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Appendix E Tidal Flow Monitoring GEMS 2008
Appendix E
Tidal Flow Monitoring GEMS 2008
Technical Report
OCEANOGRAPHIC STUDIES
FOR THE
DEVELOPMENT OF THE CAPE PRESTON
SERVICES CORRIDOR CAUSEWAY
February 2008
GEMS
G L O B A L E N V I R O N M E N T A L M O D E L L I N G S Y S T E M S
G L O B A L E N V I R O N M E N T A L M A P P I N G S Y S T E M S
G L O B A L E N V I R O N M E N T A L M O N I T O R I N G S Y S T E M S
GEMS
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GEMS CONTACT DETAILS
Melbourne Office
Telephone: +61 (0)3 9712 0016
PO Box 149
Warrandyte VIC 3113
Dr Graeme D Hubbert
Head of Oceanographic Studies
Mobile: +61 (0)418 36 63 36
Email: [email protected]
Steve Oliver
Head of Meteorological and Wave Studies
Mobile: +61 (0)408 81 8702
Email: [email protected]
Website: www.gems-aus.com
Perth Office
Telephone: +61 (0)8 6364 0880
PO Box 1432
Subiaco WA 6097
Matt Eliot
Coastal Engineer
Mobile: +61 (0)408 414 225
Email: [email protected]
Jason Catlin
Head of GIS Mapping Systems
Mobile: +61 (0)407 048 458
Email: [email protected]
ABOUT GEMS
Global Environmental Modelling Systems (GEMS), a wholly owned Australian company, has expertise
in the development and application of high-resolution computer models to realistically predict
atmospheric and oceanographic conditions for use in riverine, coastal and oceanic settings.
The GEMS team is made up of qualified and experienced physical oceanographers, meteorologists,
numerical modellers and environmental scientists. GEMS is a leading developer of numerical models
in Australia. It has developed a system of validated environmental models and rigorous analytical
procedures that provide solutions to a variety of environmental, engineering and operational
problems.
DISCLAIMER
This report and the work undertaken for its preparation, is presented for the use of the client. Global
Environmental Modelling Systems (GEMS) warrants that the study was carried out in accordance
with accepted practice and available data, but that no other warranty is made as to the accuracy of
the data or results contained in the report.
This GEMS report may not contain sufficient or appropriate information to meet the purpose of
other potential users. GEMS, therefore, does not accept any responsibility for the use of the
information in the report by other parties.
GEMS
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TABLE OF CONTENTS
GEMS Contact Details ............................................................................................................ 2
About GEMS........................................................................................................................... 2
Disclaimer .............................................................................................................................. 2
1 Introduction.................................................................................................................. 6
2 Tidal Flow Studies ......................................................................................................... 8
2.1 Tidal Modelling............................................................................................................. 8
2.2 Sources of Bathymetry and Topography.................................................................... 10
2.3 Simulation of tidal flows under Existing Conditions................................................... 10
2.3.1 Verification of Large Scale Tidal Dynamics ......................................................... 10
2.3.2 Simulation of the Local Tidal Flows through the Causeway Region ................... 12
2.4 Area of culverts required to minimise disturbance to tidal levels ............................. 12
2.5 Effects of the temporary causeway as currently designed ........................................ 15
2.6 Effects of Sea Level Rise ............................................................................................. 15
3 Storm Surge Studies ................................................................................................... 17
3.1 Existing Conditions ..................................................................................................... 19
3.1.1 Methodology....................................................................................................... 19
3.1.2 Modelling Process............................................................................................... 20
3.1.3 Description of Numerical Models ....................................................................... 21
3.1.4 Storm Surge Validation ....................................................................................... 24
3.1.5 Storm Surge Return Periods at the Causeway.................................................... 28
3.2 Effects of the temporary Causeway with culverts ..................................................... 28
3.3 Effects of the permanent Causeway with culverts..................................................... 29
3.4 Effects of sea level rise on the 1 in 100 year storm surge.......................................... 30
4 Summary of findings................................................................................................... 31
4.1 Verification of tidal/storm surge model..................................................................... 31
4.2 Existing Tidal Conditions............................................................................................. 31
4.3 Area of culverts required to minimise interference with tidal flows......................... 31
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4.4 Impacts of the Temporary Causeway with culverts on Tidal Flows ........................... 31
4.5 The Impacts of the permanent Causeway with culverts on Tidal Flows.................... 32
4.6 The Impacts of a sea level rise of 0.5 metre on the Tidal Flow results ...................... 32
4.7 The existing 1 in 100 year storm surge event at the causeway ................................. 32
4.8 Effects of the temporary Causeway with culverts on storm surges .......................... 32
4.9 Effects of the permanent Causeway with culverts on storm surges.......................... 33
5 References .................................................................................................................. 34
TABLE OF FIGURES
Figure 1.1: Plan of the proposed services corridor causeway route superimposed on
high resolution (20metres) model grid showing the two creek flat crossings. ................ 7
Figure 2.1: The domain for GEMSURGE showing the location of the verification data
(CM1b). ......................................................................................................................... 9
Figure 2.2: Very high resolution grid (3 metres) of proposed causeway region at high
tide. ......................................................................................................................... 9
Figure 2.3: Comparison of tidal levels predicted by GEMSURGE (red) with observations
(blue) from September 15 to December 31, 2006. ........................................................ 11
Figure 2.4: Comparison of tidal heights during a spring tide event before (blue) and after
(red) the causeway and culverts are installed................................................................ 14
Figure 2.5: Comparison of tidal heights during a spring tide event before (blue) and after
(red) the causeway and 109 m2 culverts are installed.................................................... 16
Figure 3.1: Components of sea level elevation during cyclones...................................... 18
Figure 3.2: Tracks of cyclones included in cyclone climatology in the vicinity of Cape
Preston. ....................................................................................................................... 19
Figure 3.3: Flowchart of model components for determination of waves and water
levels for a cyclone event. .............................................................................................. 20
Figure 3.4: Evolution of model wind field for Cyclone Orson passing close to Cape
Preston, April 1989. ........................................................................................................ 22
Figure 3.5: Model wind time series for tip of Cape Preston............................................ 22
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Figure 3.6: Tracks of cyclones Clare and Glenda. ............................................................ 24
Figure 3.7: Cape Lambert tide gauge and residual during TC Clare (courtesy WA DPI). . 25
Figure 3.8: Observed and modelled winds at Roebourne – Cyclone Clare. .................... 26
Figure 3.9: Observed winds at Roebourne during Cyclone Glenda and modeled winds for
the event with radius of maximum winds set to 30 and 35 km. .................................... 26
Figure 3.10: Observed and modeled water level at Cape Lambert Jetty during Cyclone
Clare ....................................................................................................................... 27
Figure 3.11: Modeled water level for Cyclone Glenda compared with tidal prediction at
Cape Lambert.................................................................................................................. 27
Figure 3.12: Comparison of modeled peak water level at three locations during Glenda.27
Figure 3.13: Results for the 1 in 100 year storm surge at the southern creek crossing for
the three cases studied................................................................................................... 30
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1 INTRODUCTION
CP Mining Management Pty Ltd (CPMM) are developing the Austeel Project at Cape Preston, located
some 60 km west of Dampier in North West Australia. As part of this development, CPMM propose
to construct an infrastructure and services corridor between their mine site and proposed port site at
Cape Preston. The corridor crosses a moderate sized tidal creek and associated tidal flats between
the mainland and the Cape Preston peninsula.
