Modelling coastline evolution using one line model for hoi an quang nam

THUY LOI UNIVERSITY UNIVERSITY OF LIEGECAO DUC HAI MASTER THESIS MODELING COASTLINE EVOLUTION USING ONE-LINE MODEL FOR HOI AN, QUANG NAM Major: Sustainable Hydraulic Structures Coas

THUY LOI UNIVERSITY UNIVERSITY OF LIEGE

CAO DUC HAI

MASTER THESIS

MODELING COASTLINE EVOLUTION USING ONE-LINE MODEL FOR

HOI AN, QUANG NAM

Major: Sustainable Hydraulic Structures

Coastal Engineering and Management

Supervisor: Ass Prof Dr.NghiemTien Lam

Prof Rigo Philippe

Hanoi, 2016

ACKNOWLEDGEMENT

This master course has been supported by the cooperation project between University of Liege and ThuyLoi University

First and foremost, I would like to thank to all members in my big family, specially

my father, my mother, grandfather, grandmother and my brother, which they help and promote me a lot when I had got trouble in the master course

Next I really world like to show my gratitude to my professor in Vietnam, Assoc Prof Nghiem Tien Lam; also professor at University of Leige, Prof Rigo Philippe during master research, who gives me the direction in study

In research, special thanks to Dr Nguyen Quang Chien for supporting to fix mistakes in programming open source code, and Assoc Prof Dr Vu Minh Cat for sharing data and material about Hoi An I am deeply grateful to him about his enthusiasm

Last but not least, I want to send my thanks to my site manager, Duong Minh Tinh, and all leaders in Phuc Hung Holdings Construction Joint Stock Company that they give opportunities to complete my master thesis on time

