eu edf – sopac project report 134 reducing vulnerability

EU EDF – SOPAC Project Report 134 Reducing Vulnerability of Pacific ACP States

KIRIBATI TECHNICAL REPORT Hydrodynamic Model of Tarawa

Water Circulation and Applications

October 2008

View looking north-west from the south-east of Tarawa area (Buota)

Prepared by:

Herve Damlamian

SOPAC Secretariat

September 2008

PACIFIC ISLANDS APPLIED GEOSCIENCE COMMISSION

c/o SOPAC Secretariat

Private Mail Bag

GPO, Suva

FIJI ISLANDS

http://www.sopac.org

Phone: +679 338 1377

Fax: +679 337 0040

www.sopac.org

[email protected]

Important Notice

This report has been produced with the financial assistance of the European Community; however, the views expressed herein must never be taken to reflect the official opinion of the

European Community.

Kiribati: Tarawa hydrodynamics EU EDF-SOPAC Reducing Vulnerability of Pacific ACP States – iii

EU-SOPAC Project Report 134 – Damlamian 2008

TABLE OF CONTENTS

Acknowledgements ................................................................................................................. iv

EXECUTIVE SUMMARY ........................................................................................................ 1

1. INTRODUCTION ....................................................................................................... 2

2. PHYSICAL CHARACTERISTIC ................................................................................ 3

2.1 Tarawa Lagoon .......................................................................................................... 3 2.2 Tides .......................................................................................................................... 4 2.3 Winds ......................................................................................................................... 4 2.4 Eastern Channels....................................................................................................... 4

3. MATHEMATICAL MODEL ........................................................................................ 5

3.1 Bathymetry ................................................................................................................. 5 3.2 Sources and Eastern Channels ................................................................................. 5 3.3 Lagoon Flats and Bottom Roughness ........................................................................ 6

4. CALIBRATION .......................................................................................................... 8

4.1 Surface Elevation ....................................................................................................... 8 4.2 Current Speed ............................................................................................................ 9 4.3 Current Direction ...................................................................................................... 10

5. MODEL RESULTS: water circulation over a two-week period ................................ 12

5.1 Flood ........................................................................................................................ 12 5.2 Ebb ........................................................................................................................... 14 5.3 Discharge across the Western Reef ........................................................................ 16 5.4 Time Lag and Amplitude .......................................................................................... 17

6. SEDIMENT PLUME DISPERSION ......................................................................... 18

6.1 Background .............................................................................................................. 18 6.2 Dispersion Model Description .................................................................................. 20

7. RESULTS AND DISCUSSION ................................................................................ 22

8. CONCLUSION ........................................................................................................ 23

9. REFERENCES........................................................................................................ 24

APPENDICES ....................................................................................................................... 18

Appendix 1: Simulated current during spring tide ............................................................... 25 Appendix 2: Fine-grained cohesive particles of sediment plume ....................................... 30 Appendix 3: Fine-grained non-cohesive particles of sediment plume ................................ 32 Appendix 4: Effect of causeways on lagoon water residence time .................................... 34

iv – Kiribati: Tarawa hydrodynamics EU EDF-SOPAC Reducing Vulnerability of Pacific ACP States


TABLE OF FIGURES

Figure 1: IKONOS satellite image of Tarawa. ................................................................. 3

Figure 2: Surface elevation data collected from the Betio gauge. .................................. 4

Figure 3: Tarawa wind rose during the simulated period. ............................................... 4

Figure 4: Comparison of multibeam data and satellite image extraction method. .......... 5

Figure 5: Tarawa modified bathymetry. .......................................................................... 7

Figure 6: Map of bottom roughness as a function of depth. ............................................ 7

Figure 7: ADP locations in Tarawa Lagoon. ................................................................... 8

Figure 8: Surface Elevation Calibration. ......................................................................... 9

Figure 9: Current Speed Calibration. .............................................................................. 9

Figure 10: Current Direction Calibration. ........................................................................ 10

Figure 11: Current direction in the shipping channel. ..................................................... 11

Figure 12: Circulation pattern at the flood late stage. ..................................................... 12

Figure 13: Circulation pattern at the flood peak. ............................................................. 13

Figure 14: Circulation pattern at the early flood stage. ................................................... 13

Figure 15: Circulation pattern at the ebb peak. ............................................................... 14

Figure 16: Circulation pattern at the early ebb stage. ..................................................... 15

Figure 17: Circulation pattern at the late ebb stage. ....................................................... 15

Figure 18: Discharge Sections on the Western Reef. ..................................................... 16

Figure 19: Surface elevation at Betio and in the southeast corner of Tarawa. ............... 17

Figure 20: Dredging method using suction and discharge pumps. ................................. 18

Figure 21: Resources areas 1 and 2 for aggregates mining. .......................................... 19

Figure A4.1: Comparison between water circulation prior and after causeways. ............... 34

Figure A4.2: Water residence time map with causeways. .................................................. 35

Figure A4.3: Water residence time map without causeway. ............................................... 35

Acknowledgements

I first express my thanks to Jens Krüger, Physical Oceanographer and Dr Arthur Webb, Aggregate Management Adviser of the SOPAC/EU project, for their assistance throughout.

