be too coarse to provide an accurate result
of sediment deposition especially over the working site and its surrounding areas. These areas
are likely to have the greatest variation of sediment deposition and environment impacts. A
smaller inner grid, say, less than 50 m will provide a much better definition of sediment deposition
over the area as opposed to the grid size used by TTR of 1km.
29. A smaller inner grid size of ≤ 50 m together with a better characterisation of the PSD spatial
variations over the working site could be incorporated to better predict the intensity of sediment
plumes in close proximity to the mining activity (within ~3 km) . Given the long duration of the
model run, implications for the prediction of far-field sediment plumes are less of concern. The
lack of sufficient input PSD data (derived from only a small number of sediment cores) is one the
most significant limitations of the plume model.
30. The model should describe how the sediment layer data are integrated with the ROMS model.
31. The hydrodynamics part of the model appears to be accurate. This is reflected by the reasonably
good agreement in both magnitude and phase in the tidal time series of the measured data and
model results. The hydrodynamic forces are the drivers for sediment plumes.
Evaluation
32. The lack of sample data coupled with the grid size used when carrying out this modelling makes it
difficult to have confidence in the conclusions that have been drawn by TTR regarding the
environmental effects of sediment re-distribution. After the first sediment plume review by SKM,
TTR carried out further sensitivity analyses on the effect of grid size on the distribution of
Suspended Sediment Concentration (SSC) and concluded that the 500 m grid size provides a
better resolution of SSC distribution at the work site and its adjacent areas than those
corresponding to the 1000 m grid. Further away from the work site, the two plumes are similar
(Hatfield, 2014). SKM concurs with this as, normally a smaller grid size around the source is
required to reduce the margin for error and increase confidence levels.
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33. Inaccuracies in the plume model will affect the predictions that are made in relation to sediment
deposition depth around the working site and spread over a wider area. In the near shore and surf
zone areas, silt and clay particles are unlikely to settle due to the strong wave environment. The
mining operation is unlikely to have any significant environmental effect on near shore and surf
zone areas.
34. Despite the lack of adequate sediment representation and the grid size used, the environmental
impact risks at the work site and its surrounding areas are low and TTR does not intend to mine
the areas where mud contents are high. Based on this understanding we are of the opinion that
any additional samplings or model refinement are unlikely to change the outcome of the
assessment.
References
1)
Brown, M. 2014. Before the EPA Trans-Tasman Resources Ltd Ironsands Extraction Project.
Statement of Evidence in Chief of Mr Matthew Brown on behalf of Trans-Tasman Resources Ltd.
27 pp.
2)
Coastal Engineering Manual (CEM) 2006, USACE, Vicksburg.
3)
Hadfield, M., 2014. Before the EPA Trans-Tasman Resources Ltd Ironsands Extraction Project.
Statement of Evidence in Chief of Dr Mark Hadfield on behalf of Trans-Tasman Resources Ltd.
59 pp.
4)
H. Roberts, 2004. Grid generation methods for high resolution finite element models used for
hurricane storm surge prediction. Masters Thesis, Notre Dame University.
5)
Narrows Crossing Dredge Plume Modelling, Hydrodynamic Modelling (Appendix E) 2012, Report
prepared by APASA P/L for QGC Pte Ltd, Queensland.
6)
Orpin, A.R., 2014. Before the EPA Trans-Tasman Resources Ltd Ironsands Extraction Project.
Statement of Evidence in Chief of Dr Alan Orpin on behalf of Trans-Tasman Resources Ltd., 25
pp.
7)
Vinh Tanh Power Plant Project – Dredge Plume Modelling, 2013. Report prepared by eCoast Pte
Ltd for SKM.
