MINISTRY OF EDUCATION AND MINISTRY OF AGRICULTURE TRAINING AND RURAL DEVELOPMENT THUYLOI UNIVERSITY NGUYEN VIET DUC INVESTIGATION OF MECHANISM OF MORPHOLOGICAL CHANGES IN COASTAL ZONE AND STRUCTURAL SOLUTIONS FOR STABILIZATION - APPLICATION FOR XUONG HUAN BEACH REGION IN NHA TRANG BAY Specialization: Hydraulic Engineering Code No.: 62-58-02-02 SUMMARY OF DOCTORAL DISSERTATION HANOI, 2016 The work was completed at: Thuyloi University Advisor 1: Assoc Prof Dr Nguyen Trung Viet Advisor 2: Prof.Dr People’s Teacher Nguyen Chien Reviewer 1: Prof Dr Tran Dinh Hoi, Institute for Water, Environment and Climate Change Reviewer 2: Assoc Prof Dr Nguyen Danh Thao, Ho Chi Minh City University of Technology Reviewer 3: Assoc Prof Dr Phung Dang Hieu, Vietnam Administration of Seas and Islands This dissertation will be defended at the meeting of the University Doctoral Committee in room No…………………on ……………… This dissertation is available at: - The National Library - The Library of Thuyloi University INTRODUCTION Rationale Vietnam with more than 3,260km of coastline, ranks 27th in the world, out of 157 countries adjacent to the sea Nha Trang Bay is one of the 29 most beautiful bays in the world, a center of tourism and services, with rapid economic growth of Khanh Hoa province in particular and the South-Central region in general Besides the strengths in tourism, Nha Trang beach area has currently some limitations: Beach narrowing, steep beach slope and seasonal variations; Large waves impact the nearshore Until now there is no study to fully identify the main mechanism of erosion-accretion processes, sediment transport and causes affecting the coastal zone changes Therefore, this research will focus on the hydrodynamic regime, erosion-accretion mechanisms, wave and current impacts to the coastal zone morphological changes, then based on these new findings to propose suitable technical solutions to protect and embellish the beach area of Nha Trang Bay coast effective, long-term stability Research objectives Describing and explaining the mechanism of seasonal accretion-erosion; clarifying the hydrodynamic regime, sediment transport mechanisms and other main influencing factors causing beach morphological changes; proposing and selecting the suitable structural solutions to upgrade the Nha Trang Beach area Scope of the Study Study on the hydrodynamic regime, erosion-deposition mechanisms, wave and total current impact to the coastal morphological changes in Xuong Huan Beach, Nha Trang Bay Research contents Literature review on the mechanism of morphological changes in coastal zone and coastal engineering works for beach stabilization; Scientific research proposing the suitable structural solutions to stabilize and further extend the Xuong Huan beach area, Nha Trang Bay Approach and study methods To achieve the objectives, the author carried out a literature review on the hydrodynamic modelling of wave and current as well as sediment transport, field surveys (Video-camera observation and drifter experiment to measure the total nearshore current); and practical application Scientific and practical significance 6.1 Scientific significance The drifter buoy method is introduced and successfully applied to study the hydrodynamic regime in the coastal area of Nha Trang Bay, the data is used for the calibration and validation processes of numerical models The thesis has clearly clarified sediment transport mechanisms, find out the main causes and assess the role of each hydrodynamic factor impacts the coastal morphological changes 6.2 Practical significance Successfully provide a suitable structural solution to stabilize and restore beaches in this study area in particular and any other eroded beaches in general New contributions Successful application the drifter buoy technique to measure the nearshore total current velocity; Clearly clarified the mechanism of erosion and accretion in the study areas and proposed appropriated countermeasures for the coastal stabilization; Dissertation contents In addition to the Introduction, Conclusions and Recommendations, this dissertation consists of 04 chapters as follows: Chapter 1: Overview of morphological changes in coastal zone and coastal countermeasures for stabilization Chapter 2: Scientific research basis on the mechanisms of coastal morphological changes Chapter 3: Investigation of the mechanisms of coastal evolution in Xuong Huan Beach, Nha Trang Bay Chapter 4: Investigation of the proposals of coastal structural solutions in order to stabilize the Xuong Huan Beach, Nha Trang Bay CHAPTER OVERVIEW OF MORPHOLOGICAL CHANGES IN COASTAL ZONE AND COASTAL COUNTERMEASURES FOR STABILIZATION 1.1 1.1.1 Introduction Coastal zone definition A coastal zone is the interface between the land and water These zones are important because a majority of the world's population inhabit such zones Coastal zones are continually changing because of the dynamic interaction between the oceans and the land and affecting by waves and tides The coastal zone consists of three components such as backshore, foreshore