Analysis of typhoon risk associated with storm surge and wind waves in southern vietnam

A numerical coupled model using wind field from the JMA Grid Point Value GPV reanalysis data and considering two-way interaction between storm surge and wind wave was used to confirm the

Analysis of Typhoon Risk associated with Storm Surge and Wind Waves

in southern Vietnam

A Dissertation Submitted to the Department of Transdisciplinary Science and Engineering

In Partial Fulfillment of the Requirements of the Degree of Doctor of Engineering

Le Tuan Anh

2019

ACKNOWLEDGMENT

Undertaking this Ph.D has been a truly life-changing experience for me and it would not have been possible to do without the support and guidance that I received from many people I would like to first say a very big thank you to my supervisor Assoc Prof Hiroshi Takagi, whose expertise was invaluable in the formulating of the research topic and methodology in particular I am very appreciated for all the support and encouragement he gave me, during both the field trips in Mekong Delta and also the time I spent at Takagi laboratory Without his guidance and constant feedback this Ph.D would not have been achievable

Besides my advisor, I would like to thank the rest of my thesis committee: Prof Manabu Kanda, Assoc Prof.Daisuke Akita, Assoc Prof.Takashi Nakamura, and Dr.Alvin Christopher Varquez, for their insightful comments and encouragement, which help me to widen my research from various perspectives

I would like to acknowledge my colleagues from Takagi laboratory for their wonderful collaboration You supported me greatly and were always willing to help me Thank you for the stimulating discussions and for all the fun we have had in the last three years

Last but not least, I would like to thank my family for supporting me spiritually throughout writing this thesis and my life in general

ABSTRACT

Typhoon and storm surge are the biggest hazards that threaten coastal communities in Vietnam The possibility of a large typhoon taking place in Southern Vietnam is considerably smaller than in the northern and central parts of the country, though this does not necessarily mean that Southern Vietnam is less vulnerable against typhoons The questionnaire surveys were carried out to investigate disaster awareness amongst local inhabitants in Southern Vietnam Although typhoon rarely occurs, the interview results show that the local population has a high degree of awareness about the dangers posed by those events However,

it seems that they did not necessarily consider the risk as their own problems Typhoon Linda 1997 is the historical event that caused catastrophic damage for this region, and also significantly affected local people's perception of typhoons Nevertheless, it is still required to improve preparedness against coastal disasters in Vietnam, especially in terms of the evacuation plan and coastal disaster education for fishermen as well as younger generation A numerical coupled model using wind field from the JMA Grid Point Value (GPV) reanalysis data and considering two-way interaction between storm surge and wind wave was used to confirm the accuracy of the model through investigating the physical impact of Typhoon Jebi, which struck Kansai region of Japan in 2018 The simulated results fit well with the measured water level and wave height during its passage, showing the reliability of the proposed model The same model can be used to investigate the extent of the storm surge and high waves during the worst Typhoon Linda in Southern Vietnam in 1997 However, due to the limitation of meteorological data, the wind field obtained from the parametric typhoon model is used instead of the reanalysis data The good agreement in comparison between simulation results and observed data demonstrates the applicability of the simplified model for those typhoons transiting off the coast of Southern Vietnam, where the number of fishing boats always seek productive fishing spots The scenario with a hypothetical typhoon is performed to create a spatial wave distribution map, designed for fishermen during their operation This kind of hazard map will be beneficial for disaster management officers

to assess whether they can return to the origin or neighboring safer islands to evacuate when an unexpected typhoon approaches

Table of content

ACKNOWLEDGEMENT………i

ABSTRACT……… ii

LIST OF FIGURES……… vi

LIST OF TABLES……… ix

CHAPTER 1 Introduction……….………… 1

1.1 Coastal disaster in Vietnam……… ………1

1.1.1 Tropical typhoon……….1

1.1.2 Storm Surge……… 2

1.1.3 Wave climate classification……… 2

1.1.4 Tides……….………… 3

1.2 Storm surge Model……….………3

1.2.1 ADCIRC……… 3

1.2.2 SLOSH……….………… 4

1.2.3 FVCOM……… 5

1.2.4 CH3D……… 6

1.3 Recent researches on Storm surge and typhoon associated disasters……… 6

1.3.1 Studies in storm surge in the world……….6

1.3.2 Storm surge researches in Vietnam……….………7

A Numerical model development……….………7

B Typhoon perception in Vietnam……….………… 8

1.4 Aims and Objectives……… 8

1.5 Methodology……… 9

1.6 Outlines of this study………9

CHAPTER 2 Typhoon perception in Southern Vietnam………11

2.1 Introduction……….11

2.2 Typhoon Track Analysis……….12

2.3 Storm surge awareness in Mekong Delta………14

2.3.1 Interview survey ……… 14

2.3.2 Typhoon Linda In 1997………17

2.3.3 People’s Awareness in Mekong Delta……… 19

2.4 Field survey in Con Dao……….22

2.4.1 The purposes of the survey……… 22

2.4.2 Memory of Linda and Awareness of Typhoon Disasters in Con Dao Island………23

2.5 Discussion On Disaster Risk Awareness Improvement……… 25

2.6 Conclusion……… 27

CHAPTER 3 Coupled Storm surge – Wave model using Meso-scale Data - Hindcasting with an ideal meteorological input……… 28

3.1 Introduction……… 28

3.2 Methodology……… 29

3.2.1 Field survey……… 29

3.2.2 Wind–wave and Storm Surge Hindcasting……… 30

3.2.3 Analysis of tide gauge data……… 33

3.3 Results………35

3.3.1 Field survey……… 35

3.3.2 Wind-Wave and Storm Surge Hindcasting………45

A Wind and pressure fields………45

B Storm surge simulation……… 47

C Wind-wave simulation………48

D Tidal effect on wave and storm surge simulation……… 49

3.4 Discussion……… 52

3.4.1 Improvement of the proposed 2-way coupled model compared with other conventional models……… 52

3.4.2 Wave-Storm surge ratio……….54

3.5 Conclusion……… 58

CHAPTER 4 Coupled Storm surge – Wave model using, parametric typhoon Model - Hindcasting with a limited meteorological input……….59

4.1 Introduction……… 59

4.2 Methodology……… 60

4.2.1 Applicability of wind field from Typhoon model and GEBCO08 bathymetric data in Vietnam………60

4.2.2 Coupled model description………63

4.3 Results……… 64

4.3.1 Storm Surge Induced by Linda……… 64

4.3.2 Waves Generated by Linda………66

4.4 Discussion……… 68

4.5 Conclusion……… 71

CHAPTER 5 Discussion – Application of the models for disaster management……… 72

5.1 Discussion on policy, public education, and training regarding typhoon and storm surge risk management in Vietnam……… 72

5.2 Hazard map for fishermen………75

CHAPTER 6 Conclusion………80

REFERENCES……… 81

List of Figures

Figure 2.1 (a) Landfall points along Vietnamese coast between 1951 and 2010, (b) Annual

frequency of landfalls for each one degree segment of the coastline, (c) Total number of landfalls for each one degree segment of the coastline between 1951 and

2010, and (d) Annual total number of tropical cyclones making landfall Vietnam

between 1951 and 2010……… 13

Figure 2.2 Location of surveyed areas in Vietnam……… 15

Figure 2.3 Interview with local people in Can Tho city……… 16

Figure 2.4 Age distribution of respondents in Mekong delta……… 16

Figure 2.5 Occupation of respondents……… 17

Figure 2.6 Distribution of people have experienced damage from previous disasters………… 18

Figure 2.7 Surge height, as indicated by a local resident who remembered Typhoon Linda……… 18

Figure 2.8 Wind speed during Typhoon Linda in Can Tho city……… 19

Figure 2.9 Damage in Can Tho city during Typhoon Linda 1997……… 19

Figure 2.10 Distribution of respondents who were aware of the nature of storm surges in Mekong Delta……… 20

Figure 2.11 Distribution of respondents who think that a storm surge constitutes a real danger for them in Mekong Delta……… 20

Figure 2.12 Distribution of respondents regarding whether they have taken part in evacuation drills in the last 5 years……… 21

Figure 2.13 Distribution of people who know how to evacuate in the event of a typhoon / storm surge……… 21

Figure 2.14 Houses near water in Ca Mau……… 22

Figure 2.15 Circular plots on this satellite image indicate the towns of Con Dao Island where we conducted the survey (White dots: East coast; yellow dots: Northern village.) A total of 103 questionnaires were collected in Con Dao The insets show interviews with local witnesses……… 23

Figure 2.16 Occupation of respondents……… 24

Figure 2.17 Questionnaires results regarding people awareness and preparedness in Con Dao……… 25

Figure 2.18 Do children learn about storm surge/flooding in school, and how to evacuate? 26

Figure 2.19 Tsunami instruction panel in Con Dao……… 27

Figure 2.20 Source of information on storm surge and typhoons in Southern Vietnam……… 27

Figure 3.1 Japan Meteorological Agency’s weather map immediately before Jebi made landfall (September 4, 2018, 09:00, Japan Standard Time, UTC+9)……… 29

Figure 3.2 Processes in one-way and two-way coupled models………

Figure 3.3 Simulation domains for simulating wind waves during Typhoon Jebi The indicated locations have wave monitoring stations of the NOWPHAS and wind observation stations from JMA, whose data were used in this study for model verification………

