Research on large flood management of tra bong river basin, quang ngai province master thesis major disaster management code 60 44 02 2

ACKNOWLEDGEMENT Firstly, I would like to express my sincere gratitude to my advisors Assoc.Prof Hoang Thanh Tung and Dr Pham Thanh Hai for the continuous support of my M.Sc study and related research, for their patience, motivation, and immense knowledge Their guidance helped me in all the time of research and writing of this thesis I could not have imagined having a better advisor and mentor for my M.Sc study Besides, I am especially grateful to lecturers in the Department of Hydrology and Water resources, Thuy Loi University and foreigner lectures from NICHE project who supported me for all the lectures and useful advices throughout my course My sincere thanks also goes to my colleagues in National HydroMeteorological Service and Hydro-Meteorological Department of Middle Centre region who supported me for data collection and analysis Without they precious support it would not be possible to conduct this research Last but not the least, I would like to thank my family: my parents, my wife and children for supporting me spiritually throughout the course and my life in general Hanoi, Nov 08th 2016 Nguyen Phu Luan ABSTRACT This study aimed at proposing solutions for managing large flood in Tra Bong river basin, Quang Ngai province The research includes main parts: create a flood inundation map based on historical flood marks collected and flood forecasting experiment for the downstream of river by simulating flood flow with the combination of hydrological and hydrodynamic models At the first part, 42 flood marks of the 2009 historical flood had been collected with their exactly coordinate and maximum water level The distribution and elevation of flood marks have been used to calculate with topography data (DEM) using GIS tools and the result was the 2009 flood inundation map At the second part, a rainfall – runoff hydrological model (NAM) has been firstly used to simulate flow from upstream of basin to the section at the beginning of the main river The input data were collected from rain gauge stations for simulation After that, the computed water discharge got from rainfall–runoff model have been used as the upstream boundary for simulating flood flow in the main river The model has been used was 1-dimensional hydrodynamic model After simulating the flood flow in the river, the study tried flood forecasting for the downstream part Based on the flood inundation map and the result of flood forecasting experiment, the research has proposed solutions to manage large floods on Tra Bong river basin These solutions can be used to develop large flood management plans for local authorities, in order to enhance efficiency in water resources using and reduce losses caused by large flood Abbreviation GIS Geographical Information System WMO World Meteorological Organization DHI Danish Hydraulic Institute DEM Digital Elevation Model DMC Disaster Management Cycle MONRE Ministry of Natural Resources and Environment MOC Ministry of Construction NHMS National Hydro-Meteorological Sevice NAM Nedbør-Afstrømnings-Model (Danish, meaning rainfall-runoff model) FF Flood Forecasting HD Hydrodynamic 1D, 2D 1-dimensional, 2-dimensional TABLE OF CONTENTS INTRODUCTION General Introduction Description of the Study Area Problems and Need of Study 11 Objectives of Study 11 Scope of Study 12 CHAPTER I: LITERATURE REVIEW 13 1.1 Related researches about study site 13 1.2 Flood inundation mapping 17 1.3 Geographical Information Systems in Hydrology and Water Resources 21 1.4 Flood Forecasting 23 CHAPTER II: APPROACH AND METHODOLODY: 28 2.1 Approach of study 28 2.2 Flood inundation map 28 2.3 Using GIS tools to develop flood inundation map 30 2.4 Mike 11 general description 32 2.5 Theoretical Foundation of rainfall – runoff hydrological model (NAM) 42 2.5.1 The basic parameters of NAM model 44 2.5.2 Basic modelling components 46 2.5.3 Initial conditions of the model 50 2.5.4 Model calibration 51 2.6 Flood Forecasting (Mike 11 FF): Updating procedure 51 2.6.1 Two unique features MIKE 11 FF'S updating procedure 52 2.6.2 The calibration updating parameters 55 2.7 Flood Forecasting Error 57 CHAPTER III: RESULTS AND DISCUSSIONS 59 3.1 Analysing flood features of Tra Bong river basin 59 3.1.1 Rain features 59 3.1.2 Flood features 61 3.2 Develop flood inundation mapping for Tra Bong river basin 66 3.3 Flow forming simulation using Mike NAM 74 3.3.1 Calculation layout 74 3.3.2 Data analysis 75 3.3.3 NAM model calibration and verification 77 3.4 Flood flow in the downstream using Mike 11 hydraulic model 82 3.4.1 Boundary condition 82 3.4.2 Calibration and verification of flood flow simulation model 84 3.5 Flood forecasting experiment for Tra Bong river system 90 3.6 Propose solutions of large flood managing in Tra Bong river basin 95 3.6.1 Structure methods 96 3.6.2 Non-structure methods 97 CHAPTER IV: CONCLUSION AND RECOMMENDATION 101 4.1 Conclusion 101 4.2 Recommendation 102 REFERENCES 103 LIST OF FIGURES Figure 1: Administration map of Quang