Riverbank Erosion under Boat-Generated Wave Attacks and … · 2019. 5. 21. · Professor Junji Yagisawa is gratefully acknowledged for having discussed, suggested and reviewed my

Riverbank Erosion under Boat-Generated Wave Attacks

and Proposed Countermeasures for Wave Attenuation

（河岸侵食について航走波による河岸侵食とその対策に関

する研究）

2015年 3月

埼玉大学大学院理工学研究科（博士後期課程）

理工学専攻（主指導教員田中規夫）

LA VINH TRUNG

RIVERBANK EROSION UNDER BOAT-

GENERATED WAVE ATTACKS AND PROPOSED

COUNTERMEASURES FOR WAVE

ATTENUATION

LA VINH TRUNG

A dissertation submitted in partial fulfillment of the requirements for the

degree of Doctor of Philosophy in Hydraulic and Coastal Engineering Examination committee: Prof. Norio Tanaka (Chair)

Prof. Takashi Asaeda

Prof. Ken Kawamoto

Assoc. Prof. Lee Han Soo

Hydraulic Engineering Laboratory

Department of Civil & Environmental Engineering

Graduate School of Science & Engineering

SAITAMA UNIVERSITY, SAITAMA, JAPAN

March 2015

i

ABSTRACT

Although bank erosion has been the interest of numerous studies, just a little work has

been done on the impact of waves, either natural or anthropogenic. It is well-accepted that

the bank erosion rate is dependent on the frequency of wave impacts as well as the wave

energy relative to soil resistance of riverbank. In heavily navigated rivers and channels,

high frequency of ships/boats generates waves constantly attacking towards riverbank and

therefore puts banks under the risk of being eroded. In recent years, due to considerably

increasing demand of inland waterway transportation served for daily activities such as

business, passenger and cargo loading, tourism, etc…a large number of boats/ship have

been observed. Especially, with the appearance of high speed boats getting more common,

the ones equipped with power engine that move relatively close to the riverbank and

generate high waves, wave-induced bank collapse is becoming a severe problem.

This study presents three field investigations. The first one was conducted in Arakawa

River in Japan. This work is a preliminary stage with an aim to identify whether heavy

navigation-generated waves is the main factor causing sediment movement in the river.

The results showed that the amount of sediment movement caused by a ship run (15

minutes) is smaller than that caused by tide (7 hours). However, by considering the time

consuming, the effect of ship wave on sediment transportation is much greater than tide.

And in such a heavily navigated river, this effect should be more impressive. The second

one was conducted at five rivers in Ca Mau Province, Southern part of Vietnam. At each

site, boat-generated wave and soil of riverbank characteristics were collected. The shear

stress induced by boat waves acting on riverbed is excessive the critical shear stress of

soil which means bank erosion is occurring. The observation at two sites showed that

vegetation can protect riverbank from erosion by attenuating wave energy. Based on the

field sites, wave and vegetation characteristics, such a 10 m width for sparse vegetation or

ii

an 8 m width for dense vegetation is able to dissipate completely the wave energy and

therefore prevent bank erosion due to wave attacks. In the third field investigation, the

efficiency of vegetation on wave attenuation was elucidated. Two vegetation species were

compared in aspect of wave attenuation capability, including Rhizophora apiculata and

Nypa fruticans. The results indicated that wave heights reduced exponentially and about a

half of initial incident wave height was reduced in the short distance when propagating

through vegetation. Wave reduction is dependent on vegetation porosity, incident wave

height, and still water depth. The effect of vegetation porosity, however, is more sensitive

compared to the latter. Rhizophora apiculata is better than Nypa fruticans on wave

attenuation even though its porosity is greater. It should be due to the special root

structure of this mangrove species which acts like a natural net, interrupting and breaking

up the wave train effectively. By taking ground slope effect into account, under steeper

slope condition compared to previous studies, it is explainable why the wave reduction

rate in present study is more impressive. Similar results were obtained in the study of

Brinkman (2006) because of relatively close slope condition. The last part of this study

discusses about applying artificial porous structure on riverbank protection. It rises from

the fact that in some areas, especially in cities where land is limited or soil condition is

not allowable, vegetation belt may not be planted and man-made structures are built

instead. Experimental study was conducted in combination with 1-D model to establish

the relationship between drag coefficient and porosity. The optimal θ to mitigate wave

attacks was then determined by a 2-D model taking into consideration not only cross

slope behind groins (i) but also the gap between two neighboring groins (G). In cases of i

=1/100 and 1/500, the combination of = 0.4 and L/G = 5 (L is the stream-wise length of

a groin) could bring out the best wave energy reduction. Thus, it is necessary to consider

both of i and G for designing stream-wise groins.

iii

ACKNOWLEDGEMENT

There are many people who have played a role in the compilation of this thesis, either

directly or indirectly. First of all, I wish to express my deepest gratitude and appreciation

to my supervisor, Professor Norio Tanaka for his valuable advices, kind guidances and

continuous encouragements and supports throughout the research. Also, Assistant

Professor Junji Yagisawa is gratefully acknowledged for having discussed, suggested and

reviewed my research work so that my research could be improved significantly.

Then, I would like to thank my co-supervisors, Professor Takashi Asaeda and Professor

Ken Kawamoto, for their valuable comments and suggestions on my research. My sincere

thanks must go to Assistant Professor Eiichi Furusato and Dr. Kithsiri Nandasena for their

fruitful feedback and supports in the seminars.

I am thankful all members of Hydraulic laboratory for their co-operation and help during

my entire study period, either in doing experiment or getting used to the new life in Japan,

especially to Mr. Toshiya Kaneko, Mr. Taiji Morinaga and Mr. Aki Onai.

I am also grateful to the Japanese Government for the full financial assistance provided

me during my study period in Japan. I extend my gratitude to the Water Resources

University (the second base) and Service of Agriculture and Rural Development, Ca Mau

Province for their great supports during the field investigations.

Last but not least, I wish to thank my parents and my wife for their moral supports and

sharing over the hard work. Especially, the companionship of my wife, Thuy Nguyen, and

our son, Tit, is the invaluable encouragement that makes this work accomplishable.

iv

CONTENTS

ABSTRACT ........................................................................................................................ i

ACKNOWLEDGEMENT ............................................................................................... iii

CONTENTS ...................................................................................................................... iv

LIST OF TABLES .......................................................................................................... xiii

LIST OF SYMBOLS ....................................................................................................... xv

LIST OF ABBREVIATION .......................................................................................... xix

CHAPTER 1 ....................................................................................................................... 1

INTRODUCTION ............................................................................................................. 1

1.1. Riverbank erosion and suspected reasons ............................................................ 1

1.2. Literature review .................................................................................................... 8

1.2.1. Ship/boat-generated wave issue ....................................................................... 8

1.2.2. Previous studies to cope with wave attacks .................................................... 10

1.3. Purpose of the study ............................................................................................. 12

1.3.1. Aim and objectives .......................................................................................... 12

1.3.2. Outline of Thesis ............................................................................................. 13

CHAPTER 2 ..................................................................................................................... 14

EFFECT OF SHIP WAVE AND TIDE ON SEDIMENT TRANSPORTATION ..... 14

2.1. Overview ................................................................................................................ 14

2.2. Study area and field investigation ....................................................................... 14

2.3. Equipment deployment and measurement ......................................................... 16

2.3.1. Measuring point selection ............................................................................. 16

2.3.2. Experimental equipment and deployment .................................................. 17

2.3.3. Tide observation ............................................................................................ 20

v

2.3.4. Data analysis method ..................................................................................... 25

2.4. Results and discussions ........................................................................................ 25

2.4.1. Tidal effect ...................................................................................................... 25

2.4.1.1. Characteristics of the tide ......................................................................... 25

2.4.1.2. Time variation of Reynolds stress ............................................................. 27

2.4.1.3. The particle distribution and weight of sediment collected at the field .... 29

2.4.2. Ship wave effect.............................................................................................. 32

2.4.2.1. Time variation of velocity and water depth .............................................. 32

2.4.2.2. Time variation of Reynolds stress ............................................................. 34

2.4.2.3. The particle distribution and weight of sediment collected at the field .... 35

2.4.3. Comparison between tide and ship wave effects on sediment transport .. 36

CHAPTER 3 ..................................................................................................................... 38

