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18
th
World IMACS / MODSIM Congress, Cairns, Australia 13-17 July 2009
http://mssanz.org.au/modsim09
Three-dimensional hydrodynamic modelling of a
coastal embayment for multiple use industrial
discharge and infrastructure development
J. E. Harris, J.P. Antenucci, P.N. Okely, R. Mills and P.S. Yeates.
Centre for Water Research, The University of Western Australia, M023,
35 Stirling Hwy, Crawley 6009, Western Australia, Australia
Email: harris@cwr.uwa.edu.au
Abstract:
Cockburn Sound is a coastal embayment located to the south of Fremantle on the Western Australian
coast. The region is an important industrial centre, receiving numerous discharges to the Sound
including those by Kwinana Power Station, BP Refinery, Tiwest Refinery and Water Corporation. In
addition, the embayment supports multiple uses including commercial fishing, defence, recreation and
provision of drinking water. Current and potential water quality issues facing the embayment include
reduced circulation and sediment plumes arising from dredging and infrastructure development and
thermal and saline plumes resulting from industrial discharges.
The Centre for Water Research at the University of Western Australia has conducted a series of
detailed validations of the Estuary, Lake and Coastal Ocean Model (ELCOM), coupled with the
Computation Aquatic Ecosystem Dynamics Model (CAEDYM) applied to Cockburn Sound. The high
public profile of the site and the extent of the industrial use required extensive stakeholder interaction
in the model construction and validation.
The validated and calibrated three-dimensional model has provided an integrated tool for numerous
applications, across various spatial and temporal scales, including:
• The impact of the Perth Seawater Desalinisation Plant on stratification and dissolved oxygen
dynamics in the Sound;
• The influence of harbour modifications in Jervoise Bay on flushing of the Australian Maritime
Complex harbours;
• The impact of Fremantle Ports Kwinana Quays development on circulation within the Sound;
and
• The impact of cooling water discharge from proposed High Efficiency Gas Turbines on
thermal plumes.
This paper presents the extensive validation process of ELCOM in Cockburn Sound, and the
application of this integrated tool to assess the impact of numerous interventions on varying spatial and
temporal scales. Future modelling directions will also be discussed.
Keywords: Cockburn Sound, hydrodynamic modelling, integrated tool, industrial development.
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Harris et al., Hydrodynamic Modelling of a Coastal Embayment
1. INTRODUCTION
Cockburn Sound is a semi-enclosed coastal embayment located south of Perth along the Western
Australian southern coast (Figure 1). The main basin is approximately 16 km long, 7 km wide, and has
a maximum depth of 22 m. The surface area is approximately 80 km
2
and the volume approximately
1.2 x 10
9
m
3
. Cockburn Sound is bounded to the east by the coastal mainland, to the west by Garden
Island, and has a shallow ridge, approximately 10 m deep, that extends across its northern ocean
boundary. At the southern end of the Sound, the mainland and Garden Island are connected by a solid
rock-fill causeway, with two small openings to the sea that are approximately 300 and 600 m wide.
Over seasonal time-scales, the hydrodynamic behaviour of Cockburn Sound is dominated by surface
wind stress and vertical density gradients (Yeates et al., 2007). Stratification is driven by a combination
of salinity and temperature gradients that exist throughout the year (DEP, 1996; D’Adamo, 2002).
During summer (November to March), strong south-westerly afternoon sea-breezes erode the weak
stratification. In winter, the strength of the sea-breeze decreases and increased discharge from fresh
groundwater springs along the eastern shoreline form a baroclinic flow which in combination,
strengthen the stratification (D’Adamo, 2002).
Numerous thermal discharges are released into Cockburn Sound, with maximum flow rates of 10.16 m
3
s
-1
and temperature differences between discharge and ambient waters of up to 12.7
o
C. In addition, the
Perth Seawater Desalination Plant releases a saline discharge into the Sound, at a flow rate of 2.3 m
-3
s
1
and salinity 43% greater than ambient conditions. Figure 1 indicates the locations of current and
proposed industrial discharges in the Sound, and proposed harbour developments.
