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61/23317/96676 CKI Home Island Slipway RedevelopmentBaseline Marine Ecological Studies

Appendix F

Particle Size Analysis of Marine WaterSamples – Analytical Report

61/23317/95769 Cocos (Keeling) Islands - Home Island SlipwayEnvironmental Management Plan

Appendix C

Hydrodynamic and Sediment TransportModelling

Attorney General of theCommonwealth of

Australia

Report for Cocos (Keeling)Islands Home Island Slipway

Hydrodynamic and DredgePlume Modelling

March 2010

61/23317/05/95127 Cocos (Keeling) Islands Home Island SlipwayHydrodynamic and Dredge Plume Modelling

Contents

Executive Summary 4

1. Introduction 5

1.1 Outline of the Modelling Study 5

2. Study Area 6

3. Hydrodynamic Modelling 11

3.1 Model Selection 11

3.2 Model Set Up 11

3.3 Hydrodynamic Calibration 15

3.4 Hydrodynamic Results 16

4. Dredge Plume Modelling 19

4.1 Dredge Type 19

4.2 Dredge Layout and Dredge Volume 20

4.3 Model Set Up 20

4.4 Predicted Dredge Plume TSS Concentrations 22

5. Limits of Report 26

6. References 27

Table IndexTable 1 Tide levels 7Table 2 Spatial resolution of flexible mesh grid 13Table 3 Particle classification 20Table 4 Typical particle size, settling velocity and

percentage composition per classification 21Table 5 Dredge plume material source flux into the water

column 22Table 6 Characteristic particle size per fraction using the

weighted mean method 42

Figure IndexFigure 1 South Keeling Island. 6


Figure 2 Bathymetry regions (adapted from Kench 1994). 7Figure 3 Wind roses for November (left) and December

(right) at Cocos Airport using data from the years19962009. 9

Figure 4 As per Figure 4 for January and February 9Figure 5 Circulation model of the Lagoon (source: Kench

1994) 10Figure 6 Flexible mesh of the Cocos (Keeling) Island model.

A) Complete model domain; BD) Details of flexiblemesh in zones ivi. 12

Figure 7 Model bathymetry referred to mean sea level. 13Figure 8 Detail of southern atoll model bathymetry referred

to mean sea level. 14Figure 9 Comparison between simulated (red) and predicted

(green) tidal levels at Port Refuge 16Figure 10 Simulated depth averaged velocities 6hr apart

during neap tides on February 8, 2010, for asimulation forced with tides and wind. 17

Figure 11 As per Figure 10 for spring tides during March 02,2010. 18

Figure 12 Proposed dredge ‘Gen 4’ 19Figure 13 Aggregate spatial distribution map of TSS

concentration in the absence of wind. 23Figure 14 As per Figure 14 for the simulation including wind. 24Figure 15 Study area 25

AppendicesA Monthly Wind Pattern: 1996 – 2009 Aggregated Data at Cocos

AirportB Wind Pattern for January (1996 – 2009) at Cocos AirportC Wind Pattern for February (1996 – 2009) at Cocos AirportD Sediment Sampling LocationsE Characteristic Particle Size per Sediment Fraction – Example

Calculation


Executive Summary

GHD Pty Ltd (GHD) have developed and calibrated a hydrodynamic model of the Cocos (Keeling)Islands. The simulated water currents and levels from this hydrodynamic model served as inputs for adredge plume model to predict the spatial and temporal characteristics of a turbidity plume fromproposed dredging activities near the Home Island jetty. These model results are used in a companiontechnical report to assess potential impacts to benthic primary producer habitat, specifically hard corals(GHD 2010).

The model domain encompassed a large area of the Indian Ocean under the influence of tides andwinds. The model domain was established with four open boundaries surrounding the Cocos (Keeling)Islands across latitudes of 11° to 13°30 S (distance of 275 km) and longitudes of 95°30’ to 98°30 E(distance of 320 km), a total surface area of 88,000 km2. Circulation patterns in the lagoon of thesouthern atoll (‘the lagoon’) of the Cocos (Keeling) Islands are driven primarily by tides, whereas windhas less influence. Field observations indicate minimal spatial variations (vertical and horizontal) insalinity and temperature within the lagoon, and so they were not modelled because of their limitedinfluence on circulation patterns. Because the lagoon is shallow and not vertically stratified, a verticallyaveraged twodimensional (2D) hydrodynamic model was used to simulate circulation patterns.Hydrodynamic modelling was undertaken with DHI’s MIKE 21 HD (flexible mesh) numerical model tosimulate current speeds, current directions and water levels for subsequent dredge plume modelling.Model inputs were comprised of halfhourly predicted (astronomical) tides at the open boundaries andhourly winds from Cocos (Keeling) Island Airport station for the simulation with wind effects.

