Luận văn thạc sĩ: Study on inundation due to storm surge for Con Dao island

I hereby declare that is the research work by myself under the supervision ofAssoc Prof PhD Vu Minh Cat The results and conclusions of the thesis are fidelity,which are not copied from any sources and any forms The reference documents relevant

sources, the thesis has cited and recorded as prescribed The results of my thesis have not

been published by me to any courses or any awards.

Ha Noi, date 12 July 2016Author of the thesis

Nguyen Thi Lan

First and foremost I would like to thank the supervisor Ass Prof PhD Vu Minh Catfor his great contribution in this thesis, for supporting me and guiding me stay on the rightep thanks to Ass Prof PhD Tran Thanh Tung who is maintrend [ also want to show

co-ordinator, making value contributions to success in Master course In addition, pleaseallow me to send my best gratitude to the “Research, identification scientific argumentsand proposal of Phu Quoc - Con Dao marine spatial planning for sustainabledevelopment” subject, Code: KC.09.16 /11-15, which support a lot of valuable data formy thesis.

Ha Noi, date 12 July 2016Author of the thesis

Nguyen Thi Lan

‘TABLE OF CONTENT

LIST OF FIGURES viLIST OF TABLES viiINTRODUCTION 11 The necessity of the study 1

2 Study objectives 13 Objects and scope of the study 24, Study approaches and methodology 25 Structure of the thesis 3CHAPTER |: OVERVIEW ON STORM SURGE STUDY AND GENERALDISCRIPTION ON CON DAO ISLANDS 4

1.1 LITERATURE REVIEWS 41.1.1 Intzoduetion about storm in Con Dao islands 41.1.2, International reseatches on storm surge 61.23 Storm surge researches in Viet Nam, la12 GENERAL DISCRIPTION ON STUDY AREA rm1.2.1, Natural conditions M41.22 Climate 161.2.3, Oceanographic regime 81.24, Socio-economic activities lộ(CHAPTER 2: APPLICATION OF DELFT3D TO STUDY STORM SURGE 22

2.1 Description of model 22.4.1, Delft 3D model 22.1.2 Typhoon model 232 Data used for the simulation 31

22.1, Topographical data 312.22 Hydrodynamic data 312.3, Model set up.

2.3.1, Simulated domain and computed mesh 322.3.2 Model calibration 352.3.3, Model verification 39(CHAPTER 3: SIMULATION OF STORM SURGE IN CON DAO ISLANDS 423.1 Typhoon scenarios in the areas announced by Vietnam government 43.2 Extraction of water level around study islands, 453.3 Simulation of storm surge based on suggested scenarios 473.3.1, Scenario 0: Simulation of water level in Con Dao without Durian typhoon473.3.2 Scenario 1: Simulation of Durian typhoon 483.3.3 Scenario 2: Simulation of Durian storm with changing wind speed 553.34 Scenario 3: Simulation of Durian typhoon with changing only its trajectory603.35 Comparison of scenarios 663.4, Chapter's conclusion 6CHAPTER 4: INUNDATED MAPPING DUE TO STORM SURGE 04.1, Introduction AreGIS T0

4.2 Building inundated maps and inundated calculation for research area n4.3 Build up inundation maps due to storm surge in Con Dao islands with scenariosaccordingly 73

4.3.1, Building inundated mapping and calculation of inundated area for 1* scenario

4.3.2 Building inundated mapping and calculation of inundated area for 2TM scenarion4.3.3 Building inundated mapping and calculation of inundated area for 3" scenario

1944, ChapteFs conclusion 80CONCLUSIONS AND RECOMMENDATION i

CONCLUSIONS 81

1 Major results si2 Drawback ai

RECOMMENDATION s

REFERENCES 83APPENDIX 1: RESULTS OF MODEL CALIBRATION 86APPENDIX 2: RESULTS OF MODEL VALIDATION 95APPENDIX 3: WATER LEVEL IN CON DAO ISLANDS ACCORDING TO 4SCENARIO 99

LIST OF FIGURES

Figure 1.1: Storm Surge vs Storm Tide 6Figure 1,2: Map of Con Dao Islan “Figure 2.1: System architecture of DeIf3D 23Figure 2.2: ketch of wind velocity field fora moving eyelone 28Figure 2.3: Computed Mesh created for Con Dao arca aFigure 2.4: Topography of Con Duo islands aFigure 2, 5: The water level line between observed water level and computed water level

with C= 68 mí” Ý in Storm model verification, 4I

Figure 3 1: Partition of the storm risk of Vietnam coastal area 4BFigure 3 2: Extracted water level points in Con Dao 46Figure 3 3: Diagram of water level without Durian storm at around Con Dao islandsaccording to scenario 0 47Figure 3 5: Trajectory of Durian typhoon 49

Figure 3 6: Diagram of water level at point Ì according to 1” scenario sỊ

Figure 3.7: Diagram of water level at point 2 according to 1 seenario 52

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Figure 3Figure 3.Figure 3Figure 3Figure 3.Figure 3.Figure 3Figure 3Figure 3.Figure 3Figure 3Figure 3.Figure 3.Figure 3.Figure 3,Figure 3.2:Figure 3.Figure 3

8: Diagram of water level at point 3 according to 1" scenario

9: Diagram of water level at point 4 according to 1" scenario

Diagram of water level at point 5 according to I" scenarioDiagram of water level at point 6 according to the!" scenario

Diagram of water level at point 7 according to the 1" seenarioDiagram of water level at point 1 according to 2°! scenarioDiagram of water level at point 2 according to 2"! scenarioDiagram of water level at point 3 according to 2TM scenarioDiagram of water level at point 4 according to 2TM scenario

Diagram of water level at point 5 according to scenario 2

Diagram of water level at point 6 according to 2TM scenario

Diagram of water level at point 7 according to 2"! scenario

Diagram of water level at point 8 according to 2TM scenario‘Tracks of Durian typhoon according to the 3 scenario.