32 34 Figure 3.4 Map of west Japan showing the locations of tide gauge stations considered in this study and trajectory of Typhoon Jebi from August to September of 2018……… 35 Figure 3.5 (I) Large view of field survey area, (II) Location of survey points, (III) storm surge

and overtopping height measurements inside Osaka Bay and (IV) around Kii Strait The primary mechanism of elevated sea level: wave overtopping (red) and

storm surge (yellow) The blue line indicates the trajectory of Typhoon Jebi

Locations (a) to (o) indicate the sequence of survey points……… 37 Figure 3.6 Field survey at Nanko Bird Sanctuary and Sakai (locations (a) and (b) in Figure

3.5-III) (I) Survey locations at the sanctuary, (II) trash accumulated over the main route, (III) sea dyke, (IV) scour behind the dyke due to wave overtopping, (V) damaged building near the dyke, (VI) difference in grass color demonstrating that seawater reached a height up to the withered grass and (VII) broken parapet

Figure 3.7 Field survey at Rinku Park (location (c) in Figure 3.5-III) (I) Park overview two

days after Typhoon Jebi impact showing trash and a damaged roof We measured the elevation of the ground where trash remained (II) Inner and (III) outer protection layers of the park……… 41

Figure 3.8 Field survey at Wakayama (locations (d) and (e) in Figure 3.5-IV) (I) Localization

of Kainan and Saikazaki, (II) coastal fence smashed by high waves at Kainan,

(III) broken parapet by overtopping waves at Saikazaki……… 42

Figure 3.9 Field survey at Tokushima (locations (f) and (g) in Figure 3.5-IV) (I) Damaged

roof in a village from Anan, (II) high sea dyke at Minami Awa……… 42

Figure 3.10 Field survey at Honjo river mouth (location (h) in Figure 3.5-IV) (I) Location

of training wall and (II) smashed handrail of the training wall……… 43

Figure 3.11 Field survey at eastern coast of Awaji-shima island (locations (i) to (k) in Figure

3.5-IV) (I) Location of surveyed places, (II) armored breakwater at Minami Awa Fishing Port, (III) smashed guardrail at Awa Nadakuroiwa, (IV) rubber fenders

found at Awa Nadashirosaki caused a large punching hole on a wall……… 44 Figure 3.12 Field survey at Kobe city (locations (l) to (o) in Figure 3.5-III) (I) Kobe Meriken

Park storm surge and high waves during Typhoon Jebi (rough sea screenshot from online live camera at 14:17, September 4, 2018, Japan Standard Time) (https://www.youtube.com/watch?v=lCupBcgCuO8), (II) situation after 4 days

of the typhoon at Kobe Meriken Park, (III) fallen bricks at Nishinomiya Yacht Harbor, (IV) destroyed inland floodgate at Amagasaki Port, (V) stranded large vessel at Amagasaki Port, (VI) trash gathered behind breakwater, (VII)

Koshienhama Artificial Beach, where wave overtopping was confirmed……… 45 Figure 3.13Comparison between wind speed obtained from the JMA mesoscale spectral

model and observed data on 4 September 2018 (time in Japan Standard Time,

Figure 3.14 Comparison between air pressure obtained from the JMA mesoscale spectral

model and observed data on 4 September 2018 (time in Japan Standard Time,

UTC+9)……… 46 Figure 3.15 Wind field distribution during Typhoon Jebi passage (I) over Kii Strait on

September 4, 2018, at 11:00 and (II) inside Osaka Bay at 14:00 (Japan Standard Time, UTC+9)……… 46

Figure 3.16 Comparison of calculated surge level with flow–wave interaction, no interaction

with wave, and observed values during Jebi passage……… 47

Figure 3.17 Spatial distribution of storm surge caused by Typhoon Jebi inside Osaka bay from

(I) two-way coupled model and (II) no-coupling single model (Japan Standard

Figure 3.18 Significant wave height obtained from our surge-wave model and observed data

Time is expressed in Japan Standard Time, UTC+9……… 49

Figure 3.19 Significant wave height distribution during Typhoon Jebi passage (I) over Kii

Strait on September 4, 2018, at 11:00 and (II) inside Osaka Bay at 14:00 (Japan Standard Time, UTC+9)………

Figure 3.20 Comparison of wave height from different models with different coupled

components (Japan Standard Time, UTC+9)………

Figure 3.21 Comparison of the total water level (storm tide) from coupled storm surge -wave

– tide model (Japan Standard Time, UTC+9)………

Figure 3.22 Comparison of significant wave height between 2-way couple model and other

models (Japan Standard Time, UTC+9)………

49

50

52

53 Figure 3.23 Sea level in September 2018 affected by Typhoon Jebi based on analyses of tide

gauge data (a) Original tide gauge records and tide prediction (b) SA and SD at different tide gauge 15-minute average waveforms to remove wave

Figure 3.24 Power spectrum of component waves during 12 hours including the arrival time

Figure 4.1 Comparison between wind field during Typhoon Jebi reproducing by GPV model

Figure 4.2 Comparison of Significant wave height and Storm surge during Jebi between the

2-way coupled models using using four different combination input data set (time in Japan Standard Time, UTC+9) at Komatsujima and Kaiyo Tokushima 62 Figure 4.3 Simulation grids, combining larger and smaller domains……… 63 Figure 4.4 Central pressure and wind speed during Linda (Vietnamese local time)………… 64 Figure 4.5 Distribution of the storm surge induced by Typhoon Linda, estimated by a

typhoon-storm surge integrated model……… 65 Figure 4.6 Comparison between observed and simulated water levels, including storm surge

and tide, during Linda from three locations in the Mekong Delta Observed data were derived from our previous study (Vietnamese local time UTC+07)……… 66 Figure 4.7 Simulated and observed wave heights at Huahin, Ko Chang, and Rayong as Linda

approached the Gulf of Thailand (Observed data retrieved from the previous

Figure 4.8 Wave-height distribution when Linda crossed the south side of Con Dao at 7:00

a.m on November 2nd, 1997 (Vietnamese local time)……… 67 Figure 4.9 Simulated wave height at three stations from the model using coarse and finer

Figure 4.10 Simulated wave height around ConDao from the model using coarse and finer

Figure 4.11Comparison of estimated (a) surge component with and without wave interaction

and (b) wave height with and without considering the storm surge effect at Ganh Hao from model using coarse bathymetry………

Figure 4.12 Spatial distribution of storm surge in Mekong Delta from (I) coupled model and

(II) singled model caused by Typhoon Linda………

Figure 4.13 Spatial distribution of wave in Mekong Delta from (I) coupled model and (II)

singled model caused by Typhoon Linda………

70

70

71 Figure 4.14Temporal change of wave height caused by Typhoon Linda from one-way and

Figure 5.1 Structure of the organizations that taking responsibility to provide directions and

guidance to local populations to respond to a natural disaster……… 73

Figure 5.2 Major fishing spots in Southern Vietnam (the white arrows in the right inset show

the possible route toward Con Dao that many fishing boats took to escape from high waves during Linda; those in the left inset show the escape route according

to interviews with fisherman in the local newspaper……… 76 Figure 5.3 Two similar strong typhoons approached Mekong Delta after 20 years……… 77 Figure 5.4 The wave spatial distribution caused by hypothesis typhoon at 21:00 1st November

1997 (Vietnamese local time) ……… 78 Figure 5.5 Wave-height distribution when hypothetical typhoon crossed the south side of

Con Dao at 7:00 a.m on November 2nd, 1997 (Vietnamese local time)… 79

Table 3.2 Comparison of wave height from different models with different coupled

components (The cells highlighted in yellow show the most accurate results

compared with observed data)………

Table 3.3 Comparison of max wave height from 2-way coupled model and other different

models (The cells highlighted in yellow show the most accurate results compared with obseerved data)………

36

51

53 Table 3.4 Wave height and storm surge level at 8 locations in Kii Strait and Osaka Bay and the

wave/surge ratios caused by the 2018 Typhoon Jebi………

Table 4.1 Investigation the sensitivity of different combination of input data set (The cells

highlighted in yellow show the most accurate results among four combinations)…

57

61 Table 5.1 Relation between wind speeds and wave heights……… 77

Chapter 1 Introduction

1.1 Coastal disaster in Viet Nam

1.1.1 Tropical typhoon

The two most common types of typhoon, are tropical typhoons and extra-tropical typhoons, which

have different generation mechanisms and also differ in their size and intensity The difference is important since tropical systems have the potential to quickly grow into hurricanes (when the sustained wind speed is over 74mph ~ 119 km/h), while extratropical do not Extra-tropical typhoon form in the transition zone between subtropical and polar climate zones (between latitudes 35° and 70° north and south of the equator)

It primarily gets energy from the horizontal temperature contrasts that exist in the atmosphere Extra-tropical typhoon (also known as winter storm) are low-pressure systems with associated cold fronts, warm fronts, and occluded fronts Tropical typhoon, in contrast, typically have little to no temperature differences across the typhoon at the surface and their winds are derived from the release of energy due to cloud/rain formation from the warm moist air of the tropics (Merrill, 1993)

