Impacts of the vung tau go cong sea dyke on hydrodynamic flow regime

THUY LOI UNIVERSITY IMPACTS OF THE VUNG TAU – GO CONG SEA DYKE ON HYDRODYNAMIC FLOW REGIME BUI DUC TOAN MSc Thesis on Integrated Water Resources Management August 2015... THUY LOI UNI

THUY LOI UNIVERSITY

IMPACTS OF THE VUNG TAU – GO CONG SEA DYKE

ON HYDRODYNAMIC FLOW REGIME

BUI DUC TOAN MSc Thesis on Integrated Water Resources Management

August 2015

THUY LOI UNIVERSITY

BUI DUC TOAN

IMPACTS OF THE VUNG TAU - GO CONG SEA DYKE ON

HYDRODYNAMIC FLOW REGIME

Major: Integrated Water Resources Management

THESIS OF MASTER DEGREE

Supervisor (s):

Assoc Prof Dr Nguyen Cao Don

This research is done for the partial fulfilment of requirement for

Master of Science Degree at Thuy Loi University (This Master Programme is supported by NICHE – VNM 106 Project)

Ha Noi, August, 2015

ABSTRACT

The low-lying terrain downstream of Saigon - Dong Nai River is most affected by natural disasters such as, flooding, saltwater intrusion causing difficulties in process of socio-economic development The project of building Vung Tau - Go Cong sea dyke with a length of 32km was proposed

to solve these problems, in particularly creating a reservoir for storing water and preventing saltwater intrusion, expanding urban space, industrial parks, tourism, services, shelter from the storm boats, reserving fresh water in the future However, hydrodynamic regime in this area would be altered by the construction of Vung Tau - Go Cong, causing sedimentation in estuaries, changing salt marsh ecosystems The comparison between hydrodynamic regime with two scenarios before and after construction sea dyke will be mentioned in this thesis

In this study, MIKE 21 model was used to simulate hydrodynamic regime in study area The computed domain is described as follow: Latitude:

1080000 – 1160000; Longtitude: 670000-770000 The grid which used in computation was unstructured mesh because it met the requirement of accuracy and detail computation Exported data from MIKE 11 model was used as input data for discharge boundary of model The observed water level data of Vung Tau station and global tide prediction data were used for model calibration and validation Duration time of model calibration and validation for the research site from 17/October/2000 to 20/10/2000 and 21/October/2000 to 24/10/2000 respectively The calibrated parameter was bed resistance

The application of model is considered in two scenarios: without sea dike and with sea dike Both scenarios show semidiurnal tide regime in Go Cong- Vung Tau area Moreover those confirm that current is mainly

influenced by tide and flow of estuaries in coastal area The construction of sea dike creates two distinct areas: The first area- Reservoir including main dike, branch dike and Soai Rap estuary; The second area - Ganh Rai Bay containing Long Tau, Thi Vai estuaries and branch dike There is a significant change in hydrodynamic regime between two scenarios at inside reservoir, for example considerable differences in phase, fluctuation amplitude of water level/current Except for inside the reservoir, there is a small change in phase, fluctuation amplitude of water level/current at outside reservoir

DECLARATION

I hereby certify that the work presented in this thesis entitled, “Impacts of the Dung Tau – Go Cong sea dyke on hydrodynamic flow regime ” in partial fulfillment of the requirement for the award of the Master of Science in Integrated Water Resource Management, is done by myselft under the supervision of Assoc Prof PhD Nguyen Cao Don The matter embodied in this thesis has not been submitted by me for the award of any other degree or diploma

Ha Noi, date August 2015

Bui Duc Toan

ACKNOWLEDGEMENTS

First and foremost I would like to thank the supervisor Assoc Prof PhD Nguyen Cao Don for his great contribution in this thesis, for supporting me and guiding me stay on the right trend I want to show deep thanks to Assoc Prof Dr Nguyen Thu Hien and Dr Hoang Nguyet Minh who are main co-ordinators, making value contributions to success in Master course and I would like to thank the CoMEM Mariette who help me improve English writing skill I would like to thank my wife, my family and my colleagues for their continuous encourages

