Nghiên cứu thay đổi dòng chảy và diễn biến bồi xói sông vàm nao do ảnh hưởng của hệ thống đê bao khu vực đồng bằng sông cửu long

Erosion and landslide situation of the Vietnamese Mekong Delta and Vam Nao River ..... ix LIST OF ABBREVIATIONS DONRE Department of Natural Resources and Environment DTM Dong Thap Muo

ĐẠI HỌC QUỐC GIA TP HCM

TRƯỜNG ĐẠI HỌC BÁCH KHOA

-

NGUYỄN THỊ THẠCH THẢO

NGHIÊN CỨU THAY ĐỔI DÒNG CHẢY VÀ DIỄN BIẾN BỒI XÓI SÔNG VÀM NAO DO ẢNH HƯỞNG CỦA HỆ THỐNG

ĐÊ BAO KHU VỰC ĐỒNG BẰNG SÔNG CỬU LONG

Chuyên ngành : Quản Lý Tài Nguyên Và Môi Trường

Mã số: 8850101

LUẬN VĂN THẠC SĨ

TP HỒ CHÍ MINH, tháng 02 năm 2020

CÔNG TRÌNH ĐƯỢC HOÀN THÀNH TẠI TRƯỜNG ĐẠI HỌC BÁCH KHOA –ĐHQG -HCM

PGS TS Nguyễn Thống

Luận văn thạc sĩ được bảo vệ tại Trường Đại học Bách Khoa, ĐHQG Tp HCM ngày 06 tháng 01 năm 2020

Thành phần Hội đồng đánh giá luận văn thạc sĩ gồm:

1 PGS TS Võ Lê Phú

2 PGS TS Lê Hoàng Nghiêm

3 PGS TS Nguyễn Thống

4 PGS TS Chế Đình Lý

5 TS Hà Dương Xuân Bảo

Xác nhận của Chủ tịch Hội đồng đánh giá LV và Trưởng Khoa quản lý chuyên ngành sau khi luận văn đã được sửa chữa (nếu có)

MÔI TRƯỜNG VÀ TÀI NGUYÊN

PGS TS Võ Lê Phú PGS TS Võ Lê Phú

NHIỆM VỤ LUẬN VĂN THẠC SĨ

Họ tên học viên: Nguyễn Thị Thạch Thảo MSHV: 1870065

Ngày, tháng, năm sinh: 20/01/1995 Nơi sinh: Bình Dương Chuyên ngành: Quản lý Tài nguyên và Môi trường Mã số : 8850101

II NHIỆM VỤ VÀ NỘI DUNG:

Nhiệm vụ: Nghiên cứu các tác động của đê bao trên Đồng bằng Sông Cửu Long đến chế

độ dòng chảy và chế độ bồi xói sông Vàm Nao (An Giang)

Nội dung:

+ Tổng quan tài liệu nghiên cứu

+ Xây dựng mô hình toán dòng chảy và tính toán bùn cát theo 2 kịch bản: có đê bao và không có đê bao

+ Đề xuất phương án quản lý giảm thiểu tác động của hệ thống đê bao đến chế độ thủy văn và bùn cát trên sông Vàm Nao

III NGÀY GIAO NHIỆM VỤ: 11/02/2019

IV NGÀY HOÀN THÀNH NHIỆM VỤ: 08/12/2019

V CÁN BỘ HƯỚNG DẪN : PGS TS Lê Song Giang

Tp HCM, ngày 25 tháng 02 năm 2020

CÁN BỘ HƯỚNG DẪN

(Họ tên và chữ ký) CHỦ NHIỆM BỘ MÔN ĐÀO TẠO (Họ tên và chữ ký)

TRƯỞNG KHOA MÔI TRƯỜNG VÀ TÀI NGUYÊN

(Họ tên và chữ ký)

PGS.TS Võ Lê Phú

ĐẠI HỌC QUỐC GIA TP.HCM

TRƯỜNG ĐẠI HỌC BÁCH KHOA

_

CỘNG HÒA XÃ HỘI CHỦ NGHĨA VIỆT NAM

Độc lập - Tự do - Hạnh phúc

_

i

ACKNOWLEDGEMENT

First of all, I would like to express my deep respects and sincere gratitude to my supervisor Assoc Prof Dr Le Song Giang for his devoted guidance, numerous valuable suggestions, and encouragement throughout this work He tried to guide me

to enhance the quality of this work and present it in the best possible way He taught

me how to be a good researcher He is one of few people have influenced me personally and academically over the years in my student life I thank you for helping

me on how to handle heavy work and live loads For all, thank you, professor Le Song Giang

I would like to thank M.S Tran Thi My Hong I greatly benefited from her keen scientific insight, their knack for solving seemingly intractable practical difficulties, and their ability to put complex ideas into simple terms Every result described in this thesis was accomplished with the help and support of my study group without their efforts my job would have undoubtedly been more difficult

My thanks also go to the department of fluid mechanic During this stay, I met the people who inspired me to stay in lab, research and got there in the end I would like

to thank the various members of fluid mechanic laboratory and teachers work in department with whom I had the opportunity to work and have not already mentioned for their help, moral support and cooperation which contributed in various ways to the completion of this dissertation

To complete my long journey, I also would like to thank from deep within hearts to Mom, Dad and my sister for their constant love and support Especially, sincere thanks

to Mr Vo Quang Minh Hoang for their valuable comments me, patience and understanding in difficult times of this thesis I will never forget the days near the deadline, you have always encouraged and helped me a lot Without you all, I would certainly not be here today to submit my master thesis Thank you everyone for all! Your sincerely,

