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

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 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 i 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 8th 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 6th General Seminar on Environmental Science & Technology Issues Japan, Kumamoto, Oct 2006、pp 144-152 ii CONTENTS Acknowledgement List of Publications Contents List of Tables List of Figures i ii iii v vi Chapter Introduction 1.1 1.2 1.3 1.4 Background Objectives of the Study The Study Areas Outline of the Dissertation References Chapter Effects of Development around River Mouth and Shallow Water Region in the Sea 2.1 2.2 2.3 2.4 Introduction 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 2.2.2 Materials and methods 2.2.3 Results of analyzing satellite data 2.2.4 The erosion mechanism of beaches in Site-A and Site-B 2.2.5 Numerical model Effects of Sand Extraction on Beach Erosion Conclusions References Chapter Experimental Study on Critical Shear Stress of Cohesive Bed Material for Erosion 3.1 3.2 3.3 3.4 Introduction Experimental Apparatus and Procedures 3.2.1 Non-vertical jet test apparatus 3.2.2 Procedures for determining the τc and kd Experimental Study on Remolded Samples 3.3.1 Remolded samples making process 3.3.2 Procedures for determining the τc and kd of remolded samples In Situ Experiments 3.4.1 Experimental sites 3.4.2 Temperature iii 2 6 7 11 15 19 22 24 24 26 26 26 26 27 30 30 33 34 34 35 3.4.3 3.4.4 3.4.5 3.4.6 3.4.7 3.5 3.6 Rainfall Tidal currents and waves Vegetation and aquatic animals The properties of cohesive soils of Soairap river banks Procedures for measuring the τc and kd of undisturbed samples Results and Discussions Conclusions References Chapter Estimation of Erosion Resistance of Cohesive Bank in River and around River Mouth 4.1 4.2 4.3 4.4 4.5 4.6 4.7 Introduction Flume Experiments 4.2.1 Experimental apparatus 4.2.2 Results and discussions Wave Basin Experiments 4.3.1 Experimental apparatus 4.3.2 Results and discussions 4.3.2.1 Erosion caused by waves (Case-1 to Case-3) 4.3.2.2 Erosion profiles caused by waves, wave-opposing currents, and wave-following currents Numerical Model 4.4.1 Governing equations 4.4.2 Estimation of applied shear stress around river mouth 4.4.2.1 Boundary conditions 4.4.2.2 Simulation results and applicability of the model 4.4.3 Estimation of applied shear stress on cohesive bank in river 4.4.3.1 Boundary conditions 4.4.3.2 Simulation results and discussions Estimation of Erosion Resistance along Soairap River 4.5.1 Field investigation and scour test 4.5.2 Estimation of erosion resistance along Soairap River Discussions Conclusions References Chapter Conclusions 36 36 38 39 40 41 44 44 46 46 47 47 48 50 50 52 52 54 57 57 59 59 60 68 69 71 75 75 77 79 80 80 82 iv List of Tables Chapter Effects of Development around River Mouth and Shallow Water Region in the Sea Table 2.1 Satellite data used for the study Chapter Experimental Study on Critical Shear Stress of Cohesive Bed Material for Erosion Table 3.1 Table 3.2 Table 3.3 Table 3.4 Table 3.5 Table 3.6 Table 3.7 Table 3.8 Table 3.9 Table 3.10 Table 3.11 Remolded samples with only sand and clay Remolded samples with 40% of silt Remolded samples with 10% sand Remolded samples with the change of moisture content Samples with only sand-clay content and moisture content Remolded samples with the change of salinity Remolded samples with the change of consolidation time Remolded samples with the change of dead root and leaves Properties of cohesive soils of Soairap river banks The results of experiments on effect of consolidation The results of experiments on effect of dead roots and leaves 31 31 31 32 32 32 32 33 39 43 43 Chapter Estimation of Erosion Resistance of Cohesive Bank in River and around River Mouth Table 4.1 Table 4.2 Table 4.3 Table 4.4 Table 4.5 Table 4.6 Table 4.7 Table 4.8 Table 4.9 Erosion properties of remolded samples in the flume test Experimental conditions in the basin tests Erosion properties of remolded samples in the basin tests Input conditions of laboratory scale model Calculation results around river mouth Input conditions of the model to investigate the effect of waves and wave-induced currents Input conditions of the model to investigate the effect of river flows Input conditions of the model to investigate the effect of waves, wave-induced currents and tidal currents Input conditions of the model to investigate the effect of waves with weak currents v 48 52 52 60 61 69 70 70 70 List of Figures Chapter Introduction Fig 1.1 Location of study areas Chapter Effects of Development around River Mouth and Shallow Water Region in the Sea Fig 2.1 Fig 2.2 Fig 2.3 Fig 2.4 Fig 2.5 Fig 2.6 