Vietnam is a country that is frequently hit or affected by the tropical typhoon, 80% of the disasters affecting Vietnam are typhoon related (induced) (GTZ, 2003) The peak occurrence of typhoon landfalls is different depending on the region For the central part the peak time is normally during the month of October, whereas, in the southern part, it is generally in November There were approximately 786 typhoons that approached or affected Vietnam during the twentieth century These storms generally hit the mainland, especially the coastal provinces in the north and the center of Vietnam (Kleinen, 2007) Recently, regarding the influence of climate change, an increase in the number of high-intensity typhoons affecting Vietnam was announced by the Ministry of Natural Resources and Environment of Vietnam (MONRE) It has been identified that there is a trend that typhoon tracks tend to move southward and the typhoon seasons end later

(MONRE, 2003, 2009) According to the statistic from MONRE that summarized from a number of Vietnamese reports and articles, the higher number of typhoon in the context of climate change could lead

to growing concerns over the threat posed by typhoons

There is a general belief that the frequency of tropical cyclones will progressively increase, supported

by a number of researchers, who have stated their concerns about the possibility that global warming may have affected not only quantity but also the intensity of tropical typhoons Knutson et al., 2010 found that some increase in the mean maximum wind speed of tropical typhoons is likely (+2% to +11% globally) with projected twenty-first-century warming, although increases may not take place in all tropical regions

Webster et al., 2005 alsoproved the increase in their intensity through analysis of the 30-year satellite records

of tropical typhoons Furthermore, an analysis of the trends in the upper quintiles of typhoon maximum wind speeds pointed out a significant upward trend for wind speed quintiles above the 70th percentile (Elsner et al., 2008) Such increases could have important consequences for ports and coastal areas, which would be affected by higher waves and suffer more disruption to economic activities (Takagi et al., 2011; Esteban et al., 2009) The Emergency Events Database (EM-DAT) indicates that the economic damage associated with storms has been rapidly growing in Vietnam By contrast, the fatality rate due to storm-relevant disasters has been declining in recent decades (Takagi, 2019)

1.1.2 Storm Surge

Storm surge is the result when both the high winds pushing the ocean’s surface and the low pressure

at the center of a storm system sucking in the ocean water, that combined effect leads to an increase in sea levels The effect of astronomical tide is not taken into account in the storm surge However, the combined effect of astronomical and meteorological surges is often referred to as a storm tide When typhoons hit the land during the high tide, the consequent damage will be more severe due to the combination of high tides, storm surges, and large waves; the increasing water will go further into the mainland, causing flooding in the coastal areas and destroying people’s property

The storm surges caused by the tropical typhoon are different in character to those generated by higher latitude storms (i.e extratropical storms) Tropical storm surges tend to be of smaller spatial scale (~500 km rather than ~1000 km), and have a shorter duration (hours to days rather than several days), but are much larger in amplitude (sometimes 5–10 m rather than typically 2–3 m)

The destruction caused by storm surges is also a serious concern for China, the Philippines, Vietnam, and Thailand all surrounding the South China Sea According to the statistical data, storm surge also caused severe damage in Vietnam As examples, Typhoon Kelly in 1981 made landfall in Nghe An, Central Vietnam, caused storm surge up to 3.2m, water level elevation during Typhoon Andy, 1985 in Quang Binh, Central Vietnam, was 1.7m In 1986, in Thai Binh, North Vietnam, storm surge induced by Typhoon Wayne was recorded at 2.3m (Truong et al 2016)

The southern part of Vietnam is generally less vulnerable to typhoons or storm surges compared to the northern and central parts of the country according to the statistical data It should also be noted that the low-lying nature which characterizes coastal areas such as the Mekong Delta can substantially increase the vulnerability of local communities to storm surges, even if surge heights are relatively small Thus, a given storm surge height that would not cause great inundation in central or northern parts in the country could devastate coastal settlements in the Mekong Delta or other low-lying areas (Takagi et al 2014b) Strong monsoon winds can cause high water elevations in the Mekong Delta In combination with a spring tide, this produces a storm surge with water levels that are elevated by up to 0.8 - 0.9 m The consequence of the large waves during a storm surge can cause the destruction of exposed infrastructure along the coast The sea-level rise projected by climate models will increase the height of water levels during storm surges by up to 1 m (Albers et al 2013) Water levels of this height combined with waves of 1 - 2 m will lead to the overtopping

of dykes that are built according to the current recommended dyke standards

1.1.3 Wave climate classification

The warm and humid nature which characterize summer monsoon is referred to as the SW-monsoon, while the NE-monsoon referred to the relatively cool and dry winter monsoon The east coast of Southern Vietnam including Mekong Delta is predominantly exposed to waves during the NE-monsoon (Pham, 2011) The corresponding wave climates have the seasonal characteristic due to the relatively moderate and persistent monsoon wind In winter, the higher waves from the northeast dominate the wave climate, when during summer, the dominant wave directions are from the southwest and wave heights are lower In northern Vietnam, the average height of deep ocean waves is 1.8-2.0 m in winter and 1.2-1.4 m in summer Along the southern Vietnam continental shelf (Hoang & Nguyen 2006), the average wave height is 1.6 m and the wave period about 5 s However, strong SW monsoon winds occasionally generate waves of up to 3 m in height (Dat & Son, 1998) The corresponding wave heights on the east coast of Ca Mau can be up to 2 m nearshore during the northeast monsoon (ADB, 2011)

1.1.4 Tides

The combination of the rotation of the earth with the varying gravitational impacts of the sun, the moon and the planets on the water is the mechanism that generating the astronomical tide The tide mainly travels from where it generated in the deep ocean to the coastal waters The height of the tidal wave in deep water is normally less than 0.5 m In shallow coastal waters, it is affected by shoaling and friction In restricted waters, such as the estuaries, changes of the cross-section and reflection lead to tidal ranges increase Tidal conditions in North Vietnam are mainly diurnal, which have only one-time low water and one-time high water every day, the highest can reach to 4m The semi-diurnal tide regime dominates in South Vietnam, which has two times low water and two times high water every day, the tidal range is about 3-4m (the highest in Vietnam) Mixed tides are observed in Central Vietnam where the transition from diurnal to semi-diurnal regime occurs, the tidal level increase from 1m in North Central Vietnam to 3-4m in the southern part The tide is higher than normal during the full moon and new moon (spring tide), while it is lower during the quarters (neap tide) A location with a tidal range > 1.5 m is defined as a macro tidal regime The coastal morphology at sites with a macro tidal regime and a relatively mild wave climate is normally mainly influenced by the tide (Mangor, 2004) This regime is often reflected in wide tidal flats and can be found, for example, on the east coast of the Mekong Delta

1.2 Storm Surge Model

Nowadays, a number of storm surge modeling systems have been widely used for predicting the impact of tropical cyclones (inundation in coastal areas due to tropical storms), covering a range of numerical methods, model domains, forcing and boundary conditions, and purposes Common models include the Advanced Circulation (ADCIRC) model, Curvilinear-Grid Hydrodynamics 3D Model–Storm Surge Modeling System (CH3D-SSMS), the Finite-Volume, Primitive Equation Community Ocean Model (FVCOM), and the Princeton Ocean Model (POM;) While sometimes used for the same purpose, different models are quite different in approaches, have different advantages and disadvantages, and each is likely more well-suited for a particular application In this part, the general information of the most common models will be summarized

in the Gulf of Mexico (Fossell et.al 2017, Forbes et al 2010), along the eastern US coast (Lin et.al 2010, Colle et.al 2008, Yin et.al 2016, Cialone et.al 2017, Lin et.al 2010)

The governing equation in ADCIRC is the shallow water equations (SWEs), that is using the traditional hydrostatic pressure and Boussinesq approximations and have been discretized in space using the Galerkin finite element method and in time using the finite difference method The elevation is obtained from the solution of the depth-integrated continuity equation in Generalized Wave-Continuity Equation

(GWCE) form, and the velocity is obtained from the solutions of either the 2DDI (two-dimensional integrated) or 3D momentum equations, retaining all nonlinear terms (Kolar et al 1994) The GWCE is solved using either a consistent or a lumped mass matrix and an implicit or explicit time-stepping scheme

depth-(Kolar et al 1994) ADCIRC uses a finite element unstructured triangular grid allowing for very high refinement of coastal regions (successfully modeled at scales less than 50 meters) and coarser resolution in the deep ocean Several features including within ADCIRC improve the accuracy in simulating storm surge The feature of modeling the wetting and drying of inundated areas is available It also can represent subgrid-scale obstructions to flow as weirs For the simple simulation, the model can be run on a single processor, but for other normal cases, it is mostly run in parallel on high-performance computing systems with hundreds

of processors using the Message Passing Interface (MPI) Because the ADCIRC model fully reproduces the complex physical process associated with storm surge and often uses very high-resolution grids over a relatively large domain, the model is computationally expensive to be applied to large numbers of simulations (Lin et al 2010) The computational cost of ADCIRC runs may be prohibitive when many local design and failure scenarios are to be simulated (John Baugh 2015) The small time steps required by the semi-explicit solution scheme also come with high computational cost

1.2.2 SLOSH

The SLOSH storm surge model was developed by the US National Weather Service (US NWS)

(Jarvinen and Lawrence,1985; Jelesnianski et al., 1992) SLOSH is used by NWS to implement a) real-time operational b) hypothetical (for evacuation planning) c) historical (for validation purpose) d) probable

