Thạc Sĩ Safety assessment of sea dikes in Vietnam - A case study in Namdinh Province

Master of Science Thesis
UNESCO-IHE Delft, June 2004 II
Acknowledgments
This work was performed as a part of the MSc program of the Hydraulic
Engineering Faculty, UNESCO-IHE, Delft, The Netherlands and was carried out
at UNESCO-IHE from October 2003 to June 2004. The whole MSc. program in IHE
lasted 20 months (from October 2002 to June 2004) included core courses, field
trips, group works, and the thesis.
First of all I would like to acknowledge the sponsors, NFP; CICAT, TU-Delft under
the framework of CE-HWRU project; and RWS/DWW for the financial support, and
the graduation committee for their guidance and judgement.
I owe special words of many thanks to: Mr. Krystian Pilarczyk- my supervisor from
DWW- for his concern, guidance, enthusiasm, valuable advice and assistance with
so much warmth and care, Dr. Randa Hassan - my supervisor and coordinator- for
her frequently constant support and directed guidance during my study at IHE
with plenty of warm welcome and care, Mr. Thang and Mr. Le Duc Ngan from
DDMFC for arrangement of pleasant and interesting site visit to province of
Namdinh, Mr. Hans Noppen, Mr. Wilfred Molenaar (TU-Delft) for their sharing
literature and advices in probabilistic approach, Mr. Henk Jan Verhagen (TU-
Delft) for his valuable advices in wave calculation and probabilistic design, Mr.
Paul Bonnier (PLASIX B.V) and Mr. Peter The (RWS/DWW) for their valuable
guidance of using PLAXIS for solving geotechnical problem, Mr. Bas Jonkman (TU-
Delft) for his comments on probabilistic calculation, Mr. Jurriaan Lambeek from
Delft Hydraulic for his warm welcome and friendship.
My high appreciation goes to all the teachers who have taught and armed
me with such a valuable knowledge to my future career both in Vietnam and
in The Netherlands; IHE staffs, my colleagues, friends and my classmates
for their support, assistance and for making my stay here filled with joys
and memories.
I would like to keep the great thanks to my sweet family for their great
support and always being source of encouragement, motivation and energy.
Mai Van Cong
UNESCO-IHE Delft, June 2004 Master of Science Thesis
UNESCO-IHE Delft, June 2004 III
Abstract
Vietnam has about 3260 km of coastline, primarily consisting of low-lying coastal areas
which are protected by sea dikes, natural dunes and mountains. More than 165 km of
coastline lies within the Red River Delta, a densely populated region which experiences
substantial dynamic changes and destruction due to frequent intense impacts from the
sea (typhoons, changes in sea level, currents, etc). This dynamic coastline is mainly
protected by sea dike system which has been developed for almost hundred years.
The NamDinh Province constitutes part of this coastline, with total length of about 70
km, which is protected by sea dikes. The sea dike system has been heavily damaged.
There were many times of dike breach which caused serious flooding and losses. The
situation of NamDinh sea dikes can be considered a representative for coastal area in
Northern part of Vietnam.
In recent years there has been a number of studies aiming at understanding the situation
of sea defences system in NamDinh, assess the safety of the and find the solutions to
mitigate these losses for this region. However, due to the lack of data and design tools
the results of these studies, somehow, are still limited and the problem is still poorly
understood. Therefore adjustment of safety of the existing Namdinh sea defences
system is necessary.
This study is initiated with the main focus on analysis and assessment of safety of
Namdinh sea dikes. Firstly, the historical development of sea dike system in Namdinh
province is analysed base on historical record and collected data. Based on that the
possible causes of old-dike failures are carried out. Secondly, the study investigates all
possible failure mechanisms and their causes of the existing dikes. Follows by, the
safety assessment of the dikes is performed for possible failure modes in term of
hydraulic, structural and geotechnical related problems. Finally, conclusions on safety
of Namdinh sea dikes are stated and some recommendations (guidelines) of new sea
dike design in Namdinh and in Vietnam will be carried out.
The study is based on deterministic and probabilistic approaches. The latest Vietnamese
codes and Dutch codes for design of sea dikes and revetments are the basic references
for these analyses. Comparisons will be made to applying different design codes for
design of sea dikes in Namdinh as well as in Vietnam.
In general, analytical methods are applied in this study. However for solving some
specific related problems the advanced mathematic models are also applied as
calculation tools such as CRESS and BREAKWAT for some hydraulic related
problems; GEO-Slope and PLAXIS for geotechnical related ones; VaP and MathLab
models for probabilistic calculations. By doing this study the necessary engineering
knowledge and study skill to solve a problem in practice are also achieved.
Table of contents