Hanoi, August 2016

Author

Cao Duc Hai

CHAPTER 1 : INTRODUCTION 1

1.1 Study area 1

1.2 Problem definition 1

1.3 Literature review 3

1.4 Study objectives 4

1.5 Approach and methodology 4

1.6 Thesis structure 6

CHAPTER 2 : PHYSICAL CHARACTERISTICS 8

2.1 Introduction 8

2.2 Geographic conditions 8

2.2.1 Mountain topography 8

2.2.2 Hilly topography 9

2.2.3 Delta topography 9

2.2.4 Sandy coastal topography 9

2.3 Topographic conditions 9

2.4 Climatic conditions 10

2.4.1 Wind 10

2.4.2 Temperature 11

2.4.3 Humidity 11

2.4.4 Evaporation 11

2.4.5 Rainfall 12

2.5 River basin and river network 13

2.5.1 River basin 13

2.5.2 River network 14

2.6 Observation stations and hydrologic conditions 16

2.6.1 Observation stations 16

2.6.2 Hydrologic conditions 17

2.7 Oceanographic conditions 19

2.7.1 Tides 19

2.7.2 Wind waves 20

2.7.3 Currents 22

2.8 Sediment 23

2.8.1 River sediment transport 23

2.8.2 Coastal sediment transport 23

2.9 Historical evolution of the coastline 25

2.10 Conclusions 27

CHAPTER 3 : APPLICATION OF GENERIC COASTLINE MODEL 28

3.1 Introduction 28

3.2 Theoretical background 28

3.2.1 One-line theory 28

3.2.2 Fundamental equations 30

3.2.3 Limitations 32

3.2.4 Sediment transport formulae 32

3.3 Model setup 34

3.3.1 Input data 34

3.3.2 Model settings 36

3.3.3 Computation of coastline evolution 38

3.4 Model calibration 40

3.4.1 Bed roughness 40

3.4.2 Grain diameter 46

3.4.3 Wave breaking parameters 53

3.5 Conclusions 55

CHAPTER 4 : PREDICTION OF EVOLUTION FOR HOI AN COASTLINE 56

4.1 Introduction 56

4.2 Zero-option 56

4.2.1 Scenario 1 56

4.2.2 Scenario 2 57

4.3 Coastal evolution in other coastal protection solutions 59

4.3.1 Submerge breakwaters 59

4.3.2 Immersed breakwaters 60

4.3.3 Groins 61

4.3.4 Conclusions 63

CHAPTER 5 : CONCLUSIONS AND RECOMMENDATIONS 64

5.1 Conclusions 64

5.1.2 Recommendations 65

References 66

List of figure 68

List of table 70

Appendix A 71

Appendix B 85

CHAPTER 1 : INTRODUCTION 1.1 Study area

The Cua Dai beach and the ancient town of Hoi An are located in Quang Nam province in the middle of Vietnam They play great important roles in economic development of Quang Nam province, Vietnam Hoi An city is located in the Thu Bon River Basin with 10035 km2 of total area and approximately 120,000 inhabitants Its ancient town is recognized as a UNESCO World Heritage site in Vietnam Quang Nam province shares the border with Thua Thien-Hue province in the north, Quang Ngai province in the south, Kon Tum province and the Laos in the west It is a strategic important position that can cover and monitor almost of the marine routes in the East Sea

Figure 1-1 Map of study area (Viet and Tanaka, 2015)

1.2 Problem definition

Over last decade, coastal erosion in Cua Dai Beach has been happening severely and quickly, causing a significant retreat of the coastline The Hoi An beach is lost

day by day On average, the Hoi An coastline is eroding 12m per year So a great number of resorts, restaurants and houses are either destroyed or threatened Therefore, the number of tourists visiting Hoi An is significantly decreasing, losing the great amount of money that invested to build infrastructure along coastline

In Hoi An, tourism accounts for 64% of the total municipal revenue, and it is also the province’s main source of income In 2013, Quang Nam province’s tourism earned up to 170 million USD, which is approximately 10 per cent of the gross provincial product (Vietnam News, 2013) Hoi An also contributes about 40 million USD (UNESCO, 2008) Almost local inhabitants live on tourism, hence they are affected by coastal erosion on their livelihood

In the world, climate change is a global problem This could also influence on the coastal area of Hoi An According to ASEC Consultants (2014), sea level rise around Hoi An will be 5mm per year They predicted that a quarter of the Hoi An area will be regularly flooded in 2020 Beside, typhoon tends to move toward the south along the country’s coastal line The combination between extreme events, such as storm surge, tide and sea level rise leads to more severe coastal erosion According to Marcel Stive (2015), there is a difference in sediment volume between the loss of sediment on the northern beaches and the gain of sediment on the southern beaches He claimed that the newest situationin 2015 is shown in Figure 1-

2 in Hoi An They illustrate part of the morphology situation that happened there

Figure 1-2 Erosion on study area (Hoi An Project)1

Thus, the study area needs to assess the problem in a careful way to support the decision making process for an effective solution

1.3 Literature review

Long-term prediction of coastal erosion can be done based on one-line model theory This research is intended to apply this kind of model to study the coastal erosion in Hoi An and propose the solution to cope with it

One of the commonly used one-line model is GENESIS (Hanson and Kraus, 1989) GENESIS can simulate successfully the evolution of coastline being make up a great volume of fine-grained sediment transport Chantal et al (2004) compared equilibrium beach profiles which the processes in surf zone do not affect and affect

to calculate the deviation of fine material The continuity equation of sediment transport in GENESIS was adjusted represent the volume of these losses Existing dykes in Hai Hau, Nam Dinh province was simulated in GENESIS as seawalls that erosion in the past the dykes were prevent

Murray and Ashton (2004) explored coastline evolution under such wave climates using a one-line model require some alterations to avoid discretization artefacts related to high-angle waves, and allow complex shoreline structures to emerge in the model However, it does not imply that these solutions are unique; other approaches to solving these problems are certainly possible

Regarding Hoi An coastline, there are some researches have done related to the erosion problem or the surrounding study area