2. METHODOLOGY
2.1. Estuary, Lake and Coastal Ocean Model
The Estuary, Lake and Coastal Ocean Model (ELCOM) is a three-dimensional hydrodynamics model
designed to simulate the temporal and spatial variation in temperature and salinity in surface water
bodies (Hodges et al., 2000). ELCOM solves the unsteady Reynolds-averaged Navier-Stokes equations
using a semi-implicit method similar to the momentum solution in the TRIM code (Casulli and Cheng,
1992) with the addition of a quadratic Euler-Lagrange discretisation for scalar transport using a
conservative flux-limited approach. Flow is solved using an Arakawa-C rectangular grid which has
velocities defined on cell faces and the free-surface height and scalar concentrations defined at the cell
centres. The free-surface height is solved by integrating the continuity equation for incompressible
flow and is permitted to pass vertically through grid layers as required. Heat exchange at the surface is
determined using standard bulk transfer models for nonpenetrative components of sensible heat transfer
and evaporative heat loss that act on the surface cells only, long-wave radiation inputs and losses from
the surface cells, and a penetrative flux due to shortwave radiation that heats the water column by
following an exponential decay with depth that is given by Beer’s Law. An explicit mixing model is
applied to compute the vertical turbulent transport (Hodges et al., 2000).
ELCOM was configured according to the components in Table 1. The validated model configuration
was adjusted to model scenarios described in proceeding sections.
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Harris et al., Hydrodynamic Modelling of a Coastal Embayment
Figure 1. Map of Cockburn Sound, indicating proposed and current industrial discharges, and
proposed harbour development. The outlined areas indicate the extent of the two modelling domains.
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Harris et al., Hydrodynamic Modelling of a Coastal Embayment
Table 1. Components of Cockburn Sound validation (temperature and salinity (T/S) and velocity
(U/V)) simulations.
Component Validation T/S Validation U/V
Start time 23 Feb 2005 (year-long validation)
13 Dec 2006 (desalination in operation)
9May 2006 (winter)
31 Jan 2006 (summer)
Period 400 days 62 days
Initial
conditions
Velocity: zero
Salinity: measured profile
Temperature: measured profile
Velocity: zero
Salinity: constant
Temperature: measured profile
Time-step 90 seconds 60 seconds
Bathymetry Horizontal: plaid grid 100x100m near
power station and desalination outfalls,
expanding to 200x200m.
Vertical: 0.5-1m
Horizontal: plaid grid 100x100m for
basin (Woodman Point to James Point),
200x100m extending 10km north of
Fremantle Port.
Vertical: 1m
Meteorological
Data
BOM Swanbourne and Perth airport (10
min). Solar radiation: Caversham (10
min)
Winter: BOM Garden Island (10 min).
Summer: in situ LDS (15 min). Solar
radiation: Caversham (10 min)
Boundary
conditions
Water Heights: FP Fremantle Fishing
Harbour Station- 15 min.
Water T/S: vertically averaged
measurements taken in 2005/2006.
Water heights: FP Fremantle Fishing
Harbour Station- 15 min.
Water T/S: vertically averaged
measurements taken in 2005/2006.
2.2. Observation Data
To enable validation of the model, water column temperature, salinity and velocity data were sourced.
Water column temperature and salinity for Cockburn Sound were sourced by Oceanica Consulting Pty
Ltd. The data included profile measurements at four locations throughout the Sound on nine occasions
during 2005. Weekly profiles at the same locations for the summer months (Dec- Mar) were available
from the Cockburn Sound Management Council. Intensive profiling was undertaken by the Centre For
Water Research during week-long field campaigns in Dec 2006 and Apr 2007.
Validation of the velocities in Cockburn Sound focussed on ADCP deployments at two locations: the
old spoil grounds site (“FSG”) and the northern basin site (“FNB”), during a summer and winter period
(31 January - March 10 2007 and 9 May – 10 July 2006). The FSG site is located on the eastern shelf
of the Sound and has a depth of 6.5m, while the FNB site is located in a more complex topographic
environment, approximately 500 meters west of the eastern shelf of the Sound that sees the water depth
drop rapidly from 10 to 20 meters.
3. RESULTS AND DISCUSSION
3.1. Model Validation
The temperature and salinity validation showed an excellent comparison between measured and
modelled values. For example, Figure 2 presents a transect of measured and simulated salinity along
the main shipping channel in Cockburn Sound in the morning of 27 April 2007. The field data shows a
lens with elevated salinity at the bottom of the channel associated with the saline discharge from the
Perth Seawater Desalination Plant. The simulation results show the model has captured the intensity
and extent of this feature. At all sampling locations, the correlation coefficients for temperature and
salinity ranged from 0.94 - 0.99 and 0.84 – 0.94 respectively.
