ERASMUS MUNDUS MSC PROGRAMME
COASTAL AND MARINE ENGINEERING AND MANAGEMENT
COMEM
MORPHOLOGICAL MODELLING OF LAI GIANG INLET,
VIETNAM
Trinh Dieu Huong
Delft University of Technology
June 2010
The Erasmus Mundus MSc Coastal and Marine Engineering and
Management is an integrated programme organized by five European
partner institutions, coordinated by Delft University of Technology (TU
Delft). The joint study programme of 120 ECTS credits (two years fulltime) has been obtained at three of the five CoMEM partner
institutions:
• Norges Teknisk- Naturvitenskapelige Universitet (NTNU) Trondheim,
Norway
• Technische Universiteit (TU) Delft, The Netherlands
• City University London, Great Britain
• Universitat Politècnica de Catalunya (UPC), Barcelona, Spain
• University of Southampton, Southampton, Great Britain
The first year consists of the first and second semesters of 30 ECTS
each, spent at NTNU, Trondheim and Delft University of Technology
respectively.
The second year allows for specialization in three subjects and during
the third semester courses are taken with a focus on advanced topics
in the selected area of specialization:
• Engineering
• Management
• Environment
In the fourth and final semester an MSc project and thesis have to be
completed.
The two year CoMEM programme leads to three officially recognized
MSc diploma certificates. These will be issued by the three universities
which have been attended by the student. The transcripts issued with
the MSc Diploma Certificate of each university include grades/marks
for each subject. A complete overview of subjects and ECTS credits is
included in the Diploma Supplement, as received from the CoMEM
coordinating university, Delft University of Technology (TU Delft).
Information regarding the CoMEM programme can be obtained from
the programme coordinator and director
Prof. Dr. Ir. Marcel J.F. Stive
Delft University of Technology
Faculty of Civil Engineering and geosciences
P.O. Box 5048
2600 GA Delft
The Netherlands
DELFT UNIVERSITY OF TECHNOLOGY
FACULTY OF CIVIL ENGINEERING AND GEOSCIENCE
SECTION OF HYDRAULIC ENGINEERING
DEPARTMENT OF COASTAL ENGINEERING
MORPHOLOGICAL MODELLING OF LAI GIANG INLET,
VIETNAM
Trinh Dieu Huong
Graduation committee
Prof.dr.ir. M.J.F. Stive (Chairman, Delft University of Technology)
Dr.ir. Z.B. Wang (Deltares/ Hydraulic Engineering, Delft University of Technology)
Drs. N. Geleynse (Geo-Engineering, Delft University of Technology)
Ir. M. Eelkema (Hydraulic Engineering, Delft University of Technology)
A dissertation submitted in partial fulfillment of the degree of
MSc Coastal and Marine Engineering and Management
June 2010
Morphological Modelling of Lai Giang inlet, Vietnam
SUMMARY OF RESEARCH
The Lai Giang inlet located in Binh Dinh province incorporates various features of the
estuaries in the Central part of Vietnam. Belonging to the micro-tidal and wave-dominant
coast and influenced by the monsoon regime, the inlet has a seasonal character. During
the dry season, as the river flow diminishes, the wave action causes high level of
sedimentation and closes up the inlet eventually. In flood season, as the river discharge is
high, the channel is scoured and the inlet begins to migrate. The high sedimentation level
and migration of the Lai Giang inlet has been a serious problem of Binh Dinh province for
a long time, because it is the only exit for the floodway. It is an anchorage and also the
connection between the sea and the aquaculture area of Hoai Nhon district. The high
level of sedimentation at the entrance of the inlet prevents river flood from flowing
smoothly, thus leading to overflow in lowlands and navigation issues.
In recent decades, the exploitation and protection of Lai Giang area have been studied in
various forms of scientific researches and projects by different scientists and local
professional agencies. However, the studies have only focused on hydraulics, hydrology
and on adjusting the flow of Lai Giang river. There are only general and basic studies on
the entrance of the inlet.
The main objective of this research is to understand the morphological behaviour of Lai
Giang inlet. The specific interest is focused on the main factors which are the tidal
characteristic, the wave climate and the river flow during the flood season, and the
interaction between all these factors that influence the morphological changes. The study
starts with the collecting and analysing all the documentations to come up with a
conceptual model of the Lai Giang inlet to explain how the sedimentation and the
migration processes happen. Then, the Delft3D modelling software, which can model
(tidal) flow, waves and sediment transport, has been applied to confirm the hypothesis
and gain further knowledge. According to the data analysis, the conceptual model as well
as the descriptive and quantitative result of the model, we can make the following main
conclusions: 1. The wave climate in this area has seasonal characteristic and is dominated
by two main directions; Northeast and Southeast in winter and summer monsoon,
respectively. 2. During the summer monsoon, the longshore sediment transport moves
towards the north, bypasses the entrance of the inlet and gradually builds up on the
down-drift spit due to the Southeast Wave. 3. During winter monsoon, the Northeast wave
intensifies the southward longshore sediment transport leading to the large amount of
sedimentation in front of the inlet. At the same time, the significant river flow flushes
away the sediment deposits at the main ebb channel located nearer to the up-drift spit.
Thus the sedimentation could not take place at the up-drift spit. The sediment
displacement at the up-drift and down-drift spit made the inlet migrate to the north
gradually. Finally, the possibility to stabilize the inlet is discussed to give the optimum
solution for this area.
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Morphological Modelling of Lai Giang inlet, Vietnam
ACKNOWLEDGEMENTS
First and foremost, I would like to thank the members of my graduation committee –
Prof.dr.ir. Marcel Stive, Dr.ir. Zheng Bing Wang, Drs. Nathanael Geleynse and Ir. Menno
Eelkema for their valuable comments and suggestions. Without their helps, this thesis
could not be completed. I express my deep gratitude especially to Menno for his
dedicated guidance and patience. My sincere thanks to Madelon Burgmeijer for keeping
me stay on track and helping me improve my writing.
I would like to thank Key Laboratory of River and Coastal Engineering, Vietnam Academic
for Water Resources, especially Assoc.Prof.Dr. Trinh Viet An for his helpful support and
assistance.
My fellow CoMEM mate, Serene Tay thank you for your explanation of difficult
terminology and helping me with the Delf3D model in the beginning. To my CoMEM
graduates, you all have made these two years memorable.
Last but not least, I would like to express my thanks to my beloved parents for their love,
encouragement and caring from my home town throughout the whole course of this MSc
thesis.
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Morphological Modelling of Lai Giang inlet, Vietnam
TABLES OF CONTENT
SUMMARY OF RESEARCH...................................................................................................1
ACKNOWLEDGEMENTS ......................................................................................................2
TABLES OF CONTENT .........................................................................................................3
LIST OF ABBREVIATIONS ....................................................................................................5
LIST OF SYMPOL.................................................................................................................5
LIST OF FIGURES ................................................................................................................6
LIST OF TABLES ..................................................................................................................8
CHAPTER 1. INTRODUCTION ..............................................................................................9
1.1. Study area ............................................................................................................... 9
1.2. Coastal inlet .......................................................................................................... 10
1.3. Hydrodynamic classification .................................................................................. 11
1.4. Inlet stability ......................................................................................................... 13
1.5. Objective............................................................................................................... 14
1.6. Thesis structure..................................................................................................... 15
CHAPTER 2. PHYSICAL SETTING ........................................................................................16
2.1. Climate condition .................................................................................................. 16
2.1.1. Monsoon regime ............................................................................................ 16
2.1.2. Wind .............................................................................................................. 16
2.1.3. Rainfall ........................................................................................................... 18
2.2. Hydrology condition.............................................................................................. 18
2.2.1. River basin...................................................................................................... 18
2.2.2. River flow ....................................................................................................... 20
2.3. Coastal characteristics........................................................................................... 21
2.3.1. Tide ................................................................................................................ 21
2.3.2. Wave.............................................................................................................. 23
2.3.3. Current........................................................................................................... 24
2.3.4. Typhoons........................................................................................................ 24
2.4. Sediment............................................................................................................... 26
CHAPTER 3. CONCEPTUAL MODEL ...................................................................................27
3.1. Historical changing process ................................................................................... 27
3.1.1. Based on the historical document .................................................................. 27
3.1.2. Based on the Remote sensing image .............................................................. 28
3.1.3. Based on the survey data ............................................................................... 30
3.2. Conceptual model ................................................................................................. 31
3.2.1. Flood season .................................................................................................. 32
3.2.2. Dry season...................................................................................................... 33
3.2.3. Summary of the morphological process in one year ....................................... 33
CHAPTER 4. PROCESS-BASED MODELLING .......................................................................36
4.1. Hydrodynamic model ............................................................................................ 36
4.1.1. Model domain ................................................................................................ 36
4.1.2. Calibration...................................................................................................... 39
4.1.3. Model setup ................................................................................................... 48
4.2. Wave model.......................................................................................................... 49
4.2.1. Wave data analysis ......................................................................................... 49
4.2.2. Typical year of wave climate........................................................................... 54
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Morphological Modelling of Lai Giang inlet, Vietnam
4.2.3. Scenarios and model setup............................................................................. 56
4.3. Sediment transport ............................................................................................... 58
4.3.1. Data processing .............................................................................................. 58
4.3.2. Model domain ................................................................................................ 60
CHAPTER 5. MODELLING RESULT: DESCRIPTION AND ANALYSIS.......................................63
5.1. Tide ....................................................................................................................... 64
5.2. Wave..................................................................................................................... 67
5.3. Flow ...................................................................................................................... 69
5.3.1. Tidal current ................................................................................................... 69
5.3.2. Tide- and wave- induced current .................................................................... 69
5.3.3. Current influenced by tide, wave and river flow ............................................. 74
5.4. Sediment transport ............................................................................................... 76
5.4.1. Influence of tidal regime................................................................................. 76
5.4.2. Influence of the individual wave climate......................................................... 77
5.4.2. Influence of the total wave climate ................................................................ 77
5.4.3. Influence of the interaction between the river flow and wave climate ........... 78
5.4.4. Influence of the storm .................................................................................... 82
5.4.5. Longshore sediment transport of Lai Giang inlet ............................................ 82
5.5. Limitation of the morphodynamic model .............................................................. 84
CHAPTER 6. ENGINEERING SOLUTION..............................................................................86
6.1 Increasing the basin area........................................................................................ 86
6.2 Dredging ................................................................................................................ 86
6.3 Jetties .................................................................................................................... 86
6.4 Sand fluidization..................................................................................................... 87
6.5 Optimum solution .................................................................................................. 88
CHAPTER 7. CONCLUSION AND RECOMMENDATION .......................................................89
7.1. Conclusion ............................................................................................................ 89
7.2. Recommendation.................................................................................................. 91
REFERENCES ....................................................................................................................93
APPENDIX ........................................................................................................................95
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Morphological Modelling of Lai Giang inlet, Vietnam
LIST OF ABBREVIATIONS
E
East
N
North
NE
Northeast
NW
Northwest
S
South
SE
Southeast
SW
Southwest
W
West
LIST OF SYMPOL
ρ
kg/m3
Concentration of suspended sediment
∆h
m
Tidal range in the estuary
A
m2
Cross section area
D50
mm
Medium diameter of sediment
Mmax
m3
Total littoral drift
P
m3
Tidal prism
R
m
Hydraulic radius of the channel
r
-
Parameter define the stability of the inlet
Vcr
m/s
Critical velocity
Vm
m/s
Maximum entrance channel velocity
x
m
Geometric parameter
Qmax
m3/s
Maximum river discharge
Qmin
m3/s
Minimum river discharge
Hs
m
Significant wave height
k
-
Power relation between transport and wave height
MI
-
Morphological impact
P
-
Probability of occurrence
Hmor
m
Representative morphological wave height
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Morphological Modelling of Lai Giang inlet, Vietnam
LIST OF FIGURES
Figure 1.1 Lai Giang inlet system........................................................................................ 9
Figure 1.2 Geological model of a tidal inlet (Boothroyd, et. al., 1985) .............................. 10
Figure 1.3 Hydrodynamic classifications of tidal inlets (Hayes, 1979) ............................... 12
Figure 1.4 Channel velocity geometry relationship........................................................... 13
Figure 1.5 Structure of the thesis ..................................................................................... 15
Figure 2.1 Wind rose at Quy Nhon station from 1981 to 2005 ......................................... 17
Figure 2.2 The average yearly rainfall in Lai Giang area .................................................... 18
Figure 2.3 Overview of Lai Giang river basin and the station’s position (Thanh, 2009) ..... 19
Figure 2.4 Measurement of river discharge in a flood in November 2008 ........................ 21
Figure 2.5 Correlation of water level between Tam Quan station and Quy Nhon station . 22
Figure 2.6 Correlation of water level between Hoai Hai and Quy Nhon station ................ 22
Figure 2.7 Water level differences between Tam Quan and Hoai Hai station ................... 22
Figure 2.8 Wave rose in 10 years ..................................................................................... 23
Figure 3.1 Lai Giang inlet.................................................................................................. 27
Figure 3.2 Lai Giang inlet at different times...................................................................... 29
Figure 3.3 Topography of the area in different period...................................................... 30
Figure 3.4 The changes in topography of the inlet between April 2005 and September
2007 ................................................................................................................................ 31
Figure 3.5 Conceptual model of the Lai Giang inlet .......................................................... 32
Figure 3.6 Net effect of the inlet ...................................................................................... 34
Figure 4.1 The domain of hydrodynamic model for Lai Giang inlet................................... 37
Figure 4.2 Bathymetry, boundary and observation point of the model ............................ 38
Figure 4.3 Magnitude of the velocity outside the inlet ..................................................... 40
Figure 4.4 Direction of the velocity outside the inlet........................................................ 41
Figure 4.5 Water level inside the river ............................................................................. 41
Figure 4.6 Water level at Tam Quan station (along the coast) .......................................... 41
Figure 4.7 Water level at Hoai Hai station (along the coast) ............................................. 42
Figure 4.8 Comparison of the magnitude of the velocity with the adjustment of several
factors ............................................................................................................................. 43
Figure 4.9 Comparison of the direction of the velocity with the adjustment of several
factors ............................................................................................................................. 43
Figure 4.10 Comparison of the water level with the adjustment of several factors .......... 43
Figure 4.11 Computed magnitude of the velocity of the model with the phase adjustment
forward of the North boundary ....................................................................................... 45
Figure 4.12 Computed magnitude of the velocity of the model with the phase adjustment
backward of the North boundary..................................................................................... 45
Figure 4.13 Computed magnitude of the velocity of the model with addition amplitude of
the North boundary ......................................................................................................... 45
Figure 4.14 Computed magnitude of the velocity of the model with reduction amplitude
of the North boundary ..................................................................................................... 46
Figure 4.15 Computed magnitudes of the velocity of the model with adjustment in phase
and amplitude of the North boundary ............................................................................. 46
Figure 4.16 Calibration result in the magnitude of the velocity ........................................ 47
Figure 4.17 Calibration result in the direction of the velocity ........................................... 47
Figure 4.18 Calibration of water surface elevation inside the river................................... 47
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Morphological Modelling of Lai Giang inlet, Vietnam
Figure 4.19 Wave rose for 10 years.................................................................................. 50
Figure 4.20 Characteristics of wave climate and its influence on morphodynamics of Lai
Giang ............................................................................................................................... 53
Figure 4.21 Annual wave rose from 1996 to 2005 ............................................................ 55
Figure 4.22 Wave rose in 2003 with full and sorted data.................................................. 60
Figure 4.23 Morphodynamic modelling process............................................................... 62
Figure 5.1 Water level condition at the boundary of the dominant wave scenarios ......... 64
Figure 5.2 Water level condition at the boundary of the one year scenario ..................... 64
Figure 5.3 Computed water level at the entrance of the inlet .......................................... 65
Figure 5.4 Tidal propagate in Lai Giang inlet area............................................................. 66
Figure 5.5 Wave propagate in Lai Giang inlet area under difference directions ................ 68
Figure 5.6 Tidal induced current at Lai Giang inlet area.................................................... 71
Figure 5.7 Tidal induced and NE wave induced current at Lai Giang inlet area ................. 72
Figure 5.8 Tidal induced and SE wave induced current at Lai Giang inlet area .................. 73
Figure 5.9 Water level condition at different boundary of the model............................... 74
Figure 5.10 Tidal induced current under river flow condition without and with NE wave
climate............................................................................................................................. 75
Figure 5.11 Cumulative sedimentation (positive) and erosion (negative) under tidal
influence.......................................................................................................................... 76
Figure 5.12 Cumulative sedimentation (positive) and erosion (negative) under the
influence of NE and SE Wave ........................................................................................... 79
Figure 5.13 Seasonal cumulative sedimentation (positive) and erosion (negative) in a
typical year ...................................................................................................................... 80
Figure 5.14 Cumulative sedimentation (positive) and erosion (negative) after the flood in
November 2008 with and without wave influence........................................................... 81
Figure 5.15 Cumulative sedimentation (positive) and erosion (negative) after Lingling
storm............................................................................................................................... 82
Figure 5.16 Transects and positive directions for sediment transport computation at the
inlet ................................................................................................................................. 83
Figure 6.1 The proposed position of jetties ...................................................................... 87
Figure 6.2 Flow in fluidized channel (Parks, 1993) ............................................................ 88
Figure 7.1 Illustration of morphological change in summer and winter monsoon ............ 91
Figure A. 1 Velocity at the inlet during flood in November 2008 with and without wave
influence..........................................................................................................................95
Figure A. 2 Discharge through the inlet cross section during flood in November 2008 .....95
Figure A. 3 Magnitude of the velocity during the flood in November 2008 ......................96
Figure A. 4 Cumulative sedimentation (positive) and erosion (negative) after a typical year
of tide and wave ..............................................................................................................97
Figure A. 5 Wave rose during Lingling storm ....................................................................97
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Morphological Modelling of Lai Giang inlet, Vietnam
LIST OF TABLES
Table 1.1 Hydrographical classifications of coast and tidal inlet ....................................... 11
Table 1.2 Classification of wave climate ........................................................................... 12
Table 2.1 Wave statistic in 10 years ................................................................................. 24
Table 4.1 Co-ordinate of the simulation area ................................................................... 37
Table 4.2 Tidal components of the study area from the South China Sea model .............. 39
Table 4.3 Tidal components of My A inlet ........................................................................ 39
Table 4.4 Observation data for calibrating model ............................................................ 40
Table 4.5 Flow model setup ............................................................................................. 48
Table 4.6 Wave statistic in Lai Giang ................................................................................ 49
Table 4.7 Wave height and wave direction effect on morphodynamic of Lai Giang area .. 51
Table 4.8 Wave statistic comparison between 10 years observation, 2001 and 2003....... 56
Table 4.9 Wave model setup............................................................................................ 56
Table 4.10 Tidal statistic by year ...................................................................................... 58
Table 4.11 Tidal statistic by month................................................................................... 59
Table 4.12 Comparison of tidal components value........................................................... 59
Table 4.13 Comparison of wave characteristic in between full data and sorted data in
2003 ................................................................................................................................ 60
Table 4.14 Addition setup for morphodynamic model ..................................................... 60
Table 5.1 Sediment transport through transects (m3/day) ............................................... 83
Table 6.1 Comparison between the propose solution and criteria (x: good; o: bad; -:
unknown)......................................................................................................................... 88
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Morphological Modelling of Lai Giang inlet, Vietnam
CHAPTER 1. INTRODUCTION
1.1. Study area
Lai Giang inlet (Figure 1.1) is located in Binh Dinh province that is in the southern part of
the Central of Vietnam. It connects the Lai Giang River and the Dam Gai lagoon system
with the sea. The lagoon system has a surface area of approximately 10 km2 and
elongates 10 kilometres along the coastline. Currently, only the northern lagoon is
affected by the current and the river flow, the southern part is almost isolated for the
aquaculture purposes. This area has a complex coastal topography with the Tam Quan
head land in the North and the alternatively rocky and sandy coast in the south.