Fig 2.7 Fig 2.8 Fig 2.9 Fig 2.10 Fig 2.11 Fig 2.12 Fig 2.13 Fig 2.14 Fig 2.15 Fig 2.16 Fig 2.17 Fig 2.18 Fig 2.19 Fig 2.20 Fig 2.21 Fig 2.22 Fig 2.23 Fig 2.24 Fig 2.25 Fig 2.26 Fig 2.27 Fig 2.28 Fig 2.29 Beach erosion around fish harbor Beach erosion by sand extraction Location of study areas Location of Site-A (Landsat/TM-1989, May 29) Location of Site-B (Landsat/TM-1989, May 29) NDVI image of Site-A TM5-2 image of northern Site-B Band-4 images of Landsat/TM in Site-A Shoreline changes around Huasai, Site-A (1992, 1996, and 1998) Comparison of land-use in Sept 1998 and Sept 2001 The shoreline change near Balat river mouth The shoreline change near Ninhco River The shoreline change in Haihau district The shoreline change in Vanly area The shoreline change in Giaothuy district Expected topography change around the shrimp pond Beach erosion caused by remained abandoned shrimp ponds (near Huasai, Thailand - Site-A) Beach erosion caused by remained abandoned shrimp ponds (in Haihau district, northern Vietnam - Site-B) The erosion process in front of the double sea dike system (in Haihau district, northern Vietnam - Site-B) The mechanism of longshore sediment transport in summer (without mangrove forest) The mechanism of longshore sediment transport in winter (without mangrove forest) The mechanism of longshore sediment transport in winter (with thick mangrove forest) Boundary conditions (B.C.) in summer Boundary conditions (B.C.) in winter Calculated shoreline change without mangrove forest Calculated shoreline change with mangrove forest Calculated shoreline retreat backed by sea dike Sand extraction sites and erosion place in Kochi coast, Japan Bottom topography around river mouth of Niyodo River, Kochi Coast, Japan vi 9 10 10 11 12 13 13 14 14 15 15 16 16 17 17 18 18 18 20 20 21 21 22 22 23 Chapter Experimental Study on Critical Shear Stress of Cohesive Bed Material for Erosion Fig 3.1 Fig 3.2 Fig 3.3 Fig 3.4 Fig 3.5 Fig 3.6 Fig 3.7 Fig 3.8 Fig 3.9 Fig 3.10 Fig 3.11 Fig 3.12 Fig 3.13 Fig 3.14 Fig 3.15 Fig 3.16 Fig 3.17 Fig 3.18 Fig 3.19 Fig 3.20 Fig 3.21 Fig 3.22 Fig 3.23 Fig 3.24 Fig 3.25 Fig 3.26 Non-vertical submerged jet test apparatus Diffusion principles (a) and stress distribution (b) Calibration of the initial jet velocity at nozzle Remolded sample after mixing (a) and after pouring water (b) Remolded sample before testing Mixing rates and moisture content of remolded samples Procedure for determining the τc Procedure for determining the kd Study area and measuring sites In situ submerged jet test device Compact-WH (a) and Compact-EM (b) Average air temperature from 2002 to 2005 (Tan Son Hoa Station) Annual rainfall from 2002 to 2005 (Tan Son Hoa Station) The water level from 2002 to 2005 (Phu An Station) Wave data measured at site SR2R Tidal current data measured at site SR2R Wave data measured at site SR6L Tidal current data measured at site SR6L Exposed roots and burrowed holes at SR5R Physical properties of measuring site samples Procedure for determining the τc at SR5R Procedure for determining the kd at SR5R Relationship between the τc and clay content Relationship between the τc and moisture content Relationship between the τc and salinity Relationship between the kd and τc 27 28 29 30 30 33 34 34 35 35 35 35 36 36 37 37 38 38 39 39 40 40 41 42 42 43 Chapter Estimation of Erosion Resistance of Cohesive Bank in River and around River Mouth Fig 4.1 Fig 4.2 Fig 4.3 Fig 4.4 Fig 4.5 Fig 4.6 Fig 4.7 Fig 4.8 Fig 4.9 Fig 4.10 Fig 4.11 Fig 4.12 Fig 4.13 Fig 4.14 Fig 4.15 Sketch of test cross section of the flume Velocity profile in the center of test section Erosion of remolded sample of (P1) after 2-hour testing Erosion of remolded bank sample after 18-hour testing Sketch of wave-river basin and river mouth model Erosion caused by waves after 3-hour testing Wave data of the experiment with wave-opposing currents Erosion profile in Case-4 and Case-5 Wave data at the starting time of the experiment with wave-following currents Erosion profiles in Case-6 and Case-7 The domain of laboratory scale model Distribution of current velocities (Case-3) Distribution of wave heights (Case-3) Distribution of applied shear stresses (Case-3) Distribution of current velocities (Case-6) vii 47 48 49 50 51 53 54 55 56 56 60 62 62 62 63 Fig 4.16 Fig 4.17 Fig 4.18 Fig 4.19 Fig 4.20 Fig 4.21 Fig 4.22 Fig 4.23 Fig 4.24 Fig 4.25 Fig 4.26 Fig 4.27 Fig 4.28 Fig 4.29 Fig 4.30 Fig 4.31 Fig 4.32 Distribution of wave heights (Case-6) Distribution of applied shear stresses (Case-6) Distribution of current velocities (Case-8) Distribution of wave heights (Case-8) Distribution of applied shear stresses (Case-8) Distribution of current velocities (Case-11) Distribution of wave heights (Case-11) Distribution