Study on flood risk assessment in downstream area of ke go reservoir, Ha tinh province

or: Asso. Prof. Dr. Pham Thi Huong Lan This research is done for a partial fulfilment of the requirement for Master of Science Degree at Thuyloi University This Master Programme is supported by NICHE – VNM 106 Project Hanoi, 2015 MINISTRY OF EDUCATION AND TRAINING MINISTRY OF AGRICULTURE AND RURAL DEVELOPMENT Abstract Flooding causes economic, social and environmental damages and life loss. This fact increases the great attention on flooding by government, and science in many countries around the world. As a country located in the tropical climate region, Vietnam has been facing various water related disasters since ancient time, particularly in central parts of Vietnam where featured by steep topography. In recent years, Rao Cai river basin in Ha Tinh province is frequently flooded due to climate change impact, rapid infrastructure and urbanization growth. This problem caused serious damages to human life, properties, and social – economic development activities Flood risk management is a new concept based on a proactive approach which recently becomes a robust tool for reducing flood damage. Main contents of flood risk management are flood risk assessment and flood preventive measures or flood preventive planning. Flood risk assessment is key part in flood risk management. Flood risk assessment is a function of three main variables: flood hazards, vulnerability, and coping capacity. Understanding of flood hazards, vulnerability, and coping capacity is the vital step for efficiency of flood risk assessment. Flood risk management strategies have not been developed for Rao Cai river basin for many years and there is no spatial planning approach for regional development. This research aims at flood risk assessment for Rao Cai river basin based on the new concept of flood risk management mentioned above. An incorporated hydrological modeling approach for hazard assessment for Rao Cai river basin has been adopted in this research. The research objective divides into three parts: (1) Identification of flooding and potential reasons based on available natural, social and economic data; (2) The second part involved flood simulation and inundation mapping of events with chosen return periods using a MIKE package model (MIKE UHM, MIKE 11, and MIKE 11 GIS).The model was calibrated and verified based on the data series in October, 2010. A flood from 2 nd to 6 th , October 2010 was used to calibrate the model. Another flood in October, 2010 (from 14 th to 19 th , October) was used to verify the model. Results of calibration and verification were fit to measured data. The flood simulations for selected return periods were generated for 200 and 1000 years corresponding to frequency of design and checking flood of Ke Go reservoir. (3) Flood risk assessment is combined effect of flood depth (hazard factor) and population density (vulnerability factor) by weighing factors for both of them. As for the results, the research revealed that flood risk assessment is helpful tool for flood risk management. Flood risk maps were produced for the flood of 1000 year and 200 year return period. The level of hazard and risk were determined for each community in Cam Xuyen, Thach Ha and Ha Tinh city. These maps can be used for flood risk management and mitigation planning for Ha Tinh province in general, Rao Cai river basin in particular. Declaration I hereby certify that the work which is being presented in this thesis entitled, “Study on flood risk assessment in downstream area in Ke Go reservoir, Ha Tinh province” in partial fulfillment of the requirement for the award of the Master of Science in Integrated Water Resource Management, is an authentic record of my own work carried out under supervision of Asso. Prof. Dr. Pham Thi Huong Lan. The matter embodied in this thesis has not been submitted by me for the award of any other degree or diploma. Date: February 15, 2015 Tran Ngoc Huan Acknowledgements I would like to express my sincere gratitude to my advisor Asso. Prof. Dr Pham Thi Huong Lan for her guidance, suggestion and inspiration. I would also like to acknowledge Dr. Vu Thanh Tu, Mr Duong Hai Thuan and Dr. Bui Du Duong for their comments and suggestion. I would like to thank the Hanoi University for Natural Resources and Environment, Ministry of Natural Resources and Environment, Vietnam and NICHE – VNM 106 Project for the award of a scholarship and also Hanoi Water Resources University for giving me the opportunity for this special study. I also wish to thank members of the master thesis committee consist of Prof.Dr. Nguyen Quan Kim (chairman), Asso.Prof.Dr. Mai Van Cong (examination), Asso.Prof.Dr. Nguyen Mai Dang (examination), Dr Le Viet Son and Dr Dinh Thanh Mung for their comments, examination, and corrections. Finally, I would like to express my special appreciation to my friends and colleagues for their supports, encourages and advices. The deepest thanks are expressed to my family members for their unconditional loves. i TABLE OF CONTENTS CHAPTER 1 - INTRODUCTION ........................................................................... 