THE EFFECT OF FORESHORE LOWERING ON BAC LIEU SEA DIKE

In recent years, the erosion of beaches and estuaries in Bac Lieu has gradually intensified. Especially, foreshore lowering phenomenon has been happening at area adjacent Soc Trang province. This has severely affected the safety of local inhabitants, the quality of infrastructure, the degradation of environment, and the economic development. This is the critical reason that the author do the research The effect of foreshore lowering on Bac Lieu sea dike.

THUY LOI UNIVERSITY

LE THI HIEN

THE EFFECT OF FORESHORE LOWERING

ON BAC LIEU SEA DIKE

Major: Coastal Engineering and Management Code: 62580203

MASTER THESIS

SUPERVISER 1:

SUPERVISER 2:

Dr LE HAI TRUNG Assoc Prof Dr TRAN THANH TUNG

Ha Noi, October 2016

Le Thi Hien

I hereby declare that is the research work by myself under the supervisions of Assoc.Prof Dr Tran Thanh Tung and Dr Le Hai Trung The results and conclusions of thethesis are fidelity, which are not copied from any sources and any forms The referencedocuments relevant sources, the thesis has cited and recorded as prescribed Theresults of my thesis have not been published by me to any courses or any awards

Ha Noi, October 2016

Le Thi Hien

TABLE OF CONTENTS

LIST OF FIGURES iii

LIST OF TABLES vi

ABSTRACT 1

CHAPTER 1 LITERATURE REVIEW 3

1.1.Introduction 3

1.2.Foreshore and mangrove 4

1.3.Design of sea dike with regard to foreshore and mangrove 8

1.4.Discussion 10

CHAPTER 2 FORESHORE LOWERING AND ITS IMPACTS 12

2.1.Introduction 12

2.2.Description of the Bac Lieu sea dike 12

2.2.1 Topography and geomorphology 12

2.2.2 Hydrodynamic characteristics 14

2.2.3 Foreshore and mangrove 19

2.2.4 Bac Lieu sea dike system 23

2.3.Governing factors and classification of the foreshore lowering mechanism 26

2.3.1 Governing factors of the foreshore lowering mechanism 26

2.3.2 Classification of the foreshore lowering mechanism 31

2.4.Sea dike under impact of foreshore lowering 32

2.5.Summarise 33

CHAPTER 3 MODELling of the FORESHORE LOWERING mechanism 34

3.1.Overview 34

3.2.Model set – up, calibration and verification 34

3.2.1 Introduction to Mike 21 34

3.2.2 Model set –up for a large domain 36

3.2.3 Model set - up for a small domain 38

3.2.4 Model calibration and verification 40

3.3.Boundary conditions and simulation scenarios 49

3.3.1 Boundary conditions 49

3.4.Simulation results 50

3.4.1 Wave 50

3.4.2 Current 52

3.5.Dicussion and conclusion 54

CHAPTER 4 the EFECT OF FORESHORE LOWERING ON SEADIKE 57

4.1.Introduction 57

4.2.Changes of the bathymetry 57

4.3.Changes in the wave characteristics 59

4.4.The effect of foreshore lowering on sea dike 61

4.4.1 Sea dike design without mangrove forest 62

4.4.2 Sea dike design with mangrove forest in the case of a lowered foreshore 64

4.5.Discussion 66

CONCLUSION 69

APPENDICES 75

LIST OF FIGURES

Fig.1-1: Energy dissipation of waves on higher floodplains with mangroves (Albers et

al., 2013) 10

Fig.2-2: Map of the Mekong delta 12

Fig.2-3: The location of the study sea dike 13

Fig.2-4: The average long-term wave field of Con Dao(KHCN-BĐKH/11-15, 2015) 15 Fig.2-5: The average long-term wind field of Con Dao(KHCN-BĐKH/11-15, 2015).17 Fig.2-6: Direction and speed of the winter currents(KHCN-BĐKH/11-15, 2015) 18

Fig.2-7: Direction and speed of the summer currents(KHCN-BĐKH/11-15, 2015) 18

(i)The large eroded beach (ii)Deeply eroded offshore 20

Fig.2-8: The erosion phenomenon at the Northern area of windmill field in Bac Lieu ( Sources: Le Hai Trung) 20

Fig.2-9: The mangrove forest erosion at Ganh Hao town, Bac Lieu province 22

Fig.2-10:Mangrove tree left in front of Nha Mat embankment 22

Fig.2-11:The current status of forest beach at a canal gate out to the sea 22

Fig.2-12:Preliminary locations of survey Bac Lieu sea dike in Aug- 2014 23

Fig.2-13: The sea dike hardened of Nha Mat estuary 24

Fig.2-14:Two type of protection structures applied at Nha Mat 25

Fig.2-15:The construction current status of concrete dike from Lang Ong Hai Nam to Ganh Hao estuary 26

Fig.2-16:The current status of concrete dike defence Ganh Hao town 26

Fig.2-17: The sediment accumulation and the trend of sediment transport in Mekong Delta, which is affected dominantly by northeast monsoon (Sources: Marine economy.vn) 28

Fig.2-18:The section of tidal beach is surface eroded (Loi, 2015) 31

Fig.2-19: The section of tidal beach is background eroded (Loi, 2015) 31

Fig.2-20: The section of tidal beach is deep eroded (Loi, 2015) 32

Fig.2-21: The foreshore lowering process 33

Fig.3-22:The topography data of the study area(KHCN-BĐKH/11-15, 2015) 37 Fig.3-23:The large domain gird of the calculation area (KHCN-BĐKH/11-15, 2015) 37

Fig.3-24: The large domain topography of the calculation area (KHCN-BĐKH/11-15,

2015) 38

Fig.3-25:The topography data at the study area(KHCN-BĐKH/11-15, 2015) 39

Fig.3-26:The mesh of the calculation area (KHCN-BĐKH/11-15, 2015) 39

Fig.3-27: The topography of the detailed calculation area (KHCN-BĐKH/11-15, 2015) 40

Fig.3-28:The location of the water level measuring station 41

Fig.3-29:The calculation boundaries of the area 41

Fig.3-30:The calibrated result of measured water level and computed at Dinh An inlet 43

Fig.3-31: The calibrated result of measured water level and computed at Tran De inlet 43

Fig.3-32:The location of water level measuring stations 44

Fig.3-33:The verification result of measured water level and computed at My Thanh inlet 45