I also thank Franck Magron, Reef Fisheries Information Manager of SPC, for providing the bathymetry of Tarawa lagoon which led to the success of this study.

Kiribati: Tarawa hydrodynamics EU EDF-SOPAC Reducing Vulnerability of Pacific ACP States – 1


EXECUTIVE SUMMARY

Damlamian, H. 2008: Hydrodynamic Model of Tarawa, Kiribati: Water circulation and applications. EU EDF 8 – SOPAC Project Report 134. Pacific Islands Applied Geoscience Commission: Suva, Fiji, iv + 35 p.

With the development of powerful computers, numerical models are increasingly being used to simulate processes of nature. Mike21 is one example of professional modeling software for two-dimensional free surface flows; it comes in modular form with four main application areas: coastal hydraulics and oceanography, waves, sediment processes and environmental hydraulics.

Like most Pacific Atolls, Funafuti in Tuvalu suffers from natural hazards (storm surge, tsunami), poor water quality, etc. Especially the limited availability of land, a common issue for an atoll, means vulnerability to coastal erosion.

This work was undertaken as part of the SOPAC/EU project, EDF 8, aiming to create a realistic numerical model of Tarawa lagoon. Using this management tool, issues such as water quality, sediment transport and sediment plume dispersion could be examined.

To simplify the model, the wave effect from the east was simulated by sources located in each eastern channel. That information was extracted from field work undertaken previously by the EU team and described in EU EDF-SOPAC Project Report 136). Tide-induced current controls the water circulation in Tarawa lagoon, and six patterns have been identified depending on the tidal phase.

Using this model as a management tool, one can address any future coastal management project in Funafuti. The hydrodynamic model provides a baseline for studying water quality scenarios, dredging operations and erosion. One application was set up to highlight the tool’s possibilities for SOPAC member countries: it assesses a dredging operation in order to prevent beach mining and irreversible erosion damage along Tarawa coast.

A sediment plume dispersion model was developed based on the hydrodynamic model and sediment data collected; it identifies sediment plume dispersion from a dredging operation in two possible resource areas.

The predicted impacted zone is a guide only: for accurate results calibration data are required.

To emphasize the model’s potential to assist coastal management, a water residence time study was also undertaken, assessing the lagoon’s vulnerability to a pollution scenario.

The models were built with a grid resolution of 100 m, which reflects the computation limitation faced at the time of development.

2 – Kiribati: Tarawa hydrodynamics EU EDF-SOPAC Reducing Vulnerability of Pacific ACP States


1. INTRODUCTION

Harbours and lagoons are all subject to some, or all, of the following disturbances: pollution, wave action, storm surge, seiching, tsunami, erosion and sedimentation, and sea-level rise. Studies linked to coastal management are often limited by the data sets available, seasonal variations and cost.

Numerical modeling provides an opportunity to view and analyse coastal problems and risks. It permits a valuable symbiosis between development and application, with minimal penalties for error.

This work is part of the SOPAC-EU Reducing Vulnerability Project / Kiribati work plan. The production of a baseline hydrodynamic model of Tarawa lagoon using MIKE 21 software was undertaken using new high-resolution bathymetric data collected by the Marine Survey component of the EU Project, combined with a bathymetry extracted from the IKONOS satellite image of Tarawa.

This baseline model may then be used to address further project works in relation to the Kiribati Aggregates Company Proposal. This proposal is being jointly developed by the SOPAC-EU Project’s Coastal Processes component, the Kiribati, EU Office and the Ministry of Fisheries and Marine Resource Development, Kiribati.

Water circulation is regulated by a combination of the effect of the tide, the wind and the wave. This study identifies the influence of each factor.

The model can help us estimate the aftermath of a dredging operation, caused by the dispersion of sediment plume in two surveyed aggregate sites.

Changes in water circulation and water residence time in relation to the building of South Tarawa causeways is also modelled.