Nguyen Thi Thach Thao

ii

ABTRACT

In recent 20 years, a large number of levees (flood dykes) have been built by humans

or formed naturally along the Vietnamese Mekong Delta River levees, considered as a defense structure, are constructed to prevent flooding of the basin, protect livelihood and confine the flow of the river for higher and faster flow Although the construction

of dykes helps local communities to increase agricultural production, contribute to poverty alleviation and reduce flood-driven damage, it also leads to the alteration of the natural hydrological regimes of the floodplains and the increase of flood risks to other areas in the delta Thus, hydrodynamic modelling is needed to further understand the impact of levee systems on the hydrology of the region and will help formulate appropriate water resources management and development plan options By using a 3D integrated model of F28 software and detailed application of a hydrodynamic model for the Mekong floodplains and delta the effects of levees were confirmed

TÓM TẮT

Trong 20 năm trở lại đây, một số lượng lớn đê bao (đê chống lũ) đã được xây dựng hoặc hình thành tự nhiên dọc theo Đồng bằng Sông Cửu Long Đê bao được coi là một cấu trúc phòng thủ, được xây dựng để ngăn lũ lụt đồng bằng, bảo vệ sinh kế của người dân và hạn chế tốc độ dòng chảy Mặc dù việc xây dựng đê giúp cộng đồng địa phương tăng gia sản xuất nông nghiệp, góp phần xóa đói giảm nghèo và giảm thiệt hại

do lũ lụt, nhưng nó cũng dẫn đến sự thay đổi chế độ thủy văn tự nhiên của vùng lũ và tăng rủi ro lũ lụt cho các khu vực khác ở đồng bằng Do đó, việc ứng dụng mô hình thủy động lực vào quản lý là cần thiết để hiểu thêm về các tác động của các hệ thống

đê đối với thủy văn của khu vực và đồng thời góp phần hình thành việc lựa chọn kế hoạch phát triển và quản lý tài nguyên nước thích hợp Bằng việc sử dụng mô hình tích hợp 3D của phần mềm F28 và ứng dụng chi tiết mô hình thủy động lực cho vùng đồng bằng sông Cửu Long, các tác động của đê đã được làm rõ

iii

LỜI CAM ĐOAN

Tôi xin cam đoan đây là công trình nghiên cứu của tôi Các số liệu, thông tin, tài liệu trích dẫn được sử dụng trong luận văn có nguồn gốc rõ ràng Kết quả nghiên cứu trung thực và chưa từng được ai công bố trong bất cứ công trình nào khác

Người thực hiện

Nguyễn Thị Thạch Thảo

iv

TABLE OF CONTENTS

TABLE OF CONTENTS iv

LIST OF FIGURES vi

LIST OF TABLES viii

LIST OF ABBREVIATIONS ix

CHAPTER 1 INTRODUCTION 1

1.1 Problem statement 2

1.2 Research objectives 4

1.2.1 The general objective 4

1.2.2 The specific objective 4

1.3 Scope and limitations of the thesis 5

1.3.1 Study scope 5

1.3.2 Study subjects 5

1.4 Research contents 5

1.5 The meaning of research 6

1.5.1 Scientific meaning 6

1.5.2 Practical meaning 7

1.5.3 The novelty of this research 7

CHAPTER 2 LITERATURE REVIEW 8

2.1 The general hydrology and flooding at the Vietnamese VMD (VMD) 9

2.1.1 The general hydrology at the VMD 9

2.1.2 The general flooding at the Vietnamese Mekong Delta 11

2.1.3 Mechanisms of physical erosion and sediment transport in Vietnamese Mekong Delta 13

2.2 Vam Nao River 16

2.2.1 Geographical conditions 16

2.2.2 Hydrological characterises in Vam Nao river 17

2.3 Erosion and landslide situation of the Vietnamese Mekong Delta and Vam Nao River 18

v

2.4 Levee system in Vietnamese Mekong Delta 23

2.5 Regional and local studies 26

2.5.1 Developing the physical models for experiment or use situ methods 26

2.5.2 Developing methods and calculation tools that are mainly computer software 27

2.5.3 Calculate specific math problems 28

CHAPTER 3 METHODOLOGY 29

3.1 Research Data Collection Method 30

3.1.1 Qualitative Research Data 30

3.1.2 Spatial data 30

3.1.3 References Data 30

3.2 Field Methods 30

3.3 1D2D3D integrated description 31

3.3.1 The basic formulas 31

3.3.2 Sediment transport process in river flow 33

3.4 Model construction 37

3.4.1 1D2D3D integrated setup 37

3.4.2 Hydrodynamic modeling scenarios 39

CHAPTER 4 RESULTS AND DISCUSSIONS 40

4.1 The flow structure and morphology properties in Vam Nao river before building the levee systems in the VMD upstream 41

4.2 The impacts of levee system on the flow regime of Vam Nao river 45

4.3 The impacts of levee system on the sedimentation of Vam Nao river 54

4.3.1 Bed erosion 54

4.3.2 Sediment process 61

4.4 Recommendation for water resources management 64

LIST OF RELATED RESEARCH HAS BEEN PUBLISHED 71

REFERENCES 73

APENDIX 80

vi

LIST OF FIGURES

Figure 1-1 Land use types of the Vietnamese Mekong Delta (Chapman, 2018) 2

Figure 1-2 Vam Nao River 5

Figure 2-1 The two seasons of the Mekong River’s hydrological year (Adamson, 2006) 9

Figure 2-2 The Lower VMD Zone including two main tributaries: the Mekong (Tien river) to the North, and Bassac (Hau river) to the South (Marchesiello et al., 2019) 10

Figure 2-3 The distribution diagram of bottom sediment in Tien and Hau rivers (Source: Marcello et al., 2017) 15