Lai Giang inlet reflects outstanding features of the coastal inlet in the Central of Vietnam
with micro-tidal and wave-dominated environment as well as monsoon influenced region.
The opening of the inlet is maintained by river flow in the flood season. In the dry season,
the inlet has highly amount of sedimentation, and closes up during strong wave action.
The complex dynamics between river, lagoon and sea, which lead to the movement of
sandy barrier, cause instability of the inlet.
Figure 1.1 Lai Giang inlet system
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Morphological Modelling of Lai Giang inlet, Vietnam
1.2. Coastal inlet
Coastal inlets are created by cutting through the barrier islands under storms,
hydrodynamic action or human’s interference to exchange water between the lagoon and
the sea (Dean and Dalrymple, 2002). Tidal basins, including tidal lagoons and estuaries
with their associated coastal inlets, interrupt a significant part of the world’s shorelines
(Stive and Wang, 2003).
Vietnam is bordered with 3,260 km of coastline and about more than 100 inlets. With all
the common feature of coastal inlets, coastal inlets in Vietnam also play an important role
in environment and coastal processes as well as water way. Besides their potentiality and
advantage, it has the drawback which is the significant amount of sedimentation, the
biggest existences of estuaries of Vietnam. These problems have influenced considerably
on floodway system, navigation and eco-system of the area.
Figure 1.2 Geological model of a tidal inlet (Boothroyd, et. al., 1985)
Morphodynamics of coast inlet is controlled by the interaction between the river and
ocean processes under the meteorological condition. Boothroyd, 1985 described all the
feature of inlet in Figure 1.2 above. According to Bruun (1978), inlet has three main parts
which are flood tidal delta, tidal gorge and ebb tidal delta.
•
Ebb tidal delta is the ocean part of the inlet. The overall morphology of this part
depends on the interaction between wave action and tidal current or is
10
Morphological Modelling of Lai Giang inlet, Vietnam
determined by the balance between net sediment flux driven by ebb current and
driven by wave current.
•
Flood tidal delta is the bay part of the inlet. In this area, the flood current and the
river flow interact with each other and there is a relatively small wave action.
•
Tidal gorge is the channel part between flood and ebb tidal delta.
1.3. Hydrodynamic classification
According to Hayes (1979), coastal inlet classification is based on a combination of tidal
range and wave energy classification (as described in Table 1.1 and 1.2 below) because of
the importance of the relative effect between waves and tides. He defined that there are
five classes of tidal inlet such as:
•
Wave dominant inlets have long and continuous barrier, only a few tidal inlet and
a lot of over-washes.
•
Mixed energy (wave dominant) inlets have a numbers of inlet but less over-wash
area. Moreover, the ebb tidal data becomes larger in this type of inlet.
•
Mixed energy (tide dominant) inlets have larger number of tidal inlet as well as the
increasing in size of ebb tidal area and drum stick barrier.
•
Tidal dominated (low) inlets occasionally build bars.
•
Tidal dominated (low) inlets is characterised by large ebb delta, deep inlet and
intensive salt marsh and tidal flat.
Table 1.1 Hydrographical classifications of coast and tidal inlet
Class
Tidal range (m)
Microtidal
< 1.0
Low mesotidal
1.0 – 2.0
High mesotidal
2.0 – 3.5
Low macrotidal
3.5 – 5.5
High macro tidal
> 5.5
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Morphological Modelling of Lai Giang inlet, Vietnam
Table 1.2 Classification of wave climate
Wave energy class
Mean significant wave height (m)
Low wave energy
< 0.6
Medium wave energy
0.6 – 1.5
High wave energy
> 1.5
Figure 1.3 Hydrodynamic classifications of tidal inlets (Hayes, 1979)
Figure 1.3 – Hydrodynamic classification of tidal inlet (Hayes, 1979) indicates that Lai
Giang inlet belongs to the wave – dominant inlet class with the tidal range of 0.9 meters
and the mean significant wave height mainly in a range from 1.0 to 1.5 meters.
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Morphological Modelling of Lai Giang inlet, Vietnam
1.4. Inlet stability
In 1940, Escoffier introduced the method to define the stability of the inlet cross section
by the relationship between maximum entrance channel velocity (Vm) and the geometric
parameter (x) which is combined from three variables: hydraulic radius of the channel R,
cross section area A and the tidal range in the estuary ∆h under the constant assumption
of roughness, area of the estuary and tidal range. Beside, the concept of critical velocity
Vcr was given; if the velocity in the channel is below this value, it is too low to cause
erosion.
Figure 1.4 Channel velocity geometry relationship
The stability of the inlet is described in Figure 1.4. On section A – B of the curve, the
channel is too small and the friction is too high to maintain it self, thus the inlet will be
close up eventually. If the channel belongs to the D – E section of the curve, the geometry
of the inlet becomes smaller but the channel velocity increases, so sedimentation will
take place until D is reached. On the last section B – D, the inlet is eroded until reaching
point D. D point is the stable situation.
With the Escoffier’s foundation, O’Brien (1969), Jarret (1976) and Shigemura (1980) found
the relationship between minimum cross section of the equilibrium channel A (m2) and
the volume of the tidal prism P (m3):
A = 6.56 x 10-5 P
Later, Bruun and Gerritsen (1960) and Bruun (1978) proposed a parameter r to define the
stability of the inlet based on the sediment by-passing capacity:
r = P / Mmax
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Morphological Modelling of Lai Giang inlet, Vietnam
in which P is tidal prism (m3) and Mmax is total littoral drift (m3/year)
Sediment by-passing characteristic of the inlet depends on the value of parameter r:
•
r < 20: Inlets become unstable non-permanent overflow channel;
•
20 < r < 50: The inlets area typical bar-by-passer;
•
50 < r < 150: The entrance bars are still pronounced (combination of bar-bypassing and flow-by-passing);
•
r > 150: The inlets are predominant tidal flow by-passer (little bar and good
flushing).
1.5. Objective
Due to the migration and sedimentation problem of Lai Giang inlet, the objective of this
thesis is to understand the morphological behaviours of Lai Giang inlet. The study will be
carried out in three steps as followed:
1. Based on different kinds of documents, give the main idea of the mechanism
influencing on the morphological changes of the inlet (or the migration and
sedimentation problem, specifically), which are connected with the conceptual
model.
2. Applying numerical modelling to confirm the mechanism as well as gain further
understanding of the phenomenon:
•
Influence of wave on longshore current: by running two representative wave
directions in two seasons and under extreme condition (storm).
•
Influence of wave on sediment transportation under condition of two
representative wave directions in two seasons and extreme condition (storm).
•
Influence of the interaction between river flow and northern wave on sediment
transportation during flood.
•
The morphodynamical process with one typical year of wave and representative
tide.
3. Discuss a possible solution to stabilize the inlet.
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Morphological Modelling of Lai Giang inlet, Vietnam
1.6. Thesis structure
The structure of the thesis is indicated in Figure 1.5. After Chapter 1 with the introduction
of the thesis, Chapter 2 will describe in detail the physical setting of the tidal inlet system.
Chapter 3 focuses on primary conceptual morphological model of Lai Giang inlet base on
all available documentaries. In Chapter 4 and Chapter 5, data preparation, model
calibration, model result and analysis are described. Chapter 6 proposes the Engineering
solution to stabilize the inlet. Last, Chapter 7 contents the conclusion of the thesis and the
recommendation for further research .
Figure 1.5 Structure of the thesis
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Morphological Modelling of Lai Giang inlet, Vietnam
CHAPTER 2. PHYSICAL SETTING
Lai Giang inlet is the connection between Lai Giang River and the sea. Thus, it is strongly
influenced by both river and ocean conditions. These conditions have seasonal effects
driven by meteorological factors. In the dry season, the river discharge is relatively small,
sometime negligible; ocean factor becomes dominant causing sedimentation in the inlet
area. Opposite to the dry season, river discharge in the flood season is quite large;
moreover, there is a presence of typhoons during this period leading to the complex
interaction between the sea and the river. This chapter will describe in detail these three
factors and its interaction influencing on morphodynamic of Lai Giang inlet area.
2.1. Climate condition
2.1.1. Monsoon regime
The Lai Giang area has a strong effect of monsoon regime causing two-season climates for
this area such as the winter monsoon and the summer monsoon.
Northern monsoon or winter monsoon lasts from October until February the next year.
The wind comes from the Northeast direction, travels across the South China Sea,
bringing humidity, leading to high precipitation in this period.
Southern monsoon or summer monsoon happens during the remaining 8 months of the
year with the Southeast and the Southwest wind, mainly. In this season, the Southwest
winds with high humidity cross the Truong Son mountain causing the rain in the Western
side and the dry and hot climate in the Eastern side, so called the “Font-effect”. “Fonteffect” is the reason for the hot and dry climate in Lai Giang area from March to October
every year.
2.1.2. Wind
According to the meteorological data at the Quy Nhon station from 1981 to 2005, the
highest wind speed in this area is at 40 m/s in North direction (26th October 1995) and
Northwest direction (7th November 1984).
Observation data showed that the northern wind is dominant, account for 15.31 % of the
total. The frequency of NW, SE direction is 11.13 % and 8.64 %, respectively. There is
35.39 % calm wind. In more detail, wind with the N and the NW direction dominate from
October to March next year; the SE direction in next 3 months, conversely and the W
direction in July and August.
16
Morphological Modelling of Lai Giang inlet, Vietnam
Figure 2.1 Wind rose at Quy Nhon station from 1981 to 2005
17
Morphological Modelling of Lai Giang inlet, Vietnam
2.1.3. Rainfall
The precipitation in this area is mainly driven by the monsoon regime. Besides that, it is
also affected by the steep topography with the mountain in the west and low land in the
east. Thus, the rain fall varies in space and time.
The largest amount of rain fall focuses on the last four months of the year, accounting for
80 % of the total amount in a year. October and November have the largest amount,
make up more than 50 %, leading to the flood in these two months. Accompanying with
the large precipitation, there are the presence of typhoons in October and November,
causing the strong influence in the morphology of this area. From January to August, the
amount of rain fall is quite small, less than 100 mm.
The rainfall from the North (Hoai Nhon station) to the South (Quy Nhon station) of the
study area is illustrated in Figure 2.2. Obviously, the amount of precipitation is reducing
from the North to the South with the difference of 50 mm between the two stations.
600
Quy Nhon station
Rain Fall (mm)
500
Hoai Nhon station
400
300
200
100
0
Jan
Feb
Mar
Arp
May
Jun
July
Aug
Sep
Oct
Nov
Dec
Figure 2.2 The average yearly rainfall in Lai Giang area
2.2. Hydrology condition
2.2.1. River basin
Lai Giang is the second largest river of Binh Dinh province with 1466 km2 of the
catchment’s area. It is the combination of two channels An Lao and Kim Son.
•
An Lao channel has the length of 75 kilometres and the catchment’s area of 697
km2. This channel originates from a mountainous area in the Northwest of Quang
18
Morphological Modelling of Lai Giang inlet, Vietnam
Ngai province, flows from the North to the South crossing the Hoai An and An Lao
district of Binh Dinh province.