and inshore or shoreface 1.1.2 Scientific-Technology research issues on the coastal morphological changes The main scientific-technology research issues on the coastal morphological changes are morphological characteristics; coastal sediment distribution and classification; coastal zone hydrodynamic and sediment transport; and coastal protection structures 1.2 1.2.1 Overview of overseas research on the mechanism of coastal morphological changes and stabilization structures Research history on coastal zone evolution Research in the construction of coastal protection works, the port is always associated with the development of human civilization Some ancient ports are still exist until this day The Greek and Latin documents from Herodotus, Josephs, Suetonius mentioned the description of the coastal research Ancient researchers have well understanding the coastal dynamic processes such as the flow pattern of the coastal areas of the Mediterranean, prevailing winds and the effects of wind and waves Ancient Rome was the first to set up wind rose denotes inshore winds mode 1.2.2 Research on coastal zone evolution The study on coastal zone evolution have been concerned a long time ago Research achievements have clearly indicated that natural processes are clearly understood over time, the research findings have also been compiled and published in scientific articles or the book which are valuable for referencing in this study 1.2.3 Overview of research methods on coastal zone morphological changes Previously research methods coastal zone changes are made in the following order: Measurement of waves, currents, sediment concentration, then calculate sediment transport rate by using some semi-empirical formulas and assess changes in the topography of the studied area Current research methods are mainly used numerical simulation models together with the field data measurements 1.2.4 Overview of wave and current numerical simulation models The numerical simulation models are usually classified into four categories according to the its application domain as follows: deep water - the effect of the bottom is neglected; transitional region - area between deep water and shallow water; shallow water region - where the shallow water effect is important; and human construction works - need to take into account the interaction between waves on structures In addition, the models can be divided into two types: Phase and Phase-averaged models The common numerical models for the hydrodynamic and morphological changes are: GENESIS; UNIBEST, SOBEK 2D, Delft3D; MIKE; NPM, SMS; CEDAS, EFDC, FVCOM, TELEMAC-MASCARET, etc … In Vietnam, the MIKE, DELFT3D and EFDC models are popularly used 1.2.5 Overview of nearshore sediment transport formulas Based on field observation data, physical model experiments, theoretical analysis , scientists around the world have come up with many semi-empirical formula, each formula has a certain scope of application Some typical formula are: CERC Formula ; Kamphuis Formula 1.2.6 Overview of coastal protection and stabilization structures To protect and stable the shore, beach in the world is often used single or various combinations of the following basic technical solutions: sea dye structure; groins; breakwater; beach nourishment and mangrove planting 1.3 1.3.1 Overview of Vietnam’s research on the mechanism of coastal morphological changes and stabilization structures Research on coastal zone morphological changes Research on coastal zone morphological changes have been mainly carried out in the framework of national research programs in recent years, quite detail explanation of shoreline changes, coastal erosion and deposition along to Vietnam’s Coastline were described on the map of 1/250,000 and 1/100,000, has a preliminary explanation of causes to the problems, of which exogenous impact is the most important Research on coastal protection and stabilization structures In the past, the roof paving embankment constructions are used mainly as passive solutions Recent years, many other structural solutions have applied to prevent coastal erosion and wave reduction such as - groin, but these works only built as an experiment There are many studies on the estuaries and coasts stability, but it was only given a general technical solutions and dike structures Due to the complexity and local difference so each region should have its own research, so far still lack the technical guidelines on spatial layout of structures to prevent coastal problems 1.3.2 Previous researches on Nha Trang’s coastal zone There are quite many previous studies on Nha Trang coastal zone, but no study has yet enough systematic and reliable data to come up with the hydrodynamic regime, sediment transport mechanisms and evolution of coastal zone Recently, the Protocol Project between the Thuyloi University, Vietnam and the Institute for Research Development (IRD), France has been measuring in detail of bathymetry, bottom sediments, water level and current velocity The characteristic of hydrodynamic regime, suspended sediment