Safety Assessment of Sea Dikes In Vietnam
A Case Study In Namdinh Province i


Table of contents
TABLE OF CONTENTS .I
LIST OF FIGURES .III
LIST OF TABLES .V
CHAPTER 1 INTRODUCTION 1
1.1 BACKGROUND .1
1.2 PROBLEM DEFINITIONS .2
1.3 SCOPE OF STUDY .4
1.4 AIMS OF STUDY .4
1.5 STUDY APPROACH .4
1.6 OUTLINE OF STUDY .5
CHAPTER 2 BOUNDARY CONDITIONS 6
2.1 NATURAL CONDITION .6
2.1.1 General description about study area .6
2.1.2 Delta topography .7
2.1.3 Soil characteristics and Geological features 8
2.1.4 Sediment transport conditions .8
2.1.5 Climate and Meteorology 10
2.1.6 Oceanography 10
2.1.6.1 Tides and tidal currents 10
2.1.6.2 Wind .11
2.1.6.3 Waves .12
2.2 PRESENT SITUATIONS OF SEA DIKE SYSTEM. 13
2.2.1 Sea defence system in NamDinh province .13
2.2.2 The current situation of sea dikes in Namdinh province. .15
CHAPTER 3 OVERVIEW OF PREVIOUS STUDIES AND REVIEW OF DESIGN
CONSIDERATION FOR SEA DIKE 17
3.1 OVERVIEW OF PREVIOUS STUDIES. .17
3.1.1 Historical changes of Namdinh coast 17
3.1.2 Overview of previous studies .19
3.2 DESIGN CONSIDERATION OF SEA DIKES 22
3.2.1 General .22
3.2.2 Design philosophy 22
3.2.3 Design methodology .24
3.2.4 Boundary Conditions and Interactions .25
3.2.4.2 Processes and interactions (Pilarczyk, Krystian W. 1998) .27
3.2.4.3 Consideration of slope protection 29
CHAPTER 4 POSSIBLE FAILURE MECHANISMS OF NAMDINH SEA DIKES 31
4.1 FROM HISTORICAL DEVELOPMENT OF THE SYSTEM TO FUTURE PREDICTION .31
4.1.1 General .31
4.1.2 From historical analyze of dike’s development to future prediction 32
4.1.2.1 Period from 1890 to 1971: .32
4.1.2.2 Period from 1971 to 2002: .34
4.1.2.3 Summary 36
4.2 POSSIBLE FAILURE MODES OF NAMDINH SEA DIKES 38
4.2.1 Hydraulic related failure modes 38
4.2.1.1 Wave run-up and wave overtopping 38
4.2.1.2 Failures of inner slope .40
4.2.1.3 Failures of outer slope .40
4.2.1.4 Foreshore erosion .41
4.2.2 Geo-technical related failure of dike’s body .42
4.2.2.1 Instability of inner and outer slopes 42
4.2.2.2 Local instability .43 Table of contents