(Taylor et.al 2008) and e) extratropical storm surge simulations A simplified parametric wind model is embedded in the SLOSH model That wind model uses the input parameters including pressure, size, forward speed, and track data from tropical cyclones to creat the wind field which pushes the water around model The SLOSH model consists of a set of equations derived from the Newtonian equations of motion (shallow water equations) and the continuity equation applied to a rotating fluid with a free surface The equations are integrated from the seafloor to the surface, the advective terms in the equations are discarded The coastline is served as a physical boundary Subgrid-scale water features (cuts, chokes, sills, and channels), and vertical obstructions (levees, roads, spoil banks, etc.) are parameterized The model accounts for astronomical tides by specifying an initial tide level The SLOSH is a 2-D explicit, Finite Difference (FD) model formulated on a semi-staggered Arakawa B-grid (Arakawa et al., 1977) SLOSH grids for all vulnerable areas of the US coast have been constructed by the NWS for use in emergency management (http://www.weather.gov/tdl/marine/Basin.htm) The structure of a SLOSH grid results in finer resolution near the pole of the grid (which is placed near the area of interest) and coarser at its outer boundary These grids are limited in domain size and have their open ocean boundaries located on the shelf SLOSH has the capability to simulate wetting and drying as well as parameterize sub-grid scale features such as 1-D channel flow with contractions and expansions, vertical obstructions to flow with overtopping (levees, roads, and banks that include cuts) and increased friction drag in heavily vegetated areas

However, as other numerical models, SLOSH does have limitations First, the SLOSH grid is generally limited in size to the coastal shelf surrounding the study area The use of a structured grid limits the capability to provide localized refinement While SLOSH grids do have increased refinement at their center, it is not always possible to resolve additional features that may be elsewhere (e.g., an inlet along the coastline that is away from the center of the grid) Second, the shelf based, regional nature of a SLOSH domain limits accurate specification of boundary conditions during storm surge events because of lack of knowledge of set-up at the open boundary and prevents dynamic coupling to larger basins However, the SLOSH simulation may not be able to capture some unusual water responses to storms at locations with

complex geophysical features In addition, SLOSH does not include rainfall amounts, river flow, or driven waves

wind-1.2.3 FVCOM

The finite difference (FD) schemes are the earliest techniques employed for simulating storm surge The resulting computational codes using FD schemes showed the advantage when have proven to be efficient and robust, thus they are widely applied However, FD methods also have a disadvantage, that comes from their use of structured grids That kind of grid makes the task of refining the complex coastal geometries and localized flow patterns near the coast and over floodplains is impossible It is noticeable that the grid resolution and orientation significantly affect the accuracy of surge height To overcome that limitation of

FD methods, the finite element (FE) method, that allowed use of unstructured meshes has applied to the SWE With FE method, it is able to map complex geometry quite accurately The advantage comes from the fact that using unstructured grid produces the domains that varying nodal spacing over several orders of magnitude, in order to minimize computational effort while maximizing accuracy (Blain et al., 1998; Westerink et al., 1994) This allows for high resolution grid at the interesting coastal areas such as inlets and barrier islands while using much larger node spacing further away in where not necessary The method that proved to be most successful and widely used for FE modeling of the SWE is the wave equation formulation, due to its suppression of spurious oscillations (Lynch and Gray, 1979; Kinnmark, 1986) However, the application for the shallow, nonlinearly dominated flows reveal the limitation, including mass balance error,

as the continuity equation is not being satisfied in the wave equation, but rather its time derivative (Walters and Carey, 1984; Kolar et al., 1994) Additionally, the costly simulations come from the required very small time steps below the Courant condition has been found when using this FE approach The combination of small time step and high resolution unstructured grid requires many computational resources in comparison with FD models for storm surge simulations, limiting application of FE method in time-constrained applications Recent there is the idea to combine the strengths of FD (mass conserving) and the strengths of

FE (high resolution unstructured grid) methods That resulted in various forms of finite volume (FV) techniques, but use of these models has not yet been widespread although results have been reliable In general, finite volume models require a relatively longer computational time than finite difference models (Kazuhiro Aoki et al 2007) and the same cost issues regarding the size of highly resolved grids in FE modeling also apply to FV models as well

FVCOM - the unstructured-grid finite-volume coastal ocean model (FVCOM) developed by Chen et

al (2003) - is a free-surface, three-dimensional primitive equations model that fully couples sediment-ecosystem models with options for various turbulence closure schemes, generalized vertical terrain-following coordinates, and data assimilation under hydrostatic and non-hydrostatic approximations FVCOM also includes provision for flooding and drying, a critical element of storm surge simulation (Hubbert and McInnes, 1999; Peng et al., 2006; Weisberg and Zheng, 2006a, 2006b) FVCOM was modified by Weisberg and Zheng (2006a) for the addition of atmospheric pressure gradient effects The finite-volume approach in FVCOM combines the geometric flexibility feature of finite-element methods with the simple and efficient computational structure feature of finite-difference methods FVCOM solves the primitive and turbulence equations using a second-order accurate flux calculation integrated over each model grid control volume This ensures mass, momentum, energy, salt, and heat conservation in the individual control volumes and also over the entire computational domain (Chen et al., 2003) The advantages of flexible unstructured-grid and mass conservative nature make FVCOM be suitable for interdisciplinary applications in coastal oceans FVCOM has been used by many researchers to simulate tidal and estuarine circulation (Xing et al 2011; Zheng and Weisberg 2012; Yang et al 2012; Yang and Wang 2013), storm surge predictions (Weisberg and Zheng 2006, 2008)

ice-ocean-wave-1.2.4 CH3D

Another well-developed FD model that has been applied to storm surge is the Curvilineargrid Hydrodynamics model in 3D (CH3D; Sheng, 1986; Sheng and Alymov, 2002)and has been significantly enhanced (Sheng and Kim 2009; Sheng et al 2010) It is a parallelized model capable of running large grids using MPI on high performance computing systems CH3D uses a structured curvilinear grid and has been applied on domains with resolution reaching less than 20 meters It is a fully nonlinear model that has been coupled with larger scale circulation models Various wind models have been used to provide input

The governing equations for CH3D are based on the wave- and Reynolds-averaged Navier-Stokes equations in a horizontally boundary-fitted curvilinear grid and a vertically terrain-following sigma grid, with assumptions of incompressible water, hydrostatic pressure, Boussinesq approximation, and eddy-viscosity concept The non-orthogonal boundary-fitted grid enables CH3D to more accurately represent the complex geometry than orthogonal grids used by such models as POM (Blumberg and Mellor, 1987) and ROMS (Song and Haidvogel, 1994) CH3D uses a second-order closure turbulence model (Sheng and Villaret, 1989) for vertical turbulent mixing, and in horizontal Smagorinksy-type turbulent diffusion coefficients are used Recent additions to the CH3D features include flooding and drying (e.g.,

Davis and Sheng, 2003), wave-current bottom stress (Sheng and Villaret, 1989), and wave-induced surface stress (Alymov, 2005) However, CH3D is not suitable used for the basin-scale surge, too many grid cells are required for that simulation would lead to huge increase in computational resources

1.3 Recent researches on Storm surge and typhoon associated disasters

1.3.1 Studies in storm surge in the world

Numerical modeling is a powerful tool for forecasting and hindcasting typhoon related disasters at desired temporal and spatial scales and resolutions The major damages caused by the typhoon are due to storm surges and coastal flooding Numerous researchers have studied and developed to typhoon related coastal disasters including typhoon storm surge and high wave Shen et al (2006) simulated diagnostically the effect of offshore surge on storm tide inside the Chesapeake Bay caused by Hurricane Isabel in North Carolina in 2003 using UnTRIM model, without considering the effect of waves They found that the water level increasing inside the Bay during Isabel was the combination of offshore surge propagation into the Bay and local wind Nakamura et al., 2016 used WRF +FVCOM to simulate the storm surge caused by Typhoon

Haiyan in Phillippines in 2013, but not considered the wave impact.The Global Forecast System (GFS) was used to generate the initial atmospheric conditions of the WRF in this study They employed the TC-Bogus scheme (Hsiao et al 2010) that based on a Rankin vortex (Kurihara et al 1993, 1995) in order to generate better initial typhoon conditions after removing the original typhoon from the initial atmospheric condition and creating a typhoon However, the value of the storm surge at every location is also underestimated by approximately 1 m S.K Dube et al 2009 ran the vertically integrated numerical storm surge prediction model developed by Dube et al 1994, using the wind field at the sea surface derived by using a dynamic storm model developed by Jelesnianski and Taylor (1973) to investigate storm surge caused by several typhoons in the Bay of Bengal and Arabian Sea In the present models, the cyclonic storm is the sole driving force for the dynamical processes in the sea However, the tides and wave setup have not been included in the present study Forbes et al 2010 implemented ADCIRC model without coupling to a wave model to simulate the storm surge produced by Hurricane Gustav (2008) They mentioned that the underprediction of

maximum water surface elevation in the simulations could be due to the exclusion of surface waves from the simulations, although it is well known that waves provide additional surge due to wave setup

In this regard, beside storm surge, wind-wave characteristics are very important in the design and construction of ports, waterways, bridges, oil platforms, undersea pipelines, coastal defense facilities, and other coastal engineering projects, especially during the extreme conditions Sheppard and Renna (2004)

investigated the collapse of the bridge deck of Interstate-10 crossing the Escambia Bay arm of Pensacola Bay during hurricane Ivan and concluded that it is due to the combined effect of large waves and storm surge

