estimation of erosion resistance of cohesive bank in river and around river mouth

Chapter 2 Effects of Development around River Mouth and 2.1 Introduction 6 2.2 Detection of Shoreline Retreat from Satellite Images and Analysis of the Erosion Mechanism Caused by the D

Doctoral Dissertation

ESTIMATION OF EROSION RESISTANCE

OF COHESIVE BANK IN RIVER AND AROUND RIVER MOUTH

（粘性土で構成された河川堤防及び河口周辺護岸

の侵食耐性の評価に関する研究）

BUI TRONG VINH

Department of Civil Engineering Graduate School of Engineering

Osaka University

August, 2009

Doctoral Dissertation

ESTIMATION OF EROSION RESISTANCE

OF COHESIVE BANK IN RIVER AND AROUND RIVER MOUTH

（粘性土で構成された河川堤防及び河口周辺護

岸の侵食耐性の評価に関する研究）

A dissertation submitted in partial fulfillment of the requirements for the

degree of Doctor of Philosophy in Civil Engineering

by

Bui Trong Vinh

Supervised by

Prof Ichiro Deguchi

Graduate School of Engineering Osaka University, Japan August, 2009

ACKNOWLEDGEMENT

This dissertation has been carried out under the academic advice of Prof Ichiro Deguchi

at Department of Civil Engineering, Osaka University I would like to express my deep gratefulness to him, my great supervisor, who always encourages and supports me during

my study and living in Japan

I owe special thanks to Associate Prof Susumu Araki, who taught me how to solve problems when I met and what the hard work we face in life

I would like to gratefully acknowledge Prof Keiji Nakatsuji who taught me and questioned with his excellent mind I had some good chance to go with him for taking part in some workshops, seminars in Japan, Vietnam, and Korea He also always helps and supports me during my study His excellent critics help me understand the important effects of tide in my study

I thank Prof Shuzo Nishida who gave me important advice when we joined the General Seminar of Core Program in Danang City and during the journey in Vietnam His critics helped me have some new view points about both scientific meanings and real applications of my study I also thank Prof Yasutsugu Nitta for his evaluation on my dissertation defense

I am greatly indebted to Prof Huynh Thi Minh Hang who was my supervisor when I was undergraduate and master student in Vietnam Now she passed away, but in my mind, she

is still there

I thank Assistant Prof Mamoru Arita, who helped me when I got troubles with experimental device I appreciate his enthusiasm during the time at conferences and field investigation in Japan, Portugal, Korea and Vietnam

I also had a very beautiful time when I studied with and met many Japanese students belonged to Prof Deguchi Lab Now they are working in many companies, but during the study, I learned very much from them I am sorry because I do not remember the name of all students, but special thanks to Mr Nakaue, Mr Shimizu, Mr Nomura, Mr Sabusaki,

Ms Fukuhara, Ms Yoshiyama etc I thank Mr Han James for his help to prepare the artificial bank and bed of the flume experiments and to take photos during the field investigation

I also thank Prof Fujita, Prof Ike, Prof Viet and his staffs, JSPS, and Monbusho who supported me the finance during my stay and my study in Core University Program and PhD Program I am also grateful to the faculty staffs at Department of Civil Engineering, Affair Department of International Students for their kindness

Last but not least, I would like to thank my family, friends, and colleagues of Faculty of Geological and Petroleum Engineering – Hochiminh City University of Technology – VNU-HCMC who always support and help me during the time I’ve been living and studying in Vietnam and in Japan

List of Publications

[1] Bui Trong Vinh, Ichiro Deguchi, Mamoru Arita, 2009 “Erosion Mechanisms of

Cohesive Bed and Bank Materials” Proceedings of the Annual International Offshore and Polar Engineering Conference & Exhibition (ISOPE) Osaka, Japan,

June 21-26, 2009,Vol III, pp 1305-1312

[2] Bui Trong Vinh, Ichiro Deguchi, Keiji Nakatsuji, 2008 “Beach Erosion Caused by Development in Littoral Region – Effect of Sand Extraction around River Mouth”

Proceedings of the 8 th General Seminar on Environmental Science & Technology Issues Japan, Osaka, Nov 2008, pp 114-119.

[3] Bui Trong Vinh, Deguchi Ichiro, Arita Mamoru, Fukuhara Saori, 2008

“Experimental Study on Critical Shear Stress of Cohesive Bed Material for

Erosion” Annual Journal of Coastal Engineering, JSCE, 2008, Vol.1, pp 531-535

(in Japanese)

[4] Bui Trong Vinh, Ichiro Deguchi, Mamoru Arita, Susumu Araki, 2008

“Measurement of Critical Shear Stress for Erosion of Cohesive Riverbanks” The International Conference on Marine Science and Technology - OCEANS'08 Japan, Kobe, April 8, 2008 (in CD-Rom)

[5] B.T Vinh, I Deguchi, S Araki, T Nakaue, A Shimizu, 2007 “The Mechanism of

Beach Erosion in Southern Part of Red River Delta, Vietnam” Proceedings of the Annual International Offshore and Polar Engineering Conference & Exhibition (ISOPE) Lisbon, Portugal, July 1-6, 2007, Vol III, pp 2461-2466

[6] I Deguchi, S Araki, T Nakaue, B.T Vinh, 2006 “Monitoring of the Change in Coastal Environment in Southern Part of Red River Delta from Satellite Images and

the Mechanism of Beach Erosion” Proceedings of the 6 th

General Seminar on Environmental Science & Technology Issues Japan, Kumamoto, Oct 2006、pp

144-152

Chapter 2 Effects of Development around River Mouth and

2.1 Introduction 6 2.2 Detection of Shoreline Retreat from Satellite Images and

Analysis of the Erosion Mechanism Caused by the Development of Mangrove Forests 7

2.2.3 Results of analyzing satellite data 11 2.2.4 The erosion mechanism of beaches in Site-A and Site-B 15

2.3 Effects of Sand Extraction on Beach Erosion 22 2.4 Conclusions 24 References 24

Chapter 3 Experimental Study on Critical Shear Stress of

3.1 Introduction 26 3.2 Experimental Apparatus and Procedures 26

3.2.1 Non-vertical jet test apparatus 26 3.2.2 Procedures for determining the τc and kd 27 3.3 Experimental Study on Remolded Samples 30