Figure 2-4 Hjulstrom-Sungborg diagram (Hjulstrom, 1935) 16

Figure 2-5 Online map of coastal erosion, river bank erosion and landslide sites in the VMD (Source: http://satlodbscl.phongchongthientai.vn, update on 7:00AM, 7th December 2019 ) 20

Figure 2-6 Vam Nao river after the landslide on 22 April 2017 20

Figure 2-7 The situation of sedimentation and landslide at the junction of Hau river - Vam Nao river (Ngoc, 2018) 22

Figure 2-8 Levee systems in Vietnamese Mekong Delta (Source: Vietnam News Agency) 23

Figure 2-9 Levee networks of the Vietnamese Mekong Delta (Chapman and Darby, 2016) 25

Figure 2-10 The location of high levee system (green dots) in upstream provinces of the Vietnamese Mekong Delta (Triet et al., 2017) 25

Figure 3-1 Sampling sites in Vam Nao river 31

Figure 3-2 Broad-crested spillway 32

Figure 3-3 Lateral link at river banks 33

Figure 3-4 Modes of sediment transport: bed load transport at small shear stresses (A), sheet flow (B), suspended sediment (C) 34

Figure 3-5 1D sub-model (left) and 2D sub-model 37

Figure 3-6 Computational mesh of VamNao river 3D model integrated in Mekong delta model 38

Figure 4-1 Morphology schematic of the study area 40

Figure 4-2 The morphology of the Vam Nao river bed after flood periods of 1998, 1992 and 2011 43

Figure 4-3 The velocity field on the surface (blue vector) and at the bottom (red vector) Vam-Nao river during the flood-peaks of 1998 (small flood), 1992 (moderate flood), 2011 (huge flood) 44

vii

Figure 4-4 The typical velocity field in the Vam Nao river (1998, 1992, 2011) 45 Figure 4-5 The velocity vector fields at Vam nao - Bassac river confluence during the peak-flood of 1998 (A), 1992 (B), 2011 (C) in two cases of with levees and no levees (G1: at the bottom, G5: on surface) 48 Figure 4-6 The velocity vector fields at Vam nao – Tien river confluence during the peak-flood of 1998 (A), 1992 (B), 2011 (C) in two cases of with levees and no levees (G1: at the bottom, G5: on surface) 51 Figure 4-7 The change of velocity due to levees at Vam Nao - Bassac confl at the bottom (G1) and the flow uence during floods in 1998, 1992 and 2011 (in percent) between the flow on the surface (G5 53 Figure 4-8 The water level at nodes in the flood peaks of 1998, 1992 and 2011 with two screnarios (no levees, with levees) 54 Figure 4-9 Velocity on the section plane and velocity perpendicular to the section during flood peaks in yeas of 1998, 1992, 2011 58 Figure 4-10 The change of river bed depth at 2 cross-sections in the huge flood (2011) 59 Figure 4-11 Location of cross-sections 60 Figure 4-12 The sedimentation process due to levees at Vam Nao - Bassac confluence during floods in 1998, 1992 and 2011 between without levees scenario and with present levees scenario 62Figure 4-13 The sedimentaion process due to levees at Vam Nao – Tien river confluence during floods in 1998, 1992 and 2000 between without levees scenario and with present levees scenario 63 Figure 4-14 Suspended sediment concentrations (g/l) during flood years (1998, 1992, 2011) at two scenarios 64 Figure 4-15 The relative change in the channel-bed sediment of flood years (1998,

1992, 2011) in two scenarios 64 Figure 4-16 The management Option 1 65 Figure 4-17 The line graph of the flow (Q), the velocity (V), and the water level (Z) in Vam Nao river in Option 1 66 Figure 4-18 The management Option 2 67 Figure 4-19 The line graph of the flow (Q), the velocity (V), and the water level (Z) in Vam Nao river in Option 2 68

viii

LIST OF TABLES

Table 2-1: Cumulative threats and potential risks for the Mekong river delta (Kondolf

et al., 2018) 19Table 2-2 Scale and level of riverbank erosion in An Giang province (Tuan et al., 2016) 21Table 4-1 The change of of river bed depth at 2 cross-sections in the huge flood (2011) 58

ix

LIST OF ABBREVIATIONS

DONRE Department of Natural Resources and Environment

DTM Dong Thap Muoi

IMHEN Vietnam Institute of Meteorology, Hydrology and Climate Change

LXQ Long Xuyen Quadrangle

MONRE Ministry of Natural Resources and Environment

MRC Mekong River Commission

SIWRR Southern Institue of Water Resources Research

USD United States Dollar

VMD Vietnamese Mekong Delta

VND Vietnamese Dong

1

CHAPTER 1

INTRODUCTION

2

1.1 Problem statement

In rivers, flow regime and sediment transport are the principal characteristics of river channels and play a vital role in riverbank stability, floodplain processes and the overall productivity of the Vietnamese Mekong Delta (VMD) for thousands of years Only formed and developed more than 7,000 years ago due to the sedimentation brought in from the Mekong River and the coastal sediment created, the VMD has become a “biological treasure trove” with an area of around 50,000 km2 (WWW, 2016; Liu et al., 2017) It is also known as the third widest low-lying delta plain in the world after Amazon and GangesBrahmaputra Deltas (Coleman et al.,2003) Nowadays, this place is a homeland of 20% Vietnam’s population and is a wealthy agricultural civilization The VMD accounts for over than half of the paddy rice harvested in Vietnam annually (Kontgis et al., 2015), and besides, constitutes 2.4% of the global paddy rice harvest (as per 2017) base on FAO data In addition to the great potential of agriculture, in the past years, the Mekong Delta has also contributed more than 65% of aquatic products and about 70% of the country's fruit trees (FAOstat, 2017)