Figure 2.3 Overview of Lai Giang river basin and the station’s position (Thanh, 2009)
•
Kim Son channel has almost the same catchment’s area as the An Lao channel
with the diction from Northwest to Southeast.
19
Morphological Modelling of Lai Giang inlet, Vietnam
The two channels merge together at Lai Khanh and flow into the sea through the Lai
Giang inlet with the SW – NE direction. After merging, Lai Giang River is wide and shallow.
In the right side of the river, there is a presence of the mountain running along the river
until the sea. The left side of the river is low land area which has potential inundation in
the flood season.
2.2.2. River flow
Driven by the precipitation season, the river flow also has two seasons: a flood and a dry
season. The flood season in Lai Giang river occurs mainly in October and November
(sometimes in December). The amount of river discharge in these two months account for
75% of the total river discharge in a year. The dry season starts from January until
September and the total discharge (mainly supplied from ground water) in these 9
months only equal one third of the discharge in three months of the flood season.
The unequal of monthly river discharge distribution in a year is described clearer by the
data analysis and the extreme event. The average river discharge in November is 117
m3/s, equals 29 times the average value in August (4.0m3/s). Moreover, the measurement
at An Hoa station recorded that the maximum river discharge happened in 19th November
1987 at the value of 5880 m3/s (Qmax) and the minimum value was 1.82 m3/s (Qmin) ( on 6th
June 1983). The rate between the maximum value and the minimum value is Qmax /Qmin =
3230. The great difference between the river discharge in the flood season and the dry
season cause the potential sedimentation in the river and at the entrance of the inlet.
The flood in the Lai Giang River happens in a short time, only lasts three days due to the
steep topography and is the combination of flooding in two channels: An Lao and Kim Son
making up 66 % and 34 %, respectively. Figure 2.4 is an example of a flood which occurred
in November 2008. The flood started from 21:00 18th and finished at 11:00 21st November
2008 (less than 3 days). The measurement data had been taken in 3 points which are Lai
Giang (near Lai Giang inlet area), My Thanh (at An Lao channel) and An Thuong (at Kim
Son channel). As we can see, the river discharge in An Lao channel is double the one in
Kim Son channel. According to the data from the river flow station, the velocity in the
flood season can be from 3 to 5 meter per second upstream of the river and reduce 1 ÷ 3
meter per second at the inlet area. This value of velocity leads to the increase of the
sediment transport capacity which directly affects the morphology of Lai Giang inlet area.
20
2600
2400
2200
2000
1800
1600
1400
1200
1000
800
600
400
200
0
Lai Giang
My Thanh
An Thuong
11h18/11/2008
13
15
17
19
21
23
1h19/11/2008
3
5
7
9
11
13
15
17
19
21
23
1h20/11/2008
3
5
7
9
11
13
15
17
19
21
23
1h21/11/2008
3
5
7
9
11
Discharge (m3/s)
Morphological Modelling of Lai Giang inlet, Vietnam
Figure 2.4 Measurement of river discharge in a flood in November 2008
2.3. Coastal characteristics
2.3.1. Tide
The studying of the tidal characteristic of Lai Giang area is also based on two different
sources of data such as: The long term tidal data (from 1977 to 2004) at Quy Nhon
station; and The measuring data at Tam Quan station (from 09:00 4th to 16:00 20th May
2008) and Hoai Hai station (from 9:00 4th to 9:00 15th May 2008).
The tidal of Lai Giang area is complicated with 18 to 21 days of diurnal tide and 10 days of
mixed semidiurnal tide. According to analysing the correlation between measurement
data at Tam Quan and Hoai Hai station with long-term tidal data in Quy Nhon station
(Figure 2.5 and 2.6), the average tidal range at Lai Giang inlet is 0.9 meters; the maximum
and minimum tidal range are 2.11 meters and 0.16 meters, respectively.
Furthermore, Lai Giang inlet is located in the varying tidal ranges area. In the north of Lai
Giang is Thua Thien-Hue which has the small tidal range (approximately 0.5 meters). The
southern tidal range is much higher with the value from 3.5 meters to 4 meters. Thus, the
tidal range along the Lai Giang coast also varies from the North to the South. Figure 2.7
show the water level difference between Tam Quan station (in the north of the inlet) and
Hoai Hai (in the south of the inlet). With the distance of 1350 meters, the average
difference is 0.2 meters. Hence, the tide of Lai Giang inlet has effect on the current along
the coast.
21
Morphological Modelling of Lai Giang inlet, Vietnam
Figure 2.5 Correlation of water level between Tam Quan station and Quy Nhon station
Figure 2.6 Correlation of water level between Hoai Hai and Quy Nhon station
0
-0.05
Water level (m)
-0.1
-0.15
-0.2
-0.25
-0.3
-0.35
10
/5
/2
00
8
5h
9/
5/
20
08
20
08
17
h
9/
5/
5h
8
20
0
17
h
8/
5/
20
08
8
8/
5/
5h
7/
5/
20
0
17
h
20
0
8
8
7/
5/
5h
6/
5/
20
0
17
h
20
0
8
8
6/
5/
5h
5/
5/
20
0
8
20
0
17
h
5/
5/
5h
17
h
4/
5/
20
08
-0.4
Figure 2.7 Water level differences between Tam Quan and Hoai Hai station
22
Morphological Modelling of Lai Giang inlet, Vietnam
2.3.2. Wave
Lai Giang coast has the narrow continental shelf and the steep profile, leading to the
strong wave-influence. Moreover, because of the location in the monsoon regime area,
waves have seasonal feature: N and NE direction in winter and SE and S-SE direction in
summer for on-shore waves. According to the 10 year wave data (at My A cap with
coordinate 150 Northing and 1090 Easting), the frequency for wave directions are 79.3 %
of NE and E-NE; and 20.7% of E, SE and S-SE. The main significant wave height is in the
range from 0.5 to 1.5 meters (made up 66 % of total).
When the wave travels near the coast line, the wave direction and the significant wave
height change due to the refraction and the bottom friction. The significant wave height
in winter monsoon is high comparing with summer monsoon. The wave climate is
described in the table below.
Figure 2.8 Wave rose in 10 years
23
Morphological Modelling of Lai Giang inlet, Vietnam
Table 2.1 Wave statistic in 10 years
Tp (s)
Hs (m)
34
45
56
67
78
8_
9
910
10 11
11 12
12 13
0.0 - 0.5
11
103
220
211
106
58
35
14
2
1
0.5 - 1.0
1.0 - 1.5
35
588
152
1355
665
2087
984
1398
1138
413
386
190
116
101
61
32
14
10
5
88
5
1
464
225
48
3
1
516
602
354
128
29
3
325
249
224
222
133
73
18
55
54
47
32
37
31
47
21
7
7
3
5
2
9
2
1
3
5.0 - 5.5
4
5.5 - 6.0
6.0 - 6.5
6.5 - 7.0
7.0 - 7.5
7.5 - 8.0
SUM
1
46
843
2334
4023
4274
2106
670
224
61
19
7
4
1
%
0.3
5.8
16
27.5
29.2
14.4
4.6
1.5
0.4
0.1
0
0
0
1.5 - 2.0
2.0 - 2.5
2.5 - 3.0
3.0 - 3.5
3.5 - 4.0
4.0 - 4.5
4.5 - 5.0
13 14
14 15
15 16
SUM
%
761
5.2
6219
3523
42.6
24.1
1481
1144
682
388
206
109
65
10.1
7.8
4.7
2.7
1.4
0.7
0.4
21
25
0.2
3
2
4
2
1
0
2
0
0
0
0
0
7
2
2
1
1
1
2
2.3.3. Current
The total current in this area is the combination of wave induced current, tide induced
current and river flow, under the coast line characteristic. As including wave induced
current, the total current has seasonal features affected by the monsoon regime.
•
Tidal current is the regular current and has a small magnitude in the range from
0.1 to 0.15 m/s, approximately.
•
The remaining part of the total current is an irregular current with seasonal
changes. In the winter monsoon, the current which is dominated by the
Northwest, South and Southeast direction (the survey result of the Vietnam
Institute for Water Resources Research), is driven by the Northern wave and high
river flow. In the summer monsoon, the current is only affected by the Southern
wave because of the negligible of river flow and dominated by South and
Southeast direction.
2.3.4. Typhoons
Typhoons often happen in September, October and November (northern monsoon).
Table 2.2 lists all the typhoons which happen in 30 years (from 1977 to 2006) affect Binh
24
Morphological Modelling of Lai Giang inlet, Vietnam
Dinh province. As we can see, there are 2 storms having direct effect on the coast of Binh
Dinh province, which are:
•
Faith happened from 9th to 14th December 1998 at 33 m/s wind speed.
•
Lingling happened November 2001 at 33 m/s wind speed.
Table 2.2: Listing typhoons affect Binh Dinh province during 30 years
Wind
Year
Typhoon name
Date
1977
DINAL - 7712
23rd Sep
1979
ATND
9th Aus
Province
Ninh Thuan - Qui Nhon - Tay
Nguyen
Phu Yen - Khanh Hoa
th
Speed
(m/s)
Direction
( 0)
20
NNW
18
W
1980
1981
1982
RUTH - 8105
KELLY - 8106
HOPE - 8216
15 Sep
5th Aus
7th Sep
Quang Ngai - Binh Dinh
Qui Nhon - Binh Dinh
Khanh Hoa – Dong Nai
19
14
17
NNE
W
NW
1984
1984
1985
ATND
AGNES – 8424
CECIL - 8521
7th Sep
7th Nov
15th Oct
Binh Dinh – Quy Nhon
Thua Thien Hue – Khanh Hoa
Quan Binh – Quang Ngai
14
40
10
W
NW
N
1986
1987
1988
1988
GEOGRIA - 8622
NAURY - 8721
NONAME - 8861
Skip
21st Oct
19th Nov
9th Oct
5th Nov
Thua Thien Hue – Nghia Binh
Khanh Hoa – Dong Nai
Ninh Thuan – Tay Nguyen
20
20
14
41.2
W
NE
SE
1990
1992
1992
IRA - 9022
ANGERA - 9224
Collen
3 rd Oct
23rd Oct
15th Oct
Nha Trang – Dong Nai
Quy Nhon – Nha Trang
12
24
28.2
N
SW
1993
1993
1994
KYLE - 9325
Lola
Teresa
23rd Nov
7th Dec
20th Oct
Quang Ngai – Quang Binh
34
41
41
NE
1995
1996
1998
ZACK - 9521
ATND
CHIP - 9812
31st Oct
17th Oct
14th Nov
Quang Ngai
Khanh Hoa - Ninh Thuan
Ninh Thuan – Binh Thuan
40
12
20
N
NE
NE
20th Oct
10th Dec
12th Nov
Phu Yen - Khanh Hoa
Phu Yen - Khanh Hoa
Binh Dinh - Phu Yen - Khanh Hoa
20
33
33
NE
1998
2001
DAWN - 9813
Faith
LINGLING - 012
2006
2006
Xanesane
Duriral
30th Sep
30th Nov
43.7
54
25
NE
Morphological Modelling of Lai Giang inlet, Vietnam
2.4. Sediment
The measurement of Vietnam Institute for Water Recourses Research (in 3 periods: 27th –
30th March, 2005; 18th – 21st September, 2006; 17th – 20th May, 2007) give some results:
•
Concentration of suspended sediment inside Lai Giang inlet is small: ρmax = 20 mg/l
÷ 22 mg/l and ρmin = 7.5 ÷ 8 mg/l.
•
In the beach profile, sediment is coarser form near shore to off shore. The
diameter of sediment (D50) at the coast near the inlet is approximately 0.14 ÷ 0.34
mm and 0.5 ÷ 0.87 mm in the inter-tidal area. Moreover, the gain size in the
northern coast of the inlet is larger than the southern side.
Conclusion
The physical setting of Lai Giang inlet acts an important role in giving the idea how all the
factors interact with each other leading to the influence of all factors on morphodynamics
of the area.
•
In the winter monsoon, the northern wind is dominant, bringing high
precipitation and high river discharge. At the same time, northern waves are
dominant and typhoons occur. Thus the area is influenced by both river and
ocean factors.
•
In summer monsoon, under the southern wind condition, the river discharge
dismisses, tide and waves become the dominants factors. The migration of the
inlet or sedimentation may take place during this period.
26
Morphological Modelling of Lai Giang inlet, Vietnam
CHAPTER 3. CONCEPTUAL MODEL
3.1. Historical changing process
On the basis of historical documents and the available data, the changing process in the
Lai Giang inlet is analyzed according to 10 years, 5 years and 1 year time scales. The
processes going from a broad scale to a small scale will show the effect of the
hydrodynamic condition on the closure and movement of the inlet.
3.1.1. Based on the historical document
According to historical documentation, Lai Giang inlet originally belonged to Hoai Hai
Commune a few hundred years ago (Figure 3.1). It moved northward gradually because of
the hydrodynamic interaction between the river and the sea as well as the monsoon
regime.
Figure 3.1 Lai Giang inlet
Between 1975 and the present, the inlet has been closed every 10 years in 1975, 1983,
1996, 2003 and April 2010, recently. As stated above, river discharge in the flood season
maintained the opening of the inlet. The river discharge is the first factor which needs to
be taken into account when considering the changes of the inlet. In the centre of
Vietnam, the peak period of the dry season is in July and August while the main flood
season is in November and December every year (though it may occur one month earlier
or later). The most extreme discharge occurred in 1987 (5880 m3/s at An Hoa station –
Figure 2.3); 1985 and 1986. It had the minimum discharge in 1986. Thus, the inlet does
27
Morphological Modelling of Lai Giang inlet, Vietnam
not close in the years of minimum discharge. Under low river flow condition in the dry
season, strong sedimentation is evident in the inlet; however the inlet is widened by the
extreme river flood flow. In the case that during the year, discharge is low in the dry
season and not high enough in the flood season, choking at the inlet will take place.
Under the closure condition of the inlet, there is the interference of human. When the
sedimentation level is too high which cause difficulty for the traffic, the dredging action
has been processed. Further more, during the flood season if the river discharge is too
high, the low land surrounding area of the inlet is inundated, inlet has been blasted
manually.
3.1.2. Based on the Remote sensing image
The remote sensing data are available from 1990. Every 5 years records of different kinds
are kept which are SPOT4 (taken in 1995 and 2005 with 10 m resolution), LANDSAT (taken
in 1990 and 2000 with 15 m resolution) and SPOT5 (taken in 2008 with 10 m resolution).
All the remote sensing images are shown in Figure 3.2.
In 1990, the inlet was located at the north of the river mouth (14o 24’ 37” N and 109o 05’
13” E) in the Southwest (SW) –Northeast (NE) direction. Between 1990 and 1994, it
moved 190 meters to the north, approximately, and changed direction to NW – SE in
1995. Within 5 years, there are the reorientation and the northern movement with the
speed of 38 meters per year. The inlet closed in 1996 and it is at a co-ordinate of 14o 30’
48” N and 109o 05’ 08” E in 2000 (further to the north of the river mouth), moving
northward 120 meters (26 meters per year) in comparison with the inlet’s position in
1995. The direction of the inlet also changed from NW – SE to SW – NE.
In 2001, a storm from the NE direction hit Binh Dinh province with a wind speed of 33
meters per second. This event caused the irregular extreme displacement of the inlet
2500 meters to the south between 2000 and 2005. From 2005 to 2008, the movement
returned to normal (northward) with a faster speed of 83 meters per year.
