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 i ACKNOWLEDGMENT Undertaking this Ph. 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. 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. would not have been achievable. Besides my advisor, I would like to thank the rest of my thesis committee: Prof.
Manabu Kanda, Assoc. Daisuke Akita, Assoc. 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.
i 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. ii Table of content ACKNOWLEDGEMENT……………………………………………………………………………………i ABSTRACT…………………………………………………………………………………………….ii LIST OF FIGURES…………………………………………………………………………………….vi LIST OF TABLES…………………………………………………………………………………….ix CHAPTER 1 Introduction……………………………………………………………………….1 Coastal disaster in Vietnam…………………………………………………………….3 Wave climate classification……………………………………………………….2 Storm surge Model……………………………………………………………………….3 Recent researches on Storm surge and typhoon associated disasters…………………….1 Studies in storm surge in the world…………………………………………………………….2 Storm surge researches in Vietnam………………………………………………. Numerical model development………………………………………………….
Typhoon perception in Vietnam………………………………………………….4 Aims and Objectives……………………………………………………………………….6 Outlines of this study…………………………………………………………………………………9 CHAPTER 2 Typhoon perception in Southern Vietnam……………………………………………………11 2.2 Typhoon Track Analysis…………………………………………………………………………….3 Storm surge awareness in Mekong Delta……………………………………………………………14 2.2 Typhoon Linda In 1997………………………………………………………………………17 2.3 People’s Awareness in Mekong Delta…………………………………………….4 Field survey in Con Dao……………………………………………………………………………. The purposes of the survey……………………………………………………….2 Memory of Linda and Awareness of Typhoon Disasters in Con Dao Island…………………23 2.5 Discussion On Disaster Risk Awareness Improvement…………………………………………….27 CHAPTER 3 Coupled Storm surge – Wave model using Meso-scale Data --- Hindcasting with an ideal meteorological input……………………………………………………………………….2 Wind–wave and Storm Surge Hindcasting……………………………………….3 Analysis of tide gauge data……………………………………………………….2 Wind-Wave and Storm Surge Hindcasting……………………………………………………45 A. Wind and pressure fields……………………………………………………………………45 B. Storm surge simulation………………………………………………………….
Wind-wave simulation………………………………………………………………………48 D. Tidal effect on wave and storm surge simulation………………………………………….1 Improvement of the proposed 2-way coupled model compared with other conventional models……………………………………………………………………………………….2 Wave-Storm surge ratio……………………………………………………………………….58 CHAPTER 4 Coupled Storm surge – Wave model using, parametric typhoon Model--- Hindcasting with a limited meteorological input………………………………………………………………….1 Applicability of wind field from Typhoon model and GEBCO08 bathymetric data in Vietnam………………………………………………………………………………………60 iv 4.2 Coupled model description……………………………………………………………………63 4.1 Storm Surge Induced by Linda……………………………………………………………….2 Waves Generated by Linda……………………………………………………………………66 4.71 CHAPTER 5 Discussion – Application of the models for disaster management………………………….1 Discussion on policy, public education, and training regarding typhoon and storm surge risk management in Vietnam………………………………………………………………….2 Hazard map for fishermen……………………………………………………………………………75 CHAPTER 6 Conclusion……………………………………………………………………………………80 REFERENCES…………………………………………………………………………………………….81 v 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…………………………………………………………….2 Location of surveyed areas in Vietnam…………………………………………….3 Interview with local people in Can Tho city……………………………………….4 Age distribution of respondents in Mekong delta………………………………….5 Occupation of respondents………………………………………………….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……………………………………………………………………….8 Wind speed during Typhoon Linda in Can Tho city………………………………… 19 Figure 2.9 Damage in Can Tho city during Typhoon Linda 1997…………………….10 Distribution of respondents who were aware of the nature of storm surges in Mekong Delta…………………………………………………………………….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…………………………………………………….14 Houses near water in Ca Mau…………………………………………….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………………………….16 Occupation of respondents……………………………………………………….17 Questionnaires results regarding people awareness and preparedness in Con Dao……………………………………………………………………………….18 Do children learn about storm surge/flooding in school, and how to evacuate?.19 Tsunami instruction panel in Con Dao…………………………………………….20 Source of information on storm surge and typhoons in Southern Vietnam……….1 Japan Meteorological Agency’s weather map immediately before Jebi made landfall (September 4, 2018, 09:00, Japan Standard Time, UTC+9)…………….2 Processes in one-way and two-way coupled models…………………………….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……………………………………………………………………….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……….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 vi storm surge (yellow). The blue line indicates the trajectory of Typhoon Jebi. 37 Locations (a) to (o) indicate the sequence of survey points……………………….6 Field survey at Nanko Bird Sanctuary and Sakai (locations (a) and (b) in Figure 3.
(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 at Sakai City outside…………………………….7 Field survey at Rinku Park (location (c) in Figure 3. (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.
(I) Localization of Kainan and Saikazaki, (II) coastal fence smashed by high waves at Kainan, (III) broken parapet by overtopping waves at Saikazaki………………………….9 Field survey at Tokushima (locations (f) and (g) in Figure 3. (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. (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. (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…………….12 Field survey at Kobe city (locations (l) to (o) in Figure 3.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……….13Comparison between wind speed obtained from the JMA mesoscale spectral model and observed data on 4 September 2018 (time in Japan Standard Time, UTC+9)………………………………………………………………………….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)………………………………………………….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)………………………………………………….16 Comparison of calculated surge level with flow–wave interaction, no interaction with wave, and observed values during Jebi passage…………………………….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 Time, UTC+9)…………………………………………………………………… 47 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 vii 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)………………………………………………………….20 Comparison of wave height from different models with different coupled components (Japan Standard Time, UTC+9)…………………………………….21 Comparison of the total water level (storm tide) from coupled storm surge -wave – tide model (Japan Standard Time, UTC+9)…………………………………….22 Comparison of significant wave height between 2-way couple model and other models (Japan Standard Time, UTC+9)………………………………………….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.24 Power spectrum of component waves during 12 hours including the arrival time of Typhoon Jebi………………………………………………………………….1 Comparison between wind field during Typhoon Jebi reproducing by GPV model and Typhoon model……………………………………………………………… 61 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.3 Simulation grids, combining larger and smaller domains……………………….4 Central pressure and wind speed during Linda (Vietnamese local time)………….