The hydrodynamic model without wind was calibrated against predicted astronomical tides at PortRefuge over 44 days by adjusting the bed friction coefficient. The calibration runs were only forced withtidal elevations at the open boundaries (i.e. no wind). A Manning’s number of 28 m1/3/s captured thepredicted tidal variations well. A quantitative measure of the model accuracy is the ratio of the rootmeansquareerror (RMSE) to maximum predicted tidal amplitude (Amax), which indicated accuracy withinindustry standard values (0.13). Hence, the model accurately captured the expected water levelvariations in the lagoon under the influence of no wind.

For the purposes of dredge plume predictions, simulations of 44 day duration with the 2010 predictedtide and winds from FebruaryMarch 2009 were run. The simulated circulation patterns were inreasonable agreement with other studies and published observations. Simulated ebb and flood tidalcurrents ranged from 0.150.30 m/s across the lagoon during spring tides. The current directions werealso captured well by the model, especially along the eastern side of Home Island where predominantlynorthwest and southeast current directions were in agreement with field observations.

The hydrodynamic model was coupled to the sediment transport model to simulate the fate and transportof the dredge plume from the proposed deepening. Maps of the spatial distribution maps of totalsuspended solid concentrations (TSS) for simulations with and without wind forcing are provided for the 1percent probability exceedance for a range of TSS concentration bands. Application of the 1 percentprobability exceedance to define the areal extents of dredge plume impacts was considered an adequatemeasure to compensate for not modelling the spatial locations of the Stage 2 ‘high spot’ dredging areasas the final dredge plan was not available during the numerical modelling study. These maps of TSSconcentrations were used to evaluate the areal extent of potential impacts from the dredge plume onhard corals in the companion coral report (GHD 2010).


1. Introduction

The proposed dredging program in the Cocos (Kelling) Islands will be undertaken in two stages. Stage 1will occur near Home Island jetty and will remove approximately 11,600 m3 of sediment that willpotentially impact the benthic community (particularly scleractinian “hard” corals) within the southern atoll(hereafter referred to as ‘the lagoon’). Stage 2 will involve dredging a smaller volume (7,100 m3) over alarger and elongated area (i.e. channel) that will primarily consist of removal of ‘high’ spots. Thesediment plume from Stage 2 is likely to be smaller than from Stage 1 because it is comprised of several‘high spots’ along a long ‘approach’ channel.

The dredging layout for Stages 1 and 2 were not available at the time of numerical modelling, only theapproximate location and total volume to be dredged. Hence, GHD undertook plume modelling byassuming dredging works would be concentrated in the vicinity of Home Island jetty. This plumemodelling guided the selection of coral monitoring sites in the lagoon for a field study and subsequentimpact assessment as summarised in a companion coral technical study (GHD 2010).

This report is structured into three sections. The first reviews morphology and hydrodynamic literature forthe lagoon. The second summarises the development and calibration of a hydrodynamic model of thelagoon. The third describes the likely extent and depthaveraged concentrations of a simulated dredgeplume from the proposed dredging program assuming these dredging activities are concentrated nearthe Home Island jetty.

1.1 Outline of the Modelling StudyThis numerical modelling study consisted of:

» Review of hydrodynamic literature;

» Selection of an appropriate numerical modelling approach;

» Establishment of the hydrodynamic model;

» Calibration of the hydrodynamic model;

» Simulation of hydrodynamic scenarios;

» Establishment of the plume model; and

» Interpretation of the plume model results.

The hydrodynamic model outputs (i.e. water currents and levels) served as inputs to the plume model.The plume simulations were used to predict the effect of the proposed dredging program on totalsuspended solids (TSS) concentrations in the water column that have potential to impact on the lagoon’shard coral population, which is assessed in a companion technical report (GHD 2010).


2. Study Area

Australia’s Cocos (Keeling) Islands Territory is located 2770 km northwest of Perth in the Indian Ocean(96°50'E and 12°10'S). It comprises 27 coral islands1 in the form of two large atolls: the North KeelingIsland (not shown here) and the South Keeling Island (Figure 1). The proposed dredging site is west ofHome Island; one of the most populated islands in the southern atoll.

The morphology of the southern atoll (i.e. shape and geographical distribution of islets) controls thecirculation in the lagoon, which is dominated by tides (Kench 1998). Tidal currents of 0.160.31 m/s in thelagoon and 0.260.65 m/s in the deep passages have been previously measured (Kench 1998). Theflushing time of the lagoon is estimated at 24 days, hence the numerical model developed here wasconsidered “spun up” after 4 days of simulation. Gravity waves are dissipated by the surrounding reefs,especially in the higher southern reef where only water depths >0.7 m result in surface waves beingtransmitted into the lagoon. Anecdotal evidence suggests that wind waves are typically small at thewestern side of Home Island, near the jetty.