8)
Environmental Assessment Guideline for Marine Dredging Proposals, EAG7, EPA, Western
Australia
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Appendices
Appendix A – Documents cited and reviewed
9)
Coastal stability PHASE 2 report pp1-47.pdf
10) Coastal stability PHASE 2 report pp48-135.pdf
11) EEZ000004_Assessment_of_sediment_deposition_and_re_suspension_behaviour_of_tailings_
MTI_Holland_BV_July_2013_Appended.pdf
12) EEZ000004_Draft_Schedule_N_Part_2_Sediment_characterisation_analysis_particle_size_of_s
amples.pdf
13) EEZ000004_Geological_Desktop_Summary_Active_Permit_areas_50753_(55581)_54068_and_
54272_South_Taranaki_Bight_NIWA_August_2013.pdf
14) EEZ000004_Iron_sand_extraction_in_South_Taranaki_Bight_effects_on_trace_metal_contents_
of_sediment_and_seawater.pdf
15) EEZ000004_Multibeam_Survey_in_Southern_Taranaki_Bight_NIWA_April_2013_(including_bat
hymetry_compilation).pdf
16) EEZ000004_Nearshore_Optical_Water_Quality_in_the_South_Taranaki_Bight_NIWA_May_201
3.pdf
17) EEZ000004_NIWA_sediment_plume_modelling_report.pdf
18) EEZ000004_Optical_Effects_of_an_iron_sand_mining_sediment_plume_NIWA_October_2013_
Appended.pdf
19) EEZ000004_Phase_1_Potential_effects_of_offshore_sand_extraction_on_physical_drivers_and
_coastal_stability.pdf
20) EEZ000004_Potential_Effects_of_Trans_Tasman_Resources_Mining_Operations_on_Surfing_B
reaks_in_the_Southern_Taranaki_Bight.pdf
21) EEZ000004_Satellite_ocean_colour_remote_sensing_of_the_South_Taranaki_Bight_from_2002
_to_2012_NIWA_October_2013.pdf
22) EEZ000004_Schedule_AG_Sediment_characterisation_Textural_comparison_of_grab_and_spe
ar_samples_from_RC_cores_NIWA_May_2013.pdf
23) EEZ000004_Seascape_Natural_Character_and_Visual_Effects_Assessment_Graphic_Supplem
ent.pdf
24) EEZ000004_Sidescan_Survey_of_South_Taranaki_Bight_Sediments_NIWA_April_2012
SECURE.pdf
25) EEZ000004_South_Taranaki_Bight_Baseline_Evironmental_Report_final_No_Appendices (not
for noise).pdf
26) EEZ000004_South_Taranaki_Bight_Iron_Sand_Mining_Nearshore_Wave_Modelling_Phase_4_
Studies_NIWA_October_2013 SECURE.pdf
27) EEZ000004_South_Taranaki_Bight_Iron_Sand_Mining_Oceanographic_measurements_data
report_NIWA_August_2012.pdf
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28) EEZ000004_South_Taranaki_Bight_Iron_Sand_Mining_Project_Seascape_Natural_Character_a
nd_Visual_Effects_Assessment.pdf
29) EEZ000004_Supporting_Information_for_Marine_Consent_Application_October 2013.pdf
30) Hadfield, M., 2014. Before the EPA Trans-Tasman Resources Ltd Ironsands Extraction Project.
Statement of Evidence in Chief of Dr Mark Hadfield on behalf of Trans-Tasman Resourcs Ltd.
31) TTR_Wave_Modelling.pdf
Appendix B - summary of the reports’ key findings by SKM
previous review
Name of report
Purpose of report
Key findings of review
Trans-Tasman
Overall impact assessment
Some gaps arising from
Resources Ltd South
supporting the TTR application.
uncertainties in the modelling,
Taranaki Bight offshore
distribution of a mud layer
iron sand project: impact
within sediments, and trace
assessment (TTR 2013).
metal concentrations in sea
water.
South Taranaki Bight
Summarises environmental
Generally appears to present
factual baseline
information from the South Taranaki
best available information at
environment report
Bight to provide context for the TTR
time of report preparation
(MacDiarmid et al. 2011)
proposal area.
regarding physical
environment.
Multibeam survey in
Presents results of a multibeam
Routine technical data report.
Southern Taranaki Bight
bathymetry survey covering a
Coverage of entire mine area
number of areas relevant to the
proposal.
Sidescan survey of
Presents the results of a sidescan
Routine technical data report.
South Taranaki Bight
survey to identify the extent of
Coverage of mine area not
sediments
bryozoans and sponges within an
comprehensive.
area being considered for disposal of
dredge spoil.
High resolution boomer
Presents the results of a February
Routine technical data report.
survey in South Taranaki
2013 boomer survey to understand
Survey does not cover the
12
Bight (Woelz and Wilcox
the geometry of a sedimentary
mine area. Refers to previous
2013)
wedge in the South Taranaki Bight.
survey that does cover mine
area.
Coastal stability in the
Provides an assessment of the
Comprehensive report
South Taranaki Bight –
effects of sand extraction on the
presenting best available
phase 2 potential effects
landforms and geomorphic character
information.
of offshore sand
of the shore, physical drivers (waves
extraction on physical
and currents) of coastal processes,
drivers and coastal
sediment processes and coastal
stability
stability.