transport have been studied quite well However, this project have not studied in depth the causes and mechanisms of coastal morphological changes, especially the role of waves and total neashore currents to the beach morphological changes 1.4 Dissertation’s research proposals This dissertation is aiming to combine the study of empirical field observation and numerical simulation for addressing following topics: describe and explain the mechanism of seasonal beach erosion and deposition; unravelling the hydrodynamic regime, sediment transport mechanisms and find out the main influencing factors causing beach morphological changes; proposing the best structural solutions to improve the Nha Trang Beach area 1.5 Conclusions of Chapter In this chapter the author has studied to clarify: Overview of overseas researches on the mechanism of coastal morphological changes and stabilization structures includes research history on coastal zone evolution; research on coastal zone evolution; overview of research methods on coastal zone morphological changes; overview of wave and current numerical simulation models; overview of nearshore sediment transport formulas and overview of coastal protection and stabilization structures Overview of Vietnam’s research on the mechanism of coastal morphological changes and stabilization structures; research on coastal zone morphological changes; research on coastal protection and stabilization structures; previous researches on Nha Trang’s coastal zone This dissertation is aiming to address the mechanism of seasonal beach erosion and deposition; wave impacts; unravelling the hydrodynamic regime, sediment transport mechanisms and find out the main influencing factors causing beach morphological changes based on the detail and reliable measurement data sets; proposing the best structural solutions to effectively protect and the long-term stability of Nha Trang Beach area CHAPTER SCIENTIFIC RESEARCH BASIS ON THE MECHNISMS OF COASTAL MORPHOLOGICAL CHANGES 2.1 Main factors causing Nha Trang beach morphological changes The Nha Trang Beach morphological changes are the result of interactions between endogenous , exogenous and human activities factors The analysis of key factors are included topography, geomorphology; distribution of sediment; Cai river discharge; tidal regime; wave impacts; current flow regime in Nha Trang Bay and human activities Among these the steep beach slope causes the large waves can approach the nearshore area In the rainy season, Northeast wind-waves pass through the northern coral area does not carry sediment to the south causing beach erosion in Xuong Huan area In contrast during the dry season, Southeast waves bring sediment from Southeast Nha Trang Bay Islands to deposit along Tran Phu beach, the distribution of fine sand particles gradually decrease from south to north indicating this trend East waves cause longshore sediment movement and maintain the beach slope The sediment from freshwater discharge of Cai river has a certain influence on the amount of sediment along the adjacent beach through the rainy season Thus, the preliminary analysis shows that the main causes of seasonal beach erosion and deposition changes are due to the impact of the waves and topographic conditions of this area 2.2 Data collection The main data sets are using in this dissertation are: bathymetry, water level, river discharge which were collected from the Protocol Project between the Thuyloi University and the Institute for Research Development (IRD) in May and December 2013 The author conducted a field measurement of total current velocity by using the drifter technique in November 2015 2.3 One-line model theory The one-line model is used the simulate the longshore sediment transport rate in the study area The model was first developed by Pelnard-Considere (1956) Figure 2.17 The governing equations for the longshore sediment transport are shown in Equations (2.1) and (2.7) y  Q    q  t (D C  D B )  x   y ,i  y i  Q  (D C  D B ) x  t  (2.1) (2.7) Figure 2.17 Definition sketch for shoreline change calculation 2.4 2.4.1 Research approaches of coastal zone morphological change Seasonal variation of shoreline changes by video-camera analysis In the framework of the Protocol Project between the Thuyloi University and the Institute for Research Development (IRD) entitled as "Study on hydrodynamic regime and sediment transport in estuarine and coastal zone of Nha Trang Bay, Khanh Hoa Province ", a real-time monitoring system of video cameras were deployed to observed the shoreline changes from March 2013 The photos are continuous taken and uploaded to the data server, then the shoreline change can detected by averaged out all photos in every 10 minutes The dissertation are used these image data sets and shoreline position that detected from the averaged-image results to investigate the seasonal changes of Nha Trang beach 2.4.2 Study on the total nearshore current velocity by drifting buoy method Total nearshore current velocity is the combination of tidal current flow and nontidal current