2. PHYSICAL CHARACTERISTIC

2.1 Tarawa Lagoon

Tarawa, located in the Pacific Ocean 1 degree north of the equator, is the capital and the economic, administrative and population centre of the Republic of Kiribati. This atoll is the most crowded of Kiribati, with a population of 94 000 people.

Figure 1: IKONOS satellite image of Tarawa.

The population in south Tarawa increases by 5.2 % per year, so it is crucial to understand the environment and the impact of this population on its surroundings.

The Tarawa lagoon covers an area of 359 km2 with a mean depth of about 6 m. Flushing takes place mainly on the western reef, especially through a 20-m deep shipping channel. Additional flushing occurs through 12 channels, all located on the eastern reef flat of Tarawa.

Betio



2.2 Tides

A tide gauge in Betio permanently records surface elevation data. Data are available through the South Pacific sea level and monitoring project (www.pacificsealevel.org).

Tides are semi-diurnal with a significant inequality between successive highs. The spring tides have a range of 2.4 metres and the neap tides have a range of 0.5 metres (Figure 2).

Figure 2: Surface elevation data collected from the Betio gauge.

2.3 Winds

The dominant wind is from the east. The mean direction during the simulated period (from 15 September to 2 October 2005) was 116.67 degrees. The wind blew from the east 86% of that period. The mean speed was 3.67 m/s with a maximum value of 13.2 m/s and a minimum of 0.1 m/s.

2.4 Eastern Channels

Eastern channel flow was studied prior to this work. The main findings are:

No discharge towards the ocean was observed across the eastern channels.

The effect of flushing by the eastern channels on the overall lagoon circulation is insignificant (Damlamian and Webb 2008).

Figure 3: Tarawa wind rose during the simulated period.



3. MATHEMATICAL MODEL

The hydrodynamic module in the MIKE 21 Flow Model simulates unsteady two-dimensional flows in one layer (vertically homogeneous) fluids. Details of the equations used are available on the DHI website, www.dhigroup.com.

3.1 Bathymetry

Three different sources of data were used to create the bathymetry model:

Multibeam data in the centre of the lagoon from a 1995 SOPAC survey (Smith and Biribo1995)

Multibeam data from a 2005 SOPAC/EDF survey. Data were collected in the proposed aggregate site, the shipping channel and the periphery of the atoll rim (Sharma and Krüger 2008)

Derived bathymetry from IKONOS satellite imagery using the Stumpf method (pers. comm. Franck Magron, SPC). The extraction was successful in shallow water bathymetry deeper than 15 m (Figure 6).

Figure 4: Comparison of multibeam data (blue) and satellite image extraction method (red).

To reduce computation time, a resolution of 100 m was chosen. The bathymetry model can be further simplified using the eastern channel flow model (section 4.3 and Figure 5).

3.2 Sources and Eastern Channels

To model the flow in the eastern channels, water sources are simulated in every channel. The flow for each of the 12 sources depends on the surface elevation and the tidal phase.

Table 1 shows the equation used for each sources. Those equation were extracted from Damlamian and Webb (2008).



Table 1: Eastern channel flux equations. Source: Damlamian and Webb (2008).

Neap Tide Spring Tide

Ebb Flood

Kainaba (Ch.10) Negligible y = 0.0003e4.9618x

Biketawa (Ch.6) y = 30.590x – 45.133 y = 215.11x – 411.49 y = 1.0348e2.0356x

Abatoa (Ch.2) y = 52.557x – 58.137 y = 105.56x – 187.25 y = 0.1021e3.3313x

Nabeina (Ch.5) y = 26.714x – 35.491 y = 90.782x – 160.58 y = 0.1163e2.7734x

Tabuki (Ch.3) y = 31.383x – 43.448 y = 95.385x – 168.2 y = 0.1396e2.7179x

Tabiang (Ch.4) y = 83.711x – 115.82 y = 251.41x – 443.65 y = 0.4182e2.6719x

Bikenubati (Ch.8) y = 26.714x – 35.491 y = 90.782x – 160.58 y = 0.1163e2.7734x

Bikenikaibuke (Ch.11) y = 10.971x – 16.081 y = 37.017x – 66.753 y = 0.0002e5.0041x

Bikenamori (Ch.12) y = 23.215x – 35.571 y = 72.534x – 130.8 y = 0.0002e5.054x

Nea (Ch.7) y = 42.033x – 63.295 y = 233.67x – 444.7 y = 0.3429e2.269x

Tanaea (Ch.1) y = 22.834x – 29.955 y = 77.891x – 137.78 y = 0.1448e2.6268x

Channel 9 Negligible y = 5E–07e6.7917x

3.3 Lagoon Flats and Bottom Roughness

The Eastern intertidal lagoon flat dries at low tide. In order to keep the model stable, a flood and dry parameter was switched on allowing the model to determine if a node is wet or dry.