1 1.1. Problem statement ............................................................................................ 1 1.2. Research objectives .......................................................................................... 3 1.3. Scope of study................................................................................................... 3 1.4. Structure of thesis ............................................................................................. 3 CHAPTER 2: LITERATURE REVIEW ................................................................ 5 2.1. Concepts of flood risk, hazard and vulnerability .............................................. 5 2.2. Flood risk assessment ....................................................................................... 7 2.3. Previous studies in study area ........................................................................... 9 CHAPTER 3: DESCRIPTION OF STUDY AREA............................................. 10 3.1. Physical characteristics ................................................................................... 10 3.1.1. Location of this basin ............................................................................... 10 3.1.2. Topography conditions ............................................................................. 11 3.1.3. Hydro-meteorological characteristics ...................................................... 12 3.2. Social and economic characteristics ............................................................... 16 3.2.1. Population ................................................................................................. 16 3.2.1. Rural area ................................................................................................. 17 3.3. Reservoir and current irrigation system.......................................................... 18 3.3.1. Overview of Ke Go reservoir ................................................................... 18 3.3.2. Irrigation system ....................................................................................... 21 3.4. Flooding situation in downstream area ........................................................... 22 3.4.1. The flooding events occurred in 2010 ...................................................... 22 3.4.2. The flooding events occurred in 2012 ...................................................... 25 3.4.3. The flooding events occurred in 2013 ...................................................... 26 CHAPTER 4: METHODOLOGY AND DATA USED ....................................... 28 4.1. General framework ......................................................................................... 28 4.1.1. Methods to flood risk assessment............................................................. 29 4.1.2. Method for estimating design hyetograph ................................................ 30 ii 4.1.3. Method for developing design hydrograph on lateral flow in downstream ............................................................................................................................ 31 4.1.4. Method for simulation floods corresponding to various return period .... 31 4.1.5. Method for inundation mapping ............................................................... 31 4.2. Governing equation in MIKE package ........................................................... 31 4.2.1. Rainfall runoff model (MIKE - Unit hydrograph model) ........................ 32 4.2.2. Hydrodynamic model (MIKE 11 HD) ..................................................... 33 4.2.3. Identification of inundation maps ............................................................ 34 4.3. Data used ........................................................................................................ 35 4.3.1. Data collection .......................................................................................... 35 4.3.2. Data analysis............................................................................................. 36 CHAPTER 5: RESULTS AND DISSCUSIONS .................................................. 40 5.1. The reasons cause the flooding in downstream area ...................................... 40 5.1.1. Climate change impacts............................................................................ 40 5.1.2. Infrastructure impacts ............................................................................... 43 5.2. Flood hazard ................................................................................................... 43 5.2.1. Rainfall runoff modeling .......................................................................... 43 5.2.2. Flood modeling......................................................................................... 50 5.2.3. Flood hazard maps.................................................................................... 58 5.3. Flood vulnerability.......................................................................................... 