The interaction between the river and the sea (mainly the river flow and the longshore
sediment transport) is the source of the reorientation and movement cycle of the Lai
Giang inlet. When river flow dominates, the inlet has SW – NE direction which is an
advantageous position for flowing to the sea. Under the low river flow, the long and
narrow sand-spit which runs parallel to the coast extends to the north due to the
longshore sediment from the SE leading to the reorientation from the SW – NE to the NW
– SE of the inlet. As can be seen in the last two images (2005 and 2008), the movement
speed becomes faster and the direction of the inlet changes slightly because of the
28
Morphological Modelling of Lai Giang inlet, Vietnam
influence of both factors (the river flow and the longshore sediment transport) at the
same time.
Figure 3.2 Lai Giang inlet at different times
29
Morphological Modelling of Lai Giang inlet, Vietnam
3.1.3. Based on the survey data
According to the topographic map of this area, we can see that:
•
In April 2005 (at the end of the northeast monsoon period), the inlet had a width
of 120 meters.
•
By September 2006 (at the end of the southwest monsoon period and the dry
season), the southern sandy barrier island extended to the north, leading to
northward movement (360 meters) of the inlet. The inlet became wider (180
meters).
•
The inlet moves back and forth with the seasonal change of monsoon; 90 meters
southward during the northeast monsoon (May 2007), 30 meters northward
during the southeast monsoon (September 2007).
Figure 3.3 Topography of the area in different period
Moreover, the combination of the topography in different years also describes clearer the
reorientation and the movement of the inlet during a year and seasons.
•
First period from April 2005 to September 2006: From the beginning, after went
through the flood season (April 2005), the inlet had the SW-NE direction. Within
one and a haft year (two dry seasons and one flood season), the inlet changed in
direction (to the NW-SE) and moved to the north.
•
Second period from September 2006 to May 2007: Experiencing the flood season
as well as the NE wave, inlet slightly moves to the south because of the northern
longshore current.
30
Morphological Modelling of Lai Giang inlet, Vietnam
•
The last period from May to September of 2007: Under the effect of the dry
season such as southern longshore current, there is a slight extension on the
down-drift and erosion on the up-drift spit.
Figure 3.4 The changes in topography of the inlet between April 2005 and September 2007
Another outstanding feature of the topography during the 2005 – 2007 periods is the
existence of the secondary inner spit. It moves seasonally accompanying with the
migration of the inlet.
The evolution of the Lai Giang inlet is very complex because of the movement of the inlet
an in wide range as well as the changes of the inlet area under the effect of the monsoon
regime and flood and dry season. The source of the sediment is mainly littoral caused by
longshore transport.
3.2. Conceptual model
From the physical setting of the inlet and the data analysis, the primary morphological
conceptual model given below has been developed. This model is adapted from the
conceptual model of Thuan An inlet, Vietnam (Lam, N.T., 2009).
31
Morphological Modelling of Lai Giang inlet, Vietnam
Figure 3.5 Conceptual model of the Lai Giang inlet
The Lai Giang inlet is mainly influenced by two different seasons which are the flood
season and the dry season. In the flood season (from September to December) which is
driven by the northeast (winter) monsoon and typhoon, the flood dominates leading to
the reorientation of the inlet. Moreover, the breaching process occurs in extreme even
with high discharge and small river catchments. In the dry season, when the river flow is
small (sometimes dismissible), wave action and tidal current thus dominate over the inlet
area.
3.2.1. Flood season
•
Winter monsoon
The winter (northern) monsoon lasts five months (from September to February the
following year). It crosses the South China Sea causing high amounts of precipitation
for the area. In the beginning of the winter monsoon season, typhoons with strong
wave action from NE or E-NE direction and surge can also occur, leading to irregular
longshore current. Inlet reorientation, barrier breaching and southern migration
happen in this season. However, the southern migration might not happen under the
high river discharge condition.
•
Reorientation of the inlet
After experiencing the dry season in which littoral factors dominate leading to the
Northwest (NW) – Southeast (SE) direction of the inlet, the river discharge dominates
again. There is a big difference in discharge between the two seasons. The scouring
process takes place in the inner down-drift spit and the outer up-drift spit which
reorients the inlet from a SW – NE to a NW – SE direction (Figure 3.3, September 2006
– May 2007 period). The frequency of this reorientation is once a year.
•
Barrier breach
The morphodynamics of the Lai Giang inlet are caused by the complex interaction
between the river and the sea. The river dominates in the flood season. During
32
Morphological Modelling of Lai Giang inlet, Vietnam
extreme river flood, the limited lagoon area and channel are not sufficient to maintain
the high river discharge, and the river starts overflowing the barrier island. When the
velocity reaches the critical value, the flow breaks through the barrier island at the
weakest point and creates a new inlet. The frequency of the barrier breaching is in
order of once over ten years.
3.2.2. Dry season
•
Summer monsoon
The summer (southern) monsoon is from February to September. It brings the hot and
dry weather from the equator to this area. In this season, the river discharge is small,
wave has the direction of SE and S-SE and the longshore currents from down-drift
dominate. Therefore, the channel is filling up and the inlet migrates.
•
Channel fill up
The channels are partly eroded during the flood season. In the dry season, the river
flow is relatively small; and the wave action becomes the dominant factor. With the
negligible of fluvial factor, the longshore sediment transport is no longer interrupted
and the channel becomes the sand-trapping area. The littoral sediment from the
longshore current gradually fills up the channel. When the flow is small, the fluvial
sediment carrying capacity reduces causing the accretion in the channel.
•
Barrier migration and spit extension
The Lai Giang inlet is characterised by the long narrow northern and southern barrier
island. During the dry period, the longshore current which comes from down-drift
brings sediment to accumulate at the down-drift spit. The spit extension process and
the seasonal reorientation (eroding the up-drift spit) complete the barrier migration
process.
3.2.3. Summary of the morphological process in one year
Within one year, the morphological process is summarised by the figure below:
33
Morphological Modelling of Lai Giang inlet, Vietnam
Figure 3.6 Net effect of the inlet
Conclusion
The morphodynamic conceptual model of the Lai Giang inlet is built up from the historical
data, the remote sensing image taken every 5 years and the survey of Vietnam Institute
for Water Resources Research. The model gives the governing processes and the driving
force of those processes which affect the morphodynamics of the inlet.
The river force (or river flow in detail) is the main factor during the flood season. It makes
the inlet change direction compared with the dry season, and breaching sometime
happens during such an extreme event.
34
Morphological Modelling of Lai Giang inlet, Vietnam
Unlike the flood season, during dry season wave-driven longshore current dominates
because of the relatively small river discharge. In this case, where wave action dominates
over tide and river flood, accretion in the inlet and barrier migration take place.
However, this is only the primary model, the numerical model can supply more detail
understanding of the morphodynamics of the inlet.
35
Morphological Modelling of Lai Giang inlet, Vietnam
CHAPTER 4. PROCESS-BASED MODELLING
To have further understanding at the morphodynamics of Lai Giang inlet, it is needed to
carry out a Delft3D morphodynamic model which has been constructed including Delft3DFlow, Delft3D-Wave and Sediment transport. All the basic equation of the models can be
found in Lesser, et. al, 2004.
4.1. Hydrodynamic model
4.1.1. Model domain
a. Model domain
The model domain is selected from a number of criteria such as the focusing area,
characteristics of the coast line, observation stations and the available data related to
water level, velocity, waves and sediment.
The hydrodynamic model of Lai Giang inlet includes the lagoon systems, the inlet and the
extension of 30 kilometres cross-shore (reaching 120 metres depth). The area is required
to be large enough to reduce the effects of uncertainties at the boundary and the local
longshore current. Therefore, the length of the model alongshore is 57 kilometres. The
boundary in the river of the model is chosen at the point at which tidal current is no
longer an influence (distance of 3 kilometres from the inlet).
36
Morphological Modelling of Lai Giang inlet, Vietnam
Figure 4.1 The domain of hydrodynamic model for Lai Giang inlet
Table 4.1 Co-ordinate of the simulation area
Point
Co-ordinate (m)
x
y
1
287361.827
1635096.454
2
323950.486
1635153.880
3
323745.104
1580129.190
4
303462.441
1580137.818
b. Grid
The curvilinear gird of the model has a resolution of 15 metres at the inlet to 1000 metres
at the seaward boundary (Figure 4.1).The grid has 196 x 166 nodes and 16.725 grid cells in
total. This grid also meets the requirement about orthogonality and smoothness.
c. Bathymetry
The bathymetry is established from the topographic maps supplied by the Vietnam
Institute for Water Resources Research in May 2008 and the nautical map (1/25.000
37
Morphological Modelling of Lai Giang inlet, Vietnam
scale) from Institute of Mechanics, which results in more than 350 thousand samples (the
density of the samples is higher in the inlet area and lower in off shore). The bathymetry
is shown in Figure 4.2.
Figure 4.2 Bathymetry, boundary and observation point of the model
d. Boundary
The seaward boundaries consist of three sections which are North, East and South
boundary (Figure 4.2). The boundaries conditions of the model are determined by water
level vary in time which is obtained from 2 different sources such as:
•
Result of the South China Sea model (Lien, 2009): The South China Sea model, with
the boundaries at Taiwan, Basin and Malacca strait, analyzed 8 tidal components
(Table 4.2) and computed the water level in 1986, 1987, 1988, 1993, 1994, 1995,
1998, 1999, 2001, 2005, 2006, 2007 and 2008 at 4 points of the Lai Giang
hydrodynamic model (Table 4.2). However, this is a large scale model and the
distance between the North boundary and the South boundary is small, thus the
accuracy is limited.
•
Result of My A inlet model (Huong, 2007): My A inlet locates near Northern
boundary (Figure 4.1). From the one month observation water level data, author
38
Morphological Modelling of Lai Giang inlet, Vietnam
using Mike 21 model to extract 8 tidal components (Table 4.3) and predict the
water level in interested period. Nevertheless, this prediction method still has
some restriction due to the duration of the measurement is only one month.
Table 4.2 Tidal components of the study area from the South China Sea model
Tidal
Northern Boundary
Southern Boundary
Constituents
Amplitude (m)
Phase (degree)
Amplitude (m)
Phase (degree)
K1
0.297
295.5
0.318
296.4
P1
0.072
279.0
0.075
283.6
O1
0.290
257.6
0.296
265.6
Q1
0.054
206.3
0.057
98.3
M2
0.183
294.2
0.18
297.5
N2
0.028
223.9
0.026
220.8
S2
0.097
334.9
0.084
334.1
K2
0.010
34.1
0.013
37.8
Table 4.3 Tidal components of My A inlet
Tidal Constituents
Amplitude (m)
Phase (degree)
K1
0.317
294.2
P1
0.082
294.8
O1
0.251
246.6
Q1
0.037
218.3
M2
0.169
282.8
N2
0.029
249.3
S2
0.062
316.6
K2
0.027
298.8
4.1.2. Calibration
a. Available data for calibration
For calibrating the hydrodynamic model including water level and velocity (magnitude
and direction), the measurements had been taken in two periods.
•
Period 1 is from 9 am 4th to 9 am 15th May 2008, measuring the water level along
the coast at two stations Tam Quan and Hoai Hai
39
Morphological Modelling of Lai Giang inlet, Vietnam
•
Period 2 is from 11 am 17th to 11 am 20th May 2007, measuring the water level
inside the river and the velocity at the coast (right outside the inlet).
The observation station is indicated detail in Figure 4.2 and Table 4.4.
Table 4.4 Observation data for calibrating model
Observation
station
Co-ordinate
X (m)
Y(m)
Observation period
WL Tam Quan
292473.7970 1611277.1043 from 9 am 4th to 9 am 20th May 2008
WL Hoai Hai
296660.3900 1599697.9000 from 9 am 4th to 9 am 15th May 2008
Velocity inlet
295147.1016 1603451.1618 from 11 am 17th to 11 am 20th May 2007
WL inlet
294020.8620 1601542.4168 from 11 am 17th to 11 am 20th May 2007
b. Calibrating process
During the calibration, a numbers of factor have been taken into account. The most
important factor is the boundary condition (which is the water level at the boundary).
The others are Courant number, smoothness, viscosity and roughness.
The model has been firstly computed in two cases: The water level for the boundary
condition (1) taken from the South China Sea model at the South and the My A
model at the North; (2) taken from South China Sea model at both side. The result
are presented below from Figure 4.3 to 4.7.
0.25
Velocity (m/s)
0.20
Data
Model result 1
Model result 2
0.15
0.10
0.05
0.00
6h 18/5/2007
18h
6h 19/5/2007
18h
Figure 4.3 Magnitude of the velocity outside the inlet
40
6h 20/5/2007
Morphological Modelling of Lai Giang inlet, Vietnam
360
Direction (degree)
310
260
210
160
110
60
6h 18/5/2007
Data
Model result 1
Model result 2
18h
6h 19/5/2007
18h
6h 20/5/2007
Figure 4.4 Direction of the velocity outside the inlet
0.0
Water level
-0.3
-0.6
-0.9
-1.2
Data
Model result1
Model result2
-1.5
-1.8
6h 18/5/2007
18h
6h 19/5/2007
18h
6h 20/5/2007
Figure 4.5 Water level inside the river
0.5
Measurement
Model result 1
Model result 2
Water level (m)
0
-0.5
-1
-1.5
9h
4/
5/
20
08
9h
5/ 2
5/ 1
20
08
9h
6/ 2
5/ 1
20
08
9h
7/ 2
5/ 1
20
08
9h
8/ 2
5/ 1
20
08
9h
9/ 2
5/ 1
20
9h
08
10
/5 21
/2
00
9h
8
11
/5 21
/2
00
9h
8
12
/5 21
/2
00
9h
8
13
/5 2
/2 1
00
9h
8
14
/5 2
/2 1
00
9h
8
15
/5 2
/2 1
00
9h
8
16
/5 2
/2 1
00
9h
8
17
/5 2
/2 1
00
9h
8
18
/5 2
/2 1
00
9h
8
19
/5 2
/2 1
00
9h
8
20
/5 2
/2 1
00
8
-2
Figure 4.6 Water level at Tam Quan station (along the coast)
41
Morphological Modelling of Lai Giang inlet, Vietnam
0.5
Data
Model result 1
Model result 2
Water level (m)
0
-0.5
-1
-1.5
8
/2
00
21
8
/2
9h
15
/5
/5
9h
14
/5
13
9h
00
21
08
/2
0
8
21
21
12
/5
/2
00
8
00
9h
11
9h
9h
/5
/2
21
8
/2
10
/5
5/
9/
9h
00
21
08
21
20
08
21
9h
8/
5/
20
08
21
9h
7/
5/
20
08
21
9h
6/
5/
20
08
20
21
5/
5/
9h
9h
4/
5/
20
08
-2
Figure 4.7 Water level at Hoai Hai station (along the coast)
These figures above indicate that the water surface elevation and magnitude results
of two cases do not have the difference in phase, and slight difference in amplitude (in
the magnitude of the velocity situation), but the direction results are opposite
between two model results and between model result 2 and the measurement data.
Thus, the model 1, which takes water level from My A model for the North boundary
condition and from South China Sea model for South boundary condition, is selected
for further calibration.
In the numerical model, the water surface elevation is not as sensitive with all the
forcing as the velocity (in both direction and magnitude), thus the water level will not
be influenced under the slightly change of the forcing. So, the calibration will focus on
the direction and the magnitude of the velocity. The forcing of the Delft3D-Flow
model is divided in 3 groups and sensitivity analysis have been done with each group
to get the best fit between the measurements and the model results.