In addition, wind waves also affect on the water circulation, pollutants and sediment transports in coastal areas (Lv et al., 2014) The relatively large waves, reaching over 10–20m in deep open ocean waters can be generated by hurricanes Wang et al (2005) reported that the largest waves reached 27.7m during the passage

of Hurricane Ivan, 2004 Araújo et al (2011) implemented the circulation model ADCIRC to simulate the storm surge in the Viana do Castelo coast (Portugal) Their study had taken into account the astronomical tide, the atmospheric pressure, and the wind only That resulted in the discrepancy between the computed elevation and the total water level registered by the tide gauge for the event during the storm surge peak The authors pointed out the absence of the wave set-up simulation and a too coarse grid refinement in the nearshore zone are considered the main reasons The grid resolution factor is also confirmed by Kohno and Higaki (2006) when they showed that a poor resolution in the mesh may result in underestimated sea levels Recent work by Kim et al (2010) suggests that, in some open coast locations, the surge cannot be reproduced

by simple models using only atmospheric pressure and wind The taking account of astronomic tides, atmospheric pressure, wind and wind waves is necessary The impact of wave on the storm surge has been attracted many researchers’ interest several decades ago Signell et al (1990) employed two-dimensional (2-D) numerical hydrodynamic model studied the influence of wave-current interactions on wind-driven circulation in a narrow, shallow embayment There were the drawbacks of this study when the variation of flow within the water column was not resolved, and the wave field was assumed to be constant, which limited the application of the solutions. Mastenbroek et al (1993) and Zhang and Li (1997) carried out the study of coupling of surges and waves through the wave-induced radiation stresses and surge-induced currents However, in their coupling models, the global ocean wave prediction model WAM were not considered shallow water wave transformations, including wave breaking. Davies and Lawrence (1995) showed that surface waves could play an important role in determining both near-surface and near-bottom currents as well as on water level variation over a coastal region when examined the effect of wave-current interaction

on three-dimensional (3-D) wind-driven circulation However, in their study, surface waves were also considered to be a constant external input into the current model Signell et al (1990) and Davies and Lawrence (1995) incorporated wave effect into the bottom stress of storm surge governing equations

Donelan et al (1993) developed a method to calculate the drag coefficient as a function of wave parameters According to Shibaki et al (2001), the inclusion of radiation stress in the momentum equations gave reasonable results for sea level rise when surge and wave models were separately run Morey et al (2006)

showed that it is important to use a large model domain to incorporate the effect of remote forcing contribution to storm surge during Hurricane Dennis Some researchers have another approach by employing physical experiments to investigate the wave-current interaction Simons and Maciver (1998) performed experiments with regular deep-water waves propagating obliquely across a relatively narrow jet-type current

Maciver et al (2006) designed an experiment to assess the wave-current interaction in three dimensions at a physically realistic scale However, there are few studies focusing on the influence of storm surge and inundation on waves, in particular during typhoon landfall

1.3.2 Storm surge researches in Vietnam

A Numerical model development

Storm surge risks have been investigated in developed countries, such as the United States and Japan, where are subject to severe tropical cyclones However, there is significantly few research carried out in Vietnam (Takagi et.al 2014b) In 1992, the project UNDP VIE/87/020 developed a two-dimensional

numerical model set up for the Gulf of Tonkin to forecast storm surge and drift current during typhoon events

(Pham Van Ninh et al 1992) The model was calibrated by the available data from 1960 In the framework

of Disaster management Unit, UNDP project VIE/97/002, Le Trong Dao et al (2000) submitted the summary report on storm surge disaster study in Vietnam from 1990 to 1999, including the analysis on the characteristics of storm surge and its damage along the Vietnam coast Gerrisen et.al (2001) set up the hydrodynamic model for East Sea using Delft-3D, which was calibrated and validated under normal and extreme conditions in the framework of the VCM project supporting storm surge forecasting for Vietnam Hydrometeorological Services (HMS) Takagi et.al (2014b) hindcasted storm surge levels during several

past significant typhoons in Vietnam However, the contribution of wave and wave-induced setup was not

taken into account in the calculation that leads to the underestimation of simulated increasing water level

Vu Minh Cat and Vu Van Lan (2017) simulated the storm surge and inundated mapping only for Phu Quoc

island by using Delft3D model, not considering the wave effect Tho et al (2018) using SuWAT and SWAN

models to estimate the surge height and wave caused by Typhoon Xangsane 2006 in along the coast of Central Vietnam This study confirmed the contribution of wave and tide in the total surge but did not investigate the effect of surge on the wave

B Typhoon perception in Vietnam

After coastal disaster happens, including tsunami or a storm surge, typically much effort is placed on increasing disaster preparedness, it could be the construction of defense structures, relocation of communities away from danger zones or the improvement of evacuation systems (Esteban et al., 2013) Among safety measures, evacuation systems are considered one of the most effective methods in the protection against natural disasters However, for a successful evacuation, warning systems solely are not enough, it also necessary for the local population to be aware of the dangers posed by disasters and know what to do in such

an event (Esteban et al., 2014) On another hand, the awareness about disasters is not constant, it would gradually decay with the subsequent generations and immigrants (Esteban et al., 2016) In fact, it is interesting to note that, awareness about tsunamis has become more common because it appears higher than that for other types of flooding such as storm surges (Miguel Esteban et al., 2017) In the case of Typhoon Haiyan in 2013, one of the strongest typhoons in recent times (Mikami et al 2016, Takagi et.al 2016), it appeared that local residents had a low level of awareness about the nature of storm surges (Leelawat et.al 2014) The concept of storm surge is not familiar with many local residents, with a number of individuals who preferred the authorities to describe it as a “tsunami” (Esteban et al 2015a, Esteban et al 2015b)

Esteban et al., 2013 shows the importance of experience and preparedness for disaster mitigation, in term of encouraging the evacuation process and reducing the fatality ratio However, it is not clear how aware are local communities in Vietnam and specifically in Southern Vietnam, regarding the risks posed coastal disasters, particularly, storm surges, due to the low frequency of typhoon in this area The studies on disaster conception and evacuation systems can provide good insights into awareness about coastal hazards in the vulnerable coastal area in Southern Vietnam

1.4 Aims and Objectives

The goal of this dissertation is to evaluate the coastal disaster situation in Southern Vietnam to understand how people in this area will respond to the coming threats from the sea In order to do that, the study aims to investigate the social issues and the physical impact of coastal disasters cause in the south area

of Vietnam

To achieve this aim, the following specific objectives are drawn:

• Assessment of the perception of typhoon disaster among local people in Southern Vietnam

• The development of Coupled storm surge- wave model that is beneficial in assessing the physical

impact of typhoon disaster in developing countries such as Vietnam

two-1.6 Outlines of this study

The thesis comprises of six chapters

Chapter 1 introduces the background of coastal disaster context in Vietnam and reviews the common storm surge models and recent studies in typhoon related disasters The advantages and limitations of each model or study are also discussed in this chapter The overall review of the storm surge studies in terms of numerical studies and perception studies is also addressed here The end of the chapter includes Aims and Objectives, Methodology, and Organization of the thesis

Chapter 2 discusses typhoon perception among local citizens in Southern Vietnam To do so, the field survey conducted in Mekong Delta and in the remote Con Dao island located in the East Sea is introduced The structure of the questionnaires and the method conducting interviews are summarized in the content of the chapter The result of these field surveys answers the question of how local people understand and respond to typhoon disasters

Chapter 3 introduces the proposed coupled numerical storm surge model that taking account of the impact of typhoon induced wave in the total water level In addition, this model also investigates the effect

of storm surge on the typhoon-induced wave, which was not sufficiently considered in the previous studies The first part of this chapter is the Model description, which explains the proposed structure of the coupled model, as well as all of the input parameters Due to the limitation of data using for validation in Vietnam, the model is run in the case of Japan to verify its accuracy Typhoon Jebi (2018) is chosen for this study The GPV wind data is also introduced in this chapter The detailed study of mechanisms of storm surge and high wave caused by Typhoon Jebi is discussed in the results section after verifying results with observed data

Chapter 4 consists of two part: checking the applicability of the proposed model in Vietnam and applying for the specific typhoon case The GPV wind data is not available in Vietnam, thus the model in chapter two is run again but using limited meteorological input from the parametric typhoon model, after that, the outputs of this simulation are compared with the results in chapter three to confirm the applicability

of using parametric typhoon model The next part is the application of the proposed model running for the case of Typhoon Linda (1997) in Southern Vietnam The detailed study of mechanisms of storm surge and high wave caused by Typhoon Linda is discussed in the results section after verifying results with observed data The advantages and disadvantages of using the high resolution of bathymetry model are investigated

in the last section of this chapter

Chapter 5 is a discussion about the application of the proposed models for disaster management purposes The general policy in terms of disasters in Vietnam is introduced The discussion of the situation

in the sea during Typhoon Linda (1997) and its damage based on the simulation results and interview reveals the reason why many fishermen were caught by Linda in the sea The running of model using hypothetical typhoon that follows the same track of Linda but higher intensity provides the data for creating hazard map surround Con Dao island

Chapter 6 is the summary and conclusion for this study

2014) Typhoon Haiyan, in November 2013, could be considered another event that has greatly increased

awareness about storm surges not only in the Philippines but also within the wider world (Takagi et al.,