flow such as wave-induced current Clarify this total nearshore current velocity will be able to determine the trend of sediment movement in the coastal zone Therefore the author used the drifting buoy method to measure the total current velocity 10 2.4.3.3 Model calibration with the measurement data in May 2013 The model was running for entire year of 2013 data sets and export the model results in May to compare with the measured data The model comparison results of current velocity at the Station A and Station B data shown a very good agreement The Nash index of water level comparison was reached to 96% of accuracy, it is indicated that the model were well calibrated 2.4.3.4 Model validation with the measurement data in December 2013 The measured data in December 2013 was used to validate the model again The validation results of current velocity at the Station A and Station C was comparable to the measured data The Nash index of modeled water level and measured water level at the Station A and Station C were 95% and 93% of accuracy, respectively This demonstrated that the calibrated model was good enough for applying to the study area 2.4.3.5 SWAN model calibration and validation a) SWAN model calibration with the measurement data in May 2013 The comparison of modeled wave height and wave period were in order of magnitude with the measured data The BIAS and RMSE errors were 0.23m and 0.2m, respectively b) SWAN model validation with the measurement data in December 2013 The comparison results between wave height and wave period at the Station A in December 2013 showed a better fit than the calibrated results during May 2013 due to the northeast wave was large and stability The BIAS and RMSE errors were reached to 0.13m and 0.18m, respectively 2.4.3.6 Total current velocity validation with the drifting buoy measurement From November 26, 2015 to November 30, 2015, the author has conducted a field survey to measure the total trajectory and velocity by using the drifting buoy method The survey was also trying to measure all the hydrodynamic factors at the same time with drifting buoy experiments such as river discharge, water level, tidal level as well as wind speed and direction traffic, water, offshore wave and wind parameters The compatison results of water level and total trajectory and velocity shown a good agreement to the measured data 13 500 1000 1500 2000 WRU-S02 Mo hinh Do dac WRU-S04 WRU-S05 500 1000 1500 Khoang cach (m) 2000 Van toc (m/s) WRU-S03 0.88 0.66 0.44 0.22 0.81 0.54 0.27 0.00 0.51 0.34 0.17 0.00 0.81 0.54 0.27 0.00 0.78 0.52 0.26 0.00 0.81 0.54 0.27 0.00 0.81 0.54 0.27 0.00 100 200 300 400 500 Mo hinh Do dac WRU-S01 500 1000 1500 0.75 0.50 0.25 WRU-S06 WRU-B12 0.00 0.88 0.66 WRU-S07 WRU-S08 WRU-S09 WRU-S10 Total current toc (m/s)(m/s) Vanvelocity 0.44 0.22 WRU-B13 0.60 0.45 0.30 0.15 WRU-B14 0.56 0.42 0.28 0.14 WRU-B15 0.78 WRU-S11 0.26 0.00 100 200 300 400 Khoang cach (m) Figure 2.42 Model versus data of drifter buoy tracking comparison 500 Model Mo hinh Do dac Data 0.52 WRU-B16 500 1000 1500 Distancecach (m)(m) Khoang Figure 2.43 Model versus data of drifter buoy total velocity comparison As a result, the 3D EFDC hydrodynamic model which was coupled with the SWAN model and Particle Tracking Lagrangian module has been successfully calibrated and validated with the detail measurement data sets The model results have shown a very good comparison to the data and this model is good enough to apply for the study areas 2.5 Conclutions of Chapter In this Chapter 2, many different scientific research methods have been carried out to investigate the hydrodynamic and coastal zone morphological changes The following are the main achieved results: The full detail databases were successfully compiled for studying the coastal zone morphological changes and coastal stability structures for Nha Trang Beach Successful application the drifter buoy technique to measure the nearshore total current velocity; The 3D EFDC hydrodynamic model which was coupled with the SWAN model and Particle Tracking Lagrangian module has been successfully calibrated and validated for investigating the nearshore hydrodynamic and beach morphological changes in the coastal areas in Nha Trang Bay 14 CHAPTER INVESTIGATION OF THE MECHANISMS OF COASTAL EVOLUTION IN XUONG HUAN BEACH, NHA TRANG BAY 3.1 Analysis of accretion-erosion mechanism of the beach by means of camera equipments 3.1.1 Data analysis by means of camera equipments In nearly two years of operation (2013-2014) with a duration of 10 minutes per photo, the camera equipment obtained thousands of photos of the beach in the study area adjacent to Hotel 378 with an approximate length of 300 meters By means of comparing the relative position of the concrete revetment route with the coastline, we can recognize the accretion or erosion of the beach From this database, featured photos were extracted for the purpose of observation and analysis With these photos, the qualitative variations (expansion shrinking) on a seasonal basis of the beach in the study area can be identified 