In Hoi An, the beach north of Cua Dai estuary has been eroding severely Pre-built coastal structures significantly affect the adjacent beaches Viet and Tanaka (2015) claimed that the main cause of erosion in Cua Dai beach is due to the hydropower reservoirs and dams built upstream which are keeping a great amount of sediment upstream of the dams As a result, lack of sediment supplies to downstream It is considered as the highly possible mechanism of erosion

Concerning the three researches above, they just simulate very well type of smooth coastline, but they can mention rough coastlines that changing strongly angle of coastline, and the coastlines are influenced significantly by river mounts

According to Long et al (2013), the annual runoff is quite abundant in the Vu Thu Bon river basin, but it unequally distribute in space and time The flow regime

Gia-in the study area is impacted significantly Gia-in both negative and positive ways by the construction of reservoir system upstream in order to exploit water resource efficiently

They assessed the cause of erosion from upstream due to building hydropower plants and the loss of a great volume of sediment supply to the coasts adjacent the Thu Bon estuary

1.4 Study objectives

The main objective of this research is to increase the fundamental understanding on the physical regime influencing the evolution of the coastline Specifically, the study is aimed to get further insight into the parameters which are the most sensitive

to the coastline change in Hoi An in order to propose an effective countermeasure to cope with the coastal erosion problem

To achieving the research goal, the following questions have to be answered during research:

- What is the main factors influencing the evolution of the Hoi An coastline?

- How will the Hoi An coastline tend to change in next 20, 50 years?

- What is an effective solution to deal with the erosion of the Hoi An coastline?

1.5 Approach and methodology

The research questions can be answered and the objectives can be achieved by means of a numerical coastline evolution model In this research, the Generic

Coastline Model (Roelvink and Reniers, 2011) will be utilized to model the coastline evolution for Hoi An

The Generic Coastline Model is an one-line model which allow to predict the coastline change in a long-term time scale The model uses Soulsby-Van Rijn formula (Soulsby, 1997) to calculate long-shore sediment transport taking into account the influences of different processes The model will be calibrated and validated using the actual data for the research area

Once the numerical coastline evolution model is setup and calibrated and validated,

it can be used to predict the coastline evolution for different periods in the future It can also be used to investigate the evolution of the coastline corresponding to different coastal protection measures to find an effective solution to counter the coastal erosion problem of Hoi An

Figure 1-3 Structure of study

The study approach is depicted in Figure 1-3 which include the following major components:

New coastline

Wave transformation module

1 Data collection Different types of data will be collected including:

- Basic data of water level, flow discharge and velocity, wave, wind, and sediment transport

- Basic data of bathymetry, topography, cross-shore coastal profiles

- Historical changes of the coastline based on maps and remote sensing (Google Earth, other satellite imagery and aerial photography)

2 Model setup, calibration and validation The model will be setup based on Generic Coastline Model and will be calibrated and validated using measured data Sensitivity analysis and the investigation of the influences of the different factors and processes on model results can be carried out during model calibration

Prediction of coastline change in next 20, 50 years for “zero-option” and other coastal protection measures to propose the most effective solution to counter the coastal erosion problem

1.6 Thesis structure

The structure of the thesis is shown in Figure 1-4 After this introduction chapter, Chapter 2 presents more detailed on the physical characteristics of study area and the surrounding area Chapter 3 provides application of Generic Coastline Model on study area Chapter 4 focuses on predicting the evolution of the Hoi An coastline The thesis ends with the study conclusions and recommendations in Chapter 5

Figure 1-4 Structure of the thesis

Physical characteristic

Application of generic

coastline model

Prediction of evolution for Hoi

An coastline

Conclusion and Recommendation

Thesis Structure

Introduction

CHAPTER 2 : PHYSICAL CHARACTERISTICS 2.1 Introduction

Hoi An beach is one of the most sensitive and important beaches in Vietnam because of its position and economy Due to the area is located in the tropical monsoon climate region, there are several different elements influencing the evolution of coastline all governing by the tropical monsoon systems The principle factors, which is strongly influenced, is from inland, oceanic conditions and specifically human activities The dominant influence to the change of coastline is due to lacking of sediment that mainly supply from Thu Bon River system, building hydropower plain in upstream and mining along the river To obtain a general view

on the natural condition and the evolution of coastline, this chapter will provide some basic information on the physical characteristics of the change of coastline