62 5.4. Flood risk in downstream area of the Ke Go reservoir................................... 65 CHAPTER 6: CONCLUSIONS AND RECOMMENDATIONS ................... 70 6.1. Conclusions..................................................................................................... 70 6.2. Recommendations........................................................................................... 71 REFERENCES ........................................................................................................ 73 APPENDIX .............................................................................................................. 76 Appendix 1: Frequency curve of maximum rainfall during 1 day of stations....... 77 Appendix 2: Roughness coefficient ....................................................................... 81 iii LIST OF TABLES Table 3- 1 Lists of meteorological stations ............................................................... 12 Table 3- 2: The average of monthly rainfall at Ha Tinh station from 1975 - 2010 .. 14 Table 3- 3: Monthly discharge of Ke Go reservoir from 1957 - 2010 ...................... 15 Table 3- 4: Population pattern ................................................................................... 16 Table 3- 5: Land use .................................................................................................. 17 Table 3- 6: Technical parameters of reservoir .......................................................... 19 Table 3- 7: Parameters of junction work items ......................................................... 19 Table 3- 8: Technical parameters of Irrigation channels system .............................. 21 Table 3- 9: Statistic of damages caused by rainfall and flood at Ha Tinh city, Thach Ha and Cam Xuyen district occurred from 14 October to 19, October, 2010 .......... 24 Table 4- 1 Database used for research ...................................................................... 36 Table 5 - 1: Result of frequency analysis of maximum daily rainfall ...................... 44 Table 5 - 2: Value of design rainfall distribution of Ha Tinh, Ky Anh and Huong Khe stations during 1 day corresponding to difference frequency (daily rainfall) ... 44 Table 5 - 3: Sub-catchment of Rao Cai river basin and weighting factors of meteorological station ............................................................................................... 46 Table 5 - 4: Parameters of UHM in MIKE RR model of Ke Go catchment............. 47 Table 5 - 5: Different in peaks of observed and simulated discharge for calibration mode at Ke Go reservoir ........................................................................................... 48 Table 5 - 6: Different in peaks of observed and simulated discharge in verification at Ke Go reservoir ..................................................................................................... 48 Table 5 - 7: Parameters of UHM - SCS for Rao Cai’s sub-catchments .................... 50 Table 5 - 8. Runoff link of sub-catchments into river network in MIKE 11 model . 53 Table 5 - 9: Monitoring points for the calibrating and verifying hydraulic model ... 53 Table 5 - 10: Results of flood simulation form 2 nd Oct to 6 th Oct, 2010 for calibration of MIKE 11 HD model ........................................................................... 55 Table 5 - 11: Results of flood simulation from 12 Oct to 18 Oct- 2010 ................... 57 Table 5 - 12: Maximum water level corresponding to design and checking flood of Ke Go reservoir ......................................................................................................... 57 Table 5 - 13. Designed flooding hazard level scale for the downstream of the Ke Go reservoir ..................................................................................................................... 58 Table 5 - 14. Flood hazard areas for flood event in 2010 ......................................... 60 iv Table 5 - 15. Flood hazard areas corresponding to design and checking flood ........ 62 Table 5 - 16. Criteria of vulnerability map derived from population density for the downstream of the Ke Go reservoir .......................................................................... 63 Table 5 - 17. Criteria of vulnerability map derived from population density for the downstream of the Ke Go reservoir .......................................................................... 65 Table 5 - 18. Flood risk map for the downstream area of Ke Go catchment in flood in October, 2010 ........................................................................................................ 65 Table 5 - 19. Statistic table of flood risk of different floods ..................................... 68 v LIST OF FIGURES Figure 3- 1: Map of study area .................................................................................. 10 Figure 3- 2: Topography of Rao Cai river basin in ASTER global DEM ................ 