To capture the hydrodynamic complexity of the system and minimize the errors induced by openboundaries, a large model domain was selected. This domain covered both atolls as it extended over alatitude range of 11° to 13°30 S (distance of 275 km) and a longitude range of 95°30’ to 98°30 E(distance of 320 km).

Figure 1 South Keeling Island.

1 Australian Government, Geoscience Australia (http://www.ga.gov.au/education/geosciencebasics/dimensions/externalterritories/cocoskeelingislands.jsp). [As on February 2010].

Home IslandProposedDredging Site

http://www.ga.gov.au/education/geoscience-basics/dimensions/external-


2.1.1 Bathymetry

Like most atolls, the Cocos (Keeling) Islands enclose a lagoon, which is surrounded by deep seas. Thebathymetry of the southern atoll can be broadly delineated into a large, shallow (<3 m) lagoon in thesouth and a deeper (1012 m) lagoon in the north (Figure 2). At the proposed dredging site on thewestern side of Home Island, the water depth is approximately 0.5 m Chart Datum (1.2 m below MSL).

Figure 2 Bathymetry regions (adapted from Kench 1994).

2.1.2 Tide Regime and Tide Levels

The Cocos (Keeling) Islands have mixed semidiurnal tides (i.e. two high and two low tides per day) asidentified by the amplitude ratio F = 0.56 ( 2211 SMOKF ++= ). The tides are dominated by the

principal lunar harmonic constituent M2. The mean tidal range is approximately 0.5 m at neap tide and0.7 m at spring tide, with a maximum tidal range2 of approximately 1.2 m. Comparison of tide gauge datafrom the shallow and deep lagoons suggested that tides in the shallow lagoon are attenuated (Kench1994). Also, tides in the shallow lagoon lag those of the deep lagoon independent of the tidal regime(neap or spring tides). Tide levels at Home Island are shown in Table 1.

Table 1 Tide levels

Tidal Plane Level above Chart Datum (CD) Level above CKI Height Datum(CKI HD)

Highest Astronomical Tide (HAT) +1.5m +0.85m

Mean Higher High Water +1.2m +0.55m2 Tidal ranges of 1.5 m may be present during highest astronomical tides.

Deep lagoon

Shallow lagoon


Tidal Plane Level above Chart Datum (CD) Level above CKI Height Datum(CKI HD)

(MHHW)

Mean Lower High Water(MLHW)

+0.7m +0.05m

Mean Sea Level +0.7m +0.0m

Mean Higher Low Water +0.6m 0.05m

Mean Lower Low Water +0.1m 0.55m

Lowest Astronomical Tide (LAT) +0.0m 0.65m

Chart Datum (CD) +0.0m 0.65m

2.1.3 Precipitation and Air Temperature Regime

The annual rainfall varies between 850 and 3300 mm and the annual potential evaporation isapproximately 2000 mm. Air temperatures are relatively uniform during the day, ranging from 18 to 32°C.

2.1.4 Wind Regime

Analysis of historical data indicates that the winds during the morning and afternoon are predominantlyfrom the southeast for most of the year with mean daily averages of 58 m/s. Strong southeasterly tradewinds persist for more than 85% of the year, while less than 10% of winds originate from the north andnortheast (Cocos Meteorological Bureau; GHD 2000).

Wind intensity varies seasonally. During winter (from April to October), the wind is characteristicallystrong and from the east to southeast. During summer (from November to March), the wind speedsbecome lighter and wind directions tend more easterly and northerly. However, summer is also thecyclone season hence the highest wind events have been recorded during this period.

As the dredging program is expected to be conducted in summer (i.e. February 2010 or 2011), furtheranalysis of the summer months was undertaken. Wind data from the Bureau of Meteorology stationnumber 200284 for the period 19962009 was analysed on a monthly basis. The analysis confirmed thatsoutheasterly winds are dominant during the summer months (Figure 3 and Figure 4) with January andFebruary having the lowest wind intensities. Wind roses for other months were also prepared and arepresented in Appendix A to Appendix C.


Figure 3 Wind roses for November (left) and December (right) at Cocos Airport using datafrom the years 19962009.

Figure 4 As per Figure 4 for January and February

2.1.5 Wave Regime

Waves in the lagoon are generated by wind3 and swells4. Wind waves are larger on the western side ofthe lagoon than near the jetty, which is sheltered from the predominant southeasterly winds by HomeIsland. Swells reaching the islands are generated from low pressure systems in the southern IndianOcean and enter the lagoon mainly from two gaps, the one existing between Direction Island andHorsburgh Island (northeast of the lagoon), and that existing between South Island and West Island(south of the lagoon).