Figure 1-1 Land use types of the Vietnamese Mekong Delta (Chapman, 2018)

3

In the last decades, along with the economic development and against climate change backdrop, a levee systems with thousands of kilometers were built in the VMD The formation of the levees opens a great opportunity to exploit the full potential and strength of this prosperous plain However, those levees made the human livelihoods that based on an interplay between flow regime, sedimentation and nutrient transport more become tenuous (Chapman, 2018)

Some studies in the world and Vietnam have indicated the negative impacts of the levee system development for hydraulic dynamics on rivers Firstly, the narrowing of the river channel due to the construction of levees can actually exacerbate to the impact of floods in downstream (Gerald, 1995; Chapman and Darby, 2016) These levee networks not only increases in the frequency and intensity of fluvial but also have greatly the natural hydrodynamic conditions and sediment transport regime in the Vietnamese part of the delta (Triet et al., 2017) Tran and Le (2015) have reported that the levees made increase flood discharges in the Mekong and Bassac rivers In 1998, the peak-flood discharge in Bassac river at Chau Doc went up to 23% However, discharge in Mekong river at Tan Chau has increased only 1.2% The change of discharge in these two rivers, especially the inequality of the change, could lead to significant changes of the flow in Vam Nao river located behind Tan Chau and Chau Doc and play the vital of redistributing the water between Mekong River and Bassac river Secondly, levee networks interrupted the river and floodplain within the flood season causing decreased the fertile sediment inflows, typically LXQ and DTM (Manh

et al., 2015; Dung et al., 2019) Thirdly, in the dry season, the risk of saline intrusion is caused by levees holding water in the floodplain (Smajgl et al., 2015; Hoang et al., 2016) Finally, the sudden increase in velocity of the river on the one hand due to large-scale sand mining and groundwater extraction On the other hand, the ability to retain water and sediment in the floodplains is poor due to the flood dikes That was the reasons leading to erosion and landslides in the VMD (Anthony et al., 2015; Dung

et al., 2019)

Vam Nao river area, specifically Hau river - Vam Nao river junction area in recent years is tending to frequent landslides with increasing severit It is notable to note that

4

on April 22 2017, at Vam Nao river bank (My Hoi Dong commune, Cho Moi district,

An Giang province), a serious landslide caused 14 houses to be submerged, the length

of the landslide was eroded about 70 m, 35m deep and cut off inter-commune roads

On April 26, 2017 at the landslide site, there was a tendency to expand and develop with a length of 94 houses, 01 milling factory, My Hoi Dong A Primary School affected This raises a big question whether the levee system has been built still really improve livelihoods, minimize the impact of flooding on the people or only generate short-term benefits and exacerbate instability of rivers?

While the risks that were related to the impact of the levee system were considered, a sediment database of Vam Nao river is lacking and the flow regime and sedimentation processes remail poorly understood, especially confluence between rivers This gap was filled by calculating and assessing the effects of two scenarios (have levees and no levee) through three typical floods (1992, 1998 and 1998) in this study “Numerical study of the change of flow and sedimentation in Vam Nao river due to levee systems

in Mekong Delta” Using the 1D2D3D integrated model, the change of flow structure and sediment – erosion process was investigated

1.2 Research objectives

1.2.1 The general objective

In this research, the aim was to perform calculates in detail to determine the impacts of the levee system, which was built in the past on the VMD, on the flow regime and sediment-erosion for the VMD

1.2.2 The specific objective

- Learning to proficiently use the F28 hydraulic modeling and apply it in calculating sediment transport in rivers

- Studying the change of flow of Vam Nao river due to impacts of levee system

- Studying sedimentation process of Vam Nao river due to impacts of levee system

- Proposing an appropriate management plan to narrow the impact of levee system on river banks

Figure 1-2 Vam Nao River

1.3.2 Study subjects

- The change of flow in Vam Nao river caused by the levee systems

- The change of sedimentation process in Vam Nao river caused by the levee systems

1.4 Research contents

In order to achieve the set objectives, the research content needs to be carried out in the thesis including 3 contents:

6

Content 1: Overview of the research situation

 Seeking studies related to the change of flow, erosion, and methods of implementation in Vietnam and abroads

 Collect and synthesize existing documents and data, in Vietnam and abroads,

on the causes, mechanism and situation of flow and sedimentation process in the lower Mekong region for research

 Surveying the field and collect hydrological data for study

Content 2: Set up a numerical hydraulic model and evaluate research results

 Setting model grid and calculation conditions

 Calibrate the model

 Simulate 2 scenarios to assess the change of flow and erosion progress in the study area:

+ The current levees situation

+ The situation is completely without levees

 Statistics and analysis of results to draw conclusions about the relationship of the levee system to changes in flow to Vam Nao river

Content 3: Proposal of solutions

 Propose appropriate management solutions to limit the impact of the levee systems on the stability of the study area

1.5 The meaning of research

1.5.1 Scientific meaning

The study opens a broader perspective on the causes and trends of river flow to the risk

of landslides and bank erosion

7

1.5.2 Practical meaning

Proposing solutions to control landslides, bank erosion for the Vam Nao River, and schedule of additional investment in solutions to reduce the risk of bank erosion in the future, in order to protect people living along the river

1.5.3 The novelty of this research

Assessing the relationship between bank erosion at Vam Nao river and the levee system that has been built in the past in Mekong Delta through the three-dimensional integrated model with detailed calculations and high reliability