Ngai Province Figure 2.1: Structure of literature review 13 Figure 2.2: Conceptual framework for flood hazard and risk calculations 19 Figure 2.3: Different flood map types 20 Figure 2.4: Process for developing a flood forecasting model 25 Figure 3.1: Conceptual Framework 27 Figure 3.2: Overview of study 28 Figure 3.3: Digital Elevation Model with square grid 31 Figure 3.4: Error based on the topography data in flood inundation mapping 31 Figure 3.5: The structure of NAM model 38 Figure 3.6: Channel section with computational grid 44 Figure 3.7: The shape of the computational grid around a node which has three branches 45 Figure 3.8: The shape of the grid points and the nodes in the complete model 45 Figure 3.9: Branch matrix before reducing 47 Figure 3.10: Branch matrix after reducing 47 Figure 3.11: Three-branch node with limit for continuity equation 48 Figure 3.12: River branch with discharge boundary 49 Figure 3.13: Illustration of amplitude and phase error 53 Figure 3.14: The updating results of simulations 55 Figure 3.15: Example of measured and simulated discharge at an update location 57 Figure 3.16: Example of updating parameters 57 Figure 4.1: Chart of the possibility of flooding which reach 2nd alarm level or higher in flood season - Tra Bong river basin 64 Figure 4.2: hour rainfall chart, from 19:00 27 Sep 2009 to 01:00 30 Sep 2009 – Tra Bong station 65 Figure 4.3: hour rainfall chart, from 19:00 27 Sep 2009 to 01:00 30 Sep 2009 – Chau O station 66 Figure 4.4: Hourly water level process of the flood from 28 Sep to 01 Oct 2009 – Chau O station 67 Figure 4.5: 2009 Flood marks map of Tra Bong river downstream 70 Figure 4.6: 2009 Flood inundation map of Tra Bong river basin 73 Figure 4.7: Map of hydro-meteorological stations network 74 Figure 4.8: Calculation layout 75 Figure 4.9: Weight factor distribution layout of rain gauge stations in Quang Ngai province 76 Figure 4.10: Basin parameters declaring dialog 77 Figure 4.11: Parameters calibration for rainfall-runoff model (NAM) 77 Figure 4.12: Calculated flow process at Binh Minh station 78 Figure 4.13: Rainfall – runoff model (NAM) calibration, compare observed and simulated flood discharge at Binh Minh – Tra Bong river, from 16 to 20 Oct, 2008 80 Figure 4.14: Rainfall – runoff model (NAM) calibration, compare observed and simulated flood discharge at Binh Minh – Tra Bong river, from 28 Sep to Oct, 2009 80 Figure 4.15: Rainfall – runoff model (NAM) calibration, compare observed and simulated flood discharge at Binh Minh – Tra Bong river, from 13 to 19 Nov, 2010 81 Figure 4.16: Rainfall – runoff model (NAM) verification, compare observed and simulated flood discharge at Binh Minh – Tra Bong river, from 14 to 20 Oct, 2011 81 Figure 4.17: Rainfall – runoff model (NAM) verification, compare observed and simulated flood discharge at Binh Minh – Tra Bong river, from to Nov, 2011 82 Figure 4.18: Rainfall – runoff model (NAM) verification, compare observed and simulated flood discharge at Binh Minh – Tra Bong river, from 25 to 29 Nov, 2011 82 Figure 4.19: Hydraulic routing layout of Tra Bong river downstream 83 Figure 4.20: Initial conditions calibration dialog 86 Figure 4.21: Bed resistance calibration dialog 86 Figure 4.22: Bed resistance calibration for cross-sections 87 Figure 4.23: MIKE 11 (HD) calibration, compare observed and simulated water level at Chau O station – Tra Bong river, from 16 to 20 Oct, 2008 88 Figure 4.24: MIKE 11 (HD) calibration, compare observed and simulated water level at Chau O station – Tra Bong river, from 28 Sep to Oct, 2009 88 Figure 4.25: MIKE 11 (HD) verification, compare observed and simulated water level at Chau O station – Tra Bong river, from 13 to 19 Nov, 2010 90 Figure 4.26: MIKE 11 (HD) verification, compare observed and simulated water level at Chau O station – Tra Bong river, from 14 to 20 Oct, 2011 90 Figure 4.27: Flood flow forecasting results from rainfall data at Binh Minh – The rains from to Nov, 2011 92 Figure 4.28: Observed and hour predicted flood water level process in Chau O station - from to Nov, 2011 93 Figure 4.29: Flood flow forecasting results from rainfall data at Binh Minh – The rains from 25 to 29 Nov, 2011 93 Figure 4.30: Observed and hour predicted flood water level process in Chau O station - from 25 to 29 Nov, 2011 94 Figure 5.1: Synthesis of large flood management solutions in Tra Bong river basin 97 Figure 5.2: Disaster Management Cycle 99 LIST OF TABLES Table 1.1: The morphological features of Tra Bong river and major tributaries 10 Table 2.1: Overview of methods and data for high-resolution flood-risk mapping in Germany 18 Table 2.2: Predictive performance of the models using independent calibration /validation data 26 Table 3.1: Technical requirements of elevation different between the contours with corresponding scales 32 Table 3.2: Quality of forecasting classification 59 Table 4.1: Rainfall causes flood rising in Tra Bong river basin 61 Table 4.2: Rainfall in the history flood on 28-30 Sep 2009 61 Table 4.3: Maximum daily rainfall in Tra Bong station, from 2006 to 2011 61 Table 4.4: Annual peak flood of