3.3.1 Remolded samples making process 30 3.3.2 Procedures for determining the τc and kd of remolded

samples 33

3.4.3 Rainfall 36

3.4.5 Vegetation and aquatic animals 38 3.4.6 The properties of cohesive soils of Soairap river banks 39 3.4.7 Procedures for measuring the τc and kd of undisturbed

samples 40

3.6 Conclusions 44 References 44

Chapter 4 Estimation of Erosion Resistance of Cohesive

4.1 Introduction 46

4.3.2.1 Erosion caused by waves (Case-1 to Case-3) 52 4.3.2.2 Erosion profiles caused by waves, wave-opposing

currents, and wave-following currents 54

List of Tables

Chapter 2 Effects of Development around River Mouth and

Shallow Water Region in the Sea

Chapter 3 Experimental Study on Critical Shear Stress of Cohesive

Bed Material for Erosion

Table 3.1 Remolded samples with only sand and clay 31

Table 3.4 Remolded samples with the change of moisture content 32 Table 3.5 Samples with only sand-clay content and moisture content 32 Table 3.6 Remolded samples with the change of salinity 32 Table 3.7 Remolded samples with the change of consolidation time 32 Table 3.8 Remolded samples with the change of dead root and leaves 33 Table 3.9 Properties of cohesive soils of Soairap river banks 39 Table 3.10 The results of experiments on effect of consolidation 43 Table 3.11 The results of experiments on effect of dead roots and leaves 43

Chapter 4 Estimation of Erosion Resistance of Cohesive Bank

in River and around River Mouth

Table 4.1 Erosion properties of remolded samples in the flume test 48 Table 4.2 Experimental conditions in the basin tests 52 Table 4.3 Erosion properties of remolded samples in the basin tests 52 Table 4.4 Input conditions of laboratory scale model 60 Table 4.5 Calculation results around river mouth 61 Table 4.6 Input conditions of the model to investigate the effect of waves

Table 4.7 Input conditions of the model to investigate the effect of river flows 70 Table 4.8 Input conditions of the model to investigate the effect of waves,

wave-induced currents and tidal currents 70 Table 4.9 Input conditions of the model to investigate the effect of waves

List of Figures

Chapter 1 Introduction

Chapter 2 Effects of Development around River Mouth and

Shallow Water Region in the Sea

Fig 2.4 Location of Site-A (Landsat/TM-1989, May 29) 9

Fig 2.5 Location of Site-B (Landsat/TM-1989, May 29) 9

Fig 2.8 Band-4 images of Landsat/TM in Site-A 11

Fig 2.9 Shoreline changes around Huasai, Site-A (1992, 1996, and 1998) 12

Fig 2.10 Comparison of land-use in Sept 1998 and Sept 2001 13

Fig 2.11 The shoreline change near Balat river mouth 13

Fig 2.12 The shoreline change near Ninhco River 14

Fig 2.13 The shoreline change in Haihau district 14

Fig 2.15 The shoreline change in Giaothuy district 15

Fig 2.16 Expected topography change around the shrimp pond 16

Fig 2.17 Beach erosion caused by remained abandoned shrimp ponds

Fig 2.18 Beach erosion caused by remained abandoned shrimp ponds

(in Haihau district, northern Vietnam - Site-B) 17 Fig 2.19 The erosion process in front of the double sea dike system

(in Haihau district, northern Vietnam - Site-B) 17 Fig 2.20 The mechanism of longshore sediment transport in summer

Fig 2.21 The mechanism of longshore sediment transport in winter

Fig 2.22 The mechanism of longshore sediment transport in winter

Fig 2.23 Boundary conditions (B.C.) in summer 20

Fig 2.24 Boundary conditions (B.C.) in winter 20

Fig 2.25 Calculated shoreline change without mangrove forest 21

Fig 2.26 Calculated shoreline change with mangrove forest 21

Fig 2.27 Calculated shoreline retreat backed by sea dike 22

Fig 2.28 Sand extraction sites and erosion place in Kochi coast, Japan 22

Fig 2.29 Bottom topography around river mouth of Niyodo River, Kochi

Chapter 3 Experimental Study on Critical Shear Stress of Cohesive

Bed Material for Erosion

Fig 3.1 Non-vertical submerged jet test apparatus 27

Fig 3.2 Diffusion principles (a) and stress distribution (b) 28

Fig 3.3 Calibration of the initial jet velocity at nozzle 29

Fig 3.4 Remolded sample after mixing (a) and after pouring water (b) 30

Fig 3.6 Mixing rates and moisture content of remolded samples 33

Fig 3.12 Average air temperature from 2002 to 2005 (Tan Son Hoa Station) 35

Fig 3.13 Annual rainfall from 2002 to 2005 (Tan Son Hoa Station) 36

Fig 3.14 The water level from 2002 to 2005 (Phu An Station) 36

Fig 3.16 Tidal current data measured at site SR2R 37

Fig 3.18 Tidal current data measured at site SR6L 38

Fig 3.19 Exposed roots and burrowed holes at SR5R 39

Fig 3.20 Physical properties of measuring site samples 39

Fig 3.21 Procedure for determining the τc at SR5R 40

Fig 3.22 Procedure for determining the kd at SR5R 40

Fig 3.23 Relationship between the τc and clay content 41

Fig 3.24 Relationship between the τc and moisture content 42

Fig 3.25 Relationship between the τc and salinity 42

Chapter 4 Estimation of Erosion Resistance of Cohesive Bank

in River and around River Mouth

Fig 4.1 Sketch of test cross section of the flume 47

Fig 4.2 Velocity profile in the center of test section 48

Fig 4.3 Erosion of remolded sample of (P1) after 2-hour testing 49

Fig 4.4 Erosion of remolded bank sample after 18-hour testing 50

Fig 4.5 Sketch of wave-river basin and river mouth model 51

Fig 4.6 Erosion caused by waves after 3-hour testing 53

Fig 4.7 Wave data of the experiment with wave-opposing currents 54

Fig 4.9 Wave data at the starting time of the experiment with

Fig 4.10 Erosion profiles in Case-6 and Case-7 56

Fig 4.12 Distribution of current velocities (Case-3) 62

Fig 4.13 Distribution of wave heights (Case-3) 62

Fig 4.16 Distribution of wave heights (Case-6) 63 Fig 4.17 Distribution of applied shear stresses (Case-6) 64 Fig 4.18 Distribution of current velocities (Case-8) 65 Fig 4.19 Distribution of wave heights (Case-8) 65 Fig 4.20 Distribution of applied shear stresses (Case-8) 65 Fig 4.21 Distribution of current velocities (Case-11) 66 Fig 4.22 Distribution of wave heights (Case-11) 66 Fig 4.23 Distribution of applied shear stresses (Case-11) 67 Fig 4.24 Effect of waves and river discharge on applied shear stresses 68

Fig 4.26 Effect of only waves and wave-induced currents on shear stresses 72 Fig 4.27 Effect of river flow and tidal range on applied shear stresses 73 Fig 4.28 Effect of waves and wave-following currents on shear stresses 74 Fig 4.29 Effect of waves and wave-opposing currents on shear stresses 75

Fig 4.31 Erosion site 21R of Soairap river bank in 2007 (a) and in 2008 (b) 77 Fig 4.32 Distribution of shear velocities in condition with only currents (a),