Fig.3-34: The verification result of measured water level and computed for Ganh Hao inlet 45

Fig.3-35:Wave height (left) and wave period (right) on the Bien Dong area of Wave Watch III model 46

Fig.3-36: Wave model calibration 47

Fig.3-37:The location of wave measuring station of Nha Mat estuary 48

Fig.3-38: Wave model verification 48

Fig.3-39: The distribution of the characteristic wave for the northeast monsoon (left), the southwest monsoon (right) 50

Fig.3-40: The location of extract points of calculation result 50

Fig.3-41:The significant wave height chart for climate one year (May 2011 to April 2012) at positions P1, P2, P3 51

Fig.3-42: The distribution of total currents (speed and direction) when tide going up (left), going down(right) in northeast monsoon scenario 52

Fig.3-43: The distribution of total currents (speed and direction) when tide going up (left), going down (right) in southwest monsoon scenario 52

Fig.3-44: The distribution of currents at the time of tide when going up (left) and going

down (right) - the scenario only considers to effect of tide 53

Fig.3-45:The chart of distribution of currents speed in 1 year climate (May 2011 - April 2012) at P1,P2, P3 position 53

Fig.3-46: Simulated result of the average currents distribution due to wind in northeast monsoon season (left) and southeast monsoon (right) 54

Fig.4-47: The measured topography of March 2011 (left) and January 2015 (right) (KHCN-BĐKH/11-15, 2015) 57

Fig.4-48: The calculation topography of March 2011 (left) and January 2015 (right) (KHCN-BĐKH/11-15, 2015) 58

Fig.4-49: The foreshore lowering in 2015 compared to 2011 58

Fig.4-50:The location of cross sections 59

Fig.4-51: Wave propagation and evolution of cross section 1 in 2011 and 2015 60

Fig.4-52: Wave propagation and evolution ofcross section 2 in 2011 and 2015 60

Fig.4-53: Wave propagation and evolution of cross section 3 in 2011 and 2015 61

Fig.4-54: The effect of the foreshore lowering on sea dike 68

LIST OF TABLES

Table 1-1: The area of mangrove forest in Mekong Delta (Loi, 2015) 4

Table 1-2:The area of mangrove forest in Mekong Delta from 1973 to 2012 5

Table 3-3: The parameters of the tidal model 42

Table 3-4: The coordinates of measuring stations 44

Table 3-5: The parameters of the wave model 46

Table 3-6: Simulation scenarios 49

Table 4-7:The input parameters used for Wadibe 62

Table 4-8: The parameters of mangrove forest 62

Table 4-9: Design parameters of cross section 2, without mangrove 64

Table 4-10: Design parameters of cross section 2 with topography in 2015 (with mangrove) 65

1 Rationale

In recent years, the erosion of beaches and estuaries in Bac Lieu has graduallyintensified Especially, foreshore lowering phenomenon has been happening at areaadjacent Soc Trang province This has severely affected the safety of local inhabitants,the quality of infrastructure, the degradation of environment, and the economicdevelopment This is the critical reason that the author do the research "The effect offoreshore lowering on Bac Lieu sea dike"

2 Study objectives

The aim of study is to find out the effect of mangrove foreshore lowering to Bac Lieuprovince In order to achieve the purpose of research that should deal with the bellowproblems:

- Literature review

- Foreshore lowering and its impacts

- Numerical modelling of the foreshore lowering

- The effect of foreshore lowering on sea dike

Simulation of the hydrodynamic regime, the erosion and accretion mechanism of thearea Evaluating the influences, effects of the foreshore lowering on sea dike Thestudy is applied to calculate specifically for 1 km of sea dike in Vinh Trach Dongcommune, Bac Lieu city, Bac Lieu province

3 Study approaches and methodology

Evaluation for the effect of foreshore lowering on Bac Lieu sea dike by using Mike21model for hydrodynamic regime simulation and then design a typical cross section.Methodology is shown in lower figure

Firstly, basing on previous studies and analysis, available informations use to definecurrent status of foreshore, mangrove forest and sea dike system; governing factors ofthe foreshore lowering mechanism

Secondly, collection of data: These data are used as input for calibration andverification of models These are included: topography, wave and water level dataThirdly, set up model: It includes computation network, meshes, defined boundaries.The study is using two computation domain including large domain and small domain.Large domain used calibration and verification model, small domain usedhydrodynamic simulation

Fourthly, The scenarios proposed for study are based on the impact factors foreshorelowering The next is simulation of hydrodynamic regime of study area Given change

of the wave and current when the foreshore is lowered and then calculation, design for

a typical dike cross section

Lastly, in this content, author will summary the results conducted in the study

CHAPTER 1 LITERATURE REVIEW

1.1 Introduction

For the last few years coastal erosion has become more severe in Cuu Long Delta ingeneral and in Bac Lieu province in detail Bac Lieu, one of 28 coastal provinces ofVietnam, whose total coastline is 56 km accounting for 1.27% of the coastline of thecountry Coastal and estuarine erosion in Bac Lieu has been more frequent and itsintensity has been increasingly severe through time Noticeably, the mangrove forestbelt has been gradually eroding and losing In three years, from 2011 to 2014, the 11

km sea dike from Vinh Trach Dong commune bordering Soc Trang to Nha Mat Canaltotally lost 119 meters

This area was affected by the convergence of waves and the semi- diurnal tide of BienDong regime Under the effect of waves, wind, etc…, this area with its weak

geological features is easy to get eroded and lose its coastline stability The mangrove

forest area in front of sea dike has been narrowing at an alarming rate Losing this area

of forest has put a high pressure on the stability of the sea dike system in the area ofSoc Trang and Bac Lieu Unless there is an urgent and effective measure taken toprotect and restore the mangrove forest, many parts of this sea dike will be at risk oferosion, instability or breaches

Obviously, erosion and coastline fluctuation in Bac Lieu province is an enormousobstacle holding back the social economic development, industrialization,modernization, especially in the process of accelerating marine economy Thisresearch is going to study and analyze in order to evaluate the situation as well as tofind out the causes of erosion in the area of sea dike in Bac Lieu province It will be animportant foundation to propose plans, construction, upgradation and effectiveprotection for the sea dikes to cope with the effects of climate change and sea level rise

in the future

1.2 Foreshore and mangrove

The mangrove forest plays an enormous role in preventing coastal erosion andmitigating natural disaster However, at present the area of the forest was rapidlydecreased due to many causes such as erosion Moreover, the trees are old and theirability to reproduce and grow is limited, which means the forest is easily affected bynatural factors like weather, wave, wind and tide