THE EFFECT OF BOAT-GENERATED WAVE ATTACKS ON RIVERBANK

STABILITY ..................................................................................................................... 38

3.1. Overview ................................................................................................................ 38

3.2. Study area and field investigation ....................................................................... 38

3.3. Equipment deploys and cases .............................................................................. 42

3.3.1. Boat wave generation and measurement ....................................................... 42

3.3.2. Soil characteristics measurement ................................................................... 43

3.3.3. Cases ................................................................................................................ 44

3.4. Numerical simulation for assessing the occurrence of bank erosion ............... 45

3.4.1. Governing equations ....................................................................................... 45

3.4.2. Boundary conditions ....................................................................................... 47

3.4.2.1. Boundary conditions of the waterway ...................................................... 47

3.4.2.2. Boundary condition of the ship ................................................................. 48

vi

3.4.3. Model validation by comparing with the field investigation ......................... 48

3.5. Results and discussion .......................................................................................... 50

3.5.1. Bank erosion under boat-generated wave attacks ......................................... 50

3.5.1.1. Waterway conditions in the researched area ........................................... 50

3.5.1.2. Current situation of riverbank stability .................................................... 53

3.5.2. Preliminary assessment about bank erosion mitigation by vegetation ......... 56

CHAPTER 4 ..................................................................................................................... 59

EFFICACY OF RHIZOPHORA APICULATA AND NYPA FRUTICANS ON

ATTENUATION OF BOAT-GENERATED WAVE UNDER STEEP SLOPE

CONDITION .................................................................................................................... 59

4.1. Overview ................................................................................................................ 59

4.2. Study area and vegetation characteristics .......................................................... 59

4.3. Methodology and cases ......................................................................................... 62

4.3.1. Measurement of boat-generated waves and analysis .................................... 62

4.3.2. Drag force measurements ................................................................................. 63

4.3.3. Cases ................................................................................................................ 65

4.3.4. Numerical simulation for determining the slope effect on wave attenuation

................................................................................................................................... 66

4.4. Data collection and analysis ................................................................................. 66

4.4.1. Wave propagation through Rhizophora apiculata and Nypa fruticans

forests ........................................................................................................................ 66

4.4.2. Effects of vegetation density, incident wave height, and still-water depth on

wave attenuation ....................................................................................................... 68

4.4.3. Comparison of wave attenuation capability of Rhizophora apiculata and

Nypa fruticans .......................................................................................................... 70

vii

4.4.4. Model validation and application ................................................................... 71

4.5. Discussions ............................................................................................................. 72

4.5.1. Variation of drag coefficient .......................................................................... 72

4.5.2. Wave attenuation characteristics ................................................................... 74

4.5.3. Application for vegetation design to deal with wave attacks ......................... 80

CHAPTER 5 ..................................................................................................................... 81

THE OPTIMAL POROSITY OF STREAM-WISE GROINS FOR WAVE

ATTENUATION UNDER DIFFERENT CROSS SLOPE CONDITIONS ............... 81

5.1. Overview ................................................................................................................ 81

5.2. Experimental setup and procedure ..................................................................... 82

5.2.1. Experimental apparatus ................................................................................. 82

5.2.2. Artificial porous structure .............................................................................. 83

5.2.3. Cases ................................................................................................................ 84

5.3. Numerical simulation ........................................................................................... 84

5.4. Results and discussions ........................................................................................ 86

5.4.1. Wave propagation through the porous structure .......................................... 86

5.4.1.1. Wave height reduction .............................................................................. 87

5.4.1.2. Velocity reduction ..................................................................................... 88

5.4.1.3. Effect of porosity parameter on wave attenuation .................................... 89

5.4.2. Validation of numerical simulation ............................................................... 90

5.4.3. Optimal design of porous structure for mitigating the wave attacks ............ 93

CHAPTER 6 ..................................................................................................................... 97

SUMMARY AND CONCLUSIONS .............................................................................. 97

REFERENCES .............................................................................................................. 100

viii

LIST OF FIGURES

Figure 1.1. The map of Vietnamese Mekong Delta (Source: Vietnam Academy for Water

Resources) ........................................................................................................................... 2

Figure 1.2. Occurrences of bank erosion at (a) Cai Nai River, (b) Bay Hap River, and (c)

Dam Chim River in Ca Mau Province, the Vietnamese Mekong Delta .............................. 3

Figure 1.3. Populated areas of Nam Can and Dam Doi Communes with heavy navigation

in Cai Nai, Bay Hap, and Dam Chim River ........................................................................ 4

Figure 1.4. Bed elevation change measured in 2004, 2005 and 2008 in (a) Cai Nai and (b)

Dam Chim River .................................................................................................................. 6

Figure 1.5. The peaks of flood events recorded from 2002 to 2008 .................................... 6

Figure 1.6. The relationship between the quantity of boat in use and bank erosion rate in

Cai Nai and Dam Chim River in 2004 and 2008 ................................................................. 8

Figure 2.1. Map of the field site (downstream of Arakawa River, Japan) and locations of

the measuring points .......................................................................................................... 15

Figure 2.2. The porous structures (made of wood) taken at the field survey, (a) At Senju

Sakuragi and, (b) At Higashi Yotsugi ............................................................................... 16

Figure 2.3. Experimental schema ...................................................................................... 17

Figure 2.4. Electromagnetic current meter, (a) The main body, (b) Detector ................... 18

Figure 2.5. The wave gauge, (a) Wave height meter, (b) Amplifier, (c) Detecting string 19

Figure 2.6. The sediment trap, (a) Bottle and lid, (b) Bottles installation ......................... 20

Figure 2.7. Equipment deployment in the real field .......................................................... 20

Figure 2.8. The sea level variation recored in September at Tokyo Bay ........................... 21

Figure 2.9. The sea level variation recored during observation time ................................ 21

Figure 2.10. The sea level variation recored in November at Tokyo Bay ......................... 23

ix

Figure 2.11. The sea level variation recored during observation time .............................. 23

Figure 2.12. The sea level variation recored in October at Tokyo Bay ............................. 24

Figure 2.13. Variation of flow velocity and water depth with time, (a) At Higashi Yotsugi

(9/13) and (b) At Senju Sakuragi (9/14) during the neap-tide ........................................... 26

Figure 2.14. Variation of flow velocity and water depth with time at Higashi Yotsugi

(11/15) during spring-tide. ................................................................................................. 27

Figure 2.15. Variation of Reynolds stress and water depth with time at Higashi Yotsugi,

(a) During neap-tide and (b) During spring-tide................................................................ 28

Figure 2.16. Grain size curve of sediment collected at Higashi Yotsugi during the neap-

tide (9/13) .......................................................................................................................... 29

Figure 2.17. Grain size curve of sediment collected at Senju Sakuragi during the neap-tide

(9/14) ................................................................................................................................. 29

Figure 2.18. Grain size curve of sediment collected at Higashi Yotsugi during the spring-

tide (11/15) ........................................................................................................................ 30

Figure 2.19. Variation of water depth and velocity with time at Higashi Yotsugi during (a)

low tide (a) and (b) high tide ............................................................................................. 32

Figure 2.20. Variation of water depth and velocity with time at Senju Sakuragi during (a)

low tide (a) and (b) high tide ............................................................................................. 33


(a) Low tide and (b) High tide. .......................................................................................... 34

Figure 2.22. Grain size curve of sediment collected at Higashi Yotsugi (caused by ship

wave) ................................................................................................................................. 35

Figure 3.1. Study area (a) Cai Nai River, (b) Bay Hap River, (c) Dam Doi River, (d) Dam

Chim River, and (e) Cai Lon River. .................................................................................. 39

x

Figure 3.2. The location of each transect where the field measurements were conducted.