3.1.2 Analysis of top view variation of the beach in the study area 3.1.2.1 Coastline variation in the study area Figure 3.2 Top view variation of the beach in the study area In the study area, the erosion of the beach occurs during the north-east monsoon period, while the shoreline accretion occurs during the rest This shows that the shoreline has a seasonsal variation every year 3.1.2.2 Variation mechanism of the beach width The analysis of beach width variation is conducted as shown in Figure 3.4 Beach width in the area near Hotel 378 (zone 1, x < 70 m) is greater than 30 meters, and decreases southwards (zone 4, x > 300m), which is only about meters 15 Bồi mạnh Xói mạnh Vùng Figure 3.4 Variation of the beach width at some characteristic cross-sections Based on the above analyses, it can be seen that the beach width in study areas changes on a seasonal basis with sinusoidal period and amplitudes corresponding to the bathymetry variation Accretion process mainly occurs in the period from May to August and erosion from October to January of next year 3.2 3.2.1 Calculation of longshore sediment transport in terms of quantity and direction Calculating the quantity of longshore sediment transport by means of PelnardConsidere emperical formula with the dataset obtained from camera equipments The method of calculation of longshore sediment Pelnard-Considere is presented in Section 2.3 of Chapter In formula (2.7), the temporal variable value of shoreline position (Δys/Δt) is determined by means of image analysis techniques from camera surveillance system From formula (2.7), with continuous data sets of coastline from May 2013, Qx value over time can be determined and is indicated by the diagram of longshore sediment transport as shown in Figure 3.6 50 100 150 200 250 300 Y13-06-28 Y13-07-31 Y13-08-26 Y13-09-29 Y13-10-31 Y13-11-28 Y13-12-30 Y14-01-30 Y14-03-02 Q (m3/ngày) Q (m3/ngay) -1 -2 -3 -4 -5 50 100 150 200 250 300 Khoảngcach cáchdoc dọcbo bờ(m) (m) Khoang Figure 3.6 Diagram for the determination of the longshore sediment discharge in months 16 3.2.2 Determining the direction of longshore sediment transport corresponding to the combined current regime By means of drifting buoy method as described in Section 2.4.2 of Chapter 2, using the self-made ones based on the prototype of Davis in 1985, the drifting orbits of the buoys in the study area was measured in the period from 25th to 30th November 2015 The result shows that the combined littoral currents is from north to south as shown in Figure 3.9, with the velocities shown in Table 3.2 Figure 3.9 Results of orbit measurements of combined littoral currents in the study area Table 3.2 Measurements of combined littoral currents (m/s) Buoy name WRU-B 12 WRU-B 13 WRU-B 14 WRU-B 15 WRU-B 16 3.3 3.3.1 Maximum velocity Vmax (m/s) 0.83 0.83 0.56 0.83 0.56 Minimum velocity Vmin (m/s) 0.00 0.08 0.14 0.08 0.11 Average velocity VTB (m/s) 0.21 0.45 0.47 0.47 0.44 Study on the variation of coastal bathymetry in Xuong Huan by means of numerical model Hydrodynamic regime in the study area The simulation results in the south-west monsoon period are shown in Figure 3.12 and Figure 3.13 It can be seen that in this period the combined littoral currents on both sides of Cai estuary are northward The currents from Cai river have minor impacts on the coastal zone of Xuong Huan During the north-east monsoon period, the simulation results shows that the littoral currents on both sides of Cai river are southward as can be seen in Figure 3.14 and 17 Figure 3.15 The currents from Cai River have a major impacts on the combined littoral currents in Xuong Huan as shown in Figure 3.14 (d) (a) (b) (d) (c) Figure 3.12 Effects of currents from Cai River on the study area in south-west monsoon season 23/5/2013 đến 02/6/2013 23/5/2013 đến 02/6/2013 Vận tốc (m/s) Vận tốc (m/s) 23/5/2013 đến 02/6/2013 Vận tốc (m/s) 23/5/2013 đến 02/6/2013 Vận tốc (m/s) 23/5/2013 đến 02/6/2013 Vận tốc (m/s) Figure 3.13 Current rose in the study area in south-west monsoon season (a) (b) (d) (c) Figure 3.14 Effects of currents from Cai River on the study area in north-east monsoon season 18 3/12/2013 - 13/12/2013 Vận tốc (m/s) 3/12/2013 - 13/12/2013 Vận tốc (m/s) 3/12/2013 - 13/12/2013 Vận tốc (m/s) 3/12/2013 - 13/12/2013 Vận tốc (m/s) 3/12/2013 - 13/12/2013 Vận tốc (m/s) Figure 3.15 Current rose in the study area in north-east monsoon season 3.3.2 Combined residual littoral currents in the study area In order to study the flow regime in the coastal area, a cross section of 700m wide was set up for investigation in the study area The simulation results shows that the dominant current direction is northwards during the southwest monsoon from April to September and southwards during the northeast monsoon from October to March in the next year 750 Legend Lưu lượng qua MCN/giờ Cat Ngang LưuMat lượng TB ngày Trung Binh Ngay dư 3(m chảy (m dòngchay Lưu /s) /s) Luu lượng luong dong 625 500 375 250 125 -125 -250 -375 Tháng -500 Jan-13 Feb-13 1/2013 2/2013 Mar-13 3/2013 Apr-13 4/2013 May-13 5/2013 Jun-13 Jul-13 