I would like to express deep thanks to KC.09.16/11-15 state project

namely “Research, identification scientific arguments and proposal of Phu Quoc – Con Dao marine spatial planning for sustainable development”

(Assoc Prof Dr Pham Quy Nhan is the Project Manager) that I am joining to because of its funding

Finally I would like to thank NICHE VNM-106 project (funded by NUFFIC) for finance supporting and facilities for studying process

Ha Noi, date August 2015

Bui Duc Toan

TABLE OF CONTENT

ABSTRACT 1

DECLARATION 3

ACKNOWLEDGEMENTS 4

CHAPTER 1: INTRODUCTION 10

1.1 Background 10

1.2 Problem statements 14

1.3 Objectives and Research questions 16

1.4 Methods 16

1.5 Structure of the thesis 17

CHAPTER 2: LITERATURE REVIEW 18

2.1 The studies on Mekong Delta of foreign authors 18

2.2 The related researches on the lower downstream of Sai Gon-Dong Nai river basin 20

2.3 Overview of hydrodynamic models 23

2.3.1 Possible models 25

2.3.2 Selection criteria 26

2.4 Overview of MIKE 21 27

CHAPTER 3: ANALYSIS IMPACTS OF THE VUNG TAU – GO CONG SEA DYKE ON HYDRODYNAMIC REGIME 29

3.1 Governing equation 29

3.2 Model setting 36

3.2.1 Study area 36

3.2.2 Bathymetry 37

3.2.3 Mesh generation 37

3.2.4 Water level boundaries 39

3.2.5 River boundaries 39

3.3 Model calibration and validation 41

3.3.1 Model calibration 41

3.3.2 Model validation 45

3.4 Application 48

3.5 Results and discussions 49

3.5.1 Surface elevation 51

3.5.2 Current 58

CONCLUSIONS AND RECOMMENDATIONS 64

CONCLUSIONS 64

RECOMMENDATIONS 65

REFERENCES 66

APPENDICES 68

LIST OF FIGURES

Figure 1-1 Vung Tau- Go Cong Sea dike project (Source: Google Earth 2010) 10

Figure 1-2 Sai Gon - Dong Nai river basin 12

Figure 1-3 Map showing the Vung Tau - Go Cong sea dike project (Source: Son ( 2012)) 15 Figure 2-1 Overview of the station net realized during the cruise in April 2007 20

Figure 2-2 Southeastern region (Source: Trinh (2007)) 21

Figure 2-3 Research area in Go Cong, Kien Giang province (Source: Hung (2011)) 22

Figure 3-1 Computational mesh 37

Figure 3-2 Study area and bathymetry of computational domain 38

Figure 3-3 Locations of river boundaries and sea boundaries 39

Figure 3-4 Hydrographs at river boundaries 40

Figure 3-5 Model calibration: Comparison between simulated and measured water level at Vung Tau gauge station (M=28 m 1/3 /s, 17/10/2000 01:00-20/10/2000 01:00) 43

Figure 3-6 Surface elevation in model calibration, 17/10/2000 01:00-20/10/2000 01:00 (left hand-flood tide; right hand-ebb tide) 44

Figure 3-7 Current speed in model calibration, 17/10/2000 01:00-20/10/2000 01:00 (left hand-flood tide; right hand-ebb tide) 44

Figure 3-8 Model validation: Comparison between simulated and measured water level at Vung Tau gauge station ( 21/10/2000 01:00-24/10/2000 01:00) 46

Figure 3-9 Surface elevation in model validation (left flood tide; right hand-ebb tide) 47

Figure 3-10 Current speed in model validation (left hand-flood tide; right hand-ebb tide) 47

Figure 3-11 Computational domain in the second scenario, with sea dike 48

Figure 3-12 Locations of exported results 50

Figure 3-13 Surface elevation at ebb tide, 10/20/2000 04:00, first scenario 51

Figure 3-14 Surface elevation at ebb tide, 10/20/2000 04:00, second scenario 51

Figure 3-15 Surface elevation at flood tide, 10/21/2000 17:00, first scenario 52

Figure 3-16 Surface elevation at flood tide, 10/21/2000 17:00, second scenario 52