of applied shear stresses (Case-11) Effect of waves and river discharge on applied shear stresses The domain of field scale model Effect of only waves and wave-induced currents on shear stresses Effect of river flow and tidal range on applied shear stresses Effect of waves and wave-following currents on shear stresses Effect of waves and wave-opposing currents on shear stresses Study sites in Soairap River Erosion site 21R of Soairap river bank in 2007 (a) and in 2008 (b) Distribution of shear velocities in condition with only currents (a), only waves (b), and waves-currents (c) viii 63 64 65 65 65 66 66 67 68 69 72 73 74 75 76 77 78 Case B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 B11 B12 Table 4.7 Input conditions of the model to investigate the effect of river flows and tidal level T tis tie Tide Direction of H0 (cm) (s) (cm) (cm) (cm) river flow 0.5 5.0 0.0 2.0 0.0 Following 0.5 5.0 0.0 4.0 0.0 Following 0.5 5.0 0.0 6.0 0.0 Following 0.5 5.0 0.0 8.0 0.0 Following 0.5 5.0 0.0 10.0 0.0 Following 0.5 5.0 0.0 12.0 0.0 Following 60.0 6.0 0.0 0.0 170.0 Following 60.0 6.0 0.0 0.0 -170.0 Following 40.0 6.0 0.0 0.0 170.0 Following 40.0 6.0 0.0 0.0 -170.0 Following 25.0 5.0 0.0 0.0 170.0 Following 25.0 5.0 0.0 0.0 -170.0 Following Table 4.8 Input conditions of the model to investigate the effect of waves, wave-induced currents and tidal currents Case H0 T tis tie Tide Direction of (cm) (s) (cm) (cm) (cm) river flow C1 60.0 6.0 0.0 2.0 0.0 Following C2 60.0 6.0 0.0 4.0 0.0 Following C3 60.0 6.0 0.0 6.0 0.0 Following C4 60.0 6.0 0.0 8.0 0.0 Following C5 60.0 6.0 0.0 10.0 0.0 Following C6 60.0 6.0 0.0 12.0 0.0 Following C7 60.0 6.0 7.0 0.0 0.0 Opposing C8 60.0 6.0 10.0 0.0 0.0 Opposing C9 60.0 6.0 13.5 0.0 0.0 Opposing C10 60.0 6.0 15.0 0.0 0.0 Opposing Case D1 D2 D3 Table 4.9 Input conditions of the model to investigate the effect of waves with weak currents H0 T tis tie Tide Wave - Current (cm) (s) (cm) (cm) (cm) direction 25.0 5.0 0.0 8.0 0.0 Following 40.0 6.0 0.0 8.0 0.0 Following 60.0 6.0 0.0 8.0 0.0 Following Note: tis is the water surface elevation given at the upstream boundary tie is the water surface elevation given at the downstream boundary With high incident waves (H0 = 60, 80, 100, 120 cm), the effects of big ships, or severe storms were concerned The medium incident wave height (H0 = 40 cm) indicates the effects of medium ships, or large fishing boats The small incident wave height (H0 = 25 cm) indicates the effects of small ships and fishing boats 70 When tie ≠ 0.0 cm and tis = 0.0 cm, the effects of wave-following currents were concerned When tie = 0.0 cm and tis ≠ 0.0 cm, the effects of wave-opposing currents were determined 4.4.3.2 Simulation results and discussions Effect of waves and wave-induced currents on applied shear stress (Case-A1 to A6） Figure 4.26 shows the simulation results in condition with only waves and wave-induced currents (tis = cm, and tie = cm) The incident wave heights were changed from 25 cm to 120 cm Cross-shore distributions of wave height, mean current velocity and applied shear stress are illustrated in Fig.(a), (b) and (c) of the figure, respectively As the incident wave height increases, wave breaking point shift to offshore In the shallow water region after breaking, wave height is roughly proportional to the total water depth (still water depth + wave set-up) and the amount of wave set-up increases as the increase in the incident wave height The difference in wave height in very shallow water region is caused by the difference in the wave set-up In this case, the mean current is a so-called longshore current caused by the obliquely incident waves and the velocity monotonically increases as the increase in the incident wave height The maximum shear stress in each case also increases from 1.5 to 5.8 N/m2 according to the increase of the incident wave height Applied shear stresses on the bank change from 1.1 to 1.9 N/m2 This means that the bank will be eroded if the critical shear stresses of cohesive bank material are smaller than these applied shear stresses A1 A4 A2 A5 A3 A6 A1 A4 A2 A5 A3 A6 1.0 1.6 wave height (m) Current velocity (m/s) shoreline 0.8 0.6 0.4 1.2 0.8 0.4 0.2 0.0 0.0 12 16 Cross-shore distance (*0.25m) (a) 12 Cross-shore distance (*0.25m) (b) 71 16 A1 A4 A2 A5 A3 A6 Applied shear stress (N/m2) 0 12 Cross-shore distance (*0.25m) 16 (c) Fig 4.26 Effect of only waves and wave-induced currents on shear stresses (a): current velocities; (b): wave heights; (c): applied shear stresses on the bank Effects of river flow (tidal currents) and tidal level on applied shear stress (Case-B1 to B6 and Case-B7 to B12) The results of numerical simulation are shown in Fig 4.27 ((a): mean current velocity, (b): applied shear