2.1.6 Circulation Patterns in the Lagoon

The following circulation patterns have been noted by Kench (1994, 1998):3 Wind waves are waves generated by local winds that are still under the influence of local winds.4 Swells are waves that have moved out of their area of generation. They are also usually generated by wind.


» Currents are tidally modulated and dominated by the M2 tide. A westward net flow has beenobserved during neap tides. During spring tides the ebbing pattern is slightly altered to a northwestnet flow which is reversed to southeastward during rising spring tides that penetrate 1 km into thelagoon.

» Tidal elevation plays a significant role on the flushing mechanism of the lagoon. A greater nettransport of water masses is observed during neap times than during spring tides.

» Only up to 15% of the currents in the lagoon can be explained by the influence of the wind. Thesoutheast trade winds play a major role in driving these currents.

» The unidirectional ocean to lagoon flow observed in the southern passage is explained by themorphology of the atoll and the elevation of the southern reef crest. However, the influence of thisflow on the currents on the eastern side of the lagoon is minimal.

» Tidal currents of 0.160.31 m/s in the lagoon and 0.260.65 m/s in the deep passages have beenmeasured.

A general circulation model of the lagoon is shown in Figure 5.

Figure 5 Circulation model of the Lagoon (source: Kench 1994)

2.1.7 Water Density Structure

Water temperature profiles collected as part of GHD’s coral monitoring program (GHD 2010) indicated avertically well mixed and horizontally homogeneous lagoon. Typical water temperatures fluctuatedslightly around 27°C.


3. Hydrodynamic Modelling

3.1 Model SelectionBecause of the shallow depth of the lagoon the water column is vertically wellmixed and currents aredominated by tides and wind. There is no evidence that threedimensional processes play a role indriving circulation processes in the lagoon. Hence, a twodimensional (2D) (vertically averaged)hydrodynamic model driven by tides and wind is appropriate for modelling the water currents andelevation in the lagoon.

Hydrodynamic modelling was undertaken with DHI’s MIKE 21 HD (flexible mesh) numerical model toprovide the current speeds, current directions and water levels for the dredge plume modelling. Thismodel readily simulates sea levels and currents in response to tides and wind. The numerical solution isbased on the 2D incompressible Reynolds averaged NavierStokes equations, assuming densityvariations are only important in the gravity term (Boussinesq approximation) and vertical accelerationsare negligible (hydrostatic pressure assumption).

3.2 Model Set Up

3.2.1 Model Domain

To minimize the errors induced by open boundaries, a large model domain was selected. This domaincovered both atolls as it extended over latitudes of 11° to 13°30 S (distance of 275 km) and longitudes of95°30’ to 98°30 E (distance of 320 km), totalling a surface area of 88,000 km2.

3.2.2 Vertical and Horizontal References

The mean sea level (MSL) at Port Refuge (Home Island) was the adopted vertical datum. The horizontaldatum was the Geocentric Datum of Australia 1994 (GDA94), Zone 47.

3.2.3 Computational Grid

The hydrodynamic model used a flexible mesh grid with triangular elements of varying sizes. Elementsizes ranged from 0.02 to 30 km2 depending on the zone (i.e. 6 zones named ivi in Figure 6). Each zoneenclosed regions of similar depths. Higher spatial resolution (i.e. smaller element area) was utilised in thevicinity of the jetty (zone i). The spatial resolution of the model was decreased stepwise from zone i. Acoarser resolution was assigned to the outermodel region (zone vi), which covers areas of less interest(Figure 6 and Table 2). The model grid has 32525 elements and 16518 nodes.

To avoid numerical stability issues arising from small grid cells and to increase the real to run time ratio(i.e. increase the numerical time step without compromising the CFL condition), the jetty near HomeIsland was excluded from the model domain. As the jetty itself is relatively short (approximately 200 mlong), its effect on the overall circulation in the lagoon is low. Simulations including and excluding thejetty yielded similar areal sediment plume extents for TSS concentrations <5 mg/L (i.e. the zone ofinterest for coral monitoring). However, simulations undertaken with the model grid that excluded the jettywere faster thereby allowing for multiple tests to be conducted in a shorter period of time withoutcompromising the validity of the results.


Figure 6 Flexible mesh of the Cocos (Keeling) Island model. A) Complete model domain; BD)Details of flexible mesh in zones ivi.

A

B

B

C

C

D

D

vi

v

iv

iii

ii

i


Table 2 Spatial resolution of flexible mesh grid

Zone i ii iii iv v vi

Maximum element surface area [km2] 0.020 0.125 0.5 1 5 30

3.2.4 Model Bathymetry

Bathymetry data was sourced from the software package Mike CMap (DHI’s digital Global ocean depthrepository) and used to construct the horizontal flexible mesh representation of the Cocos (Keeling)Island (Figure 7 and Figure 8). The bathymetry was refined using data published by the AustralianHydrographic Service, chart AUS 607.