Diagram of water level difference when Durian typhoon occurredDiagram of water level difference when Durian typhoon occurredDiagram of water level difference when Durian typhoon occurred

s2626363Diagram of water level difference when Durian typhoon occurred at point 4

according to 3" scenario.

Figure 3 26: Diagram of water level differe when Durian typhoon occurred

64Figure 3.27: Diagram of water level difference when Durian typhoon occurred at point 6

according to 3" scenatio.

64

Figure 3, 28: Diagram of water level difference when Durian typhoon occurred at point 7according to scenario 3 “Figure 3 29: Diagram of water level difference when Durian typhoon occurred at point 8according to scenario 3 6sFigure 3,30: Diagram of water level difference at point 1 66Figure 3.31: Diagram of water level difference at point 2 66Figure 3.32: Diagram of water level diference at point 3 orFigure 3.33: Diagram of water level difference at point 4 7Figure 3 34: Diagram of water level difference at point 5 _Figure 3.35: Diagram of water level difference at point 6 68Figure 3 36: Diagram of water level difference at point 7 68Figure 3,37: Diagram of water level difference at point 8 68Figure 4 1: DEM map in Con Dao islands 1Eigure 4.2: Contour line in Con Dao islands 4Figure 4, 3: Inundated mapping in Con Dao islands due to storm surge in Durian typhoon

according to I" scenario T6

Figure 4 4 Inundated mapping in Con Dao island due to storm surge in Durian typhoon

according to 2TM scenario 7

Figure 4, 5: Inundated mapping in Con Dao island due to storm surge in Durian storm

according to 3" scenario 79

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LIST OF TABLES

Table 1,1: Statistic of storm in Con Dao in the last 50 years‘Table 1.2: The average temperature of air in the Con Dao IslandTable 2 1: Data of model

Table 2.2 Parameters of Con Dao grid

Table 2.3: Tidal harmonic constants at borders.‘Table 2 4: Parameters of model

Table 2 5 Table synthesis error values when calibration models 38Table 3 1 Basic characteristics and the potential affected by storm to coastal areas inVietnam

‘Table 3,2 Potential of storm surge and total water level for Vietnam coastal areasTable 3.3 Co-ordination and depth of extracted water lever points in Con Dao.“Table 3.4: General information ofthe Durian typhoon

‘Table 3.5 Parameters of Dusian typhoon,

Table 3.6 The highest WL difference and appeared time at 8 points

‘Table 3.7 Parameters of Durian typhoon according to the 2" scenario.

5sTable 3 8 The highest water level difference and appeared time at 8 extracted points

according to 2TM scenario.

Table 3 9: Parameters of Durian typhoon according to 3” scenario

Table 3 10 The highest water level difference and appeared time.‘Table 4 1 Parameters of DEM map in Con Dao.

Table 4, 2, Inundated area in Con Dao islands according to 1" scenario.

Table 4 3 Inundated area in Con Dao islands according to 2TM scenario.

‘Table 4 4 Inundated area in Con Dao islands according to 3” scenario

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1 The necessity of the study

years, the impact of global climate change, natural disasters hasbecome more complex, especially storms, accompanied by sea level rise, which causedinundation in coastal area, Storm surges can cause inundation to coastal areas and eancause dike break, especially if storms occur during high tides Therefore, studying,calculation and forecasting on the extreme water level during storm at coastal locationsand assessment of inundated risk by storm is very important for finding the solutionsand positive measures to prevent and mitigate damages,

‘Storm surge is a dangerous natural phenomenon, which caused a lot of damageto people and property In Vietnam, the highest water level has recorded as 3.6 m in1989 by Dan storm Storm surge is particularly dangerous when the storm occurs atthe same time with spring tides and in tis situation, total water level rising combinedWith strong wave ean cause dike breaks and wave overtopping and also cause heavydamages to people and property.

In Vietnam in general and Con Dao islands in particular, although there are a

lot of studies of sea level fluctuations in storm, but most of which focus on water levelfluctuations on a large scale The fluctuations of water level in coastal locations andinundated risk have not been interested adequately yet, especially when assessing theinundated risk of coastal area Therefore, studying of extreme water levels in storm, in

., waves and inundated

consideration of tides, storm surg isk to coastal area takes a‘great scientific meaning, The result of study will contribute to mitigate theunfavourable impact of storm surges, serve for protection and maintenance, upgrading‘of coastal structures, shoreline protection, requirement for socio-economicdevelopment, environmental protection and sustainable development,

2 Study objectives

‘To assess situation of inundation due to storm surge for Con Dao islands.

3 Objects and scope of the study

~ Objects of the study: The extreme water level during storm at the shoreline,including tides, storm surges and inundated risk from the sea.

~ Scope of the study: The area around Con Dao archipelago,4 Study approaches and methodology

To achieve the above mention objectives, the methodology of this thesis has been

developed base on the characteristics of Con Dao islands The thesis has been

‘Simulation storm surges

Simulated Results of Storm

Observation =P Analysis and Discussion Expert Interview

Conclusions and Recommendations

5 Structure of the thesis

Besides the introduction, conclusion, recommendation and annexes, the study isconsisted 4 chapters as following:

Chapter 1: Overview on storm surge study and general description on Con Dao

Chapter 2: Application of DELFT3D to study storm surge

Chapter 3: Simulation of storm surge for Con Dao islands:

Chapter 4: Inundated mapping due to storm surge for Con Dao islands

CHAPTER 1: OVERVIEW ON STORM SURGE STUDY ANDGENERAL DISCRIPTION ON CON DAO ISLANDS.