Chapter 1

Introduction

1.1 Background

Coastal regions in south-eastern countries have been developing with a run and on a large scale Typical developments are constructions of shrimp ponds, paddy fields and so on in mangrove forests Harbor facilities have also been constructing along down stream side

of large river and continuing dredging must be conducted to keep the depth of fairway Great volume of sand has been exploited around river mouth for construction materials and barrow sand to construct artificially replenished beach

Banks of newly developed shrimp ponds and so on are usually constructed using consolidated cohesive materials Thick mangrove forests once existed were replaced by thin bank of cohesive soil These developments draw down the vulnerability in the coastal region and, as a consequence, bank erosion near river mouth and around newly developed coastal region is an increasingly serious problem for coastal environment and economic activities

The hydrodynamic processes in these regions are extremely complicated because of the interaction of waves, wave–induced currents, tidal currents, and weathering processes Simultaneously, the erosion processes become very complex objects and difficult to understand because of the nature of cohesive materials in cohesive banks The erosion processes in cohesive banks are distinctly different from those of non-cohesive ones because of the irreversible processes In the river mouth, estuaries, and newly developed coastal regions, there are several physical factors acting on the banks, bed and shores that can influence erosion rates including wind waves, ship waves, tidal currents, surface runoff, etc

The cohesive behavior of sediments is generally observed at sizes less than 0.074 mm (Dean and Dalrymple, 2001) In most cases, the degree of consolidation, physicochemical conditions (temperature, pH, cation exchange capacity) and the electrochemical bonds between the individual particles control the erodibility of cohesive materials The cohesive sediments settle to the bed with a low bulk density In time, as the sediment consolidates, its strength increases Since the erodibility of cohesive soils is primarily a function of the shear strength of the soil, several models quantifying erosion rate as a function of the shear strength of the soil have been developed

The cohesive bank erosion has been studied many years since Most researchers focused

on bank erosion caused by river flows and weathering processes (Thorne C.R., 1982; ASCE., 1998; Simon et al., 2000) Some studies have been carried out to determine the

tidal currents, and weathering processes to cohesive bank erosion around river mouth and newly developed coastal zones

Besides, various studies are also conducted concerning the transport phenomena of mud

or silt on bottom (Krone, R.B., 1962; Partheniades, E., 1975; Mehta, A.J., 1989; van Rijn, L.C., 1993, Berlamont J.E et al., 2000) In these studies, they considered cohesive properties of only bed materials

Other researchers developed the procedure for predicting topographic change around river mouth by using numerical models (Sawaragi T et al., 1985, Deguchi I et al, 1988) The numerical simulation models mainly focus on predicting bar formation/destruction at the river mouth The model for predicting various kinds of beach profile are also established However, all of these studies focus on non-cohesive bed materials Therefore, it is necessary to develop the numerical model to predict the cohesive bank erosion in this region

1.2 Objectives of the Study

The main objective of this study is to investigate the erosion mechanism of cohesive bank

in river and around river mouth to evaluate erosion resistance of cohesive soil and to establish the procedure for stabilizing and utilizing the coastal regions effectively

To achieve the aim, three research objectives have been carried out as following

(1) Assess the effects of development, preservation and disaster prevention on coastal erosion in some Asian countries

(2) Establish procedure for evaluating critical shear stress of cohesive soils for erosion (3) Study on erosion mechanism of cohesive bed, bank and shore and propose procedure for examining erosion resistance

1.3 The Study Areas

The study areas consist of Vietnam, Thailand, and Japan In Vietnam, two main parts of southern coast of the Red River Delta and coastal region around Soairap river mouth (southern Vietnam) were concerned In Thailand, the southern coast was studied In Japan, a series of laboratory experiments were accomplished at Osaka University and field data of Kochi Coast were used

1.4 Outline of the Dissertation

The material of Chapter 1 serves, in the first place, as a general introduction Background, research objectives, study areas and the outline of the study are introduced

In Chapter 2, the effects of development, preservation and disaster prevention on coastal erosion in some Asian countries [5, 7] are assessed The south-eastern coast of Thailand and the southern coast of the Red River Delta (Vietnam) were mainly concerned because

of severe erosion In these regions, many mangrove areas have been cut down to construct shrimp ponds The changes of shoreline were detected and analyzed by satellite images The erosion mechanism of the beach in some parts was reproduced by numerical models based on one-line theory On the other hand, beach erosion caused by sand extraction from river mouth and sand dunes were also concerned Around Lap river mouth (Vung tau city, southern Vietnam), beach erosion caused by sand extraction has been at the rate of greater than 10 m/year In Koichi coast of Japan, around Niyodo river mouth, beach erosion took place at the rate up to 8.1 m/year was calculated after sand extraction proscription from 1997 The process of beach erosion in this region has been lasting long

Pacific Ocean

Kochi coast Osaka University

Shongkhla

Vietnam

Thailand

Soairap River

Fig 1.1 Location of study areas

moisture, and salinity were made and examined by using non-vertical submerged jet test device The procedure was first introduced by Hanson et al (2002) and was reproduced

by Deguchi et al (2007) The banks of the Soairap River (southern Vietnam) were chosen

to carry out the in situ experiments This region has been severely eroded caused by wind waves, ship waves, tidal currents, aquatic animals etc This chapter identifies the relation between critical shear stress of cohesive soils to mixing rates of sand, silt and clay, and to moisture contents, salinity, consolidation, and vegetation

In Chapter 4, erosion mechanism and erosion resistance of cohesive bed, bank and shore [4] are investigated Various kinds of remodeled samples of cohesive bed material were made and examined in 2-D flume with unidirectional currents and 3-D wave basin under the action of waves and currents The property of erosion resistance is investigated by using numerical models to calculate the erosion rates of artificial and real bed, bank and shore In 2-D flume, the remolded samples were tested to determine the critical shear stress first, and then they were experimented to determine the erosion rates The effect of slope was also concerned in this study In 3-D wave-current basin system, many tests were done with different conditions The first test was done with the impact of only waves The second test was carried out with the action of waves and both wave-opposing currents and wave-following currents After all, the erosion resistance and erosion rate of cohesive bed, bank, and shore was reproduced by numerical models These models were applied in both laboratory scale and in the real conditions of Soairap river banks

Finally, conclusions of this dissertation are presented in Chapter 5

References

[1] Arulanandan et al., 1980 “Development of a quantitative method to predict

critical shear stress and rate of erosion of natural undisturbed cohesive soils,”

USACE, Waterways Experiment Station Technical Report GL-80-5, Vicksburg,

MS

[2] ASCE, 1968 “ASCE Task Committee on Erosion of Cohesive Materials Erosion

of cohesive sediments,” Journal of the Hydraulics Division, ASCE Vol 94 (1968)

