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— Universitéde Liége

THUY LOI UNIVERSITY UNIVERSITY OF LIEGE

CAO DUC HAI

MASTER THESIS

MODELING COASTLINE EVOLUTION USING ONE-LINE MODEL FORHOI AN, QUANG NAM

Major: Sustainable Hydraulic Structures

Coastal Engineering and Management

Supervisor: Ass. Prof. Dr.NghiemTien LamProf. Rigo Philippe

Hanoi, 2016

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‘This master course has been supported by the cooperation project between

University of Liege and ThuyLoi University

First and foremost, [ would ike to thank to all members in my big family, speciallymy 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, Assoe.Prof, Nghiem Tien Lam; also professor at University of Leige, Prof. Rigo Philippeduring master research, who gives me the direction in study

In research, special thanks to Dr. Nguyen Quang Chien for supporting to fixmistakes in programming open source code, and Assoc. Prof. Dr. Vu Minh Cat forsharing data and material about Hơi 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 Tĩnh,and all leaders in Phục Hung Holdings Construction Joint Stock Company that they

give opportunities to complete my master thesis on time.

Hanoi, August 2016

‘40 Due Hai

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CHAPTER 1: INTRODUCHON.

L1 Study area.

1.2 Problem definition1.3 Literature review.

224 Sandy coastal topography’

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26 Observation stations

2.62 Hydrologic conditions...

2.7. Oceanographic conditions27.1 Tides

282

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34.3 Wave breaking pafametefs...e.ecccecseece 183

List of table T0

Appondix A nAppendix B. 85

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CHAPTER1: INTRODUCTION

11 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 km? of total area and approximately 120,000

inhabitants, ts ancient town is recognized as a UNESCO World Heritage site in

Vietnam. Quang Nam province shares the border with Thua Thien-Hue province inthe north, Quang Ngai province in the south, Kon Tum province and the Laos in theWesL Its a strategic important position that can cover and monitor almost of the‘marine routes inthe East Sea

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

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day by day. On average, the Hoi An coastline is eroding 12m per year. So a greatnumber 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 itis also

the province's main source of income. In 2013, Quang Nam province's tourismearned 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 focal inhabitants live on tourism, hence they ane

affected by coastal erosion on thei 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 fends to move toward thesouth 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~

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

"the Dutch ster stent group fom Tw-Delt Universi took the pstares on he Hs site2

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‘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 t,

‘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

reat volume of fine-grained sediment transport, Chantal et al. (2004) compared

equilibrium beach profiles which the processes in surf zone do not affect and affectto calculate the deviation of fine material. The continuity equation of sediment‘transport in GENESIS was adjusted represent the volume of thes

Existingin Hai Hau, Nam Dinh province was simulated in GENESIS as seawall that‘erosion in the past the dykes were prevent

Murray and Ashton (2004) explored coastline evolution under such wave climatesusing a one-line model require some alterations to avoid discretization artefactsrelated to high-angle waves, and allow complex shoreline structures to emerge inthe model. Howe rit does not imply that thes solutions are unique; other

approaches to solving these problems ate certainly possible.

Regarding Hoi An coastline, there are some researches have done related to the

crosion 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 thatthe main cause of erosion in Cua Dai beach is due to the hydropowerreservoirs and dams built upstream which are keeping a great amount of sedimentupstream of the dams. As a result, ack of sediment supplies to downstream. It is‘considered as the highly possible mechanism of erosion.

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Concerning the three researches above, they just simulate very well type of smoothcoastline, but they can mention rough coastlines that changing strongly angle of

coastline, and he coastlines are influenced significantly by river mounts

According to Long et al (2013), the annual runo[f is quite abundant in the Vu ‘Thu Bon river basin, but it unequally distribute in space and time, The flow regimeinthe study area is impacted significantly in both negative and positive ways by theconstruction of reservoir system upstream in order to exploit water resourceefficiently.