The velocity validation utilised contour plot comparisons (to display the temporal and vertical
variability) and histogram comparisons (an integrative summary of the model performance at each
depth) of current speed and direction over the full depth at both stations. Figure 3 presents current
velocity and direction at the FSG site between 19 March and 25 March 2007. At the FSG site in
summer, the field data show the currents are generally unidirectional over depth, with velocities
slightly slower at depth due to bottom friction. Current direction is strongly related to the wind
direction, with the predominant currents in summer to the north. At the FNB site (not shown),
velocities are far more complex, with numerous examples of three-layer flow operating despite the
relatively well-mixed conditions. The simulation results reflect the current patterns in both summer and
winter periods, and in two topographically different locations within the Sound. Root mean squared
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Harris et al., Hydrodynamic Modelling of a Coastal Embayment
error values ranged from 0.019 m s
-1
to 0.047 m s
-1
and mean absolute error values ranged from 0.015
m s
-1
to 0.037 m s
-1
, indicating the model is fit for purpose for scenario modelling.
Figure 2. Transect of measured (upper panel) and simulated (lower panel) salinity along the main
shipping channel in Cockburn Sound in the morning of 27 April 2007. Source: Okely et al., 2007.
Figure 3. Measured (ADCP) speed, simulated (ELCOM) speed, measured direction and simulated
direction for six days at the Spoil grounds location for 19 Mar – 25 Mar 2007. Source: Antenucci et al.,
2008.
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Harris et al., Hydrodynamic Modelling of a Coastal Embayment
3.2. Perth Desalination Discharge
The Perth Seawater Desalination Plant (PSDP) obtains water from the Sound at a location
approximately 160 metres from the coastline, and discharges the waste brine via a 40 port diffuser
approximately 300 metres from the coast. Detailed field investigations were carried out in 2006 and
2007 to determine the near and far-field mixing of the saline plume generated by the plant, and to
provide validation data for the modelling.
A key factor in the environmental approvals process was the accurate representation of the horizontal
scale of the saline plume. Simulation results averaged over an annual cycle (Figure 4) show the
average density difference induced by the discharge. The plume is generally confined to the shallow
eastern margin of the sound, where it tends to reside in the dredged shipping channels until being
vertical mixed by wind events. The maximum density difference observed was 1 kg m
-3
, equivalent to
a salinity difference of approximately 0.75 PSU. The shape of the plume to the north and south follows
the dredged shipping channels, as the plume moves under gravity to these deeper locations. The
transect in Figure 2 intersected the saline plume in the main shipping channel. Dissolved oxygen
conditions under the stratification did not draw down to low levels, as the stratification is periodically
broken down by wind mixing and oxygen is therefore replenished.
Figure 4. Average density difference between the base case and desalination discharge. Source: Yeates
et al., 2006.
3.3. Jervoise Harbour Modifications
A study is currently being undertaken to determine the flushing times for each of the months December
2006 to March 2007 of the northern and southern harbours in the AMC complex of Jervoise Bay. The
velocity validation configuration was modified to assess the impact of several small modifications to
the harbour breakwater on flushing times. The 100x100m horizontal resolution bathymetric data was
interpolated to 50x50m over the Sound, down to 20x20m over the harbours. The overall domain was
cut down to cover only the eastern shelf including Jervoise Bay, to keep run times reasonable with the
increased model resolution. Spatially and depth-averaged flushing times were determined for each of
the harbours. Flushing times were also determined for a series of profile points throughout each of the
harbours to gain some insight in to spatial variability, of which there was some noted in the northern
harbour in particular. Preliminary results suggest that the inclusion of a 100m gap down to 2m below
mean sea level in the southern breakwater of the northern harbour provides a significant reduction in
the flushing time, with associated water quality benefits.
3.4. Kwinana Quays Offshore Development
Due to increasing industrial pressure on Fremantle Harbour, Fremantle Ports are currently planning the
development of a harbour (Kwinana Quays) in Cockburn Sound. Extensive dredging and reclamation
operations will be involved in the construction of the harbour. There are a number of issues to be
addressed in the environmental approvals process in relation to this development including the fate of
nearby thermal and saline plumes, the impact on circulation of the Sound, the impact on residence time
in the AMC Harbours and the ability of works to meet their assigned Low Ecological Protection Area
(LEPA) management areas with respect to temperature and salinity. The validated ELCOM model has
been applied to simulate a number of scenarios to assess the impact of the development, during each
stage of its construction. The model configuration has been extended to cover a full one-year period
during the construction of the harbours. A nested grid set up was utilised where water height,
temperature and salinity of the 100x100m plaid grid was provided as open boundary conditions to the
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Harris et al., Hydrodynamic Modelling of a Coastal Embayment
50x50m inner model grid. The velocity fields produced by the model have also been exported for use
by a third-party (APASA Pty Ltd) in the simulation of dredge plumes associated with the construction.