The width of the forest belt in some areas was significantly reduced, which leads to theinability to support the sea dike, no regeneration tree belt develops toward the sea.There have been some domestic studies conducted on the erosion situation and itscauses and the reduction of mangrove forest in Mekong Delta By the end of 2014,seven provinces in Mekong Delta have totally 91.168 hectare of mangrove forest (Loi,2015), the majority of which is protective forest as shown in the Table 1 -1

Table 1-1: The area of mangrove forest in Mekong Delta (Loi, 2015)

Planted forest

Natural forest

Planted forest

Natural forest

Planted forest

used software to extract fluctuation forest area and it was shown that the mangroveforest area has reduced by 100.000 hectares for the last 40 years.

Table 1-2:The area of mangrove forest in Mekong Delta from 1973 to 2012

Area (hectare) 163.343,37 90.714,62 58.980,6 50.429,86

In the study, the author also made a comment that the coastal mangrove forest erosionphenomenon has a growing trend in recent years Before 2000s, the erosionphenomenon only occured in some certain regions where erosion and accretionphenomenon are intertwined Since 2000, erosion phenomenon is sharply increasing inboth area and intensity Erosion hardly occured in coastal regions of Bac Lieu andSoc Trang in the past, but this has happened in tens of kilometers since 2000s up tonow

Mangrove forest erosion previously occurred only in coastal regions of the Bien Dongand accretion appeared in the Bien Tay in Ca Mau, now the mangrove forest erosion inthe Bien Tay area is also happening at a very high rate, up to 40 m/ year in someregions The author also pointed out some causes of reducing the mangroves forest.First, many parts of the forest have been used for agriculture land or aquaculture.Deforestation, overexploitation of mangroves forest, coastal fishing activities,construction, and natural factors are also listed as the causes

Recently, there has been research about the effect of coastal foreshore lowering onwave height growth in Bac Lieu province, Vietnam The surveyed bathymetric dataand wave climate and water level data at site were used for numerical modelling withthe Mike21 software suite The study on foreshore lowering along Bac Lieu coastlinewith surveyed data in 2011 and 2015 showed that the local beach is eroded and thisallows waves to propagate further landward, at the same time raises the wave height.Results showed that with an offshore design wave of 5.59 m, when the foreshorelowers by 1.3 m, the nearshore wave will grow by 0.49 m at most Therefore, instudying coastal erosion processes, it is necessary to consider the lowering offoreshore and beaches, the causes of erosion by wave growth (Tung et al., 2015)

In the research "Shoreline change of the Mekong River delta along the southern part ofthe Bien Dong coast using satellite image analysis (1973-2014)", Besset et al.(2015)used Landsat satellite images in order to track recent deltaic shoreline changes alongthe Bien Dong coast south of the mouths of the Mekong Besides, the technique wasused for determining changes in mangrove cover liable to be an exacerbating factor inshoreline erosion and determining of shoreline change rates (m/year) between 1973and 2014 The analysis showed that nearly 70% of the 160km - long shoreline of thedelta has been strongly eroded The coastal surface losses attained 4.40 km²/year overthe period 1973-2014, and even more between 2003 and 2011 with a loss of 6.98km²/yr Land-use changed and vegetation lost between 2003 and 2011, mainlyconsisting of seafront fringe mangroves, along a sector of the eroding coast in BacLieu Province An area of 60 ha of vegetation were lost during the period 2003-2011.

In these sectors of eroding coast, mangroves have been replaced by aquaculture,particularly shrimp farming

Linking rapid erosion of the Mekong River delta to human activities using resolution satellite images showed that the Mekong delta is now dominated by rampanterosion The rate of land loss is nearly 2.3 km2 /year in 2003–2012 along the BienDong coast, the shoreline retreat over this period having affected over 50% of the large600km long coast, and even up to 90% of the muddy Bien Dong coast Decreasingriver sediment supply to the coast is deemed to be the prime cause of this erosion, andmost likely due to existing dam retention of sediment and to massive channel-bed sandmining in the delta (Anthony et al., 2015)

high-Monitoring the shoreline change in coastal area of Ca Mau and Bac Lieu provincefrom 1995 to 2010 had been conducted using remote sensing and GIS It was shownthat coastal erosion and accretion in Ca Mau and Bac Lieu in this period was verycomplex Erosion and accretion process were periodly intertwined According tofieldwork result, Nha Mat area of Bac Lieu province was rapidly eroded in the period

of 2000-2005, and from 2005 to 2010 the trend was going down, with 0.5 to 5 metersper year, due to the construction of a breakwater (Diem et al., 2013)

The research of shoreline change in Southern Vietnam from Ho Chi Minh to KienGiang by Hung et al (2012) used the technique of overlaying maps and analyzingsatellite images to describe a full view of coastal erosion and accretion in areas from

Ho Chi Minh City to Kien Giang province The authors divided the researched areainto 9 small areas and analyzed the shoreline erosion process The research resultsshowed that the western coastal area was quite stable and accretion process wasdominant In the eastern sea area, the process of erosion and accretion was mutuallyinterleaved The article also drilled down the main factors affecting erosion andaccretion process including natural factors and human activities, which served as thebase for solutions to prevent this situation and preserve the area

In the research "Modeling transport and deposition of the Mekong River sediment" theauthors applied a coupled wave–ocean–sediment transport model to simulate theMekong shelf circulation the transport and dispersal of Mekong - derived finesediments in 2005 In summer, a large amount of fluvial sediment was delivered anddeposited near the Mekong river mouth In the following winter, strong ocean mixingand coastal current lead to resuspension and south-westward dispersal of a smallfraction of previously deposited sediments Strong wave mixing and downwellingcoastal current associated with the more energetic northeast monsoon in the winterseason are the main factors controlling the southwestward along-shelf transport (Xue Z

et al., 2012)

