Appendix 6
Technical Report 6: Benthic Primary
Producer Habitat Loss Assessment
April 2012
Bunbury Port Berth 14 Expansion and Coal Storage
and Loading facility (Assessment No. 1886)
Technical Report 6: Benthic Primary
Producer Habitat Loss Assessment
Prepared for Lanco Resources Australia Pty Ltd
Prepared by Wave Solutions
23 April 2012
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Suite 1, 475 Scarborough Beach Road Osborne Park, WA 6017
PO Box 1756, Subiaco, WA 6904
Phone: +61 (08) 9204 0700 Fax: +61 (08) 9244 7311
Email: [email protected] Web: www.wavesolutions.com.au
Standard Report
Project Brief
Job Number
2419 – Bunbury Port: Berth 14 Environmental Approvals
Work Pack
03 – BPPH Loss Assessment
Project Brief
Benthic Primary Producer Habitat Loss Assessment undertaken
in accordance with Western Australian Environmental Protection
Authorities (EPAs) Environmental Assessment Guideline No. 3.
This report addresses elements of the Marine Benthic Habitat
scope identified in the requirements of WA EPA Environmental
Scoping Document: Bunbury Port Berth 14 Expansion and Coal
Storage and Loading facility (Assessment No. 1886).
Client Contact
Pranab Thakur
Client Address
Lanco Resources Australia Pty Ltd
C/- GPO Box G474
PERTH WA 6841
Document Reference
2419-003-001-001
Prepared By
Kris Waddington
Signature
Reviewed By
Damian Ogburn
Signature
Approved By
Damian Ogburn
Signature
Document Status
Rev
Date
Description
A
23/09/2011
Client Review
0
1/12/2011
Final Report
1
23/04/2012
Final Report Revision 1
Disclaimer
This document has been produced on behalf of, and for the exclusive use of the nominated recipient, and
is issued for the purposes of the proposed works only. Wave Solutions accepts no responsibility or liability
whatsoever in respect to use of this document by any third party.
The information contained within the document is confidential and subject to copyright.
This document shall not be copied, transmitted or divulged to other parties without the prior written
consent of Wave Solutions’ duly authorised representative.
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Table of Contents
1
2
Executive Summary .........................................................7
Introduction ......................................................................9
2.1 Project Description........................................................................9
2.2 Relevant Legislation ....................................................................12
2.3 Objective ......................................................................................12
3
Background Conditions .................................................13
3.1 Key Receptors .............................................................................15
3.2 Background Conditions ..............................................................18
3.2.1
3.2.2
4
Water Quality .............................................................................................. 18
Sediment Characteristics ............................................................................ 21
Predicted Loss of BPPH ................................................23
4.1 Sediment Transport Modelling Outputs.....................................23
4.2 Impact of Dredging on BPPH ......................................................25
4.2.1
4.2.2
5
6
Direct Effects of Dredging ........................................................................... 25
Indirect Effects of Dredging ........................................................................ 25
Conclusions ....................................................................29
References ......................................................................31
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List of Figures
Figure 1: Project area including dredge footprint, existing and proposed dredge spoil
disposal locations and existing BPA anchorages..................................................... 11
Figure 2: Project area including dredge footprint, existing and proposed dredge
material placement grounds and existing BPA anchorages. .................................... 14
Figure 3: Quantitative habitat map showing distribution of benthic biota across the
Project area. ............................................................................................................ 15
Figure 4: Predictive certainty of habitat modelling across the Project area. ............. 16
Figure 5: Distribution of seagrass across the Project area....................................... 18
Figure 6: MODIS image showing concentration of suspended sediments across the
Project area and influence of outflow from the Preston River and Leschenault
Estuary on nearshore environments. ....................................................................... 20
Figure 7: Integrated NTU profile of Koombana Bay showing occurrence of nepheloid
layer near the seafloor. ............................................................................................ 21
Figure 8: Surficial sediment map in Koombana Bay. ............................................... 22
Figure 9: Outputs from the sediment transport modelling based on scenario one. .. 23
Figure 10: Showing limestone reef structure on the north-eastern margin of
Koombana Bay and the location of receptors used to examine the time series of
sediment concentrations. ......................................................................................... 24
Figure 11: Time series of sediment concentrations at location A for the two model
runs. ......................................................................................................................... 24
Figure 12: 12-hour time sequence of depth-averaged dredge plume behaviour for
Scenario 1................................................................................................................ 25
Figure 13: Near bottom turbidity across Koombana Bay showing higher turbidity near
The Cut, likely due to outflow from the Leschenault Estuary. .................................. 26
List of Tables
Table 1: Key characteristics of the proposed works. ................................................ 10
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1
Executive Summary
A benthic habitat loss assessment for the proposed Bunbury Port Berth 14 development was
undertaken following the Western Australian Environmental Protection Authorities (EPAs)
Environmental Assessment Guideline No. 3 - Protection of Benthic Primary Producer Habitats in
Western Australia’s Marine Environment.