2.2 Geographic conditions

Generally, the topography of the Thu Bon River basin change in a complex and dissected way with the direction of the slope from the western to the eastern, and there are 4 types of main topography

2.2.1 Mountain topography

The type of mountainous topography accounts for most of the Thu Bon River basin’s area, and the topography belongs to Truong Son mountain range with the popular height from 500 to 2000 meter The waterline of the basin is the peak of mountains with the height about 1000 to 2000 meter along Hai Van pass to the west, then the southwest and the south, creating a kind of topography as C shape For the type of topography, the area is affected by the northeast monsoon and the morphological weathers from eastern sea, hence it can create several heavy rain, flash floods in upstream and flooding in downstream

2.2.4 Sandy coastal topography

The type of sandy soil in coastal zone tends to be brought gradually to the western

by wind, creating sandy hill with the wave shape

is distributed in the margin of structure, characterized by granitoid formations of Dai Loc complex, while the red continental sediments of Tan Lam formation is only exposed in Long Đại zone The Upper Paleozoic - lower Mesozoic complex includes extrusive terrigenous sediments of Bung River formation, intrusive magma formations of Ben Giang - Que Son complex, grabroid formations of Cha Val complex, granttoid formations of Hai Van complex, weakly metamorphic, weakly dislocated rocks and continental activated complexes mainly include intrusive magma formations of Deo Ca, Ba Na complex Nông Sơn zone is located in center

of research area, limited by Vu Gia River fault in the north, Thang Binh – Hiep Duc

fault in the south, Song Tranh fault in the west This zone consists of four complexes: Pre-Cambrian complex includes Kham Duc formation exposed in Thanh My The few lower chlorite sericite complex includes: chlorite sericite schist, quartz chlorite sericite schist of A Vuong formation, upper Paleozoic - lower Mesozoic activated complex plays an important role in the formation of Nong Son zone, is characterized by a terrigenous - extrusive sediment combination of Bung river formation, the magmatic formations of Ben Giang - Que Son complex Upper Mesozoic complex includes coal-bearing sediments of Nong Son formation and sediments of Ban Co, Khe Ren, Huu Chanh formations Kham Duc zone is a complex structural zone, being transformed many times, limited to other zones by Tam Ky - Phuoc Son fault in the north, Hương Nhượng - Ta Vi fault in the south,

Po Ko fault in the West This zone consists of complexes with following texture: Pre-Cambrian complex include terrigenous – extrusive magmatic, terrigenous - carbonate, terrigenous - magmatic to felsic extrusive formations of Kham Duc formation The rocks were strongly corrugated and dislocated Lower Paleozoic complex: characterized by A Vuong formation containing alternating extrusive layers Lower Cenozoic complex included basalt and Quatenary sediment formations

2.4 Climatic conditions

2.4.1 Wind

The study area belongs to tropical zone, and there are 2 types of tropical monsoon annually, including dry and wet seasons In winter from October or November to the March or April after next year, the dominated winds are the east and the southwest, in which the dominated winds are the southeast and the southwest from April or May to September or October The mean rate of wind in the mountainous area and coastal plain are approximately 0.8÷1.7m/s and 1.3÷3.7m/s respectively Furthermore, the rate of wind in wet season is higher than dry season, and the rate

of wind can be up to 40m/s during typhoon

2.4.2 Temperature

The temperature in the study area tends to increase gradually from the south to the north, from the west to the east, and from the high altitude to the low altitude The mean temperature in the mountainous area and coastal plain is approximately 24.5-