6/2013 7/2013 Date Aug-13 8/2013 Sep-13 Oct-13 Nov-13 Dec-13 9/2013 10/2013 11/2013 12/2013 Figure 3.17 Residual flow through the cross section in 2013 3.3.3 Results of study on the impacts of waves and combined littoral currents on the beach in the study area 3.3.3.1 Simulation results of top view bed shear stresses distribution 19 The simulation results shows that the total bed shear stress during the north-east monsoon season is much greater than those during south-west monsoon season; and the largest distribution is found in Cai estuary and decreases southwards Figure 3.18 Total bed shear stress distribution during north-east monsoon season 3.3.3.2 Figure 3.19 Total bed shear stress distribution during south-west monsoon season Simulation results of bed shear stresses distribution at characteristic points The variations of total bed shear stresses, induced only by waves and only by currents over time are extracted at survey points (Figure 3.1) Figure 3.1 Survey points for bed shear stresses 20 Table 3.1 Wave-induced and combined current-induced bed shear stresses (N/m2) Bed shear stresses in NE monsoon Min Average Max 0.02606 2.59335 11.04081 0.00107 1.50541 7.04616 0.00041 0.58920 3.47106 0.18818 6.98954 24.69806 0.00037 1.24569 6.05634 0.00068 0.51585 3.08132 0.27523 8.19320 29.98253 0.00008 1.25419 6.07702 0.00085 0.46295 2.82096 Point No Point 01 Point 02 Point 03 Point 04 Point 05 Point 06 Point 07 Point 08 Point 09 Bed shear stresses in SW monsoon Min Average Max 0.00018 0.84029 6.14089 0.00006 0.07451 0.62306 0.00014 0.05706 0.48170 0.00006 0.55704 4.03345 0.00006 0.04720 0.41821 0.00006 0.05644 0.43925 0.00008 0.62912 4.65869 0.00013 0.04910 0.41629 0.00002 0.06002 0.42575 5.7 3.8 1.9 0.0 0.33 0.22 0.11 0.00 0.45 0.30 0.15 0.00 3.6 2.4 1.2 0.0 0.219 0.146 0.073 0.000 0.42 0.28 0.14 0.00 4.5 3.0 1.5 0.0 0.222 0.148 0.074 0.000 0.42 0.28 0.14 0.00 2013-05-22 22/5/2013 Ứng suấtdo dodong dòngchay chảy Ung suat Ung suat Ứng suấtdo dosong sóng Diem 01 01 Điểm Diem 02 02 Điểm Diem 03 03 Điểm Diem 04 04 Điểm Diem 05 05 Điểm Diem 06 Điểm 06 Điểm Diem 07 Diem 08 Điểm 08 Điểm Diem 09 2013-05-26 2013-05-30 26/5/2013 30/5/2013 Thoi gian Thời gian 2013-06-03 03/6/2013 Figure 3.30 Variations of bed shear stresses during SW monsoon season 3.3.4 Ung suat (N/m2) đáy(N/m2) suất day Ứng Ung suất suat day (N/m2) đáy(N/m2) Ứng Ứng suấtdo dodong dịng chảy Ung suat chay Ung suat Ứng suấtdodosong sóng 8.7 5.8 2.9 0.0 2.52 1.68 0.84 0.00 22.2 14.8 7.4 0.0 5.1 3.4 1.7 0.0 2.19 1.46 0.73 0.00 25.2 16.8 8.4 0.0 4.6 2.3 0.0 1.8 0.9 0.0 Diem 01 01 Điểm Diem 02 02 Điểm Diem 03 03 Điểm Diem 04 04 Điểm Diem 05 05 Điểm Diem 06 Điểm 06 Điểm Diem 07 Diem 08 08 Điểm Điểm Diem 09 2013-12-02 02/12/2013 2013-12-06 2013-12-10 06/12/2013 10/12/2013 Thoigian gian Thời 2013-12-14 14/12/2013 Figure 3.31 Variations of bed shear stresses during NE monsoon season Simulation results of accretion-erosion situation in the study area Simulation results of accretion-erosion situation of the coastal zone in the study area in 2013 are shown in Figure 3.32 The accretion process has the greatest intensity from May until reaching the maximum amount of aggradation in August Then an erosion zone is found at the location of Y-shaped revetment near Hotel 378 It extends southwards until the end of December 21 Bồi xói (m) -0.3 02/2013 0.5 Bồi xói (m) -0.3 03/2013 0.5 Bồi xói (m) -0.3 05/2013 0.5 Bồi xói (m) -0.3 06/2013 0.5 Bồi xói (m) -0.3 08/2013 0.5 Bồi xói (m) -0.3 07/2013 0.5 Bồi xói (m) -0.3 09/2013 0.5 Bồi xói (m) -0.3 12/2013 0.5 Figure 3.32 Variations of the coastal bathymetry in the study area in 2013 3.4 Conclusion of Chapter In this chapter, the following issues have been clarified: (i) quantitative description and explanation of seasonal accretion-erosion mechanism in the study area, (ii) clarification of the hydrodynamic regime and sediment transport mechanism in the study area (iii) clarification of the main cause and determination in detail of the impacts of each factor: waves, combined littoral currents on the variation of the bathymetry (iv) in order to limit the variation of the coastal bathymetry and to replenish the beach in the study area, structural measures must be introduced to reduce the impact of east and north-east waves on the beach, to prevent the southward longshore sediment transport in winter and retain the sediment from the south during the summer 22 CHAPTER INVESTIGATION OF THE PROPOSALS OF COASTAL STRUCTURAL SOLUTIONS IN ORDER TO STABILIZE THE XUONG HUAN BEACH, NHA TRANG 4.1 Present situation of the study area and objectives of the replenishment 4.1.1 Present situation of the beach in the study area The replenished beach is located at the southward side of Yersin revetment, from Hotel 378 in north-south direction along Tran Phu street to Tran Phu – Yersin junction The beach has a frequently seasonal variation In winter, the beach is eroded severely and becomes narrower with a width of 10 meters with a slope of 2% In summer, the beach is restored with an approximate width of 40 meters with a gentler slope of 2% In