........................................................................................................................................... 40

Figure 3.3. The characteristics of Rhizophora apiculata observing at Cai Lon River,

transect 14-14..................................................................................................................... 41

Figure 3.4. Wave measurement equipments (a) Boat used for wave generation, (b) Staff

gauge and, (c) Magnetic velocity device. .......................................................................... 42

Figure 3.5. Soil experiment at site. (a) Equipment for in-situ measurement, (b) Collecting

soil sample and, (c) Measurement ..................................................................................... 44

Figure 3.6. Validation of numerical simulation used in this study: (a) Comparison

between the time series of wave propagation calculated by numerical simulation and the

observed one obtained at transect 10-10, Dam Chim River, (b) Comparison between

observed and calculated maximum wave height at some representative transects ........... 49

Figure 3.7. Maximum wave height and current velocity measured at the sites................. 50

Figure 3.8. Calculated critical shear stress of soil at each transect .................................... 53

Figure 3.9. The situations of riverbank observed at (a) Cai Nai River, (b) Bay Hap River,

(c) Dam Doi River, and (d) Dam Chim River ................................................................... 54

Figure 3.10. The relationship between the wave-induced shear stress acting on the river

bed and the erosion rate ..................................................................................................... 55

Figure 3.11. The representative case (transect 10-10, Dam Chim River) showing the effect

of vegetation width and density on the wave-induced shear stress acting on the river bed

........................................................................................................................................... 57

Figure 4.1. Locations of the field investigation, R. apiculata in the Cai Lon River (A, B)

and N. fruticans in the Ong Doc River (C, D) ................................................................... 60

Figure 4.2. The measurement deployment in the field. (a) At Cai Nai River and, (b) At

Ong Doc River ................................................................................................................... 62

xi

Figure 4.3. The schematic of drag force measurement apparatus (L1 = L2 = 0.5 m) and

drag force measurement at site .......................................................................................... 64

Figure 4.4. Representative time series of boat-generated wave at location A, pole 1

position, under high wave high tide (HWHT) condition, and the definition of maximum

boat-generated wave height. .............................................................................................. 66

Figure 4.5. Wave height reduction along the distance from boat-line to each pole at (a)

Location A: R. apiculata (dense case), (b) Location C: N. fruticans (dense case) ............ 67

Figure 4.6. Comparison of efficiency of N. fruticans with R. apiculata in wave

attenuation: (a) high wave high tide (HWHT) and low wave high tide (LWHT), (b) high

wave medium tide (HWMT) and low wave medium tide (LWMT) ................................. 69

Figure 4.7. Comparison of calculated and measured maximum wave height for model

validation ........................................................................................................................... 71

Figure 4.8. Comparison of drag coefficient from field investigation with study of

Takemura and Tanaka (2007) and Tanaka and Yagisawa (2010) ..................................... 73

Figure 4.9. Wave reduction rate per meter vegetation width in comparison with Mazda et

al. (1997 & 2006), Quartel et al. (2007) and Brinkman (2006) ......................................... 77

Figure 4.10. The correspondence between vegetation width and ground slope condition 80

Figure 5.1. The sketch of experimental setup .................................................................... 83

Figure 5.2. The image of the porous structure and porosity adjustment by using steel

cylindrical boxes ................................................................................................................ 84

Figure 5.3. Representative showing the effect of porous structure on the free surface

elevation (H = 10 cm) ........................................................................................................ 87

Figure 5.4. A representative of (a) Time profiles of water surface elevation measured at 4

positions ( = 0.2, H = 10cm, h = 35 cm) and (b) Wave height reduction through the

porous structure ( = 0.2) .................................................................................................. 88

xii

Figure 5.5. Representative of velocity reduction through the porous structure with

porosity of 0.5 ................................................................................................................. 89

Figure 5.6. The relationship between porosity and wave height reduction (H = 7 cm) .... 90

Figure 5.7. Comparison of the maximum wave height measured in the experiment and

calculated by numerical simulation at position 4 by changing the drag coefficient, (a) =

0.7, (b) = 0.5, and (c) = 0.2 .......................................................................................... 92

Figure 5.8. The representative of free surface elevation with time ( = 0.7, h = 0.25 m, H

= 0.04 m): (a) At position 1, (b) At position 2, (c) At position 3, and (d) At position 4. .. 93

Figure 5.9. Wave energy reduction corresponding with porosity of structure under

different slope conditions .................................................................................................. 94

Figure 5.10. The relationship between wave energy reduction and L/G (a) i = 1/500, (b) i

= 1/100, and (c) i = 1/50 .................................................................................................... 96

xiii

LIST OF TABLES

Table 1.1. The statistics for transportation used in the waterways of Ca Mau Province up

to July 2011 ......................................................................................................................... 7

Table 2.1. Overview of each location ................................................................................ 16

Table 2.2. Observation time and tide level at Tokyo on the 13th and 14th of September ... 21

Table 2.3. Schedule of measurement on the 13th and 14th, September during the neap-tide

........................................................................................................................................... 22

Table 2.4. Observation time and tide level at Tokyo on the 15th of November ................. 22

Table 2.5. Schedule of measurement on the 15th of November during the spring tide ...... 24

Table 2.6. Tide level forecast in Tokyo on the 10th of October ......................................... 24

Table 2.7. Sediment collected in trap in each stage of tide during neap tide at Higashi

Yotsugi (9/13) and Senju Sakuragi (9/14) ......................................................................... 31

Table 2.8. Sediment collected in trap in each stage of tide during spring tide at Higashi

Yotsugi (11/15) .................................................................................................................. 31

Table 2.9. Sediment collected in trap in each stage at Higashi Yotsugi (10/10) and Senju

Sakuragi (available data) ................................................................................................... 36

Table 2.10. Comparison of sediment movement caused by tide and ship wave ............... 36

Table 3.1. Summary of site characteristics. ....................................................................... 41

Table 3.2. Frequency of boat at each transect during the monitoring time (8.00 AM-5.00

PM) .................................................................................................................................... 51

Table 3.3. Summary of soil characteristics at each transect of this study and Watts et al.

(2003) ................................................................................................................................. 52

Table 4.1. Vegetation characteristics at selected locations ................................................ 61

Table 4.2. Hydraulic conditions and vegetation types in the field .................................... 65

Table 4.3. Summary of wave height reduction for all cases .............................................. 70

xiv

Table 4.4. Summary of experimental condition of each study .......................................... 76

Table 4.5. Percentage change of wave height reduction under mild and steep slope in the

case of with and without vegetation .................................................................................. 79

Table 5.1. Summary of all experimental cases in the study .............................................. 84

xv

LIST OF SYMBOLS

Symbol Description

A Cross-sectional area of flow in unit width under the water surface

Am Measured area

a Wave amplitude

Bs Ship width

b Porosity for the permeable layer of the river bed

C Wave celerity

CD Drag coefficient

CM Inertia coefficient

D Diameter of cylinder

Dt Average diameter of tree trunk

d Average diameter of root

dS Ship draft

E Energy flux

F1 Drag force

F2 Applied force

fD Drag force

fM Inertia force

fx Drag force in x-direction

fy Drag force in y-direction

G Space between cylinders in the cross-stream direction

g Gravitational acceleration

H Wave height

xvi

HA Wave amplitude

Hc Calculated maximum wave height

Hm Observed maximum wave height

Hn Wave height at position 2, 3, and 4

Hs Wave height at a sea side station

HL Wave height at a station 100 m further inshore

H1 Wave height at position 1

h Still water depth

ie The grid number at the end in x- direction

is The grid number at the start in x- direction

je The grid number at the end in y- direction

js The grid number at the start in y- direction

k A constand (=2/)