Gia-‘They assessed the cause of erosion from upstream due to building hydropowerplants and the loss of a great volume of sediment supply t0 the coasts adjacent the

‘Thu Bon estuary,

14. Study objectives

“The main objective ofthis research isto in ase 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 duringresearch:

‘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

1.5. Approach and methodology

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

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Coastline Model (Roelvink and Reniers, 2011) will be utilized to model the

‘coastline evolution for Hoi An.

‘The Generic Coasline Model is an one-line model which allow to predict thecoastline change in a long-term time scale. The model uses Soulsby-Van Rijnformula (Soulsby, 1997) to calculate long-shore sediment transport taking intoaccount the influences of different processes. The model will be calibrated andvalidated using the actual data forthe research area

‘Once the numerical coastline evolution model is setup and calibrated and validated,it ean be used to predict the coastline evolution for different periods in the future. Ttcan 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,

<small>Wave transformation module</small>

<small>solve Matric</small>

Figure 1-3 Structure of study

‘The study approach is depicted in Figure 1-3 which inelude the following major

<small>‘components:</small>

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1. Data collecdon. 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, eross-shore coastal profiles.

<small>= Historical changes of the coastline based on maps and remote sensing</small>

(Google Earth, other satelite 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 usingmeasured data, Sensitivity analysis and the investigation of the influences ofthe different factors and processes on model results can be carried outdưng 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 thecoastal erosion problem.

1.6 Thesis structure

“The structure of the thesis is shown in Figure 1-4, Afler this introduction chapter,Chapter 2 presents more detailed on the physical characteristies of study area andthe 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,

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CHAPTER2: PHYSICAL CHARACTERISTICS

‘on the natural condition and the evolution of coastline, this chapter will providesome basic information on the physical characteristics ofthe change of coastline.

2.2 Geographic conditions

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

2.2.1 Mountain topography

‘The type of mountainous topography accounts for most of the Thu Bon Riverbasin’s area, and the topography belongs to Truong Son mountain range with thepopular height from 500 to 2000 meter. The waterli of the basin is the peak ofmountains with the height about 1000 to 2000 meter along Hai Van pass to theWest, then the southwest and the south, ereating a kind of topography as C shape

For the type of topography, the area is affected by the northeast monsoon and themorphological weathers from eastern sea, hence it ean ereate several heavy rain,flash floods in upstream and flooding in downstream,

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2.2.2 Hilly topography

Hilly zone, there is the type of topography as wave, tends to be lower gradually

towards the eastern, and some places are quite lat

2.2.3 Delta topography

‘Topographical delta just appear in the eastern with narrow coastal plain along fromthe southern to the northern, The soil is the accumulation of ancient alluvial from

sea and river.

2.2.4 Sandy coastal topography

‘The type of sandy soil in coastal zone tends to be brought gradually to the westernby wind, creating sandy hill withthe wave shape.

2.3 Topographic conditions

Vũ Gia ~ Thu Bồn Basin is located in stratigraphic units of 3 tectonie zones: Khim"Đức, A Vương ~ Sé Công and Nông Sơn. A Vuong ~ Sẽ Công zone occupied mostThe‘of the Northern area of the basin and formed a large fold with latitudinal axizone is bounded by Son Tra - A Trep fault in the north and Tam Kỳ ~ Phước Sơn.

fault in the south, This complex is characterized by a combination of maficcextrusive rocks alternating siliceous sediments ete. Lower Paleozoic complexsts of sericite schist, chlorite sericite, quartz sericite schist alternating mafic tofelsic extrusive lens, activated vi quarzide limestone. Middle Paleozoie complexis distributed in the margin of structure, characterized by granitoid formations of

Dai Loe complex, while the red continental sediments of Tan Lam formation is onlyexposed in Long Đại zone. The Upper Paleozoic - lower Mesozoic complexincludes extrusive terri -nous sediments of Bung River formation, intrusive magmaformations of Ben Giang - Que Son complex, grabroid formations of Cha Valcomplex, granttoid formations of Hai Van complex, weakly metamorphic, weaklyslocated rocks and continental activated complexes mainly include intrusive‘magma formations of Deo Ca, Ba Na complex. Nông Son zone is located in center‘of research area, limited by Vu Gia River fault in the north, Thang Binh ~ Hiep Duc