Roughness is expressed by the Manning number and is chosen according to depth: from 0 to –10m: 1/M=28; from –20 to –10m: 1/M=30; from –100 to –20 m: 1/M=32; for a water level down to –100, 1/M=35.

In shallow-water areas such as Tarawa lagoon, an accurate Manning number can significantly improve the model calibration.

First, a bed roughness map was developed from the bathymetry. The map was then tuned to better match the current data collected. A smooth bed surface (1/M = 60) was used in the shipping channel to increase the current speed, while a rough bottom surface was implemented on the western reef (1/M=28) (Figure 6).



Figure 5: Tarawa modified bathymetry

Figure 6: Map of bottom roughness as a function of depth.

Legend Ocean side 1/M = 40 0m<x<–10m 1/M = 35 –10<x<–20 1/M = 55 Western reef 1/M = 28

Shipping channel 1/M = 60

1

2



4. CALIBRATION

The Betio tide gauge was used to calibrate the surface elevation in the model while four acoustic Doppler profilers (ADP) were deployed in the lagoon during September and October 2005 to assist the current speed and direction calibration.

Table 2: ADP locations (Figure 7).

Easting Northing

ADP 1 715488.35 163511.3

ADP 2 716574.53 15442.19

ADP 3 713751.77 156068.19

ADP 4 717830 150686.11

Figure 7: ADP locations in Tarawa Lagoon.

4.1 Surface Elevation

Figure 8 shows a comparison between the tide gauge data and the data from the model. The surface elevation in the model matches the data collected by the gauge well. However, on average, the tidal range in the model (the difference between high tide and low tide), is 7 cm greater than in reality.

When adding land (green in Figure 5), we directly deform the water motion in areas 1 and 2. During normal conditions, however, the equatorial current, the wind and the waves are mainly oriented towards the west and water passing across this area is not likely to enter the lagoon.

Loc1

Loc3

Loc2

Loc4



Figure 8: Surface Elevation Calibration.

4.2 Current Speed

Figure 9 compares the simulated model current speed in the four locations with the collected ADP data.

Figure 9: Current Speed Calibration.

Phase and amplitude of simulated current speed match the collected data for every location. A greater difference can be seen in the shipping channel (location 3) where the simulated speed amplitude is around 85% of the real data. The calibration is nevertheless considered satisfactory.



4.3 Current Direction

Figure 10: Current Direction Calibration: from top to bottom: ADP 1, ADP 2, ADP 3, and ADP 4, with model values to the left and actual records at the right-hand side.

The model matches the main current directions for locations 1, 2 and 4. Due to the grid size limitation, some differences occur when comparing the residual current directions. These are not expected to influence the overall model accuracy.

However, in the shipping channel (location 3), there is a difference of 45 degrees in the westward current direction between the real data and the simulated data.



At the adjacent node in the shipping channel, the difference now occurs for the eastward mean direction (Figure 11). This indicates that the bathymetry of the shipping channel does not suit the applied 100 m grid space. By contrast the calibration results above show that model provides a good representation of the actual water circulation in the lagoon.

Figure 11: Current direction in the shipping channel.



5. MODEL RESULTS: water circulation over a two-week period

5.1 Flood

Three different water circulation patterns were extracted from the simulation. Each pattern is related to a specific flood period: early stage, peak and late stage (Figures 12–14).

A common feature for the entire flood phase is the mean direction of the water circulation. Water enters the lagoon from the western reef, directed toward the southeast corner.

The main difference occurs along the shore. During the early stage, a longshore current takes place, pointing towards the southeast corner of Tarawa. As the water level increases, the flood current becomes stronger and stops the longshore current along north Tarawa. Then, the flood current decreases as the water level is about to reach high tide. Waves push the water across the eastern reef towards the lagoon while ebb longshore currents are starting to take place.

The importance of each circulation pattern was quantified by extracting the current direction along the shore during a flood period for spring tide and neap tide (Table 3). During flood, the strongest current occurs in the shipping channel with a peak speed of 1.8 m/s.

Table 3: Flood circulation dominancy.

Spring Neap

Early stage 17% 35%

Peak 50% 55%

Late stage 15% 10%

Figure 12: Circulation pattern at the late flood stage.



Figure 13: Circulation pattern at the flood peak.