1.1 LITERATURE REVIEWS,

1.1.1 Introduction about storm in Con Dao islands

Souther Vietnam is the region where has rare storm, but if the storm occursthen the damage is very serious because people have not experience in stormprevention as in Central and Northern coastal provinces For over 50 years, severalstorms have occurred, of which 3 hurricanes caused severe damage, such as Thintyphoon in 1904, Linda typhoon in 1997 and Durian typhoon in 2006 Below is a

statis ical able of typhoon occurs in Con Dao in the last 50 years (from 1962 to 2012)Table 1: Stati sor in Con Dao in he is 5039

No-] Name of storms Types tin

i 8 [Typhoon 25 NOV-01 DEC, 19622 | Rrpieal Storm 7 FEB, 1965

3 [Tropical Storm Ti DEC, 19654 wat [Tropical Storm 2737 NOV 19703 #80 — | Typhoon 3ONOV05 DEC, 1972D #6 — ÍTamelSom THISNOV, 17

7 5 [repeal Som N2 DEC Bội

: mm — ÏameTvmom LI0NOY, 1589D #0 — TTmhem 322N0V 9ê0 WH TepelSem 2431 OCT, 1956in #1 Í Tropical Storm 29 OCT-18 NOV, 1996,

4

No | Nameofstorms Types Time

2 LINDA | Typhoon 31-03 NOV, 1997B GL Tropical Storm 10-11 DEC, 1998im 31W Í Tropical Depression 01-01 DEC, 199915 32W ‘Tropical Depression | 09-11 DEC, 199916 MUIFA ‘Typhoon 14-26 NOV, 20041 DURIAN Super Typhoon 26 NOV-05 DEC, 200618 | TWENTYFIVE | Tropical Depression 14-14 NOV, 2012

- In principle, typhoons attacked Viet Nam Coasts are originated from west

pacifie ocean and entering to East sea and typhoon track is seasonally changed Thefirst half ofthe storm season, typhoons approach mostly to northern and central coasts‘of Viet Nam, but the latter half of the season, storms move gradually to the south andaffected to south-central and southern coasts of Viet Nam On average, typhoons areless likely to affect Vietnam from January to May From June to August, they are mostlikely to affect the Northern regions and moving to central and southern regions fromSeptember to December annually.

~ In the latter half of the year, typhoon’s trajectories often more complicatedthan first half of typhoon season, The trajectory of the storm in the East Sea can bedivided into five main types: stable, complex, parabolic, weaken at sea and becomesstronger as be near the shore Among them, the complex form and the form ofbecomes stronger as be near shore are the most unpredictable.

Table 1.1 shows the storm landed in Con Dao from October to December cách.year belonging latter half of the storm season At this time, trajectory of typhoon‘becomes more complex and less predictable.

1.1.2, International researches on storm surge

Along the coast, storm surge is often the greatest threat to life and property from atyphoon In the past, large death tolls have resulted from the rise of the oceanassociated with many of the major typhoons that have made landfall Typhoon Katrina(2005) isa prime example of the damage and devastation that can be caused by surgeAt Teast 1500 persons lost their lives during Katrina and many of those deaths occurreddirectly, or indirectly, as a result of storm surge

Storm surge is an abnormal rise of water generated by a storm, over and above thepredicted astronomical tides Storm surge should not be confused with storm tide,which is defined as the water level rise due to the combination of storm surge and theastronomical tide This rise in water level can cause extreme flooding in coastal areasorm tides

particularly when storm surge coincides with normal high tide, resulting in

reaching up to 20 fe

high teean sea level

Currently research of storm surge (and monsoon) has achieved a lot results, Thecountry suffered damage by storm surges as US, Japan, Russia, China has self=construction and development of appropriate software and also provided other

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countries use as India, Bangladesh There are several methods of calculation andforecast storm surge such as method uses semi-empirical formula, diagram method,artificial neural network method and numerical model methods

In the method using semi-empirical formula (Ippen and Hallerman, 1966),

surge magnitude is calculated based on ground level according to wind speed, wind

fetch Iength, the angle between the wind direction and the axis perpendicular to theshoreline and the water depth This method is very simple, but precision is not highbecause which did not describe all the factors impact on storm surges.

Diagram method (Yang et al, 1970, Horikawa, 1985) is often used to forecaststorm surges for some ports, where have many monitoring data on typhoons and stormsurges The content of the method is to construct the monogram based on the

relationship between monitoring data of water level with parameters of typhoon storm

(the largest wind speed, wind direction, reduce of pressure in the centre) Therefore,there is very limited when there is insufficient data or data series is not long enough(usually around 100 years if requirement of high precision result) and often only truefor the nearest observation station

Numerical models method was created to overcome the deficiencies ofempirical measurement data The advantage of this method is reduction of costcompared with experimental measurement methods In addition, this method alsoallows calculation, forecast the evolution of the phenomenon based on a lot ofproposed scenarios, which is not yet existed in reality, but likely to happen in thefuture,

In “Damage Assessment from Storm Surge to Coastal Cities: Lessons from the

Miami Area” Genovese, Hallegatte and Dumas focused on the two contiguous

counties of Miami, Dade and Broward In this research, authors considered the impact

of different storm surges predicted by the computerized model SLOSHI and

investigate flood risks with current sea level, considering different typhoon parameters(storm category, track, wind speed, and tide level) For each impact, authors apply a