L Space between neighboring cylinders

LS Ship length

L1 Space between neighboring mangrove roots

l Vegetation width

N Vegetation density

n The Manning roughness coefficient

nr Root density

Qx Depth-integrated velocity components in x direction

Qy Depth-integrated velocity components in y direction

R2 Coefficient of determination

Rbx Breaking wave damping term in x-direction

Rby Breaking wave damping term in y-direction

xvii

r Wave reduction rate

S The transverse section area of the ship below still water level

S0 The mid-ship section area

T Wave period

T* Transition time

t Time

t0 Time that breaking was initiated

U1 Velocity measured at position 1

Un Velocity measured at position 2, 3, and 4

US Ship speed

u Current velocity

'u Average fluctuating velocity

V Control volume

Vs Volume occupied by solid objects

xS The distance from the mid-ship

W Distance between the toe of riverbank and a position in the riverbed at which vegetation still can survive

z Vertical co-ordinate positive upward, origin at still water level

Mid-ship section coefficient

Correction factor of the dispersion term

b Mixing length coefficient

Solid volume portion of the forest

Water surface elevation

I

t Initial value of breaking

F

t End value of breaking

xviii

t Rising rate of water level

*t Parameter that determines the onset and cessation of breaking

Wavelength

Porosity of vegetation, porous structure

Density of water

Wave-induced shear stress

c Critical shear stress of soil

bx Shear stresses acting on the bed in x direction

by Shear stresses acting on the bed in y direction

f Shear strength of soil

R Reynolds stress

Wave velocity

e Eddy viscosity

Angular frequency

xix

LIST OF ABBREVIATION

Symbol Description

VKD Vietnamese Mekong Delta

HWHT High wave high tide

LWHT Low wave high tide

HWMT High wave medium tide

LWMT Low wave medium tide

RD Rhizophora Dense case

RS Rhizophora Sparse case

ND Nypa Dense case

NS Nypa Sparse case

1

CHAPTER 1

INTRODUCTION

1.1. Riverbank erosion and suspected reasons

While the causes of riverbank erosion have been the interest of numerous studies, just a

limited work has been done on the effect of boat-generated waves even though they are

considered as one of the reasons threatening the bank stability. Nowadays, on many rivers

and channels, the appearances of high speed boats have become commonly that increase

both magnitude and frequency of boat waves towards the riverbanks. When a boat runs on

a river, the magnitude and form of waves generated are dependent on many factors, such

as the shape of hull, the clearance between the hull and riverbed, the speed and direction

of boat relative to currents in the river, etc...The wave period and direction of propagation

towards riverbank depend on the boat speed and still water depth (Nanson et al., 1994).

Bank erosion occurs once the wave energy is sufficient to exceed the soil resistance of

riverbank. Yet, the erosion rate increases with the increase of frequency of wave impact

which depends on the navigation situation in the rivers/channels. The heavier the

navigation is, the more waves are generated and attack riverbank.

Located in the South of Vietnam, the Vietnamese Mekong Delta (VMD) covers 12

provinces (Fig.1.1). The topography is flat with the average elevation is from 3 to 5 m,

some areas have low elevation which is just 0.5 to 1 m higher than sea level. VMD has a

typical feature of humid tropical climate and get influenced by both the southwest and

northeast monsoons. In general, the rainy season runs from May to November while the

dry season spans December to April. During the dry season, it is almost no rain and as a

consequence, puts this area into salinity intrusion trouble. To be favored by nature, VMD

has large and fertile plain as well as rich natural resources. For years, this region has

2

contributed more than 50% of total food production of Vietnam. In addition, the VMD is

also a major source of fish and fruits.

In the VMD in general and Ca Mau Province in particular, due to specifically

geographical conditions, the land transportation systems are not able to be developed

properly. However, with the dense and complicated river networks, inland waterways

have been built as transport infrastructures to support local communities. In recent years,

it is noticeable that bank erosion has occurred more and more seriously in both intensity

and scale. This phenomenon can be observed along the main rivers and channels, river

junction and estuaries.

Figure 1.1. The map of Vietnamese Mekong Delta (Source: Vietnam Academy for Water

Resources)

The damages caused by bank erosion are very severe and influencing on not only human

livelihood but also the ecosystem. According to the surveys conducted so far, bank

erosions occurring in Ca Mau Province have destroyed many constructions as barrier,

dyke…causing salinity intrusion, inundation and adverse influences on the natural

3

balance of ecological conditions, therefore threatening fishery sources as well as

vegetations. Hundreds of houses were collapsed into rivers, thousands of households must

evacuate from the place where they were born and grown up to safer places (Fig.1.2).

(a)

(b)

(c)

Figure 1.2. Occurrences of bank erosion at (a) Cai Nai River, (b) Bay Hap River, and (c)

Dam Chim River in Ca Mau Province, the Vietnamese Mekong Delta

4

Besides, bank erosion also increases the amount of sediment in the flow and causes

deposition in other places that will affect badly on waterways, irrigation and flood

drainage. There are various reasons causing bank erosion dependent on spatial and

temporal conditions. However, the previous reports (Center for River Training and

Natural Disaster Mitigation, 2002, 2008) listed 4 dominant reasons causing bank erosion

in Ca Mau Province (1) The current velocity exceeds the critical velocity, (2) Wind waves,

(3) Ship-induced wave attacks, and (4) Overload on the bank. While the two former

reasons are natural phenomenon, the two latter ones are due to human activities.

Figure 1.3. Populated areas of Nam Can and Dam Doi Communes with heavy navigation

in Cai Nai, Bay Hap, and Dam Chim River

(Source: Vietnam Institute of Meteorology, Hydrology and Environment)

In the river reaches and channels further inland, the effects related to man-made acts

become more apparent due to the increase of population density. Especially, in the

Dam Chim

Cai Nai

Bay Hap

5

crowded residential areas as markets, school…the frequency and quantity of boats serving

for transportation are already high. It is noticeable to see that in recent years, due to

rapidly socioeconomic development in the Vietnamese Mekong Delta, there has been a

remarkable increase of transportation. In order to meet the demands of passage among

areas, means of transport is more developed while the widths of rivers and channels do

not change much that makes the traffic density becomes denser and denser. Being

considered to have the most populated waterway in Vietnam, the transport in Ca Mau

Province is diversified in terms of type of means as engine-driven boats, canoes, high-

speed boats, etc…The increase in both quantity and speed of boats induces waves

attacking towards riverbank and causes bank erosion severely. In the main waterways as

Cai Nai, Bay Hap, and Dam Chim (Fig.1.3), bank erosion is more and more serious. This

is not surprised and can be foreseen because these rivers are the life-line routes serving

for navigation in not only this province but also the vicinity. In addition, the crowded

population living along riverbanks by cutting down vegetation to build up houses reveals

the bare banks which are eroded easier by boat-generated waves. According to previous

reports of Center for River Training and Natural Disaster Mitigation, cross sections of

these rivers change significantly in the last few years due to erosion problem. Fig.1.4

shows the change of elevation at cross sections measured at Cai Nai and Dam Chim River

in each period of time. It can be seen that bank erosion occurred more severely from 2005

to 2008 compared to that in 2004 in both rivers. Considering the bank retreat within 3

years from 2005 to 2008, the amount of land collapsed into rivers is considerable with the

bank retreat being more than 9 and 11 m in Cai Nai and Dam Chim River, respectively.

6

(a)

(b)

Figure 1.4. Bed elevation change measured in 2004, 2005 and 2008 in (a) Cai Nai and (b)

Dam Chim River

(Source: Center for River Training and Natural Disaster Mitigation, HCM city)

The flood events recorded from 2002 to 2008 in Tan Chau and Chau Doc Hydro-

meteorological stations (Fig. 1.5) shows that the peaks of flood during these years are not

different. Thus, there should be another reason that increases the erosion rate.

0

2000

4000

6000

8000

10000

12000

2001 2002 2003 2004 2005 2006 2007 2008 2009

Time (year)

Mea

n di

scha

rge

(m3 /s

)

Tan Chau Chau Doc

Figure 1.5. The peaks of flood events recorded from 2002 to 2008

(Source: Provided by Southern Institute of Meteorology, Hydrology and Environment)

11/2008

01/2005

01/2004

12/2008

01/2005

01/2004

Bank retreat

Bank retreat

7

Table 1.1 shows the number of ships/boats used in Ca Mau Province in details based on the

statistical data of boat registration of the Bureau of River Management, Ca Mau. It can be

realized that there has been a jump of boat use demand recently at all areas of operation.

Dam Doi and Nam Can where have Dam Chim and Cai Nai River are the two communes

which not only have got the highest boat use in Ca Mau Province but also have been

observing the remarkable increase of boats in use from the period of 1995-2004 to 2005-

2011. This boat increment may concern with the increase of bank erosion in this area. Fig.

1.6 shows the bank erosion rate corresponding with the increase of number of boats in use

in different year. Obviously, bank erosion is getting more severe since the number of boats

in use increases. Therefore, it is urgently necessary to investigate the effect of boat-

generated waves which has been underestimated in ages on riverbank erosion.