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‘This zone consists of fourfault in the south, Song Tranh fault in the wes

complexes: Pre-Cambrian complex includes Kham Due formation exposed in

‘Thanh My. The few lower chlorite sericite complex includes: chlorite sericite schist,‘quartz chlorite sericte schist of A Vuong formation, upper Paleozoic - lowerMesozoic activated complex plays an important role in the formation of Nong Sonzone, is characterized by a terrigenous - extrusive sediment combination of Bungriver formation, the magmatic formations of Ben Giang - Que Son complex. Upper

Mesozoic complex includes coal-bearing sediments of Nong Son formation andsediments of Ban Co, Khe Ren, Huu Chanh formations. Kham Duc zone is @‘complex structural zone, being transformed many times, limited to other zones by‘Tam Ky - Phuoe 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 texturePre-Cambrian complex include (emignous ~ extrusive magmatic, terrigenous -carbonate, temigenous - magmatic to felsic extrusive formations of Kham Dục

formation. The socks were strongly corrugated and dislocated. Lower Paleozoiccomplex: characterized by A Vuong formation containing alternating extrusivelayers. Lower Cenozoic complex included basalt and Quatenary sedimentformations

24 Climatie conditions2.4. Wind

‘The study area belongs to tropical zone, and there are 2 types of tropical monsoonannually, including dey and wet seasons. In winter from October or November tothe March or April after next year, the dominated winds are the east and thesouthwest, in which the dominated winds are the southeast and the southwest fromApril or May to September or October. The mean rate of wind in the mountainousarea and coastal plain are approximately 0.82lL7in/s and I.323/7m/s respectively

Furthermore, the tate of wind in wet -ason is higher than dry season, and the rate‘of wind can be up to 40nvs during typhoon,

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2.4.2 Temperature

‘The temperature in the study area tends to increase gradually from the south to thenorth, 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

°C and 25.6°C, respectively. The spans of temperature among months in coastalplain and mountainous areas are 729°C and S=7'C 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 30°C and spanning from 33+34°C in July respectively. The

monthly mean of the daily minimum and lowest temperature are approximately

from 18 to 23°C, and from 18 to 20°C, respectively in January or December:

2.4.3 Humidity

‘The humidity of air is related closely to the air femperature and the amount of rain,“The variability of humid

ty 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 andmountainous area, the relative humidity is below 80% and a span from 80 to 85%.

“The lowest relative humidity drops about 20 to 30%.

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2.4.5 Rainfall

“The annual rainfall is from 2000 to 4000mm. It often appears in high mountainousregion, such as Tra My, TienPhuoe. 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

With approximately 3-4 months of the short wet season from September toDecember, the amount of rainfall accounts for 70280 per cent of the total

‘amount of year. The months that the greatest amount of rain from October toNovember account for 4050 percent ofthe annual total amount of rainfall

<small>= With approximately 829 months of the dry season from January to August,</small>

the volume of rainfall accounts for 20230 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 ofthe annual total amount of rainfall

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

2.8 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 asthe Vu Gia:Thủ Bon river basin because itis connected by Quang Hueand Vinh Dien rivers in downstream. In recent years, the siver upstream also has

‘been connected by the Dak Mi hydropower project through an interbasin transferpipe 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.

B

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‘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 ofsubsidiary crops and cash-crop trees, Before 2013, «total of 13 hydropower put into‘operation, while another 31 were in the construction or planning phase (NSHD-M,2013).