stress) The current velocities were changed from 30 cm/s to 187 cm/s near the shoreline (Fig 4.27(a)) The maximum applied shear stresses caused by only tidal currents were from 0.3 to 8.3 N/m2 (Fig 4.27(b)) In case of the pure action of waves, the maximum applied shear stress larger than 2.0 N/m2 appears when the incident wave height higher than 0.4 m On the other hand, the maximum applied shear stress more than 2.0 N/m2 appears in the case where the mean current velocity larger than 1.0 m/s (Case-B4 in Fig 4.27(a) and (b)) Figure 4.27(d) illustrates the effect of tidal ranges with different wave heights The maximum applied shear stresses got value of 2.9 N/m2 nearly the same in condition with incident wave height H0 = 60 cm and tidal levels are +170 cm, 0.0 cm, and -170 cm The difference here is the movement of shoreline in spring tide or neap tide However, in spring tide, the bank can be saturated by river water and can be eroded quite easier than in neap tide with the same condition of wave effects B2 B5 B1 B4 B3 B6 shoreline 2.0 Applied shear stress (N/m 2) Current velocity (m/s) B1 B4 1.6 1.2 0.8 0.4 0.0 12 Cross-shore distance (*0.25m) 16 4 12 Cross-shore distance (*0.25m) (b) 72 B3 B6 (a) B2 B5 16 Applied shear stress (N/m2) B12 B8 B11 B7 B10 A3 B9 0 12 16 20 Cross-shore distance (*0.25m) 24 (c) Fig 4.27 Effect of river flow and tidal range on applied shear stresses (a): current velocities; (b) & (c): applied shear stresses on the bank Effect of waves, wave-induced currents, and wave-following currents on applied shear stress (Case-C1 to C6 and D1 to D3) Figure 4.28 indicates the calculation results under the conditions of waves and wavefollowing currents The incident wave height H0 is kept constant to be 60 cm, the current velocities are changed from 50 cm/s to 210 cm/s close to the bank (Figure 4.28(a)) by adjusting the water level at upstream boundary The incident waves break in the deeper region when compared with the case where there is no river flow (Fig 4.28(b)) However the current velocity does not affect the wave breaking point and wave decay after breaking The maximum applied shear stress (Fig.4.28(c)) is larger than N/m2 even in the case of incident wave height is 25 cm The mean current in river increases the shear stress caused by waves remarkably and if the waves attack on a relatively weak current, the shear stress on the bank also increases significantly Figure 4.28(d) shows the effects of changed wave heights and wave-following currents with the same velocity (about 120 cm/s) With the same velocities, the higher wave height has the higher applied shear stress C2 C5 C3 C6 C1 C4 C2 C5 C3 C6 0.8 2.0 shoreline 1.6 Wave height (m) Current velocity (m/s) C1 C4 1.2 0.8 0.4 0.0 0.6 0.4 0.2 0.0 12 Cross-shore distance (*0.25m) 16 (a) 12 Cross-shore distance (*0.25m) (b) 73 16 C1 C4 C2 C5 C3 C6 D1 D2 D3 Applied shear stress (N/m2) Applied shear stress (N/m 2) 6 shoreline 4 0 12 Cross-shore distance (*0.25m) 16 12 Cross-shore distance (*0.25m) 16 (c) (d) Fig 4.28 Effect of waves and wave-following currents on shear stresses (a): current velocities; (b) wave heights; (c), (d): applied shear stresses on the bank Effect of wave, wave-induced currents, and wave-opposing currents on applied shear stress (Case-C7 to C10) Figure 4.29 indicates the calculation results in the case of waves and wave-opposing current The incident wave height is again kept constant to be 60 cm, the maximum current velocities are changed from 30 cm/s to 120 cm/s (Fig 4.29(a)) When the current velocity increases further, waves can not propagate against currents As can be seen from Fig 4.29(b) incident waves break closer to the shore when compared with the cases of no current and following current The maximum applied shear stress is again larger than 2.0 N/m2 even the case of incident wave height of 25 cm The maximum shear stress in this case is larger than that in the cases of no current under the same incident waves However, the increase in the shear stress with existence of opposing current is smaller than the increase in the shear stress with the following current C7 C8 C7 C10 C8 C9 C10 1.6 shoreline 1.6 Wave height (m) Current velocity (m /s) 2.0 C9 1.2 0.8 0.4 1.2 0.8 0.4 0.0 0.0 12 Cross-shore distance (*0.25m) 16 (a) 12 Cross-shore distance (*0.25m) (b) 74 16 Applied shear stress (N/m2) C7 C8 C9 C10 0 12 Cross-shore distance (*0.25m) 16 (c) Fig 4.29 Effect of waves and wave-opposing currents on shear stresses (a): current velocities; (b) wave heights; (c): applied shear stresses on the bank 4.5 Estimation of Erosion Resistance along Soairap River 4.5.1 Field investigation and scour test In 2007 November, 12 erosion pins which have 1.5 m in length and colorful markers were installed perpendicularly