8

CHAPTER 2

LITERATURE REVIEW

9

2.1 The general hydrology and flooding at the Vietnamese VMD (VMD)

2.1.1 The general hydrology at the VMD

The Mekong is one of the ten largest rivers on over the world both in terms of its flow discharge and its sediment load (World Resources Institute 2003, Gupta and Liew, 2007) The total catchment Mekong area of 795,000 km2 with an annual runoff of over

475 billion cubic meters of water at its mouth in the South China Sea (MRC, 2005) The southern Vietnam where the Mekong River drains into the South China Sea is the largest part of VMD with an area of 39,000 km2 Moreover, the region also supports the livelihood for more than 17 million people (Kondolf et al., 2018; GSO, 2018) The hydrology of the Mekong River is defined by a vast mean annual discharge through dry season and flood season, which is shows at Figure 2-1 (Adamson, 2016) The annual minimum daily discharge usually occurs in early April (point 1) The first transition season starts in late May with the doubling discharge of point 1 (point 2) The flood season starts within a few days at the end of June (point 3) The transition season 2 occurs the period between the end of the flood season (point 4) and the start

of the dry season (point 5) Generally, the rainfall regime is symbolized by a dry season between November and April, and a flood season from May to October

A = Transistion Season 1 B = Transistion Season 2 Figure 2-1 The two seasons of the Mekong River’s hydrological year (Adamson,

2006)

10

The Mekong River flows into the territory of Vietnam with an average flow of 13650m3/s plits into two major channels which are the Mekong (or Tien river) and the Bassac (or Hau river) rivers (Figure 2-2) The Vam Nao River downstream is a section

of the Mekong River flows to the Bassac River The water flow is almost equally distributed between the two major rivers After the Hau river receives water from the Tien river through the Vam Nao river, the average discharge of the two rivers are 51% and 49%, respectively (The Netherlands Delta Development team, 1974) However, in the dry season, the flow on the Vam Nao river varies with the tide, so the hydrological regime in this area is relatively complicated

Figure 2-2 The Lower VMD Zone including two main tributaries: the Mekong (Tien river) to the North, and Bassac (Hau river) to the South (Marchesiello et al., 2019) The Mekong flood flows into the VMD along the mainstream and from flooded areas

of Cambodia According to the study results of the IMHEN in 2013, the average peak

11

flood discharge is about 38,000 m3/s (corresponds to the water level of Tan Chau 4.40

m and Chau Doc 3.88 m) In years of heavy flooding, the flow can reach 45,000 m3/s, of which through the main stream about 32,000-34,000 m3/s (accounting for 75-80%), overflow the border from 8,000-12,000 m3/s (accounting for 20-25%), of which the LXQ is 2,000-4,000 m3/s and DTM is 6,000-9,000 m3/s On the main stream, the flow through Tan Chau is 24,000-26,000 m3/s (accounting for 82-86%) and through Chau Doc 7,000-9,000 m3/s (accounting for 14-18%) The total flood into the VMD is about 350-400 billion m3, of which the main flow is 80-85%, the flow over the border is 15-20%

40,000-Hydrological regime in the VMD also depends on the influence of two tidal sources in the East and West Sea The tides of the East Sea have an irregular semi-diurnal regime and the West Sea has an irregular diurnal regime Tides always fluctuate in cycles, from short (days) to medium (half months, months) and long (years, years) In addition, human impacts on hydrology connectivity and flow regime such as the unnatural channel, dams, levee systems or dyke systems have enormously altered the natural hydrodynamic conditions in the Vietnamese part of the VMD (Kondolf et al., 2006) These barriers will reduce flooding in mainstream rivers, which means reducing tributary-derived fine sediments accumulated on the bed and reducing permeability (Kondolf và Wilcock, 1996; Rahel 2006)

2.1.2 The general flooding at the Vietnamese Mekong Delta

The floodplain inundation plays an important role in the agricultural ecosystem and social and economic benefits of VMD They do not only cater natural flood retention but also reduce the discharge peaks in the flood season Floodwaters stay on floodplain long enough to recharge groundwater and provide water to floodplain forests (Kondolf

et al., 2006) Besides, slow-velocity water on floodplain provides habitat juvenile fish benefit from food availability Furthermore, silt deposits on floodplain, providing natural soil fertility

Flooding in the VMD can be divided into 3 periods At the beginning of the flood season (May-August), flood discharge on the main river rises quickly and follows the canals flowing into the fields to fill up the fields During this period, floodwaters

of floods

There are three aspects influenced to the inundations in the VMD (Hung et al., 1998): (1) The hydrograph of flood starting in the Mekong basin upstream of Kratie (2) The shielding of the flood wave in the Tonle Sap lake networks

(3) The tides of the Thailand Gulf and the South China Sea

The Tonle Sap hits the most vital part with regard to the flood duration in the Delta

In the flood season, the flow is divided into two sections: flow to the Tonle Sap Lake and flow draining into the Delta The Tonle Sap Lake discharge flows back to the Mekong River when the rain of Mekong River is lower than the lake rain level This buffer system reduces the negative impact of floods, but it extended floods compared

to the floods at Kratie The input flows of Tonle Sap beginnings around the middle of June, while the return discharge to the Mekong River typically initiates at the beginning of October The average annual inflow and outflow of the Tonle Sap Lake are around 79.0 km3 and 78.6 km3, respectively (Kummu and Sarkkula, 2008) The early peak enters between mid-July and mid-August and the other from September to October which is correlated with the typhoons from the South China Sea