CPMM propose to construct a causeway across the tidal creek and flats and have engaged URS
Australia (Perth office) to assist them obtain environmental approvals for construction of the
causeway. URS has subsequently subcontracted Global Environmental Modelling Systems (GEMS) to
provide advice on the design of culverts for the Causeway and to assess potential impacts of the
Causeway on storm surge levels.
In EPA Bulletin 1056 (EPA, 2002) which assessed the acceptability of the Austeel project, the EPA
expressed concern that the tidal creek crossing for the services corridor between the mainland and
Cape Preston should not significantly reduce the area of valuable mangrove habitat to the east of the
crossing, or its ecological function. Bridging structures or culverts were considered necessary for this
crossing to maintain tidal flushing to protect mangroves located upstream.
The major concerns expressed recently by regulators regarding the services corridor causeway were
that:
1) The causeway should have minimal effect on the normal tidal flushing as it presently occurs
upstream of the causeway;
2) The causeway should also allow for normal tidal flushing under a global warming sea level
rise ; and
3) The causeway should preferably not increase storm surge levels above those that the
mangroves are currently exposed to.
Therefore in response to the above concerns, this GEMS report describes the modeling work that
was undertaken to estimate the cross sectional area flushing requirements for both normal tidal
flushing and with a 50 cm Sea Level Rise component (items 1 and 2 above). The report also addresses
the effect of the new causeway design on storm surge levels (item 3).
CPMM propose to construct the causeway in two stages.
1. Stage 1 proposes to establish interim access to the Cape from the mainland through
construction of a temporary Causeway approximately 33 metres wide and 3.0 meters
high (to RL 4.5m AHD). This stage will commence in mid 2008.
2. Stage 2 will convert the temporary Causeway into a permanent Causeway by
widening the footprint to 80 meters wide and raising the causeway another 2.5 m (to
RL 7m AHD). This stage is not expected to commence until 2010.
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A range of studies have been carried out by GEMS to assist in the design of the proposed services
corridor causeway at Cape Preston (Figure 1.1). As seen in Figure 1.1 the causeway will cross two
creek flats, the southern one being much larger than the northern one.
To address the regulators’ concerns, the tidal flow was modelled through the natural topography and
again with the causeway in place to determine the location and capacity of the drainage opening(s)
required to maintain the tidal flow, and the water level and flow velocities.
The storm surge return periods were also studied for the existing baseline conditions and again with
the causeway in place, including the culverts, to determine the storm surge levels and the wave
impact for the rock armour design. These studies were undertaken for both the temporary (Stage 1)
and permanent (Stage 2) causeway design.
Figure 1.1: Plan of the proposed services corridor causeway route superimposed on high
resolution (20 metres) model grid showing the two creek flat crossings.
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2 TIDAL FLOW STUDIES
GEMS has been measuring sea levels near Cape Preston since October, 2006 with an Acoustic
Doppler Current Profiler (ADCP) located approx 0.5 km offshore from Cape Preston in 15 m of water
(location CM1b in figure 2.1). The data collected so far has been analysed and the local LAT (Lowest
Astronomical Tide) has been derived as -2.40 metres AHD. HAT (Highest Astronomical Tide) was
derived as 2.35 metres giving a tidal range of approximately 4.75 metres.
It is important to note that LAT is not the lowest sea level which can be experienced, due to the
atmospheric effects of wind set-down and high pressure systems. This is most extreme during
tropical cyclones when negative storm surges of several metres can occur. During non-cyclonic
meteorological events much smaller set downs can occur with the depression in sea level being
approximately 1 cm for each hPa atmospheric pressure above Mean Sea Level Pressure.
2.1 TIDAL MODELLING
To investigate the changes in sea levels and tidal flows which may occur due to the services corridor
causeway development, the GEMS storm surge model (GEMSURGE) was run over three nested
domains ranging from a simulation of the larger scale tidal dynamics in the Cape Preston region to
very high resolution simulations of the flows along the creeks to be crossed by the proposed
causeway.
GEMSURGE (Hubbert et al, 1999) is a recognized storm surge and tidal model which has been used
for studies of these processes in many locations in Australia and around the world over the past 15
years. It has been used by the United States Navy for coastal tide and storm surge modelling for the
past 10 years as part of the coastal ocean forecasting system.