A detailed benthic habitat mapping study indicated that Koombana Bay is characterised by bare sand,
with biota restricted to a limestone reef area occurring on the north-eastern margin of the bay. This
limestone reef covers an area of approximately 15 ha and is characterised by foliose algae, turf algae
and filter feeders. No seagrasses occur in Koombana Bay with the nearest seagrasses observed
further offshore in at least 9 m water depth.
In terms of background water quality, the Inner Harbour and Koombana Bay is a complex system
characterised by high turbidity, a near bottom nepheloid layer and frequent wind driven and
anthropogenic sediment resuspension. Background TSS/TSM levels in Koombana Bay periodically
exceed 20 mg/L with key drivers of turbidity including outflow from the Leschenault Estuary and
resuspension from vessel movement and wind.
A calibrated, validated sediment transport model was used to predict the extent and intensity of the
sediment plume from proposed dredging activities. This model indicated the sediment plume will be
largely confined to the Inner Harbour with limited dispersion into Koombana Bay. Under modelling
scenarios where the plume extends to the nearby limestone reef, increases in suspended sediment
concentrations are predicted to be low (2-5 mg/L) and of short duration (in the order of days), and are
highly unlikely to result in the indirect loss of benthic habitat from the Project area. Given there is
unlikely to be any direct or indirect loss of BPPH as a result of development activities, there was no
requirement to establish a local assessment unit (LAU) for the Project area and undertake a formal
loss assessment.
This prediction is supported by the fact that similar communities occurring near The Cut (to the north
of Koombana Bay) are frequently exposed to elevated suspended sediment concentrations
associated with outflow of highly turbid water from the Leschenault Estuary. Microphytobenthos are
unlikely to occur in the inner harbour and Koombana Bay due to frequent sediment resuspension
particularly when wave height >1 m, wind speed >8 m/s, and wind direction from the north-northwest.
In addition the low light climate in Koombana Bay due to a persistent nepheloid layer would further
hinder the benthic photosynthesis. Finally, all biotic groups considered in the loss assessment are
capable of rapidly re-colonising areas of bare habitat where suitable substrate occurs.
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2
Introduction
2.1 Project Description
Construction and operation of a coal export terminal at Berth 14 within the Inner Harbour of Bunbury
Port will facilitate the export of up to 15,000,000 tonnes per annum (tpa) of coal for power generation
in India and other countries. In order to handle this increased volume of coal, a new rail loop,
materials handling infrastructure and berthing arrangements are required at the Bunbury Port.
Whilst outside the scope of this Public Environmental Review (PER), Lanco Resources Australia Pty
Ltd (Lanco) also plans to expand the production capacity of the Griffin Coal Mine from under
5,000,000 to 20,000,000 tpa to meet local market demand for coal and allow the export of up to
15,000,000 tpa. of Griffin coal. The current rail network to the Port has limited capacity, so there is
also a need to duplicate the line from the Collie Basin to the Port.
This PER only assesses the works associated with the Port; separate assessments will be
undertaken for works associated with the mine expansion and upgrade of the existing rail line from
the Collie Basin to Bunbury Port.
Works assessed in this PER are summarised in Table 1. These works include: a coal handling facility
including a new rail loop, two enclosed stockpile sheds, conveyor systems, ship loading facilities, and
a new berth (including dredging of the seabed). It is proposed that the new rail loop would
accommodate a train length of 950 m of coal loaded wagons to be unloaded at a rate of 8,000 tonnes
per hour.
To increase flexibility and maintain efficiency, the proposed coal handling facility is designed to
receive coal by rail and unload either directly to a berthed ship, or to the proposed enclosed stockpile
sheds. The enclosed sheds would allow up to a five day supply of stockpiled coal. The stockpiled coal
would act as a buffer between the unloading and loading processes to ensure a waiting ship is loaded
as quickly as possible, as well as allowing train unloading to proceed if a ship is not available.