The research “Building mangrove protection forest in coastal zone, reality andsolution” indicated that although the mangrove forest belt in this area grew in a goodway since 20 years ago on clayey sand, and extending up to 500m seaward (the treesare matured with trunk diameters of 20÷30 cm), there have been visible forestdegradation The erosion occurs first in narrow trenches, 20÷ 30 m wide perpendicular

to the shoreline The trenches then gradually expands, causing land loss on both sides.The eroded phenomenon in mangrove forest still occur in some segments of sea dike

of Soc Trang, Bac Lieu provinces (Que, 2012)

Aresearch about the tendency of sediment transport in coastal Mekong delta showedthat sediment transport process has heavily influenced by longshore current regime

due to influence of wind, tide and wave activity in close to coastal area That sediment

is transported to southwest, when passing Ca Mau cape, sediment current travel towardthe northwest along the west coast of Ca Mau peninsula Where the most sedimentaccumulation is a wet delta around Ca Mau peninsula (from Bo De inlet to Cua Lonarea), and potential erosion zone is Soc Trang- Bac Lieu coastal area, especially thesea level rise (Thanh et al., 2011)

1.3 Design of sea dike with regard to foreshore and mangrove

In the Mekong Delta, sea dikes function to prevent salinity intrusion and flooding, itoften works directly under the effect of waves and flows Boundary conditions fordesign are hydrodynamic characteristics such as water level, wave, flow regime.Besides, several loads such as means of transport on the dike must not be ignored incalculation According to several studies, foreshore element and mangrove forestshould be taken into consideration in design process

The foreshore is a buffer zone, where reciprocating movement of sediment takes placefrom high beach down to low beach or vice versa, which depends on features ofseasonal wave The foreshore plays the role of reducing wave impact on high beachand constructions and infrastructure built along the shoreline In the summer, under theimpact of small wave, sediment moves from low beach up to high beach and thereversal happens in the winter under the effect of swell If the foreshore is a widemudflat area, a gentle slope, with the growth of mangrove, used as an aquaculture area,both show is protect for land region inside beach under effect of wave and flowcurrent

The research about the safety, stability of the foreshore has direct effect on safety andstability of coastal in general and sea dike construction, coast protection construction

in particular The effect of foreshore on the safety and stability of sea dike isunderstood as effect and impact caused by its narrowing and lowering process Theforeshore is often affected by hydrodynamic elements such as wave, storm surge, tideand longshore currents The effect of foreshore on the safety and stability of dike isreviewed by two main factors including narrowing the width of the dike due to erosionand lowering the height of the dike compared with the original surface elevation

Besides, the foreshore is extremely important for safety of dike, especially for highenergy beach areas Therefore, to stabilize the sea dike, foreshore protection is moreimportant, needs more attention and investment (Tien, 2015).

A recent research by Trung (2015) showed that in the Mekong Delta, sloping earthendike has been built for tens of years and seems to be consistent with material conditionand foundation of the region Preliminarily, surveys have found out that rubble –mound structure, gabion is suitable for sea roof, grow grass embankment can beapplied for field roof The common condition in front of the dike is existence of a strip

of mangrove forest, which makes lower the power of the waves hitting to the foot ofthe dike, so it is not necessary to use complex structures and burdensome Moreover,the foundation here is quite weak, which is not suitable for large dike sections andheavy structures

According to the research and the proposed forms of the sea dike section (part 2) theforeshore has effects on sea dike design The foreshore has a special influence on theamount of wave overtopping through dike and therefore, on the safety of construction.The research showed that in the same design loading condition when depth offoreshore increases, the amount of wave overtopping over dike will increase according

to exponential law, and to maintain the level of safety, the crest elevation of dike mustalso be raised equivalent to the increase of freeboard by 1 to 3 times with one of depthforeshore So, in sea dike design, it is necessary to consider the foreshore inseparable

to ensure the safety and long term stability of construction Forecasting and monitoringchanges in foreshore morphology need to be regularly carried out as a part ofmaintaining the sea dike construction (Cat, 2012)

Besides, the mangrove forest is also an important factor to be taken into considerationthroughout the design process The Decision 667/QD-TTG released by the PrimeMinister on approving consolidation program, upgrading sea dike system from QuangNgai to Kien Giang claimed that new sea dikes must be built on a principle of havingmangrove forest stretch at least 500 meters wide

On coasts with low-lying floodplains, consisting either of marsh or a mangrove belt,the floodplain is an important stabilizing element of the coastal protection system It

protects against coastal erosion and flooding The higher the floodplain, the greater thewave dissipation on it, and as a result, the wave load on the dike is decreasedsignificantly In the presence of mangroves, the wave reduction effect is even larger(Fig 1 -1) Mangroves also reduce storm surge water levels by slowing the flow ofwater and reducing surface waves (Albers et al., 2013).

Fig.1-1: Energy dissipation of waves on higher floodplains with mangroves

(Albers et al., 2013) 1.4 Discussion

Most researches have focused on assessing the degradation of mangrove forest,erosion and retreat of the Mekong Delta coastline, and particularly Bac Lieu area.Basing on the analysis of satellite image, we can consider the coastline evolutionprocess, degradation of mangrove forest area over the years A research in 2015 usingnumerical modelling to assess the foreshore lowering of Bac Lieu area Thephenomena of coastal erosion, strong degradation of protective mangrove forests havepushed sea dike to the brink of danger and threatened human life For that reason,giving causes, trend of coastal erosion and propose timely solutions are imperative.Generally, coastal erosion and mangrove forest attenuation in Mekong Delta,specifically in Bac Lieu are a problem particularly in need of attention and opportunesolutions The coastal erosion and loss of mangrove forest will cause damage to thefoundation of a sea dike system So, designing sea dikes must take foreshore andmangrove forest into consideration In the scope of this thesis, the author primarily

focus on the current status of foreshore lowering, trend of sediment transport of thestudy area and the impact of foreshore lowering on the behind sea dike system Toclarify the issue of the study, the author choose numerical modelling to simulate thecirculation and wave field for the study area.