Table 1.1. The statistics for transportation used in the waterways of Ca Mau Province up

to July 2011

Category

Number of ships/boats in use Sum Before

1975 From

1975 to 1984

From 1985 to

1994

From 1995 to

2004

From 2005 up to now

Purpose of use 87387 - - 15 29837 57535 Household 81128 12 28296 52820 Manufacture 3384 3 894 2487 Passenger 1225 - 219 1006 Cargo shipping 1441 - 365 1046 Public services 239 - 63 176 Area of administrative operation

87387 - - 15 29837 57535

Ca Mau city 3662 - - 3 1708 1951 U Minh 5612 - - - 1777 3835 Thoi Binh 10637 - - 5 4282 6350 Tran Van Thoi 12909 - - 3 4290 8616 Cai Nuoc 9041 - - - 1849 7192 Dam Doi 16946 - - 2 4139 12823 Nam Can 10556 - - 1 1646 8909 Phu Tan 10763 - - - 3305 7458 Ngoc Hien 7243 - - 1 1841 5401 (Source: Provided by the Bureau of River Management, Ca Mau Province)

8

0

1

2

3

4

5

0 2000 4000 6000 8000

Number of boats in use

Ave

rage

ero

sion

rat

e (m

/yea

r)

Cai Nai_01/2004-01/2005Cai Nai_01/2005-11/2008Dam Chim_01/2004-01/2005Dam Chim_01/2005-12/2008

Figure 1.6. The relationship between the quantity of boat in use and bank erosion rate in

Cai Nai and Dam Chim River in 2004 and 2008

1.2. Literature review

1.2.1. Ship/boat-generated wave issue

Waves generated by boats contain a massive amount of energy that can erode seriously

the riparian and coastal environment (Belibassakis, 2003; Bonham, 1983; Coops et al.,

1996). Once bank erosion occurs, it may adversely affect not only human livelihood but

also the ecosystem. Among the reasons causing riverbank erosion, little work has been

done on the influence of waves, although they induce high energy events. Recently,

because the number of motor-boats, some capable of high speed, becomes common, the

wave magnitude and frequency of boat wave affecting riverbanks significantly increase.

Kirkegaard et al. (1998) showed that waves generated by high-speed boat in shallow

water are substantially different from the waves generated by conventional ships as a

consequence of the higher speed and the size of these modern vessels. Tanimoto et al.

(2000) found out that the ship waves in a shallow and narrow channel such as a canal are

greatly different from those in a deep open sea. Surprisingly, most damage is not caused

9

by the large ships, which often move slowly and relatively far from the banks. Small,

high-speed boats with powerful engines passing near the bank actually do more harm

(Schiereck, 2005). Moreover, study of Nascimento et al. (2010) reported that in heavily

navigated channels, the combination of two or more wave trains, generated by boats

moving either in the same direction or in opposite directions, causes much more damage.

Such studies were largely based on computational simulations and improved algorithms

of wave transformation over shoaling seabed. Although this approach is able to support a

large amount of calculations, there are always some certain limitations due to initial

assumptions for simplicity and therefore cannot reflect exactly the happening in the real

field with the effects of complex internal and external factors. There are just a few studies

on ship/boat waves based on field investigation, although these studies have not yet

included enough information to compare the theoretical results with the field data.

Velegrakis et al. (2007) conducted a field observation of waves generated by passing

ships and compared how influence the wave, induced by conventional ferry and fast ferry,

on beach sediment dynamics. Their results demonstrated that the fast ferry could generate

a much more energetic event, which not only did include much higher waves, but it was

also an order of magnitude longer. Therefore it might affect both beach sediment

dynamics and nearshore benthic ecosystem. Nanson et al. (1994) tried to link bank

erosion rates with measured wave characteristics, conducted on the lower Gordon River,

Tasmania, with the aim of river management. McConchie and Toleman (2003) carried-

out a field investigation along the Waikato River, New Zealand to determine the main

reason causing bank erosion and bed elevation change. They found out that the wakes

induced by a boat were more effective at suspending and transporting sediment than

wind-generated waves, particularly in the cases of shallow water. Therefore, in order to

complement more information regarding ship wave attack obtained from the field

10

observation, it is necessary to urgently study boat-generated wave characteristics and how

it affects riverbank stability in the real field in the cases of bare and vegetated riverbanks.

1.2.2. Previous studies to cope with wave attacks

To prevent wave attacks from eroding riverbank, while riprap, soil, and concrete

embankments have been applied for years as countermeasures, non-constructional

methods utilizing eco-friendly vegetation are not very common. In the last decade,

interest has been growing in understanding the impact of vegetation on wave attenuation.

Vegetation is considered one factor contributing to bank stability, in particular by

reducing wave energy during the transmission of waves through the vegetation (Bonham

1983; Kobayashi et al. 1993; Coops et al. 1996). Bonham (1983) conducted hydraulic

experiments and field tests about the attenuation of ship waves in well-surviving beds of

emergent river plants. In his study, both wave type and vegetation characteristics were

considered. Three types of boat wash were identified including surge waves, ship waves,

and a wake of transverse and reflected waves. The wave heights of ship waves were in the

range of 13.5-22.5 cm depending on the boat hull form. Wakes of transverse and reflected

waves were relatively small with wave heights not greater than 7 cm. The surge waves

were the smallest. However, 2 boats out of 75 could produce a surge height exceeding 10

cm. To deal with bank erosion caused by ship wave attacks, four types of emergent river

plants were tested for wave dissipation capability. His results showed that any of those

species could dissipate almost two-third of the boat wash ship wave energy and inhibit

wave-break. Kobayashi et al. (1993) developed a model to study the effect of submerged

vegetation on wave propagation. In addition, Coops et al. (1996) by conducting an

experimental study in a wave tank; found out a positive impact of emergent vegetation

characteristics on both sediment reinforcement and wave attenuation. While small waves

11

(10 cm) could cause considerable alteration to the slopes in the unplanted sections, such

waves did not greatly affect slopes under vegetation cover. Moreover, due to more finely

distributed root structure into the soil, Phragmites australis (Cav.) Trin. ex Steudel could

withstand the attacks of 23 cm waves while Scirpus lacustris L. was uprooted, followed

by increased erosion of the soil. Furthermore, actively capturing mud to create sediment

deposition by maintaining the intense turbulence in the water flow through the forest were

also studied (Wolanski 1992; Furukawa and Wolanski 1996).

It is well-accepted that wave attenuation depends on the characteristics of the plant

(geometry, buoyancy, density, stiffness, degrees of freedom, and spatial configuration),

ground slope as well as wave parameters (mainly wave height, period, and

direction)(Augustin et al. 2009). Several laboratory experiments to investigate the

interaction between waves and vegetation models have been reported. Although the

results all showed that vegetation appears to attenuate waves, these studies were not

consistent in concluding which parameter is the most effective for wave attenuation. By

conducting laboratory and numerical studies of wave damping by emergent and near-

emergent wetland vegetation, Augustin et al. (2009) showed that wave height decays

appear to be most dependent on the ratio of stem length to water depth and stem density.

By considering incident wave height, vegetation density and width with an emergent,

rigid vegetation condition, Huang et al. (2011) also claimed that vegetation porosity is

much more significant than vegetation width in wave attenuation. Contrariwise, by

changing the incident wave height, vegetation density as well as vegetation width, Ismail

et al. (2012) demonstrated that, in general, vegetation width and densities damped waves,

but forest width has a stronger effect on wave height reduction than vegetation density.

Other studies focused on numerical simulations to clarify wave energy dissipation in non-

uniform mangrove forests of arbitrary depth (Luong and Massel 2008), or considered

12

regular and irregular incident waves as well as took into account the effect of vegetation

motion on wave attenuation (Méndez et al. 1999). In addition, the effect of the ground

slope on vegetation performance in dissipating tsunami energy was also investigated, and

revealed that a very mild ground slope was more vulnerable to thrashing by tsunami

waves than a relatively steep ground slope. Moreover, ground slope was more effective

than vegetation on wave attenuation under a steep ground slope condition (Nandasena et

al. 2008). There are only a few field observations on how waves dissipate due to

interaction with vegetation. Mazda et al. (1997, 2006) investigated the wave reduction in

a mangrove forest of Kandelia candel and Sonneratia sp. with the bed slope of 1/2000

and 1/1000, respectively. Their studies demonstrated that the drag coefficient CD is

strongly dependent on the configuration of vegetation species. Quartel et al. (2007)

pointed out that in the mangrove vegetation (bed slope of 1.9/1000), the dense network of

trunks, branches, and above ground roots causes a great drag force. Bao (2011) analyzed

the relationship between wave height and a mixed mangrove forest structure with the aim

of defining the minimum mangrove forest band width for coastal protection from wind-

wave attacks.