0C and 25.60C, respectively The spans of temperature among months in coastal plain and mountainous areas are 7÷90C and 5÷70C respectively The change of temperature follows different seasons The monthly highest temperature is always from June to July with the monthly mean of the daily maximum and highest temperature are above 300C and spanning from 33÷340C in July respectively The monthly mean of the daily minimum and lowest temperature are approximately from 18 to 230C, and from 18 to 200C, respectively in January or December

2.4.3 Humidity

The humidity of air is related closely to the air temperature and the amount of rain The variability of humidity is similar to the variability of rainfall and opposite to the variability of air temperature For the months in the wet season in the coastal plain and the mountainous area, it can be up to from 86% to 89% and from 90% to 93%, respectively Although during the months in the dry season in coastal plain and mountainous area, the relative humidity is below 80% and a span from 80 to 85% The lowest relative humidity drops about 20 to 30%

2.4.4 Evaporation

The evaporated capacity of the basin depends on surface condition and climatic elements, such as the air temperature, sunshine, wind, humidity According to observed data from the gauging stations in the study area, the capacity of evaporation is approximately 1086mm/year There is a tendency of increment in evaporation from the west to the east The evaporation is lower in mountainous area which is about 667mm, and it is higher in coastal plain with a value of 1031mm

2.4.5 Rainfall

The annual rainfall is from 2000 to 4000mm It often appears in high mountainous region, such as Tra My, TienPhuoc The annual rainfall in the coastal plain just remain from 1700 to 2200mm

For the features of annually tropical climatic monsoon, it creates two distinguish seasons:

- With approximately 3÷4 months of the short wet season from September to December, the amount of rainfall accounts for 70÷80 per cent of the total amount of year The months that the greatest amount of rain from October to November account for 40÷50 percent of the annual total amount of rainfall

- With approximately 8÷9 months of the dry season from January to August, the volume of rainfall accounts for 20÷30 percent of the annual total amount

of rainfall The months which the lowest amount of rainfall is from February

to April account for 3÷5 percent of the annual total amount of rainfall

Figure 2-1 Monthly average rainfall (1979-2013)

2.5 River basin and river network

2.5.1 River basin

The Vu Gia-Thu Bon river basins are located in central Vietnam They are often combined as the Vu Gia-Thu Bon river basin because it is connected by Quang Hue and Vinh Dien rivers in downstream In recent years, the river upstream also has been connected by the Dak Mi hydropower project through an interbasin transfer pipe and canal The western part of the basin is mountainous and sparsely populated, while the flat delta area in the east is used for agriculture

Figure 2-2 Overview of the Vu Gia-Thu Bon Basin

The topographic conditions are quite favourable for water resources development projects: hills and mountains cover 75 per cent of the basin, numerous reservoirs and weirs are located in the basin irrigating over 30,000 ha of rice and 10,000 ha of subsidiary crops and cash-crop trees Before 2013, a total of 13 hydropower put into operation, while another 31 were in the construction or planning phase (NSHD-M, 2013)

province (Figure 2-3) The average annual discharge of Thu Bon River is approximately 327m3/s However, the flow regime in the river has a large seasonal fluctuation The flow during the flood season is about 62-69 per cent of the annual volume, with 26-31 per cent occurring in the peak month, November

Figure 2-3 River network of Vu Gia-Thu Bon basin (Long, Tung, Huy, 2013)

In the downstream area, there is an exchange of flow between the two rivers In the wet season, the Quang Hue River diverts part of the flow from the Vu Gia into the Thu Bon via the Quang Hue cross-connection, and further downstream, Vinh Dien River returns part of the water back from the Thu Bon to the Vu Gia In the downstream stretch, the river network is rather dense Apart from the flow exchanges, the mainstreams are also supplied with additional water from other branches (Long, et al, 2013)

2.6 Observation stations and hydrologic conditions

2.6.1 Observation stations

In the Vu Gia-Thu Bon river basin, there are eighteen gauge stations (on average, 600km2/ station), in which data in Da Nang gauging station is available from 1907, but data was interrupted before 1975

Table 2-1 The properties of Stations

N 0 Station River

Catchm ent area (km 2 )