Schedule N, Part 2:
Contains aspects of work on
Results are only reported for
Sediment
sediment characterisation include:
two cores. It is unclear how the
characterisation analysis:
-
Analysis of samples from core
horizons
-
Sampling methodology
-
Characterisation of particle sizefrequency distribution
-
Reporting related to sediment
plume modelling.
particle size of samples
from cores (Orpin 2012)
findings informed modelling.
Schedule AG: Sediment
Quantifies the potential loss of fines
Report has significant
characterisation – textual
through handling and storage of
implications for reliability of
comparison of grab and
muddy sediment in polyweave sacks.
sediment plume modelling, to
spear samples from RC
the extent that PSDs assumed
cores (Orpin 2013)
in the modelling rely on spear
samples that underestimate
fines content.
Geological desktop
Provides a geological summary of
summary
the South Taranaki Bight area.
South Taranaki Bight
Presents a synthesis of the
Methodology and data appear
iron sand mining:
oceanographic field measurements.
to be best available
oceanographic
information.
measurements data
report
Nearshore optical water
Presents a synthesis of
Methodology and data appear
quality in the South
measurements of background optical
to be best available
Taranaki Bight
water quality and suspended
information.
sediment concentrations in the
13
nearshore region of the South
Taranaki Bight.
Satellite ocean-colour
Presents results from earth-
remote sensing of the
observing ocean colour satellite
South Taranaki Bight
sensors to provide information on
material naturally occurring in the
water column.
Seascape and natural
Assessment of natural character,
Unfinished draft report. Relies
character and visual
landscape/seascape and visual
on long-term median model
effects assessment
amenity with regard to effects of the
results without consideration of
proposal.
variability under differing
environmental scenarios.
Appears to make conclusions
regarding potential ecological
impacts related to natural
character on superficial
consideration of long-term
median changes without
reference to ecological
sensitivities.
Assessment of sediment
Presents modelling on the near-field
Uses appropriate model to
deposition and re-
behaviour of tailings arising from the
assess near-field sediment
suspension behaviour of
proposal.
deposition and predict source
tailings (MTI Holland
terms (fluxes of suspended
2013; note: 31 July 2013
and deposited sediment) for
report reviewed, draft
input into far-field modelling.
version on website was
not reviewed separately)
South Taranaki Bight
Presents estimations of the
Further explanation and
iron sand extraction
concentration and deposition rates of
justification for the PSD and
sediment plume
sediments released from the
grid sizes applied to modelling
modelling – phase 3
ironsand extraction operation.
would increase confidence in
studies (Hadfield 2013).
model predictions.
South Taranaki Bight
Survey data (including site selection
Provides new data on
iron sand mining:
and methodology) for 11-month
shoreline dynamics that
shoreline monitoring data
beach monitoring programme.
represents best available
report
information.
14
South Taranaki Bight
Describes Phase 4 of numerical
Detailed wave modelling using
iron sand mining:
modelling study to investigate the
appropriate models and
nearshore wave
impacts of the proposal on wave
resolution. Represents best
modelling phase 4
conditions in the South Taranaki
available information.
studies (Gorman 2013).
Bight.
Potential effects of
Assessment of the effects of the
Valid assessment representing
Trans-Tasman
proposal on surfing breaks in the
best available information.
Resources mining
Southern Taranaki Break.
operations on surfing
breaks in the Southern
Taranaki Bight
Iron sand extraction in
Presents results of laboratory tests
Appropriate methods.
the South Taranaki
with sediment from the mining areas
Omission of mercury from
Bight: effects on
Christina and Diana.
analyte list represents
seawater trace metal
potentially significant gap.
concentrations (Vopel et
Compares nickel elutriate
al. 2013).
concentrations to ANZECC/
ARMCANZ trigger value for
95% species protection
whereas these guidelines
recommend use of the 99%
species protection value.
Estimates that 160-fold dilution
required for copper to meet
99% species protection value.
Coastal stability in the
Provides an assessment of the
Comprehensive review
Southern Taranaki Bight
stability of the shoreline of the
representing best available
– Phase 1: historical and
present day shoreline
change
South Taranaki Bight extending 130
information.
km from about Opunake to
Wanganui.
Optical effects of an iron
Considers the optical effects of
Detailed experimental and
sand mining sediment
suspended sediment concentrations.
modelling study representing
plume in the South
best available information
Taranaki Bight region
derived from model predictions
of sediment plume. Model
transects do not coincide with
key sensitive receptor (North
15
and South Traps). Predictions
based on long-term median
suspended sediment
concentration predictions, do
not address variability.