In short, the previous studies showed that vegetation has potential of protecting the

riverbank from wave attacks. However, the results accumulated from field studies are not

only limited but also seldom consider the slope effect since most of them were conducted

in mild slope conditions. In addition, because each tree species has different

characteristics that induce different drag forces, the results of one species cannot be

applied to others.

1.3. Purpose of the study

1.3.1. Aim and objectives

13

The aims of this study are to raise the awareness of community about the effect of boat-

generated waves on riverbank stability and propose countermeasures to deal with this

problem. The outcome of this study is not only meaningful for river management but also

for engineering field to protect the bank stability from the risk of being eroded.

Targeting to the aims mentioned above, following are the objectives of present study:

(1) Investigate the boat-generated wave characteristics and assessing the risk of riverbank

erosion by boat-generated wave attacks.

(2) Investigate the efficiency of two vegetation species, Rhizophora apiculata and Nypa

fruticans in aspect of wave attenuation capability.

(3) Investigate and design artificial porous structure to deal with the wave attacks.

1.3.2. Outline of Thesis

The thesis includes 6 Chapters. In Chapter 1, the bank erosion and its relation to boat-

generated waves is raised up. Literature reviews about ship wave issue as well as wave

attenuation by vegetation are discussed. In Chapter 2, the dominant effect of ship wave in

the heavily navigated river is elucidated by comparing with the tide effect. In Chapter 3,

field investigation in combination with numerical simulation was carried out so as to

understand the boat-generated wave characteristics and assessing the bank situation under

boat-generated wave attacks. Next, Rhizophora apiculata and Nypa fruticans as

countermeasure for wave attenuation and riverbank protection from erosion are

considered and compared in Chapter 4. The effect of artificial porous structure and how to

determine such an optimal design of groin are discussed in Chapter 5 that brings out

another solution to deal with wave attacks. Conclusions and some recommendation for

future study are mentioned in Chapter 6.

14

CHAPTER 2

EFFECT OF SHIP WAVE AND TIDE ON SEDIMENT TRANSPORTATION

2.1. Overview

In the downstream of Arakawa River, artificial works have been built and placed along

the riverbank to protect the reeds from wave attacks and therefore, preserve an

environment for aquatic habitats. The field investigations were conducted at two locations,

Higashi Yotsugi and Senju Sakuragi. The tidal current and ship wave characteristics were

measured. In addition, the amount of sediment transported by tide and ship wave was also

collected. Under the view point of sediment movement, the dominant factor, either tide or

ship wave, causing sediment entrainment can be identified.

2.2. Study area and field investigation

The downstream of Arakawa River (Fig.2.1) is an artificial channel built to protect Tokyo

Metropolitan area from flood damage. For a long time, it has been a natural shelter for

many birds, fish and aquatic vegetation. However, due to economic growth, the channel

was renovated and widened that reduces the living area of reed, a plant which can be seen

commonly here. In recent years, because the ships have sailed daily in this channel

generating waves that threatens the safety of reeds by eroding sediment, artificial works

have been placed along the river to prevent reeds from being destroyed. This type of work

has a wooden square framework filled with natural stones and enables to dissipate wave

energy, and therefore reduces erosion problem. After applying the wooden work,

unfortunately, erosion still occurs at a few points that brings out a question that whether

not only the ship wave but also the tidal regime is a factor causing erosion. The aim of

this study is to elucidate that.

15

Figure 2.1. Map of the field site (downstream of Arakawa River, Japan) and locations of

the measuring points

Higashi Yotsugi

Senju Sakuragi

Site monitoring

16

The field investigations were conducted in Higashi Yotsugi and Senju Sakuragi (Fig.2.2).

(a) (b)

Figure 2.2. The porous structures (made of wood) taken at the field survey, (a) At Senju

Sakuragi and, (b) At Higashi Yotsugi

At these locations, wooden work is applied. The distances from the wooden work to the

shore under high tide and low tide are sufficient for the wave measurement (avoid wave

reflection from shore). Table 2.1 below shows the overview of each location.

Table 2.1. Overview of each location

Location Position Scale Geographical conditions Erosion situation

Distance (km)

Length (m)

Width (m)

Area (ha)

Mean slope Soil Currently

Prediction in future

Higashi Yotsugi

7.7 – 8.05 350 30 0.66 1/14 Silt No Expansion

Nishiarai (right bank)

13.7 – 14.75 1100 66 3.91 Flat Mud Yes Reduction

2.3. Equipment deployment and measurement

2.3.1. Measuring point selection

The selected measuring point is 30 m far from the wooden work and 10 m far from the

reed area as shown in Fig.2.3. The point is in the central line of the gap between two

17

Figure 2.3. Experimental schema

artificial works. Wave height, velocity and sediment collection were all carried out right here.

2.3.2. Experimental equipment and deployment

Wooden work

Wave

Reeds

30m

10m

Observation point Sediment trap

(A1 A2 A3….) Wave height measurement Velocity measurement

18

For velocity measurement: An electromagnetic current meter (KENEK VM2000)

(Fig.2.4) was used. It was set so that it could measure velocity in two directions, one is to

the shore, and the other is with flow downstream.

(a) (b)

Figure 2.4. Electromagnetic current meter, (a) The main body, (b) Detector

19

The detector has two heads, one is the sensor (black) which should be put into water, the

other connects to the main body by wire.

For the wave height measurement (Fig.2.5): An outdoor wave gauge was used. This

apparatus consists of three parts, the wave height meter which shows value of wave

height in voltage unit, a head amplifier, and a detecting string.

(a) (b)

(c)

Figure 2.5. The wave gauge, (a) Wave height meter, (b) Amplifier, (c) Detecting string

Sediment trap (Fig.2.6): Sediment transported by wave/tide was collected by a bottle with

a height of 14 cm and diameter of opening of 3 cm. Bottles were buried in the riverbed

with the opening being the same level of the bed elevation. During the measurement, lids

were taken off so that sediment could get into bottles. Collected sediment later will be

used for soil analysis.

20

(a) (b)

Figure 2.6. The sediment trap, (a) Bottle and lid, (b) Bottles installation

Fig.2.7. below shows how all equipment deployed in the field.

Figure 2.7. Equipment deployment in the real field

2.3.3. Tide observation

The observation time and tide level forecast at Tokyo Hydrological Station on the 13th and

14th of September are shown in Table 2.2 and Fig.2.8 (neap-tide). Due to phase lag, same

tide level will be observed at Higashi Yotsugi and Senju Sakuragi about 10 to 20 minutes

later.

21

Table 2.2. Observation time and tide level at Tokyo on the 13th and 14th of September

Date High tide Low tide Time Tide level Time Tide level Time Tide level9/13 11:37 152 4:03 63 18:32 135 9/14 13:52 159 5:48 66 18:32 136

Figure 2.8. The sea level variation recored in September at Tokyo Bay

The graph of sea level of September is then zoomed in for specific time of field

observation (Fig.2.9). The average sea level is determined based on recorded data.