2014) According to Murty, 1986 , storm surges were the main reason for 60% of deaths in the low-lying

arable coastal areas of the countries bordering the Bay of Bengal and the adjoining Andaman Sea Bangladesh suffers the widespread damage due to the cyclone Sidr in 2007, even though it has greatly improved its disaster preparedness with the construction of a large number of cyclone shelters (Shibayama, 2009) Countries were disaster preparedness is low, such as when Myanmar was hit by cyclone Nargis in

2008, can suffer great damage due to these events (Shibayama, 2009) It is noticeable that the powerful weather systems are not only the threats for the developing countries, but also for even rich countries such

as the United States and Japan, and it is feared that their causing damage will become greater in the future

(Hoshino et al., 2012, Mikami et al., 2012) Although some areas do not suffer from frequent typhoons, they are not necessarily less vulnerable from a disaster management perspective, which considers social

vulnerability in addition to physical hazards (Takagi, 2019)

The climate in Vietnam is characterized by the tropical monsoons, dividing the country into 3 geographical areas, namely the northern, central and southern regions The white sandy beaches are found

in Nha Trang and Phan Thiet, while muddy coastlines are predominant in the Southern part of Vietnam, around the Mekong Delta The delta is very flat and low with an average elevation of only about 1 m above mean sea level (Toan, T.Q et.al 2010) The region is extremely vulnerable to the influence of sea-level rise, flood, and typhoon storm surges (Takagi et al 2016)

The maximum storm surge offshore the Red River Delta that located in the Northern part can be 1 to 1.5 m above mean sea level, but as the surge progresses towards the coast it typically grows higher (Larson

et al 2014) The possibility of a large typhoon taking place in Southern Vietnam is considerably smaller than in the northern and central parts of the country, though this does not necessarily mean that Southern Vietnam is less vulnerable against typhoons

In fact, storm surges can be considered to pose the greatest risk to low-lying coastal areas of the Mekong Delta, whereas fluvial and pluvial flood events appear to be more predominant in the upper part of the delta, close to the Cambodian border (Takagi et al 2014a)

For example, Typhoon Linda (meteorologically, categorized as severe tropical storm) formed in late October 1997 in the East Sea of Vietnam, eventually causing extensive damage to coastal areas in the south end of Vietnam, as many fishermen and sailors were caught at sea in the path of the storm and were unable

to escape it (UNDP, 2003) Linda is considered to be the worst storm to have hit the Southern part of Vietnam

in the past several decades and resulted in 3,111 people being killed, and the total damage was estimated at

$385 million (USD) The flooding caused by Linda damaged or destroyed about 200,000 houses and left about 383,000 people homeless Though the storm surge during the passage of Linda has not been sufficiently investigated, it could have reached about 0.7m, excluding wave-setup, near the mouth of Hau

River (Bassac River), one of the distributaries of Mekong River (Tab 2.1) (Takagi et al 2014b)

In the present paper, we tried to make an assessment of the state of awareness and preparedness of the population of Southern Vietnam against typhoons and storm surges In order to attempt such an analysis,

I analyzed typhoon tracks in the last six decades and also conducted field surveys and interviewed a variety

of local residents and officials in Southern Vietnam The extent of storm surge caused by Typhoon Linda was also investigated by interviewing those who had direct experience of it The results show that it is important to improve disaster awareness education and also put in place feasible prevention measures to mitigate the damage caused by future typhoon storm surges

Table 2.1

Estimated storm surges at the river mouth of the Mekong River

2.2 Typhoon Track Analysis

We used the so-called Typhoon Best Track Data, obtained from Joint Typhoon Warning Center (JTWC) to analyze the typhoon tracks between 1951 and 2010 around the East Sea of Vietnam The data consists of time, geographical position of the storm center, minimum sea level pressure at the storm center and the maximum sustained wind speed in knots To explore the occurrence of tropical cyclones approaching the coasts of Vietnam in more detail, the authors used a numerical code for detecting the point of landfalls, defined as the place where a tropical cyclone track intersects with the coastline (Takagi et al 2015) Fig

2.1 was created to try to analyze in detail both the temporal and spatial patterns of tropical cyclone landfall

along the entire Vietnamese coastline Table 2.2 shows the number of typhoons that passed through

Vietnam’s coasts in the last six decades Each tropical cyclone was categorized into three latitude zones (North: N21.5°- 18°, Center: N18°- 14° and South: N14°- 9.5°) according to the point when it made landfall

in order to attempt to identify any trends over time

It appears that tropical cyclones have made landfall even in the southernmost part of Vietnam, Mekong Delta, although the chance of landfall is substantially smaller than in the northern or central coasts Based on these analyses, it is obvious that tropical cyclones in the Mekong Delta are not negligible when planning future disaster management strategies

A series of field surveys were conducted in many small coastal towns along the coastline of the

Mekong Delta and in its regional capital, Can Tho city, located along the bank of Hau river (Fig 2.2) The

visited locations are the places that have ever directly suffered damage or affected by the previous typhoon The majority of places are undeveloped rural areas except Can Tho city The purpose of the survey was twofold: (1) identifying the damage caused by past typhoons and (2) understanding disaster awareness of the inhabitants To do so, a structured questionnaire survey was distributed to individuals encountered on an

opportunistic basis (Fig 2.3), focusing on awareness and past damage during typhoons and storm surges A

wide variety of locations and situations were covered, from people staying at home, sitting in coffee shops,

to people casually walking around

As a result, a total of 172 valid questionnaire responses were obtained (n=172) Due to the

opportunistic nature of questionnaires, no effort was made to preserve a male/female balance, however, coincidentally, there was 51% of the respondents being male, and 49% of females The majority of respondents were over the age of 40, which could be considered to be part of the generation that has more

experience and memory about previous disasters, as opposed to the younger groups (Fig 2.4) Fishermen

and farmers, who have directly suffered damage by disasters, occupied 6% and 18% of the sample,

respectively Fig 2.5 also shows how self-employed, housewives and laborers constituted other major

occupational groups (12%, 9%, and 9%, respectively)

1951-1960

1970

1961- 1980

1971- 1990

1981- 2000

1991-

North (21.5 o -18 o )

10 (53%)

9 (27%)

20 (48%)

15 (36%)

20 (45%)

16 (57%)

90 (43%) Northern

Central (18 o -14 o )

4 (21%)

16 (49%)

14 (33%)

15 (36%)

11 (24%)

6 (22%)

66 (32%) Southern

Central (14 o -11 o )

5 (26%)

7 (21%)

7 (17%)

10 (24%)

10 (22%)

5 (18%)

44 (21%)

South (11 o -9.5 o )

0 (0%)

1 (3%)

1 (2%)

2 (4%)

4 (9%)

1 (3%)

9 (4%)

Figure 2.2

Location of surveyed areas in Vietnam

Figure 2.5

Occupation of respondents (n=172)

2.3.2 Typhoon Linda in 1997

Regarding the damage suffered from previous disasters, 63% of respondents reported that they had

experienced some sort of damage (Fig 2.6) It is well known that the community in the low-lying area in

Mekong Delta is particularly vulnerable to any type of flooding events, whether from the river or a storm surge due to a typhoon Furthermore, respondents typically indicated that the disaster which caused the most damage was Typhoon Linda, which made landfall in the area at the beginning of November 1997 A moderate storm surge, approximately 1 m high, was generated by this typhoon in Ca Mau, according to a

local resident (Fig 2.7) This agrees well with the results of numerical simulations, which indicate a storm

surge height of around 0.7 m at the mouth of the Mekong River (Takagi et al 2014b) (Tab 2.1)

Aside from the questionnaire on disaster awareness, the authors also conducted a separate more limited survey on the damage caused by Typhoon Linda, amongst 102 residents in Can Tho city (n=102)

Fig 2.8 shows that Linda caused strong winds (classified as either of moderate or stronger by 86% of

respondents, who indicated that it could knock trees down) in Can Tho city, even though it is located about

170 km away from the place where the typhoon made landfall It was also clearly recognized that places near the river bank suffered stronger winds than that inland parts of the city The insufficient structural strength

of the houses meant that many roofs were blown away, which was the most common type of damage in Can

Tho (50%) (Fig.2.9) Strong wind in combination with heavy rain also caused flooding in places adjacent to

the riverbank, while there was no noticeable damage recorded in the higher areas and further inland

Fishing/Fisheries, 6% Service workers (with

customers) , 7%

Domestic worker/maid, 5%

Housewife , 9%

Office workers, 5%

Farmer , 18%

Laborer, 9%

Self-employed, 12%

Others, 29%

Figure 2.6

Distribution of people have experienced damage from previous disasters (n= 172)

Figure 2.7 Surge height, as indicated by a local resident who remembered Typhoon Linda

Yes 63%

No 37%

Figure 2.8

Wind speed during Typhoon Linda in Can Tho city (n=102)

Figure 2.9

Damage in Can Tho city during Typhoon Linda 1997 (n=102)

2.3.3 People’s Awareness in Mekong Delta

In terms of awareness, over half of respondents (55%) understood the nature of storm surges (Fig