(HY4), pp 1017–1047

[3] Berlamont, J.E & E.A Toorman (editors) (2000) COSINUS Final Scientific

Report, Hydraulics Laboratory, K.U.Leuven

[4] Bui Trong Vinh, Deguchi Ichiro, Arita Mamoru, 2009 “Erosion Mechanisms of

Cohesive Bed and Bank Materials” Proceedings of the Annual International Offshore and Polar Engineering Conference & Exhibition (ISOPE) Osaka, Japan,

June 21-26, 2009, Vol III, pp 1305-1312

[5] Bui Trong Vinh, Ichiro Deguchi, Keiji Nakatsuji, 2008 “Beach Erosion Caused

by Development in Littoral Region - Effect of Sand Extraction around River

Mouth,” Proceedings of the 8 th

General Seminar of the Core University Program,

Osaka, Japan, pp 114-119

[6] Bui Trong Vinh, Deguchi Ichiro, Arita Mamoru, Fukuhara Saori, 2008

“Experimental Study on Critical Shear Stress of Cohesive Bed Material for

Erosion” Annual Journal of Coastal Engineering, JSCE, Vol.1 (2008), pp

531-535 In Japanese

[7] Bui Trong Vinh, Ichiro Deguchi, Mamoru Arita, Susumu Araki, 2008

“Measurement of Critical Shear Stress for Erosion of Cohesive Riverbanks” The International Conference on Marine Science and Technology - OCEANS'08 Japan, Kobe, April 8, 2008 (in CD-Rom)

[8] Couper P., 2003 “Effects of silt–clay content on the susceptibility of river banks

to subaerial erosion,” Geomorphology Vol 56 (2003), pp 95–108

[9] Deguchi Ichiro, Sawaragi Toru, 1988 “Effects of structure on deposition of

discharged sediment around river mouth,” Proc of 21 st

International Conference

on Coastal Engineering, Vol.2, pp 1573-1587

[10] Dean Robert and Dalrymple, (2002) Coastal Processes with Engineering

Applications, Cambridge University Press

[11] Gaskin et al., 2003 S.J “Erosion of undisturbed clay samples from the banks of

the St Lawrence River,” Canadian Journal of Civil Engineering Vol 30 (2003),

pp 585–595

[12] Grissinger et al., 1981 “Erodibility of streambank materials of low cohesion,”

Transactions of the ASAE (1981), pp 624–630

[13] Hooke J.M., 1979 “An analysis of the processes of river bank erosion,” Journal

of Hydrology Vol 42 (1979), pp 39–62

[14] Krone R.B., 1962 "Flume Studies of the Transport of Sediment in Estuarial

Processes," Final Report, Hydraulic Engineering Laboratory and Sanitary Engineering Research Laboratory, University of California, Berkeley

[15] Mehta A.J., et al (1989) “Cohesive sediment transport I: Process description,”

Journal of Hydraulic Engineering Vol 115, pp 1076-1093

[16] Partheniades E., 1992 "Estuarine Sediment Dynamics and Shoaling Processes," in

Handbook of Coastal and Ocean Engineering, Vol 3, J Herbick, ed., pp

985-1071

[17] Sawaragi, T., 1995 “Coastal Engineering – Waves, Beaches, Wave-Structure

Interaction”, ed T Sawaragi, Elsevier, Amsterdam

[18] Simon et al., 2000 “Bank and near-bank processes in an incised channel,”

Geomorphology Vol 35 (2000), pp 193–217

[19] Thorne C.R., 1982 “Processes and mechanisms of river bank erosion,”

Gravel-bed Rivers, Wiley and Sons, Chichester, UK (1982), pp 227–259

Fig 2.1 Beach erosion around fish harbor

Chapter 2

Effects of Development around River Mouth

and Shallow Water Region in the Sea

2.1 Introduction

Sandy beaches maintain their configuration based on a very delicate balance of shore and longshore sediment transport If this balance is lost, erosion or unexpected accretion will take place In the case where the amount of discharged sand from the river decreases on the equilibrium beach by for example a newly constructed dam, erosion will occur

cross-Constructions of coastal structures such as

harbors, groins, reclamations and so on in

improper place often disturb the continuity

of longshore sediment transport and bring

about erosion in the lee side of the

longshore sediment transport We can see

such kinds of beach erosion all over the

world Figure 2.1 shows a typical example

of beach erosion of this type caused by

blockage of longshore sand transport in

Aomori Prefecture, Japan After the

construction of Misawa Fishery Port, a

large amount of sand accreted on its

southern side while erosion took place on

its northern side Northward longshore sand transport (upward in the photograph) is evidently predominant along this coast [4]

Many coastal countries around the world have suffered from severe erosion In recent years, Asian countries, especially Japan, Vietnam and Thailand, have many coastal areas eroded intensely There are many factors that bring about unbalance of sediment transport and cause coastal erosion They can be divided into two main causes: natural causes and human-induced causes Natural causes comprise of climate changes and rising sea level Human-induced causes are sand extraction, coral mining, constructing structures, and cutting down mangroves for shrimp farming

In Southeast Asia, mangrove forests play an important role in protecting residents from natural disasters such as beach erosion, wave overtopping, tsunami etc and supplying foods and fuel woods However, many mangrove areas have been cut down rapidly to construct shrimp ponds because of high productivity and economic values These human actions have weakened the ability of coastal protection from erosion

Predominant longshore sediment transport

accretion erosion

On the other hand, mangrove plantations have also been carried out by various organizations Mangrove plantation around river mouth may trap discharged sediment from the river that will be the source of the bed material of the sandy beach around the river mouth This is the negative effect of mangrove plantation from the viewpoint of the shore protection However, this effect also has not been evaluated yet

Another kind of beach erosions caused by

the extraction of bed material in the beach

has become social problems in many

countries In sandy beach, especially near

the river mouth where huge amount of the

volume of sand is usually deposited, people

dredged a large volume of sand for building

material This is not a direct development of

the coastal region but an implicit

development or a kind of exploitation

Figure 2.2 is one example of beach erosion

caused by sand extraction around Lap river

mouth – Vungtau city, southern Vietnam

The sand extraction around Lap river mouth caused beach erosion about 10 m/year The sediment extraction for deepening the navigation channel integrated with natural conditions such as wind waves, ship waves, and tidal currents also caused erosion of Soai Rap river banks (southern Vietnam) at the rate of greater than 10 m/year[2]

In order to deal with the negative effects brought about by these developments and take steps to meet the situation, it is necessary to quantitatively evaluate the amount of erosion Here, at first, the author proposes a procedure for detecting water line from satellite images to evaluate the amount of erosion in two coasts in Thailand and Vietnam and investigates the mechanism of erosion Then the author investigates the effects of sand extraction on beach erosion using the fund of information of the bottom topography around the river mouth of Niyodo River in the Shikoku Island