11 Figure 3- 3: Hydro-meteorological station network of Rao Cai river basin ............. 13 Figure 3- 4: The average of monthly rainfall at Ha Tinh station from 1975 - 2010 . 14 Figure 3- 5: Monthly discharge flow into Ke Go reservoir ...................................... 16 Figure 3- 6: The spillway of Ke Go Reservoir, there are two arc gates ................... 19 Figure 3- 7: The Emergency spillway of Ke Go reservoir ........................................ 19 Figure 3- 8: Flood at Ha Tinh city in October, 2010 ................................................ 23 Figure 3- 9: Percentage of damages in terms of money for various categories occurred from 14 th to 19 th , October, 2010 ................................................................. 25 Figure 3- 10: Flood at Ha Tinh city in October, 2012 .............................................. 26 Figure 3- 11: Inundation in the downstream of Ke Go reservoir in June 2 nd 2013... 27 Figure 3- 12: Flood in Ha Tinh city in October, 2013 .............................................. 27 Figure 4 - 1: Illustration of the research methodology.............................................. 29 Figure 4 - 2: Mass flow rate in and out of an elementary control volume ................ 34 Figure 4 - 3: MIKE 11 GIS Input and Output ........................................................... 35 Figure 4 - 4: Inflow of Ke Go reservoir in October, 2010 ........................................ 37 Figure 4 - 5: Inflow of Ke Go reservoir in October, 2013 ........................................ 37 Figure 4 - 6: Water level at Phu and Hoi Bridge station in October, 2010 ............... 38 Figure 5 - 1: Annual rainfall change on the Rao Cai river basin from 1975 – 2005 41 Figure 5 - 2: Changing of maximum rainfall of Ha Tinh station .............................. 42 Figure 5 - 3: Inflow of Ke Go reservoir and actual rainfall 2013 ............................. 43 Figure 5 - 4: Rao Cai watershed sub-basin schematizations and Thiessen polygon weighting computation of mean rainfall of sub-catchment in Rao Cai river basin .. 45 Figure 5 - 5: Parameters of UHM in MIKE RR model of Ke Go catchment ........... 47 Figure 5 - 6: Observation and simulation of hourly discharge of Ke Go reservoir from 2 Oct to 6 Oct – 2010 for calibration model .................................................... 47 Figure 5 - 7: Observed and simulated hourly discharge of Ke Go reservoir from 14 Oct to 19 Oct – 2010 – Verification .......................................................................... 48 Figure 5 - 8: Flood at Ke Go reservoir in 16 October 2013 ...................................... 49 Figure 5 - 9: Hydraulic calculation network in downstream of Ke Go reservoir ..... 51 vi Figure 5 - 10: Storage capacity of floodplain in downstream ................................... 52 Figure 5 - 11: Calculated and measured water level at Cau Phu (2 nd to 6 th October, 2010).......................................................................................................................... 54 Figure 5 - 12: Calculated and measured water level at Cau Ho (2 nd to 6 th October, 2010).......................................................................................................................... 55 Figure 5 - 13: Calculated water level and measured water level at Cau Phu(12 Oct to 18 Oct- 2010) ............................................................................................................ 56 Figure 5 - 14: Calculated water level and measured water level at Hoi Bridge (12 Oct to 18 Oct- 2010) ................................................................................................. 56 Figure 5 - 15. Some typical picture to determine flood hazard threshold................. 60 Figure 5 - 17. Flood hazard map of 0.5% design flood event ................................... 61 Figure 5 - 18. Flood hazard map of 0.1% design flood event ................................... 62 Figure 5 - 19: Frequency distribution of population of study area ........................... 63 Figure 5 - 20: Frequency distribution of population density of study area ............... 63 Figure 5 - 21: Vulnerability map in Rao Cai river basin .......................................... 64 Figure 5 - 22. Designed risk level for the downstream of the Ke Go reservoir ........ 65 Figure 5 - 23. Flood risk map for the downstream area of Ke Go river basin of flood in October, 2010 ........................................................................................................ 66 Figure 5 - 24. Flood risk map for 0.5% design flood ................................................ 67 Figure 5 - 25. Flood risk map for 0.1% checking flood ............................................ 