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damage function and determine if the considered storm surges potentially lead to assetloss, considering both properties and their contents, The results show that, in absenceof protections, losses will be very high for large storm surges reaching up to tens ofUSD billions In the second part of the analysis, authors demonstrate how theeconomic impact changes when protections are built up, considering different dams"heights The authors concluded that raising flood defences would be beneficial, sincethe consequences of a storm surge could be enormous,

In the “De clopments in storm and in combination with tide modeling and riskassessment in the Australian region” Bruce Harper, Thomas Hardy, Lucian MasonandRoss Fryar emphasized the need for integrated planning and forecasting approachesfor storm tide risk assessment, The importance of the meteorological forcing and the

appropriate modeling of each of the storm tide components, namely astronomical tide,

storm surge, breaking wave setup and coastal inundation is discussed The critical roleof tropical cyclone “best track” datasets for risk assessment studies and the potentialimpacts on design criteria and risk assessment studies is highlighted, together with thechallenge of developing credible enhanced-greenhouse climate change scenarios It isconcluded that storm tide modeling needs to be undertaken in a holistic frameworkthat considers the relative uncertainties in each of the various elements - atmospheric,hydrodynamic and data, as well as addressing operational forecasting, design andplanning needs.

‘Methods used to simulate and assess storm surge

(1) Analytical methods to determine storm surge: The linear relationship betweenstorm and storm surges can not cover the storm surge phenomenon fully Previously

when computer technology and methodology has not developed the analytical methods

have been mentioned This method can be found in the documents of Russia, Spainand Japan Currently, this method is almost undeveloped, except for someestablishments want to have very fast approximate forecast results,

(2) Experimental methods: Using the results of the analytics model throughexperiments create expressions to describe storm surge phenomena, These results maybbe put into operation quickly Experimental property is shown in the observation ofreal high storm surge in off-shore, near-shore region more difficult to implement.‘Then, using the analytic relationship of wave propagation and water depth, the heightof storm surge can be determined in coastal areas,

(3) Statistical and regression methods: This is one of the methods to be applied widelyin the world, where measured data is available or the monitoring of storm surgessimultaneously with other environmental factors Statistical methods are used incombination with probability theory and extreme method Murty (1978), Harris (1962)‘was the first authors to propose statistical methods to apply in forecast, calculation ofstorm surge Firstly Haris created datasets of storm surges including monitoring data,ata of interpolation, extrapolation and additional data from the source of results fromnumerical model, which is got by the linear model From these data sets, the author‘had built equations of linear regres sion to predict surges.

Although there are many advantages to forecast storm surges, however statisticalmethods only offer best results once applied on defined area To build a regressionequation to forecast of storm surges, we need to consider the impact relations follow

the impact funetion.

‘The form of equation of general linear regression could be written as follows (Murty1978).

We “ c6 ay

In which

= 11: Altitude of rising water at x”, yŸ station in time L

~ At: Time step

~ i Time step between monitoring time periods

9

=i j Indexes corresponding to monitoring stations

~ k: indexes of monitoring stations

~ FŠ(L= At: Quant ies characterize for meteorological parameters, which has kform from j monitoring stations at time (ti At)

~ a (x,y): Coefficients corresponding to dependent parameters This coefficients

can be determined by theory or empirical datas

‘To determine the height of water rise, people often use empirical formulas, which issummarized from the actually observed documents for each area In Vietnam, BuiXuan Thong and Nguyen Tuong have based on survey of relationship between heigh‘of storm surges with wind speeds from 1959 to 1970 in Norther coastal area, theyhave given the calculated expression as follows:

Ah = 0.175 x Weng a2)In which:

Wo, - The average wind speed of storms (m/s)

‘Ah ~ the height of storm surge (em)

In the past time, in order to calculate the height of storm surge when design thewarehouse, Soviet Union has often applied the formula of Karausev A.V, LabzovskiNA esp dally norms like:

+ According to the ,06 04.82 normative of Soviet Union

Ah = ky (W2X/gH).cos a q3)

+ According to the 22 TCN 222-95 branch standard of Ministry of Transport

Ah =k, (W?X/g(H40.5 Ah) cos ơ q4)

+ According to QPTLC - 1 - 78 Norm of Ministry of Irigation (Ministry ofAgriculture and Rural Development)

Representing the group of analog electrochemical methods is the Russian authors as

Macarov and Menzin (1970), Ishiguro (1972) In principle, the electric network isdesigned as real hydraulic network and electric parameters is replaced as hydraulicsystem such as water level, discharge ete Since dynamic processes are simulated onthe analog network, Ishigura was suggested as follows (1) The extemal forces makeup water flow in the hydrodynamic system has similar nature like current electric incleotric network, (2) current electric flow through the absorption of citical powergenerated voltage, likewise due the dynamic nature of currents will ereate differentwater levels, (3) the law of energy conservation through the continuity of line appearin both real and same network and (4) Timing is the same in both network systems.

(5) Hybrid method: Holz (1977), Funke and Crookshank (1979) te

the tidal estuary The mathematical and physical meaning of Hybrid model show in theed hybrid model in

combination of propagation of long wave through coastal area by 2 models, whichsimulate at the same time in deep water and shallow water and through dynamic modeland hydraulic model Two models simulate at the same time, water level at the

abutting area between (wo models is determined empirically and put this value intodynamics numerical models.

ime of twoIn fact, the application of Hybrid models is done continuously at the same

model types as physical models and hydraulic model Information is exchanged

directly through the interface of two model regions.

(6) Methods of numerical simulation: This is the main content of the mathematical

modeli techniques to simulate storm surge This technique includes the following

= The first step is to build an understanding about the nature of the physical

phenomena of storm surge and procs expressed in real terms, This step, theresearcher completely can do and must do it.