The proposed dredging of Berth 14 and its approaches is necessary to provide sufficient space to
allow bulk carriers to enter and depart the new berth. Dredging works below sea level are estimated
to take up to 40 weeks plus five weeks for rock removal if rock is encountered and would include both
marine and terrestrial footprints. The berth will have a local berth pocket and the side slopes for the
berthing area will be stabilised using a rock or a precast revetment to suit the design slopes.
The key characteristics of the proposed works are identified in Table 1. Construction of the Project is
required to be completed in 2014 for the export of coal.
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Table 1: Key characteristics of the proposed works.
Marine Components
Description
Berth pocket
Berth pocket dredged to - 12.7 m Chart Datum (CD) to
accommodate Panamax sized vessels.
Associated approach navigational area dredged to - 12.2 m CD.
Dredge footprint is approximately 11.5 ha, including both
terrestrial and marine areas.
Dredging
Dredge volume of up to 1,900,000 m³. Underwater rock
• Capital
fracturing may be required to remove 20,000 m³ of rock.
•
Maintenance
Dredge material placement
ground
Berth structure
Terrestrial Components
Materials handling
infrastructure
Rail
Throughput (design capacity)
Construction period
Water requirements
Vegetation loss
Terrestrial ground
disturbance
Required approximately every 2-3 years.
Final dredging quantities will be determined as the final
designs for Berth 14 are prepared.
An offshore dredge material placement ground has been
identified in Commonwealth waters and, as such, does not
form part of this assessment.
Suitability of this site, as well as the disposal of dredge
material, will be assessed by the Department of
Sustainability, Environment, Water, Population and
Communities under the Environmental Protection (Sea
Dumping) Act 1981. Other disposal options include the
landside placement of material for reuse for onsite
construction requirements
Likely to comprise of a reinforced concrete jetty structure
supported on circular steel piles. The piles will be
constructed by installing the steel tubes as a bored pile
casing removing soil within the tube until basalt is reached,
rock sockets typically penetrating 2 to 3 diameters into sound
basalt will be bored into the rock using auger type equipment
and after base cleaning the piles will be filled with reinforced
concrete.
The jetty structure will be fitted with fenders, rails for the ship
loaders, handrails lighting and all other ancillaries for safe
operation.
Description
Train unloader, conveyors, stackers, coal storage facility and
ship loading equipment.
New rail loop and unloading station within the site boundary
to the northwest of the Preston River.
15,000,000tpa.
Approximately 18 months.
Still to be determined as designs for the Berth 14 are still
under preparation.
Approximately 6 ha of disturbed native vegetation will be
removed.
Approximately 30 ha.
The marine components of the project include deepening of the seabed at Berth 14 through dredging
of sediments and potentially, rock fracturing of the underlying material. The berth pocket is proposed
to be dredged to approximately - 12.7 m CD and navigational areas to approximately - 12.2 m CD to
accommodate bulk carriers with at least 225 m LOA. They will access the berth via the existing
shipping channel through Koombana Bay (Figure 1). The total volume of material required to be
removed for establishment of the berth is estimated to be up to 2,700,000 m³ of which up to
1,900,000 m³ may be placed at sea. The dredging and rock excavation program is estimated to last
up to 45 weeks. It is estimated that up to 20,000 m³ of rock excavation may be required to finalise
dredge depths within the berth pockets.
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Figure 1: Project area including dredge footprint, existing and proposed dredge spoil disposal
locations and existing BPA anchorages.
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2.2 Relevant Legislation
Loss of BPPH has been assessed according to the Western Australian EPAs Environmental
Assessment Guideline No. 3 - Protection of Benthic Primary Producer Habitats in Western Australia’s
Marine Environment. This guidance statement requires the proponent to address the protection and
maintenance of ecological integrity and biodiversity through a framework for assessing cumulative
irreversible loss or serious damage to BPPH in Western Australia’s marine environment that may
potentially result from the proposed development. BPPHs are defined as seabed communities within
which algae (e.g. macroalgae, turf and benthic microalgae), seagrass, mangroves, corals or mixtures
of these groups are prominent components. BPPHs also include sections of seabed that can support
these communities.