The three domains setup for these studies were:
• Domain 1 - To simulate the tidal dynamics in the region of Cape Preston a 100 metre
resolution model grid was setup on the domain shown in Figure 2.1;
• Domain 2 - To simulate the tidal flows around Cape Preston more accurately a 20 metre
resolution GEMSURGE grid of Cape Preston was setup over the region shown in Figure 1.1.
• Domain 3 - To simulate the tidal flow through the region to be crossed by the services
corridor causeway a 3 metre resolution GEMSURGE grid was set up surrounding the
causeway region as shown in Figure 2.2.
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Figure 2.1: The domain for GEMSURGE showing the location of the ADCP meter (CM1b).
Figure 2.2: Very high resolution grid (3 metres) of proposed causeway region at high tide.
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2.2 SOURCES OF BATHYMETRY AND TOPOGRAPHY
There were four main sources of bathymetry and topography utilized in setting up model grids.
These were:
• Digital marine chart data from Geosciences Australia;
• High resolution 3D bathymetric survey data for the shipping lane out from Cape Preston;
• Medium resolution bathymetric data acquired for near shore regions by Sandwells using a
differential GPS and a depth sounder;
• Topographical and bathymetric data derived by Whelans for the proposed services corridor.
The major weakness in these data was that, although the data for the proposed services corridor was
very high resolution, it did not extend west or east to the open waters. Hence the waterways which
give the tidal or storm surge flows access to the services corridor region were not defined at very
high resolution and relied on the digital chart data augmented by some of the data provided by
Whelans.
Under normal conditions it seems that the flood tide approaches the services corridor region from
the west as the topography to the east is just a bit too high to allow normal tidal flows from the east.
Therefore under normal tidal flows it is important to accurately resolve the waterways along which
the flood tide flows, not just the region around the proposed causeway.
Under storm tide conditions the mechanisms are very different as the storm surge breaches the
topography to the east of the proposed services corridor and, since cyclones predominantly
approach Cape Preston from the north or northeast, the storm tide flooding comes more from the
east than the west. In this case the waterways and low-lying topography to the east become
important to studies of impacts in the services corridor region.
2.3 SIMULATION OF TIDAL FLOWS UNDER EXISTING CONDITIONS
2.3.1 VERIFICATION OF LARGE SCALE TIDAL DYNAMICS
To verify the accuracy of predictions of the existing tidal regime the model was run over domain 1
(Figure 2.1) for the period September 15 to December 31, 2006 and compared with the observed
data at the site off Preston Island. The regional tidal parameters used to drive the open boundaries
of this domain were derived from the GEMS Australian region tidal data-base which was developed
for Australian Search and Rescue in Canberra.
The resulting agreement of the model predictions with the data was excellent (Figure 2.3).
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Figure 2.3: Comparison of tidal levels predicted by GEMSURGE (red) with observations (blue) from September 15 to December 31, 2006.
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2.3.2 SIMULATION OF THE LOCAL TIDAL FLOWS THROUGH THE CAUSEWAY REGION
To simulate the high resolution flows through the region to be crossed by the proposed causeway
GEMSURGE was run on the full nested grid system with boundary conditions from Domain 1 driving
Domain 2 and boundary conditions from Domain 2 driving Domain 3.
This nesting process is important in order to maintain connection to the large-scale tidal dynamics in
the Cape Preston region simulated and verified on Domain 1.
The strongest flows through the causeway cross-section occur during high spring tides around the
time that the sea level is at mid-tide (close to zero AHD) on the flood and ebb tides as the sea level is
rising towards HAT or falling from HAT respectively.
The model simulations were therefore run for the period surrounding the high spring tides occurring
in Figure 2.3 (centered on day 24 and day 53). During these simulations the maximum current
speeds through the cross-section were logged.
The main findings from the study of existing conditions were that:
• The length of causeway exposed to water at HAT is approx. 1900m;
• The flood tide approaches the region of the proposed causeway from the west and even at
the highest spring tides does not connect directly to the ocean on the eastern side of the
proposed causeway alignment.
• At HAT (2.35m AHD) the cross-sectional area of water along the causeway alignment is
approximately 2400 m2.
• Flow speeds along the major creek bed can approach 1 knot during spring tides.
• At MHWS (1.75m AHD) the cross-sectional area of water along the causeway alignment is
approximately 1260m2 (note that the exact cross-section is very dependent on the accuracy
of the topography and bathymetry)
2.4 AREA OF CULVERTS REQUIRED TO MINIMISE DISTURBANCE TO TIDAL LEVELS
The effects of the temporary causeway were studied by filling the topography along the causeway
alignment in Domain 3 to a height of 4.5 m AHD.
The temporary causeway is to be built with culverts installed in the major creek crossing and other
low-lying sections to allow tidal flow through. The task requested of GEMS by URS was to determine
the minimum cross-sectional area to be provided by culverts which would meet the condition that
water levels on the eastern side of the causeway were not prevented from reaching MHWS tidal
levels (RL 1.75 m AHD) 80% of the time. The culvert cross-sectional area would also be dependent on
the design of the culverts as frictional issues with smaller pipe culverts will require a larger cross-
sectional area than for large box culverts which have lower frictional impacts on the flow.
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The modelling undertaken in this study cannot take into account this issue because it simply
represents the culvert as one big box culvert of a given cross-sectional area. The use of smaller
culverts will therefore require the addition of more cross-sectional area to compensate for
frictional issues.
To determine the minimum single box culvert cross-sectional area required to enable tides to reach
the level of MHWS on the eastern side of the causeway at least 80% of the time they do under
normal conditions, the simulations of the spring tides around day 24 and day 53 in Figure 2.3 were
repeated with GEMSURGE with the temporary causeway included in Domain 3.