Safety Assessment of Sea Dikes In Vietnam
A Case Study In Namdinh Province ii


4.2.2.3 Piping .43
4.2.2.4 Deformation and settlement of dike’s body 44
4.2.2.5 Liquefaction and softening 44
4.2.3 Structural failure modes (revetment) .45
4.2.3.1 Instability of armour layer. 45
4.2.3.2 The filter layers 46
4.2.3.3 Toe foot instabilities 47
CHAPTER 5 DETERMINISTIC ASSESSMENT OF THE SAFETY OF NAMDINH SEA DIKES 48
5.1 DEFINITION OF BOUNDARY CONDITION 48
5.1.1 Load boundary conditions .48
5.1.1.1 Design water levels 49
5.1.1.2 Design wave heights 52
5.1.2 Strength boundary conditions 54
5.2 SAFETY OF THE DIKES BY APPLYING VIETNAM AND DUTCH DESIGN CODES 55
5.2.1 Impact of wave run-up, wave overtopping and crest level to the related failures .55
5.2.1.1 Investigation of Wave run-up and wave overtopping computation 55
5.2.1.2 Investigation of design crest level .62
5.2.1.3 Failure mechanisms related to insufficient design crest level 65
5.2.2 Design of revetments and safety investigation for related failure modes .65
5.2.2.1 General information .65
5.2.2.2 Namdinh revetments and applied boundary conditions 67
5.2.2.3 Safety of slope protection of the dikes by applying Vietnamese Design Codes 68
5.2.2.4 Safety of slope protection of the dikes by applying Dutch Design Codes 77
5.2.3 Geotechnical related stability of the dikes 90
5.2.3.1. Generally geotechnical conditions, limit states and boundary conditions .90
5.2.3.2 Analyses of seepage through the dikes and subsoil. .92
5.2.3.3 Analyses of stress-strain and displacements. 94
5.2.3.5 Overall safety analysis .100
5.2.3.6 Slope stability analysis 102
5.2.3.7 Piping. 105
CHAPTER 6 PROBABILISTIC ASSESSMENT OF THE SAFETY OF NAMDINH SEA DIKES .106
6.1 INTRODUCTION .106
6.2 GENERAL BACKGROUND OF PROBABILISTIC CALCULATION .108
6.3 PROBABILISTIC ASSESSMENT OF THE SAFETY OF NAMDINH SEA DIKES .109
6.3.1 General reliability function and failure probability calculation 109
6.3.2 Statement of the problem 111
6.3.3 Probability of failure mechanism .112
6.3.3.1 Overtopping .112
6.3.3.2 Instability of armour layers of revetment 117
6.3.3.3 Piping .120
6.3.3.4 Sliding of dike slopes (outer and inner slopes) .123
6.3.4 Probability of dike failure 126
6.3.5 Conclusion .127
CHAPTER 7 CONCLUSIONS AND RECOMMENDATIONS .128
7.1 CONCLUSIONS 128
7.1.1 Conclusions on safety of the sea dikes in Namdinh 128
7.1.2 Conclusions on design of sea dikes in Vietnam 130
7.2. RECOMMENDATIONS 131
REFERENCES 133
APPENDICES .135 Table of contents