Figure 3-17 Comparison water level elevation between two scenarios at P1-P4 53

Figure 3-18 Comparison water level elevation between two scenarios at P5-P8 54

Figure 3-19 Comparison water elevation between inside and outside reservoir , 55

Figure 3-20 Comparison water elevation between inside and outside reservoir, 55

Figure 3-21 Current speed at ebb tide, 10/24/2000 02:00, first scenario 58

Figure 3-22 Current speed at ebb tide, 10/24/2000 02:00, second scenario 58

Figure 3-23 Current speed at flood tide, 10/21/2000 17:00, first scenario 59

Figure 3-24 Current speed at flood tide, 10/21/2000 17:00, second scenario 59

Figure 3-25 Comparison current between two scenarios at P1-P4 60

Figure 3-26 Comparison current between two scenarios at P5 -P8 61

LIST OF TABLES

Table 1-1 Distribution of annual monsoon 12

Table 3-1 Locations of river boundaries and Vung Tau gauge station 39

Table 3-2 Results in Nash - Suteliffe coefficient 43

Table 3-3 Parameters after model calibration 45

Table 3-4 Design parameters of sea dike and sluice in the second scenario 49

Table 3-5 Locations of points to export results 50

Table 3-6 Comparsion estreme watel level between two scenarios at P1-P8 56

Table 3-7 Comparison extreme current speed between two scenarios at P1-P8 62

CHAPTER 1: INTRODUCTION

1.1 Background

The downstream of the Dong Nai river and the Vam Co river covering

10000 km2 plays an important role in the society and the economy of Viet Nam (Figure 1-1) This area includes Ho Chi Minh City, Dong Thap Muoi (belonging to Mekong Delta River), Vung Tau - Go Cong area and Tien Giang, Long An province with dense populations and concentration of business as well as intensely agricultural productions

Figure 1-1 Vung Tau- Go Cong Sea dike project (Source: Google Earth 2010)

The relative low elevation of this area in combination with the increasing extraction of drinking water and the flooding problem from the effect of sea level rise as well as salt intrusion are now of the primary concern of these areas The low-lying land area is located near the mouth of the large branches

of the Dong Nai and Vam Co River system (Figure 1-2) and therefore it is

strongly affected by the variation of the flow in the river and, an even more dominant factor, the tidal current Large reservoirs have been built upstream retaining the flood flow, leading to a decrease in the amount of average flood flow Conversely the effect of the tidal current is increasingly higher, resulting in salt intrusion and a lack of fresh water Human intervention in this area also diverts the direction of the flow and tidal current into the river, increasing water level at high tide and reducing water level at low tide causing an increase in the tidal range in the river mouths The higher the tidal ranges, the more tidal energy, the less time tides need to transport from the sea to the river basin The overall impact is that more areas are suffering from tidal flooding Moreover, the effect of the sea level rise will add to these impacts and make it more severe

In short, in the present situation, the study area is located in a complicated system of many estuaries which consist of intensive rivers and channel networks therefore it is strongly affected by tide from the sea Moreover this is a low land area therefore it is difficult for draining Human intervention including groundwater withdrawal and channel accretion have also contributed to land subsidence and, even lowers the elevation of the area

In other words, tidal influences and salt intrusion problems due to sea water penetrates are two main concerns of this area

Figure 1-2 Sai Gon - Dong Nai river basin

Natural conditions

Wind regime:

The study area is affected by two main direction of wind: Southwest monsoon and Northeast monsoon(Table 1-1)

Table 1-1 Distribution of annual monsoon

The southwest monsoon often starts in May and lasts to the end of September At offshore region, the main direction of wind is west with the maximum velocity of 10-12m/s In coastal area, due to continent the main direction of wind is Southwest and the average wind velocity and maximum wind velocity are 4-6 m/s and 8-10 m/s respectively

The northeast monsoon often starts in September and lasts to the end of March At offshore region, the main wind direction is Northeast and average wind velocity is 9-11 m/s, the maximum wind velocity is above 20 m/s In the coastal area, the main direction is Northeast and East, the average velocity and maximum velocity are 8-10 m/s and 12-14 m/s respectively

Oceanic regime and sedimentation:

The study area is strongly affected by tide from the East sea, the monsoon as well as flow regime in the Mekong and Sai Gon – Dong Nai rivers The total flow includes tidal flow, ocean current, river flow and coastal drift