2.2 Detection of Shoreline Retreat from Satellite Images and Analysis

of the Erosion Mechanism Caused by the Development of Mangrove Forests

2.2.1 Sites of investigation

Two coastal areas of south-eastern Thailand (Site-A) and southern part of Red River Delta in Vietnam (Site-B) were chosen because of severe erosion The locations of these areas are shown in Fig 2.3

Site-A is the long straight sandy beach with many small river mouths bordering the thick mangrove forest (Fig 2.4) There are many shrimp ponds constructed close to the sandy beach Beach configuration is mainly controlled by waves The significant wave height reached a maximum about 2.0 m and period averaging 4-5 seconds in summer season A 700-m jetty was constructed in 1968 at the south of Shongkhla inlet caused accretion in the south of jetty and severe erosion in Nakhon SiThamrat [14]

sand extraction around river mouth

eroded beach

sand dune dead roots

Fig 2.2 Beach erosion by sand extraction

Site-B is also the sandy beach with many shrimp ponds inside the sea dikes and two large river mouths named Ninhco and Balat river mouths (Fig 2.5) In front of the dikes, the beach slope is very gentle and varies along the coastline from 1:40 on eroding beaches to 1:200 in other places [15] The beaches consist of very fine sand with an average grain size of about 0.08 mm In Balat river mouth, there is a thick mangrove forest advancing offshore In 2005, Typhoon 7 (international name is Damrey) with wind velocity of 133 km/h, attacked all coastal areas of the Red River Delta and caused severe erosion in these areas Along the coast, 25 km of sea dikes were broken and 800-m sea dike of Site-B was washed out [5]

Fig 2.3 Location of study areas

2.2.2 Materials and methods

The satellite images used in this study are listed in Table 2.1 SAR (Synthetic Aperture Radar) images of JERS-1 (Japan Earth Resources Satellite) were used to detect the change in the location of shorelines in both sites SAR is an active sensor which transmits microwave and observes characteristics, inequality, slope in the surface of the earth, etc without being influenced by the weather day and night due to scattered waves from the Earth [10]

ASTER (Advanced Space Thermal Emission and Reflection Radiometer) is one of the five state-of-the-art instrument sensor systems on-board Terra a satellite launched in December 1999 It was built by a consortium of Japanese government, industry, and

research groups The ASTER/VNIR data consist of three bands with high resolution of 15 meters [8]

The Landsat (Land Satellite) Program is a series of Earth-observing satellite missions jointly managed by NASA and the U.S Geological Survey The Landsat multi-spectral scanner (MSS) image data consist of four spectral bands and the resolution for all bands

is 79 m The Landsat Thematic Mapper (TM) image data consist of seven spectral bands with a spatial resolution of 30 meters for bands 1 to 5 and band 7 Spatial resolution for band 6 (thermal infrared) is 120 meters, but band 6 data are oversampled to 30 meter pixel size Landsat Enhanced Thematic Mapper Plus (ETM+) image data consist of eight spectral bands (band designations), with a spatial resolution of 30 meters for bands 1 to 5 and band 7 Resolution for band 6 (thermal infrared) is 60 meters and resolution for band

8 (panchromatic) is 15 meters [17] Although the SAR images are monochrome, they have high spatial resolution (15 m/pixel) while the spatial resolution of Landsat images are about 30 m/pixel

Table 2.1 Satellite data used for study Satellite Sensor Acquisition date Resolution (m/pixel)

Ninhco river mouth

Site-B

Namdinh

25

2 5

Band Band

Band Band

In both sites, Landsat data were used to investigate the change in the land-use by

analyzing NDVI (Normalized Difference Vegetation Index) This index can be calculated

by Eq (2.1)

(2.1)

where NIR represents near-infrared energy and RED represents visible energy The value

of NDVI is usually between -1.0 (for water) and +1.0 (for strongest vegetative growth)

A so-called supervised landform classification method was also employed After that, the

location of shorelines was determined by using images of band 5 (1.55 -1.75 μm) and

band 2 (0.76-0.90 μm) of Lansat/TM and evaluated the parameter TM5-2 defined as the

following equation:

By using the TM5-2 images, the radiance values in the pixel between land and sea are

changed suddenly so that the shoreline change can be extracted easily (Deguchi et al.,

2005 [6], [7]) Figure 2.6 shows an example of NDVI image of Site-A in 1994 Figure 2.7

shows another example of TM5-2 image of south-eastern Balat river mouth of Site-B in

2001

Fig 2.7 TM5-2 image of northern Site-B Fig 2.6 NDVI image of Site-A

RED NIR

RED NIR

NDVI

+

−

=

2.2.3 Results of analyzing satellite data

Change in the shoreline location of Site-A

Figure 2.8 shows the Band-4 images of Landsat/TM (1989 and 1994) The dark part of the image is the water surface Just landward of the shoreline, a lot of dark part spreads like patchwork This means that a large part of the mangrove forest along the shoreline has been cut down to construct shrimp farming ponds

Fig 2.8 Band-4 images of Landsat/TM in Site-A

Figure 2.9 shows the results of the shoreline changes in 1992, 1996, and 1998 in Site-A The vertical and horizontal axes are the north latitude and east longitude, respectively In section A of the figure, two parts of the section have been retreated about 100 m during six years (1992 to 1998) around 8.041 and 8.044 degrees of north latitude In section B, the shorelines have been quite stable In section C, one part near 8.069 degrees of north latitude has been eroded more than 50 m This position just coincides with the area shown

in Fig 2.17 Such kind of shoreline retreat can be seen at many places in Site-A

Fig 2.9 Shoreline changes around Huasai, Site-A (1992, 1996, and 1998)

Detected change in land-cover and the location of shoreline around river mouth of Site-B

Figure 2.10 illustrates the classification of land-cover in Sept 1998 and Sept 2001 obtained using supervised classification From this figure, it is found that the area of mangrove forest coloring red increased rapidly in both sides of the river mouth

Figure 2.11 shows that the change of shoreline from 1960 to 2001 After 1960, sand bars have been formed around river mouth and kept continuing up to now

Figures 2.10~2.11 also show that the area of land increased significantly on the southern part of the river mouth because of the development of mangrove forest This is the results

of reforestation of Vietnamese governmental policies [13]

east longi (deg.)

8.065 8.063 8.061 8.059 8.057 8.055 8.053 8.051

100.320 100.322 100.324 100.326

1992 1998 1996 660m

east longi (deg.)

8.079 8.077 8.075 8.073 8.071 8.069 8.067 8.065

100.324 100.326 100.328 100.330

1992 1998 1996 660m

east longi (deg.)