~ The second step is to find out the formation proc of mathematical equation todescribe storm surge.

‘The third step is the techniques to identify forming forces of storm surge, the closed.system of equations Determine the type of boundary conditions including hardboundary, liquid boundary and side boundary

~ The fourth step is the technique to solve a system of equations, which have been setup This is the digital part, which relating to calculated grids, difference method, select

the programming language

~ The fifth step is the techniques to calibrate model, the stability of the model

1.2.3, Storm surge researches in Viet Nam

Vietnam is a coastal country, where have high storm surges risk so researchesof storm surges have been done for a long time and many models and technology ofstorm surge forecast were built, Besides the development of numerical models tosimulate the storm surge, some studies tend to use commercial models or open source

model to calculate storm surges to coastal areas of Vietnam The business model may

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be mentioned as MIKE package developed by Denmark Hydraulic Institute (DHI) andDelfi-3D model by Delft Technology University, Netherlands, ete

In researching methods by numerical models, three main research directionshave been implemented They are (i) built particular models; (ji) research and develop

‘open source models of abroad and (ii) use commerce models of abroad In addition

there have many projects, research projects in different levels, which are implementedand developed models, technology to calculate and forecast of storm surges inVietnam.

In the project namely “Nghién cứu để cập nhật, chỉ tết hóa bộ số liệu cơ bản về

triều, nước dâng dọc bở biển từ Quảng Ninh đến Quảng Nam phục vụ tính toán thiếtning cắp tuyén để biển” conducted by Dinh Van Manh and partners, based on database of tides, storm surges and total levels of tide and storm surges along the coasts

from Quang Ninh to Quang Nam were established, in which, data of statistical arisingstorm was built by Monte Carlo method based on probability distribution of theparameters storms occurred in the past,

Following this direction, in "Nghiên cứu xây dụng hệ thing dự báo tc nghiệpkhí tượng thủy văn biển (gồm đồng chảy, sóng và nước ding do bio) vùng biển Đông

xà ven biển Vigt Nam” projet, Le Trong Dao and partners hay used Delft 3D model{0 set up and simulation, forecasting storm surges to coastal areas in Vietnam.

In addition, Nguyen The Tuong, Tran Hong Lam et al (2007) in the framework‘of cooperation between Vietnam - China research project used a package of modelssuch as 3D Delft of the Netherlands, IMA (Japan Meteorological Agency storm surgemodel) of Japan and CTS (China typhoon Surge) of China to calculate and provideforecasting water surges fo the coasts of Viet Nam,

1.2 GENERAL DISCRIPTION ON STUDY AREA1.2.1 Natural conditions

1.2.1.1 Geographical location

Con Dao archipelago consists of 16 islands with total of 76 km*, which located in thefar southern sea and far from Vung Tau city of 185km, from the Ho Chi Minh City

230 km and from the Hau estuary (Can Tho) about 83km Geographical coordinates of

Con Dao island is 106°31 to 106'45' East longitude and 8°34' to 8°49" North latitude.The largest island is Con Lon (Con Dao) with area of 51.52 km? and is also the

economic, cultural, politcal and social center of the district Population of Con Daoistrict about 7,000 people in 2013, there is no administrative units of wards and‘communes, only the residential area

Figure 1.2: Map of Con Dao Island

Con Dao district has great potentials for development of tourism, exploitation

and processing of seafood, port development, petroleum and gas complex services andshipping Coast with a 200 km of length, with many beautiful beaches like Dat Doc,

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Bai Canh, Dam Trau, Hon Cau, Hon Tre In addition, the Con Dao National Park isan area of nearly 6,000 ha on mainland and 14.000 ha on sea with a variety of treesand rare animals,

1.2.1.2 Characteristics of topography and geomorphology

Con Dao archipelago was formed in stage of volcanic activity in the late Jurassicand lasted until late Paleogenic era, Becau: tectonic activity of volcano in the periodmentioned above, the archipelago was formed and exists to the present day Con Daozhas soil texture and terrain more complex than other islands.

Floatin Island Region: Con Dao Islands consists of 14 floating islands,including Con Son Island, which is the largest island with area of approximately

5SkmỶ, three other islands with area of over 5 km? as Hon Ba, Hon Bay Canh and Hon‘Cau and ten remaining islands has an area of less than 1km? Con Dao has low

mountainous terrain (Thanh Gia mountain with the highest peak of 577 m), mountains‘was formed by magma eruption.

Coastal zone: The shoreline of Con Dao Island has complex structure, texture‘of rock mailly formatted by magma eruption and intrusion, The distribution and

positions of shoreline are not fixed, depending on tide, sedimentation and e sion.

Coast are composed of erupted magma distributed in the northeast of Con Dao andHon Tai, Bay Canh, Hon Cau and some other small islands inthe northwest Features‘of eruptive rocks are highly alkaline Coast are composed by shallow intrusion magmais the stretch of shore in Ben Dam, Thanh Gia mountain, Ta Be mount, Chim cape andnorthwest shore of Bay Canh island, Coast is composed of discrete Quaternary rocks4istributed in central areas of Con Dao - the first coast of Co Ong airport valley Thesediment here mostly is fine sand originated by sea

The seabed: The seabed have abrasives and accumulate surface in depths from3-10m: It is not great area, mainly distributes in the northeast coast, central and southwestern of Con Dao islands, This is the underground shelf of Northeast gulf, Con Sonbay, Ben Dam bay Largely of surface is sand, fine-grained sand mixed with organism