The tidal regime is semi–diurnal tidal, tidal range is 3.5-3.6m The velocity of flood tide is 0.8-0.9 m/s, up to 1.2 m/s and velocity of ebb tide is 1.5-1.8 m/s

Respective to inconstant distribution of annual volume water, flow regime in Mekong and Sai Gon – Dong Nai rivers fluctuates following seasons For example, at Tan Chau station the discharge is up to 20.000 m3/s

in wet season, meanwhile the discharge is 3.000 m3/s in dry season

In wet season, the sediment content is higher than that in dry season The sedimentation in which rivers supply to sea is mostly in wet season

Topography:

In the area, topography is flat plain with the average elevation of (+0.7 - +0.8), the highest elevation of (+1.3 - +1.4), the lowest elevation of (+0.4 – 0.5) (Hung, 2011)

Social – economic features

The economic advantages of the location:

- Near to East Sea, the southern key economic region (fishing exploitation, marine eco-tour, shipping industry)

- Rich land resources with many types of land in which there is high fertility

- Due to the availability of many hydraulic structures, surface water resources are not saline

The disadvantages of the location:

- Low amount of rainfall, high evaporation

- No fresh ground water

- Salt intrusion during the whole year

- The old dike is strongly eroded making it unsafe in wet season

1.2 Problem statements

With the aim of solving the tidal flooding and salt intrusion (which are the most important problems of the study area), the Ministry of Agriculture and Rural Development (MARD) has proposed to construct a 32 km long system dike connecting Go Cong and Vung Tau (Figure 1-3) The main goals

of the dike procject are:

- To prevent inundation and saltwater intrusion for the whole are in short term and long term;

- To improve the capacity for drainage water;

- To create fresh water reserves in the future for the region to prevent any fluctuation in upstream and protec against the natural disaster from the sea;

- To shorten the transport distance between provinces in the West to Vung Tau The system dike will connect Vung Tau with the Southwest, creating a driving force to form new urban areas;

- To develop tourism and make socio-economic development for the entire region;

- To offer places to storm shelter for ships

Figure 1-3 Map showing the Vung Tau - Go Cong sea dike project (Source: Son ( 2012))

However, the sea dike could also bring in many disdadvantages regarding to the environment and navigation Some consequences could be mentioned:

- The Dong Tranh bay and Ganh Rai bay will become reservoirs to restore waste from uper stream;

- The change in tidal regime and salinity will destroy the ecological mangrove which is available from Soai Rap estuary to Long Tau estuary;

- The alteration in hydrodynamic flow could lead to sedimentations at Long Tau and Thi Vai estuaries Therefore deep water ports at these estuaries would be influenced

One of issues is the change in hydrodynamic flow regime in the area in case of having the project The thesis, therefore will focus on how the dike impacts to the hydrodynamic flow in research site

1.3 Objectives and Research questions

Objectives of the study are to:

- Analyze the impacts of the Vung Tau – Go Cong sea dyke on hydrodynamic flow regime

- Assess the influence of hydrodynamic flow regime changes on the coastal erosion in the study area

The methods are applied in this research could be mentioned:

- To analyze marine, meteorological, hydrological data in Go Cong – Vung Tau area

- To apply a mathematical model to simulate hydrodynamic flow in Go Cong – Vung Tau area

- To compare differences in the hydrodynamic flow regimes before and after having sea dyke

1.5 Structure of the thesis

The thesis consists of three chapters

The first chapter introduces background information, including: Locations; information of Meteorology, hydrology, topography; current situation; Problem statement; Objectives and research questions

The second chapter reviews some researches, projects regarding to estuaries in Mekong delta and study area Besides, over view about hydrodynamic models are also presented

The third chapter concluded: Governing equation; Model setting (using MIKE 21 model) for study area: Setting up calculation grid and scenarios; setting up parameters of model; Model calibration/validation; Application; Result and discussions