The EPA expects the following hierarchy of principles to be addressed by all proponents applying this
EAG and the EPA will apply these to its consideration of proposals that could cause damage/loss of
benthic primary producer habitats:
1. All proponents should demonstrate consideration of options to avoid damage/loss of benthic
primary producer habitats, by providing the rationale for selection of the preferred site and
broad project design for example.
2. Where avoidance of benthic primary producer habitats is not possible, then design should aim
to minimise damage/loss of benthic primary producer habitats (e.g. through iterative design
and demonstrable application of Principle 3 below). Proponents will be required to justify that
design in terms of operational needs and environmental constraints of the site.
3. Proponents will need to demonstrate ‘best practicable’ design, construction methods and
environmental management aimed at minimising further damage/loss of benthic primary
producer habitats through indirect impacts and maximising potential for recovery.
4. The EPA’s judgement on environmental acceptability with respect to damage/loss of benthic
primary producer habitats and the risk to ecological integrity will be based primarily on its
consideration of the proponent’s application of principles 1 to 3 and calculations of cumulative
loss of each benthic primary producer habitat type within a defined local assessment unit (the
most ‘realistic’ scenario), together with supporting ecological information, and expert advice,
as required.
2.3 Objective
The report has the following objective:
• To predict the loss of BPPH from the Project area according to the Western Australian
Environmental Protection Authorities (EPAs) Environmental Assessment Guideline No. 3 Protection of Benthic Primary Producer Habitats in Western Australia’s Marine Environment.
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3
Background Conditions
2
The Project area is shown in Figure 2 and is approximately 240 km Benthic habitats in the Project
area were quantified using a combination of high resolution satellite imagery, a remotely operated
vehicle (ROV) and videography of benthic habitats. For a full description of the survey, including
methods used to quantify benthic habitats and outputs, please refer to Technical Report 5. The
sampling plan for benthic habitat surveys was based on ground-truthing discrete habitats identified
from selected feature assessments of a multi-spectral image in the Project area. A total of 132 sites
were selected covering the suite of habitats, bathymetry and spatial extent of the Project area.
Further, a greater density of sites were sampled in Koombana Bay and at the proposed offshore
dredge material disposal location to improve confidence in the training sets and subsequent
interpolation at these potentially more impacted sites. Habitat modelling was undertaken with the
interpolation algorithm Topo-to-Raster in ESRI GIS method used to model habitats, with modelled
data tested against test data to determine the certainty of habitat predictions across the Project area.
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Figure 2: Project area including dredge footprint, existing and proposed dredge material
placement grounds and existing BPA anchorages. The Leschenault Estuary is seen on the
right of the Project area.
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3.1 Key Receptors
The distribution of subtidal benthic biota occurring across the Project area is shown in Figure 3. A
spatial representation of predictive certainty is shown in Figure 4.
Figure 3: Quantitative habitat map showing distribution of benthic biota across the Project
area.
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Figure 4: Predictive certainty of habitat modelling across the Project area.
In terms of substrate composition, 62.3% of the Project area was comprised of sand while 37.7% of
the Project area was comprised of reef. Biotic cover occurring in the Project area was typically low
(32.0%) compared to surrounding areas such as Binningup where 60. 5% of habitats were vegetated
(Scott and Hillman, 2009). Coverage of biota occurring on sand (25.0% ± 1.1% SE) was lower than
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biota occurring on both reef (51.1% ± 2.2% SE) and sand inundated reef (52.8% ± 2.2% SE). Key
biotic groups occurring in the Project area included seagrasses, macroalgae and filter feeders.
The dominant seagrasses occurring across the Project area included Amphibolis spp., and Posidonia
spp., and were typically associated with sand areas (Figure 5). Seagrasses occurred a considerable
distance from shore in a minimum of 9 m water depth (Figure 5). Canopy forming macroalgae were
found associated with reef substrata and had a ubiquitous distribution across the Project area, though
typically occurred in low densities (<2.5% coverage). The density of foliose algae (predominantly red
forms) was consistently the highest across the Project area, particularly on the nearshore reef
complex to the north of the Project area where medium densities (25-50%) of foliose algae were
common (Figure 3).