68 vii LIST OF ARCONYM HCFSCS Ha Tinh Committee for Flood and Strom prevention and Control and Search and Rescue CCFSC Central Committee for Flood and Storm Control UHM Unit Hydrograph Model RR Rainfall Runoff HD Hydraulic Dynamic SCS Soil Conservation service VHDIC Vietnam Hydro-meteorological Data and Information Center DEM Digital elevation model 1 CHAPTER 1 - INTRODUCTION 1.1. Problem statement In recent years, the situation of flooding and tropical becomes more and more severe, especially in Vietnam’s Central provinces. With the rain increasing quickly both in quantity and intensity, many large floods as well as deforestation in the upstream appears. Besides, there is also the impact of the socioeconomic development, such as the process of rapid urbanization, infrastructure construction (roads, channel systems), which are factors hindering the flow of water and increasing damage caused by floods. Unsafe reservoirs contain a high risk. According to the Steering Committee for Flood and Storm Control Central, in 2013, floods and typhoons have caused 264 deaths and 800 injured people, about 12,000 collapsed and damaged houses, and the loss of more than 300,000 ha of rice, 2 broken irrigation dams, etc. The estimated total material damage amounted to approximately 25,000 billion dongs (2013), 16.000 billion dongs (2012) and 12.000 billion dongs (2011) (Hoai, 2013). It is undeniable that the effects of climate change have a significant impact on the weather in recent years and cause significant damage both to people and property. The Ke Go reservoir, located on the Rao Cai river in Cam My commune, in the Cam Xuyen district of the Ha Tinh province, about 20 km from Ha Tinh city to the West, is selected as a case study. The reservoir is located on one of the larger rivers of Ha Tinh province: the catchment area to the Ke Go hydrological station is 230 km 2 with the total length is 27 km. The Rao Cai area to estuary is 892 km 2 , including the whole Cam Xuyen district, Ha Tinh city and a part of Thach Ha province. The Ke Go reservoir has the particularly important task to irrigate 20,896 ha of arable land of the two districts of Thach Ha and Cam Xuyen, to supply water for Ha Tinh city and for industry, combining power generation, fish growth and flood control for the downstream. This is the largest irrigation headworks system of the central Vietnam and is constructed for a long time. 2 The flood risk research and assessment has particularly important implications for the prevention and mitigation of natural disaster. Firstly, flood hazards as part of the management of flood risk can be understood as the probability that flood prone areas will be inundated for a given time period with a specific return period (Alkema, 2007). Flood modeling is a relatively new approach, which is used in many countries for flood hazard and risk assessment. Flood hazard and risk based spatial planning must be applied to flood prone areas (Pender, 2007). Measures of flood control aimed at lowering the vulnerability of people and their property, also include a list of means, i.e. river engineering works, such as dams, levees, embankments, and/or river training works, such as retention polders (Klijn, 2009). Traditionally, management on flood risk focuses on preventing floods by river training and dykes system. There are several disadvantages to this approach, such as dyke break caused by erosion or overtopping of the embankment. Nowadays alternative and more resilient management strategies are applied in many countries in the global (Bruijn, 2005). The Decision Support Systems (DSS) are supposed to be a robust tool for flood risk management; DSS is not only meant for experts, as it is a new trend to represent the final output of the experts’ research in way to meet the decision makers’ skills and requests (Klijn, 2009). However, for many countries DSS is unfeasible, due to the lack of data and techniques as well as experts, and the country of Vietnam is no exception. Actually, there is little research on flood risk assessment in Vietnam, for instance in the case of the Ke Go Catchment there has been only one study that has focused on the effects of flood scenarios to downstream areas without any detailed assessment information about the level of risk that can cause for people to have the mitigation measures in place (Thai et al., 2011). The research of the topic ―Study on flood risk assessment in downstream area of Ke Go Reservoir – Ha Tinh province‖ will be a useful tool for decision-makers in view of spatial planning and future risk assessment for the region. 3 1.2. Research objectives General objective: Flood risk assessment in the downstream of the Ke Go reservoir, Ha Tinh province, to have measures in preventing and controlling damage due to floods for study area. The specific objective of this research can be determined as: - Analyzing potential reasons cause flooding in the study area to have accurate estimation and suggestion for this research and local authority. - Understanding the flood risk assessment method to choose a suitable method apply for the study area. - Generating flood risk maps of the downstream of the Ke Go reservoir based on hazard maps corresponding to flood scenarios and vulnerability maps to estimate risk level for each area. Based on local authority can determine where should emergency action being concentrated or having prepare plans and measures when flooding. 