CHAPTER 2: LITERATURE REVIEW 2.1 The studies on Mekong Delta of foreign authors

In the continental shelf of Vietnam in general, coastal area from Binh Thuan to Ca Mau in particular, some international surveys were conducted in which NAGA (1959-1961) with Stranger vessel belong to Scripps Institution

of Oceanography, California NAGA programs finished 5 surveys from 11/1959 to 2/1961, from latitude 40N ÷ 160N with 6 sections perpendicular to shoreline and out to offshore 250 nautical miles, depth ≈ 4000m (3895m) The survey provided a large amount of raw data on the hydrodynamic characteristics, geology, biology, ecological-environment, contributing to further elucidate natural conditions in study area The results of the survey were published, the most significant of which was the work of Wyrtki (1961) However, due to a large scale of survey and huge gap between stations, the results can only reflect the large- stable oceanographic processes, According to Wyrtki (1961) pointed out that the entire East Sea was dominated by the monsoon regime, free laminar flow often had two opposite directions in seasons while major ocean currents did not flowed into

In recent years, many studies have focused on the areas affected by freshwater (Regions Of Fresh Water Influence-ROFI) A comprehensive summary report on ROFI dynamics was presented in the article by Simpson (1996) that in most of ROFI there was a mixture between stratification effects and stirring effect of wind, wave and tide It is clear that this mixture is difficult to determine than the thermal mixture, because the buoyancy of fresh water is not united in space but depending on separate inlet sources Due to such complexity, Simpson said that the understanding of the system ROFI was one of the biggest challenges that the oceanographers have faced today

When river water flows into estuaries, it is affected by the Coriolis force, most of them directs to Kelvin wave (in the northern hemisphere) But Simpson proposed that this flow can be diverted by the prevailing wind Another feature of the ROFI is under low friction conditions, buoyance driven flow may express condition of baroclinic instabilities resulting in meandering flow However, in shallow water or areas with strong tidal current, bottom friction often eliminates this uncertainty Another important feature of the ROFI is a distinct variation in vertical stratification which depends on tidal cycle In this condition, the tidal deformation plays a crucial role It could be proved that this feature may be caused by the difference between oblate ellipse of tidal surface and tidal bottom layer (Carbajal et al., 2004) Simpson also mentioned the tropical ROFI in Mekong estuaries According to Simpson, the sea shelf around Southeast Asia has to be interested because of the variation

in fresh water flowing into sea by the monsoon cycle

Hein et al (2007) was initially studied the dispersion of water in Mekong based on data field measurements in April/2007 Mekong River can be seen

as a typical example for a region of freshwater Influence (ROFI) The shallow region of the ROFI is defined by a great horizontal density gradient, characters the frontal zone in the ROFI This zone also shows the region of strongest vertical mixing The observations of a continuous station over 25 hours present the situation slightly off the shallow region of the ROFI The results point out that the stirring effect of tides, i.e a complex variation of vertical stratification during a tidal cycle To conduct these measurements accurately it is important to invest in three-dimensional hydrodynamic simulations Modeling of hydrodynamics in a ROFI is obviously linked with a good concept of the frontal processes between two water masses and their mixing Some simple simulations of frontal behavior show the basic necessity

to improve the advection scheme of hydrodynamic models The major problem is the numeric diffusion created by the algorithm of the advection scheme A basic one- dimensional model shows that an advection scheme using a limiter function is able to simulate the interaction between the Mekong discharge and the coastal water in a manner, that the diffusion and moreover the mixing process in the model is controlled by physical processes Altogether, the fundamental results of the ship experiments and the advection scheme studies allow an implementation of a hydrodynamic model, which approaches the governing physical processes of the ROFI and their variability

Figure 2-1 Overview of the station net realized during the cruise in April 2007

2.2 The related researches on the lower downstream of Sai Gon-Dong Nai river basin

So far, researches related to flow regime in the lower downstream of Sai Gon – Dong Nai river basin is limited Some of the searches such as: Trinh (2007), Hung (2011), Linh et al (2013) and Kim (2014) can be summerized

as follows:

Trinh (2007) conducted master thesis "Study on hydrodynamic regime and environment in southeastern region" The author has clarified hydrodynamic regime in Southeast, i.e: Difference in water level between months (July and November) about 40cm and trend in rising sea level with 0.5cm / year speed period from 1979 to 2003; Complicated attribute of tide and magnitude of tide is quite high along the coast of Vietnam; Current speed depended on season and direction its depended mainly on position; Wave regime rested on season, i.e: In winter (prevailing northeast monsoon) from November to April, observed wave direction was northeast, the average wave height of 1.25m and reached at maximum of 4m; Meanwhile in summer (prevailing southwest monsoon) from June to September, observed wave direction was southwest, the average wave height was low and maximized to 1.25m