A single large box culvert (variable cross-section) was included across the major creek crossing and
three smaller ones (2 south and 1 north with fixed cross-section at 21m2 each) across low-lying
regions north and south of the creek crossing. A large number of simulations were then carried out
with incremental increasing of the cross-sectional area (10m2 steps starting at 50m
2) of the box
culvert in the major creek crossing.
For each simulation the tidal levels on the eastern side of the causeway were stored and plotted
against the corresponding time series for the simulation without the causeway in place. In this
manner the minimum cross-sectional area of a single box culvert which would still allow high tides to
reach MHWS at least 80% of the time they do under normal conditions was determined.
The main findings were:
• The reduced cross-sectional areas across the creek crossings will create a head on the
western side during the spring high tides and generate faster flow speeds through the
culverts and potentially restrict the total mass of water passing through;
• The minimum internal total cross-sectional area of the box culverts which would allow tidal
levels of MHWS to be attained 80% of the “normal” time was 250 m2, made up of a 190m
2
culvert in the major creek crossing, a 40m2 culvert south of the major creek crossing and a 20
m2 culvert north of the major creek crossing;
• With the proposed box culverts in place, the water levels on the eastern side during high
spring tides did not reach the same levels as for the existing case and the maxima were
reduced by approximately 10% at high tides that approach HAT (see Figure 2.4);
• During the lower tidal levels the restriction is reduced and there is not any significant
variation in sea levels when comparing existing tides with changed tides below
approximately 1.3 metres.
• The currents through the culverts at high spring tidal flows can reach 2 m/sec and could
potentially cause some local scouring without appropriate mitigation measures.
The causeway design engineers were therefore advised to include sufficient culverts to allow a cross
sectional area of 250 square metres to enable relatively unhindered tidal flushing upstream.
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•
Figure 2.4: Comparison of tidal heights during a spring tide event before (blue) and after (red) the causeway and culverts are installed.
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2.5 EFFECTS OF THE TEMPORARY CAUSEWAY AS CURRENTLY DESIGNED
The engineers responded that such a large area was impracticable and would necessitate substantial
excavation of the tidal flat to install, not to mention the time and cost requirement. Hence meetings
were held with the client’s engineers to arrive at a compromise solution. The objective of the
solution became to allow sufficient flushing to ensure survival of the mangrove habitat lining the
creek upstream of the causeway. From interpretation of available topography along the causeway
alignment it appeared that the landward margin of these mangroves occurred at about RL 1.30 m
AHD. A practicable and feasible maximum area culvert design of 109 square metres was provided by
the client’s engineers. The model was subsequently re-run to assess the effects of a 109 m2 culvert
allowance on tidal water levels upstream of the causeway and Figure 2.5 presents the results.
Figure 2.5 shows that with the proposed culverts in place:
• The causeway will restrict water flow to the east, but will not cause ponding upstream of the
causeway;
• Maximum water levels upstream of the causeway will be reduced to RL 1.5 m AHD (from
2.35 m AHD at HAT);
• The period of wetting at RL 1.3 m AHD during spring tides (horizontal axis figure 2.5) will be
reduced by approximately 50%;
• There will be no effect on neap tides below about RL 1.0 m AHD.
2.6 EFFECTS OF SEA LEVEL RISE
A further requirement by URS of the study was to estimate the impacts of a 0.5 metre sea level rise
on the size of culverts required to achieve MHWS levels under these changed climatic conditions.
An increase in Mean Sea level (MSL) of 0.5 metre would translate to values of HAT of approximately
2.85 metres and of MHWS of 2.25 metres.
To investigate this problem the bathymetry grids for each of the three Domains were deepened by
0.5 metre and then the method described in the previous section was repeated to determine the
minimum cross-sectional which would still allow high tides to reach MHWS 80% of the time under
these new conditions.
The main findings were:
• The cross section of water at HAT along the alignment of the causeway is increased by
approximately 1,000 m2
to 3400m2;
• The cross section of water at MHWS along the alignment of the causeway is increased by
approximately 840 m2
to 2100m2;
• The minimum total internal cross-sections of box culverts required to allow tidal levels of
MHWS to be attained 80% of the time is increased to approximately 400 m2.
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Figure 2.5: Comparison of tidal heights during a spring tide event before (blue) and after (red) the causeway and 109 m2 culverts are installed.
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3 STORM SURGE STUDIES
The tropical cyclone season in Australia occurs typically from November to April. A tropical cyclone
in the southern hemisphere is defined as a rotating low pressure system originating in the tropics in
which the ten minute average winds exceed 63 km/hr or 34 knots. There is characteristically a large
area of convective cloud and heavy rain associated with the system; in the more intense tropical
cyclones there may also be a clear region, the ‘eye’, situated near the cyclone centre. The strongest
winds are located in a band surrounding this eye although, within the eye itself, winds are usually
very light.
When such a storm approaches the coast it can cause an abnormal elevation of sea level. The
maximum sea elevation usually occurs close to the point of maximum winds as the cyclone crosses
the coast, but the general dome of raised water can affect an area up to 50-100 km off the coast with
the effect lasting for several hours.
In determining the sea level elevation associated with a particular event, it is necessary to include the
contributions of the:
• storm surge, resulting directly from the combined action of wind and relatively low
atmospheric pressure;
• astronomical tide, and
• breaking waves at the coastlines, which include an increase in the mean sea level known as
set-up as well as the intermittent effect of wave run-up.
These processes are illustrated schematically in Figure 3.1. The component of sea level elevation
attributable to storm surge and tide is called the still water level. The peak steady water level
combines the still water level and the wave set-up but excludes wave run-up.
The magnitude of sea level elevation is dependent on, inter alia, the:
• intensity of the cyclone, as measured by its central pressure;
• size of the cyclone, usually indicated by the distance from the centre to the region of most intense
winds;
• cyclone track, including the direction of movement, its forward speed and proximity to the point of
interest, and
• shape and depth of the sea floor.