Figure 7 Model bathymetry referred to mean sea level.


Figure 8 Detail of southern atoll model bathymetry referred to mean sea level.

3.2.5 Model Initialization and Time Step Selection

The model was initialised with zero currents and the water level over the entire model domain at meansea level (i.e. ‘cold’ start). A time step of 60 seconds was implemented, which is appropriate to simulatethe hydrodynamic circulation patterns in the lagoon with the adopted mesh. Because the flushing time ofthe lagoon is 24 days, the initial 4 days of the simulation was considered sufficient for the circulationfrom tidal and wind forcing to become established in the lagoon from the ‘cold’ start, after whichsimulated results were used for subsequent dredge plume modelling.

3.2.6 Boundary Conditions

Tidal ForcingAstronomical tide elevations at halfhourly intervals were forced at the open boundaries. The time seriesof tidal elevations were generated with a global tidal model with a resolution of 0.25° by 0.25° that formspart of the DHI’s Mike Zero Toolbox. Four semidiurnal (M2, S2, K2, and N2) and four diurnal (K1, O1, P1,and Q1) harmonic constituents were used.

Wind ForcingFor the scenario driven by wind (Section 3.2.9), halfhourly wind data from the Cocos (Keeling) IslandAirport5 was applied uniformly over the surface area of the model domain. Wind data from February andMarch 2009 were directly applied to the same period during 2010.

5 Bureau of Meteorology station number 200284


Wind Waves and Swell ForcingBecause wind wave heights are typically low near the jetty their effect on the sediment plume transport issmall compared to the effects of tides and wind and therefore were not considered.

Swells were not considered in the simulations as the dredging program will be conducted during the calmsummer months when swells are typically low. In addition, swells from outside of the model domaincannot be accounted for here without further ocean scale modelling inputs, which exceeds the currentscope.

Coriolis ForcingThe effect of the earth’s rotation on currents (i.e. Coriolis force) was simulated in the model.

3.2.7 Modelling Period

The modelling period was started on February 01, 2010 and it spaned 44 days (including 4 days for spinup) over the months of February and March 2010. February is typically the calmest month of the year,with wind intensities seldom exceeding 10 m/s. Lagoon circulation patterns are dominated by tides duringlow wind periods, therefore this period represents the best conditions for model calibration with predictedwater levels. February is also the proposed dredging month.

3.2.8 Parameterization of Horizontal Dispersion Processes

The selected horizontal eddy viscosity was a Smagorinsky type with a constant Smagorinsky coefficientof 1.

3.2.9 Scenario List

Once calibrated, two hydrodynamic scenarios were run with the numerical model, namely:

» The model forced by tides at the open boundaries; and

» The model forced by tides at the open boundaries and winds as described in section 3.2.6.

3.3 Hydrodynamic CalibrationThe hydrodynamic model was calibrated with predicted astronomical tides at Port Refuge (Home Island)over the modelling period. This is the preferred calibration parameter in the absence of current speedand current direction measurements. Calibration consisted of adjusting the bed friction coefficient in thestandard quadratic friction law of the hydrodynamic model within practical ranges. Model runs weretested with the Manning’s number (M) in the range of 1550 m1/3/s. The calibration runs were only forcedwith tidal elevations at the open boundaries (i.e. no wind or atmospheric pressure applied over the watersurface).

Comparisons of the simulated and predicted water levels are shown in the left panel of Figure 9 for M =28 m1/3/s. Qualitatively, the semidiurnal tidal signal at Port Refuge station is well captured by the model,which only differs slightly from predicted neap tides.

To further investigate the accuracy of the model, the correlation between simulated and predicted (orastronomical) water levels at Port Refuge is shown in the right panel of Figure 9. Red points correspondto halfhourly outputs during the simulation period that correspond to the halfhourly astronomical tidalrecord. No correction for phase lags was applied in Figure 9, hence this correlation reflects the combined


tidal phase and amplitude accuracy of the model. This is a more rigorous comparison than the standardindependent correlations of ‘amplitude’ or ‘phase’. The correlation plot shows that the simulated waterlevels match fairly well with those predicted. For M = 28 m1/3/s, the results yield a rootmeansquareerror(RMSE) of 0.07 m and a RMSE to maximum predicted amplitude (0.54 m) ratio of 0.13; both valueswithin industry standard accepted ranges for twodimensional models forced by predicted tides only. Thediscrepancies between simulated and predicted water levels observed during neap tides are mainlyattributed to the effect of residual bottom friction forces on the mean currents. Bottom friction is inverselyproportional to the water depth (i.e. the shallower the region the higher the effect of bottom friction on themean volume flux) and directly proportional to the depth average velocity squared, hence shallow areasare prone to errors especially during low flow conditions (e.g. during neap tides). However, as mentionedabove, the errors encountered in the hydrodynamic simulations are within practical ranges and do nothave a substantial impact on the processes investigated in this project (i.e. areal extent of sedimentplume generated by dredging).