CHAPTER 2 FORESHORE LOWERING AND ITS IMPACTS

2.1 Introduction

The foreshore lowering phenomenon would be caused by many factors and its impact

on mangrove forest and sea dike system is considerable Soc Trang, Bac Lieu coastalareas are affected of wave, longshore currents, which are at risk of coastal erosion,especially in the trend of sea level rise Multiple shoreline segments areas seriouslyeroded and foreshore surface is being gradually lowered vertically, which causesmangrove forest to die as their roots are pulled up As a result, mangrove forest areawas reduced and alarmingly degraded And sea dike is affected in return

2.2 Description of the Bac Lieu sea dike

2.2.1 Topography and geomorphology

As a part of Ca Mau peninsula, Bac Lieu is a coastal province in the Mekong Delta,see Fig 2 -2 Bac Lieu borders Can Tho, Kien Giang to the north-west, SocTrang tothe north-east, Ca Mau to the south-west, and Bien Dong to the south - east Theprovince has a coastline of 56 km long connecting three large estuaries: Ganh Hao, CaiCung and Nha Mat The natural land area is 2.521km2

Fig.2-2: Map of the Mekong delta

Bac Lieu consists of 5 districts: Vinh Loi, Hong Dan, Gia Rai, Phuoc Long, Dong Haiand Bac Lieu town Bac Lieu has relatively even and flat terrain, most of which is1.2m higher than the sea level Bac Lieu has sand dunes from 1.0 m to 3.0 m high,running parallel along the shoreline Mudflats appear along the coastline, most ofwhich are temporarily submerged at high Outside of the mudflats is shallow sea area.Strips of mangrove forest along the coastline serve as protective shields againstharmful effect from wave, wind and sea level The terrain, which is tendentiously steepfrom the coastline towards the inland, from north-east to south-east, is separated by anetwork of rivers and canals There are many large canals such as Quan Lo, PhungHiep, Canh Den, Pho Sinh and Gia Rai The topography and geomorphology along theshoreline are heterogeneous among its areas, which is the result of the interactionbetween the natural elements such as geological structure and tectonics, the process ofmarine activities, the impact of the river, the role of mangrove forest and humanactivities modifying the land surface The typical beach segment involved in thesurvey belongs to the dike section from K0 to K1+200, starting from the border ofSoc Trang – Bac Lieu extending north towards Nha Mat inlet, with a total length of 1

km It borders the Bac Lieu wind power project area to the north and the protectivemangrove forest to the South The surveyed sea dike located in Vinh Trach Dongcommune, Bac Lieu city, Bac Lieu province

2.2.2 Hydrodynamic characteristics

The tidal regime

Bac Lieu has the Bien Dong tidal characteristics, consisting of two different regions inthe east and in the west Namely, the East bank takes the unevenly semi-diurnalcharacteristics Every day, the sea level goes up and down twice with 3 m to 4 m oftide amplitude, ranked the highest in the country The average water level is 1.7 m andthe maximum water level difference two tide periods is from 1.5 m to 2 m while theminimum one is from 0.5 m to 0.6 m In contrast, the west bank has the mixed tidalregime inclined irregular tide, with small tidal amplitude

Wave conditions

The wave in Bac Lieu is similar to coastal wave in eastern Mekong Delta It istypically the mixing wave under the direct influence of local wind The elevation andyear cycle are 1.6 m and 5.5 seconds respectively Besides, the maximum elevationand maximum cycle observed can be up to 10.5 m and 11.5 seconds

During the northeast monsoon, waves whose frequency is less than 1m high accountfor 82%, in which the north-east ward and the northward ones accounts respectivelyfor 49% and 24% The waves whose height is from 1 to 1.5m account for 12% Swellswith height between 1.9 m and 3.7 m high, comprise 20%, in which the northward oneaccounts for 19% Higher swells 3.7 m accounts for 7%, the frequency was 65% calm

On the southwest monsoon, waves whose frequency is less than 1m accounts for 77%,

in which the south-west ward and the southward ones accounts respectively for 50%and 15% Waves whose height is from 1 to 1.5m account for 14% The swell which isfrom 0.3 to 1.8 m high accounts for 17%, in which the southward accounts for 9% andsouthwest ward comprises 7% The swell with height from 1.9 to 3.7 m with frequency15% in which the southwest account for 8%, the south 7% The swell with more than3.7 m high account for 9% The frequency calm wave is 69%

January February Marc h

Fig.2-4: The average long-term wave field of Con Dao (KHCN-BĐKH/11-15, 2015)

The wind regime

The studied area is located in the equatorial tropics A year comprises two distinctseasons: rainy season and dry season The rainy season mainly experiences thesouthwest monsoon whereas the dry season is dominantly influenced by northeastmonsoon, with and average speed of 1.77 m/s

The northeast wind, called counter-wind by local people, usually comes from the east,sometime from northeast or southeast This type of wind appears from November toApril the following year The frequency and speed gradually increase during thisperiod, reaching the maximum in February or March and then gradually reduce toterminate in April The wind speed is around 2 m/s to 3 m/s, inparallel direction withlarge rivers.

Coastal districts are severely affected by counter-wind as the wind speed can be up to

14 m/s to 16 m/s Combining with high tides,the strong wind may break the sea dikesystem and coastal aquaculture constructions Besides, the counter-wind and high tidesalso cause surges As a result, sea water invades the land, which affects daily lifeagricultural production

The southwest wind appears from May to October, the south - southwest winds iscoming early or late, that is important contribution to which decide whether rainyseason comes early or late

July August September

Fig.2-5: The average long-term wind field of Con Dao(KHCN-BĐKH/11-15, 2015)

Longshore currents

The surface currents generated mainly due to effect of wind Therefore, the regime ofsurface currents at coastal Mekong Delta has two main trends due to the effect ofnortheast monsoon and southwest monsoon

In the winter, the surface current has southwest direction Fig 2 -6 The averagecurrents peed largest is 0.4 m/s at south sea zone of Con Dao, maximum is 0.3 m/s atnorth of Con Dao and about 0.1 ÷ 0.2 m/s in the coastal zone

Fig.2-6: Direction and speed of the winter currents( KHCN-BĐKH/11-15, 2015)

In the summer, under the effect of southwest monsoon, the coastal currents from CaMau toward the Mekong river mouth is very weak The average currents speed in thesummer is only 0.25 m/s at the north and south of Con Dao Island The coastal studyarea, the speed is usually smaller than 0.05 m/s as Fig 2 -7

Fig.2-7: Direction and speed of the summer currents (KHCN-BĐKH/11-15, 2015)