Figure 14: Circulation pattern at the early flood stage.



5.2 Ebb

Three different water circulation patterns were extracted from the simulation. Each pattern is related to a specific period of the ebb: early stage, peak and late stage (Figures 15–17).

A common feature for the entire ebb phase is the mean direction of the water circulation. The water is flushed out across the western reef directed parallel to the North Tarawa coast on the north part of the lagoon; it gradually becomes parallel to South Tarawa coast as we go south.

The main difference occurs along the shores. During the early stage, water level is high enough to allow waves to push ocean water across the eastern reef. As the water level decreases, the ebb current becomes stronger and stops the longshore current along the north end of North Tarawa. Then, the ebb current decreases as the water level is about to reach low tide. No flushing occurs on the eastern reef and longshore current takes place, pointing towards the western reef of Tarawa.

The importance of each circulation pattern was quantified by extracting the current direction along the shore during a flood period for spring tide and neap tide (Table 4). During ebb, the strongest current occurs in the shipping channel, with a peak speed of 1.6 m/s

Table 4: Ebb circulation dominancy.

Spring Neap

Early stage 10% 22.5%

Peak 65 % 50%

Late stage 25 % 27.5%

Figure 15: Circulation pattern at the ebb peak.



Figure 16: Circulation pattern at the early ebb stage.

Figure 17: Circulation pattern at the late ebb stage.



5.3 Discharge across the Western Reef

The simulated discharge across the western reef was calculated during the flood and the ebb of a spring tide. During flood, 0.92x109 m3 of ocean water comes in the lagoon while 5.68x106 m3 of lagoon water goes to the ocean. During ebb, 0.98x109 m3 of lagoon water exits the lagoon while 36.2x106 m3 of ocean water enters the lagoon.

Five sections were defined and the simulated discharge was calculated for each section in order to understand their relative importance in the water circulation (Figure 18).

Figure 18: Discharge Sections on the Western Reef.

The discharge through the shipping channel (sector 3) represents between 10 and 15 % of the total discharge while its length is only 3% of the western reef.

The mean lagoon volume is estimated to be 2.1x109 m3. During spring tide, at the flood phase, 44% of the mean lagoon volume is filling with the ocean water coming across the western reef. The amount of water leaving the lagoon, during the ebb, through the western reef represents 46.6% of the mean lagoon volume. Considering a mean depth of 6 m and a tidal range during spring tide of 2.5 m, flushing in and out of the lagoon should represents 41–42% of the mean lagoon volume.

Table 5: Discharge on the western reef during flood.

Section Percentage discharge to lagoon Percentage discharge to ocean

1 13.73 16.38

2 10.75 13.71

3 65.48 56

4 4.67 7.63

5 5.35 6.31

5

4

3

2

1



Table 6: Discharge on the western reef during ebb.

Section Percentage discharge to lagoon Percentage discharge to ocean

1 8.4 13.91

2 15.2 11.147

3 74.8 64.08

4 0.76 5.14

5 0.77 5.7

5.4 Time Lag and Amplitude

The tidal wave travels from the western reef to the southeast lagoon corner. The wave travels from the west to the east in 15 minutes. In other words, the average of the tidal wave propagation in the Tarawa lagoon is about 24.5 m/s. Also, the tidal range in the south corner of Tarawa is 0.2 m greater than in Betio.

Figure 19: Comparison between surface elevation at Betio (red) and in the southeast corner of Tarawa (black).



6. SEDIMENT PLUME DISPERSION

6.1 Background

6.1.1 Dredging Method

Dredging consists of excavation of material from the sea, river or lakebed and its relocation elsewhere. This method is usually applied to improve the navigable depths in ports, harbours, and shipping channels; or to win minerals from underwater deposits. Other applications are to improve drainage, reclaim land, improve sea defence, or remove and relocate contaminated materials.

The dredge stirs up sediment and causes sediment plumes to be dispersed. Current and gravity influence the sediment-water mixture that forms the plume.

Several negative impacts of a sediment plume on water quality, marine ecology, fish and shellfish can be expected:

Elevated turbidity causes reduction of photosynthesis by phytoplankton, algae and rooted vegetation. Reduction of visibility makes feeding difficult for some fish.

Increase of suspended sediment will cause reduction of dissolved oxygen levels, and release absorbed heavy metals or toxic organics from fine-grained suspended solids. This will cause interference with the respiration and feeding of fish, impediment to mobility, or irritation to tissues, making infection or invasion by parasites more likely.