Safety Assessment of Sea Dikes In Vietnam
A Case Study In Namdinh Province iii


List of figures
FIGURE 1.1: A DAMAGED DIKE SECTION .3
FIGURE 1.2: HAITRIEU VILLAGE IN 1995 3
FIGURE 1.3: ABANDONED HAITRIEU IN 2001 .3
FIGURE. 2.2: SIEVE CURVE OF BEACH MATERIAL IN HAIHAU COAST .9
FIGURE. 2.3: LOCAL SEDIMENT BUDGET AT NAMDINH COAST (PRUSZAK ET AL. 2001) .9
FIGURE.2.4: MAIN SEASONAL WIND DIRECTIONS IN NORTHERN VIETNAM .11
FIGURE. 2.5: SKETCH OF DOUBLE DIKE SYSTEM AT HAIHAU BEACH 14
FIGURE. 2.6: SEA DIKE SYSTEM IN NAMDINH PROVINCE .14
FIGURE 2.7: SEVERELY ERODED DIKE WITH PLANTED CASUARINAS TREES AT HAIHAU BEACH. .15
FIGURE.2.8: CHARACTERISTIC CROSS-SECTION OF AN ERODED DIKE NEAR VANLY VILLAGE .15
FIGURE 2.9 REPRESENTATIVE CROSS SECTION OF SEA DIKES IN NAMDINH 16
FIGURE 3.1: COASTLINE CHANGE AT NAMDINH PROVINCE FROM 1912 TO 1981 17
FIGURE 3.2: COASTLINE CHANGE AT HAIHAU BEACH FROM 1905 TO 1992 (HUNG ET AL., 2001) .18
FIGURE 3.3: A FAILURE OF SEA DIKES AT HAIHAU IN NAMDINH(APRIL 1995) 18
FIGURE 3.4: SEDIMENT TRANSPORT ALONG THE NAMDINH COAST (PRUSZAK ET AL. 2001) 20
FIGURE 3.5: POSSIBLE FAILURE MECHANISMS 23
FIGURE 3.6: SIMPLIFIED EVENT TREE FOR A DIKE (PILARCZYK, KRYSTIAN W., 1998) .23
FIGURE 3.7: OVERVIEW OF DETERMINATION OF HYDRAULIC BOUNDARY CONDITIONS .26
FIGURE 4.1 SHORELINE DEFINITIONS. .31
FIGURE 4.3: RETREAT OF COASTLINE DURING FROM 1972 TO 2002 .36
FIGURE 4.5: HEAVY DAMAGE OF REVETMENT AND OUTER .41
FIGURE 4.5. EROSION OF OUTER SLOPE LEADED TO FAILURE OF DIKE BODY AND COLLAPSED REVETMENT 42
FIGURE 4.6: POSSIBLE LOCAL INSTABILITY DUE TO EXCEEDING CRITICAL LIMIT STATE. .43
FIGURE 4.7: PIPING MECHANISM IN SAND LAYER UNDERNEATH THE DIKE .43
FIGURE 4.8. MECHANISM OF POSSIBLE LIQUEFACTION AT NAMDINH SEA DIKES .44
FIGURE 4.8: DAMAGE OF COVER LAYER, THE FILTER LAYER EXPOSURES ( HAICHINH SECTION ) .45
FIGURE 4.9: FAILURE OF REVETMENT AT TRANSITION 46
FIGURE 4.9 FAILURE OF FILTER LAYER AT VANLY SECTION .46
FIGURE 4.10: FAILURE OF TOE STRUCTURE LEADS TO DAMAGE OF REVETMENT (HAITRIEU SECTION) .47
FIGURE 5.2: DEFINITION SKETCH FOR WAVE RUN-UP AND WAVE RUN-UP ON A SLOPE OF A DIKE 55
FIGURE 5.3: WAVE OVERTOPPING AT A DIKE 60
FIGURE 5.4: COMPONENTS CONTRIBUTE TO DESIGN CREST LEVEL OF THE DIKES 63
FIGURE 5.5: MIXED RIPRAP BLOCK REVETMENT- APPLIED AT NAMDINH .67
FIGURE 5.6: HEXAGONAL CONCRETE BLOCK REVETMENT- APPLIED AT NAMDINH .68
FIGURE 5.8: STABILITY OF REVETMENTS BY FIRST CHINESE FORMULA (11A) 72
FIGURE 5.9: STABILITY OF CONCRETE REVETMENT BY SECOND CHINESE FORMULA (12/12A) 73
FIGURE 5.9: APPLIED PILARCZYK’S FORMULA IN VIETNAMESE DESIGN CODE .74
FIGURE 5.10: COMPARISON BETWEEN PILARCZYK’S AND FIRST CHINESE FORMULA .75
FIGURE 5.11: VAN DER MEER’S AND PILARCZYK’S FORMULAE FOR ROCK REVETMENT .78
FIGURE 5.12: OBSERVATION DATA SUPPORTED TO VAN DER MEER FORMULA(17) .80
FIGURE 5.13: EXAMPLE OF RESHAPED PROFILE REACHED THE EQUILIBRIUM. 81
FIGURE 5.14: SIMULATION OF RESHAPED PROFILES BY BREAKWAT 82
FIGURE 5.15: PORE PRESSURE IN THE SUBSOIL DURING WAVE RUN-DOWN (PILARCZYK ET AL, 1998) 82
FIGURE 5.16: SCOUR MECHANISM NEAR THE TOE OF SLOPING STRUCTURE 84
FIGURE 5.17: SCHEMATIZATION OF SCOUR MECHANISM AT NAMDINH REVETMENT AT LWL .85