2.5.2 River network

‘The Vu Gia, Thu Bon River system is one of the largest river systems in central‘Vietnam, and consists of 2 principal branches, Vu Gia and Thu Bon Rivers. Water‘of the both rivers originates from the high mountains in the west side of Quang Nam

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province (Figure 2-3). The average annual discharge of Thu Bon River is

approximately 327m"Vs. 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 annualvolume, with 26-31 per cent occurring inthe 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 DienRiver returns part of the water back from the Thu Bon to the Vu Gia. In thedownstream stretch, the river network is rather dense, Apart from the flow‘exchanges, the mainstreams are also supplied with additional water from otherbranches (Long, et al, 2013).

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2.6 Observation stations and hydrologie conditions

2.6.1 Observation stations

In the Vu Gia-Thu Bon river basin, there are eighteen gauge stations (on average,

.60ÖkHỦ/ 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

Observed | Observed | Period of

N' Staion | River canh VỐN "

x 49 — | %

13-43,61-6676-AT...

Thôn xe a | 1976-1998

7 | Cia

xX 2 | 1976-19985 Bane

x 17 | 19771996

9 [Ten Prue

x 20 | 19711996

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10 | Cima0 | Cam Le XH 2 1916-1998TT | AiNehia

Xa 2 1916-199812 | Thăng

<small>x „ 1977-1998</small>

BìhT1 | Son Tân

_year, In the beginning, most of the gauge stations located in coastal plain.

2/6/24 Anmual flow

2.6.2 Hydrologic conditions

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.

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‘Table 2-2 Hydrologic characteristics of the Thu Bon River

‘Table 2-4 Designing flood discharge corresponding to frequency

River Station

Thu Bon Nong Son

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Table 2: Flow discharge through a cross — section at Cua Dai

Qualtns) positive] “ (positive

Duration of observation : nà

downstream) upstream)

1490 1240From 15/8 to 3/8/2009

(12h 1718/2009) (23h 2378/2009)

2406 -983From 239 to 8/10/2010

23h -7/10010) (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.¢ 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 20230 per eent of

the annual amount of water.

2.7 Oceanographic condi

2.7. Tides

‘Along the coast of Quang Nam province and Da Nang c

‘complex. This are is the place of tidal transformation between mainly semi-diurnalin the north and fully diurnal in the south, There are 10 days of fully diurnal in a

month. The tidal range changes accordingly to different month. The lowest tidal

range in March is 116cm, the highest tidal range in September is 201em,

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

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Month tom om fav fy ve |v] van 1X [x [XI fxm

Ho (em) 2 a1 |3 |ä4|47|-8|-a (3 [33 [ar {20

1.5m. From October to April next year, dominated waves mostly from NE direction‘and accounts for 75 per cent of the time with the mean significant wave height of

about 0.5 ~ 2.5m, In deep waters off Ho An, the mean wave height is from 1 = 2m,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

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magnitudes of Ma, S; ate higher than of the K, and O, components

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

Date’ | Meteorological condition in| Depth) Level] velocity, NoteTime the observed area

(my | (m) | vs)

0045 |The direction of3/10/2010) Incoming wave angle:

v current flaw

100M 06H | 0.045 Hows

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3:00AM | Mean sigileant height 08H | 0,019 | rising tide19m

Incoming wave angles =| 1,2 |0.2H The direction of

M : bạn

= (8H ve tide

height Hs=1.tm 00a

23:00PM | Incoming wave angle s=|L2 |02H|0053 |+ The direction of}

OP mean significant wave current flow SE:NW

0,6H | 0,032

height: Hs=1,81m

osit|oooa | Hstesing 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 ofsand tends to decrease gradually from upstream to downstream. Upstream, sedimentthác the grain size of sand quite big from 0.5+Smm. Sediment has the grain size of,

sand in average from 0.1 + 20mm downstream and there are a small amount of‘grain size of clay which is less than 0001mm, In addition, the total sedimenttransport 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 witha 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 earries a lot

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‘of sediment downstream towards the river mouth. In this season, the river is seen as‘a main source of sediment supply tothe coastal system.