Erosion in the VMD usually occurs in two forms including erosion and bank erosion Erosion is a phenomenon when the surface of a riverbed or riverbank is eroded and banked when the riverbank is unstable and slipped downstream Erosion and sedimentation occur when particles of sediment separate from the surface and move by rolling on the river bed or floating in the stream When it comes to locations where the flow velocity decreases, sediments are no longer carried away and accumulate, resulting in deposition On the contrary, the places where flow velocity increases, the ability to separate sediment particles at the bottom (or river banks) increases while the amount of sediment that is not enough to compensate will cause erosion Thus, erosion and sedimentation phenomenon is an unbalance in the movement of sediment and to evaluate sedimentation or erosion, it is often used the following sediment balance method:

Where: q – sediment flux, - the discharge changes the sediment flux in , -

time period, – the porosity, – the bottom height, - the river bed inclination, – the river length

14

As can be seen from the sediment balance formula:

 , : If the amount of sediment discharged is more than the amount collected, it will lead to erosion

 , : If more sediment is collected than is released, it will lead to accretion

Thus, in addition to the impact of the flow causing the sediment particles to move, the phenomenon of erosion also depends on the amount of sediment escape and advent or

in other words depends on the capacity of sediment transport (q) The ability to convey sediment q according to many studies has shown the dependence on factors related to velocity, volume, particle size, sediment type (Chih, 1996) Therefore, the mechanism

of erosion and sedimentation depends on the following main factors: Flow velocity, size of sediment particles and concentration of sediment in the flow

Erosion and sedimentation are natural phenomena that can be extremely affected by human activities Natural aspects like steep unstable slopes, greatly erodible soils and high rainfall intensities often play an important role in developing percentages of erosion and sediment loads Nonetheless, natural erosion is normally a very slow process that takes place over centuries or even millennia Human-induced or accelerated erosion and associated increases in sediment inputs to rivers can result in major increases in sediment flux and vital impacts on water quality and sediment capacitys (UNESCO and IRTCES, 1998) In the Mekong river, sediment discharge is about 110 million t/y (Milliman and Farnsworth, 1998) The annual sediment flow has been decreasing in the last decades, mainly because of dam construction, flood dykes, sand mining, and dredging (Anthony et al., 2015; Gugliotta et al., 2017) Recent studies report lower sediment yields, such as 87 MT/y (Darby et al., 2016), reflecting new measurement/calculation techniques and reductions in sediment transport already evident in the river

15

According to a survey by Marcello et al (2017), the distribution of sediments along the Tien and Hau rivers as shown in Figure 2-3, the size of sediment particles decreases from the upstream to the estuary In Hau river, at Chau Doc, the bottom has large grain sand (d = 0.05 –0.1mm) and the size decreases to fine sand (d = 0.01 - 0.05mm) at the confluence of Vam Nao river After the Vam Nao River junction, the bottom sediment on the Hau river is large due to receiving water and silt from the Tien river, then the sand size decreases gradually to the tidal influence area (after Can Tho) where the river bottom mostly mud

Figure 2-3 The distribution diagram of bottom sediment in Tien and Hau rivers

(Source: Marcello et al., 2017) Hjulstrom-Sungborg diagram (Figure 2-4) shows that with a particle size of 0.1mm, the velocity of 0.30 m/s can erode and cause erosion If the particle size is 0.05mm, the velocity of 0.5m / s will cause erosion The flow on Tien and Hau rivers in Chau Doc and Tan Chau areas has an average velocity 1.6 m/s in flood season and 0.6 m/s in the dry season Thus this section of the river is likely to be eroded year-round

16

Figure 2-4: Hjulstrom-Sungborg diagram (Hjulstrom, 1935) Sediment transport plays a vital role in maintaining fluval ecosystems such as marshes, floodplains, wetlands and estuaries, and the equilibrium between erosion and deposition usually arises along rivers in natural systems Similarly, soil erosion must

be seen as a natural process and in undisturbed landscapes, rates of soil loss or surface lowering are generally balanced by rates of soil formation (Nowacki et al., 2015; Xing

et al., 2017) However, natural stability is easily disrupted by intense climatic conditions and human activities, like land clearance which cause increased inputs of both runoff and sediment to river networks Sediment materials in the VMD Basin are

gravels, sands, silts and clays (Piman and Shrestha, 2017)

2.2 Vam Nao River

2.2.1 Geographical conditions

Vam Nao river flows through three communes of An Giang province which are Phu

My commune (Phu Tan district), Kien An commune (Cho Moi district), Binh Thuy commune (Chau Phu district) in the northeast-southwest direction The river is about 7

km long, 700 m wide, the depth of the river bed is over 17 m This is the only part of

17

the river connecting the two rivers are Tien river and Hau river (DONRE An Giang, 2014) Vam Nao river has many swirling and fast-flowing water, the river bed is very deep Due to the high flow of this river and the deep basin lead to the formation of vortices that can be tens of meters wide

Vam Nao river is not only the only river connecting Tien river and Hau river but also the shortest river in the Vietnamese river system Vam Nao River plays an important role in daily life as well as the regional economy In transport, Vam Nao helps facilitate trade in the VMD

This river has no islets located in the channel so there is no phenomenon of diversion and branching to create creeks The water flows from the upstream and flows to the Tien river, after which, the Tien river is divided into two roughly equal branches with

a width of about 600m each at the western islet

2.2.2 Hydrological characterises in Vam Nao river

The Vam Nao river confluence area in the lower Mekong region has a very large flow when it flows into An Giang (the average flow is 13650 m3/s) along two branches, namely Tien river and Hau river (Huy and Tu, 2015) Vam Nao River has the flood season usually starting from July or August and the dry season usually starts from February to May annual with the amount of river water accounting for only 15-25% Due to the influence of the terrain and the southwest monsoon regime, the flow in the area varies greatly in the rainy and dry seasons