2.10), with the great majority (59%) believing that they represented a moderate or high danger to them (Fig 2.11) This result shows that local people have a high degree of knowledge regarding storm surges, this

contrasts to their less experience about such event due to low frequency The reason for this could be explained by the fact that the low elevation and flat nature which characterizes coastal areas can substantially increase the vulnerability of local communities to storm surge, even if surge heights are relatively small Thus, a given storm surge height that would not cause great inundation in Central or Northern parts of the country could devastate coastal settlement in the Mekong Delta (Takagi et al 2014b)

Ships in the river were overturned

Roads were flooded Houses and/or

facilities were damaged

Furthermore, Category 5 super typhoon Haiyan in 2013, which also was mentioned by many respondents during the interview, could be considered as a defining event in raising awareness about storm surges, not only in the Philippines but within the entire world including Vietnam (Esteban et al 2014) On the other hand, this 59% appears similar to the 63% of people who have experienced damage from previous disasters This means that in the Mekong Delta local resident tend to know about the disasters only when they have observed or suffered damage from those events That explains the fact that most interviewees with

higher awareness are at the age over 40 (Fig 2.11) There are respondents even had already experienced but

do not afraid of disasters because of the minor damage in the past was not strong enough

Figure 2.10

Distribution of respondents who were aware of the nature of storm surges in Mekong Delta (n= 172)

Figure 2.11

Distribution of respondents who think that a storm surge constitutes a real danger for them in Mekong Delta (n= 172)

Despite the relatively high awareness about storm surge, the preparedness in Mekong Delta seems to

be not adequate with the potential risk posed by such an event Even though several evacuation drills are

Yes 55%

No 45%

conducted in annually, just over one-fifth of people (21%) said that they had taken part in that events in the

last 5 years (Fig 2.12), thus the majority of respondents (62%) indicated that they do not know how to evacuate when a typhoon disaster event happens (Fig 2.13) It is noticeable that participation in that drills

is not mandatory for local people, the majority of attendants are local governments and a small part of local residents Furthermore, when asking local people in Can Tho city what they did during Typhoon Linda, many answered that they were working and keep working Essentially, it appears that they underestimated the danger from the typhoon In addition, there are still a number of people living by the coastline or along the riverbank, who would directly face the storm surge and little time for evacuation unless an effective early

warning system is put in place (Fig 2.14)

No 79%

Yes 38%

No 62%

Figure 2.14

Houses near water in Ca Mau

2.4 Field survey in Con Dao

2.4.1 The purposes of the survey

We also conducted a similar interview survey in Con Dao Island, which is located off the southeastern tip of Vietnam—approximately 230 km south of Ho Chi Minh City (HCM), where during Typhoon Linda passage, many fishermen were reported death or missing We administered another structured questionnaire

to the local inhabitants of Con Dao in March 2017 Our primary intent was to investigate the residents’ memory of Typhoon Linda, as well as how Linda affected the general awareness of typhoon disasters The questionnaire consisted of a number of different questions, including 5 main questions:

III Do you fear typhoons?

There are two main residential areas in Con Dao—one on the east coast and the other, a northern

village on the mountain (Fig 2.15) As a result of the field survey conducted in the two areas, a total of 103

valid responses were collected It should be noted that tourists and residents who recently relocated to Con Dao were intentionally excluded from the survey

Figure 2.15

Circular plots on this satellite image indicate the towns of Con Dao Island where we conducted the survey (White dots: East

coast; yellow dots: Northern village.) A total of 103 questionnaires were collected in Con Dao The insets show

interviews with local witnesses

2.4.2 Memory of Linda and Awareness of Typhoon Disasters in Con Dao Island

Because Linda occurred in 1997, we interviewed only those who might remember the event As a result, most of the questionnaires (92.2%) were collected from people over 30 years of age Nevertheless, a small number of the younger population were also interviewed to investigate their general perception of typhoon disasters As could be expected from the target age group and the socio-economic characteristics of

the area (Fig 2.16), most adults were employed in the tourist (service worker) (30%) and fishing industry

(12%,), and just small percentage of the young defining themselves as students (3% of respondents) The results of the questionnaire also show how the economy of this settlement on the island relies mostly on tourists and fishing that sensitively affected by the extreme weather in the sea

Fig 2.17 shows the results from the questionnaires It is remarkable that 83.5% of respondents in

Con Dao remembered Typhoon Linda The 16.5% that did not recall Typhoon Linda include the young respondents who were born after the event and those who had recently migrated from the mainland and did not live on the island during Linda However, only small part of respondents was aware of the nature of storm surge (12%), that significantly lower than that percentage in Mekong Delta The reason for the remote location probably restricts local people to approach the information regarding coastal disasters including storm surge Of the respondents, 84.5% confirmed that they are afraid of typhoons in general The level of existing awareness in Con Dao is rather remarkable and also high than in Mekong Delta (59%) According

to one local resident, it was likely that fear for typhoons did not exist among the local population before

Linda Regarding preparedness after the event, 88.3% of respondents confirmed that they know how to evacuate in the event of a typhoon and storm surge This fact shows how strong Typhoon Linda affected local people's awareness when considering that there has been almost no typhoon related damage in recent times in this area

Despite the strong winds caused by Linda, more than half of the respondents (55.8%) did not evacuate There were two reasons for this: First, people did not know where to evacuate because this was the first time they had experienced a storm Second, concrete buildings in the east coast area—where a large population lived—were used to accommodate soldiers during the Vietnam War On this basis, those residents believed that their house was the safest place, and thus, stayed home

However, it appears that local residents underestimated and were not well-prepared for Linda In fact, strong winds caused many wooden houses to collapse Although there were no reported casualties in the inland communities, many fishing boats were wrecked by strong winds and high waves when anchored at the pier

Figure 2.16

Occupation of respondents (n=103)

Fishing/Fisheries 13%

Office workers 10%

Village Leader 3%

Driver 3%

Service workers (with customers) 32%

Figure 2.17

Questionnaires results regarding people's awareness and preparedness in Con Dao

2.5 Discussion on Disaster Risk Awareness Improvement

Many researchers have shown how typhoons can be considered a great threat to Vietnam (Takagi et

al 2014b, Esteban et al 2012) The authors also found out through their field surveys that sometimes local

people even ignored the threats posed by natural hazards (10% of respondents, according to Fig 2.11) Those who do not know how to evacuate when disasters happen (62%) (Fig 2.13) could drown due to the storm

surge as a result of placing themselves in high-risk situations The high population density and low-lying estuarine nature of the delta are also considered a disadvantage for people to try to find safe places to evacuate, especially those who have not ever taken part in evacuation drills The number of people exposed

to storm surges is also increasing due to the rapid growth of the population around hazardous areas in the coastal zone of Mekong Delta Thus, raising awareness among local inhabitants is the most urgent thing that needs to be considered in the disaster management plan

Fig 2.18 shows that in Mekong Delta, only 19% of respondents assumed disaster issues were taught

at school, while around 26% of people did not know and 55% said that this was not taught This can offer some insight into why the younger generation might be less aware of disasters, given that many were probably not taught about them in school, and were too young to have experienced this during their lives

According to the questionnaires, in Con Dao, (Fig 2.17) the quiet small percentage (7.8%) of children learn

about storm surge in school Typically, the level of disaster awareness in a country changes throughout time, with awareness being reinforced by recent events However, after a given event awareness gradually fades

in time unless appropriate educational efforts are made (Esteban et al 2015) In order to raise awareness of children, it is necessary to enhance education and training about coastal disasters in school and maintain this kind of knowledge by organizing activities For the case of Con Dao, it was once an unpopulated island used only for political prisoners After the Vietnam War, people moved from the mainland, leading to the present population of approximately 7,000 New immigrants—who did not experience Typhoon Linda—gradually became the majority on the island, thus inhibiting the dissemination of the experience from older to younger generations

One possible solution would be to add a compulsory subject on coastal and other life-threatening

disasters into the school curriculum The student should be taught the basic information of Do's and Don'ts

during the disasters Besides, it is advisable to organize school events to visit places that suffered damage from past disasters and talk with those who experienced disasters Being able to observe these areas can help students understand the potential consequences of disasters and the risks they pose to human life, hopefully increasing their awareness For adults, evacuation drills are considered as a key component of disaster risk management and should be conducted as a part of a disaster mitigation programme We found a tsunami

evacuation instruction panel (Fig 2.19) near the pier that exemplifies efforts by the local government in Con

Dao to raise awareness of coastal disasters among local inhabitants and tourists

Regarding the sources of information about disasters, most respondents in Mekong Delta indicated that they used TV or radio (84%), internet (51%), or got it from authorities (including village elders, firefighters or police) or public address systems (Fixed Loudspeakers, Mobile Loudspeakers, etc.) during

flooding events (Fig 2.20) This shows that television is also an efficient way to inform people in Vietnam

about disasters, and disaster education tied up with a TV programme could be effective due to the fact that information for TV programme can reach millions of people – but only for a few minutes at a time and especially it is free for people to access Furthermore, there is also a need to develop warning stations

Figure 2.18

Do children learn about storm surge/flooding in school, and how to evacuate? (n= 172)

Yes 19%

No 55%

No Answer 26%

of tropical cyclones in Southern Vietnam is not negligible, although the frequency is substantially lower than that in Northern or Central Vietnam The results of the questionnaire surveys revealed the gap between the understanding of the nature of storm surge of local residents in Mekong Delta and Con Dao due to the location Generally, it shows that the local population has a relatively high degree of awareness about the dangers posed by typhoons However, residents are not well prepared to face these natural disasters, and the present study highlights the necessity to enhance the education about natural hazards of the younger generation in the Mekong Delta, Con Dao island and the rest of Vietnam