Chau O station (2006-2011) 62 Table 4.5: Typical flood intensity and amplitude in Chau O station (with the peak flood higher than first alarm level) 62 Table 4.6: The typical floods in 2006-2011 period of Chau O station 63 Table 4.7: hour rainfall data, from 19:00 27 Sep 2009 to 01:00 30 Sep 2009 – Tra Bong river basin 65 Table 4.8: Detail characteristics of the flood from 28 to 30 Sep 2009 66 Table 4.9: The data of 2009 flood marks in Tra Bong river basin 67 Table 4.10: The rain gauges used for hydrological calculating 75 Table 4.11: Results of parameters calibration for rainfall – runoff model 79 Table 4.12: Results of NAM model calibration and verification at Binh Minh 79 Table 4.13: Position of nodes in hydraulic calculation layout of Tra Bong river 83 Table 4.14: Analysis of model calibration efficiency and error 87 Table 4.15: Analysis of model verification efficiency and error 89 Table 4.16: Results synthesis of flood forecasting of Tra Bong river in Chau O station 95 INTRODUCTION General Introduction Large flood is a kind of disaster occur regularly and seriously annually around the world Flood has some benefits such as bring fertile soil to replace nutrient-poor soils, but it also impacts and causes enormous damage, constantly threatening people living and the economic and social development People use many methods to prevent and reduce the impact of flood including management, structures and non-structures methods Therein, large flood management is always an important objective, which requires detailed and specific research for each region The research “Research on Large flood management of Tra Bong river basin, Quang Ngai province” aims at approaching a new point of view in flood management, in order to achieve high efficiency in reducing the harmful effects of flood to the economic development and environmental protection of study area Description of the Study Area Quang Ngai province is located at the latitudes 14°32'- 15°25' North, longitudes 108°06' - 109°04' East, lean on Truong Son mountain range, overlooking the Eastern sea Quang Ngai abuts Quang Nam province in the North, Binh Dinh province in the South, Kon Tum province in the West, and Eastern sea in the East Located in the middle of the country, it is 883km from Hanoi capital to Quang Ngai and 838km from Ho Chi Minh City Due to the steep topography and poor vegetation, so the transfer speed of flood is very high, flood’s damage is huge Every year floods have caused extensive damage and losses of life and property 600,000 people affected by flooding, especially some communes which suffered flood inundation elevation more than m with the frequency of floods are 20% and 10% The model verification results based on the observed data showed that the simulated results of downstream flood water level relatively consistent with observed data Thus, the model can be applied to simulate and forecast floods in downstream of Tra Bong river system 3.5 Flood forecasting for Tra Bong river system Flood forecasting is a necessary part of flood management Provision of flood forecasting will also form part of flood management planning and development strategies, which recognize that there are occupied flood plain areas where nonstructural measures can be effective This can include the use of temporary defences (flood gates or demountable barriers), domestic protection (sandbagging) and local evacuation (to flood shelters) Flood management requires a variable degree of response from the water management agency, local or municipal authorities, transport and communications operations and emergency services Flood forecasting has to provide information to these users both for preparation and response (WMO, 2011) In this part of study, we used the combination of hydraulic and hydrological models Mike NAM and Mike 11 HD with the parameters have been calibrated and verified in the previous steps to forecast flood for the downstream of Tra Bong river In practical application, when we have the actual monitoring data of rainfall of stations in the catchment, we can use these models for forecasting flood water levels in the downstream before a period of time Forecasting information can help local authorities timely warning to residents, in order to take the appropriate measures for preventing or relocating if necessary For the prevention of flooding in downstream areas of Tra Bong river effectively, requires predefine the flood water level along the river downstream for a certain time However, as mentioned above, Tra Bong river basin has only hydrological station – Chau O, so we primarily forecast flood for this hydrological station Time of prediction is long or short depending on the forecasting methodology, time of water convergence in the basin and flood transmission time on the river The most effective forecasting method for large flood management in the basin is short-term forecasting According to National Technical Standards on Flood Forecasting, short-term forecasting has maximum prediction time equal to time of water convergence in the entire basin (MONRE, 2008) This time in Tra Bong