Sedimentation will cause covering of the bottom near the dredging site, smothering bottom-dwelling organisms, reducing or eliminating food supply, or reducing habitat diversity.

To investigate the impact from the sediment plume, the model simulates a system with two pumps. One extracts the aggregates from the site using a suction pump while the other pump discharges water and particles near the seabed (Figure 20).

Figure 20: Dredging method using suction and discharge pumps.



6.1.2 Resources Areas

Beach mining significantly increases the erosion impact on the coast. In order to discourage this, SOPAC delimited two resource areas suitable for a proposed dredging operation. In 1995, a survey was undertaken to collect sediment sample in a resource area close to Betio (Site 1). In September 2005, another survey collected sediment samples in the second resource area (site 2) (Figure 21).

The possible impact of such a project can be modeled if the distribution of sediment grain size in the resource area, as well as parameters defining the particle (settling velocity, density, etc.) are known. The model can simulate the dispersion of the sediment plume from the dredging operation.

The dimension of those resource areas is 2 km2 with water depths ranging from 1 to 14 m, and 16 km2 with water depths ranging from 4 to 25 m.

This model looks at dispersion of only the finest sediment particles released from the dredging overflow. Heavier particles will settle on the bottom close to the water column where they have been released. Here, we look at the finest particles, representing non-cohesive sediment, with a size around 62.5 μm (sand), and cohesive sediment describing the dispersion of the mud, with a grain size 4.0 μm (approximated limit before flocculation).

Figure 21: Resources areas 1 and 2 for aggregates mining.

Site1

Site2



6.2 Dispersion Model Description

This section gives a quick overview of the main parameters chosen for the simulation.

6.2.1 Time Step and Simulated Period

As a rule of thumb, a particle should not move more than one grid cell in each timestep, otherwise the numerical integration of the stochastic differential equation that controls the particle motion becomes unstable.

In the resource area, the maximum current speed, Umax, does not exceed 0.9 m/s and the grid spacing, x, is 100 metres. From the definition of the Peclet number the maximum timestep can now be calculated as t < x/Umax. Hence, a 100 second time step was chosen. The model simulates a 12-day period.

6.2.2 Sources

To simulate the overflow discharge, a source is inserted in the dredging site releasing the overflow sediment plume emerging from the near-bottom discharge pump. The source is defined by its horizontal and vertical position, and its flux of fine sediment at its location.

6.2.2a Sources Position

For each particle type, an overflow discharge was simulated in several key locations in the resource area, with all sources located 2 m above the seabed.

6.2.2b Flux

For a realistic idea of the overflow discharge for each resource area, the pump system needs to be known. Because at this stage no dredging method has been chosen, the pump used in this model is similar to the one described by Cruickshank and Morgan (1996). Its overflow discharge can be estimated as follows:

Solids pumped = 19 m3/h

Water pumped = 170 m3/h

The overflow of fine particles is assumed to be the percentage of the fine-grained particles in the sediment multiplied by the volume of solids pumped. This gives the predicted flux for each grid point in the resource area. As the dredging is to be conducted for three hours per day, a time series file was also created for each grid point in the resource area.

6.2.3 Dispersion

The model uses dispersion proportional to the current. The proportional factors are:

Longitudinal Direction fLong = 1

Transversal Direction fTrans = 1

Vertical Direction fVert = 0

For the short-term conditions of interest, we can assume the plume to maintain a fairly constant thickness as it is transported by tidal and wind current and dispersed laterally by turbulent processes. We do not need to include vertical dispersion in this model, which simplifies the model and is a conservative assumption: any significant vertical dispersion will lower the predicted suspended particulate concentrations.



6.2.4 Type of Particles

The dispersion of two different types of particles was investigated; one focused on the fine-grained non-cohesive sediment (grain size 62.5 μm) and one on cohesive sediment (4.0 μm). The chosen dimension allows us to ignore the flocculation and develop the worst-case scenario.

The last parameter is the Critical Shields parameter for motion. A typical value is 0.045 for sand, and around 0.2 for mud. In the absence of a local value for the density of sediments, 1.667 was chosen (Cruickshank and Morgan 1996). Although the common density for sand is 2.65, the smaller value used in this model allows the sediment particles to travel further, thus illustrating a worse-case scenario.

6.2.5 Settling Velocity

Settling velocity depends on the size of the particles: of a single free particle it can be estimated using Stokes’ law, which can be represented by, for example:

Where s = sediment density (kg/m3), = viscosity (m2/s), = density of water, ws = settling velocity (m/s), g = acceleration due to gravity (m/s2 ) and d = grain size (m).