Figure 2-2 Southeastern region (Source: Trinh (2007))

Hung (2011) conducted the research "study flow regime, sediment distribution in coastal area from Soai Rap estuary to Cua Tieu estuary, proposal solutions to prevent erosion at Go Cong dike in Tien Giang province

" One of the main tasks in this project is to find out the causes of coastal erosion by using computational model The simulation results have identified that natural factors were the main causes of coastal erosion in study area, including: (1) river flow and tide inlet at Soai Rap estuary; (2) waves during northeast monsoon The drawback of this research is limited sediment input data at upper boundary, therefore results express more trend than quantity Besides, the mathematical model used in the study did not consider the role of mangroves in limiting coastal erosion

Figure 2-3 Research area in Go Cong, Kien Giang province (Source: Hung (2011))

Linh et al (2013) did a research on "Research on the impacts of Go Cong Vung Tau sea dike on water quality at Saigon-Dong Nai estuary" The author has presented research results of water quality changes in Saigon - Dong Nai estuary with the application of 3D numerical models, the hydrodynamic and water quality EFDC The disadvantage of this article was that did not collect sufficient data about waste discharge sources in basin,

therefore water quality in reality could be worse than simulated model results On the other hand the model has just simulated water quality in flood season, dry season when this region has run the risk of being polluted, has not been concerned

Kim (2014) has done a research on “Study about the integrated solutions for controlling inundation and salt intrusion in the lower of Dong Nai – Sai Gon basin and adjacent to areas” The main contents of the project include: 1) Calculation in features of designed meteorology-hydrology (flood, rainfall and tide) to identify boundary conditions of hydraulic system; 2) Assessment and revision to effectiveness of solutions for controlling inundation according

to approved planning for preventing inundation in HCM city; 3) Evaluation the ability of controlling flood in whole river basin in the context of climate change/rising sea level in the long term (30 years, 50 years and 100 years); 4) Prediction for changes in hydrodynamic regime and water quality in reservoirs and main river system In order to answer the question should or should not we build the systems of Go Cong – Vung sea dike, Ministry of Science and Technology proposed a group of six projects concerning to the impacts of super sea dyke on hydrodynamic regime, navigation, ecological mangrove, environment and socio-economic developments in Go Cong – Vung Tau area The project which is conducted by Professor.Dr Nguyen Quang Kim is a key research to provide database data for five other projects

2.3 Overview of hydrodynamic models

To understand deeply what issues relating to a system, researchers could

do a field trip and measure This is called monitoring Monitoring is not always feasible due to it is expensive Moreover the researchers cannot not control the direction or the magnitude of the boundary conditions for the

whole area at the same time scale For these reasons they choose modeling There are two methods of modeling: 1) Physical and 2) Numerical

Physical modeling is a scaled model which is built in the laboratory to represent the prototype When the physical model is built, the modeler imposes the boundary conditions and records the model response, e.g wave height Some advantages of physical modeling are: Good visualization; Freedom from instability issues; However, there are still some drawbacks of physical modeling including: High setup and operating cost; Physical space issues

Numerical modeling is numerical simulation of the governing phenomena based on the solution of mathematical equations that approximate the physical conservation laws Computational Fluid Dynamics, CFD in brief, has been started in the 1940s and has been improved each year A typical CFD application contains three major steps: pre-processing, processing and pos processing Pre-processing covers procedures that modelers take to set up

a model, such as grid generation and making boundary conditions Processing includes the act of running the simulation Post processing involves visualization of results plus calculating other parameters of interest Keep in mind that numerical models solve the gorverning partial differental equations

as algebral equations with values at dicrete points; therefore the results are not continous Some advantages of numerical modeling could be mentioned: Low setup and operation costs; convenience in changing the geometry; Ability to make simultaneous runs In constrast, disadvantages of numerical modeling could be: Poor visualization capability for non-professinal audience; Computational limitations, for example truncation errors in the mathematical formulation