Figure 2.9. The sea level variation recored during observation time

4:03 63

11:37 152

5:48 66

13:52 159

4:03 63

11:37 152

5:48 66

13:52 159

観測期間観測期間

Sea surface Mean sea level

Time (day)

Sea

leve

l (cm

)

ObservationObservation

22

In addition, flow velocity, wave height, and sediment measurement were also conducted

on these two days (Table 2.3)

Table 2.3. Schedule of measurement on the 13th and 14th, September during the neap-tide

Location Date Flow velocity and wave measurements Sediment measurement

Higashi Yotsugi 9/13

7:50 – 8:05 7:50 – 8:05 (A1, A2, A3) 8:10 – 8:25 8:30 – 8:45 8:50 – 9:05 8:50 – 9:05 (B1, B2, B3)

Senju Sakuragi 9/14

9:45 – 10:00 9:45 – 10:00 (A1, A2, A3) 10:05 – 10:20 10:25 – 10:40 10:45 – 11:00 11:05 – 11:20 11:05 – 11:20 (B1, B2, B3)

Another tide level forecast recorded at Tokyo Hydrological Station on the 15th is shown in

Table 2.4 and Fig.2.10 as well as being zoomed-in in Fig.2.11 for more specific

Table 2.4. Observation time and tide level at Tokyo on the 15th of November

Date High tide Low tide Time Tide level Time Tide level Time Tide level11/15 15:09 186 9:10 92 21:40 34

23

Figure 2.10. The sea level variation recored in November at Tokyo Bay

Figure 2.11. The sea level variation recored during observation time

9:10 92

15:09 186

21:40 34

Sea surface Mean sea level

Time (day)

Sea

leve

l (cm

)

観測期間 9:10 92

15:09 186

21:40 34

Observation

24

During the spring tide, the flow velocity, wave height, and sediment measurement were

conducted only at Higashi Yotsugi (Table 2.5). Since the data at Senju Sakuragi during

spring tide are available, there is no need to measure it again.

Table 2.5. Schedule of measurement on the 15th of November during the spring tide

Location Date Flow velocity and wave measurements Sediment measurement

Higashi Yotsugi 11/15

11:55 – 12:10 11:55 – 12:10 (A1, A2, A3)12:15 – 12:30 12:15 – 12:30 (B1, B2, B3)

- 12:30 – 18:05 (C1, C2, C3)- 18:05 – 18:20 18:05 – 18:20 (D1, D2, D3)18:25 – 18:40 18:25 – 18:40 (E1, E2, E3)

For similar reason, the wave height, flow velocity and sediment by ship wave were only

conducted at Higashi Yotsugi as shown in Table 2.6 and Fig.2.12

Table 2.6. Tide level forecast in Tokyo on the 10th of October

Date High tide Low tide Time Tide level Time Tide level Time Tide level10/10 8:43 176 1:47 33 14:01 114

Figure 2.12. The sea level variation recored in October at Tokyo Bay

1:47 33

8:43 176

14:01 114

25

2.3.4. Data analysis method

Reynolds stress can be used to describe the transport of sediment in the river/channel.

Thus, from the raw data of flow velocity measured at field, the Reynolds stress is

calculated. For the sand analysis, collected sediment was then dried and weighed. After

that, sieve screening method was applied to get the sand particle distribution.

2.4. Results and discussions

2.4.1. Tidal effect

2.4.1.1. Characteristics of the tide

The time variation of water depth and flow velocity during the neap tide at Higashi

Yotsugi (9/13) and Senju Sakuragi (9/14) are shown in Fig.2.13a and b, respectively.

Here the velocities were measured at two directions, one is heading on shore; the other is

following downstream. It can be seen clearly that at Higashi Yotsugi, the velocity heading

on-shore is much greater than the one following downstream, especially under the flood

tide (when the high tide is ensuing). When the tide nearly reaches the highest, the

difference of these two velocities is reduced. In the case of Senju Sakuragi, the same trend

can be observed except the very first moment of flood tide, velocity in downstream

direction is relatively close to that heading on-shore. It is suspected to be due to an

unexpected ship appearing at that moment which might affect the measured results.

26

-8

-6

-4

-2

0

2

4

6

8

7:40 7:55 8:09 8:24 8:38 8:52 9:07 9:21時間

流速

(cm

/s)

0

5

10

15

20

25

30

35

40

45

水深

(cm

)

流速(岸沖)流速(流下方向)観測地点水深

(a)

-50

-40

-30

-20

-10

0

10

20

30

40

9:36 9:50 10:04 10:19 10:33 10:48 11:02 11:16 11:31

時間

流速

(cm

/s)

0

5

10

15

20

25

30

35

40

45

50

水深

(cm

)


(b)

Figure 2.13. Variation of flow velocity and water depth with time, (a) At Higashi Yotsugi

(9/13) and (b) At Senju Sakuragi (9/14) during the neap-tide

The time variation of water depth and flow velocity during the spring tide at Higashi

Yotsugi (11/15) is shown in Fig.2.14. Obviously, it can be seen that the heading on-shore

velocity is dominant compared to that following downstream.

Wat

er d

epth

(cm

)

Vel

ocity

(cm

/s)

Time

Velocity (on shore)

Velocity (downstream)

Water depth at observing point

Time

Vel

ocity

(cm

/s)

Wat

er d

epth

(cm

)

Velocity (on shore)



27

-20

-15

-10

-5

0

5

10

11:31 12:43 13:55 15:07 16:19 17:31 18:43 19:55

時間

流速

(cm

/s)

0

20

40

60

80

100

120

水深

(cm

)


Figure 2.14. Variation of flow velocity and water depth with time at Higashi Yotsugi

(11/15) during spring-tide.

In short, it can be accepted that in either neap-tide or spring-tide, velocity in the on-shore

direction is always dominant. Sediment transport in this direction therefore is also greater.

By this reason, for assessing the amount of sediment transported by tide, only on-shore

direction is considered.

2.4.1.2. Time variation of Reynolds stress

The Reynolds stress is calculated by using this equation 2'.uR , where R is Reynolds

stress (N/m2), is water density (kg/m3), and 'u is the average fluctuating velocity (m/s).

Fig.2.15a and b show the variation of Reynolds stress with time during the neap-tide and

spring-tide at Higashi Yotsugi, respectively.

Vel

ocity

(cm

/s)

Wat

er d

epth

(cm

)

Velocity (on shore)



Time

28

(a)

(b)


(a) During neap-tide and (b) During spring-tide.

It can be seen that Reynolds stress is the greatest in flood tide, more than 5 N/m2 during

spring-tide and about 2.2 N/m2 during neap-tide. It is also noticeable that when water

level is high, it does not affect much on Reynolds stress. Thus, it may not affect on

sediment transport either.

Rey

nold

s stre

ss (N

/m2 )

Time

Wat

er d

epth

(cm

)

Reynolds stress


Wat

er d

epth

(cm

)

Rey

nold

s stre

ss (N

/m2 )

Time

Reynolds stress


29

2.4.1.3. The particle distribution and weight of sediment collected at the field

Fig. 2.16 – 2.18 shows the particle size distribution of sediment collected from trap under

the neap-tide and spring-tide conditions at Higashi Yotsugi and Senju Sakuragi.

Approximately, the diameter of moved sediment ranges from 37 to 840 m. As the fine

sand is in the range of 0.02-0.2 mm while the coarse sand is in the range of 0.2-2 mm,

sediment collected is a mix of fine and coarse sand.

Figure 2.16. Grain size curve of sediment collected at Higashi Yotsugi during the neap-

tide (9/13)

Figure 2.17. Grain size curve of sediment collected at Senju Sakuragi during the neap-tide

(9/14)

Parti

cle

size

dis

tribu

tion

(%)

Sieve (m)

Parti

cle

size

dis

tribu

tion

(%)

Sieve (m)

30

Figure 2.18. Grain size curve of sediment collected at Higashi Yotsugi during the spring-

tide (11/15)

Reynolds stress is the greatest during flood tide, that is why not only more fine sand but

also more coarse sand can be moved at this period. It accounts for bigger sand size were

trapped in A1, A2, and A3 (which corresponding to flood tide) compared to that in B1,

B2, and B3 (which corresponding to high tide). However, according to the sediment grain

size collected at Senju Sakuragi, the difference between group A and B is not much. As

explained in previous section, it may be due to unexpected ship passing by at the high tide

that caused more turbulence.

The amount of sediment is shown in Table 2.7 and 2.8. The lower water level is, the more

sediment collected in trap. Even during the spring tide, a lot of sediment is moving when

water is low. It is understandable as turbulence occurs easier at the flood tide and ebb tide.

When water level is high, the flow velocity near the bed becomes smaller, soil therefore is

harder to be moved. That accounts for the amount of sediment collected in batch C is not

so different even though the duration of measurement is much longer than other batches.