Densities of turf algae up to 50% were observed across the Project area (Figure 3). Density of turf
algae was highest on the nearshore and midshore reef complexes in the central part of the Project
area though turf were also abundant on the nearshore reef systems just north of Koombana Bay and
The Cut. Finally, filter feeder communities in the Project area were dominated by sponges and were
generally sparse (<2.5%), though were widely distributed across the Project area (Figure 3). The
density of sponges was highest on the nearshore reef systems just north of Koombana Bay and The
Cut. This is likely due to the high current speeds and nutrient rich water associated with outflow from
Leschenault Estuary through The Cut.
White mangroves, Avicennia marina are also present in the Project area. This species is present in
the Leschenault Inlet and represents the most southerly occurrence of this species in Western
Australia with their nearest neighbours located approximately 500 km to the north at the Houtman
Abrolhos Islands. The Leeuwin current is thought to be responsible for the occurrence of this species
in the inlet through the delivery of seedlings and warm water. The location of the mangroves in the
inlet means the population should not be potentially impacted by the proposed development activities.
No hard corals are known to occur or were observed in the Project area during surveys.
Biotic groups occurring in the Project area are known to be persistent at the community level (Andrew,
1999; Wernberg et al, 2003). While some loss of kelp and associated macroalgae may occur through
processes such as herbivory (Vanderklift et al, 2009), disease (Andrew, 1999), or through physical
detachment (Wernberg and Connell, 2008), rapid colonisation by turfing and opportunistic species
such as Lobophora spp. and Zonaria spp. sees these gaps in the canopy rapidly colonised as part of
the successional processes operating on temperate rocky reefs (Andrew, 1999). Further, the
seasonal dynamics of temperate reef systems in the region are well understood (Kennelly, 1987;
Kirkman, 1989; Andrew, 1999; Kendrick et al, 1999; Wernberg et al, 2003; Wernberg and Connell,
2008; Wernberg and Goldberg, 2008).
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Figure 5: Distribution of seagrass across the Project area.
3.2 Background Conditions
3.2.1
Water Quality
A review of background water quality in the Project area was undertaken by Wave Solutions using a
combined approach of field measurements, data loggers and remote sensing (MODIS analysis)
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provided by GRAS. A detailed description of water quality in the Project area is provided in Technical
Report 3. Here, an overview of turbidity conditions is provided and key drivers of turbidity in the
Project area are discussed.
Analysis of MODIS images was used to provide a regional analysis of quarterly Total Suspended
Matter (TSM) statistics in marine waters of the Project area and surrounds. The years 2005, 2006,
2007, 2008 and 2010 were selected to represent high and low annual rainfall periods for the Bunbury
area. Generally, the highest average TSM levels within Koombana Bay were recorded during the third
(July to September) and fourth (October to December) quarters with TSM levels between 10 mg/L
and 30 mg/L. The second quarter (April to June) had the lowest TSM levels and the lowest variation
with TSM concentrations in the bay ranging from 4 mg/L to 15 mg/L. The first quarter (January to
March) had high variability with average TSM ranging from approximately 2 mg/L to 30 mg/L. Even
during periods where there is minimal freshwater discharge from the Leschenault Estuary, such as
during summer months, the turbidity of the Project area was found to be high compared to nearby
marine systems such as Binningup to the north and Geographe Bay to the south.
TSM levels in Koombana Bay and nearshore environments were also influenced by river flow from the
Preston River. The Collie River flow is regulated by constructed dams and therefore less responsive
to rainfall in the catchment. The Preston River flows into the Leschenault Estuary which is connected
to the Indian Ocean via The Cut just north of Koombana Bay. At times of high outflow from the
Preston River, TSM levels in the upper water column of the bay averaged 15 mg/L to 25 mg/L while
during periods of low rainfall, the TSM levels were considerably lower (approximately 5 mg/L to 15
mg/L). Figure 6 shows elevated turbidity levels associated with outflow from the Preston River and
Leschenault Estuary during a recent flood event.
The Project also used acoustic backscatter analysis from data loggers to further investigate TSS
conditions within Koombana Bay and coastal waters. This analysis was able to reconcile patterns in
total suspended sediment (TSS) concentrations on a fine scale and showed that Koombana Bay and
nearshore environments are influenced by sediment re-suspension from ships wakes, along with tidal
exchange and outflow from the Leschenault Estuary, such that TSS levels within the Project area are
highly dynamic in both time and space. Wind speed and direction was also shown to influence TSS in
Koombana Bay through re-suspension of bottom sediments within the bay. TSS was generally in
exceedance of 20 mg/l when wave height is >1 m or when wind speed is >8 m/s. This is particularly
apparent during the third quarter when higher average wind speeds from the north and north-west
induce waves within the bay. Ship-wake sediment re-suspension also induces turbidity spikes in the
vicinity of the shipping channel.