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Wave setup
Mean Sea Level
Lowest Tide
Storm Surge
Wind Waves
Highest Tide
Figure 3.1: Components of sea level elevation during cyclones.
The probability of a cyclone of a particular intensity occurring in the region can be readily estimated.
However, we have seen that this is only one component of an estimation of the probability of a
particular sea level elevation being exceeded.
For example, a significant surge might be caused by a weak cyclone with its maximum winds crossing
directly over Cape Preston near high tide. The same sea level could also be achieved by a strong
cyclone crossing further along the coast, but at lower tide.
Individual events such as Cyclone Orson (1989) and Cyclone Vance (1999) are well documented, but
such storms need to be put into perspective. To accurately represent storm surge and wave events
at Cape Preston it is necessary to specify the long-term storm climate of the region.
The Bureau of Meteorology holds data relating to cyclone behavior in the Australian region that
dates back to the early part of the 20th
century. However, data for storms before the 1950s are
considered less reliable due to the relative paucity of observation sites during this earlier period.
Accordingly, only storms for the 56 seasons from 1950 to 2006 were considered for the storm surge
studies at Cape Preston. A region of influence for storms affecting Cape Preston was defined based
on experience gained in studies of a similar nature carried out in the region. Figure 3.2 shows tracks
of storms which occurred in this region.
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Figure 3.2: Tracks of cyclones included in cyclone climatology in the vicinity of Cape Preston.
3.1 EXISTING CONDITIONS
A detailed storm surge study has been carried out separately to this work to quantify recurrence
intervals for wave heights and storm surge levels for the design of the load out breakwater at Cape
Preston. For the purposes of the present study the results of the larger study were interrogated to
determine the results for the location of the services corridor causeway under existing conditions.
Studies of storm surge levels under changed conditions (inclusion of causeway, sea level rise)
required specific modelling studies to be undertaken.
3.1.1 METHODOLOGY
The simplest approach to develop the required recurrence statistics would be to model the storms
that occurred over the 56 year period described in the previous section. Surge and wave heights
could then be computed from the modelled levels. However, this method is very likely to be
inaccurate since the relatively short period for which data is held does not adequately capture the
range of storm-tide events that may occur over a period of many centuries.
For example, the most intense storm, since accurate recordings have been available, to cross the
coast in the vicinity of Cape Preston is Cyclone Orson – but modelling a single event like the Orson
event does not allow for variations of tracks and storm intensity of similar storms that may occur
over longer time scales.
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To obtain results that address these issues, a method has been developed to create ‘synthetic’
storms based on the actual storm tracks, but which allows for variations in track, intensity and time
of occurrence (which allows for tidal cycle). This methodology has been successfully employed in
many other storm surge assessment studies.
Synthetic storms are produced by the following procedure:
(i) storm central (minimum) pressures are aggregated for all storms in the region
and an extreme value distribution is fitted to the data;
(ii) for each storm event, a pressure is selected randomly from the pressure
distribution;
(iii) a storm track is chosen from the subset of storms of ‘similar’ intensity to the
chosen pressure;
(iv) all pressures for that storm are adjusted up or down based on the selected event
pressure and the minimum recorded for the selected track;
(v) the track is allowed to shift randomly in space to the extent that it remains
within the study area
(vi) a start time is selected from anywhere in the cyclone season over the 18 year
period of a full astronomical tidal cycle
(vii) the equivalent number of storm years is determined based on the number of
storms generated and the measured rate of storm frequency (within the region
of influence).
3.1.2 MODELLING PROCESS
Once the storm database has been established, each storm event can be modeled according to the
procedure set out schematically in Figure 3.3. A suite of models, described in the next section, is
applied to each event so as to represent the processes of interest – that is, the evolution of the
atmospheric pressure and wind fields and the ocean response to these fields across the life of the
storm.
Figure 3.3: Flowchart of model components for determination of waves and water levels for
a cyclone event.
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Procedurally, wave parameters and water levels are stored as a time series at each designated
location of interest for each event. After all the events have been so modeled, the time series data
are processed and recurrence intervals are assigned. This is achieved by relating the modeling
results to the average frequency of the storm events.
3.1.3 DESCRIPTION OF NUMERICAL MODELS
A suite of numerical models has been employed to study the impacts of tropical cyclones impacting
upon Cape Preston. A description of the individual models is provided in this report section along
with the specification of their set-up for the Cape Preston region.
Cyclone Winds
The GEMS tropical cyclone model is based on the empirical model developed at the Australian
Bureau of Meteorology (Holland, 1980). The model treats the wind field as an asymmetric vortex.
Wind directions and speed are a function of the storm central pressure and the environmental
pressure in which the storm is embedded. The spatial distribution of winds is controlled by the
Radius of Maximum Winds (RMW), which defines the distance from the storm centre to the region of
strongest winds. Physically, this region of strongest winds is found around the cyclone ‘eye-wall’; the
eye region is the calm centre of the storm. Typically the radius of maximum winds is of the order of
30-50 km.
Another parameter (the so-called ‘B’- parameter) defines the extent to which the strongest winds are
concentrated around the eye-wall or otherwise extend outwards from the storm centre.
The model is demonstrated for Tropical Cyclone Orson, which crossed the coast near Cape Preston in
April 1989. Figure 3.4 shows the spatial distribution of model wind field as snapshots in time while
for Figure 3.5 shows the predicted winds at Cape Preston.
Storm Surge Model
The two-d coastal ocean model, GEMSURGE (Hubbert and McInnes, 1999; McInnes and Hubbert,
1995) solves the depth-averaged hydrodynamic equations over a region defined by detailed
topographic and bathymetric information to provide currents, sea level heights due to tidal and
meteorological conditions.