Figure 9 Comparison between simulated (red) and predicted (green) tidal levels at Port Refuge

3.4 Hydrodynamic ResultsThe calibrated model was run for 44 days (including 4 days to spin up) with tide and wind (whenapplicable) forcing as described in Section 3.2.9.

There was reasonable agreement between the simulated depth averaged currents (Figure 10 and Figure11) and Kench’s (1994) circulation model (Figure 5). Simulated ebb and flood tide currents ranged from0.150.30 m/s across the lagoon during spring tides. The current direction is also well captured by themodel especially on the eastern side of Home Island with simulated currents predominantly in the northwest and southeast direction in agreement with field observations. In addition, the unidirectional oceanto lagoon flow in the southern passage reported by Kench (1944) is replicated well by the model (Figure10).


Figure 10 Simulated depth averaged velocities 6hr apart during neap tides on February 8, 2010,for a simulation forced with tides and wind.


Figure 11 As per Figure 10 for spring tides during March 02, 2010.


4. Dredge Plume Modelling

Dredge plume modelling was undertaken with DHI’s Mike 21 MT (Mud Transport) model. The model canbe readily coupled to DHI’s Mike 21 HD (flexible mesh) hydrodynamic model described in the precedingsection of this report. Mike 21 MT allows for modelling of erosion, transport and deposition of mud orsand/mud mixtures under the action of currents and waves, although transport and deposition are theonly two processes investigated in this study. Because Mike 21 MT can simulate up to eight fractions ofsand/mud mixtures, it is widely considered appropriate for simulations of dredge plumes.

Basic model parameters as well as initial concentrations of sediment fractions and settling velocitieshave been derived from sediment data collated during the coral monitoring campaign or estimated on thebasis of GHD’s experience of similar dredging projects. Data regarding the dredging schedule anddredging layout were either scarce or unavailable at the time the modelling works were undertaken. Themodel set up and assumptions made for the simulations are described next.

4.1 Dredge TypeA floating dredge with a bucket wheel cutter has been proposed for the dredging program. The proposeddredge is ‘Gen 4’ (Figure 12), a Neumann built 300 mm Heavy Duty Dredge (20 m long and 6.9 m widewith a 465 KW main engine) with a free flowing maximum sediment dredging capacity of 80 L/s at 30%(i.e. 30% of the 80 L/s flux is sediment). Here, a conservative and precautionary approach wasconsidered where the dredging capacity of the dredge was assumed to be 15%. Hence, the simulateddredging program doubles the period of time at its maximum capacity of 30%. For a dredging capacity of15%, the sediment content of the waterdredged material slurry is 12 L/s. To minimise the amount of siltescaping to form the dredge sediment plume, the cutting head of the dredge will be covered with a siltshroud.

Figure 12 Proposed dredge ‘Gen 4’


4.2 Dredge Layout and Dredge VolumeThe dredge layout and volume were unknown at the time the modelling works were undertaken. Theinformation made available to the modelling team was:

» The approximate dredging site (in the vicinity of Home Island jetty); and

» The approximate dredging volume (20,000 m3).

4.3 Model Set Up

4.3.1 Dredging Schedule

The dredge was assumed to operate continuously (e.g. no maintenance stoppages) during the dredgingprogram. This assumption is considered conservative as higher TSS concentrations will occur relative tonondredging periods (e.g. maintenance, refueling). A continuous dredge program also assumes goodweather conditions, which is likely during the summer months.

The simulated duration of the dredging program was 20 days, which is based on the total dredgingvolume of 20,000 m3 and a dredge sediment volume flux of 12 L/s (see Section 4.1). The simulateddredging volume is slightly larger than the recent estimates of the combined dredging volumes of Stages1 and 2 (18,700 m3), hence the modelling is considered conservative.

4.3.2 Sediment Fractions

Four particle sizes (i.e. clay, silt, fine/medium sand and coarse sand) were simulated. Particle sizedistribution curves from six sediment samples near Home Island jetty during the second coral monitoringcampaign (GHD 2010) were used to develop the characteristic percentage fractions of each particle sizeclass for dredge plume modelling (i.e. labelled 13, 14, 15, 16, 17 and 18 in Appendix D). Further detailson the sediment sampling sites can be obtained from GHD (2010).