river due to monitoring data at hydrological stations is the following: 90 - Time of water convergence in the basin to Binh Minh is about hours; - Flood transmission time from Binh Minh to Chau O is about hours Thus, to forecast by hydraulic method of simulating flood from upstream to downstream station with good results, the anticipated duration should be only hours, if extended, the accuracy will much decrease To prolong further forecasts, it is necessary to accurately predict rainfall Because the accuracy of rainfall prediction has not been so high today, so the results of flood forecasting with predicted time longer than hours are usually not high, and used only for flood warning bulletins To ensure flood protection directing timely and effective, Tra Bong river floods forecasting usually before hour, flood warning capabilities before 1- days Based on the actual situation of rainfall – flood mechanism, available monitoring stations network, the application of MIKE-11 model in flood forecast for Tra Bong River Basin was conducted as follows: Collecting rainfall data at every station of Tra Bong River basin and the adjacent stations There are totally stations here include Chau O, Tra Bong and Tay Tra Appling hydrological rainfall - runoff model (Mike 11 NAM) with parameters calibrated and verified above to simulate and forecast flow process at Binh Minh Use the simulated flow data at Binh Minh as upstream boundary condition and use the water level data at Sa Can estuary as downstream boundary condition Calculate the flood flow from Binh Minh to Sa Can using hydrodynamic model MIKE-11 (HD) and forecast flood with the parameters that have been verified and calibrated Verified forecast results as follows: The floods are forecast verified include: - The flood from to Nov, 2011; - The flood from 25 to 29 Nov, 2011 Flood forecasting verification process is shown in Figure 4.28 to 4.31 91 Figure 4.27: Flood flow forecasting results from rainfall data at Binh Minh – The rains from to Nov, 2011 Figure 4.28: Observed and hour predicted flood water level process in Chau O station - from to Nov, 2011 92 Figure 4.29: Flood flow forecasting results from rainfall data at Binh Minh – The rains from 25 to 29 Nov, 2011 Figure 4.30: Observed and hour predicted flood water level process in Chau O station - from 25 to 29 Nov, 2011 93 The reliability of the plan is assessed base on the National Technical Standards on Flood Forecasting (MONRE, 2008) Specifically, in the flood data series in Chau O, the acceptable error (S cf ) is calculated using the following formula: S cf = 0.674*σ (4.2) Where: σ is the standard deviation of forecasting factor, calculated by the formula: n σ= ∑ (H i − H tb ) (4.3) n −1 H i is the peak flood water level of the flood i in the data series H tb is the average peak flood water level Apply to calculate error and reliability of forecasting plan in Chau O stations The results are shown in Table 4.16 Table 4.16: Results synthesis of flood forecasting of Tra Bong river in Chau O station Floods From to Nov, 2011 From 25 to 29 Nov, 2011 Predicted time is hours Total True The Median forecasting forecasting reliability absolute times times (%) error (m) Maximum absolute error(m) 90 85 94 0.06 0.53 65 55 85 0.10 0.54 The results of forecasting experiment for floods on Tra Bong river in 2011 showed that the application of MIKE-11 model in forecasting with predicted time of hours i.e not exceeding flood transmission time from upstream to downstream are pretty good, with the reliability (true/total forecasting times) is higher than 80%, the median absolute error is less than 0.2m However, in some cases when the flood process significant changes and continuously variable, it will have significant error (> 0.5m) 94 With the complexity of Tra Bong river flood regime, the forecasting results by the model MIKE-11 as above is pretty good Compared to the current forecasting methods, MIKE-11 model application method to forecast flood for downstream with predicted time less than hours has a higher reliability In particular, when forecasting flood by the model MIKE-11, the predicted time is longer, and it is so important for flood prevention in the locality 3.6 Propose solutions of large flood managing in Tra Bong river basin Flooding is a natural process, flooding itself not only causes negative impact on the environment, economy and human society but also bring a lot of resources In other words, flooding on the river basin includes two aspects: pros and cons Large flood management on the basin is reducing the damages caused by floods and maintain, promote its benefits Large flood management for a basin includes structure and nonstructure methods, presented in the Figure xxx 95 Figure 5.1: Synthesis of large flood management solutions in Tra Bong river basin 3.6.1 Structure methods Structure methods for large flood management in Tra Bong river basin include: • Reservoir construction and management: The construction of the reservoir causes a large impact on the environment, climate, river ecosystems and the socio-economic development of the