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Figure 3.4: Evolution of model wind field for Cyclone Orson passing close to Cape Preston,
April 1989.
0
10
20
30
40
50
21-Apr-89 22-Apr-89 23-Apr-89
Wind Speed (m/s)
0
60
120
180
240
300
360
21-Apr-89 22-Apr-89 23-Apr-89
Wind Direction (deg from)
Figure 3.5: Model wind time series for tip of Cape Preston.
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GEMSURGE features a grid generator to facilitate the setting up of model grids over any specified
geographical region and grid resolution.
It can be run over successively higher resolution regions, using the results of lower resolution outer
simulations as boundary conditions. This so-called ‘nesting’ technique is an economical way of
maximizing grid resolution. The user interface is capable of incorporating output from a wide range
of atmospheric models to obtain surface winds and pressure. Meteorological conditions required by
GEMSURGE includes the 10 minute winds and surface pressure. These must be derived from a
cyclone model, archived analyses or regional atmospheric model simulations. The winds are
interpolated both spatially and temporally from the atmospheric model grid to the GCOM coarse and
fine mesh grids.
The model can run with or without tidal forcing. The tidal prediction model included in GEMSURGE
reads the astronomical constants for each tidal constituent and calculates the tidal heights. These are
applied as lateral boundary conditions in the coarse resolution GEMSURGE simulation.
GEMSURGE Set-up for Cape Preston
GEMSURGE has previously been established and tested extensively for applications in Australia and
the Pacific. For the current study, model grids were established at 1km and 50m spatial resolution
based on best available topographic and bathymetric data.
Wave Model (SWAN)
To model wave processes occurring in the near-shore zone, it is first necessary to establish the
evolution of waves over the open ocean. Typically, significant tropical storm winds affect a region up
to a few hundred kilometres from the storm centre and this area changes with the movement of the
storm.
Depending on the intensity of the cyclone, the winds in the affected area have the capacity to
generate large ocean waves, which in turn propagate away from the generation region. In order to
model these processes, it is necessary to establish a wave model over a regular grid, with a
sufficiently large spatial extent to capture these processes.
Once these large-scale wind and wave generation processes are captured, the results can then be
used to focus on the interaction of the ocean scale waves with coastlines. This latter task involves
modelling the wave processes at higher spatial resolutions as the waves intersect shallower water
depths.
GEMS has previously used two spectral wave models, WAM and SWAN (Booij et al, 1999) for tropical
cyclone studies. Since WAM is essentially a deep-water model, we have preferred to apply the third
generation spectral model, SWAN which was originally developed to model near-shore processes.
Later versions of SWAN have improved large scale /deep water algorithms and successfully predict
tropical cyclone wave behavior.
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SWAN also incorporates a smooth nesting process in which model scales can be effectively
“telescoped” from spatially coarse large scale grids to small high resolution grids established over
particular areas of interest.
GEMS has previously validated SWAN for tropical cyclone generated wave events in Australia.
3.1.4 STORM SURGE VALIDATION
GEMS has validated its storm surge modelling system over many storm events at a variety of
locations in North Western Australia. Unfortunately there is no data at Cape Preston during a
cyclone and so for the current study we have included details of two storms that produced
significant surge events at Cape Lambert, northeast of Cape Preston, where good validation data is
available.
The 2005-06 cyclone season was a particularly active one for the region and included two significant
storm surge events. The tracks of the storms producing these surge events, Clare and Glenda are
shown in Figure 3.6. Water levels in the Cape Lambert region (Figure 3.7) for these two events have
been used to validate the models.
Figure 3.6: Tracks of cyclones Clare and Glenda.
Clare was a relatively moderate cyclone (minimum pressure, 960hPa) but crossed the coast just west
of Dampier so that its strongest winds passed close to Cape Lambert Figure 3.7 shows predicted and
measured water levels at the Cape Lambert tide gauge during Cyclone Clare. The time series data
plotted in Figure 3.7 shows that the storm surge residual was of the order of 1.6m and that the peak
of the storm surge occurred close to high tide on 9 January 2006
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The track of Glenda was similar to that of Clare, but it was a more intense storm (minimum pressure,
910 hPa) and crossed the coast further to the west. No tide gauge data is available for the storm due
to instrument malfunction, but there is significant evidence of inundation accompanying the event.
This evidence, documented for the Shire of Roebourne (GEMS, 2007), indicates water levels of at
least 3.7m at Point Sampson and up to 5.0 m at Cossack (Bond Store) and 4.8m at Johns Creek Boast
Harbour.
Figure 3.7: Cape Lambert tide gauge and residual during TC Clare (courtesy WA DPI).
Results from applying the cyclone model to the ‘best’ track provided for each storm by the Bureau of
Meteorology are shown in Figures 3.8 and 3.9. Model winds were plotted with observed winds at
Roebourne; the wind model was run with a ‘regional average’ value of 30km for the radius of
maximum of wind for both cyclones. For Glenda, an additional wind plot for RMW= 35km is shown.
The additional plot is included to demonstrate how model settings may be adjusted to change the
wind field with a given storm track.
The model winds (RMW=30km) provide excellent correlation for Clare but appear to overestimate
(by as much as 20 % at peak) for Glenda.
Results from the storm surge model simulations for the two cyclones Clare and Glenda are shown in
Figures 3.10 and 3.11 respectively. For Clare, the model plot is shown against the tide gauge data
measured at Cape Lambert – with close correlation between the model and observations. For
Glenda, in the absence of measured data, plots are shown for water level modeled at Cape Lambert
with and without the cyclone. The results show that the surge residual was of the order of 1.8m – a
little higher than for Clare. The difference in outcome for the two storms relates to the tidal phasing
– both storms occurred near high tide but Glenda occurred close to the peak of a spring tide (not far
below highest astronomical tide).