1. Courant number, smoothness and viscosity
Courant number depends on the grid spacing and the time step of the model. It can
be changed by adjusting the time step (from 30 seconds to 15 seconds) and refining
(with the new grid dimension is 197 x 283) the grid in a required range (less than 10)
of the Courant number. The smoothness in M and N direction is adjusted by minor
adjusting the grid. Viscosity parameter is also changed (form the default value of the
model, 1, to 0.4). From the result in Figure 4.8 to 4.10, we can see that these factors
do not influence the velocity significantly in this model.
42
Morphological Modelling of Lai Giang inlet, Vietnam
0.30
Velocity (m/s)
0.25
0.20
Data
Original model
Refine
Time step 0.25 mins
Viscosity
0.15
0.10
0.05
0.00
6h 18/5/2007
18h
6h 19/5/2007
18h
6h 20/5/2007
Figure 4.8 Comparison of the magnitude of the velocity with the adjustment of several factors
350
Direction (degree)
300
250
200
150
Data
Original model
Refine
Time step 0.25 mins
Viscosity
100
50
0
6h 18/5/2007
18h
6h 19/5/2007
18h
6h 20/5/2007
Figure 4.9 Comparison of the direction of the velocity with the adjustment of several factors
0.1
Water level (m)
-0.2
-0.5
-0.8
-1.1
-1.4
-1.7
-2.0
6h 18/5/2007
Data
Original model
Refine
Time step 0.25 min
Viscosity
18h
6h 19/5/2007
18h
6h 20/5/2007
Figure 4.10 Comparison of the water level with the adjustment of several factors
43
Morphological Modelling of Lai Giang inlet, Vietnam
2. Roughness coefficient
In theory, roughness has a main role in the amplitude of the velocity. The increasing of
the roughness parameter mainly cause the reducing of the amplitude. In this model
the Manning’s roughness coefficient is preferably used in the range from 0.025 to
0.035. After the water level, the direction and the phase of magnitude of the velocity
are calibrated; roughness coefficient adjustment can be applied to get the better
result of the amplitude of the velocity.
3. Boundary condition
One of the alternatives for the boundary condition has been taken in to account is
dividing the East boundary to 3 sub-boundaries due to the length of 57 kilometres of
East boundary. The water level condition for the sub-boundary is interpolated base on
the water condition at the North and the South boundary. However, the division of
the East boundary to 3 sub-boundaries do not affect the model results.
Due to some inaccuracy of the boundary condition which mentioned in Section 4.1.1d, changes of the boundary condition in phase and amplitude are considered. The
water level in the North boundary have been shifted forward and backward in
different time such as 10 minutes, 15 minutes and half an hour; and added the certain
amount in amplitude (± 2, 5, 7, 10 and 12 centimetres) to find out the sensitive factor
which effect the magnitude of the velocity.
Figure 4.11 and 4.12 describe the phase adjustment effect on the velocity of the
model Delft3D-Flow for Lai Giang inlet. To compare with the observation data, the
computed velocity have smaller range with the different between two maximum
velocities of 0.1 meter per second and there is one hour phase lag. Moreover, unlike
the data, model result consists of two peaks in one day. While moving the phase of
the water level in Northern boundary forward lead to the increasing of the
“uninterested” peak (which does not exist in the measurement data) and decreasing
of the “interested” peak (which also appear in the measurement data), moving the
phase backward makes the amplitude of the velocity higher and the phase lag
between the observation and the model result smaller.
Figure 4.13 and 4.14 show how changing the amplitude of the water level at the
boundary affect the velocity. Figure 4.15 examines both the sensitivity of phase and
the amplitude to the velocity.
Among all alternatives, moving phase of the Northern water level 30 minutes
backward gives the most appropriate result compared with the measurements. The
44
Morphological Modelling of Lai Giang inlet, Vietnam
change in Manning Roughness coefficient is applied (from 0.3 to 0.35) (Figure 4.16,
4.17 and 4.18) to get the same range between the data and model results.
0.30
0.25
Data
Phase
Phase
Phase
Phase
0
+
+
+
min
10 mins
15 mins
30 mins
Velocity (m/s)
0.20
0.15
0.10
0.05
0.00
6h 18/5/2007
18h
6h 19/5/2007
18h
6h 20/5/2007
Figure 4.11 Computed magnitude of the velocity of the model with the phase adjustment
forward of the North boundary
0.35
0.30
Velocity (m/s)
0.25
Data
Phase
Phase
Phase
Phase
0 min
- 10 mins
- 15 mins
- 30 mins
0.20
0.15
0.10
0.05
0.00
6h 18/5/2007
18h
6h 19/5/2007
18h
6h 20/5/2007
Figure 4.12 Computed magnitude of the velocity of the model with the phase adjustment
backward of the North boundary
0.30
Velocity (m/s)
0.25
0.20
Data
Amplitude
Amplitude
Amplitude
Amplitude
+
+
+
+
0 cm
2 cm
5 cm
10 cm
0.15
0.10
0.05
0.00
6h 18/5/2007
18h
6h 19/5/2007
18h
6h 20/5/2007
Figure 4.13 Computed magnitude of the velocity of the model with addition amplitude of the
North boundary
45
Morphological Modelling of Lai Giang inlet, Vietnam
0.40
0.35
Velocity (m/s)
0.30
0.25
Data
Amplitude - 0 cm
Amplitude - 2 cm
Amplitude - 5 cm
Amplitude - 7 cm
Amplitude - 10cm
Amplitude - 12 cm
0.20
0.15
0.10
0.05
0.00
6h 18/5/2007
18h
6h 19/5/2007
18h
6h 20/5/2007
Figure 4.14 Computed magnitude of the velocity of the model with reduction amplitude of the
North boundary
0.40
0.35
0.30
Velocity (m/s)
0.25
Data
0 min 0 cm
- 5 cm - 15 mins
+ 5 cm - 15 mins
+ 10 cm - 15 mins
+ 10 cm + 10 mins
+ 10 cm -10 mins
- 10 cm + 10 mins
- 10 cm - 10 mins
0.20
0.15
0.10
0.05
0.00
6h 18/5/2007
18h
6h 19/5/2007
18h
6h 20/5/2007
Figure 4.15 Computed magnitudes of the velocity of the model with adjustment in phase and
amplitude of the North boundary
46
Morphological Modelling of Lai Giang inlet, Vietnam
0.30
Data
Phase - 30 mins Roughness 0.3
Phase - 30 mins Roughness 0.35
0.25
Velocity (m/s)
0.20
0.15
0.10
0.05
0.00
6h 18/5/2007
18h
6h 19/5/2007
18h
6h 20/5/2007
Figure 4.16 Calibration result in the magnitude of the velocity
360
Direction (degree)
310
260
210
160
Data
Phase - 30 mins Roughness 0.3
Phase - 30 mins Roughness 0.35
110
60
6h 18/5/2007
18h
6h 19/5/2007
18h
6h 20/5/2007
Figure 4.17 Calibration result in the direction of the velocity
0.1
-0.2
Water level
-0.5
-0.8
-1.1
-1.4
Data
Phase - 30 mins Roughness 0.3
Phase - 30 mins Roughness 0.35
-1.7
-2.0
6h 18/5/2007
18h
6h 19/5/2007
18h
Figure 4.18 Calibration of water surface elevation inside the river
47
6h 20/5/2007
Morphological Modelling of Lai Giang inlet, Vietnam
The calibration figures above describe the computed result and the observation data. It
can be seen that the water level along the coast line of the model almost fit with the
observations in Figure 4.6 and Figure 4.7 at Tam Quan station (in the north) and Hoai Hai
station (in the south). Nevertheless, Figure 4.18 shows that the water level result of the
model inside the river is not as good as the coastline results, but basically has the same
phase as well as the value of the high water level. This result is reasonable because the
model does not consider the river flow. In the flood tide, the tide-induced and waveinduce current dominate; the river flow is neglected, so the water level of the model
result during this period is almost the same with the measurements. But in ebb tide, river
flow have more influence leading to the water level between observation and calculation
to be different. Beside, the measurements around the inlet area only cover the short
period and have the limitation in accuracy.
In the velocity calibration result, Figure 4.16 and 4.17, the direction gives the good result
in both phase and amplitude. The magnitude result is more complicated. In the flood tide,
the model result is almost the same with the data, but there is a different of 0.2 meter
per second between the data and the model result create the an additional peak.
4.1.3. Model setup
The setup value for the model are shown in the table below, all other values which are
not mentioned are set as the default value.
Table 4.5 Flow model setup
Bathymetry and Grid
- Interpolated the topography May 2008
- Grid dimension: 197 x 166, 16.725 nodes
- Latitude: 14
- Orientation: 0
Time
- Time step: 0.5 minute
- Local time zone: 7
Initial condition
Average of water level at 4 boundary points
Roughness
Manning: 0.035 (Uniform)
North boundary
Water level in time series
Point 1 and point 2 have the same value
Predicted from My A tidal components and
move the phase backward 30 minutes
East boundary
Water level in time series
48
Morphological Modelling of Lai Giang inlet, Vietnam
Linear from point 2 to point 3
South boundary
Water level in time series
Point 3 and point have same value
Result of Water level South China Sea model
4.2. Wave model
In studying morphological processes, wave action is an important factor for sediment
transportation. Longshore sediment transport is caused by longshore current which is
driven by the radiation stress of wave approaching under an angle in the surf zone (D’
Angremond and Pluim-Van der Velden, 2001).
4.2.1. Wave data analysis
This section will select two main directions which represent for two seasons (winter and
summer) as input for the model to describe the different of flow condition in two
seasons.
a. Wave statistic
The wave data used as input for the model is the observation data at the coordinate 150
Northing and 109.50 Easting collected in 10 years (from 1996 to 2005) every 6 hours
(Kellogg Brown & Root, 2006). When considering the wave data, only the wave come
from the East is taken in to account. Beside, the coastline deviates 1540 with the North
direction. Thus, the effective direction of wave is in the range from –260 to 1540, the data
are sorted in effective directions and listed in Table 4.6 below.
Table 4.6 Wave statistic in Lai Giang
No
Wave
Frequency (%)
1
N-NE
2
Averaged wave parameters
Hs (m)
Tp (sec)
MWD (o)
2.25
2.20
8.44
26.19
NE
27.06
2.14
8.99
48.97
3
E-NE
41.02
1.29
8.89
64.60
4
E
10.08
0.85
8.91
88.52
5
E-SE
7.07
0.79
8.07
112.33
6
SE
7.98
0.81
7.12
135.62
7
S-SE
3.48
0.80
6.73
150.29
8
N
0.83
1.94
7.56
136.71
9
N-NW
0.23
1.83
7.48
343.54
49
Morphological Modelling of Lai Giang inlet, Vietnam
Figure 4.19 Wave rose for 10 years
b. Morphological impact of wave climate
Using the method of Elias, 2006 to estimate the dominant component of wave climates
on sediment transport, the wave data observation from 1996 to 2005 have been analyzed
in wave direction and wave height. The data are sorted in 9 directions and 7 classes of
wave height. For each class, the representative morphological wave height (Hmor) is
determined by the equation below:
1
1 n
k
=  ∑ H k s (i ) 
 n i =1

H mor
Where n is the number of observation
Hs is significant wave height (m)
k is the power relation between transport and wave height (taken here as k = 2.5)
The morphological impact (MI) of each class is calculated by multiplying the
representative morphological wave height with probability of occurrence (P):
MI = Hmor x P
The analyses result is shown in the table and figure below:
50
Morphological Modelling of Lai Giang inlet, Vietnam
Table 4.7 Wave height and wave direction effect on morphodynamic of Lai Giang area
<1m
1-2m
2-3m
3-4m
4-5m
5-6m
>6m
N-NE
NE
E-NE
E
E-SE
SE
S-SE
N
N-NW
Hmor
0.77
0.79
0.8
0.75
0.72
0.7
0.73
0.77
0.69
P (%)
0.23
3.49
16.73
7.79
5.64
6.01
2.82
0.14
0.05
MI
0.18
2.75
13.43
5.87
4.08
4.2
2.06
0.11
0.03
Hmor
1.53
1.56
1.42
1.18
1.18
1.24
1.24
1.55
1.71
P (%)
0.91
9.5
18.84
2.16
1.37
1.95
0.65
0.31
0.08
MI
1.39
14.82
26.76
2.56
1.61
2.41
0.81
0.47
0.14
Hmor
2.48
2.46
2.39
2.53
2.64
2.12
2.02
2.42
2.58
P (%)
0.68
9.12
4.44
0.08
0.05
0.02
0.01
0.28
0.1
MI
1.7
22.46
10.59
0.21
0.13
0.05
0.02
0.68
0.25
Hmor
3.47
3.44
3.36
3.21
3.14
0.00
0.00
3.51
0.00
P (%)
0.21
3.62
0.88
0.03
0.01
0.00
0.00
0.07
0.00
MI
0.74
12.45
2.96
0.11
0.03
0.00
0.00
0.23
0.00
Hmor
4.46
4.42
4.48
4.07
0.00
0.00
0.00
4.54
0.00
P (%)
0.16
1.15
0.11
0.01
0.00
0.00
0.00
0.02
0.00
MI
0.74
5.07
0.48
0.03
0.00
0.00
0.00
0.07
0.00
Hmor
5.3
5.21
5.12
0.00
0.00
0.00
0.00
5.24
0.00
P (%)
0.03
0.16
0.02
0.00
0.00
0.00
0.00
0.01
0.00
MI
0.17
0.86
0.13
0.00
0.00
0.00
0.00
0.04
0.00
Hmor
6.28
7.66
0.00
6.79
0.00
0.00
0.00
0.00
0.00
P (%)
0.02
0.02
0
0.01
0.00
0.00
0.00
0.00
0.00
MI
0.1
0.13
0
0.06
0.00
0.00
0.00
0.00
0.00
51
Morphological Modelling of Lai Giang inlet, Vietnam
52
Morphological Modelling of Lai Giang inlet, Vietnam
Figure 4.20 Characteristics of wave climate and its influence on morphodynamics of Lai Giang
53
Morphological Modelling of Lai Giang inlet, Vietnam
c. Conclusion
As stated in Chapter 2 (Physical setting), the dominant wave directions are N-NE, NE, E-NE
and E in winter monsoon and E-SE, SE and S-SE in summer monsoon. From the statistic
result above, there are two main directions in winter monsoon which are NE and E-NE
with the frequency 27.06 % and 41.02 %, respectively. However, the morphological
impact (MI) of NE direction (more than 58 % in total) is higher than E-NE direction (54 % in
total). Thus, wave with NE direction has the biggest influence on sediment transportation.
Following the same methodology, SE wave direction is selected as the dominant wave
direction in summer monsoon.
4.2.2. Typical year of wave climate
To have the complete picture about the influence of the ocean factors on the inlet, it
requires the operation of the numerical model with a typical year of wave climate. The
wave statistics have been made to have a typical wave climate year.
Figure 4.21 illustrates annual and full wave climate in wave rose. It can be seen that the
NE and E-NE are the dominant direction in all the years, obviously. And comparing with
the 10 year wave rose, the wave rose in 2001 and 2003 are almost the same in terms of
distribution in direction as well as significant wave height. So, further analysis for this two
year is carried out for quantitative selection. From table 4.8, it can be seen that the wave
climate in 2003 is more suitable in the consideration of direction distribution, signification
wave height and peak wave period.