entire region Besides flood control works, reservoirs can serve other purposes of economic development such as hydropower, irrigation, 96 aquaculture So the construction of reservoir should have a thorough research of scientists and policy makers • Flood diversion and delay areas: Similar to the reservoir, the flood diversion and delay areas also greatly affect the natural conditions and economic development of the region In addition, recent studies show that the flood diversion and delay areas usually occupies a very large area, hindering the socio-economic development in that area, while the effectiveness of this method seem to be less than the other new methods So the flood diversion and delay area method is hardly mentioned in large flood management this time • River dykes and embankments: Based on the flood forecasting results and flood flow simulating by Mike 11 models, we can determine the positions where the difference between top elevation of dykes and highest elevation of flood water are smaller than safety difference So the authorities should have plan for reinforcing the dykes The embankment is an effective solution to stabilize riverbanks, avoid landslides caused by large flooding • Building a system of drainage culvert across the roads and highway to enhance the effectiveness of flood drainage The reason is that in the process of socio-economic development, the systems of roads and highway have been constructed, the roads base have been heighten created a dyke prevents natural flood drainage Generally, Tra Bong river basin is characterized by small, short and steep The floods come and rise quickly The use of structural measures in the management of major floods in this basin will therefore lower effective than non-structural measures 3.6.2 Non-structure methods Non-structural methods for large flood management include methods suitable for all stages of the disaster management cycle Disaster Management Cycle illustrates the ongoing process by which governments, businesses, and civil society plan for and reduce the impact of disasters, react during and immediately following a disaster, and take steps to recover after a disaster has occurred The cycle includes four phase: mitigation, preparation, response and recovery (Warfield, 2008) 97 Figure 5.2: Disaster Management Cycle (Dunnell, 2016) • Mitigation phase: In this phase, the methods of large flood management aim at mitigating the damage in the event of a flood The suitable methods of this phase can apply for Tra Bong River Basin include: - Develop institutions and policies on large flood management specifically and in accordance with regional; - Based on flood inundation maps of Tra Bong river downstream, planning land use, relocation of residential areas located in areas of high flood risk; - Enhancing public education in prevention and mitigation of damage caused by large flooding Strengthening preventive health care capacity in the region • Preparation phase: The goal of emergency preparedness programs is to achieve a satisfactory level of readiness to respond to any emergency situation through programs that strengthen the technical and managerial capacity of governments, organizations, and communities These measures can be described as logistical readiness to deal with disasters and can be enhanced by having response mechanisms and procedures, rehearsals, developing long-term and short-term 98 strategies, public education and building early warning systems (Warfield, 2008) The methods include: - Develop plans to save lives, minimize disaster damage, and enhance disaster response operations; - Build flood warning marker corresponding with alarm levels to provide visual information for people to timely prevention; - Be sure that food, equipment, water, medicines and other essentials are maintained and stored in safety places - Develop the lessons and organize training for people to be ready to cope with the flood and avoid the damage caused by floods • Response phase: The aim of emergency response is to provide immediate assistance to maintain life, improve health and support the morale of the affected population Such assistance may range from providing specific but limited aid, such as assisting refugees with transport, temporary shelter, and food, to establishing semi-permanent settlement in camps and other locations (Warfield, 2008) There are some methods can applied for this phase: - Propaganda, guiding people to actively implement the four on-the-spot motto (Government, 2006) to respond to the flood: leadership on-spot; human resources on-spot; means on-spot; and logistics on-spot - Develop plans for flood response, include: relief and rescue, emergency evacuation, provision of food, fresh water and supplies, emergency repair and construct damaged key constructions • Recovery phase: In the recovery phase, the local authorities and affected people need doing number of activities aimed at restoring their lives and the infrastructure that supports them The solutions for this phase are: - Reorganization of public activity was interrupted by the flooding, such as