Fig 2.10 Comparison of land-use in Sept 1998 and Sept 2001

Fig 2.11 The shoreline change near Balat river mouth

Based on the analysis of the satellite images, the results of shoreline extraction are described in Figs 2.12~2.15

Figure 2.12 shows that the river mouth sand bar has grown down toward south and the shoreline in the northern part (eastern part further than 62600 m in longitude) has retreated about 150 m to 200 m since 1975 This means that the discharged sediment from Ninhco River did not contribute to the north beach This fact is confirmed by the spread

of murky water toward south-west in the satellite images

Mangrove forest

Rice field

Soil Fresh water

Sea Others

river mouth

2211600 2212600 2213600 2214600 2215600 2216600

Figure 2.15 shows that most of the beach was eroded more than 200 m in Giaothuy district from 1975 to 1989 A part of the beach in the south of Giaothuy district was artificially reclaimed From 1989 to 2001, the beach was stable

Fig 2.12 The shoreline change near Ninhco River

Fig 2.13 The shoreline change in Haihau district

2220600 2221600 2222600 2223600 2224600 2225600 2226600 2227600

Fig 2.15 The shoreline change in Giaothuy district

2.2.4 The erosion mechanism of beaches in Site-A and Site-B

Erosion mechanism in Site-A and Site-B

The typical beach transformation observed in Site-A and Site-B is shown schematically in Fig 2.16 Phase-1 is the original beach profile In this phase, the shoreline has been stable because of the role of mangrove forest Phase-2 illustrates the cross-section just after the construction of shrimp pond near shoreline In this phase, many mangroves were cut down for constructing shrimp pond The seaward bank of the pond is usually not strong enough to protect land from severe waves Once the seaward bank is destroyed, the location of the shoreline retreats by the width of the pond Phase-3 shows that the shoreline retreats to the landward pond bank This bank now becomes the first bank to defend the areas inland

The seaward bank is usually constructed using cohesive material to keep water in ponds Therefore, the property of erosion resistance of the cohesive material is important to evaluate the strength of the sea dike

Figures 2.17~2.19 show two examples of beach erosion caused by the above mentioned mechanism The seaward bank of the pond in Fig 2.17 (Site-A) is nearly destroyed and the retreated shoreline reached the route by the destruction of the pond

Figures 2.18~2.19 show the erosion process in front of a double sea dike system in Haihau district (Site-B) can be understood easily The first defense sea dike has become weaker every year because of the continuous erosion During extreme events such as typhoons and storm surge, the first sea dike has been destroyed under strong wave attack

In order to prevent inundation, the second defense sea dike has been built When the first sea dike fails to defend inundation, the second defense sea dike becomes the new first one The erosion process has continued and caused heavy damage to coastal owners and environment

Fig 2.17 Beach erosion caused by remained abandoned shrimp ponds

(near Huasai, Thailand - Site-A)

Fig 2.18 Beach erosion caused by remained abandoned shrimp ponds

(in Haihau district, northern Vietnam - Site-B)

Fig 2.19 The erosion process in front of the double sea dike system

(in Haihau district, northern Vietnam - Site-B)

Conceivable mechanism of shoreline change Site-B

Based on the above-mentioned results, it seems that there are two kinds of beach erosion

in the objective sites One is the erosion in the northern part and another is the erosion which is progressing from south to north These erosion mechanisms in this region can be explained as follows:

Erosion in north region

In summer, the incident waves from the southeast are dominant and cause the longshore sediment transport to the Balat river mouth Besides, a huge amount of the sediment is discharged from Balat River to the sea and a large part of the sediment deposits near the mouth

In winter, the incident waves from the northeast are dominant and the amount of discharged sediment from the river is small because of the dry season If there is not any

Present first sea dike

Present second sea dike

Recent destroyed sea dike

Old first destroyed sea dike

EAST SEA

Beach erosion

LAND

sediment in the mouth to the south again Therefore, the beaches are thought to be stable These processes are roughly illustrated in Figs 2.20~2.21

Fig 2.20 The mechanism of longshore sediment transport in summer

(without mangrove forest)

Fig 2.21 The mechanism of longshore sediment transport in winter

(without mangrove forest)

Fig 2.22 The mechanism of longshore sediment transport in winter

(with thick mangrove forest)

Balat River Ninhco River

∂

q y

Q D

t

X

s s

H

g K

16

2

=

However, if there is a thick mangrove forest, it will trap sediment within itself and the

sediment in the river mouth will not move to the south again Besides the river discharge

is also small in winter Therefore, the beach will be eroded rapidly as shown in Fig 2.22

Erosion progressing from south to north

As mentioned above, the discharged sediment from Ninhco River does not contribute to

the shoreline in the southern part of the river mouth where the double sea dike system has

been constructed Around the northern part of the Ninhco river mouth, shoreline has

retreated in winter However, the retreated shoreline does not recover to the original

position in summer due to the unbalance on the longshore sediment transport This

shoreline retreat propagated toward the north

2.2.5 Numerical model

Governing equation and boundary conditions

The numerical procedure based on one-line theory is established to reproduce two kinds

of erosion between Balat river mouth and Ninhco river mouth (Site-B) The governing

equation is the conservation of sediment equation as following[3, 9]:

In which Xs is the offshore distance of the shoreline position (m); y is the longshore

distance (m); Ds is the closure depth (m); Q is the volume rate of longshore sediment

transport (m3/s); q is the rate of sediment entering and leaving the profile from the

landward and seaward boundaries (m3/s/m); and t is the time (s) The value of Q is

evaluated by the following relation (CERC, 1991):

where ρ is the density of sea water (1020 kg/m3); g is the acceleration of gravity (= 9.81

m2/s); Hb, αbs, Cgb are the wave height (m), wave angle to the local shoreline, and group

velocity (m/s) at wave breaking point, respectively; and K is the empirical coefficient

(K=0.4)

According to the previous studies (Ton That Vinh et al., 1996 [15]; Nagai et al., 1998

[12]; Mathers et al., 1999 [11]), following two representative waves are applied:

ƒ In summer (about six months), the average deep water wave height is about 1.5 m

and wave period is about 5 seconds coming from southwest directions Sediment load

of river discharge is about 0.002 m3/s

ƒ In winter, the average deep water wave height is about 2.0 m and wave period is

about 5 seconds coming from northeast directions Sediment load of river discharge

The boundary conditions applied are shown in Figs 2.23~2.24 In case where there is a sea dike behind the shoreline, the distance between the shoreline and the sea dike assumed is 100 m and the shoreline cannot retreat more than 100 m It is assumed that the sea dike is in good condition

When there is a rich mangrove forest at j=js (north boundary) and a large part of sediment

is entrapped there, open boundary condition is applied In case where there is not such mangrove forest at j=js (north boundary), a close boundary condition is applied

At the south boundary (j=je), there is no sand supply from Ninhco River in summer and sediment transported toward south is continuous