4. CONCLUSIONS
A key outcome of this work has been the acceptance of the model performance by both the regulatory
agencies and the industries concerned. This has been achieved in less than a two-year timeframe via a
heavy reliance on targeted field campaigns for model validation, which have greatly increased
confidence in the predictions. The model has now followed a development path whereby each new
project builds on existing work, as each of the industries sees the cost effectiveness of this “open”
resource approach, both in terms of reduced effort in re-establishing a model from scratch, but more
importantly as it has simplified the environmental approvals process.
The reasons for the success of the model can also be traced to the flexibility of the model in its
application to problems of varying spatial and temporal scales. The baroclinic nature of the model and
its ability to accurately handle stratification and vertical mixing has been a critical factor, due to the
buoyancy sources from the desalination and thermal plant discharges.
Future uses of the validated model include modelling of proposed industrial discharges into the Sound,
for example, thermal plume modelling of cooling water discharge from proposed High Efficiency Gas
Turbines. In addition, the Computation Aquatic Ecosystem Dynamics Model (CAEDYM) has been
coupled with ELCOM to model dissolved oxygen, nutrients, phytoplankton and seagrasses, in
association with the hydrodynamics of the Sound (Okely et al., 2006). This will continue to provide
industries and the community with an integrated tool to assess the impact of developments on the
Sound, and enable management of this heavily utilised area.
5. ACKNOWLEDGEMENTS
The authors would like to acknowledge the support of the Water Corporation of Western Australia (Dr
David Luketina, Steve Christie), Fremantle Ports (Lyle Banks), Oceanica Consulting Pty Ltd (Mark
Bailey), Verve Energy (Peter Christian), the Western Australian Department of Industry, Parsons
Brinkerhoff (Neville Blesing), and the Cockburn Sound Management Council (Dr Tom Rose).
6. REFERENCES
Antenucci, J.P., Mills, R., Botelho, D.A. (2008). Kwinana Quays: Hydrodynamic Modelling of
Cockburn Sound. Final Report. Centre for Water Research, Perth, Western Australia.
Casulli, V. and Cheng, R.T. (1992). Semi-implicit finite methods for three-dimensional shallow water
flow. Int. J. Numer. Methods Fluids, 15: 629-648.
D'Adamo, N. (2002). Exchange and mixing in Cockburn Sound, Western Australia: a seasonally
stratified, micro-tidal, semi-enclosed coastal embayment. PhD Thesis, Dept. Civil Engineering,
University of Canterbury, Christchurch, New Zealand.
Department of Environmental Protection of Western Australia (DEP). (1996). Southern Metropolitan
Coastal Waters Study (SMCWS) (1991-1994) Final Report. Report 17. Department of
Environmental Protection of Western Australia, Perth.
Hodges B.R., Imberger, J., Saggio A. and Winters, K.B. (2000). Modelling basin-scale internal waves
in a stratified lake. Limnol. Oceanogr, 45(7): 1603-1620.
Okely, P., Antenucci, J.P., Imberger, J., Marti, C.L. (2007). Field investigations into the impact of the
Perth Seawater Desalination Plant discharge on Cockburn Sound. Final Report. Centre for Water
Research, Perth, Western Australia.
Okely, P., Yeates, P.S., Antenucci, J.P., Imberger, J., Hipsey, M.R. (2006). Modelling of the impact of
the Perth Seawater Desalination Plant discharge on dissolved oxygen in Cockburn Sound. Final
Report. Centre for Water Research, Perth, Western Australia.
Yeates, P.S., Okely, P., Antenucci, J.P., Imberger, J. (2006). Hydrodynamic modelling of the impact of
the Perth Seawater Desalination Plant discharge on Cockburn Sound. Final Report. Centre for Water
Research, Perth, Western Australia.
Yeates, P.S., Okely, P., Dallimore, C., Antenuci, J.P, Hipsey, M., Imberger, J. (2007). Three-
dimensional modelling of a seawater desalination plant discharge into Cockburn Sound, Western
Australia. Asian and Pacific Coasts, September 21-24 2007, Nanjing, China.
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