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Figure 3.12 shows the modelled surge component during Glenda for three locations – Cape Lambert
Jetty, John Creek and Cossack – this show the highest surge component would be expected at
Cossack (0.5 m higher than at the Jetty) – this outcome is expected as a result of the general
topography of the area and the fact that the jetty site is some distance offshore. The overall peak
water level model prediction at Cossack Bond Store was predicted to be 8.3m (Chart Datum) or 5.1m
AHD– close to the observed level. However, the wind validation suggests that the model winds were
over-predicting, so that we might expect a lower water level model prediction for the event at
Cossack for an adjusted wind field; this would then allow for some wave run-up contribution to the
assessed level of 5.0m at the Bond Store. The predicted value for Cape Lambert is also close to the
assessed value at Pt Sampson.
Roebourne - Clare
0
5
10
15
20
25
30
35
1/7/06 0:00 1/8/06 0:00 1/9/06 0:00 1/10/06 0:00 1/11/06 0:00
Wins Speed (m/s)
Obs
Model
Figure 3.8: Observed and modelled winds at Roebourne – Cyclone Clare.
Roebourne - Glenda
0.00
5.00
10.00
15.00
20.00
25.00
30.00
35.00
3/29/06 0:00 3/30/06 0:00 3/31/06 0:00 4/1/06 0:00 4/2/06 0:00
Wins Speed (m/s)
Obs
RMW35
RMW30
Figure 3.9: Observed winds at Roebourne during Cyclone Glenda and modeled winds for the
event with radius of maximum winds set to 30 and 35 km.
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Page | 27
CL-Jetty - TC Glenda
0
1
2
3
4
5
6
7
8
29-Mar 30-Mar 31-Mar
SWL (m - Chart D
atum)
Figure 3.10: Observed (blue) and modeled (red) water level at Cape Lambert Jetty during
Cyclone Clare
0
100
200
300
400
500
600
700
9-Jan-2006 9-Jan-2006 10-Jan-2006 10-Jan-2006 11-Jan-2006
SWL (mm - C
hart D
atum)
Figure 3.11: Modeled water level for Cyclone Glenda (green) compared with tidal prediction at
Cape Lambert (blue).
-0.5
0
0.5
1
1.5
2
2.5
1/9/06 0:00 1/9/06 6:00 1/9/06 12:00 1/9/06 18:00 1/10/06 0:00
Height (m
)
Cape LambertJohns CreekCossack
Figure 3.12: Comparison of modeled peak water level at three locations during Glenda.
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Page | 28
3.1.5 STORM SURGE RETURN PERIODS AT THE CAUSEWAY
The methodology discussed earlier was used to simulate storm-tide events at Cape Preston for a
period equivalent to 1000 years. For each of these events, time series data were extracted at the
services corridor causeway location and subjected to analysis to determine the return periods of sea
levels.
The storm surge model also allowed determination of depth-averaged currents for the each of
simulated events. Since tidal forcing is included in the model simulations, the currents include
contributions from both winds and tides.
The 1 in 100 year event at the causeway, including wave setup but excluding wave runup, was
determined to be 4.9 metres.
3.2 EFFECTS OF THE TEMPORARY CAUSEWAY WITH CULVERTS
The temporary causeway is to be built to a level of 4.5 metres AHD, which is 0.4 metre below the 1 in
100 year level without the causeway in place.
To investigate the impact of installing the causeway with culverts in the southern section having a
total internal cross-sectional area of 250 m2, the storm surge model was rerun on Domain 3 with an
altered fine grid to include the causeway and its culverts.
The major findings of the analysis of the results were:
• The cross-sectional area of the culverts is insufficient to handle the strong flows during the
larger tropical cyclone events;
• The storm surges with higher return periods than 100 years pass over the top of the
causeway near the peak of the event;
• The causeway restricts the flows for all events, during the build up and draining of the storm
surge. As the levels increase above 4.5 metres, there is overtopping and the restriction
reduces.
• The 1 in 100 year storm surge level is unchanged, however the sea level associated with
return periods shorter than 100 years are increased due to the restriction offered by the
causeway.
• During cyclone events the storm surge usually builds up from the eastern side of the
causeway and so for the bigger events, which overtop the causeway, there is a cascading
flow producing stronger currents and potentially more erosion due to the lower sea level on
the western side.
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3.3 EFFECTS OF THE PERMANENT CAUSEWAY WITH CULVERTS
The final design of the permanent causeway involves raising the causeway height to 7 metres. The
final causeway would include the required box culverts discussed in earlier sections to allow
sufficient tidal flows to pass through. The raising of the causeway from a level of 4.5 metres to 7
metres will have no further impact on normal tidal flows but will have an impact on storm surges.
To evaluate this impact a case study was undertaken by selecting an event which produces the 1 in
100 year storm surge level of 4.9 metres from the original simulations of 1000 years of storms.
The case was rerun with two causeway configurations:
• a 7 metre causeway with no culverts; and
• a 7 metre causeway with box culverts with total internal cross-section of 250m2.
For each of these cases the maximum storm surge levels at the causeway were stored
The results are displayed in Figure 3.13 and showed that:
• under existing conditions the 1 in 100 year storm surge is 4.9 metres;
• with a 7 metre causeway and no culverts this level increases to 6.0 metres; and
• with 250 m2 of box culverts in place allowing some flow through the causeway the 1 in 100
year level is reduced to 5.6 metres, some 0.7 metres above the existing levels.
(Addendum: Note that modeling of the current causeway culvert design of 109 m2 has not been
undertaken, but it is clear that it will increase the restriction on water flows and will probably result
in a storm surge level of about 5.8m resulting in an increase of up to 1m above existing levels.)
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Page | 30
Figure 3.13: Results for the 1 in 100 year storm surge at the southern creek crossing for the
three cases studied.