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debris, Some surface areas have coral reefs developing rapidly such as Con Son Gulf,Ben Dam Bay Particularly in Con Son Gulf, which near the pier exists negligibleaccumulation, is mud,

‘The bottom area has abrasion accumulates surface at depths from 10 to 20 m:

Focusing mainly in Northeast bay and Con Son Gulf Bay which in southwest Con Dao

island, Bottom topography in the north-eastern and central regions is relatively flat,bottom sediments are mostly fine sand and sand-sea creatures The bottom topographyof southwest of Con Dao has more complex surface roughness Bottom sediments aremainly sand, gravel and mixed into sea creatures, coarser composition of grain

122 imate

1.2.2.1 Temperature

Con Dao locates in the area of tropical monsoon climate and has two distinct

avons and dry seasons, surrounded by sca, 0 climate of Con Dao isinfluenced by ocean climate regime therefore Con Dao climate regime is more

moderate than the mainland, Air temperature in Con Dao doesn’t not oscillates largely,

the average temperature of months in year ranges between 26-29°C, full-year average

temperature is 27,8°C high absolute temperatures is 36C and low absolutetemperatures 18°C) (Dang Ngoc Thanh, 2003) This is the most moderated areas of

coastal zone in Vietnam with the oscillation amplitude of temperature annually is Tow,

not over 3,1°C - 4°C Monthly average temperature is highest in May with 28,3°C and

lowest in January with 25,2°C (table 1.2)

“an 1.2: The average temperature of air in the Con Dao Island

‘Source: Nguyen Van Au, 2002

6 7 8 9 10 M12Month 12 3 4

TỨC) 252257 269 282 283 278 215 275 272 269 267 257

1.2.22 Rainfall regime

Rainfall regime of Con Dao region depends heavily on air masses, especially‘humid equatorial air masses Moreover, the impact of storm and the role of topographyalso contribute significantly decide to the rainfall of Con Dao Island According to

statis document show that the average rainfall for many years in Con Dao Istand isquite large, about 2055 mm/year The rainy season lasts from May to October withaverage rainfall reached 200-250 my/month, accounting about 70% of annual rainfall.The dry s

days of rain/month, average rainfall for month is about 40-66mm/month,

-ason lasts from November last year to April next year, an average of 6-7

1.2.2.3, Wind regime

‘Con Dao archipelago locates in the South East Sea climate area, the climate is‘topical oceanic equatorial monsoon Every year, climate and weather here are affectedalternately by two monsoon types, they are northeast monsoon and the southwes

+ The northeasterly wind is usually northeast and east (lo the prevailingdirection is east), blows from late October to early April next year The average wind

speed of 35-45 mis, the strongest winds to 18-20 mis, when a storm wind speeds ean

be up to 30-35 mis

4+ The southwest monsoon is usually south west and south, blowing from lateApril to early October, the average wind speed reaches 2,5-3,5 mvs, the strongestwinds reaches 20-25 m/s Typhoons usually appear in 10-12 months (low frequency),winds can reach 25-27 mis

‘The transition period between the two seasons, the prevailing wind is east tosoutheast, with an average speed of approximately 4 mis,

1.2.3, Oceanographic regime

1.2.3.1 Tide

Tidal regime at Con Dao Sea is of mixed type including some diurnal days, but proneto irregular tide Tidal range in Con Dao in spring period is from 3.0 - 4,0 m and 1.5 -2.0 m in ebb tide, Fluctuation band of relatively large sea level (0,5 - 0.7 m),fuctuating around 0.45 in average The maximum water level was recorded at 4.1 mand minimum is 0,21m, 2,47m average, average tide recorded in Con Dao is 2,29m.1.2.3.2, Wave regimes

Due to the impact of the monsoon circulation and influence of geographicalconditions, the wave regime of Con Dao has relatively complex behavior and therehave two large wave seasons annually Direction, wave height and wave length arerelated closely to the ind regime in the year In the northeast monsoon, the prevailing.wave direction is northeast and northeast-east with wave frequency is 60% and theaverage wave height is of 0.5-1.8 m In the southwest monsoon, the prevailing wavedirection southwest and west-southwest with an average wave height of 0.3-1.5 m.

1.2.3.3 Ocean circulation,

Besides oscillations by wind, waves, Con Dao island has movement of sea currents,Under the acting of flows, sea water in Con Dao islands is circulated and exchangedwith surrounding sea areas, which have changed and regulated climate, formation ofsand bars and contribute to increase the area of alluvial around island.

Flow around Con Dao Island is dominated mainly by tidal current and topography Inwinter, the direction of flow in Con Dao area is northeast - southwest with averagevelocity of 048 to 1.5 mí and in summer flow has the opposite direction, with amedium flow rate approximately 0.7 to 1.5 mis,

1.2.4, Socio-economic activities

In 2014, the socioeconomic situation in Con Dao district has many positive changes.The economic structure of developed properly orientation; Service-tourism: 87.25%,industry-construction: 7.96%, agriculture- forestry - fisheries: 4.8% The industrialproduction value is estimated at 136 billion, up 9.64% compared to 2013; total servicerevenues is 1,079 billion, up 10.09%; fishery production value estimated at 35.63billion, up 11.8% compared to last year Estimated total budget revenues reached285.87 billion in 2014 At the same time, Con Dao district has also done good work ofhealthcare, education, culture, sports, public administration reform, and policies tocensure social security, environmental protection, national defense, security

1.2.4.1 Population and labour

According to the document of People's Committee of Con Dao district, the years from1980 to 1990, population of Con Dao district ranged between 2000-3000 people, to the‘years 2000-2010, population of Con Dao district increased to 4000-6000 people andWhich reached 6600 people in 2012 They live in ten neighborhoods Con Dao townfocus of residential, resort for tourists and administrative units of Con Dao district.‘The proportion of male / female is 51% and 49% In addition to the mechanicalpopulation growth as above, the population in Con Dao has increased by natural way.Because more than 70% of residents ate officials and civil servants are seriouslyimplementing the Policy on Population and Family Planning, so the rate of naturalpopulation growth in Con Dao is very low, only 1-3% a year.