Observed element

Observed years

Period of records

Most of the gauge stations have observed data from 1976 to 1998 Some of another has observations from the beginning of the 1920s, but most of them is observed rainfall Other meteorological elements have been continually developed year by year In the beginning, most of the gauge stations located in coastal plain

2.6.2 Hydrologic conditions

2.6.2.a Annual flow

According to data of gauging stations in the study area from 1977 to 2012, the characteristics of flow is calculated as shown in Tables 2-2 and 2-3 below

Table 2-2 Hydrologic characteristics of the Thu Bon River

Flv(km2)

X0(mm)

Y0(mm)

Q0(m3/s)

Month Qo Cv Cs

Qp(%) m3/s

10% 25% 50% 75% 90%

Nong Son 3150 77-12 268.3 0,33 1,12 386.9 315.8 252.2 202.8 171.3

2.6.2.b Flow in flood season

Basing on observed data at gauging stations of Nong Son, GiaoThuy, Cau Lau, one can see that flood season is approximately 4 months, starting from September with the main floods occur in October or December On average, there are 3 floods exceeding the first alarm The number of second alarm is approximately 1 or 2 The floods exceeding the third alarm is approximately 0.6 or 1 annually

Table 2-4 Designing flood discharge corresponding to frequency

Table 2-5 Flow discharge through a cross – section at Cua Dai

Duration of observation Qmax(m

3/s) (positive downstream)

Qmax(m3/s) (positive upstream)

From 15/8 to 30/8/2009

1490 (12h - 17/8/2009)

1240 (23h -23/8/2009)

From 23/9 to 8/10/2010

2406 (23h - 7/10/2010)

- 953 (12h - 27/09/2010)

In recent years, great floods appear more often, giving severe consequence in 1986,

1996, 1998, 1999, 2005, 2009

2.6.2.c Low flow in dry season

In the study area, the dry season is from January to August, and the amount of rainfall at least just account for 1÷3 per cent of the total annual volume of rainfall The total amount of flow discharge in the dry season accounts for 20÷30 per cent of the annual amount of water

Table 2-6 The characteristics of water level and tidal range in Hoi An

Month I II III IV V VI VII VIII IX X XI XII

In March and September, prevailing E wave height just reaches up to 0.5m and accounts for 30 per cent of the time

Table 2-7 The distribution of deepwater waves in the study area

0.0- 0.5 3 18 163 182 173 158 49 2 0 1 8 4 761

Figure 2-4 Annual wave rose of Hoi An coast 2.7.3 Currents

In winter, under the influences of the northeast monsoon, the flow currents tend to

go from the north to the south The semi-diurnal currents dominate mainly with the magnitudes of M2, S2 are higher than of the K1 and O1 components

Table 2-8 The characteristics of currents along Cua Dai coastline (Oct 3, 2010)

Level (m)

velocity (m/s)

+The direction of current flow :SE÷NW

3:00AM Mean significant height

0,062 0,041 0,024

+The direction of current flow: SE÷NW +rising tide

23:00PM Incoming wave angle Өs =

00 mean significant wave

height: Hs=1,81m

1,2 0,2H

0,6H 0,8H

0,053 0,032 0,024

+ The direction of current flow SE÷NW + Increasing tide

2.8 Sediment

2.8.1 River sediment transport

According to data of gauging stations at Nong Son and Thanh My, the grain size of sand tends to decrease gradually from upstream to downstream Upstream, sediment has the grain size of sand quite big from 0.5÷5mm Sediment has the grain size of sand in average from 0.1 ÷ 2.0mm downstream and there are a small amount of grain size of clay which is less than 0.001mm In addition, the total sediment transport in flood and dry seasons account for about 75 ÷ 85 and 10 ÷ 15 per cent of the total annual sediment transport, respectively