to five severely eroded sites of Soairap river banks (southern Vietnam, Fig 4.30) In 2008 August, the second field investigation was carried out and undisturbed samples were taken to determine the physical properties and critical shear stresses Some physical properties of these undisturbed samples were tested by ASTM (The American Society for Testing and Materials) standards The critical shear stresses were analyzed based on nonvertical jet test device developed by Hanson et al (2002) and reproduced by Deguchi et al (2007) The results of field investigation show that the erosion rates of some positions of Soairap river banks have been greater than 10 m/year The author failed to find all erosion pins installed in the five positions These mean that the horizontal erosion depth was greater than 1.5 m 75 12.0 12.0 31L 6.0 100 cross-shore distance (*50m) 21R 41R 50 51R 150 6.0 3.0 200 9.0 250 3.0 After Hirose et.al., 2002 [8] 300 6.0 7.3 350 Sampling sites 61L Study area 400 7.3 6.0 25 50 75 100 125 150 175 7.3 200 225 longshore distance(*50m) Fig 4.30 Study sites in Soairap River The results of scour test and soil properties are shown in Table 4.10 From the table, the τc is quite low with the range of 0.075 to 2.47 N/m2 The sand contents of undisturbed samples change from 1.4 to 7.7% The appearance of high silt and clay contents in all undisturbed samples taken from Soairap river banks shows that they have strong cohesive properties with high silt and clay contents With high moisture content during the neap tide convinced that weathering processes impact not much on erosion mechanisms of these banks The values of permeability of site 51R and 61L are higher than those of remain sites However, the erosion rates of sites 31L and sites 51R are greater than those of 21R, 41R, and 61L These data show that not only the soil properties effect on erosion but also the shear stresses of other factors such as river flows, wind waves, ship waves and tidal regimes affect erosion mechanisms of these regions Figure 4.31 shows an example of erosion processes of site 21R of Soairap river bank in November, 2007 and August, 2008 After nearly one year, almost the grass-cover layer and bank materials have been eroded with horizontal erosion distance of 2-3 meters 76 Table 4.10 Physical properties of undisturbed samples Samples 21R 31L 41R 51R 61L τc (N/m2) 2.31 1.01 0.075 2.47 0.231 kd Sand Silt Clay MC (cm3/ N-s) (%) (%) (%) (%) 5.21 1.4 46.0 52.6 104.3 3.73 3.2 43.1 52.7 110.5 4.81 1.8 40.4 57.8 108.3 0.42 2.4 31.3 67.3 94.8 8.79 7.7 51.3 41.0 87.5 K (x10-6 cm/s) 1.08 2.77 1.25 33.20 54.70 Note: τc is the critical shear stress, kd is the erodibility; MC stands for moisture content and K is the permeability (a) November, 2007 (b) August, 2008 Fig 4.31 Erosion site 21R of Soairap river bank in 2007 (a) and in 2008 (b) 4.5.2 Estimation of erosion resistance along Soairap River In order to investigate the erosion resistance of Soairap river bank, the following three numerical simulations have been carried out The first calculation is the case under the condition with only tidal currents The boundary condition was set up with surface elevation at the upstream boundary of the river E0 = 2.5 m, tidal height is 2.0 m Figure 4.32(a) shows that the significant shear velocities mainly appear in the upper part of the Soairap River and in the Dongtranh River (near river mouth) The second calculation is carried out under the condition of the incidence of wind wave The boundary condition was set up with incident wave height H0 = 1.5 m, wave period T0 = s, and tidal height is 2.0 m Figure 4.32(b) shows that the significant shear velocities only appear at the river mouth and very low in the upper part of the Soairap River The last simulation is in condition with both current and wave action The boundary condition was set up with surface elevation E0 = 2.5 m, incident wave height H0 = 1.5 m, wave period T0 = s, and tidal height is 2.0 m Figure 4.32(c) shows that the significant shear velocities appear both in the upper part of the Soairap River and at the river mouth 77 These results indicate that the distribution of shear velocities agree quite closely with the field survey data (a) (b) N/m2 N/m2 (c) N/m2 Fig 4.32 Distribution of applied shear stress in condition with only currents (a), only waves (b), and waves-currents (c) 78 4.6 Discussions Along the Soairap River, both banks are almost covered with mangrove trees but have been eroded severely with high intense The erosion rates of some parts are greater than 10 m/year Many factors cause bank erosion, however, hydrodynamic processes are the main cause with some other factors such as many holes of aquatic animals and dead roots and leaves of mangrove trees In both flume