3.4 EFFECTS OF SEA LEVEL RISE ON THE 1 IN 100 YEAR STORM SURGE
The effect of a 0.5 metre sea level rise needs only to be assessed against the permanent causeway
option since the sea level will not rise significantly before it is built. If the permanent causeway, with
culverts, is built to a level of 7 metres above existing MSL then the impact on the 1 in 100 year storm
surge of 5.6m will be to raise it by 0.5m to 6.1m. The causeway will still act as a barrier to storm
surge and water levels will be 1.2 metres above the current 1 in 100 year storm surge level of 4.9m
AHD.
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4 SUMMARY OF FINDINGS
4.1 VERIFICATION OF TIDAL/STORM SURGE MODEL
• The agreement of the model predictions with the tidal data measured off Preston Island was
excellent.
4.2 EXISTING TIDAL CONDITIONS
• The length of causeway exposed to water at HAT is approximately 1900m.
• The flood tide approaches the region of the proposed causeway from the west and even at
the highest spring tides does not connect directly to the ocean on the eastern side of the
proposed causeway alignment.
• At HAT (2.35m AHD) the cross-sectional area of water along the causeway alignment is
approximately 2400 m2.
• Flow speeds along the major creek bed can approach 1 knot during spring tides.
• At MHWS (1.75m AHD) the cross-sectional area of water along the causeway alignment is
approximately 1260m2 (note that the exact cross-section is very dependent on the accuracy
of the topography and bathymetry).
4.3 AREA OF CULVERTS REQUIRED TO MINIMISE INTERFERENCE WITH TIDAL FLOWS
• The minimum internal total cross-sectional area of the box culverts which would allow tidal
levels of MHWS to be attained 80% of the “normal” time was 250 m2, made up of a 190m
2
culvert in the major creek crossing, a 40m2 culvert south of the major creek crossing and a 20
m2 culvert north of the major creek crossing;
• This reduced cross-sectional areas across the creek crossings will create a head on the
western side during the spring high tides and generate faster flow speeds through the
culverts. This will potentially restrict the total mass of water passing through.
• The currents through the culverts at high spring tidal flows can reach 2 m/sec and could
potentially cause some local scouring without appropriate mitigation measures.
4.4 IMPACTS OF THE TEMPORARY CAUSEWAY WITH CULVERTS ON TIDAL FLOWS
With the proposed culverts (109 m2 ) in place:
• The causeway will restrict water flow to the east, but will not cause ponding upstream of the
causeway;
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Page | 32
• Maximum water levels upstream of the causeway will be reduced to RL 1.5 m AHD (from
2.35 m AHD at HAT);
• The period of wetting at RL 1.3 m AHD during spring tides will be reduced by approximately
50%;
• There will be no effect on neap tides below about RL 1.0 m AHD.
4.5 THE IMPACTS OF THE PERMANENT CAUSEWAY WITH CULVERTS ON TIDAL FLOWS
• There are no extra impacts of the permanent causeway on tidal flows.
4.6 THE IMPACTS OF A SEA LEVEL RISE OF 0.5 METRE ON THE TIDAL FLOW RESULTS
• The cross section of water at HAT along the alignment of the causeway is increased by
approximately 1,000 m2
to 3400m2;
• The cross section of water at MHWS along the alignment of the causeway is increased by
approximately 840 m2
to 2100m2;
• The minimum total internal cross-sections of box culverts required to allow tidal levels of
MHWS to be attained 80% of the time is increased to approximately 400 m2;
4.7 THE EXISTING 1 IN 100 YEAR STORM SURGE EVENT AT THE CAUSEWAY
• The 1 in 100 year event at the causeway, including wave setup but excluding wave run-up,
was determined to be 4.9 metres.
4.8 EFFECTS OF THE TEMPORARY CAUSEWAY WITH CULVERTS ON STORM SURGES
• The cross-sectional area of the culverts is insufficient to handle the strong flows during the
larger tropical cyclone events;
• The storm surges with higher return periods than 100 years pass over the top of the
causeway near the peak of the event;
• The causeway restricts the flows for all events, during the build up and draining of the storm
surge. As the levels increase above 4.5 metres there is overtopping and the restriction
reduces.
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Page | 33
• The 1 in 100 year storm surge level is unchanged, however the sea level associated with
return periods shorter than 100 years are increased due to the restriction offered by the
causeway.
• During cyclone events the storm surge usually builds up from the eastern side of the
causeway and so for the bigger events, which overtop the causeway, there is a cascading
flow producing stronger currents and potentially more erosion due to the lower sea level on
the western side.
4.9 EFFECTS OF THE PERMANENT CAUSEWAY WITH CULVERTS ON STORM SURGES
• under existing conditions the 1 in 100 year storm surge is 4.9 metres.
• with a 7 metre causeway and no culverts this level increases to 6.0 metres.
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5 REFERENCES
Booij, N., Ris, R.C. and L.H. Holthuijsen, 1999: A third generation model for coastal regions. Part I:
Model description and validation, J. Geoph. Research, Vol. 104, No. C4, 7649-7666.
GEMS, 2007. West Pilbara Cyclonic Storm Surge Inundation Modelling Study Report. In preparation
for Shire of Roebourne.
Hubbert, G.D., L.M.Leslie and M.J. Manton, 1991. A storm surge model for the Australian region.
Quart.J. R.Met Soc., 116,1005-1020.
Hubbert, G. D., and K.L. McInnes, 1999. A storm surge inundation model for coastal planning and
impact studies. Journal of Coastal Research, 15 (1): 168-185.
Holland, G.J, 1980. An analytical model of the wind and pressure profile in hurricanes.
Mon.Wea.Rev.,108, 1212-1218.
Ris, R.C., N. Booij and L.H. Holthuijsen, 1999: A third-generation wave model for coastal regions. Part
II: Verification, J. Geoph. Research, Vol. 104, No. C4, 7667-7682.
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