2.8.2 Coastal sediment transport

There are two distinct seasons in Hoi An, the wet season and the dry season There

is a strong weather climate with a lot of rain and storms which have a severe impact

to the shore The high and energetic waves break down the upper shore face and transport an amount of material towards the lower shore face further offshore The abundance of rainfall causes flash floods in the Thu Bon River, which carries a lot

of sediment downstream towards the river mouth In this season, the river is seen as

a main source of sediment supply to the coastal system

In the dry season, the wave direction is from mostly the north, and the wave climate

is very mild During this season, the offshore sandbars pushed back towards the coast, which partly restore the original profile of the beach During this time of the year, the rainfall is less and therefore the river discharge, sediment transport also is less

Due to the research of Hoi An project from The Netherlands, they collected sample

of particle diameter in 14 different positions corresponding to changing shape of coastline and kind of construction, as show in Figure 2-5 Collecting sandy samples

in study area can present distribution of sand and tend of moving sediment transport

in a dynamic way

Figure 2-5 Positions collecting sandy samples

Figure 2-6 Distribution of sediment size grain along the coast Table 2-9 A typical distribution of sediment size grain

D50 (mm) 0.34 0.38 0.42 0.23 0.45 0.29 0.27 0.3 0.48 0.083 0.061 0.084 0.132 0.127 0.31 D90 (mm) 0.77 0.79 0.78 0.63 0.87 0.54 0.48 0.62 0.84 0.12 0.085 0.13 0.55 0.46 0.66

All information of the sediment particle diameter collected in 14 positions are shown in appendices

2.9 Historical evolution of the coastline

Generally, the Hoi An coastline has been changing yearly due to the changing of sediment transport According to Tanaka et al (2015), the morphology of Hoi An beach has been very dynamic The morphological observation started from 1975 based on an analysis using Landsat images The spit situated on the Thu Bon river mount is rather small in 1975 It tended to widen due to the great volume of sand that migrated along the shore and merge each other in 1979 and 1991 (Figure 2-7 A) The coast was divided into two spits in which the B spit was the remainder of

the sandbank, continued to merge with the coast Meanwhile, on the right side of the river, the volume of sand migrated from the right, and an outcrop formed on this side between 1995 and 2002 From 2003 to now, the outcrop has been eroding quite fast, at the same time, the amount of sand migrate from the right (Figure 2-7 C) In the observed period of the spit left of the river, there was a cumulative changes The spit had firstly accreted after 1975, and then it has been eroding back in 2015 to be about the same size as in 1975

Approaching and merging sand bank (A)

Double spit formation (B)

Overall erosion (C)

An shoreline tends to change the shape of shoreline in a quickly and severely way

CHAPTER 3 : APPLICATION OF GENERIC

COASTLINE MODEL 3.1 Introduction

In this research, Generic Coastline Model is applied to study the evolution of Hoi

An coast Generic Coastline Model is one of the open source models in Matlab developed by Delft Hydraulics (now Deltares) It can predict the evolution of a coastline based on One-line theory and Soulsby-van Rijn sediment transport formula In model calibration, different coefficients can be adjusted in the model in order to assess their influence on the model results, from that, we can point out the sensitivity of each different coefficient From the model results during model calibration, the most sensitive coefficients are adjusted to be relevant between the real problem and the simulation in the model by means of comparison the calibration - output results with measured data

3.2 Theoretical background

3.2.1 One-line theory

From sediment transportation aspect, the basic assumption of One-line theory is based on the equilibrium and stability of long-term waves induced only longshore component of sediment transportation to shape the shore profile, and incident wave angle is strongly related to longshore sediment transportation rate (Hanson, 1987) The second assumption is that sediment movement is observed up to a depth beyond which no more change of bottom profile can be observed due to sediment transportation This depth is designated as the depth closure Dc

Figure 3-1 Depth of closure

In sediment transport, there is a distance that it can be active According to Hallermeier (1981)

The formula can rewrite as: Dc = (2,28 – 6,85 × S) H

Where:

- Hs is the effective wave height just seaward of the breaker zone that is exceeded for 12 hours per year, i.e the significant wave height with probability of yearly exceedance of 0.137%