Above Vam Nao, the right branch of Tien river has the width of 700m and the depth of the riverbed about 12m, the right bank is stable to light accretion, the left bank is eroded at an average speed of 2m/year, while while the width of Vam Nao is narrowed

to 226m (the narrowest section) and 8m deep, the left bank has a strong accretion at 5m/year, the right bank has serious erosion at 6m/year The section of Tien river above Vam Nao is 886m wide and 21m deep This typical river bed morphology leads to the accumulation of water into the Vam Nao river and the reduction of the flow of the Tien river downstream The river bed of the right Tien branch below Vam Nao is at risk of losing the flow due to the narrowing and sedimentation of alluvium

786 km, of which 42 areas with a total length of 148 km are particularly dangerous However, in 2019, the number of landslides increased to 681 points, an increase of nearly 7 times (Hoai et al., 2019)

There are four classed erosion are often applied for classification of erosion problems: 1-5m/year (I), 5-10m/year (II), 10-20m/year (III), > 20m/year (IV) In Mekong Delta, specifically Thuong Phuoc area above Tan Chau, Sa Dec and Vinh Long area, there are a number river reaches with an erosion rate of 40m/year or even more Results of the delta were loss of land, hundreds of casualties and thousands of households relocated due to bank erosion and protective measures

Channel incision downstream of dam

- Destabilizing infrastructure

- Stranding irrigation works and water intakes

- Declining groundwater levels

- Impacts on floodplain vegetation and ecosystem

Sediment Trapping - Loss of future hydropower and water storage potential

- Loss of habitable and arable land in the delta

- Intercepting of sediment-bound nutrients for delta agriculture and ecosystems

- Loss of terrestrial nutrient subsidies

- Disrupts hyporheic exchanges (ground water)

- Impacts floodplain obligate fisheries

- Loss of delta building material to shore (and consequent loss of delta land)

off Incentivizes artificial fertilizers because of the loss of river

- sourced nutrients in the flood plains Sand mining Local incision and bank

Accelerated land subsidence - See under dam sediment trapping

- Accelerated delta subsidence

- Land loss in the delta

- Salt water intrusion in the delta

Global climate

change

Sea level rise - Greater vulnerability during high tides

and storm surges and salt intrusion Global climate

20

Figure 2-5 shows erosion and landslide stations in the VMD River bank erosion and landslides affected on provinces such as An Giang, Dong Thap, Vinh Long, Can Tho and Hau Giang with increasing density These disasters have destroyed and threatened hundreds of houses along Tien and Bassac river banks Riverbank erosion often occurs

in areas with complicated riverbed topography

Figure 2-5 Online map of coastal erosion, river bank erosion and landslide sites in the VMD (Source: http://satlodbscl.phongchongthientai.vn, update on 7:00AM, 7th

December 2019 )

Figure 2-6 Vam Nao river after the landslide on 22 April 2017

(level 3)

Serious (level 2)

Note (level 1)

Weak

is short and impacts on erosion

Short slide bank, intermittent frequency

Flood season:

progress is infrequent or unpredictable

However, the extent of the impact on land and housing damage is alarming

Accretion bank is short and impacts

on erosion Short slide bank, scattered and discontinuous Flood season: intermittent progression Not cause great damage

to land and houses

Medium

(1-5m/year)

Accretion bank

is long and impacts on landslide

Short slide bank and appear fairly continuous

Flood season and dry season:

happening regularly or unpredictably

Serious damage level

to land and houses Strong

(6-15m/year)

Accretion bank

is long and impacts on landslide Long slide bank and appear

continuous

Flood season and dry season:

Huge damage to land and houses

22

inter-commune roads This disaster afflicted 108 house and destroyed other many infrastructures It was estimated to have caused losses of about 88 billion VND (identical about 3.95 million USD) (Thuy, 2019) According to Vietnam Disaster Management Authority in 2019, erosion in An Giang is getting more serious with 51 landslide spots, affecting 20,000 households were along the Tien, Hau and Vam Nao rivers as well as on canals connecting with the main rivers

Figure 2-7 The situation of sedimentation and landslide at the junction of Hau river -

Vam Nao river (Ngoc, 2018)

The erosion hotspots downstream within An Giang are mostly associated with the downstream migration of mid-channel banks, which creates a shifting zone of erosion downstream and to the sides of the bank, and a zone of accretion to its upstream

Possible causes for accelerated bank erosion:

- Human causes: On the one hand, an increase of population and the associated increase of occupation of riverbank areas, also for economic activities in combination with destroying or removing of the natural vegetation has probably resulted in

of the river in that area These have made riverbank erosion more complex and unpredictable (Jory and Guillaume, 2014; Van et al., 2016; Wang et al., 2019)

These disasters have resulted in major disruptions to local livelihoods, and financial burden on the provincial government by necessitating the relocation of inhabitants and localised bank protection works

2.4 Levee system in Vietnamese Mekong Delta

According to the Dyke Law, levee (flood dyke) is a protection dyke for a specific area Since 1995, investment in the construction of levees (flood dykes) has formed four sub-regions with 76 sub-regions based on the terrain of rivers, canals and natural embankments in VMD

Figure 2-8 Levee systems in Vietnamese Mekong Delta (Source: Vietnam News

Agency)

24

There are two kinds of levees in VMD:

- Low levees (mezzanine flood-dykes): also known as the "August levee", is low, small, invests less capital, and prevents floods while receiving floods According to the Five-Year Development Plan between 1996 and 2000 for VMD (Government of Vietnam, 1996), the majority of low levees constructed in 1996s which provide protection against the early flood peak arriving around mid-July to mid-August and keep floodwater in the paddy fields after the summer crop It ensures that the famers can grow two paddy crop annually (Winter-Spring and Summer-Autumn crops)