In the dry season, the wave direction is from mostly the north, and the wave climateis very mild. During this season, the offshore sandbars pushed back towards the‘coast, which partly restore the origina profile of the beach, During this time of theyear, the rainfall is less and therefore the river discharge, sediment transport also isless.

Due to the research of Hoi An project from The Netherlands, they collected sample‘of particle diameter in 4 different positions corresponding to changing shape ofcoastline and kind of construction, as show in Figure 2-5, Collecting sandy samplesin study area can present distribution of sand and tend of moving sediment transportina dynamic way.

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Figure 2-6 Distribution of sediment size grain along the coast

‘Table 2-9 A typical distribution of sediment size grain

Diner cir xe

<small>Lam [1[2[3|4|[5|[6|[T[Xj9[m[HnIEB[BIH</small>

80mm | O54 03W| 048033 |046| 035 PORT] US '04g MENS | 0i POUR MAE OTT) lai

<small>2mm | OFF 9| 07%. HAF PORT] OS [OAR | OR ORT] OAD [OME | ĐỊT 055 046, D66</small>

All information of the sediment particle diameter collected in 14 positions areshown in appendices,

2.9 Historical evolution of the coastline

Generally, the Hơi An coastline has been changing yearly due to the changing ofsediment transport. According to Tanaka et al, (2015), the morphology of Hơi An

beach has been very dynamic, The morphological observation started from 1975based on an analysis using Landsat images. The spit situated on the Thu Bon rivermount is rather small in 1975, It tended to widen due to the great volume of sandthat migrated along the shore and merge each other in 1979 and 1991 (Figure 2-7A), The coast was divided into two spits in which the B spit was the remainder of

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the sandbank, continued to merge with the coast. Meanwhile, on the right side oftheriver, the volume of sand migrated from the right, and an outerop formed on this

side between 1995 and 2002. From 2003 to now, the outerop has been eroding quite

fast, at the same time, the amount of sand migrate from the right (Figure 2-7 C). Inthe observed period of the spt left of the river, there was a cumulative changes. The

spit had firstly acereted after 1975, and then it has been eroding back in 2015 to be

about the same size asin 1975,

Approaching and merging sand bank (A)

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Figure 2-7 Evolution of the Hoi An coast based on Landsat images (Tanaka etal,2015)

mm

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CHAPTER3: APPLICATION OF GENERICCOASTLINE MODEL

41 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 Matlabdeveloped by Delft Hydraulies (now Deltares). It can predict the evolution of acoastline based on Or ne theory and Soulsby-van Rijn sediment transportformula. In model calibration, different coefficients can be adjusted in the model inorder to assess thị influence on the model results, from that, We ean point out the

Sensitivity of each different coefficient, From the model results during modelcalibration, the most sensitive coefficients are adjusted to be relevant between thereal problem and the simulation in the model by means of comparison thecalibration - 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 isbased on the equilibrium and stability of long-term waves induced only longshore‘component of sediment transportation to shape the shore profile, and incident waveangle 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‘ransportation, This depth is designated as the depth closure D..

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Figure 3-1 Depth of closure

Ín sediment transport, there is a distance that it can be active. According toHallermeier (1981)

2,28 ~ 6,85 x S) H

~ Hs is the effective wave height just seaward of the breaker zone that isexceeded for 12 hours per year, ie. the significant wave height with

probability of yearly exceedance of 0.137%.

= Tis the wave period associated with H,

~ isthe acceleration due to gravity

<small>= Sis bed slope corresponding to H..</small>

From geometrical aspect, in the first assumption, the beach profile tends to moveparallel 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 itis sufficient to specify the location of the entire profile withrespect to a baseline (Figure 3-1). Change in the beach plan shape and volume asthe beach erodes and accretes that can be described by one contour line, so the

”

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‘contour line is conveniently taken as the readily observed shoreline. The secondassumption, sand jis transported alongshore between well-defined limiting elevations

‘on the profile. The top of active berm (Dy in Figure 3-2) is the limited location inshoreward, and where the change of depth occurs no significantly, the so-called

depth of closure (Ð, in Figure 3- 2), Restriction of profile movement between thesetwo limited positions provide the simplest way to speci the perimeter of a beach‘cross-sectional area by which changes in volume, lea 1g 10 shoreline change, cạnbe computed (coastalwiki.org, 2016).