Accordingly, the settling velocity value is 1.57 m/s for sand and 6.8x10–3 m/s for mud. The Viscosity value was taken from Ramsing and Gundersen (2001).



7. RESULTS AND DISCUSSION

Dredging operations can affect the ecosystem of Tarawa in many ways. This model looked at the rate of suspended particles added to the water column as well as their sedimentation, providing an understanding of the possible impact of such an operation.

Two dredging sites were looked into: site 1 and site 2. Also, two types of sediment were used to simulate the sediment plume dispersion: cohesive with a grain size of 4 μm (mud) and non-cohesive with a grain size of 62.5 μm (sand).

Coral reefs are very sensitive to increased sedimentation and turbidity. Destruction of coral reefs may lead to increased coastal erosion and to a significant reduction in fish population. When looking at an impacted zone from a dredging operation, a limit of acceptable damage has to be defined.

No coral reef is in constant growth. For instance, during major tropical storm events, all reefs undergo losses in coral cover and often erosion of their physical structure. Recovery time from such natural events can be used as a measure of acceptable damage, usually represented by a recovery time of one year.

Two acceptable limits have to be set up in the model: one limit of sedimentation rate and one limit of suspended particles. The outcome of the study strongly depends on how accurately those acceptable limits are determined. Investigation using laboratory experiments was beyond the scope of this and crucial information such as ambient rates of suspended particles and sedimentation was not available.

Some assumptions have to be made. In general, corals can clear themselves from 100 mg/cm2/day (Lasker 1980) without the help of current, or in the Indo Pacific Region even up to 230 mg/cm2/day (Pastorok and Bilyard 1985). In the Tarawa lagoon, a large enclosed shallow-water environment, the natural recovery rate is also expected to be high. Therefore a sedimentation rate of 10 mg/cm2/day above the natural condition was presumed not to cause irreversible damage to the coral and is used as the acceptable limit of sedimentation rate.

In the Great Barrier Reef, corals have become acclimatised to higher sediment loads, exceeding 100 mg/L (Hopely et al. 1991). For Tarawa lagoon, a limit of 10 mg/L (or 0.01 kg/m3 ) above the natural condition was considered sensible and acceptable for the corals.

The model shows that no dispersion is expected from the non-cohesive sediment grain with a size of 62.5 μm. All those particles settle quickly after being released.

The model predicts sediment plume dispersion on the reef flat of Betio if a dredging operation occurs in site1 (Appendix 2). Most of the simulated sediment plume dispersion from site 1 seems to be below the chosen limit of acceptable damage. However the current being relatively strong at this location, the sediment plume is widely dispersed. One location in site 1 produces, after dredging operation, an alarming result – locations (715000, 153800, UTM59) and (714415,153977,UTM59). Because of its high rate of fine sediment, a large amount of particles is released from the source, leading to a suspended particle rate and sedimentation rate well above the acceptable limits. This simulation indicated that the rate of suspended particle can exceed in some locations 1.5 kg/m3 and reach to a sedimentation rate above 35 kg/m2. This pollution could irreversibly damage the marine life in Tarawa lagoon. However, the dispersion from dredging in this specific location seems to be localised in front of Betio, where corals are not expected to grow.

Dredging in site 2 seems to be much safer as the current speed is much lower. This allows the finest particles to remain close to their release and settle down quickly (Appendix 3).

The results of this model should be interpreted carefully, as this model application is only a preliminary study. Extensive data collection is required, for example of the ambient



sedimentation rate, the ambient suspended particle rate, the dredging system used, habitat mapping of the lagoon, particle density, and data to calibrate the model.

However, this model already gives us some important clues: dredging in site 2 seems to be much safer than in site 1; and if dredging in site 1, it is recommended to avoid locations with high content of fine sediment.

8. CONCLUSION

The hydrodynamic model Mike21 was applied to simulate the water circulation in Tarawa lagoon. Simulation results were used to extract the dominant circulation patterns for different tide phases.

The different patterns of water circulation that occur in the Tarawa lagoon highlight the dominance of tidal currents compared with those induced by wind and waves. However, this model does not predict the water motion during storm or predominant westerly winds.

The model highlights the crucial role of the flow passing through the western reef and especially through the shipping channel. The amount of water passing in and out of the lagoon through the western reef during ebb and flood of the spring tide peak was estimated to be around 50% of the mean lagoon water volume. Moreover, between 10% and 15% of this water goes in and out through the shipping channel, which represents less than 5% of the total western reef length.