- T is the wave period associated with Hs

- g is the acceleration due to gravity

- S is bed slope corresponding to Hs

From geometrical aspect, in the first assumption, the beach profile tends to move parallel to itself that it is translated from shoreward to seaward and vice versa without change in the course of eroding and accreting If the profile shape do not change, any point on it is sufficient to specify the location of the entire profile with respect to a baseline (Figure 3-1) Change in the beach plan shape and volume as the beach erodes and accretes that can be described by one contour line, so the

contour line is conveniently taken as the readily observed shoreline The second assumption, sand is transported alongshore between well-defined limiting elevations

on the profile The top of active berm (DB in Figure 3-2) is the limited location in shoreward, and where the change of depth occurs no significantly, the so-called depth of closure (Dc in Figure 3- 2) Restriction of profile movement between these two limited positions provide the simplest way to specify the perimeter of a beach cross-sectional area by which changes in volume, leading to shoreline change, can

be computed (coastalwiki.org, 2016)

3.2.2 Fundamental equations

3.2.2.a Continuity equation

Continuity equation, which depends on the assumption that in a controlled volume, the amount of sediment going into a portion of beach, cell, should be equal to the plus of the total amount of sand in the cell that remained in the cell and left the cell,

is the fundamental equation of One-line theory:

DB : average berm height above mean water level,

Q :longshore sediment transport rate,

x :longshore coordinate of shoreline,

q : source and/or sink along the coast

Figure 3-2 Sand Continuity Equation Sketch Depth of closure, Dc, is also an important parameter defining the sediment motion boundaries as mentioned above

3.2.2.b Motion equation

The motion equation of sediment transport depends on the evolution of the amount

of sediment transport along coastline, ∂Qx/∂x Most of the important variables can

be changed when examining the morphology along coastline, such as wave height, wave angle It will be limited in the change of wave angle that affecting the coastline, assuming that evolution of waves in depth wave is seemed to not change along shoreline

In fact, it can exam the sediment transport with direction-fixed waves that affecting the direction-different segments in comparison with coming wave angle, thus, for the change of small value of coming wave angle, φ, in sediment transport formulae,

∂Qx/∂x can define its value empirically

Similarly, the wave angle is limited in a range of its small evolution, thus ∂Qx/∂φ can assume that is constant From that, the movement equation can describe as shown in the following equation

In the other hand, the assumptions during building the continuity equation are so significant The change of wave height and the direction of wave along coastline, tidal influence and other several components in Bijker’s sediment transport formula were ignored Assumption about impacting coastline with small angle is very limited, especially when bed in surf zone mainly is influenced by sediment transport along shoreline, in which the impacting angle has certain value, so the assumption just is value in the outside of surf zone

3.2.4 Sediment transport formulae

Van Rijn (1984) presented comprehensive formulas for calculating the bed load and suspended load, and only a short description of the method is given in the followings For the bed load he adapted the approach of Bagnold assuming that sediment particles jumping under the influence of hydrodynamic fluid forces and gravity forces dominate the motion of the bed load particles

∗   ) 12

-34 5

  1 * $67

In which Hs is the significant wave height

The depth integrated suspended load transport in the presence of current and waves

is defined as the integration of the product of velocity and concentration from the edge of the bed-load layer to the water surface yields

8 :9 ;9∗ 7 * <=< >?@ 7 * AA >

B 

speed due to turbulence and β is a coefficient quantifying the influence of the centrifugal forces on suspended particles

Figure 3-3 Digitalizing coastline on Google Earth The coastline is digitalized for 2004 and 2011 to assess the evolution of coastline In

2004, the change of the coastline shape started to develop, and it tends to erode landward In 2011, facing with erosion, all resorts stay in the beach have built structures to protection themselves individually So the development of the coastline does not naturally To conduct model calibration, the coastline in 2004 is used as initial condition to simulate the behaviour of the coastline until 2011 The coastline

in 2011 is refered to fit the parameters that can impact on the change of coastline