54
Morphological Modelling of Lai Giang inlet, Vietnam
Figure 4.21 Annual wave rose from 1996 to 2005
55
Morphological Modelling of Lai Giang inlet, Vietnam
Table 4.8 Wave statistic comparison between 10 years observation, 2001 and 2003
Probability (%)
Average
Significant Wave Height
(m)
10years 2001 2003
2.2
2.63
2.28
N-NE
10years
2.25
2001
1.85
2003
2.13
NE
E-NE
E
27.06
41.02
10.08
29.31
43.56
8.86
26.3
42.26
9.16
2.14
1.29
0.85
2.07
1.3
1.03
E-SE
SE
S-SE
N
7.07
7.98
3.48
0.82
4.75
8.05
2.98
0.4
7.35
7.35
5.21
0.24
0.79
0.81
0.8
1.94
0.73
0.81
0.72
1.15
N-NW
0.23
0.24
0
1.83
1.15
Average
Wave Direction (degree)
Average
Peak period (s)
10years
26.19
2001
26.65
2003
26.83
10years
8.44
2001
8.43
2003
8.86
2.13
1.35
0.76
48.97
64.6
88.52
50
62.66
87.57
49.01
64
87.16
8.99
8.89
8.91
9.28
9.18
9.91
8.89
8.92
8.56
0.65
0.76
0.87
2.11
112.33
135.62
150.29
136.71
112.33
136.73
150.88
4.22
112.08
135.96
150.01
123.93
8.07
7.12
6.73
7.56
9.01
7.5
7.94
7.25
7.86
7.18
6.35
9.15
343.54
343.73
7.48
6.32
4.2.3. Scenarios and model setup
According to wave data analysis, typhoon record and the purposes of modelling, the wave
model is executed with the following scenarios:
•
Monsoon scenarios with two main wave directions which are NE and SE;
•
Typical year scenario for 2003 wave climate to understand the net effect of all
factor;
•
Storm scenario with the storm in 2001 (Lingling) which hits directly the province
with NE wind at the speed of 33 m/s for the extreme situation;
•
Flood scenario with the flood in November 2001 under the NE wave climate.
All the scenarios follow the wave model setup given in the table below.
Table 4.9 Wave model setup
Hydrodynamics
Select water level and current result from flow
Bathymetry and Grid
- Computation grid and bathymetry: same with
Hydrodynamics model
- Spectral resolution
+ Number of direction: 16
+ Frequency space: from 0.02 to 1 with 25
frequency bins
Time frame
Using the water level result from
Hydrodynamics model every 2 hours
Boundaries
- NE wave scenario: North and East boundary
with the same wave condition:
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Morphological Modelling of Lai Giang inlet, Vietnam
+ Significant wave height: 2.14 m
+ Peak period: 8.99s
+ Direction: 47.970
+ Directional spreading: 300
- SE wave scenario: South and East boundary
with the same wave condition:
+ Significant wave height: 0.81 m
+ Peak period: 7.12 s
+ Direction: 135.620
+ Directional spreading: 300
- Typical year scenario: North, South and East
boundary with the same wave condition which
vary in time.
Obstacles
The Delft3D-Wave does not have function to
indicate all the dry points in Delft3D-Flow, the
obstacles along the land boundary have been
introduce to keep the Wave model stable.
Physical parameters
- Wind: Inactive
- Wave
+ Spectrum: JONSWAP
+ Peak enhance factor: 3.3
+ Setup: Inactive
+ Forcing: radiation stress
+ Generation mode: 3 rd generation
+ Bottom friction: JONSWAP type; coefficient=
0.067
+ Depth-induced breaking (B&J model): α= 1,
χ=0.73
+ White capping: Active
+ Quadruplet: Inactive
+ Refraction and Frequency shift: Active
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Morphological Modelling of Lai Giang inlet, Vietnam
4.3. Sediment transport
4.3.1. Data processing
Besides the river flow, the changes in morphology are mainly produced by tides and
waves. In order to reduce the computation time, input reduction and a morphological
factor have been applied. However, this process may dismiss the unusual events of the
area such as storm and flooding. To solve this problem, the input is reduced to yearscenarios, a storm scenario and a flood scenario.
The processing input data is focused on water level data and the wave climate which is
described detail below:
a. Water level data
Different to wind and wave data, tidal data is predictable. Thus, according to Latteux
(1995) and Winer (2006), it is possible to running the model with representative tide
accompanying with increasing the morphological factor (from 1 to 24 for the one year
scenario) instead of running the model with full required duration.
The 14-year tidal data have been sorted (by month and by year) and compared (by
average tidal range, the maximum and minimum tidal range). As the result in table 4.10
and 4.11, November 2006 has the best fit. For the further confirmation, the tidal
constituents within the one month water level (November 2006) are analyzed and
compared with the 14-year series data. They are almost the same (table 4.12). One tidal
cycle in November 2006 is selected as representative tide for the input of the model.
Table 4.10 Tidal statistic by year
Average Tidal range
Max Tidal range
Min Tidal range
1984
1986
1987
81.98
78.83
84.68
167.61
167.96
168.16
3.34
2.74
3.1
1988
1993
1994
91.92
77.09
65.96
178.24
152.59
144.88
7.06
2.15
2.59
1995
1998
1999
64.9
65.29
70.53
139.51
135.22
143.18
2.65
2.5
2.12
2001
2005
2006
72.45
83.16
79.73
156.07
172.84
166.89
2.75
2.64
2.32
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Morphological Modelling of Lai Giang inlet, Vietnam
2007
2008
87.21
83.94
170.82
173.52
3.41
2.56
Table 4.11 Tidal statistic by month
All year
Jan
Average Tidal range
79.33
86.2
Max Tidal range
184.66
184.23
Min Tidal range
2
2.06
Feb
Mar
May
77.44
71.51
79.11
171.57
148.29
178.73
2.13
2.03
2.1
Oct
Nov
Dec
73.75
77.2
87.05
160.17
178.24
184.28
2.02
2.12
2
Table 4.12 Comparison of tidal components value
Tidal
1-month data
14-year data
Constituents
Amplitude (m)
Phase (degree)
Amplitude (m)
Phase (degree)
K1
0.319
296.4
0.318
296.4
P1
0.073
282.6
0.075
283.6
O1
0.296
265.6
0.296
265.6
Q1
0.057
98.4
0.057
98.3
M2
0.18
297.5
0.18
297.5
N2
0.026
220.9
0.026
220.8
S2
0.084
334.2
0.084
334.1
K2
0.013
37.7
0.013
37.8
b. Wave data
Following the tidal data, the wave data also need to reduce. From the previous section,
the wave data in 2003 have been selected. The data were observed every 6 hours (4 data
everyday) To reduce data from every 6 hours to 24 hours, 4 data each day are averaged.
The comparison (between full and sorted data) results in visualization by wave rose
(figure 4.22) and in quantity (table 4.13) are acceptable.
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Morphological Modelling of Lai Giang inlet, Vietnam
Figure 4.22 Wave rose in 2003 with full and sorted data
Table 4.13 Comparison of wave characteristic in between full data and sorted data in 2003
Probability (%)
N-NE
NE
E-NE
E
E-SE
SE
S-SE
Full
data
2.13
26.3
42.26
9.16
7.35
7.35
5.21
N
0.24
Sorted
data
1.89
26.18
41.96
9.15
7.89
8.2
4.73
Average
Significant Wave
Height (m)
Full
data
2.28
2.13
1.35
0.76
0.65
0.76
0.87
Sorted
data
2.2
2.15
1.35
0.77
0.74
0.72
0.81
2.11
Average
Wave Direction
(degree)
Full
data
26.83
49.01
64
87.16
112.08
135.96
150.01
Sorted
data
28.37
48.91
64.07
88.22
111.9
135.55
150.11
123.93
Average
Peak period (s)
Full
data
8.86
8.89
8.92
8.56
7.86
7.18
6.35
Sorted
data
8.61
8.97
8.83
8.68
8.1
7.18
6.2
9.15
4.3.2. Model domain
a. Sediment and morphology setup
The addition setup of sediment and morphology is described in table below.
Table 4.14 Addition setup for morphodynamic model
Boundaries
There is no sediment going in or out of the Lai
Giang area
Physical parameters - Sediment
Overall sediment data
- Sediment Sand
- Reference density for hindered settling: 1600
(kg/m3)
Data for non-cohesive Sediment Sand
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Morphological Modelling of Lai Giang inlet, Vietnam
- Specific density: 2650 (kg/m3)
- Dry bed density: 1600 (kg/m3)
- Median sediment diameter (D50): 240 (μm)
- Initial sediment layer thickness at bed: 5 (m)
(Uniform)
Physical parameters - Morphology
General
- Update bathymetry during FLOW simulation
- Equilibrium sand concentration profile at
inflow boundaries
- Morphological factor: 1 (for the individual
dominant wave scenarios) or 24 (for the typical
year scenario)
- Spin-up interval before morphological
change: 720 minutes
Sediment transport parameter
- Van Rjin’s reference height factor: 1
- Threshold sediment thickness: 0.050000001
(m)
- Estimated ripple height factor: 2
Multiplication factor
Keep the default number
b. Mophordynamic modelling process
The morphodynamic model is computed based on the current field and wave field result
to predict the morphodynamics of the area after a certain period under the river
discharge, the tide and the wave climate. The sedimentation or the erosion that takes
place cause the changing in the bathymetry, the bathymetry is updated. The detail of the
modelling process is described in Figure 4.23 below.
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Morphological Modelling of Lai Giang inlet, Vietnam
Figure 4.23 Morphodynamic modelling process
Conclusion
This chapter is the preparation for the morphodynamic modelling of Lai Giang area
including the Flow model, the Wave model and Sediment transport. All calibration, data
analysis and input preparation have been described. However, the morphodynamic
model of Lai Giang inlet still has some restriction such as:
-
The velocity calibration occurred the unexpected peak due to the restriction of
measurement data as well as the predicted water level for the boundary
condition;
-
Furthermore, it is impossible to get the full data for the river discharge in flood
season. The only available data is 3-day water level and river discharge in
November 2008 measured near the river boundary of the model;
-
The selection the representative tide is limited, only base on the average tidal
range, maximum and minimum tidal range.
However, this model is still able to describe the basic morphology of Lai Giang inlet area.
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Morphological Modelling of Lai Giang inlet, Vietnam
CHAPTER 5. MODELLING RESULT: DESCRIPTION AND ANALYSIS
In order to confirm the explanations about the morphological chances in the conceptual
model of Lai Giang inlet in Chapter 3 and give more detail explanations, several questions
have been raised:
1. What is the influence of the tidal regime on sediment transportation?
2. How does the wave climate in the tidal regime condition control the sediment
movement of this area?
3. How do the interaction between the tide, the wave and the river flow change the
morphology of Lai Giang inlet area?
4. Does the extreme event such as a storm make the significant chances in
morphology?
To answer all these question with the consideration of the availability of the data and the
hydrodynamic condition, modelling of Lai Giang inlet area has been carried out with
difference scenarios which are:
•
Tidal induced flow and the influence on non-cohesive sediment: from 1:00 1st to
6:00 16th November 2006;
•
Tidal induced and dominant (NE and SE) wave induced flow, and the influence on
non-cohesive sediment: from 0:00 4th to 11th November 2006;
•
Tidal induced and wave induced (with all wave direction) flow and the influence
on non-cohesive sediment in a year (by using morphological factor): from 1:00 1st
to 6:00 16th November 2006;
•
Storm scenario for the Lingling storm (from 18:00 7th to 12:00 13th November
2001);
•
Flood event with a flood from 21:00 18th to 6:00 21st November 2008 without and
with northern wave effect.
The results of all scenarios are presented in the map of interested area in the
representative times which are high and low tide during the diurnal-tidal day (9th and 12th
November 2006) and semidiurnal-tidal day (5th November 2006).
For the dominant wave scenarios, the results are extracted in 6 time steps including 4
time steps in semidiurnal-tidal day (4 am, 11 am, 3 pm and 10 pm in 5th November 2006)
and 2 steps in diurnal-tidal day (8 am and 11pm in 9th November 2006) (Figure 5.1)
In one year scenario, the results also are extracted in 6 time steps: 4 am, 11 am, 3 pm and
10 pm on 5th November 2006 and 0 am and 10 am in 12th November 2006 (Figure 5.2).
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Morphological Modelling of Lai Giang inlet, Vietnam
Figure 5.1 Water level condition at the boundary of the dominant wave scenarios
Figure 5.2 Water level condition at the boundary of the one year scenario
To gain more understanding about the morphology of Lai Giang inlet area, modelling
results will be showed in 4 sections such as tidal characteristic; wave propagation process;
current under influence of tide, wave and river flow; and sediment transport
characteristic of this area in a typical year and under extreme climate event.
5.1. Tide
According to the computed water level result during one spring and neap tide cycle (half
of a month) at the entrance of the inlet showed in Figure 5.3, we can see that there are
five semi-diurnal tidal days which are 3rd, 4th, 5th, 6th and 7th November and ten diurnal
tidal days from 7th until 16th of November. The tidal range in spring tide and neap tide is
1.8 meters and 0.2 meters, respectively. Compared with the tidal characteristic base on
analysing the available observation data, the model results are almost the same.
Moreover, the water level calibration result is quite accurate. Thus, the model result can
be use to understand how the tide propagates in this area during different times.
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Morphological Modelling of Lai Giang inlet, Vietnam
Figure 5.3 Computed water level at the entrance of the inlet
Figure 5.4 describes clearly how the tide propagates during the high tide and the low tide
of the semidiurnal and diurnal tidal day.
•
At 4:00 5th, the ebb period, the lower low water occurred, tide propagated from
the North to the South because water level in the Northern boundary is higher
(0.1 meters) than the Southern one. However, the tidal range between the lower
high tide and low tide is small with the value of 0.2 meters, so it is impossible to
see the tide propagate from onshore to offshore during the ebb period. Opposite
to the ebb period above, during the flood period of the lower high tide (at 11:00
5th), water level of South boundary is higher (0.25 meters) than the North one that
lead to the North ward movement of the water level.
•
In the remaining four pictures, there is a light difference in water level,
approximately 0.05 meters between two boundary conditions but the tidal range,
with the value from 0.5 meters to 1.4 meters, is much higher than the previous
period (0.2 meters). Hence, we can see water level moves to the offshore
direction in the ebb period (15:00 5th and 0:00 12th) and moves to onshore
direction during flood period (22:00 5th and 10:00 12th).
The tidal propagation in this study area is decided by the difference in water level
between the two boundary condition and the tidal characteristic or tidal range in detail.
When the differential of boundary condition is large accompany with the small tidal
range, the differential will drive the tidal movement which is moving North ward or South
ward. During the period when the tidal range is dominated, the propagation is mainly
either shore ward or sea ward. This movement will decide the tidal induced current in this
area which will be showed in the next section.
65
Morphological Modelling of Lai Giang inlet, Vietnam
Figure 5.4 Tidal propagate in Lai Giang inlet area
66
Morphological Modelling of Lai Giang inlet, Vietnam
5.2. Wave
In theory, as wave propagate to ward the shore, the wave length decreases with the
reducing of the water depth. Thus the wave speed also decreases because the wave
period is fixed. When wave is travelling to the shallower water, the refraction occurs with
the result is wave’s orientation to the shoreline orientation (away from the deep water),
focusing on headlands and shoals. The propagation of wave tends to be parallel with the
bathymetry. Besides refraction, shoaling also happens when wave starts feeling the
bottom when wave travelling to the shallower water. So, under the influence of
bathymetry, the wave height and the wave direction change when moving shore ward.