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Safety Assessment of Sea Dikes In Vietnam
A Case Study In Namdinh Province iv


FIGURE 5.18: MAXIMUM SCOUR DEPTH ACCORDING TO SUMER AND FREDSOE 2001 86
FIGURE 5.19: SOME ALTERNATIVE TOE PROTECTIONS (PILARCZYK ET AL, DIKES& REVETMENTS, 1998) 89
( Y M,E =SCOUR DEPTH; H= LOCAL WAVE HEIGHT) .89
FIGURE 5.20: GEOTECHNICAL GEOMETRY OF NAMDINH DIKE SECTION 90
FIGURE 5.21: BOUNDARY CONDITION FOR CALCULATIONS OF GEOTECHNICAL RELATED PROBLEMS 92
FIGURE 5.22. SEEPAGE FLOW FIELD 93
FIGURE 5.23. FLOW FIELD OF SEEPAGE IN ZONE A 93
FIGURE 5.24. ACTIVE GROUNDWATER PRESSURES .93
FIGURE 5.25: TOTAL DISPLACEMENTS OF THE PROBLEM IN 3 RESULT MODES 95
FIGURE 5.27: ADMISSIBLE HEAD FOR AVOIDING INSTABILITY 98
FIGURE 5.28: PLASTIC AND TENSION CUT-OFF POINT DEVELOP IN DIKE BODY AND SUBSOIL .99
FIGURE 5.29: STRESS CIRCLE TOUCHES COULOMB'S ENVELOPE .99
FIGURE 5.30: TOTAL INCREMENTAL DISPLACEMENTS INDICATING THE POSSIBLY FAILURE MECHANISM .101
FIGURE 5.31: SAFETY FACTOR IN RELATION OF LOADING STEPS AND DISPLACEMENT AS WELL 102
FIGURE 5.32: STABILITY OF OUTER SLOPE – GLE AND BISHOP METHODS .103
FIGURE 5.29: STABILITY OF INNER SLOPE – GLE AND BISHOP METHODS 104
FIGURE 6.1: FRAME WORK OF RISK ANALYSIS (SEE CUR 141, 1990) .107
FIGURE 6.2: DEFINITION OF A FAILURE BOUNDARY Z=0 108
FIGURE 6.4: FAULT TREE OF NAMDINH SEA DIKE. 111
FIGURE 6.5: DISTRIBUTION OF MHWL BASED ON STATISTICAL DATA BY USING BESTFIT 113
FIGURE 6.6: CONTRIBUTION OF VARIABLES TO OVERTOPPING FAILURE MODE. .116
FIGURE 6.10: CONTRIBUTION OF RELATED STOCHASTIC VARIABLE TO INSTABILITY OF ARMOUR LAYER. 119
FIGURE 6.11: PIPING AT A DIKE (CUR 141, 1990) 120
FIGURE 6.12: INFLUENCE OF THE STOCHASTIC VARIABLES TO FAILURE MODE OF PIPING 121