addition, because the beach is too narrow and steep in winter, the wave have significant impacts on the coastal edge and the promenades, endangering the swimmers and causing instability for coastal structures 4.1.2 Objectives of the replenishment The objectives of the beach replenishment are to limit the erosion, restore and increase the width, lower the steepness of the beach in the study area It must ensure the aesthetic requirements, be environmentally friendly and have no adverse effects on the adjacent areas 4.2 Scientific bases for the proposals According to the results of experimental studies and numerical simulations in Chapter 3; Vietnamese Standard TCVN 9901: 2014 Irrigation works - Requirements of sea dike design and Chapter of the Project - Preliminary design of structures to renovate and upgrade the beach at the location of Khanh Hoa Provincial People's Committee 4.3 Proposals of master plans for sand-retaining and wave-attenuating structures The proposal is to build a breakwater for the attenuation of north-east waves at the start point of the beach A system of offshore submerged breakwaters which are parallel to the shoreline is introduced in order to attenuate the waves from the east, transport the sediments induced by south-east waves onto the beach and form the tombolos; together with the groynes, they can retain the sediment transport induced by the littoral currents during the north-east monsoon period southwards 4.4 Evaluation of technical efficiency of the layouts for structural measures 4.4.1 Evaluation methods The technical efficiency of the layout plans for countermeasures is assessed by means of simulations conducted by the calibrated and verified numerical model introduced in 23 Chapter on the basis of effects on wave attenuation and aggradation 4.4.2 Bathymetry changes corrensponding the layout plans for coastal structures 4.4.2.1 Simulation results of top view accretion-erosion The results of image extraction for the study of accretion-erosion in the study area after 12 months (Figure 4.10) shows the 5th altenative has the best efficiency in aggradation as a result of the southward groyne located at the end point of the system in addition to the northward groyne and offshore submerge breakwaters, which hinders the combined littoral currents and retains the sediments in the study area Figure 4.1 The accretion-erosion situation in the study area after 12 months corrensponding to various layout alternatives 4.4.2.2 Comparison of the simulation results of accretion-erosion corresponding to different cross-sections of the alternatives The cross sections were selected in order to extract the results of bathymetry variation in 2013 to find out the impacts of different structural measures on the accretion-erosion processes in the study area The simulation results show that alternative no has the best efficiency 4.5 Conclusion of Chapter In this chapter, the structural solutions has proposed for the purpose of stabilizing and replenishing the beach in the study area It includes the evaluation of the effectiveness of the proposed alternatives and the identificaton of the most efficient one The selected structural solution is a combination of submerged and semi24 submerged sand-retaining and wave-attenuating structures, and as a result it has no significant impacts on the landscape of the beach CONCLUSIONS AND RECOMMENDATIONS I The obtained results From the results of overview study, litterature review, field observations and numerical modelling, the dissertation has achieved the following outcomes Overview of the study on mechanism of coastal zone evolution and the structural solution for beach stabilization Detailed research on typical results as well as the universal methods and procedures for the studies on the mechanism of coastal zone evolution The common numerical models have been applied in order to simulate the flow and the propagation and transformation of waves Longshore sediment transport and structural solutions for coastline stabilization, beach replenishment have also been studied as well Thereby, the problem was identified and selected; in addition suitable approaches and methodology for the study were determined as well Clarifying the scientific bases for the study of the mechanism of coastal zone evolution by means of field observations and numerical models i) Synchronous adoption of many modern, advanced and specialized devices and monitoring methods The drifting buoy technology has been applied successfully in Vietnam for the first time in order to study the combined littoral currents, which opens up the opportunity for the affordable adoption in large study areas in the conditions of Vietnam ii) Selecting and applying the open source hydrodynamic model EFDC successfully coupled with the wave propagation model SWAN and the Lagrangian particle model for the simulation of combined flows into an integrated tool, which can be applied to study the hydrodynamic regime, the longshore sediment transport mechanisms and the bathymetry change in the study area Quantification of coastal