The model was then used as a baseline to estimate the impacted zone of a dredging operation in two different sites called site 1 and site 2. Site 2 appears to be much safer since there is no significant dispersion of the sediment plume: particles released at site 2 will settle quickly close to the water column where they are released. This contrasts with the sediment plume dispersion in site 1 which is much larger due to a stronger current. According to the model, if dredging is carefully planned by avoiding high-risk locations within site 1 (where the rate of finest particle is high), no significant damage should occur.

The first aim of the dispersion plume model was to illustrates what can be done using the hydrodynamic model as a management tool. The dispersion model presented here is only a preliminary study.



9. REFERENCES

Cruickshank, M.J. and Morgan, C.L. 1996. Proposed dredging in basin A offshore from Nuku’alofa, Kingdom of Tonga. Environment impact assessment for the Kingdom of Tonga. Ministry of Lands, Survey & Natural Resources, Appendix to trip report 234.

Damlamian, H, 2008. Hydrodynamic model of Funafuti, Tuvalu: Water circulation and Application, EU EDF 8 – SOPAC Project Report 133. Pacific Islands Applied Geoscience Commission: Suva, Fiji.

Damlamian, H. and Webb A. 2008. Intertidal channel flow in North Tarawa, Kiribati. EU EDF 8 – SOPAC Project Report 136. Pacific Islands Applied Geoscience Commission: Suva, Fiji.

DHI 2004, Mike 21 Reference Manual

Lasker, H. R. 1980. Sediment rejection by reef corals: the roles of behavior and morphology in Montastrea cavernosa (Linnaeus). Journal experimental marine Biology and Ecology 47: 77–87.

Pastorok, R.A. and Bilyard, G.R. 1985. Effects of sewage pollution on coral-reef communities. Marine Ecology Progress Series 21: 175–189.

Ramsing, N. and Gundersen, J. 2001. Seawater and gases, tabulated physical parameters of interest to people working with microsensors in marine systems. Limnology and Oceanography.

Sharma A. and Krüger, J. 2008. High-resolution bathymetric survey in Kiribati.EU EDF 8 – SOPAC Project Report 114. Pacific Islands Applied Geoscience Commission: Suva, Fiji.

Smith R. and Biribo, N. 1995. Marine aggregate resources, Tarawa lagoon, Kiribati, SOPAC technical report 217.



APPENDICES

Appendix 1: Simulated current during spring tide



Please note that figures showing “Scale1:xxxxxx” have been further reduced throughout this appendix.



Appendix 2: Fine-grained cohesive particles of sediment plume

source (102,48)

source (97,59)

source (97,70)

source (104,70)



source (111,69)

source (106,60)



Appendix 3: Fine-grained non-cohesive particles of sediment plume

source (106,60)

source (102,48)

source (97,70)



source (97,51)

source (111,51)



Appendix 4: Effect of causeways on lagoon water residence time

Since causeways were built in south Tarawa, water circulation in the south part of the lagoon has changed. To undertake this comparative study, causeways have been removed from the initial bathymetry and a new water circulation pattern was extracted.

By comparing the two main water circulation patterns of Tarawa with and without causeway, changes can be highlighted (Figure A4.1).

Changes in water circulation are only shown in the south part of the lagoon. However, South Tarawa is the most vulnerable area since it strongly interacts with human activity.

Prior to the causeways, water was flushed across the south channel. Nowadays this dynamic has been replaced by a longshore current.

Flood regime without causeways Flood regime with causeways

Ebb regime without causeways Ebb regime with causeways

Figure A4.1. Comparison between water circulation before and after causeways.

To quantify the effect of causeways, residence time maps were created for both configurations. The residence time is the time a particle of water, initially located at a given point, requires to leave the lagoon. Water residence time is spatially dependent and requires the use of numerical modelling. It shows the lagoon’s potential to clean itself by replacing lagoon water with clean ocean water.

A significant difference of residence time in the south half of the lagoon is easily identified by comparing Figures A4.2 and A4.3. Some interesting points to highlight:

Water, initially in the south corner of Tarawa used to be flushed out between 1 and 5 days. Nowadays, the model shows a residence time of around 70 days.



Significant increase of residence time due to causeways is seen along the south coast, with a residence time increasing by a factor 10 in several locations.

Despite obvious limitations of this study (the actual depth of the south channels being unknown), results show the increase of vulnerability inherent to the building of causeways.

Flushing into and out of the lagoon across the south reef has stopped, reducing the overall flushing potential of the lagoon.

Figure A4.2: Water residence time map with causeways.

Figure A4.3: Water residence time map without causeways.

eu edf – sopac project report 134 reducing vulnerability

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