Results of numerical application

Figures 2.25~2.26 illustrate the calculated shoreline change with and without mangrove forest around the river mouth In the case where the discharged sediment from the river is effectively distributed to the neighboring coast, the shoreline near the river mouth advanced 9 km during 150 years and the retreat of shoreline at the other end is 4 km during 150 years (Fig 2.25)

Fig 2.23 Boundary conditions (B.C.) in summer

Wave incident direction

Fig 2.24 Boundary conditions (B.C.) in winter

Wave incident direction

-8000 -4000 0 4000 8000

On the other hand, if all discharged sediment from the river is trapped at the river mouth and is not supplied to the neighboring coast, the location around the shoreline does not change and the retreat of shoreline comes to 5 km during 150 years (Fig 2.26)

Fig 2.25 Calculated shoreline change without mangrove forest

Fig 2.26 Calculated shoreline change with mangrove forest

Figure 2.27 is the numerical result of the shoreline change backed by a sea dike In this case, the shoreline cannot retreat more than 100 m The shoreline near the south end retreats continuously three years after wave attack After that, the reason of shoreline retreat propagated toward north at the speed of about 2 km/year just like the shoreline retreat shown in Figs 2.13~2.14

From these numerical results, the erosion between Balat River and Ninhco River detected from the satellite images can be reproduced by the numerical model with the proper boundary conditions qualitatively It is needless to say that detailed wave and current conditions and beach information are required to increase the accuracy of the numerical simulation

-150 -130 -110 -90 -70 -50 -30 -10 10 30 50

erosion progresses

Anyway, if the sea dike is destructed, it is surely that the retreat of shoreline becomes far larger than 100 m

Fig 2.27 Calculated shoreline retreat backed by sea dike

2.3 Effects of Sand Extraction on Beach Erosion

Figure 2.28 shows the eroded sites caused by sand extraction in Kochi coast – Kochi Prefecture, Japan [2] The average bottom slope of Kochi coast in the shallow water region (shallower than 5 m) is 1/7-1/10 In the deeper region (deeper than 10 m), the bottom slope is 1/40-1/50 A longshore bar usually exists between the region of the depth

5 m and 10 m The mean grain size of bed material on steep beach is 0.07 cm and 0.02 cm

on the deeper region The volume of discharged sand from Niyodo River is estimated to

Location of sand extraction numbers indicates

start year - end year

Erosion place from 1974 to 1997

N

3000m

N

iyodo R

Figure 2.29 shows bottom topography in front of the Niyodo river mouth from 1974 to

1997 From 1974 to 1988, total volume of extracted sand was more than 4.6 million cubic meters in front of Niyodo river mouth There was a clear river mouth terrace in front of the opening of river mouth sand bar in 1974 At peak of the extraction of sand during

1981 and 1984, river mouth terrace perfectly disappeared and some hollows appeared After the extraction was proscribed in 1988, a clear hollow still remained in 1992

Almost 10 years after proscription of sand extraction (from 1989 to 1997), the river mouth terrace did not recover in 1997 Based on measuring the change of bottom topography, the extraction of sand was the cause of shoreline retreat with annual rate of 8.1 m/year The effect of excessive sand extraction around Niyodo river mouth has been lasting long

of shorelines and land-use in these regions effectively

By comparing the detected location of the shorelines for past 30 years, two kinds of erosion were found out Mangrove reforestation was one of the main factor caused beach erosion in the southern coast of Red River Delta Beach erosion in the southern coast of Red River Delta could be reproduced by numerical models based on one-line theory with properly set boundary conditions

These results will be a great help to work out countermeasure against beach erosion However, in this study, only sandy beach composed of non-cohesive materials have been concerned It is needless to say that the erosion resistance of the developed area deeply depends on the property of cohesive materials that comprise sea dike The erosion mechanisms of cohesive bed, bank, and shore in coastal regions with river mouth have been studied more detail in Chapter 3 and Chapter 4

[3] CERC., (1991) “GENESIS: Generalized Model for Simulating Shoreline Change,”

US Army Corps of Engineers, Report 2 of series

[4] Committee on Coastal Engineering, 1994 “Japanese Coasts and Ports” Japanese Society of Civil Engineering, No.2, pp 99

[5] Cong V Mai, Marcel J.F Stive, and Pieter H.A.J.M Van Gelder (2009) “Coastal

Protection Strategies for the Red River Delta” Journal of Coastal Research, Volume

25, No 1, pp 105-116

[6] Deguchi I., Araki S., Nakaue T., Shimizu A and Hattori H., (2005) “Detection of shoreline by ASTER image and the difference between detected and surveyed

shorelines,” Civil Engineering in the Ocean, Vol.21, pp 439-444

[7] Deguchi I., Araki S., and et al., 2005 “Beach erosion caused by the change of land

use - Detection of the shoreline by JERS-1 satellite image” Proceedings of the 4 th General Seminar of the Core University Program, Osaka, Japan, pp 111-116

[8] Earth Resources Observation and Science (EROS) Webpage: http://edc.usgs.gov/

[9] Horikawa K., 1988 “Nearshore dynamics and coastal processes” University of

Tokyo Press

[10] Japanese Earth Resources Satellite Webpage: http://www.eorc.jaxa.jp/JERS-1/

[11] Mathers S., J Zalasiewiccz (1999) “Holocene sedimentary architecture of the Red

River delta, Viet Nam,” Journal of coastal research, Vol.15, No.2, pp 314-325

[12] Nagai, K S Kono and D X Quang, (1998) “Wave characteristics on the central

coast of Viet Nam in the South China Sea,” Coastal Engineering Journal, Vol.40,

No.4, pp 347-366

[13] Nakaue T., 2005, “Using satellite image to analyze coastal environmental change in

southern part of Red River Delta and clarification of coastal erosion mechanism”

Master thesis, Dept of Civil Engineering, Osaka University

[14] Nutalaya P., (1996) “Coastal erosion in the Gulf of Thailand” GeoJournal, Vol.38,

No.3, pp 283-300

[15] Ton That Vinh, G Kant, N N Huan and Z Pruszak (1996) “Sea dike erosion and

coastal retreat at Nam Ha Province, Viet Nam,” Proc 25 th

ICCE, ASCE, Vol.3, pp

2820-2828

[16] Vu Thanh Ca, Tran Thuc, and Nguyen Quoc Trinh (2006) “Study on the Regime of

Dynamic and Sediment Transport Processes for the Solutions of Coastal Erosion

Problems in Nam Dinh, Vietnam,” Vietnam-Japan Estuary Workshop, pp.78-86

[17] The Landsat Program National Aeronautics and Space Administration Webpage:

http://landsat.gsfc.nasa.gov/;