1.3. Scope of study This study focuses on considering population density and flooding depth to assess flood risk in the downstream area of the Ke Go reservoir. 1.4. Structure of thesis The thesis structure includes 6 chapters. The brief explanation of those chapters is as followed: Chapter 1 introduces problem statement of the research, the object of the research and scope of study. Chapter 2 reviews several studies about concept of flood risk, hazard and vulnerability, flood risk assessment methods and some previous researches relate to study area. 4 Chapter 3 reviews the physical characteristics as well as social and economic characteristics of the study area. The chapter also indicates the Ke Go reservoir and current irrigation system and flooding situation in recent years. Chapter 4, the general framework of this research will be mentioned including both methods and theory. Besides, the data collected during the research was summarized and analyzed. Chapter 5 shows results corresponds the research objective about potential reasons caused flooding and flood risk assessment based flood hazard and vulnerability factors on the maps. MIKE package model setup, calibration and validation are described. Chapter 6 focuses on the main findings and recommendations for further studies and local authority. 5 CHAPTER 2: LITERATURE REVIEW 2.1. Concepts of flood risk, hazard and vulnerability  Flood risk In the series of document ―Living with Risk‖, the International Strategy for Disaster Reduction (ISDR) of United Nation describes risk is ―the probability of harmful consequences, or expected losses, resulting from interactions between natural or human-induced hazards and vulnerable conditions‖ (UN, 2004). This definition emphasizes relevant vulnerabilities through that risk can be also defined as a function of hazard and vulnerability. Risk is defined: Risk = Hazard × Vulnerability Emphasis to risk retention, Asian Disaster Reduction Center (ADRC, 2005) mentioned flood risk as a function of probability of loss and loss: Risk = probability of loss × loss Focus on the resilience capacity of society against to risk, ADRC in the report of ―The role of local institutions in reducing vulnerability to recurrent natural disasters and in sustainable livelihoods development ‖developed a new term of risk generally, particularly flood which is illustrated in the function below: Risk = Hazard x vulnerability Capacity of societal system Generally, the term of flood risk is variable according to the purpose of particular research. In this research, flood risk is understood as being a function of a probability of a specific flood event and vulnerability of societal systems.  Flood hazard According to Baas.S, et al (2008) and United Nations (2004) hazard can be determined as ―potentially damaging physical event, phenomenon or human activity 6 that may cause the loss of life or injury, property damage, social and economic disruption or environmental degradation‖. Hazards have different origins: natural (geological, hydro-meteorological) or can be provoked by humane (environmental degradation and technological hazards). Each hazard is characterized by its location, frequency and probability of occurrence in a specific region within a specific time and magnitude. The investigation of assessment of hazard is associated to study of physical aspects and phenomenon of the given hazard through collection and analysis of historical records, this process is defined as assessment of hazard (Geohazards, 2009). Aspects of exposure and vulnerability are not considered in the hazard term, since it focuses on the event or physical situation (Tamar, 2010). Flood hazard is a function of: flood magnitude, depth of water and velocities, water rise rate, dur...tegorized as: Low, Medium, High and very high based on the range of value of hazard factor. Risk means the expected degree of loss or damage of a community due to the specific hazard. Risk factor is determined as: Risk factor (RF) = Hazard factor (HF) x Vulnerability factor (VF) The Asian disaster preparedness defined vulnerability as ―the degree to which an area, people, or physical structure or economic assets are exposed to loss, injury or damage caused by the impact of a hazard (Tu, 2009). Normally, population, economics activities, public services, infrastructure are the element oat risk due to flooding. The vulnerability is also divided into four level (low, medium, high and very high). The risk zone is determined from risk factors are used to present the risk level; the risk level is categorized as: Low, medium, high and very high based on the range of risk factor value. Flood hazard and flood risk maps are developed for study is based in the results of hazard and risk factor. 