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Figure 6: MODIS image showing concentration of suspended sediments across the Project
area and influence of outflow from the Preston River and Leschenault Estuary on nearshore
environments. Image captured on 24 August 2011.
Turbidity may also be described in terms of Nephelometric Turbidity Units (NTU). To investigate water
quality across the Project area, a water quality depth profiling study was undertaken as part of the
Project (Technical Report 3). This study identified a high turbidity layer close to the seafloor (Figure
7). It has been previously reported that high organic material from seagrass and macroalgal wrack
transported during winter storms and fine silt from the Leschenault Estuary accumulate in the shipping
channel in the bay (SKM, 2001). This high turbidity layer will be important in structuring benthic
assemblages in Koombana Bay. The presence of this layer is consistent with findings of previous
studies in Geographe Bay which observed a persistent and distinct nepheloid layer close to the
seafloor during winter and spring (Babcock et al, 2006).
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NTU
0
50
100
150
200
250
300
0%
Percent depth below surface
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
Figure 7: Integrated NTU profile of Koombana Bay (n=24 sites) showing occurrence of
nepheloid layer near the seafloor.
3.2.2
Sediment Characteristics
Koombana Bay is predominantly comprised of bare sand and silt with the exception of a line of reef
on the north-eastern margin of the bay (Figures 2-4). The nearshore environment in the Bunbury
region is highly modified. The opening of the Leschenault Estuary to the Indian Ocean at “The Cut” in
1951 and the realignment of the Preston River to allow for the construction of the Inner Harbour,
removed the capacity for normal estuary sediment filtration processes to occur in the lower reaches of
the Leschenault Estuary. The Cut also transformed the Leschenault Estuary from a tidally influenced
estuary to an estuary dominated by wave influences (McComb et al, 2001; DoW, 2007). Nowadays,
the discharge of water is from the central mud-basin of the estuary. It is estimated that of the average
3
170,000 m of sediment that is estimated to accumulate annually in Koombana Bay, over 50% of this
sediment is fine silt material, suggesting delivery from the estuary via The Cut (SKM 2001, Shore
3
Coastal 2009). Analysis of siltation rates suggests deposition of 45,000 m /yr of sediment in the sand
3
trap and Outer Harbour, with a further 125,000 m /yr deposited in the Main Channel and Inner
Harbour. The rates appear to be primarily driven by wave heights and influenced by water level
fluctuations (Shore Coastal 2009).
The characteristics of surface sediments in Koombana Bay were also assessed. An interpolated
sediment map of surficial sediments in Koombana Bay is shown by Figure 8. The centre of the bay is
characterised by fine silt (<65 µm) while fine sand (66 µm -250 µm) occurs along Koombana Beach,
along the entrance to the Inner Harbour and along Power Station Beach. Fine sand also occurs
between McKenna Point and the shipping channel. Medium (250 µm -500 µm) and coarse (501 µm 800 µm) sand occur at the opening of The Cut.
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Figure 8: Surficial sediment map in Koombana Bay.
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4
Predicted Loss of BPPH
This assessment considers the loss of BPPH arising due to proposed capital works dredging at Berth
14 in the Inner harbour area. Disposal of dredge material is proposed to occur in Commonwealth
waters and so are not considered as part of this assessment (instead see Technical Report 2). Loss
of benthic habitats during dredging are considered in terms of direct loss and indirect loss due to
effects of the sediment plume (EPA, 2010).
