INVESTIGATION OF MECHANISM OF MORPHOLOGICAL CHANGES IN COASTAL ZONE AND STRUCTURAL SOLUTIONS FOR STABILIZATION APPLICATION FOR XUONG HUAN BEACH REGION IN NHA TRANG BAY

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

MINISTRY OF EDUCATION AND

TRAINING

MINISTRY OF AGRICULTURE 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

This dissertation is available at:

- The National Library

- The Library of Thuyloi University

INTRODUCTION

1 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

2 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

3 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

4 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

5 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

6 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

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 1 OVERVIEW OF MORPHOLOGICAL CHANGES IN COASTAL ZONE AND COASTAL COUNTERMEASURES FOR

STABILIZATION

1.1 Introduction

1.1.1 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 Overview of overseas research on the mechanism of coastal morphological changes and stabilization structures

1.2.1 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

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 Overview of Vietnam’s research on the mechanism of coastal morphological changes and stabilization structures

1.3.1 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

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 1

In this chapter the author has studied to clarify:

1 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

2 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

3 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 2 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)

1t

y

B C

(2.1)

xt

yy)DD(

, i B

Figure 2.17 Definition sketch for shoreline change calculation

2.4 Research approaches of coastal zone morphological change

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

Total nearshore current velocity is the combination of tidal current flow and tidal 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

non-2.4.2.1 Drifter buoy selection and design

a) Drifter buoy selection

After reviewing a number of drifting buoys design, the author has selected the drifting buoy proposed by Davis in 1985 and applying to determine the total nearshore current velocity measurement in coastal areas Nha Trang Bay

b) Drifter buoy design (Davis 1985)

Figure 2.20 X-type drifting buoy

configuration by Davis 1985 Figure 2.21 Drifting buoy design

2.4.2.2 Scenarios and field measurements

a) Scenarios: Based on the tidal characteristics and local flow conditions

b) Field measurements in the Cai estuary and coastal areas of Nha Trang Bay

Figure 2.1 Preparation, setup and deployment of drifting buoy in the

2.4.3 Coastal erosion and deposition by using the numerical simulation models

2.4.3.1 Introduction

Developing 3D hydrodynamic EFDC (Environmental Fluid Dynamics Code), open source model has built for Nha Trang Bay areas The model is then coupling with the SWAN wave model and the Lagrangian Particle Tracking models (Figure 2.24)

Figure 2.24 Flowchart of numerical model linkages

2.4.3.2 Model grid and boundary conditions

Figure 2.25 Model grid and boundary conditions

• Wind speed and direction

• Wave radiation shear…

• Boundary condition of discharge, water level…

• Atmospheric condition

• Bed sediment distribution

• Critical bottom shear stresses

EFDC MODEL

• Drifter configurations

• Initial condition

L.P.T MODEL

SEDIMENT TRANSPORT MODULE

• Total current velocity and its tracking

• Suspended sediment concentration

• Morphological changes

• Bed sediment distribution

• Critical bottom shear stresses

Dong Trang

Flow boundary

Tran Phu Bridge

Xuong Huan Area

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

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

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