and wave basin experiments, the erosion depths of all remolded samples were not the same along the upper line, center line and lower line These mean that the weak parts of the samples will be eroded first, after that the remained parts will become weak and be eroded continuously The testing time may influence on the sample conditions In the flume experiments, the erosion depth of remolded bank samples is greater 2~10 times than that of the bed samples This indicates that the erosion mechanism of river bank is different from that of river bed In the bank, mass erosion (aggregates) is mainly erosion mechanism but the surface erosion is predominant in the bed The water surface fluctuation also increases the erosion rates of bank samples Gaskin et al (2003) discovered that mass erosion was the most significant erosion process in undisturbed samples of Champlain Sea clay of the St Lawrance river banks In some part of the Soairap river banks, the mass erosion process is also the predominant erosion mechanism The heavy rainy water during along continuous periods can help increase mass erosion process However, this effect was not concerned in this study In the wave basin experiments, the appearance of dead roots and leaves increases the critical shear stresses of the remolded samples but also increases the erosion rates because the dead roots and leaves can disturb the samples when waves and currents attack Waves and wave-following currents can increase the erosion rate when compared with waves and wave-opposing currents Some other available materials and eroded materials in water such as dust and sand can be the exterior factors crashing the samples and cause erosion under wave breaking and wave-current interaction The peaks of local wave heights increase the applied shear stresses on the bank and bed samples and cause much erosion Bui et al (2008) investigated that the critical shear stress of both in situ undisturbed samples and remolded samples depend on the sand-silt-clay contents, moisture contents, salinity, consolidation, and vegetation The critical shear stresses of the samples are directly proportional to clay contents, salinity, consolidation, and vegetation; and inversely proportional to the moisture contents These mean that the erosion mechanisms of the bank and bed will be influenced significantly by these factors Couper (2003) also studied the effect of silt-clay content on the susceptibility of river banks to subaerial erosion The results indicate that river banks with high silt-clay contents are the most susceptible to erosion by subaerial processes However, the subaerial processes have been observed at only site 21R and are not significant erosion mechanism of banks and bed in Soairap River Other factors such as dead roots and leaves, holes of aquatic animals play important role in weakening the bank before erosion process occur According to numerical simulations on the applied shear stress around river mouth, the highest applied shear stress appeared on the left bank and bed of the channel in the case 79 of the following current on the incident waves In the experiments, there are some cases where the critical shear stresses of the remolded samples are greater than the calculated maximum shear stress, for example 1.7 N/m2, but the samples were eroded It can be said that dead roots and leaves played an important role in weakening the samples because of their movement by waves and wave-current interaction In other cases, the calculated applied shear stresses are high enough to erode the bank and bed materials with low critical shear stresses These results coincide with the experimental results obtained in the wave basin experiments The calculated applied shear tress on cohesive bank in river also becomes large under the condition of incident waves coupled with the following current Therefore, it can be judged that the numerical model for predicting applied shear stress used in this study can apply to the case under the coexisting system of waves and relatively strong current 4.7 Conclusions Three erosion mechanisms of cohesive bank and bed materials have been investigated by in situ investigation, laboratory experiments and numerical study Surface erosion usually occurs on the surface of bed samples or on surface of the bank samples when the low waves and currents attack The mass erosion can take place in bank samples attacked by strong waves and currents The erosion by dead roots and leaves is also the important mechanism when the bank and bed samples and in the water environments consist of dense dead roots and leaves Field investigation indicated that the erosion processes of Soairap river banks and bed are being occurred severely with erosion rates of 1.5 to over 10 