Particle sizes were classified into ‘larger than coarse sand’, ‘coarse sand’, ‘fine and medium sand’, ‘silt’and ‘clay’ (Table 3). Particles in the classification ‘larger than coarse sand’ were not simulated as theseparticles are too heavy to be transported any substantial distance by currents. For the remaining fourfractions a weighted average method was used to determine the characteristic particle size for eachclassification (i.e. coarse sand, find sand, silt and clay) as illustrated via example in Appendix E. Theweighted average method was applied to the 6 sediment samples and a typical particle size classificationwas estimated as the arithmetic mean of the 6 characteristic particle sizes (Table 4).

Table 3 Particle classification

Particle size (mm) Classification

37.5

19

9.5

4.75

Larger than coarse sand

2.36 Coarse sand


Particle size (mm) Classification

1.18

0.6

0.425

0.3

0.15

0.075

Fine and medium sand

0.057

0.040

0.020

0.010

0.005

0.004

Silt

0.001 (or smaller) Clay

Table 4 Typical particle size, settling velocity and percentage composition per classification

Classification Particle size (mm) Settling velocity (m/s) Composition (%)

Larger than coarse sand 10.87

Coarse sand 1.076 0.993 29.30

Fine and medium sand 0.218 0.041 55.17

Silt 0.040 0.0014 4.33

Clay (or smaller) 0.001 0.000001 0.33

Assuming that particles settle under Stoke’s Law, the settling velocity of a particle is given by Equation 1,where g is the acceleration due to gravity (=9.81 m/s2), is the dynamic viscosity of water (=103 kg/m/s),d is the particle size (m), S is the particle density (2600 kg/m3), and W is the water density (1025 kg/m3).Settling velocities for each classification utilised in the numerical model are shown in Table 4.

Equation 1 )(18

2

WSgdVs ρρ

µ−=

The average composition for each particle size classification across the 6 sediment samples is shown inthe final column of Table 4.


4.3.3 Dredge Plume Source Fluxes

As the dredge material will be pumped to shore, a nooverflow spillage condition was assumed. Hencethe only source of dredge plume material into the water column is from material that escapes at thebucket wheel cutter head and the silt shroud. Unfortunately, no data on the efficiency of the silt shroudwas available. Hence, it was assumed that 20% of the volume of dredged clay, silt, and fine/mediumsand material would escape from both the wheel cutter and silt shroud to be the source of the dredgeplume. This ‘escape’ percentage was reduced to 10% and 5% for ‘coarse sand’ and ‘larger than coarsesand’ fractions, respectively. From the final column of Table 5, the total mass flux of dredge plumematerial into the water column was approximately 3.5 kg/s (note that the ‘larger than coarse sand’fraction was not included in the simulations).

Table 5 Dredge plume material source flux into the water column

Classification Composition(%)

Dredgematerial fluxin pipe (L/s)(total = 12 L/s)

% of dredgematerial intowater column(%)

Dredgematerialvolumesource flux(L/s)

Dredgematerial masssource flux(kg/s)

Larger thancoarse sand

10.87 1.30 5 0.07 0.110

Coarse sand 29.30 3.52 10 0.39 0.625

Fine andMedium Sand

55.17 6.62 20 1.66 2.648

Silt 4.33 0.52 20 0.13 0.208

Clay (orsmaller)

0.33 0.04 20 0.01 0.016

4.4 Predicted Dredge Plume TSS ConcentrationsRecall that the locations of the Stage 2 ‘high spot’ dredging areas were not simulated because the finaldredge plan was not developed prior to this numerical modelling study. Hence, spatial impacts havebeen defined as the area where a particular TSS concentration is exceeded for only 1 percent of thesimulation duration (i.e. the 1 percent probability exceedance area). Normally, the 20 percentileexceedance area (i.e. area in which a TSS concentration is greater than a particular concentration for20% of the simulation) is utilised. Application of the 1 percent probability exceedance to define the arealextents of dredge plume impacts was considered an adequate measure to compensate for not modellingthe spatial locations of the Stage 2 ‘high spot’ dredging areas. However, the total volume dredged(20,000 m3) at the Stage 1 location is an accurate reflection of the final planned ‘actual’ dredge volume of18,700 m3. Further, the 20 day duration of the dredging program was also considered realistic.