2.2.3 Foreshore and mangrove

Bac Lieu’s coastline is 56 km long bordering both the Bien Dong and the Bien Tay.Recently, erosion along Bac Lieu coastal zone has occurred more frequently, especially

in the area of Ganh Hao The eroded zones due to the influence of the natural processand human activities make the erosion process become more complex andunpredictable Besides, the mangroves have been seriously degrading The currentstatus of foreshore and mangroves will be presented in the following section

Foreshore

In Bac Lieu province, the area of foreshore accounts for 4% of the natural land area inthe whole province, approximately 100 hectares, distributed on Bac Lieu city andDong Hai district Amplitude of the coastal strip of Bac Lieu province is commonlyfrom 1.0 m to 1.5 m For the landward side, the bathymetry is higher from 1.5 m÷ 3.0

m The coast of Bac Lieu has three forms regarding the beach sediment composition,which appear in different areas: sandy beach with and mainly fine grains (sandproportion of 70% ÷ 80%) though locally coarse grains (sand proportion over 80%);alluvial sediment coast and beaches with liquid field mud of variable layer thickness.Generally Bac Lieu coastline has encroached seaward since 1964, except an area, 5kmlong, bordering Soc Trang and another, 9 km long, bordering Ca Mau, which havebeen eroded The trends of erosion and accretion are described in detail

The interlacement between erosion and accretion is about 19km including:

The segment from 30/4 canal (Nha Mat, Bac Lieu town) to the fourth canal (VinhThinh commune, Hoa Binh district) with the length of 16km; the segment close to CauChay canal (Dien Hai commune, Dong Hai district) to the third canal of Ganh Haotown with the length for 3km This coast is accretion all year that has 22km of totallength, from the fourth canal to close to Cau Chay canal

The sea segments have being eroded all year with the total length of 15 km

From the coastline bordering of Soc Trang to close to 30/4 canal with the total length

of 11km (The area of Bac Lieu city including Vinh Trach Dong commune, Hiep Thanh

commune and Nha Mat ward) The final segment from the third canal of Ganh Hao

town to Ganh Hao estuary is about 4 km (the area of Ganh Hao town – Dong Hai

district) The average speed of annual erosion is from 20 m÷30 m of the horizontal and0.5 m ÷ 1 m of the vertical.

From 1965 to 2001, the coast adjacent to Soc Trang to the south which has the length

of about 6 km regressed about 160m, which means the speed of movement shoreline isabout 7 m/yr The next coastal segment to the area of Dien Hai commune far fromGanh Hao estuary is about 9km, the accretion activity is happening dominantly, at thespeed of 100 m/yr, the coast was being encroaching upon the sea from 350 m to2300m

The coast of Ganh Hao which is 9 km long in the estuary area of Long Dien Taycommune, Dong Hai district, is severely eroded, at the speed of about 10 meters peryear Especially, the area of Ganh Hao estuary having the erosion speed of about 1meter per year in period 1965 – 1989 grew up to 50 m/yr in 1989-2001

Along the road 31 towards Bac Lieu, far from Nha Mat about 10km will see windmillfield area The distance from the road to the dike approximately 2km Fig 2 -8 showedthe beach eroded about 200m toward Northeast of the windmills project, this area islocated Soc Trang province Beach eroded each thin layers from sea into, accordinglytree is also lost, meandering water edge line, depending on the extent beach erosioneach location, has place into depth like Fig 2 -8(ii)

In the period 2001 up to now, the coast of Bac Lieu is quite stable Only some areashave been internally eroded at low speed, about 5 meters per year (KHCN-BĐKH/11-

15, 2015)

(i)The large eroded beach (ii)Deeply eroded offshore

Fig.2-8: The erosion phenomenon at the Northern area of windmill field in Bac

Lieu ( Sources: Le Hai Trung)

Mangrove forest plays the huge role in ensuring the elocogical balance function forcoastal wetland as well as protecting the stability of zone coast simultaneously Thearea of forest and land forest of Bac Lieu account for about 2% of the natural landarea, which is mainly protective forest The whole province has over 4600 hectares ofprotective forest, reducing by 400 hectares compared to one in 2009 (reported by BacLieu functional organization, 2012) Bac Lieu forest is alum flooded mangrove forest,which is highly biological productive, bringing big value to protection andenvironment Bac Lieu is located in the area that was strongly effected by wave andthe Bien Dong tide Under the impact of wave, tide, inshore currents in the period ofnortheast monsoon and southwest monsoon, the coast are always eroded and fluctuatedsignificantly In the present, many segments along the sea dike of Bac Lieu have lostprotective forest belt (Ganh Hao estuary)

According to the result of satellite image analys is, since 2011, the littoral zone of BacLieu has been eroded of about 327 hectares On average the area of mangrove forestper year loses 25.1 hectare of its area, particularly Ganh Hao inlet is the worst eroded

259 m Therefore, the present mangrove forest is much less densed, the fluctuationforest belt in the range of 50 ÷ 300 m, the height of forest belt about 3 m The

dominant trees are Rhizophora sp and Avicennia, which do not meet the requirements

to protect the sea dike (according to decision 667/ QD-TTg 27 May 2009 of PrimeMinister on approving strength program, upgrade the sea dike system from QuangNgai to Kien Giang, in front of dikes mangrove forest planting is needed, with aminimum width of 500 m)

The segment has the greatest a mount offorest area is from the edge Bien Dong dikes

to the sea at around 1.3 km long Meanwhile, the place has smallest mount of forestarea 60 m long, like the forest segment has length of about 6 km in Vinh Trach Dongcommune Bac Lieu province In 1997, the protective forest of this segment has thelength of 120m, after 15 years the forest segment reduced by 50%, and the trend hasbeen continuing reduced Furthermore, the road has length of about 7 km at Ganh Haotown, Dong Hai district has been lost, and the forest is very thin Unless there are

solution preventing lost forest status due to sea wave, the area of protective forest ofBac Lieu province will continue decreasing (Nguyen, 2015).