Numerical model can be one, two or three dimensional depending on which variations in dependent varialbe is more important For instance, the case of modeling flow in a river, the water level along the canal is more important than across the channel Obviously the gauge height might not be constant across a cross-section especially on a bend, but the 1-D information

is enough to solve many problems Two dimensional model to slove the gorverning equations in both x and y directions For instance, in the case of modeling a lake, a 1-D model cannot show the spatial variability in the flow while a 2-D model work for many interested parameters Three dimensional models have the capability to solve equations in x,y and z direction For example, if vertical salinity distribution through the water column in a lake is

of concern then a 3-D model will be used (Sina, 2014)

2.3.1 Possible models

After reviewing several available models, three models were selected for further consideration The Delft3D was developed by the Deltares Academy; MIKE 3 and MIKE 21 were developed by DHI Water and Environments One

of these models will be chosen in this thesis

a Delft3D

Delft 3D is able to model hydraulics, sediment transport, water quality, waves and morphology It uses a structured curvilinear orthogonal grid Delft3D supports wetting and drying simulation making it powerful model for flooding simulations Delft 3D has powerful pre-processing and post-processing tools

This model could apply for:

- River flow simulations

- Fresh- water river discharge in bays

- Salt intrusion

- Transport of dissolved material and pollutants

- Wave – driven currents (WL/Delft, 2009)

b MIKE 3

MIKE 3 is a computer program that simulates cohesive sediments, flows, water quality and ecology in lakes, rivers, bays, coatal areas and sea in three dimentions MIKE 3 was developed by Danish Hydraulic Institue (DHI) Water & Environment (Denmark) MIKE 3 is a fully 3D model and solves the momentum equation and continuity equations in the three Cartesian directions (DHI, 2007)

c MIKE 21

The model was developed by Danish Hydraulic Institue (DHI) Water & Environment (Denmark) in which rectangular mesh transforming to flexible mesh based on finite volume method An unstructured grid provides an optimal degree of flexibility in the presentation of complex geometries and enables smooth representation of boundaries Small elements are used in areas where more detail is desired, optimizing information for a given amount of computational time Moreover MIKE 21 model could simulate the mutual interaction between waves and currents using a dynamic coupling between the Hydrodynamic Module and the Spectral Wave Module (DHI, 2007)

2) Be freely or inexpensively available to non-commercial researchers

A selected model would preferably:

1) Have unstructured grid to be able of capturing complex geometry 2) Be able to run in parallel

3) Be easy to work with

4) Be easy to show the results

5) Have pre-processing and post processing capabilites included

The advantages of Delft3D and MIKE 3 models are the acuracy of results, however they require complex input data Meanwhile, MIKE 21 requires simple input data, it is user-frienly and fexible to show results Moreover it also satisfies the requirements of this thesis Therefore MIKE 21 will be chosen to study in this research

2.4 Overview of MIKE 21

MIKE 21 is one of the most effective tools for coastal modelling in the World This model could apply for: Design data for coastal and offshore structures; Desalination and recirculation analysis; Environmental impact assessment of marine infrastructure, etc…Some simulation engines are used

in MIKE 21, including: Single grid; Multiple grids and flexible mesh MIKE

21 integrates many modules that could be mentioned:

HD- Hydrodynamics: Simulating water level variations and flows in response to a variety of forcing functions

SW- Spectral waves: Simulating the growth, decay and transformation

of wind generated waves and swell

AD-Advetion dispersion: Simulating the transport, dispersion and decay

of dissolved or suspended substances

In order to simulate this research, two kinds of MIKE module could be chosen: MIKE 21 HD and MIKE 21 HD FM (FM, in brief of Flexible Mesh) MIKE 21 HD module uses rectangular grid meanwhile MIKE 21 HD FM uses triangular grid Because esturies are not usually rectangles, it is so difficult to present the boundaries of the estuary with a rectangular grid The advantage

of a triangular grid is that it is more flexible in the presentation of the mesh, because the mesh can be locally refined In this research, the boundary of rivers is complex, with advantage of a triangular grid, as a result MIKE 21

HD FM module will be used (DHI, 2007)