While simulations yielded depth averaged hourly snapshots of the dredge plume extent, the spatialdistribution of TSS over the dredging program duration is the key output of interest. Simulated TSS iscomprised of the four modelled particle size fractions. Maps of the spatial distributions of a range of TSSconcentration bands (i.e. 13, 35, 510, 1015, and >15 mg/L) for the dredging program over February2010 without (Figure 13) and with (Figure 14) wind summarise the simulations over the 20day dredgingprogram. As discussed previously these maps are representations of the 1 percent probability


exceedance area, where for only 1% of the time during the dredging program was the TSS ‘less than’the concentration band outside of these areal footprints. Alternatively, the maps indicate the areal extentin which TSS is less than the concentration band for 99 percent of the dredging program duration (i.e.99th percentile frequency). These ninetyninth percentile maps are an aggregate spatial distributioncomposed of hourly ‘instantaneous’ snapshots over the simulated 20day dredging program. This is notto be confused with ‘instantaneous’ spatial distributions of TSS, which would comprise a substantiallysmaller areal extent than the ‘aggregate’ areas shown in these maps at any given instance.

Figure 13 Aggregate spatial distribution map of TSS concentration in the absence of wind.


Figure 14 As per Figure 14 for the simulation including wind.

The potential areal extent of impacts to corals considered the predicted TSS spatial distributions of the 1percent probability exceedance area from simulations with and without wind. The potential area ofimpacts to hard corals was delimited with the 13 mg/L TSS concentration band as suggested by thecoral monitoring team (Figure 15), which was subsequently used by the coral monitoring team for fieldand impact assessment (GHD 2010).

DIRECTION ISLAND

HOME ISLAND

COCOS (KEELINGS)ISLAND

WEST ISLAND

265,000

265,000

270,000

270,000

275,000

275,000

8,650

,000

8,650

,000

8,655

,000

8,655

,000

8,660

,000

8,660

,000

8,665

,000

8,665

,000

G:\61\23317\GIS\Maps\MXD\61_2331706_G001.mxd

LEGEND

0 500 1,000 1,500 2,000250

MetresMap Projection: Transverse Mercator

Horizontal Datum: World Geodetic System 1984Grid: Universal Transverse Mercator, Zone 47S o

© 2010. While GHD has taken care to ensure the accuracy of this product, GHD and AGD make no representations or warranties about its accuracy, completeness or suitability for any particular purpose. GHD and AGD cannot accept liability of any kind (whether in contract, tort or otherwise) forany expenses, losses, damages and/or costs (including indirect or consequential damage) which are or may be incurred as a result of the product being inaccurate, incomplete or unsuitable in any way and for any reason.

Attorney General's DepartmentCKI Home Island Slipway EPBC Referral (CKI Port)

Figure 15

Job NumberRevision 0

612331706

22 MAR 2010

Location Map & Study Area

Date

Data Source: AGD: Cocos Island Imagery 2003; GHD: Study Area 20100304. Created by: nnikmohdkamil, xntan

GHD House, 239 Adelaide Terrace Perth WA 6004 T 61 8 6222 8222 F 61 8 6222 8555 E [email protected] W www.ghd.com.au

1:50,000 (at A3)

Study Area

WESTERNAUSTRALIAINDIAN OCEAN

COCOS (KEELINGS)ISLAND

Locality Map

AttorneyGeneral'sDepartment

mailto:[email protected]

http://www.ghd.com.au


5. Limits of Report

Results presented in this report should be treated as indicative as the final dredging program differs fromthat simulated here. Despite the simulated volume of dredge material (20,000 m3) being approximatelyequivalent to the planned dredging volume (i.e. 18.700 m3), the modelling did not account for dredging ofthe ‘high spots’ for Stage 2. However, the errors induced by the underlying assumption are considered tobe low for 2 reasons:

» The dredge volume of Stage 2 is to occur over a large areal extent because of small dredgingvolume for any 1 ‘high spot’; and

» Use of the 1% probability exceedance rather than the standard 20% probability exceedance is aconservative measure to increase the potential impact area to account for the ‘high spot’ dredgingnot explicitly simulated.


6. References

Australian National Tide Tables (2008) Australian Hydrographic Services, Department of Defence,Australia

Falkland, A. C. (1994) “Climate, hydrology and water resources of the Cocos (Keeling) Islands”, CSAIllumina, Ecology and Geomorphology of the Cocos (Keeling) Islands, no. 400, pp 23.

GHD (2000) Cocos (Keeling) Islands Proposed Freight and Passenger Facilities at Rumah Baru (Noticeof Intent), Commonwealth Department of Transport and Regional Services (Territories & RegionalSupport Division).

GHD (2010) CKI Home Island Slipway Redevelopment Baseline Marine Ecological Study, AttorneyGeneral of the Commonwealth of Australia.

Kench, P.S. (1994) “Hydrodynamic observations of the Cocos (Keeling) Islands Lagoon”, NationalMuseum of Natural History, Smithsonian Institution, Washington, DC, USA.

Kench, P.S. (1998) “Physical processes in an Indian Ocean atoll.”Coral Reefs, 17, 155168.

appendix f particle size analysis of marine water samples – … · mesh€in€zones ivi. 12 ......

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