The propagation of the wave in the model also follows the theoretical phenomenon.
Figure 5.5 indicates how the wave in the Delft3D-Wave model travelling in the study area
with under two dominant wave directions (which are Northeast wave and Southeast
wave) of wave climate. Within this research, due to limitation of the measurement data, it
is impossible to calibrate the Delft3D-Wave model. However, the model result is able to
describe the basic theory of the wave propagation (refraction and shoaling). Thus results
can be useful and be used to process the next modelling step (Morphological model).
According to the model results (Figure 5.5), in both NE wave and SE wave scenarios, when
wave is travelling toward the coast line, there are two wave focusing points: Tam Quan
head land and Southern head land (near Hoai Hai Commune). The focusing regions have
significant changes in wave direction and have higher wave energy which leads to the
strong displacement of sediment in these regions. Under the Northeast wave condition,
along the coast line of the study area, the significant wave height has the value in the
range from 1.6 to 1.8 meters. These values are higher in the head land and the inlet
regions, and slightly smaller at the south of the head lands where the wave are diverge.
As mentioned above, the mean wave direction also changes from 490 (off shore) to 550 ÷
900. In the Southeast wave condition, the significant wave height is much smaller than in
the northern wave in a range from 0.45 to 0.65 meters accompany with the mean wave
direction change from 1350 to 1050 ÷ 1200 when moving to the coast.
Over all, the northern wave has the significant wave height higher than the southern
wave which implies that the current caused by northern wave have more ability in
movement sediment. However, the Northeast wave season only lasts in four months, half
a the Southeast wave period as well as the complex river flow regime occurs in Northern
wave season. Hence, the conclusion about the dominant direction of the longshore
sediment transport only can be carried out after analysing the results of the
morphological model.
67
Morphological Modelling of Lai Giang inlet, Vietnam
Figure 5.5 Wave propagate in Lai Giang inlet area under difference directions
68
Morphological Modelling of Lai Giang inlet, Vietnam
5.3. Flow
To understand the current characteristic in the study area based on the modelling results,
this section is going to describe separate the tidal induced current, the interaction
between the tidal induced current and the dominant wave (NE and SE) induced current
and the over all current including the river flow during flood.
5.3.1. Tidal current
Figure 5.6 describes the tidal induced current at different tidal period focusing on the
inlet and the surrounding area.
•
As the result of the different gradient of the water level, the tidal current is
directly affected by the water level and the tidal propagation. The same as the
tidal propagation, current at the sea side is strong and at the entrance of the inlet
is weak during the lower high tide (11:00 5th) and low tide (5:00 5th) in semidiurnal tidal day. During the higher high tide (22:00 5th) and low tide (15:00 5th) of
semi-diurnal tidal day and the diurnal day (8:00 and 22:00 9th), the situation is
opposite (the sea side current is weak and the inlet current is strong). This period
might have the slightly sedimentation in front of the entrance of the inlet. In the
Northern lagoon, there is almost no existence of the current during one cycle of
spring and neap tide of the simulation time. The only time the current occur in the
lagoon is during the spring tide (22:00 9th). This phenomenal happens because of
the existence of the secondary inner spit from 2005 (remote sensing image of Lai
Giang inlet in 2005 – Figure 3.2). This inner spit is partly submerged make the
north lagoon became partly enclose, it only has the current when the water level
is extremely high.
•
Tidal forcing in Lai Giang inlet in November 2006 is quite weak with the tidal
current magnitude along the coast line of 0.1 ÷ 0.25 m/s, the maximum value of
0.4 m/s. The velocity at the entrance of the inlet is higher, in the range from 0.2 to
0.5 m/s, sometimes reaches 0.6 m/s. The velocity along the coast is quite small,
probably hardly have capacity to carry sediment sand (with the diameters of 0.24
mm), but the current at the entrance of the inlet is higher and might able to carry
the small amount of sediment.
5.3.2. Tide- and wave- induced current
Total current without the consideration of river flow is the result of the interaction
between tidal induced current and wave induced current. The total current has seasonal
characteristic caused by the seasonal characteristic of wave climate with the Northeast
wave in the winter monsoon and the Southern Wave in the summer monsoon. Under the
influence of the wave climate, there is a the region along the coast with the distance from
69
Morphological Modelling of Lai Giang inlet, Vietnam
200 meters to 600 meters from the coast line which have longshore current changing by
season and depending on wave direction.
•
As can be seen in Figure 5.7, in the Northeast wave with the significant wave
height of 2.14 meters and the peak period of 8.44 second (according to the
wave analysis in Section 4.2.1), the speed of total longshore current is quite
high from 0.4 m/s to 0.6 m/s and the current tends to move South ward
especially in the northern part of the inlet. However, there is a presence of the
extreme velocity (4 m/s in 15:00 5th November) at several point in the
southern coast of the inlet. It could definitely not happen in reality. The reason
for this is the unstable of the wave model under the high significant wave
height condition (2.14 meters in this scenario). In six periods of the computed
flow fields are extracted, the total current just differs from the tidal current in
longshore current. This current is intensified when the tidal current moving
south ward (5:00 and 22:00 5th, 22:00 9th) and reduced when tidal current
moving north ward (remaining periods). During the simulation of the NE wave,
there are almost no sign of the current flowing in to the inlet because of high
sedimentation level at the entrance and the choking starts to occur.
•
Under the Southeast wave ( with the significant wave height of 0.81 meters
and the period of 7.12 seconds) condition, opposite to the Northeast wave
condition, there is a presence of the longshore current moving to the north
with the speed of 0.2 ÷ 0.5 m/s (Figure 5.8). The north-ward longshore current
is not strong but it occurs continuously in eight months. Hence, it is still have
sedimentation in front of the inlet but the level the sedimentation is not high
enough to make the inlet close up. It can be proved by the Figure 5.8 with the
current flowing in and out of the inlet alternatively during flood and ebb
period.
The outstanding feature of the total current is the seasonal longshore current flowing the
north and south in summer and winter monsoon, respectively. This feature will mainly
contribute in the longshore sediment transportation of this area.
70
Morphological Modelling of Lai Giang inlet, Vietnam
Figure 5.6 Tidal induced current at Lai Giang inlet area
71
Morphological Modelling of Lai Giang inlet, Vietnam
Figure 5.7 Tidal induced and NE wave induced current at Lai Giang inlet area
72
Morphological Modelling of Lai Giang inlet, Vietnam
Figure 5.8 Tidal induced and SE wave induced current at Lai Giang inlet area
73
Morphological Modelling of Lai Giang inlet, Vietnam
5.3.3. Current influenced by tide, wave and river flow
As mentioned in those previous chapters, the river flow acts an important role in
morphology of the study area in the winter monsoon. However, the measurement data
during flood is restricted. The only available data were recorded for the flood in
November 2008 with the peak discharge is approximately 2300 m3/s at Lai Giang (Bong
Son) bridge near the river boundary of the model. The flood lasted three days (from 21:00
18th to 6:00 21st November 2008).
Figure 5.9 Water level condition at different boundary of the model
Modelling result of the flood with and without the northern wave influence scenarios are
extracted in 3 periods (Figure 5.10) which are the peak flood period (15:00 19th), before
(9:00 19th) and after (00:00 20th) that correspond to the ebb and flood tide.
As we can see from Figure 5.10 (more detail in Figure A.3), the speed of the velocity
flowing through the entrance of the inlet depends on the tidal regime (flood and ebb) and
the river flood regime. In the peak of the flood (at 15:00 19th), the velocity is not affected
by tidal current, the value is significant from 1.5 to 2.5 m/s. The influence area is large
with the distance of 800 meters (approximately) from the entrance. During the two
remaining periods, the water level caused by flood from the river is almost the same, but
the velocity at 9:00 19th (1 ÷ 2 m/s) is higher than the one at 0:00 20th (0.5 ÷ 0.9 m/s)
because the velocity is intensified during the ebb tide (9:00 19th) and reduced during flood
tide (0:00 20th). As the result, the “affected” area also changes due to tidal regime with
the distance of 200 ÷ 600 meter from the entrance.
Beside the significant velocity at the entrance during flood, the velocity at the curved part
of the river is also high (from 1 ÷ 2 m/s, can reach 2.4 m/s at peak flood).
Under the influence of Northeast wave, the velocity is not affected; however, the “affect
area” is shortened.
74
Morphological Modelling of Lai Giang inlet, Vietnam
Figure 5.10 Tidal induced current under river flow condition without and with NE wave climate
75
Morphological Modelling of Lai Giang inlet, Vietnam
The sudden change in the magnitude of velocity at the entrance and the wandering part
lead to the increasing carrying sediment capacity from the river, widening the entrance or
carrying out the amount of sedimentation by NE wave maintaining the open of the inlet.
All the sediment the sediment will be carry out and deposit out side the “affected area”.
Thus the intensity of the northern wave will control the place of deposition.
5.4. Sediment transport
After analysing all the main features of tide, wave and flow which govern the morphology
in Lai Giang area, the sediment transportation results will give the full picture of the idea
of morphology of this area, how the morphology of the area is driven by the seasonal
wave, the flood and the storm.
5.4.1. Influence of tidal regime
Base on analysing the horizontal tide in Section 5.3.1 above, the velocity a long the coast
is quite small with the value of 0.1 ÷ 0.25 m/s. At the entrance of the inlet, the velocity is
slightly higher with the value from 0.2 to 0.5 m/s. With this intensity of the tidal forcing,
there is no sediment displacement along the coast or the tidal induced longshore current
is not strong enough to transport sediment. But at the inlet area, during ebb tide, the
inlet is scoured 2mm and that amount of sediment is deposited with the thickness of 4
mm further in front of the inlet. All the erosion and deposition take place nearer to the
up-drift spit. Figure 5.11 which indicates clearly the influence of tidal regime on sediment
transportation has been proved it.
Figure 5.11 Cumulative sedimentation (positive) and erosion (negative) under tidal influence
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Morphological Modelling of Lai Giang inlet, Vietnam
5.4.2. Influence of the individual wave climate
Figure 5.12 shows the cumulative sedimentation (positive) and erosion (negative) under
the influence of NE and SE Wave interact with tidal regime after one week (from 0:00 4th
to 11th November 2006). Obviously, the northern wave causes much more sediment
transportation compared to the southern wave.
•
Under the NE wave climate condition which produce the longshore current south
ward, sedimentation with the thickness of 1 meters happens on shore along the
coast line (with the approximate distance of 50 meters) and the erosion of 0.5
meters further off shore (with the distance from 50 to 200 meters from the coast
line). Sedimentation of 1 meter within a week is impossible in reality. Thus with
the strong significant wave height (2.14 meters) coming from the north, beside the
“real” amount of sediment transport, there are a large amount of sediment
transportation caused by the adjustment to reach the equilibrium state of the
beach profile of the model. However, we still can see that the northern spit of the
inlet is built up a larger amount compared to the southern spit.
•
With the SE wave climate, the significant wave height is moderate, so the model is
more stable (it does not need to adjust itself to reaching the equilibrium state).
The displacement of sediment focuses on the entrance area of the inlet with the
sedimentation of 30 centimetres, approximately. The consequence is the channel
is shallower and narrower; bars have been built up and the one connected to the
southern spit is larger than the one connected to the northern spit.
The dominant wave (NE and SE) influenced on non-cohesive sediment transport scenarios
partly describe the movement of sediment which are moving southward and building up
the northern spit in NE wave and moving north-ward and building up the southern spit in
SE wave.
5.4.2. Influence of the total wave climate
The duration of the NE and SE wave within a year is different. Thus, to further understand
the process, the typical year has been carried out. Figure 5.13 indicates the sedimentation
and erosion in three periods: January (winter monsoon with the occurrence of NE wave),
February to September (summer monsoon with the wave climate mainly has SE direction)
and October to December (winter monsoon with the NE wave condition).
•
After January, the sediment transport direction can not be seen clearly because
the adjustment to the equilibrium state of the model is dominating. There are
mostly crossshore transport in shore-ward direction.
•
From February to September, assuming that the model reached its the equilibrium
state of the beach profile, deposition in the inlet area takes place focusing in front
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Morphological Modelling of Lai Giang inlet, Vietnam
of the entrance and the south of the inlet. The sedimentation of the bed level is
from 1.5 to 2 meters and some certain part in front of the entrance the level can
be 2.5 meters.
•
In the October to December period, with the strong northern wave, there are
higher deposited sediment and the sedimentation process focus on the northern
spit. The area is accumulated 2.5 meters more after three months.
So, without the influence of the river flow, the channel is completely closed up after
one year (assuming that the sedimentation level to reach equilibrium state of the
model is excluded) and fills up 4 more meters at the entrance compared to the initial
bathymetry (Figure A.5).
5.4.3. Influence of the interaction between the river flow and wave climate
A flood in November 2008 scenarios will make clear how the river flow contributes to the
morphology of this study area. Figure 5.14 shows the changes of morphology after a
three-day flood without the wave action (left) and interacted with wave climate (right). As
a result of high river flow (left of Figure 5.14), the entrance is eroded from 0.8 to 1.2
meters after flood and the sediment is brought and deposited further out side of the
inlet. The entrance is deepened at the up drift spit area because of the existence of the
submerged bars which direct the flow focus on the left of the entrance. When the river
flow interacts with the wave action, the amount of erosion at the entrance is reduced.
There are more sediment deposited in the left side of the main ebb channel, but the up
drift spit is not prolonged. It is caused by the main ebb channel located more to the left
side of the entrance.
According to all the results that have been described and analysed above, we can confirm
the assumption in the conceptual model. During the summer monsoon, the inlet migrates
to the north because SE wave influence. During the winter monsoon, the interaction
between the river flood and NE wave make the inlet can not migrate. This leads to the net
migration of the inlet has north ward direction after one year.
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Morphological Modelling of Lai Giang inlet, Vietnam
Figure 5.12 Cumulative sedimentation (positive) and erosion (negative) under the influence of NE and SE Wave
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Morphological Modelling of Lai Giang inlet, Vietnam
Figure 5.13 Seasonal cumulative sedimentation (positive) and erosion (negative) in a typical year
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Morphological Modelling of Lai Giang inlet, Vietnam
Figure 5.14 Cumulative sedimentation (positive) and erosion (negative) after the flood in November 2008 with and without wave influence
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Morphological Modelling of Lai Giang inlet, Vietnam
5.4.4. Influence of the storm
According to the 30-year typhoon’s statistic (Table 2.2), the Lingling storm, which hit Binh
Dinh province directly with the NE wind at speeds of 33 m/s, has been selected for the
modelling scenarios.
Under the extreme wave condition, with the significant wave height in the range of 3 ÷ 6
meters (Figure A.5) and the period of 10 ÷ 12 second coming from NE direction, the inlet
closed up after 5 days of storm (Figure5.15). The deposition level can go up to 3 meters.
Figure 5.15 Cumulative sedimentation (positive) and erosion (negative) after Lingling storm
5.4.5. Longshore sediment transport of Lai Giang inlet
The movement of the longshore sediment transport of Lai Giang inlet area is indicated by the
calculated result at transects A, B and C (only for the further consideration). The positions of
the transects and the positive direction of sediment transport are shown in Figure 5.16 and
the quantitative value of the longshore transport is presented in Table 5.1.
In the result of NE wave scenario, the net transport at transect A and B have to be a positive
value which means the longshore sediment transport is moving to the south under the NE
wave condition. But, there is the north ward movement of the longshore at transect B (76.77 m3/day). This result is caused by several points with the extreme velocity (Figure 5.7)
due to the instability of the northern wave model.