Report of Ministry of Home Affairs on situation of the first six months in 2012: In ConDao, the number of people in working age is 3,780 people, of which more than 2900Service: 1970

people have stable jobs Occupational structure includes: Officers

people; Industry - construction: 583; Agriculture: 370 people; Homemaker: 384

people Moreover there are 29 old men have age over 80 year old, and more than 1,500

students from preschool to high school The total number of households is 1577.

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‘has 336m and it capable of receiving vessels of 2,000 tons.

* Commerce and Services

Domestic and foreign tourists come to Con Dao islands more and more, Currently, inaddition to to flights from Tan Son Nhat (Ho Chi Minh) and Vung Tau, Con Dao hasfive large ships, which have 100 beds and three cargo ships from 50 to 250 tons ‘Thereare also Phi Yen hotel, Saigon Tourist hotel and guest houses of Con Dao NationalPark.

‘As planned, Ben Dam bay will build four ports: seafood port, petroleum technicalservices port, maritime services port and military por Con Dao will prioritize

incourage the development of services of training for human resources and wellness,

Currently, Con Dao is transferred gradually toward focus develop service, tourism to

promote potentials and advantages of the district

CHAPTER 2: APPLICATION OF DELFT3D TO STUDY STORMSURGE

21 Description of model

3.1.1 Delft 3D model

Delf3D is an integrated modelling suite, which simulates two-dimensional (in eitherthe horizontal or a vertical plane) and three-dimensional flow, sediment transport and

morphology, waves, water quality and ecology and is capable of handling the

interactions between these processes The suite is designed for use by domain experts‘and non-experls alike, which may range from consultants and engineers of contractors,to regulators and government officals, all of whom are active in one or more of thestages of the design, implementation and management cycle.

Delft3D-FLOW is a multidimensional (2D or 3D) hydrodynamic (and transport)simulation program which calculates non-steady flow and transport phenomena thatresult from tidal and meteorological forcing on a rectilinear or a curvilinear, boundaryfited grid, In 3D simulations, the vertical grid is defined following the sigma co-

ordinate approach.

‘The Delft3D program suit is composed of a set of modules (components) each ofwhich covers a certain range of aspects of a research or engineering problem Eachmodule executes independently or in combination with one or more other modules.‘The information exchange between modules provides automatically by means of a so-

called communication fle; each module writes results required by another module to

this communication file and reads from the file the information required from othermodules Other, module-specific, files contain results of a computation and used forvisualisation and animation of results,

‘The hydrodynamic conditions (velocities, water elevations, density, salinity, verticaleddy viscosity and vertical eddy diffusivity) calculated in the Delft3D-FLOW moduleare used as input to the other modules of Delf\3D, which are (see Figure 2-1)

2

Delft3D-WAVE short wave propagationDelf3D-WAQ far-field water quality

Delfi3D-PART mid-field water quality and particle tracking.

Delft3D-ECO ecological modelling

+ Delft3D-SED cohesive and non-cohesive sediment transportDelft3D-CHEM chemical components and interactionsDelft3D-MOR morphodynamic simulations

(Overall Meu

T T T T T T 1

[wave]| | [vao] | [parr]| | [Eco] | | [Sep] | |[cmw]| | [mor]

I I T i I I T

Visualization and other tok]

Figure 2 1: System architecture of Delft3D

In the thesis, Delft 3D-Flow module is used to simulation storm scenarios in Con Dao

Basic equations

‘The hydrodynamics of continent shelf in the storm conditions is simulated by solvingthe system of two-dimensional of shallow water equations that consists two ofhorizontal momentum equations and one continuity equation:

Conservation of momentum in x-direction (depth and density averaged)

tan giU|x

pe

Conservation of momentum in y-direction (depth and density averaged)

Lập „ch

m _£t _ 1 _„ °

per C+ pen

O88 WH © 6) 6

‘The depth and density averaged continuity equation is given by:

Onn , Old +n , Old +iDy

(8): External force by wind

(9): Depth averaged turbulent stress

In which:

C: Chezy coefficient4: Bottom depth£: Coriolis parameter

«8: Diffusion coefficient (eddy viscosity)

vi: Water level above a reference leveluy: Depth averaged velocity

23)

Pu: Mass density of water

U: Absolute magnitude of total velocity, U=(u"+v")"

Tax Tay? Xs Ye Components of wind shear stress, Wind shea stress is determined by the

widely used quadratic expression, t,=p,C,W? , where p, : air density, Cy: wind dragcoefficient, W: wind speed at 10m above the free surface.

‘The vertical momentum equation is reduced to the hydrostatic pressure relationVertical accelerations are small compared to the gravitational acceleration (), so it isnegligible The fluid is incompressible and using Boussinesq approximation Nodynamic coupling between changes in topography and flow In small-scale flow,complete Reynolds stress tensor is used In vertical direction, the so-called ø-coordinate is used that means the number of layer is constant over horizontal‘computational area In this study, only depth- averaged model is used corresponding to‘one layer in vertical

Bed shear stress:

2 =2,C,|WW|W with W = JW HH?Press wind components

"` @-12)Fu, =,C,lW|NW, = 2,C, [WW sind =x, sind 13)

Calculated surge water by storm

Storm surge in shallow water influence of water depth and coastal shape:

dy 1 aP z 6-14)đt pg di pgh

In order to simulate flows under typhoon condition, it is necessary to set up a typhoonmodel to simulate storms including two separately parts: pressure field and wind field,‘The results of the model are pressure field and wind field on the sea surface understorm condition that will be the data set required putting into hydrodynamic model+ Typhoon pressure computation

“The decrease of pressure causes a rise of water level In the equilibrium state, a water

level has one centimetre increase for every millibar decline of atmospheric pressure.