Chapter 3

Coupled Storm surge – Wave model using Meso-scale Data

Hindcasting with an ideal meteorological input

3.1 Introduction

Tropical cyclones are very hazardous and extreme meteorological phenomena affecting most coastal countries worldwide In fact, strong winds and heavy rainfall from tropical cyclone landfall can cause major disasters Among others, storm surges can have the most life-threatening impact during the course of a major storm For example, Hurricane Katrina in 2005 caused over 1000 fatalities in Louisiana and 200 in Mississippi due to the storm surge that exceeded 10 m in several locations along the Mississippi coastline

(Fritz et.al., 2007) Likewise, Typhoon Haiyan caused enormous damage to the Philippines in 2013, with more than 6000 reported death (NDRRMC, 6 March 2014), given the storm surge reached over 6 m in the inner-most part of Leyte Gulf (Mikami et al., 2016, Takagi et al., 2016a) Although the number of the casualty was relatively low, Typhoon Hato in 2017 generated about 2.5-m storm surge in Macau and significantly impacted Macau’s economy, especially regarding the casino industry (Takagi et al., 2018) Strong winds during the course of a typhoon can also generate high waves, which may cause the predominant physical impact The maximum hindcast wave heights during the passage of Typhoon Haiyan reached 20 m

at eastern Samar (Bricker et al., 2014) In addition, Roeber and Bricker (2015) investigated the destructive tsunami-like wave that devastated the town of Hernani, Eastern Samar, the Philippines during Haiyan Annually, an average of 2.9 tropical cyclones (from 1951 to 2016) have hit Japan (Takagi and Esteban 2015; Takagi et al 2017) The recent Typhoon Jebi in September 2018 has been the strongest tropical cyclone to come ashore in the last 25 years since Typhoon Yancy (the 13th typhoon to hit Japan, in 1993), severely damaging areas in its trajectory Typhoon Jebi was the fourth to hit Japan in the 2018 season, notably affecting Kansai area, Japan’s second-biggest, populous, and prosperous region, which is prone to typhoons and storm surges Jebi caused 13 deaths and 741 injured people as of September 14, 2018 (Fire and Disaster Management Agency, 2018) Furthermore, power outages occurred in the wider region of Kansai, affecting approximately 2.2 million residences The bridge connecting Kansai International Airport to mainland Japan was damaged following the collision of a large freighter, which was stranded due to the rough sea state caused by Typhoon Jebi Thus transportation was interrupted to this, the largest international airport in western Japan, located on an artificial island in Osaka Bay Moreover, Kansai International Airport was severely flooded during Typhoon Jebi, and around 5,000 people were forced to remain at the airport overnight

A wind radius of 50-kt was estimated around 220 km (in the longest axis) when Jebi was about to

make landfall (Fig 3.1) Jebi maintained maximum wind speed of 75-85 kt (139-157 km/h) when it hit

Osaka The sea level pressure during the passage of Jebi over Osaka Bay was of 950-975 hPa As Typhoon Jebi swept through the Osaka Bay and the south of Honshu Island, it caused heavy rainfall, high waves, and storm surges Regarding the increase in water level during the typhoon, the highest tidal level in Osaka reached 3.29 m above the mean sea level, exceeding the previous record of 2.93 m during Typhoon Nancy

in 1961, according to data from JMA (Japan Meteorological Agency) (JMA 2018) In addition, strong winds from the typhoon disrupted cities in the Kansai region, including Osaka, Kyoto, and Kobe In Kyoto, part of

the glass roof over the main rail station collapsed, causing several injuries The strong winds also damaged infrastructure in downtown Osaka and adjacent cities, where roofs were blown away and vehicles overturned,

as evidenced from videos recorded by local people Floods at coastal residences in Kobe and adjacent cities were also investigated and reported by a Japanese survey team (Takabatake et al., 2018), with reported depths of 0.18–1.27 m caused by the typhoon Furthermore, many shipping containers were displaced by the storm surge and waves in Ashiya city Overall, the JMA reported that Typhoon Jebi caused the highest storm surges above the mean sea level ever reported at Osaka (3.3 m), Kobe (2.3 m), Gobo (3.2 m), Shirahama (1.6 m), Kushimoto (1.7 m), and Awayuki (2.0 m)

We carried out a reconnaissance survey 2 days after Typhoon Jebi in the affected area and observed many damaged structures and inundations that were apparently caused by the high waves combined with storm surges The combination of these two phenomena may have exacerbated the damage on the coasts and even in the innermost part of Osaka Bay However, no comprehensive study has been conducted to reveal the combined impact of this destructive typhoon to date This paper reports the situation that we observed during the field survey The hindcast analysis is also reported to describe the spatial distribution of high waves and storm surge during Jebi In addition, we present the analysis of tide data provided by the JMA to investigate the significance of storm surges generated by Typhoon Jebi Based on these observations, we emphasize the importance of adequate coastal designs against high waves, because the associated disaster risk appears to have been underestimated regarding plausible storm surges occurring in semi-closed bays such as Osaka Bay

Figure 3.1

Japan Meteorological Agency’s weather map immediately before Jebi made landfall (September 4, 2018, 09:00, Japan

Standard Time, UTC+9) (The red line is Jebi track)

3.2 Methodology

3.2.1 Field survey

We conducted field surveys for 3 days from September 6 to 8, 2018, a few days after Typhoon Jebi made landfall at the Tokushima Prefecture around noon of September 4 The survey aimed at identifying the

damage extent in the typhoon aftermath around the bay of Kansai area including parts of Shikoku Island and

Awaji-shima Island (Fig 3.3) Laser range finders (TruPulse 360; Laser Technology, Inc.) were used for

determining the distance and the elevation of the broken dykes or fences, debris, fallen trees, and remaining water mark In addition, real-time kinematic GPS receivers (ProMark 100; Ashtech, Inc.) provided ground elevation, and trained staff used handheld GPS receivers (GPSMAP; Garmin Ltd.) to collect the coordinates

at the survey points Checking the abovementioned physical evidence provided information about flooding, wave height, and damage extent at each location (with reference to the local sea level at the time of the survey) When a sign of wave overtopping was observed but no visible water mark or damage was available, the height of protection infrastructure was considered to estimate the wave height However, the actual wave height should have been larger than the estimated height The retrieved heights of protection infrastructure, inundation depth, and ground elevation acquired through the laser range finders were corrected to the tidal height above the sea level at the time of the survey by using data from the nearest tidal station Furthermore, these data were adjusted from the mean water level in Tokyo Bay (TP) as common reference level The elevations measured using the GPS receivers were corrected to TP by calibrating with the survey control points provided by the Geospatial Information Authority of Japan (Oshima et al 2013)

3.2.2 Wind–wave and Storm Surge Hindcasting

There is a two-way interaction between storm surges and waves Wave height is limited by wave

breaking, and waves can also be affected by the increase in total water depth caused by a storm surge, wave

setup, and tide On the other hand, radiation stresses generated by the presence of waves increase the peak water level due to wave setup (Longuet-Higgins and Stewart, 1960, 1962) Xie et al (2008) applied the Princeton Ocean Model and Simulating Waves Nearshore (SWAN) model and confirmed the contribution

of wave setup to inundation predictions in Charleston Harbor during the 1989 Hurricane Hugo Funakoshi

et al (2008) applied a coupled model known as ADCIRC (Advanced Circulation Model) and SWAN, finding that wave-induced radiation stresses contributed 10–15% increase in peak water levels during Hurricane Floyd in 1999 Chen et al (2008) found that the local wind forcing was responsible for 80% of the maximum surge, while the combined effects of tides, surface waves, and offshore surge accounted for the remaining 20% during Hurricane Katrina in 2005

In this study, waves were simulated using the Delft3D-WAVE module, which uses the SWAN spectral wave model SWAN is a third-generation wave model to compute random, short-crested, and wind-generated waves in coastal regions and inland waters (Booij et al., 1999) We used the SWAN model based

on the action balance equation with sources and sinks (Equation 3.1) and provided a nesting application to

the parent grid To investigate the influence of this depth-limited condition on wave height, we used the hydrodynamics module Delft3D-FLOW to simulate the combined impact of wave and storm surge FLOW

solves the continuity (Equation 3.2) and Navier–Stokes equations (Equation 3.3 and 3.4) for an

incompressible fluid under shallow water and hydrostatic assumptions Although the Delft3D FLOW module can be applied to three-dimensional phenomena, we used a two-dimensional horizontal grid, establishing a shallow-water wave model, which is commonly used to simulate long waves such as storm surges, tsunamis, and tidal propagation (Takagi et al., 2019)

In Equation 3.1, the first term in the left-hand side represents the local rate of change of action

density in time, the second and third term represent the propagation of action in geographical space (with

propagation velocities c x and c y in x- and y-space, respectively) The fourth term represents the shifting of

the relative frequency due to variations in depths and currents (with propagation velocity c σ in σ-space) The

fifth term represents depth-induced and current-induced refraction (with propagation velocity c θ in θ-space)

The expressions for these propagation speeds are taken from linear wave theory (Whitham, 1974; Mei, 1983; Dingemans, 1997) The term S (= S(σ; θ)) at the right-hand side of the action balance equation is the