Besides, in recent years, the exploitation of mangrove forest resources, thedevelopment of shrimp farming, the land-use change of doing aquaculture, agriculture,environmental pollution aggravated by the indiscriminate discharge of solid and liquidwastes from in daily life and industry, the impact of natural disasters such aswindstorms, landslides coast, sea level rise, etc has seriously affected and declinedthe mangrove forest area as well as the diversity of coastal ecosystems of Bac Lieuprovince

Fig.2-9: The mangrove forest erosion at

Ganh Hao town, Bac Lieu province Fig.2-10:Mangrove tree left in front of Nha Mat embankment

(i)The cannal flowing into the sea (ii)The forest beach eroded each layer Fig.2-11:The current status of forest beach at a canal gate out to the sea

2.2.4 Bac Lieu sea dike system

Basing on the survey results of Bac Lieu sea dike system in August 2014 (Trung,2015) The land dikes is 52 km long, K0 from Nha Mat to Ganh Hao The 667program has been hardened about 40 km, with crest elevation is +3.5 m and crestwidth is 6.5 m There are 24 canal gates through the dike in total, including 4 largeestuaries Currently, sewer is being constructed over three estuaries Nha Mat, ChuaPhat and Cai Cung

Along the dike, the mangrove forest strip has width fluctuating from 0 to 2 km.Approximately 16 to 20 km which has being eroded consists of two segments Thefirst segment is from K0 to K11, eroded forest foreshore is processed by nourishmentcombine with geotube T shape The second segment is from K40 - 46 to K52, there areabout 6 to 10 km eroded under a other mechanism Closer to Ganh Hao is the situation

is more serious

Fig.2-12:Preliminary locations of survey Bac Lieu sea dike in Aug- 2014

In previous years, the sea dike have only one task of flood control and salinitypreventing for rice farming area behind it However, in 2000s, about 40.000 hectares

of rice farming was converted into shrimp farming Some of issues were noted is thefact that the bridge construction over the Nha Mat canal is the main cause of losing

transportation demand, the dike crest was rapidly degraded Besides, that is somesegment of dike running parallel and close to the traffic roads Crest level is not toomuch disparity compared with the road surface Fig 2 -12shows overall of Bac Lieusea dike.

The study has implemented detailed survey at two main locations: Nha Mat and GanhHao

Nha Mat

At the left bank of Nha Mat estuary, a dike segment is invested for hardening in HiepThanh commune, Bac Lieu city The seaface is reinforced by concrete armour layer,there is crest wall on above Fig 2 -13 and the dike surface poured concrete combinewith traffic road Currently, crest dike surface and the inner dike slope is takenadvantage for seafood market and eating purpose

(i)The seaface dike (ii) Amour layer and crest wall Fig.2-13: The sea dike hardened of Nha Mat estuary

The phenomenon of erosion and forest loss in this area was overcome by solution ofreducing wave submerged breakwater This dike a has geotube structure, the outershell of the eroded beach area is shown in Fig 2 -14(i) When water level is low, thecrest dike can float On of given solutions has been tested to increase accretion speed

is one type of groyne combined with melaleuca pile such Fig 2 -14(ii)

(i)The geotube submerged breakwater (ii)Soft groyne

Fig.2-14:Two type of protection structures applied at Nha Mat

The soft dike has geotube structure was tested to protect a short sea dike segment andhas the effect to reduce extention of beach erosion.Currently, the submergedbreakwater is degrading and there are some disrepaired problems which are difficult torepair thoroughly The cause may be weather condition, a number of local residentactivites and perhaps the most important is the construction method and structure ofthe dike Two final factors need to be reviewed and improvement for the next project

Ganh Hao

Ganh Hao is located at the end of Bac Lieu sea dikes, where erosion issue haveoccurred for many years and its extension is increasingly powerful and complicated.Two surveyed locations were Lang Ong Nam Hai (constructing) and town area (havefinished)

The sea dike was invested to be constructed solidly with main material is reinforcedconcrete and deep-pile structure (10m), the seaface is put soil for creating steep slopeand protected by Asperities armour layer

(i)The concrete dike is constructing (ii)Asperities concreting in place Fig.2-15:The construction current status of concrete dike from Lang Ong Hai

Nam to Ganh Hao estuary

Ganh Hao sea dike actually was invested for construction and strengthening in a longtime with significant funding However, the current status shown works itself is always

in danger of being damaged (erosion toe, armour layer lost) and having problems(subsidence, landslides and broken) The land area protected behind certainly is takingthe risk of flooding and landslides

This reality leads to a concern about the rationality of dikes planning Probably needchoose the position of the construction route, where have many disadvantage factorsabout waves, wind, currents, sediment transport Moreover, living residents and civilbuilding close to the dike, even on the inner dike roof and part of dike crest This isalso factor caused influence to the monitoring, management and maintenance ofworks

(i)The concrete dike (ii)The amour layer protecting Fig.2-16:The current status of concrete dike defence Ganh Hao town

2.3 Governing factors and classification of the foreshore lowering mechanism

2.3.1 Governing factors of the foreshore lowering mechanism

The foreshore is frequently effected by the wave, wind, currents lead to the foreshorewas uninterruptedly eroded and gradually, foreshore lowering phenomenon appeared.The erosion of coastal zone is a complex natural process, and an interactiveconsequence between many factors, which are divided into two main groups: naturalfactor and human impact The indispensable natural factors in finding out and

identifying the coastal erosion Coastal erosion would be caused by a number offactors as discussed below (Hung, et al., 2011).

1/ Coastal structures and shoreline direction

The coast from Ho Chi Minh city to Kien Giang (including Soc Trang, Bac Lieu)composed of alluvial sediment (Holocene modern age - Pleistocene) with maincomponent is brown clay mud With this structure of geology just moderate force, isenough to break the surface structure if there is no surface covering or the surfacecovering is very sparse

Because the composition of bead is very smooth, most coastal material after beingbroken will be converted into sediment and easily be swept to other place by wave andlongshore currents Beside, coastal zone structure, the coastline direction is also adecisive element of accretion process and erosion The fact showed that, at the coastsegments having the opposite with the northeast monsoon (at Ganh Hao) erosionprocess occurred strongly On the other hand, the places has shoreline covered by part

of something with above wind direction is often aggraded or only slightly eroded(Hung et al.,2011)

2/ Wind

Soc Trang, Bac Lieu are located in tropical monsoon area, so annually Bac Lieu wasregulated by monsoon with main directions are northeast and southwest Wind is mainexogenous element create wave, the monsoon regime above is also create oceancurrents and longshore currents which are opposite each other: summer currents(southwest monsoon) movement is from south up north, and winter currents (northeastmonsoon) has direction is from north down to south

Northeast monsoon often begins from November to March of next year with averagewind speed is about 8÷10 m/s, the highest is about 15÷16 m/s With its dominantfrequency, wind speed is also greater than wind speed of the other directions northeast,wind direction is almost opposite with the open coastline of the Bien Dong Therefore,

it can be determined as a main dominant wind direction affecting the erosion process

of coastline in the area The waves created by northeast monsoon stir up sedimentwhich is accreted during the southwest monsoon, as well as abrasive the cliffs are not

protected by vegetation, creating longshore currents along with the tidal currents andocean currents sediment transport toward the south This is the real sediment transportdirection in the coastal of Soc Trang, Bac Lieu areas (Hung et al.,2011).