‘The larger the water depth the stronger the influence of atmospheric pressure field, Inshallow water although the effect of atmospheric pressure itself exerts the watersurface is small compared to wind stress, it plays an important role in driving wind

26

field, Therefore, setting up a model to simulate pressure field under typhoon conditionisan important work.

An atmospheric pressure field can be given based on observation or on forecast In theabsence of data, we may use a field associated with an ideal typhoon or cyclone modelsuch as:

For all of the above models, p, has a value of p= 1 at 1013 mb The value of py is‘obtained from Best Track The magnitude of R varies in time during the development‘of a typhoon, It can be taken directly from observed data of atmospheric pressure field‘or estimated by minimising the error between observation and computed values.

“Wind field

Surface wind stress terms represent the drag force produced by wind over the watersurface, This is important for shallow water areas in storm conditions where verystrong winds occur It is even much more important than the role of pressure in drivingstorm surge Actually, typhoon wind fields are usually intensive, spatiallyinhomogeneous and directionally varying The large gradients in wind speed andrapidly varying wind direction of typhoon vortex can generate very complicated flow.However, for practical application, the wind field data may be taken from observationor forecasts using -veral simple parametric wind models as an ideal typhoon model.

Figure 2 2: Sketch of wind velocity field for a moving eyclone

Actually the wind speed (W) has two components: one is related to the typhoon centremovement and the other is the gradient wind speed component and present inCartesian co-ordination Fully wind speed model is described,

ÍW,=W, +, =W, cosp+ CW, cos(90" + + 8)

<1, sự, -W, sing + CW sin sự +)Wher

W,, Wy: x ~ (east), y — (north) components of the typhoon wind speed ataltitude of 10m above sea level

28

E: wind speed component related to moving centre of typhoon at a distance rfrom the center of the typhoon.

F, Fy x: , ý components of velocity related to moving centre of typhoon,Wz: typhoon gradient wind speed at a distance r from the centre of the typhoon

ws Angle between x-axis and the line connecting calculation point andtyphoon center,

£8: Angle made by the gradient wind speed with sopestc line.Co: empirical coefficient in the range of 06 to 08

Wind speed components ofthe typhoon:

++ The first component of wind speed related to moving centre can be calculated by

following formula (Masami, 1962)

F=cye 221)‘Where C¡ is a coefficient in the order of 4/7 to 6/7 and depends on R, Cy = 4/7 if Risrelatively large, otherwise its values is 6/7 According to this formula, wind speed500

caused by moving typhoon centre decrease from C¡ x Vụ at centre to C¡ xe" ate

par density p= 1.293 kgim’ at 0°C and 1 atms

£ Coriolis coef tị F 2osing= 0.525sing in which w is the angular speed of Earth;gis the latitude

‘There are some well-known parametric wind models presented as follow:~The modified Rankine vortex model (1947):

tới LòZÌj/0<r<R)

Ø0)=1301+a5'ÍT —1Ì)48 <r<1⁄2R“Ir:“ícỳ

25; >1.28)

In which Wyax: the maximum wind speed X is shape parameter ranging 0.3 < X< 0.8

4o adjiust the wind speed distribution in radial direction can be determined empirically

from observed data.

~ SLOSH model (1992)

Above two models require user specified R and W,„„ R is taken from fitting with

pressure observation or relation, Wa is available from best track.

~ Holland model (1980)

{nate of (2) } 226where B isa parameter and ean be obtained from

a= 900 for 1<B<2.5

~ DeMaria et al (1992):

rr) fal, (rWTì

wo=ws.(2)ee1- (Wan olf (] 0-21)(2Ý | a

bisa parameter change the shape of profile, which can be vary from 0.2 <b <0.8

30

To estimate the wind direction, it is necessary to take into consideration a bias angle

Abetween geostrophic wind and real wind, as regards typhoons, in the literature itis

sometimes assumed that the wind velocity is directed toward its center and makes annorthem hemisphere For s stationery topical cyclone, the inflow angle at the surfaceis approximated as Bretschneider in (Phadke at al 2002)

(R<r<12R (2⁄28)

¬ -8+ Varies from 10° at the centre to 20° at R and then increase linearly to 25° at 1.2R

and remains at 25° beyond 1.2R,2.2, Data used for the simulation2.2.1 Topographical data

‘Topography and seabed elevation in the study area is provided by KC09.16/11-15

project, For deep water, seabed topography is the map with scale of 1:50.000 to

1:100/000 provided by other sources.2.2.2 Hydrodynamic data

Tidal boundary ‘The tidal harmonic constants (M2, S2, N2, K2, OL, KI, Pl, Q1) areused to calculate the predicted tide at the open boundaries as input data forhydrodynamic model, Data from the harmonic constants were taken from DelftDashboard model

‘The information of storm: The information of Durian typhoon such as the storm’s

trajectory (typhoon position, movement speed, measure of reduced pressure at thecentre, the largest wind speed) is taken from collection of data BEST TRACK of theUS National Oceanic and Atmospheic Administration (NOAA)(hup/iweather unisys.conJhurricane’).

31