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Morphological Modelling of Lai Giang inlet, Vietnam
Figure 5.16 Transects and positive directions for sediment transport computation at the inlet
Table 5.1 Sediment transport through transects (m3/day)
Scenario
Transect
NE wave
SE wave
Flood + NE wave
Typical
wave
climate
year
Jan
Feb - Sep
Oct - Dec
North ward
transport (-)
South ward
transport (+)
Net
A
-4.61
168.09
163.49
B
-100.94
24.17
-76.77
C
A
B
-24.93
-128.10
-50.69
54.91
0.00
0.00
29.99
-128.10
-50.69
A
B
C
-5.52
-101.89
-31.58
159.71
25.05
100.21
154.19
-76.72
68.67
A
B
A
-3.44
-8.02
-2.90
28.26
13.26
1.69
24.83
5.25
-1.21
B
A
B
-1.63
-0.49
-1.01
0.74
24.96
9.25
-0.90
24.47
8.24
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Morphological Modelling of Lai Giang inlet, Vietnam
However, if we consider the transportation at the transect C which is located further to the
south to avoid the extreme velocity, the sediment transport has south ward movement
direction. Based of the gross transport between transect A and C, the difference is 133.5
m3/day, we can confirm that there is a large amount of sediment deposited at the inlet area.
In the SE wave scenario, the southern wave model is more stable than the northern one.
Thus, there is no sediment moving to the south under the SE wave condition. The net
longshore sediment transport at the inlet is 50.69 to 128.1 m3/day in North direction.
In river flood scenario, the rate of the south ward transport (transect C/transect A=1/2,
approximately) is higher than the NE wave scenario one (transect C/transect A=1/6,
approximately) leading to the conclusion that the river flow prevents sedimentation at the
inlet area.
The seasonal variation of longshore sediment transport is described in the typical wave
climate year scenario. In January (the winter monsoon), the longshore sediment moves to
the south with the total amount of 25 m3/day crossing the transect A and 5 m3/day crossing
at the transect B. In the area between transect A and B, deposition also take place during this
period. From February to September which is the summer monsoon period, the southern
wave dominates creating an approximate sediment transport magnitude of 1 m3/day in
North direction. October to December is the period of winter monsoon or wet season, again.
Like the January period, this period last in three months causing the same amount of
sediment moving to the south, about 24.5 m3/day and 8 m3/day at transect A and B,
respectively. After one year, the longshore sediment tend to move to the south and deposit
the large amount at the inlet area without the interaction with the river flow in the flood
season which is significant during the three months from October to December. Under the
river flow influence, the deposition level might reduce. However to calculate the accurate
reducing level, further flood measurement data are required.
5.5. Limitation of the morphodynamic model
Due to some uncertainty in the conditions, the morphodynamic model of Lai Giang inlet has
some limitations which are:
The lack of measurement data
•
As the recorded the river flow data during flood season only lasts three days, the
value of the river is not certain to model the full interaction between the tide, the
wave and the river flow.
•
The off-shore wave data (with the distance of 75 kilometres from the inlet) was
recorded in ten years (from 1996 to 2005), but the on-shore wave data and the
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Morphological Modelling of Lai Giang inlet, Vietnam
measured sediment information is not available to calibrate the propagation of the
wave in the Delft3D-Wave and Sediment transport model leading to the self
adjustment of the model to reach the equilibrium state of beach profile and the
inaccuracy in quantitative result of the model. Furthermore, the wave model is not
stable under the high significant wave height condition leading to some points which
have extreme velocity value along the coast.
Limitation in methodology
Within this study, due to the time restriction, some part of the study methodology is not
appropriate:
•
During the typical scenario, to reduce the computation time, the morphological factor
has been applied with the value of 24. It means that, the model only runs for 15 days
which is one spring and neap cycle (or one representative tide). The selection the
representative tide is limited, only base on the average tidal range, maximum and
minimum tidal range.
•
When applying the morphological factor, the sensibility analysis needs to be done.
However, within this thesis, due to the time limitation, the morphological factor of 24
is assumed below the limit value.
•
This model use the Van Rijn’s sediment transport formula which is the default method
of the sediment model. The sensitivity analysis in selecting the suitable sediment
transport formula needs to be carried out in further research.
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Morphological Modelling of Lai Giang inlet, Vietnam
CHAPTER 6. ENGINEERING SOLUTION
Based on the finding of the morphological behaviour of Lai Giang inlet in the previous
chapters, this section proposes the possible solution to stabilize the inlet as well as to reduce
the sedimentation level.
6.1 Increasing the basin area
As mentioned in Chapter 1, the southern lagoon behind the inlet is almost isolated for the
aquaculture purposes. Currently, only the northern lagoon is affected by the current and the
river flow. However, the existence of the secondary inner spit (from 2005, according to the
remote sensing data – Figure 3.2) makes the north lagoon partly close, the exchange of the
water only occurs during spring tide. By removing the secondary inner spit, the northern
lagoon is re-opened, which means that the basin area increases, leading to the increasing the
tidal prism. The dredging action might be processed to reach the required tidal prism value (r
= P / MMax > 150 – Chapter 1) for the equilibrium state of the inlet. This solution helps the
water in the lagoon exchanges frequently, creates the good condition for the eco-system of
the area. However, after the dredging action, the bathymetry of the northern lagoon is
deeper than the average water depth in other part of the basin, the river flow might focus on
the northern lagoon and create the new inlet further at the north.
6.2 Dredging
Dredging is deepening or widening a river, a harbour or a channel by removing sand, mud or
silt. This is a temporal solution for this situation. As there are large amount of deposition
(caused by longshore current) in front of the entrance of the inlet, the dredging action at the
ebb channel is needed to be taken yearly to maintain the required depth for good flushing
and navigation. From the result of the typical scenario (Figure A.4), the annual dredging
depth is at least 4 meters. Thus, it is a costly solution and effects the environment in the
surrounding area (increase the suspended sediment dramatically leading the reduction of the
water clarity) during the dredging process.
6.3 Jetties
Jetties are usually stone structures constructed at the navigational channel to prevent
sediment deposit in the channel and prevent the wave action affected the vessels crossing
the channel (Dean and Dalrymple, 2002). For the Lai Giang inlet, it is proposed to place the
jetties at two spits (Figure 6.1). And the end of the jetty must reach outside the deposited
zone along the coast (the distance of 300 meters, approximately). The direction of the jetties
is not perpendicular with the coast, it parallels with the main ebb channel creating the good
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Morphological Modelling of Lai Giang inlet, Vietnam
condition for flowing of the channel. Jetties is long-term solution and cheaper compared with
other engineering solution (such as break water). However, there will be the accretion in
front of the jetty placed at the northern spit and the erosion after the jetty placed at the
southern spit. The long-term erosion might cause the retreat of the southern spit.
Figure 6.1 The proposed position of jetties
6.4 Sand fluidization
Another possible solution is “Sand fluidization”. According to Parks (1993), fluidization is
defined as the pumping of additional water beyond the quicksand point to make a 50:50
sand-water slurry that will flow down a slight grade and is pumpable for significant distances.
In the inlet, the pipes, which carry sand-water mixture, are buried along the axis of the ebb
channel, staring from two sides of the channel and declining to the mid-length position of the
channel. The jet-eductor pump is also placed in the mid-length position. When the clear
water is pumped through the fluidization pipe, part of the clear water mix with the sand is
pumped out. This process helps prevent the sedimentation in the ebb channel accompanying
with stabilize the inlet. Beside that, by eroding the main ebb channel, the cross section of the
inlet increases leading to the intensifying of the discharge flowing out through the inlet.
Furthermore, it tends to be less expensive on the long-term than dredging and
environmentally friendly. The pipe line for this method allows the cheap and plastic one.
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Morphological Modelling of Lai Giang inlet, Vietnam
Figure 6.2 Flow in fluidized channel (Parks, 1993)
6.5 Optimum solution
Selection of the optimum solution is based on the following criteria:
1. Reduce the sedimentation level and stabilize the inlet
2. Preference to the soft solution
3. Minimum environmental impact/ Enhancing the natural processes
4. Cost-effectiveness
Achieving the inlet stability and reducing the sedimentation is the most important criteria for
the solution. However, we still need to consider the impact on the environment of this area
and the balance between the cost and effectiveness. Table 6.1 below indicates how these
solutions of the criteria meets. Increasing the basin area seems to be the good solution. But if
the dredging action also takes part in, it might cause the bad influence for the entire
morphology of this area by creating the new inlet further at the north. Dredging is a soft
solution but it has an effect on the water clarity during the dredging action and it is costly in
long-term. Jetties also have certain environmental impact by depositing in one side of the
inlet and eroding the other side but it less expensive than dredging. Only the sand fluidization
meets almost all the criteria. However, to make sure this solution really works, further
research needs to be carried out.
Table 6.1 Comparison between the propose solution and criteria (x: good; o: bad; -: unknown)
Solution
Criteria 1
Criteria 2
Criteria 3
Criteria 4
Increasing the basin area
x/o
x
x
x
Dredging
x
x
o
o
Jetties
x
o
o
x
Sand fluidization
x
-
x
x
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Morphological Modelling of Lai Giang inlet, Vietnam
CHAPTER 7. CONCLUSION AND RECOMMENDATION
7.1. Conclusion
The main objective of this thesis is to understand the physical processes governing the
morphological behaviour of Lai Giang inlet. To obtain that, the physical setting of the area is
studied by collecting and analysing data, and the general picture about the physical processes
of Lai Giang inlet is created. It also addresses the main driving force of the morphology which
is the tidal characteristic, the wave climate and the river flow.
On the basis of all the available documentation, the hypothesis of the conceptual model is
developed to explain the role of the driving forces in the morphological behaviour of the
study area. The river force is the dominant main factor during the flood season. It makes the
inlet change direction from Northwest-Southeast (the direction in dry season) to SouthwestNortheast, and breaching sometime happens during such an extreme event. Although the
Northeast waves occur in this period, the river flood that makes the inlet can not moving
southward. Unlike the flood season, during dry season wave-driven longshore current
dominates because of the relatively small river discharge. In this case, where Southeast wave
action dominates over tide and river flood, accretion in the inlet and northward barrier
migration takes place.
Based on the description and analysis of the result of the morphodynamic model for Lai
Giang inlet, which include Delft3D-Flow, Delft3D-Wave and sediment transport, all the
questions related to prove the hypothesis of the conceptual model, have been answered
resulting in a picture of the morphological chances of Lai Giang inlet with the following main
ideas:
1.
Under the weak tide forcing with the tidal current speed of 0.2 ÷ 0.5 m/s at the inlet and
0.1 ÷ 0.25 m/s along the coast, the erosion (with the thickness of 2 mm after haft a
month) only takes place at the entrance of the inlet and deposition (with the thickness
of 4mm after haft a month) happens further outside. There does not have any sediment
displacement a long the coast. The sedimentation and erosion happen at the entrance of
the inlet, nearer to the up-drift spit.
2.
The morphology of the inlet is mainly influenced by waves which have a seasonal
character, leading to the seasonal sediment transport. Under the strong NE wave
condition (high significant wave height), the longshore sediment transport moves to the
south, depositing a large amount of sediment at the inlet area and close up the inlet,
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Morphological Modelling of Lai Giang inlet, Vietnam
eventually. Under the SE wave condition, the sediment moves to the north and gradually
build up the down-drift spit. The combination of four month northern wave (winter
monsoon) and eight month of southern wave reflect the influence of wave climate
within one typical year on Lai Giang inlet. After experiencing the summer monsoon
(happens in eight months from February until the end of September), the sedimentation
of bed level is 1.5 meters. Although the winter monsoon only lasts four months, the
sedimentation of the bed level is higher (2.5 meter) which means the area accumulates
2.5 meters in four months. The total sediment transport across two transects located
the north and the south of the inlet are 2250 m3 and 750 m3, respectively. So, without
the influence of the river flow, the channel is completely closed up after one year.
3.
Influence of the river flow accompanied by the tidal regime and Northeast wave climate
causes the sedimentation level in front of the inlet to be reduced. Another interesting
result obtained from the model result is that, although the total longshore transport is
moving south ward, the strong flood river with the main ebb channel nearer to the
northern spit keeps flowing and prevents the deposition at the northern spit. Moreover,
the southern spit of the inlet is built up during the southern wave season. Thus this
interaction makes the inlet gradually migrate to the north.
4.
Storm is an extreme condition of the Northeast wave. So its influence on the
morphology is almost the same as the normal Northeast wave condition with higher
intensity, which means that the inlet is closed up after a short storm period.
The morphodynamic model of Lai Giang inlet has some limitations. Due to a lack of
measurement data, the wave model is not calibrated, leading to the instability of the
northern wave model, the sediment transport model is also not calibrated and the discharge
of the flood river is not enough to computed the flood scenario in long duration.
Furthermore, due to time restriction, the sensibility analysis for the morphological factor and
the sediment transport formula has not been considered.
After gaining understanding of the morphological behaviour, possible solutions to stabilize
the inlet are recommended. Sand fluidization satisfies all the criteria (stabilizing the inlet with
the consideration of environment). However, the deeper investigation of this solution is
required.
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Morphological Modelling of Lai Giang inlet, Vietnam
Figure 7.1 Illustration of morphological change in summer and winter monsoon
7.2. Recommendation
To improve the morphological modelling of Lai Giang inlet, it is given several suggestions such
as:
First of all, the temporal resolution of documentations in Lai Giang inlet area which is the
foundation of the conceptual model is not precise enough, only mention the taken year or
the year happened the event, but the period (or the season) in that year is not given. So,
better temporal resolution of the document is needed to investigate to improve the
conceptual model.
Secondly, the Deltf3D-Wave and Sediment transport model is not calibrated leading to the
self adjustment of the equilibrium profile of the model during the strong northern wave.
Moreover, the data of the river flow is not long enough to compute the typical year scenario
with the full interaction of the tide, the wave and the river flow. Thus, to be more
quantitative and more accurate, more detail in observation data need to be taken. The
significant wave height and the wave direction at one or two stations along the coast could
be useful to calibrate how the wave propagates from off-shore to on-shore. Based on the
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Morphological Modelling of Lai Giang inlet, Vietnam
observed sediment transport through the cross section, the sediment transport model might
be adjusted to give the more accurate result. The complete river flow data in one season
could be applied to model the complete typical scenario.
Besides, for the further research, the sensitivity analysis of the morphological factor and the
sediment formula need to taken into consideration. The selection of the representative tide
needs to do in more appropriate way.
Furthermore, the morphological modeling for the Lai Giang inlet only partly explain why inlet
migrate to the north, but it could not be able to see the migration in the model result. Hence,
the model might need to compute in longer duration (such as 5 years of 10 years) to see the
migration and the morphological behaviour of the inlet in broader scale.
Finally, the potential engineering solution also needs the detail research before applying it to
stabilize the inlet.
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Morphological Modelling of Lai Giang inlet, Vietnam
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APPENDIX
Figure A. 1 Velocity at the inlet during flood in November 2008 with and without wave influence
Figure A.2 Discharge through the inlet cross section during flood in November 2008
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Morphological Modelling of Lai Giang inlet, Vietnam
Figure A. 3 Magnitude of the velocity during the flood in November 2008
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Morphological Modelling of Lai Giang inlet, Vietnam
Figure A.4 Cumulative sedimentation (positive) and erosion (negative) after a typical year of
tide and wave
Figure A.5 Wave rose during Lingling storm
97