Sieve (m)

Parti

cle

size

dis

tribu

tion

(%)

31

Table 2.7. Sediment collected in trap in each stage of tide during neap tide at Higashi

Yotsugi (9/13) and Senju Sakuragi (9/14)

Location/Date Batch Stage of tide Duration of measurement

Total amount of sediment movement (g)

Ave. amount of sediment movement (g)

Higashi Yotsugi (9/13)

A Flood tide 15 minutes 2.74

2.64 2.58 2.60

B High tide 15 minutes 1.59

1.41 0.92 1.73

Senju Sakuragi

(9/14)


2.26 2.22 2.07


1.48 0.92 2.21

Table 2.8. Sediment collected in trap in each stage of tide during spring tide at Higashi

Yotsugi (11/15)






2.74 2.98 2.17


1.03 1.21 0.84

C High-slack-low

06 hours 2.16

2.78 3.42 2.77

D Low tide 15 minutes 1.92

1.84 1.63 1.97

E Ebb tide 15 minutes 2.22

2.84 2.06 4.26

32

2.4.2. Ship wave effect

2.4.2.1. Time variation of velocity and water depth

0

2

4

6

8

10

12

14

16

18

-20

-15

-10

-5

0

5

10

15

20

0 200 400 600 800 1000

水深

(cm

)

流速

(cm

/s)

時間

流速(流下方向)

流速(岸沖方向)

観測地点水深

(a)

012345678910

-20-15-10-505

101520253035

0 50 100 150 200

水深

(cm

)

流速

(cm

/s)

時間



観測地点水深

(b)

Figure 2.19. Variation of water depth and velocity with time at Higashi Yotsugi during (a)

low tide (a) and (b) high tide


Velocity (on-shore)


Vel

ocity

(cm

/s)

Wat

er d

epth

(cm

)

Time


Velocity (on-shore)


Vel

ocity

(cm

/s)

Wat

er d

epth

(cm

)

Time

33

0

2

4

6

8

10

12

14

-30

-20

-10

0

10

20

30

40

0 20 40 60 80 100 120 140

水深

(cm

)

流速

(cm

/s)

時間



観測地点水深

(a)

02468101214161820

-15

-10

-5

0

5

10

15

0 50 100 150 200

水深

(cm

)

流速

(cm

/s)

時間



観測地点水深

(b)

Figure 2.20. Variation of water depth and velocity with time at Senju Sakuragi during (a)

low tide (a) and (b) high tide

Fig.2.19 and 2.20 show the variation of velocity and water depth at Higashi Yotsugi and

Senju Sakuragi recorded at low tide and high tide, respectively. Since the velocity

Vel

ocity

(cm

/s)

Wat

er d

epth

(cm

)

Time


Velocity (on-shore)


Vel

ocity

(cm

/s)

Wat

er d

epth

(cm

)

Time


Velocity (on-shore)


34

towards the shore is pretty greater than that going downstream for most of the cases,

sediment is supposed to mainly transport on shore-direction. Only the last case

(Fig.2.20b) does not show the difference of velocity in two directions that should be

related to the big wind effect at site.

2.4.2.2. Time variation of Reynolds stress

(a)

(b)


(a) Low tide and (b) High tide.

Wat

er d

epth

(cm

)

Rey

nold

s stre

ss (N

/m2 )

Time (s)

Reynolds stress


Reynolds stress

Water depth at observing point Wat

er d

epth

(cm

)

Time (s)

Rey

nold

s stre

ss (N

/m2 )

35

Fig.2.21 shows the variation of Reynolds stress of ship wave when water level is low and

high. When ship waves appear, water depth is about 4 cm (low tide) and 8 cm (high tide).

It can be seen that Reynolds stress increases significantly at low tide condition, maximum

value is more than 10 N/m2 whereas it is just about 2 N/m2 at high tide condition. Thus,

more sediment is expected to move during the low tide.

2.4.2.3. The particle distribution and weight of sediment collected at the field

Fig.2.22 shows the particle size distribution of sediment collected in trap caused by ship

wave at Higashi Yotsugi (10/10). Since batch A was collected at low tide, with greater

Reynolds stress induced by ship wave, more coarse sand was obtained compared to batch

B.

Figure 2.22. Grain size curve of sediment collected at Higashi Yotsugi (caused by ship

wave)

Considering the amount of sediment collected shown in Table 2.9, with the same duration

of measurement, average amount of sediment movement in trap under low tide condition

is greater than that under high tide. This result is reasonable and can be anticipated

because Reynolds stress induced by ship wave is very high at low water depth.

Sieve (m)

Parti

cle

size

dis

tribu

tion

(%)

36

Table 2.9. Sediment collected in trap in each stage at Higashi Yotsugi (10/10) and Senju

Sakuragi (available data)





A Low tide 15 minutes 5.42

3.16 1.67 2.38


2.05 2.49 2.18

Senju Sakuragi

A Low tide 15 minutes -

4.89 - -

B High tide 15 minutes -

2.69 - -

2.4.3. Comparison between tide and ship wave effects on sediment transport

In summary, the results of sediment collected at each stage of tide indicate that the

highest amount of sediment moving during the flood tide rather than other stages. It is

corresponding with the highest value of Reynolds stress calculated at this stage. Looking

back the average amount of sediment collected in batch C, at Higashi Yotsugi (11/15),

approximately 2.8 g in 6 hours. This amount is very small by considering the duration of

measurement. Thus, it can be said that when the tide approaches high level, velocity

heading on-shore becomes much smaller that makes soil more difficult to move.

Table 2.10. Comparison of sediment movement caused by tide and ship wave

Reason Duration of measurement Average amount of sediment movement (g)Ship wave 15 minutes 4.89

Tide 7 hours 11.24

Comparing the amount of sediment moved by ship wave and tide, Table 2.10 shows that

it is about 5 g of sediment by ship wave within 15 minutes and more than 11 g of

37

sediment by a spell of tide which is 7 hours. Obviously, the effect of ship wave is more

impressive compared to tide on sediment transportation by considering time consuming.

In the area where there is semi-diurnal tidal regime, two spells of tide can be observed per

day whereas there are many ships moving in the navigated river/channel. In that situation,

the effect of ship wave on sediment movement is dominant and bank failure if happens, is

mainly due to the ship transportation.

38

CHAPTER 3

THE EFFECT OF BOAT-GENERATED WAVE ATTACKS ON RIVERBANK

STABILITY

3.1. Overview

As mentioned in Chapter 1, even though there have been many researches on the damage

along the coastal line and/or river caused by tsunami and storm surges in the world, very

limited studies about the effect of ship waves on the bank stability were carried out. In

addition, the results showed in Chapter 2 indicate that in the heavily navigated

rivers/channels, boat-generated wave is the dominant factor causing bank failure. It is

noticeable that in recent years, due to the considerable increase of numbers of boats/ships

joining in the waterway, there is a great jump in both wave height and wave frequency

that hastens the bank erosion process. In order to assess the effect of boat-generated wave

attacks on riverbank stability, the field investigation in combination with numerical

simulation were conducted. The objectives were to 1) clarify the relationship between the

shear stress induced by boat waves and bank erosion rate and 2) determine an appropriate

vegetation design to protect the riverbanks from erosion.

3.2. Study area and field investigation

In the Vietnamese Mekong Delta (VKD), due to specifically geographical condition, the

road traffic systems are not able to be developed properly. Instead, with a dense river

network, inland waterways have been built to support the demands of passages among

areas. Recently, due to rapidly socioeconomic development in the Vietnamese Mekong

Delta, there has been a remarkable increase of transportation in order to meet the demands

of passage among areas. The width of river and channel does not much change whereas

39

the waterway density becomes too dense due to the presence of a large number of boats

joining in the transportation. Considering as the most populated waterway in Vietnam, the

transport in Ca Mau Province is diversified in terms of type of means as engine-driven

boats, canoes, high-speed boats, etc…In recent years, it is noticeable that together with

the remarkable increase in both quantity and speed of boats, the more and more severe

bank erosions have been occurring. The constant attacks of boat-g

Documents

Riverbank Erosion under Boat-Generated Wave Attacks and … · 2019. 5. 21. · Professor Junji Yagisawa is gratefully acknowledged for having discussed, suggested and reviewed my