Fig.2-17: The sediment accumulation and the trend of sediment transport in Mekong Delta, which is affected dominantly by northeast monsoon (Sources:

Marine economy.vn) 3/ Wave

Wave activity is a main agent to stir up sediment from the coastal seabed At the sametime, the wave – related processes also create the currents for sediment transport Asmentioned above, for the coastal area of Soc Trang, Bac Lieu is coastal area which isadjacent to Bien Dong of Vietnamese In this area, the waves of the east directionpredominate, the nearshore average wave height is about 0.8 m÷1.0 m, the largest is2.0 m (Hung et al.,2011)

4/ Tidal regime

The Bien Dong region is dominated by the semi-diurnal irregular wave regime, largetidal amplitude is about 2 ÷ 4 m With tidal regime and tidal amplitude as above, thespeed of tidal propagation is very fast The tidal current is fast, especially in theestuary, which caused the erosion of seabed Because of these characteristics, theforming shoreline process in this area is clearly affected by river - tide Besides,because most of the shoreline in the region is very flat, as well as the tidal regime has

large amplitude and change many times during the day For that reason, the effectingscope of wave - tide on coastal zone is very wide (Hung et al.,2011).

5/ The degradation of mangrove forest and protective forest in the coastal area

Mangroves forest plays a very important role in terms of ecology and environment.Besides, the mangroves forest is also one of the main factors of coordination formingcoastline process The mangroves forest has the effect of reducing wave and currents,creating a favorable condition for the rapid sediment accumulation and, as well as it isalso make sediment cohesive better When mangroves forest is degraded and cannot berecovered itself, the erosion process will occur and continue Overall, the whole SocTrang, Bac Lieu coast are protected by a strip of mangroves forest with the widthfluctuating significantly, depending on each location However, the fact that manyforest areas are getting to strongly degraded (such as Vinh Chau, Ganh Hao) hasdirectly influenced the degradation of sea dike and shoreline

At the east of Ca Mau peninsula, northeast monsoon (counter - wind) along with hightide has create erosion on about tens of kilometers from Ganh Hao down to Dat Muihamlet The high tide transports sediment into the shore, caused many mangrovespecies having breath root on the ground be buried and die standing (Hong et al.,1999)

6/ Sea level rise due to climate change

The research results forecasts and, evaluate the extent of the damage cause by seasurge in Mekong Delta coastal area In scenario of sea rising 1.0m at the southernregion, the total submerged area at southern area will be 16.128,06 In which, manyprovinces of Mekong Delta area will be most affected by largest submergenceincluding Bac Lieu Sea rising along with monsoon, storm and high tide willcontribute to the coastal erosion (Thanh and Nam, 2008)

7/ Human activities

In the southern coastal area, the main impact of human on the coastline erosion andaccretion process Which need to be mentioned firstly is many activities relating and

degrading mangroves forest The mangroves forest of Mekong Delta area reducedfrom 250.000 hectares in 1950 to 45.790 hectares in 2001(Hoi et al., 2003).

The overexploitation, especially the severe deforestation for catching marine creatureand aquaculture, has changed the natural environment, limited the development space

of mangroves and, significantly contributed to the degradation of mangroves Theforest belt exploited by human has become very thin There is no belt regeneration,which cause the reduction of canopy layer sand wave energy, as well as it is easilyuprooted by ocean wave and swept away The consequence is not only the ecosystemimbalance, but also the dynamic imbalance of coastal zone which leads to seriouserosion in many places

Beside, other human activity evaluated as a significantly contributing effects onerosion process in the Mekong Delta area, is the lack of sediment Regarding to theconstruction of many dams in the basin of Mekong and Sai Gon- Đong Nai riversystems Until this time, there are 24 hydroelectric dams and reservoirs on the riverbranches of upstream countries and 3 dams on the mainstream in China in total whichhave been built or completed since 1966 (the report of Mekong river committee, worldBank, 2004; MRC, 2010) The construction of these dams directly impact sandsignificantly reduce the amount of the sediment supplied to the sea Since the Manwandam on the mainstream of China came into operation in 1993, it has reduced theamount of sediment at the downstream seriously The sediment attenuation hassignificantly contributed to mangroves lost which causes coastal erosion (Lu and Siew,2005)

Erosion and accretion of coastline caused by combination of many complex factors.However, the above analysis are mainly qualitative but we can also preliminaryconclusion of main reasons:

(i) the effect of waves in the northeast monsoon

(ii) the effect of the tide

(iii) the impact of human protective forest degradation

In the scope of thesis, the author focus on study and analysis of hydrodynamic factorsusing numerical modelling to clarify clearly the effect of factors toward the trend ofsediment transport in study area

2.3.2 Classification of the foreshore lowering mechanism

Nowadays, the coastline fluctuation in many provinces of Mekong Delta is happeningvery fast and complexly due to the continuously variable effects of many causes Most

of the tidal flats have being eroded at various levels, and the fact that they areintertwined makes it difficult to be delimited However, in order to consider it in ageneral way, we can classify the erosion at Mekong Delta into 3 types (Loi, 2015):

Tidal flat of surface erosion: appears at areas which still have mangroves belt, its

width is about a few tens meters, soil platforms of mangroves was lowered 0.8÷1.4mcompared with the original soil platform

Fig.2-18:The section of tidal beach is surface eroded (Loi, 2015)

Tidal flat of background erosion: appear at areas, have strong coastal currents or

hard dike, where forest belt has disappeared or it is very thin (a few tens meters),erosion degree between the platform surface current state with initial beachbackground is from 1.5 m to 2.2 m;

Fig.2-19: The section of tidal beach is background eroded (Loi, 2015)

Tidal flat of deep erosion: usually occur at estuary, rivulet, or big drain gate, where

there is cumulative wave, the speed of inshore currents is large and there is no