4.1 Sediment Transport Modelling Outputs
Outputs from the sediment transport modelling indicate that the Inner Harbour and Koombana Bay
are low dispersive environments due in part to low tidal ranges and currents but also because the
Inner Harbour and Koombana Bay are protected from the prevailing south-westerly winds and swells
typical in the region. During summer, the currents are generally to the ENE, due to the prevailing
winds. In the winter months, the current direction is more varied, and aligns with the ENE – WSW
direction. The depth averaged currents are generally in the range of 4 cm/s to 11 cm/s, occasionally
peaking above 20 cm/s. There is a seasonal variation in the peak significant wave heights, with larger
waves occurring during the winter months. The local sea component wave heights are generally lower
than the swell component. Peak swell heights reach 2 m, and the sea component has peak wave
heights on the order of 1.5 m adjacent to the entrance to the inner harbour. The swell component has
a period in the range of 12 to 14 sec, whereas the sea component is much less, on the order of 3 to 4
sec. The waves generally propagate from the southwest, west and north directions. The swell
component is minimal since offshore swell rarely propagates to the inner harbour due to the sheltered
location. Locally generated sea component wave heights are generally low but can reach nearly 1 m
at times.
As dredging activities will be confined to the Inner Harbour, this further constricts the dispersion of the
sediment plume. During the sediment transport modelling, four scenarios were simulated (Technical
Report 4). All scenarios were modelled for a period of 40 weeks with the different simulations covering
a range of seasons and wet and dry dates to encompass the range of likely scenarios that may occur
due to dredging (Refer to Technical Report 4). Scenario one (simulated from January 1 2009 to
October 8 2009) resulted in slightly larger dredge plume impacts and so is presented in Figure 9 with
the other scenarios included in Technical Report 4.
Concentration exceeded 5% of the time (i.e.
sediment plume will be less than shown 95%
of time).
Concentration exceeded 25% of the time (i.e.
sediment plume will be less than shown 75%
of time).
Figure 9: Outputs from the sediment transport modelling based on scenario one.
Biota in Koombana Bay is restricted to a limestone reef area occurring on the north-eastern margin of
the bay (Figure 10). To determine the duration of effects of this plume on biota occurring on this reef
system, the time series of suspended sediment concentrations was examined at this location (Figure
11). A 12-hour time sequence of plume behaviour is also presented for Scenario 1 (Figure 12).
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Figure 10: Showing limestone reef structure on the north-eastern margin of Koombana Bay and
the location of receptors used to examine the time series of sediment concentrations. Location A
corresponds to the nearest benthic habitat to the proposed development.
Figure 11: Time series of sediment concentrations at location A for the two model runs. Location
A corresponds to the nearest benthic habitat to the proposed development as shown in Figure
10.
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Figure 12: 12-hour time sequence of depth-averaged dredge plume behaviour for Scenario 1.
4.2 Impact of Dredging on BPPH
4.2.1
Direct Effects of Dredging
The area directly affected by dredging is the dredge footprint located at Berth 14 in the Inner Harbour,
an area of 13 ha. The inner harbour is an artificial harbour constructed in the early 1970’s following
the diversion of the Preston River and is comprised of sandy silt substrata with no biota. The depth of
the inner harbour is maintained through frequent maintenance dredging. Thus no BPPH is likely to be
directly lost as a result of dredging.
4.2.2
Indirect Effects of Dredging
Indirect effects of dredging arise from the movement of the sediment plume into areas where benthic
communities occur (EPA, 2010). Indirect effects of dredging include elevated turbidity levels through
an increase in TSS in the water column and sedimentation. Elevated TSS in the water column leads
to a decrease in water transparency and corresponding decrease in light available to benthic
communities. This increase in TSS may influence biota, not only through a decrease in light
availability and photosynthetic capacity, but also through the abrasion of soft tissues and interference
with filter feeding mechanisms (Philipp and Fabricius, 2003, Erftemeijer and Lewis, 2006). Increased
sedimentation arising from dredging may smother benthic fauna and hinder prey capture in sessile
invertebrates (Philipp and Fabricius, 2003, Erftemeijer and Lewis, 2006). For the purposes of
determining loss of benthic habitat, irreversible loss is defined as lacking a capacity to return or
recover to a state resembling that prior to being impacted within a timeframe of five years or less
(EPA, 2010).
As shown by the outputs from the sediment transport modelling, the sediment plume is predicted to
be largely confined to the inner harbour with limited dispersion into Koombana Bay (Figure 9). Where
the plume does extend into Koombana Bay, the suspended sediment concentrations are predicted to
be low (2 mg/L -5 mg/L) and of short duration (Figures 10 and 11). Relative to background levels of
turbidity in Koombana Bay where turbidity routinely exceeds 20 mg/L, the effect of such increases in
turbidity are considered negligible. Therefore the loss of BPPH due to the indirect effects of dredging
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must be considered in terms of cumulative impacts where the additive effects of dredging and
background turbidity exceed the biological threshold of the organisms to elevated TSS. Indirect
effects of elevated turbidity as a result of transporting dredge material offshore to the proposed
placement ground are expected to be minimal as hopper barges will not be overflowing at this time.