medical, education and public information to help the community back to normal life as soon as possible 99 - Gather and report the damages and the characteristics of the flood just occurred to assess the extent of the floods and provide the data for flood warning, forecasting for the region in the future 100 CHAPTER IV: CONCLUSION AND RECOMMENDATION 4.1 Conclusion With the mentioned objectives, the study has obtained some achievement: - Gave an overview of concepts and methodologies in large flood management via studies which have been done in the world Therefrom, study selected the approach of flood inundation mapping and simulating flood flow in the downstream for forecasting experiment for the Tra Bong river basin - Evaluated rain and floods features, the situation of the flood impact on economic welfare from 2006 to 2011 Study have evaluated the flood regime in recent years, especially the historic flood of 2009, the changes compared to the general rules, in order to support the flood management, monitoring, alerting and forecasting - 2009 flood inundation map of Tra Bong river downstream Flood map was developed by actual survey data associated with ArcGIS software, to ensure the accuracy This is an important tool in flood management and prevention, socio-economic development planning for downstream in a sustainable way - Applied hydrological rainfall-runoff model Mike NAM to simulate the flow forming in the upstream of Tra Bong river The result was discharge data series at Binh Minh which was calibrated and verified with observed data and used as upstream boundary for simulating flood flow in the downstream - Applied hydraulic model Mike 11 to simulate flood flow in the downstream of Tra Bong river basin The calculation used 32 sections within a range of 23.65 km Sa Can estuary back to upstream The result after calibrating and verifying closely match the observed data - Flood forecasting experiment for Tra Bong river system The reliability of forecasting results is relatively high with predicted time was longer This can be used for flood forecasting for the downstream of Tra Bong river basin 101 - Proposed potential solutions include structure and non-structure measures on large flood management of Tra Bong river basin The solutions were suitable for all the phases of Disaster Management Cycle 4.2 Recommendation Flooding in the downstream areas of Quang Ngai province in general and Tra Bong river in particular is strongly affected by the tides and storm surges However, at present there is not any measuring station of these factors in the coastal area of Quang Ngai This is one of the difficulties exist for forecasting floods It is necessary to study in accurate forecasting the tide in the estuary and storm surges to additional support materials for the on-sea disaster warning, coastal areas flood forecasting with higher accuracy On the other hand, the exact tide forecasts at the river estuary and the key economic sectors will be very helpful for the socio-economic activities along the coast Flood flow simulation part of the study using Mike 11 model obtained good results But the research has not developed the flood inundation maps for every historic flood and the designing frequency of rainfall In a further study, with the better condition of researching the model and GIS, the flood maps would be created, providing visual flood forecasting and warning for the users The flood maps of the study were only inundation maps To evaluate the capacity of flooding and the potential damages for a certain area, we need flood risk maps Besides the flood hazard maps, the very important factors to create flood risk maps are the vulnerability of the area The definition of vulnerability is very complicated, require the socio-economic data of all elements in the area, and fully analysing with specific methods This requires a lot of time and effort to collect data, study but it is a right way to develop future research 102 REFERENCES Büchele, B., Kreibich, H., Kron, a., Thieken, a., Ihringer, J., Oberle, P., … Nestmann, F (2006) Flood-risk mapping: contributions towards an enhanced assessment of extreme events and associated risks Natural Hazards and Earth System Science, 6(4), 485–503 doi:10.5194/nhess-6-485-2006 Chien, P Van, & Thiem, N H (2012) Research for correcting flood risk maps and flood warning mark following the new flood alarming levels in order to prevent and 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World Meteorological Organization 104 ... of floods are 20 % and 10% Figure 1: Administration map of Quang Ngai Province Tra Bong river is one of the biggest rivers of Quang Ngai province including Tra Bong, Tra Khuc, Ve and Tra Cau Tra. .. features of Tra Bong river and major tributaries Average Name of rivers or Length of Length of Area of tributaries rivers (km) basins (km) basins (km2) Tra Bong 45 56 697 12. 4 Sa Thin 12 50 6 .2 Tra. .. of Tra Bong river basin, flood season and the typical flood • Develop flood inundation mapping for Tra Bong river basin based on flood mark’s distribution and characteristics of the major flood