m/year The hydrodynamic regimes such as wind waves, ship waves, tidal currents (river flow) are main factors cause bank and bed erosion The appearance of many dead mangrove roots and many holes of aquatic animals result in weakening the cohesive materials of the banks The experimental study and numerical study show that erosion mechanisms of the cohesive bank and bed remolded samples occur like in the field The simulation results of both around river mouth and in the river can reproduce the erosion both in the experiments and in the field It is found that numerical models for estimation of erosion resistance of cohesive bank in river and around river mouth can be applied to Soairap River (southern Vietnam) and other rivers with cohesive bank properties provided that the soil properties of the cohesive bank, such as sand-clay content, moisture content and so on, are given References [1] Ariathurai, R and Arulanandan, K., 1978 “Erosion Rates of Cohesive Soils,” Journal of Hydraulic Division, Proc of ASCE, Vol.104, HY2, pp 279-283 [2] 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 80 [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, Vol.1 (2008), pp 531-535 In Japanese [4] Couper, P., 2003 “Effects of silt-clay content on the susceptibility of river banks to subaerial erosion,” Journal of Geomorphology, Vol.56 (1-2), pp 95-108 [5] Deguchi Ichiro, Sawaragi Toru, 1988 “Effects of structure on deposition of discharged sediment around river mouth,” Proc of 21st International Conference on Coastal Engineering, Vol.2, pp 1573-1587 [6] Gaskin et al., 2003 “Erosion of undisturbed clay samples from the banks of the St Lawrence River,” Canadian Journal of Civil Engineering Vol.30 (2003), pp 585– 595 [7] Hanson, G J and K R Cook, 2002 “Non-vertical jet testing of cohesive streambank materials,” ASAE paper No.022119 [8] Hirose K., et al., 2002 “Geo-environmental research for Cangio mangrove forest - Vietnam” Proc of workshop on natural research on earth science in the south of Vietnam, orientation in the research and training in response to the aims of regional sustainable development Vietnam National University –HCMC and Committee of Earth Science, pp 235-246 [9] Japan Society of Civil Engineers (JSCE), 1971 “Hydraulics Formulae” The forth edition pp 163 [10] Sawaragi, T., 1995 “Coastal Engineering – Waves, Beaches, Wave-Structure Interaction”, ed T Sawaragi, Elsevier, Amsterdam [11] Yamada, H., 1957 On the highest solitary waves, Rep Res Inst Applied Mech Kyushu Univ., Vol.5, pp.53-67 81 Chapter Conclusions The erosion mechanism of cohesive bed, bank, and shore near river mouth and around newly developed coastal region is distinctly different from those of non-cohesive ones because of the irreversible processes The erosion processes of these regions become very complex objects and difficult to understand because of the nature of cohesive materials Therefore, this study has been carried out to understand the erosion mechanism of cohesive bed, bank, and shore clearly In Chapter 1, the background, research objectives, study areas and the outline of the study were introduced In Chapter 2, the effects of development around river mouth and shallow water region in the sea on coastal erosion in some Asian countries have been assessed Many coastal regions have been eroded because of sand extraction, construction of improper harbors, and so on In Kochi coast of Kochi Prefecture (Japan), around Niyodo river mouth, beach erosion took place at the rate up to 8.1 m/yr was calculated after sand extraction proscription from 1997 and lasting long Around Lap river mouth (Vung tau, Vietnam), beach erosion caused by sand extraction and sand dunes has the rate of 10 m/year was investigated In the southern coast of Thailand and the southern coast of Red River Delta (Vietnam), severe erosion also took place in several years In these regions, many mangrove areas have been cut down to construct shrimp ponds The shoreline retreat of some places were detected and analyzed by satellite images got the values of at least to 30 m/year The erosion mechanism of the beach in some parts was reproduced by numerical models based on one-line theory with proper results However, in these regions, only non-cohesive materials were concerned Therefore, Chapter and Chapter have studied more detail about the cohesive properties of river banks and materials around river mouth In Chapter 3, in the laboratory study, forty-nine remolded samples with different contents of sand, silt, clay, moisture, salinity were made and examined by using non-vertical submerged jet test device In situ experiments were conducted in the banks of Soairap River (southern Vietnam) The relationship between critical shear stress of cohesive soils and