4.1.2. Method for estimating design hyetograph Based on the rainfall data collected from meteorological station (Ha Tinh, Huong Khe and Ky Anh) in basin and surrounding area, using the probabilistic distribution, the total rainfall corresponding to various return periods such as: 200 years, 100 years for each station. They are frequency corresponding to design and checking flood of Ke Go reservoir. The method can be chosen for calculation. In this study, Person Type III distribution is used to analyses rainfall frequency. Choosing a measured rainfall time-series (hyetographs) which has total rainfall is similar to design rainfall. The design hyetograph is obtained by using the ratio: 31 Where: Rr is the rainfall ratio Xd is the total design rainfall Xa is the total actual rainfall as the same period of design rainfall. Then, Design hyetograph = Actual hyetograph x Ratio (Rr) 4.1.3. Method for developing design hydrograph on lateral flow in downstream This thesis used release of Ke Go reservoir corresponding with design and checking flood frequency of Ke Go reservoir from ―Establishing Ke Go reservoir operation rule, 2012‖ project. The unit hydrograph model (UHM) in MIKE will be used to calculate lateral flow in downstream with different scenarios. This model is often used for small catchments, and the selected rainfall for analysis is short to ensure rainfall intensity does not change over spatial and temporal dimensions (Chow, 1964). 4.1.4. Method for simulation floods corresponding to various return period To calculate the flood scenarios in the river network, MIKE 11, the hydraulic model, is calibrated and verified by using collected data in the study area. The module is fully integrated with the 1DFLOW module for accurate flooding simulation (upstream, lateral inflow, downstream flow). The hydrodynamic simulation engine underneath is based upon the complete Saint Venant - Equations. It can simulate steep fronts, wetting and drying processes and sub critical and supercritical flow. 4.1.5. Method for inundation mapping The results of maximum water level at nodes are along the rivers obtained from output of MIKE 11 and digital elevation map (DEM) with cell size 30 m x 30 m for land elevation was built based on topography of downstream in scales 1:10,000) are used for illustration the inundation maps by MIKE 11 GIS. 4.2. Governing equation in MIKE package 32 4.2.1. Rainfall runoff model (MIKE - Unit hydrograph model) Rainfall runoff can be input in the MIKE 11 HD module as a point lateral discharge to a channel network. SCS method of Unit hydrograph model was used to calculate rainfall runoff for sub basin. The runoff curve number (also called a curve number or simply CN) is an empirical parameter used in hydrology for predicting direct runoff or infiltration form rainfall excess (USDA, 1986). The curve number method was developed by the USDA Natural Resources Conservation Service, which was formerly called the Soil Conservation Service or SCS — the number is still popularly known as a "SCS runoff curve number" in the literature. The runoff curve number was developed from an empirical analysis of runoff from small catchments and hill slope plots monitored by the USDA. It is widely used and is an efficient method for determining the approximate amount of direct runoff from a rainfall event in a particular area. The runoff curve number is based on the area's hydrologic soil group, land use, treatment and hydrologic condition. References, such as from USDA indicate the runoff curve numbers for characteristic land cover descriptions and a hydrologic soil group. The runoff equation is: { Where: Q is runoff ([L]; in) P is rainfall ([L]; in) S is the potential maximum soil moisture retention after runoff begins ([L]; in) Ia is the initial abstraction ([L]; in), or the amount of water before runoff, such 33 as infiltration, or rainfall interception by vegetation; historically, it has generally been assumed that Ia = 0.2S, although more recent research has found that Ia =0.05S may be a more appropriate and accurate relationship (Hawkins, R.H; et al, 2002). The runoff curve number CN, is then related CN has a range from 30 to 100; lower numbers indicate low runoff potential while larger numbers are for increasing runoff potential. The lower the curve number, the more permeable the soil is. As can be seen in the curve number equation, runoff cannot begin until the initial abstraction has been met. 4.2.2. Hydrodynamic model (MIKE 11 HD) The model provides a choice between 3 different flow descriptions: dynamic wave approach, diffusive approach, and kinematic ware approach. In the research, the author used dynamic wave approach which solved the vertically integrated equations of conservation of continuity and momentum (the Saint-Venant equations). These equations were used because the water surface slope, bed slope and bed resistance forces of rivers in Rao Cai catchment are small. The MIKE 11 HD is built based on the following assumptions - The water is incompressible and homogeneous, ie. Negligible variation in density; - The bottom –slope is small, thus the cosine of the angle it makes with the horizontal may be taken as 1, - The wave lengths are large compared to the water depth. The ensure that the flow everywhere can be regarded as having a direction parallel to the bottom, ie vertical accelerations can be neglected and a hydrostatic pressure variation along the vertical can be assumed; - The flow is subcritical. 