CHAPTER 3: ANALYSIS IMPACTS OF THE VUNG TAU – GO CONG

SEA DYKE ON HYDRODYNAMIC REGIME

In previous chapter, a wide range of numerical models which are appicable for the problem have been reviewed Finally, MIKE 21 HD FM has been selected This chapter will present the appication of chosen model to investigate the impacts of the Vung Tau – Go Cong sea dyke on the hydrodynamic regime with two scenarios of pre- and poste- project The contents of this chapter consist of the gorverning equation; the model setting such as computational domain, unstructured mesh generation, boudary condions, parameter setting; model calibration/validation; appication; result and discussions for the two scenarios with and without project

t Time (s)

x, y Cartesian co-ordinated in horizontal plane (m)

S Magnitude of discharge due to point source

η Water level above reference plane (m)

h Total water depth, h = d + η (m)

d Depth below plane of reference (m)

u, v Averaged depth velocity regarding to x, y direction (m/s)

ρ0 The reference density of water (kg/m³)

τsx, τsy The x and y components of the suface wind

τbx, τby The x and y components of the bottom stresses

xy

T ,T xy,T yy The lateral stresses, estimated using an eddy viscosity

S xx , S xy , S yx , S yy Radian stresses

u s , v s Velocity due to point source (m/s)

The discretization in solution domain is performed using a finite volume method The spatial domain is discretized by subdivision of the continuum into non-overlapping cells/elements In the two-dimensional case the elements can be arbitrarily shaped polygons, however, here only triangles and quadrilateral elements are considered In the three-dimensional case a layered mesh is used:

domain a structured discretization is used The elements can be prisms or bricks (hexahedrals) whose horizontal faces are triangles and quadrilateral elements, respectively The elements are perfectly vertical and all layers have identical topology

The integral form of the system of shallow water equations can in general form be written

Where Uis the vector of conserved variables, F is flux vector function, and

S is the vector of source terms

In Cartesian co-ordinate the system of 2D shallow water equations can be written





F hA

v hA

v u F

h

v v h

η z

Here U i and Si , respectively are average values of U and S over the ith cell

and stored at the cell centre, NS is the number of sides of the cell, nj is the unit

outward normal vector at the jth cell and ΔҐi the length/area of the jth interface Both a first order and a second order scheme can be applied for the spatial discretization

For the 2D case an approximate Riemann solver is used to calculate the convective fluxes at the interface of the cells Using the Roe’s scheme the dependent variables to the left and to the right of an interface have to be estimated Second – order spatial accurary is achiedved by empoying a linear gradient-reconstruction technique To avoid numerical oscillations a second order TVD slope limiter is used

For the 3D case an approximate Riemann solver is used to calculate the convective fluxes at the vertical interface of the cells (x’y’-plane) Using the Roe’s scheme the depedent variables to the left and to the right of an interface have to be estimated Second-order spatial accurancy is achieved by employing

a linear gradient- reconstruction technique To avoid numerical oscillatios a second order TVD slope limiter is used The convective fluxes at the horizontal interfaces (vertical line) are derived using first order upwinding for the low order scheme For the higher order scheme the fluxes are approximated by the mean value of the fluxes calculated based on the cell values above and below the interface for higher order scheme

shallow water equations and the transport equations: A low order method and a higher order method The low order method is a first order expicit Euler method

Where the h and v subscripts refer to horizontal and vertical terms,

respectively and the superscripts refer to invicid and viscous terms, respectively

As for 2D simulations, there is a lower order and a higher order time integration method

The low order method used for the 3D shallow water equations can written

The horizontal terms and the vertical convective terms are integrated using

a first order explicit Euler method and the vertical viscous terms are integrated using a second order implicit trapezoidal rule The higher order method can be written

The horizontal terms and the vertical convective terms are integrated using

a second order Runge Kutta method and the vertical terms are integrated using a

second order implicit trapezoidal rule for the vertical terms

(DHI, 2007)

3.2 Model setting

3.2.1 Study area

Based on the Vung Tau- Go Cong map and collected data of bathymetry\

an area of interest was chosen The computed domain is described as follow: Latitude: 1080000 – 1160000; Longtitude: 670000-770000 Figure 3.2 shows the area of study picked to generate the grid calculation