Chapter 3

Experimental Study on Critical Shear Stress of

Cohesive Bed Material for Erosion

3.1 Introduction

River bank erosion near river mouth is an increasingly serious problem for economic activities and natural environment of Southeast Asia There are many factors impact on erosion process such as river bank composition, mangrove trees, aquatic animal burrows, meteorological and hydrodynamic conditions The erosion mechanism of cohesive river banks is very complicated and difficult to understand because of physico-chemical internal forces of cohesive soils and the reactions of seawater and clayed sediments (ASCE, 1998a)

social-Previous studies have shown that soil silt-clay content has effected on the susceptibility of

a riverbank to subaerial erosion process Couper (2003) found that riverbanks with high silt-clay contents are most susceptible to erosion by subaerial processes The resistance of

a bank to fluvial erosion and mass failures processes tended to increase with increasing silt–clay content (Osman and Thorne, 1988) Thorne and Osman (1988, 1990) also presented that subaerial processes of the soil are associated with moisture conditions within the material and the physical state of this moisture

Crag (1992) found that an increase in soil moisture content acts to decrease the magnitude

of interparticle forces within the material, reduce the resistance of the bank face to fluvial shear forces Matsuoka (1996) recommended that soils with a silt-clay content greater than 20% are considered “frost-susceptible” Ariathurai et al (1978) investigated the erosion rates of cohesive soils by the rotating cylinder apparatus and shown that the sodium adsorption ratio, temperature, and pore fluid concentration have a profound effect and the moisture content has little effect on the erodibility of cohesive soils

The objective of this study is to investigate the effects of sand-silt-clay content, moisture content, salinity, consolidation, and vegetation on critical shear stress for erosion of cohesive remolded samples and compare with the in situ test results of Soairap river banks, southern Vietnam

3.2 Experimental Apparatus and Procedures

3.2.1 Non-vertical jet test apparatus

To measure the critical shear stress of cohesive bank materials and remolded samples for erosion, a non-vertical submerged jet test apparatus was used This apparatus was

developed based on Hanson et al (2002) and consists of a pump, adjustable head tank, jet submergence tank, a point gauge, jet tube, and other support materials (Fig 3.1)

The jet submergence tank is 30 cm in diameter and 30 cm in height with a skirt of 25 cm that the tank can be embedded into the soil tightly The lid of submergence tank was designed to hold water in the tank and opened to measure the scour depth Besides, the lid also holds the jet tube perpendicular to the soil surface and holds the discharge tube and pressure gage

The jet tube (0.8 m in length, 0.1 m in diameter) was installed in the center of the submergence tank to allow a point gauge to pass through the nozzle, to shut off the jet flow, and to take soil surface measurements as scour occurs during testing The jet tube receives flow from a head tank, pump, or strong water tap The jet nozzle height was kept

at 10 cm above the initial surface of soil samples The nozzle diameter is 6.4 mm

The head tank (0.8 m in length, 0.1 m in diameter) was used to control the hydraulic shear stress and to dampen fluctuations in pumping level The valves help to clean the air in the jet tube and measure the differential head The water supply for conducting tests is a pump or a strong water tap

local scouring jet flow

valve

Fig 3.1 Non-vertical submerged jet test apparatus

3.2.2 Procedures for determining the τ c and k d

The non-vertical submerged jet test apparatus is operated based on the diffusion

and (b) describe the detail of the diffusion principles and stress distribution where d0 is

the nozzle diameter, Je is the equilibrium depth (m), Ji is the initial distance from the

nozzle to soil surface (0.1 m), and Jp is the potential core length (0.037 m) The rate of

erosion ε (cm/s)) is proportional to the effective shear stress in excess of the critical shear

where kd is the erodibility coefficient (cm3/N.s), τe is the applied shear stress (N/m2), and

τc is the critical shear stress (N/m2)

When the rate of change in the depth of scour diminishes to zero, equilibrium depth Je is

attained and critical shear stress is calculated by Eq (3.2) as following:

In this equation, Cf is the friction coefficient and taken a value of 0.00416, ρ is the density

of fresh water (1000 kg/m3) and water of Soairap River (1025 kg/m3) U0 is the velocity at

the nozzle and calculated by Eq (3.4) [3]

4899 0

where H is the differential head (m) and was calibrated shown in Fig 3.3

Fig 3.2 Diffusion principles (a) and stress distribution (b)

diffused jet

initial bed

submergence tank

J

J

τ

τ for Ji>Jp

0 2 4 6

Fig 3.3 Calibration of the initial jet velocity at nozzle Field data were recorded in the spreadsheet This spreadsheet estimates the equilibrium

depth using the scour depth versus time The difficulty in determining equilibrium scour

depth is that the length of time required to reach equilibrium can be very large (Blaisdell

et al 1981) Therefore, the spreadsheet estimates the equilibrium depth using the scour

depth data versus time and a hyperbolic function for estimating equilibrium depth were

developed by Blaisdell et.al (1981) The general form of the equation with an asymptote

from which the ultimate depth of scour can be computed with Eq (3.5) [2] In this

equation, A is the value for the semi-transverse and semi-conjugate axis of the hyperbola;

f0, f, and x are calculated by Eqs 3.6, 3.7, and 3.8, respectively

The spreadsheet routine determines the minimum standard error for this function versus

the jet test data The spreadsheet routine conducts the minimization search from starting

values of A=1 and f0 = 1 Once equilibrium depth Je is determined, based on the value of f,

the critical shear stress is then determined by Eq (3.2) The erodibility coefficient kd is

determined based on the scour depth, time, pre-determined τc and the dimensionless time

function as described by Eq (3.9) In this equation, the dimensionless terms are defined

as Ji* = Ji/Je, and J* = J/Je (J is the distance from the nozzle to the centerline depth of

scour), tm is measured time (s), Tr = Je/(kd*τc) is a reference time

d

t U

11

1ln2

1

i i

i r

J

J J

J

J T

U0 = 4.046*H0.4899

R2 = 0.9987

3.3 Laboratory Experiments on Remolded Samples

3.3.1 Remolded samples making processes

Forty-nine remolded soil samples with different amounts of sand-silt-clay contents, moisture contents, salinities, consolidation, and vegetation were made by compaction in a 10-cm diameter and 10-cm high polyvinyl chloride cylinder tubes The processes of making remolded samples are shown in Figs 3.4 ~ 3.5

The Toyoura standard sand with grain size smaller than 0.02 cm was used The silt used

in this study was marine silt taken from Okinawa supplied by Encore-ann company The kaolin clay used was crown clay of Southeastern Clay Company All remolded samples were made and consolidated at a normal stress of about 88.3 kN/m2

After consolidation, the samples were stored at the laboratory within 48 hours for reaction between internal forces of cohesive soils and stabilization of soil fabric The moisture contents were measured by using SM200 soil moisture sensor

(a) (b)

Fig 3.4 Remolded sample after mixing (a) and after pouring water (b)

Fig 3.5 Remolded sample before testing

To investigate the effects of sand-silt-clay content on the critical shear stress, the author carried out with three types of remolded samples In the first type (Table 3.1), the silt