3.2.2 Fundamental equations

3.2.2. 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 theplus 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

st arp let y=?

y shoreline position,

1D, depth of elosure,

Da : average berm height above mean water level,

Q :Jongshore sediment transport rate,

x longshore coordinate of shoreline,4 : source and/or sink along the coast.

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Figure 3-2 Sand Continuity Equation Sketch

Depth of closure, D,, is also an important parameter defining the sediment motionboundaries as mentioned above.

3.2.2.) Motion equation

‘The motion equation of sediment transport depends on the evolution of the amount‘of sediment transport along coastline, AQ/@x. Most of the important variables eanbe 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‘coastine, assuming that evolution of waves in depth wave is seemed to not change

along shoreline

Infact, it can exam the sediment transport with direction-fixed waves that affecting

the ditection-different segments in compatison with coming wave angle, thus, forthe change of sini value of coming wave angle, in sediment transport formulae,@Q./ex can define its value empirically.

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Similarly, the wave angle is limited in a range of its small evolution, thus ÔQ,/êo'‘can assume that is constant. From that, the movement equation can describe as

shown in the following equation

side of coastal constructions.

Inthe 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 formulaWere ignored. Assumption about impacting coastline with small angle is verylimited, especially when bed in surf zone mainly is influenced by sediment transportalong shoreline, ia which the impacting angle has certain value, so the assumption

{ust is value in the outside of surf zone.

3.2.4 Sediment transport formulae

Van Rijn (1984) presented comprehensive formulas for calculating the bedload and suspended load, and only a short description of the method is given in thefollowings. For the bed load he adapted the approach of Bagnold assuming thatsediment particles jumping under the influence of hydrodynamic fluid forces and

_aravity forces dominate the motion of the bed load particles

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"` ` [Bae te]

` Ps e Ther

In which D* isthe dimensionless gran diameter, tye isthe effective bed shear

set for waves and cunt combined, clolated according to Van Rijn’s owa

D. = dey E =»:

y=i- lý

In which Hs is the significant wave height

“The depth integrated suspended load transport in the presence of current and wavesis defined as the integration of the product of velocity and concentration fom the

‘edge of the bed:-load layer to the water surface yields

Ba) (LY mPa [2e v4 9o s0

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speed due to turbulence and ÿ is a coefficient quantifying the influence of the‘contrifugal forces on suspended particles.

Figure 3-3 Digitalizing coastline on Google Earth

‘The coastline is digitalized for 2004 and 201 to assess the evolution of coastline. In2004, the change of the coastline shape started to develop, and it tends to erodelandward, In 2011, facing with erosion, all resorts stay in the beach have builtstructures to protection themselves individually, So the development of the coastine‘does not naturally. To conduct model calibration, the coastline in 2004 is used asinitial condition to simulate the behaviour ofthe coastline until 2011. The coastlinein 2011 is refered to fit the parameters that can impact on the change of coastline.

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3.3.L.b Cross-shore profiles

Based on the actual data, the coastal profiles are created on five different positionsin which the evolution of coastline may be different. In general, the coastal profilesof the Hoi An beach is uniform, but coastal profile mumber 1 tends to be flat‘compared to others.

-Figure 3-4 Coastal profiles on Hoi An beach

‘Coastal profile | is taken at river mount, Thus the evolution of morphology and itisalfected by several components, especially sediment from river.

In the model, the coastal profiles are digitalized by in Google Earth in different

yeats, and then the coastal profiles can be converted to ASCH format to utilize inthe model. The ASCI file format includes column one contains the distance along

the profile and column two contains the corresponding elevation (Figure 3-5).

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