mixing rates of sand, silt and clay, moisture contents, salinity, consolidation, and vegetation were determined In both laboratory and in situ experiments, when the moisture contents increased, the critical shear stresses tent to decrease with different rates When the rates of silt-clay contents increase, the critical shear stresses also increased The critical shear stress is proportional to consolidation time and density of dead roots and leaves The critical shear stresses also increased rapidly when the salinity increased from 0% to 1% However, this trend was slowly when the salinity increased from 1% to 3% 82 In Chapter 4, the erosion resistance of cohesive bank in river and around river mouth was investigated Many cohesive remolded samples were also made and examined by using non-vertical submerged jet test device, flume experiments with unidirectional currents, wave basin experiments with effects of only waves, waves and wave-following currents, and waves and wave-opposing currents The erosion resistance of cohesive bank was also studied by using numerical models to calculate the applied shear stresses on artificial and real bed, bank and shore In flume experiments, the remolded samples were tested to determine the critical shear stress first, and then they were experimented to determine the erosion rates The erosion depth of remolded bank samples is 2~10 times greater than that of the bed samples These indicate that the erosion mechanism of river bank is different from that of river bed In the bank, mass erosion (aggregates) is the main erosion mechanism but the surface erosion is predominant in the bed The water surface fluctuation also increases the erosion rates of bank samples In wave basin experiments, many tests were carried out with different conditions The first test was done with the impact of only waves The second test was carried out with impact of waves and both wave-opposing currents and wave-following currents The appearance of dead roots and leaves increases the critical shear stresses of the remolded samples but also increases the erosion rates because the dead roots and leaves can disturb the samples when waves and currents attack Waves and wave-following currents can increase the erosion rate when compared with waves and wave-opposing currents Some other available materials and eroded materials in water such as dust and sand can be the exterior factors crashing the samples and cause erosion under wave breaking and wavecurrent interaction The erosion resistance of cohesive bank in river and around river mouth was also reproduced by numerical models According to numerical simulations on the applied shear stress around river mouth, the highest applied shear stress appeared on the left bank and bed of the channel in the case of the following current on the incident waves In the experiments, there are some cases where the critical shear stresses of the remolded samples are greater than the calculated maximum shear stress, for example 1.7 N/m2, but the samples were eroded It can be said that dead roots and leaves played an important role in weakening the samples because of their movement by waves and wave-current interaction In other cases, the calculated applied shear stresses are high enough to erode the bank and bed materials with low critical shear stresses These results coincide with the experimental results obtained in the wave basin experiments The calculated applied shear tress on cohesive bank in river also becomes large under the condition of incident waves coupled with the following current Therefore, it can be judged that the numerical model for predicting applied shear stress used in this study can apply to the case under the coexisting system of waves and relatively strong current As for the cohesive bank in Soairap River, a rough figure of the property of erosion resistance is shown by comparing measured critical shear stress and calculated applied shear stress under typical conditions Five eroded sites of Soairap River have the low critical shear stresses of a range from 0.075 N/m2 to 2.47 N/m2 and calculated applied shear stress under usual conditions becomes larger than those Therefore, they can be eroded by high waves and strong currents 83 It is found that numerical models for estimation of erosion resistance of cohesive bank in river and around river mouth can be applied to Soairap River (southern Vietnam) and other rivers with cohesive bank properties provided that the soil properties of the cohesive bank, such as sand-clay content, moisture content and so on, are given 84 ... 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. .. 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... 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