zone evolution in the study area under the impacts of waves and combined littoral currents Qualitative description and interpretation of the seasonal accretion-erosion mechanism of the beach were given, in addition to the clarification of the hydrodynamic regime 25 and longshore sediment transport mechanisms as well as the main cause, and identification of the impacts of each factor in detail: waves, combined littoral currents on the bathymetry change Thereby, a scientific basis is provided to propose the structural solutions for beach stabilization in the study area Proposing the structural solutions for the stabilization and replenishment of the beach in the study area Structural solutions for beach stabilization and replenishment in the study area have been proposed The evaluation of effectiveness of various alternatives was conducted by means of an integrated hydrodynamic model which is calibrated and verified in Chapter 2; as a result, the most effective alternative is identified thereby The selected structural solution is a combination of submerged and semisubmerged sand-retaining and wave-attenuating structures, and as a result it has no significant impacts on the landscape of the beach II Limitations and recommendations for future research Limitations Not including all the harsh natural conditions affecting the study results Not taking the cross-shore sediment transport into consideration The duration for monitoring the combined littoral currents in the field is not long enough The drifting buoys have not been calibrated by means of the Acoustic Doppler Current Profiler (ADCP) Recommendations for future research Further studies should be conducted in order to perfect the technology and to establish a procedure for the application of drifting buoys for the researches on combined littoral current, providing firm bases for the calibration and verification of numerical models applied in the simulation of combined littoral currents in Vietnam Continuing further studies on the selected structural solution in order to proceed to the realization of the alternative Expanding the scope of research in order to apply the methods and set of tools used for the other locations in Nha Trang Gulf as well as the eroded areas along the coast in Central Vietnam 26 LIST OF PUBLICATIONS N.V Duc, N.X Tinh, N.T.Viet and B.M Hoa (2015) Investigation of Hydrodynamic Regimes for Nha Trang Bay using the 3D Open-source EFDC Model Proceedings of Vietnam-Japan Workshop on Estuaries, Coasts and Rivers, 7-8 September 2015, Hoi An, Vietnam, pp 147-151 N.V Duc, N.T Viet, D.H Thuan, D.V Hung, N.T Binh, Almar R., Lefebvre J-P (2014) Evaluation of Long-term Variation of Intertidal Topography of Nha Trang Beach Based on High Frequency Video Processing Proceedings of the 19th IAHRAPD, 21-24 September 2014, Hanoi, Vietnam N.V Duc, N.T Viet, D.H Thuan, H Tanaka, N Chien (2014) Study on hydrodynamics of Nha Trang bay by using FVCOM model Proceedings of the 19 th IAHR-APD, 21-24 September 2014, Hanoi, Vietnam N.T Viet, N.V Duc, L.T Binh, D.H Thuan, T.T Tung, N.V Thin, D.V Uu, Almar R., Lefebvre J-P and H Tanaka (2014) Seasonal Evolution of Shoreline Changes in Nha Trang Beach, Vietnam Proceedings of the 19th IAHR-APD, 21-24 September 2014, Hanoi, Vietnam N.T Viet, N.V Duc, V.C Hoang, H Tanaka, D.V Uu, T.T Tung, Almar R., Lefebvre JP (2014) Investigation of Erosion Mechanics of Nha Trang Coast, Vietnam Proceedings of the 19th IAHR-APD, 21-24 September 2014, Hanoi, Vietnam Almar R., Bonneton N., Bonneton P., Lefebvre J-P, D.V Uu, N.T Viet, L.T Binh, N.V Duc (2014) Swash Hydro-Morphodynamics at a Low-Tide Terrace Beach during Post-Typhoon Recovery Period, Nha Trang Bay, Vietnam Proceedings of the 19th IAHR-APD, 21-24 September 2014, Hanoi, Vietnam N.T Viet, N.V Duc, V.C Hoang, H Tanaka (2014) Study on mechanism of coastal erosion in Nha Trang Bay Proceedings of National Fluid Mechanics 2014 N.T Viet, N.V Duc, V.C Hoang, Tanaka H (2014) Study on Seasonal Erosion of Nha Trang Coast, Vietnam Journal of Coastal Engineering, JSCE, Vol 70, pp 1456-1460 Lefebvre J.P, Almar R., N.T Viet, D.V Uu, D.H Thuan, L.T Binh, R Ibaceta, N.V Duc (2014) Contribution of the swash generated by low energy wind waves in the recovery process of a beach impacted by extreme events: Nha Trang, Vietnam Journal of Coastal Research, Special Issue 70, pp 663-668 10 L.T Binh, N.V Duc, N.T Viet, D.H Thuan, N.V Thin, T.T Tung, Đ.V Uu, R Almar, J.P Lefebvre H Tanaka (2013) Application of VIDEO-CAMERA technique to analyze the impact of Nari Storm Nari (No 11) to the shoreline changes in Nha Trang beaches Journal of Water Resources and Environmental Engineering, Special Issue on the 10 Year Anniversary of Faculty of Marine and Coastal Engineering, pp 81-88 11 L.T Binh, N.V Duc, N.T Viet, D.H Thuan, N.V Thin, T.T Tung, Đ.V Uu, R Almar, J.P Lefebvre (2013) Some initial findings on the shoreline changes of Nha Trang Beach by the image mornitoring and analyzing technology Proceedings of Annual Scientific Conference 2013, Thuy Loi University (ISBN: 978-604-82-00664) Pp.179-181 27