- High levees (enclosed flood-dykes): was built based on hydraulic calculations and the demand of local farmers to protect the floodplains against after the dreadful flood

in 2000 These dikes are mainly made of soil and pair along the main channels These levees completely control the flow into the floodplains during flood season, and allows farmers can conduct three crops per year in the provinces where are in the upper part

of the delta such as An Giang and Dong Thap In some places, they are also combined

as residential areas to escape floods or roads in the commune An Giang province is representative of provinces in the VMD were change the levee height from low (0–2 m) to high (3.5 m+) since 2000s (Chapman and Darby, 2016) Today, in An Giang province, there are two kinds of high levees including usual high levees and 2-layer high levees which comprise an overall levee ring and their sub-levees

The whole VMD has levee systems with a total length of over 13,000 km, of which

8000 km low levee with high varies from 1.5 to 4.0 m (Figure 2-8) While high levee areas are mainly concentrated in the provinces where located in the upstream of VMD includingAn Giang and Dong Thap (Figure 2-9) The cultivated fields in An Giang and Dong Thap which are protected by high levees with crest levels of 4.0–6.0m, are 65% and 40%, respectively (SIWRR, 2010)

25

Figure 2-9: Levee networks of the Vietnamese Mekong Delta (Chapman and Darby,

2016)

Figure 2-10: The location of high levee system (green dots) in upstream provinces of

the Vietnamese Mekong Delta (Triet et al., 2017)

26

It is a fact that levees increase flood stage and flooding in downstream coastal areas due to they disconnect the main channel from the delta plain (Alexander et al., 2012) High levee systems along the Mekong and Bassac distributaries declined the threat of local flooding, but it increased flood period downstream in coastal provinces (Le et al., 2007; Triet et al., 2017) Although extensive networks of floodplain dykes allow farmers to conduct a third rice crop, it prevents the natural sediment which has many nutrients conveyed by flood-water fluxes (Hung et al., 2012; Dang et al., 2016; Chapman et al., 2016) Moreover, the flows and sediment transport also have been changed As flow velocities increased within levee reaches are able to exacerbate erosion caused by uncoordinated development of other water infrastructure and large irrigation schemes in VMD (Le et al., 2007) This effect will increase the pumped irrigation demand in VMD's upstream and that is the main reason for water scarcity in downstream (Koldolf et al., 2018)

2.5 Regional and local studies

Floodplain flow regime and sedimentation has received much scientific attention These studies around the world and Vietnam are currently conducted in three directions:

2.5.1 Developing the physical models for experiment or use situ methods

There are many mathematical models have been applied in the study of sedimentation

in the floodplain since the 1980s Studies of Van Rijn (1984) established models of non-cohesive sediment transport calculations Vincent et al., (1981) and Tanaka and Shuto (1981) studied sediment transport under the simultaneous action of waves

In the studies of Steiger et al., (2003) and Middelkoop (2005), sediment traps have been used to represent patterns of floodplain deposition However, this method lacked linkage between the deposition measured in the trap and hydraulic conditions during the flood season Hence, erosion and sedimentation are notoriously difficult to monitor

Lu et al., (2014a) conducted field measurements in three branches of the Mekong at the conjunction to explore sediment dynamic at Chaktomuk (Cambodia) where is a

27

confluence of large rivers For all that the in situ sediment sampling method cannot indicate detailed sediment dynamics in the river section because the hydraulic frame at this site is very complicated

Fleiflea (2013) used satellite images to complete to estimate suspended sediment load along the Mekong River from Chiang Sean to Phnom Penh, where is upstream of the Mekong river, rather than to study the variation of sediment in space

Nowacki et al., 2015 has also been an extrapolation to estimate the annual sediment yield of the Mekong River However, due to this research relied on consideration for a short term so it does not allow accurate estimates Hung et al., (2014) and Nguyen et al., (2015) simulated the delta floodplains by using a large scale quasi-2D hydrodynamic model to study sediment distribution However, results from these studies, was done with a limited amount of measured data (only from 2008 – 2009) Triet et al., (2017) used large-scale hydrodynamic models to study the impact of dyke systems and hydropower dams on water levels and sediment distribution in the floodplains and delta

2.5.2 Developing methods and calculation tools that are mainly computer software

There are many studies focusing on the development of methods to solve differential equations of flow and sediment transport Some of them have been developed into popular software such as:

- MIKE 21 was developed by the Danish Institute Hydraulics (DHI) This is a module

in the MIKE model used to simulate biological, chemical and physical processes of shallow water with a 2-dimensional model The latest version allows calculations on unstructured grids Dao et al (2018) used MIKE 21 model to simulate the flows, sediment propagation and sedimentation, result initially showing the area where erosion level occurs most often when floods have occurred in the Tien river

- DELF3D of Deltares (Netherlands) This model simulates processes in hydrodynamics by calculating the unstable flow and its transport characteristics under

However, sedimentation and the flow regime in the river channel are constantly changing over time and the coastal protection structures in nature are often diverse This limits the accuracy of the numerical model

2.5.3 Calculate specific math problems

Currently, the calculations of changes in flow and flow of various rivers have been carried out quite a lot There are many studies in Vietnam Typically, the study assesses the impact of sand mining on riverbank erosion in Ho Chi Minh City (1998), studying changes in Soai Rap and Long Tau rivers after digging Hiep Phuoc Canal (Hoang, 2010) or research on the development of the Saigon-Dong Nai river bed (Nguyen and Tran, 1998) Le Manh Hung's study (2013) on the impact of sand mining activities to change the river bed of Tien river and Hau rivers and propose appropriate management solutions and planning The research of Nguyen (2014) about the causes

of landslides on the Co Chien river and Tien river