The biotic groups occurring in Koombana Bay that will potentially be exposed to these increases in
suspended sediments include foliose algae, turf algae and filter feeders (Figure 3). There is a paucity
of information regarding the tolerance of foliose algae, turf algae and filter feeders to suspended
sediments with no information available regarding the cumulative effects of increased sedimentation
in a complex water quality regime such as Koombana Bay. However, foliose algae, turf algae, and
filter feeder communities are common on nearshore reefs adjacent to “The Cut” on the northern
margin of Koombana Bay where they are exposed to outflow of turbid waters from the Leschenault
Estuary (Figure 6), leading to elevated near bottom turbidity (Figure 13) exceeding cumulative levels
likely to be generated from dredging. As such, it is highly unlikely that the low levels of suspended
sediments will lead to the loss of biotic groups from this area due to cumulative effects. Further, a
study at nearby Binningup made the determination that sessile invertebrate species present in
environments exposed to high background levels of turbidity such as these are adapted to a high
degree of sediment movement and suspended sediments, with patchiness in distribution evidenced
by frequent bare areas on reefs indicating removal and re-colonisation of sessile biota is an ongoing
process (Scott and Hillman, 2009). Given there is unlikely to be any direct or indirect loss of BPPH as
a result of development activities, and following engagement with the EPA, it was determined that
there was no requirement to establish a local assessment unit (LAU) and to undertake a formal loss
assessment for the Project (Ray Masini personal communication September 16, 2011). However,
while highly unlikely, even with the complete loss of biota occurring on reef habitat in Koombana Bay,
2
this would result in a maximum loss of 0.5% of benthic primary producers in the surrounding 50 km
area based on the EPAs Environmental Assessment Guideline No. 3 Protection of Benthic Primary
Producer Habitats in Western Australia’s Marine Environment.
Figure 13: Near bottom turbidity across Koombana Bay showing higher turbidity near The Cut,
likely due to outflow from the Leschenault Estuary.
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Microphytobenthos may also occur on bare sand within Koombana Bay. Microphytobenthos are
temporally variable photosynthetic algae with high turnover rates. The productivity of
microphytobenthic communities are known to be negatively affected by reductions in light availability,
reductions in temperature and increases in sediment re-suspension (Barranguet et al, 1998;
Sundbäck et al, 2000; Schreiber and Pennock, 1995).
Koombana Bay is a highly dynamic environment characterised by high turbidity (Figure 13), a nearbottom nepheloid layer and frequent wind driven and anthropogenic sediment re-suspension. This low
light climate and continued re-suspension of the surface sediments in Koombana Bay means
microphytobenthos are unlikely to occur in the area. Microphytobenthos are also characterised by
high productivity and turnover rates, with turnover in the order of 4-10 days (Sundbäck et al, 2000;
Webster et al, 2002), indicating any microphytobenthos that may be lost due to indirect effects of
dredging would rapidly recover.
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5
Conclusions
The proposed development of the Berth 14 area is occurring in the Inner Harbour area of Bunbury
Port, an artificial harbour constructed in the early 1970’s and devoid of benthic habitat, meaning there
will be no direct loss of benthic habitat due to dredging.
There is predicted to be no loss of benthic habitat due to indirect effects of dredging. Outputs from the
sediment transport modelling indicate that biotic groups occurring in Koombana Bay will be exposed
to small, short duration increases in suspended sediment concentrations (2-5 mg/L) against
background levels of turbidity that routinely exceed 20 mg/L. Loss of benthic habitat will therefore only
be due to cumulative effects though this is also unlikely given the occurrence of similar communities
that occur to the north of Koombana Bay adjacent to The Cut where they are exposed to more turbid
background conditions due to the outflow from the Leschenault Estuary in this area. The loss of
microphytobenthos was also considered as part of the loss assessment. It was considered unlikely
that extensive microphytobenthos occur in this area due to frequent wind driven and anthropogenic
sediment re-suspension and low light climate in Koombana Bay. Finally, all groups considered in the
loss assessment are capable of rapidly re-colonising areas of bare habitat.
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6
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