34 MIKE 11 is based on the one dimensional solution of the Saint Venant Equations. Figure 4 - 2: Mass flow rate in and out of an elementary control volume Continuously equation Momentum equation ( ) | | Where: Q = discharge, (m 3 /s) A = flow area, (m 2 ) q = lateral inflow, (m 2 /s) h = stage above datum (m) C = Chezy resistance coefficient, (m 0.5 /s) R = hydraulic or resistance radius, (m) α = momentum distribution coefficient. Input data 1. River network 2. Cross – section 3. Hydrological data: discharge (Q) or water depth (h) time series at the inflow boundary of the model and a Q (h) relation at the outflow boundary of the model. 4.2.3. Identification of inundation maps There are many methods to carry out flood risk assessment based on different 35 applied modeling. In this research, the author selected MIKE 11 GIS is a tool for the spatial presentation and analysis of one-dimensional (1D) flood model results for use in the flood risk assessment process. The MIKE 11 GIS system integrated the MIKE 11 with the spatial analysis capabilities of the ArcView Geographic Information System (GIS) gives us an advanced tool to assess flood risk. The outputs were developed from using MIKE 11 GIS are important inputs for a range of floodplain management undertakings including flood risk assessment, flood control, flood forecasting, floodplain preservation and restoration, etc. Figure 4 - 3: MIKE 11 GIS Input and Output The software requirements for developing inundation maps based on using MIKE 11GIS include MIKE 11 HD and ArcView. Alternately, in the research, some steps in building up flood risk maps, ArcGIS is evolved to visualize more clearly the pictures of flood. 4.3. Data used 4.3.1. Data collection To reach goals listed in the introduction part of this research the different type of dataset must be used: topographic data, hydro-meteorological information, profiles of the riverbed, historical information on the past flood events, spatial information on infrastructure, population. (Table 4 - 1) 36 Table 4- 1 Database used for research No Data Type Date/ Measure Sources 1. Rainfall - Daily rainfall (Ha Tinh, Ky Anh and Huong Khe station) 1980 - 2010 VHDIC - Hourly rainfall (Ha Tinh, Ky Anh and Huong Khe station) Oct - 2010 VHDIC - Hourly rainfall (Ke Go reservoir) Oct - 2010 Ke Go irrigation company 2. Evaporation - Daily evaporation (Ha Tinh, Ky Anh and Huong Khe station) 2010 VHDIC 3. Discharge - Hourly discharge (Release discharge of Ke Go reservoir) - 04-11 Oct -2010 ( Flood 1) - 14 - 20, Oct - 2010 (Flood 2) - Oct 13 th 2013 Ke Go irrigation company - Hourly discharge flow into Ke Go reservoir - Oct 13 th 2013 Ke Go irrigation company 4. Water level - Hourly water level ( 11 stations in downstream area) - 04-11 Oct -2010 ( Flood 1) -14 - 20, Oct - 2010 (Flood 2) Ke Go irrigation company - Hourly water level of Ke Go reservoir - Oct -2010 Ke Go irrigation company 5 Topographic maps CAD file, 1:10 000 scale 2010 Surveying and mapping department 6 Reports Hard copy 2011 VWRAP, HCFSCS 7 Satellite image Image. Jpg June 2 nd 2013 4.3.2. Data analysis 4.3.2.1. Hydro-meteorological data a. Runoff data The hourly discharge flow into Ke Go reservoir was calculated from water level and release of Ke Go reservoir based on the water balance equation (shown Appendix 1). The results on hourly discharge flow into Ke Go reservoir of flood event in 2010 and 2013 are shown in Figure 4 - 4 and Figure 4 - 5. The calculated results are used to setup rainfall runoff model (UHM) to determine model parameters for Ke Go catchment and sub-basin of downstream area. 37 Figure 4 - 4: Inflow of Ke Go reservoir in October, 2010 Figure 4 - 5: Inflow of Ke Go reservoir in October, 2013 b. Water level All of water level measuring stations in the study is Cam Nhuong, Do Ho, Thach Dong which are affected by the tide and are the station measuring water level in short period from 1970 to present. Besides, there are local stations belong Ke Go 0 500 1000 1500 2000 2500 3000 3500 4000 4500 10/2 10/4 10/6 10/8 10/10 10/12 10/14 10/16 10/18 10/20 10/22 D is ch ar ge (m 3/ s) Time Inflow of Ke Go reservoir in October, 2010 Inflow 0 50 100 150 200 250 300 350 4000 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 0 5 10 15 20 25 R ai nf al l (m m ) D is ch ar g e (m 3 /s ) Hour Measured rainfall and flood at Ke Go reservoir in Oct 16th 2013 Rainfall (mm) Discharge (m3/s) 38 irrigation company to control water level in downstream area when releasing flood of Ke Go reservoir. They are only measured when flood coming. This thesis collected water level data all of above station in October, 2010. The main stations are Phu and Hoi bridges are shown in Figure 4 - 6. They will be used to calibrate and verify numerical model in next part. Source: Ke Go Irrigation Company, 2010 Figure 4 - 6: Water level at Phu and Hoi Bridge station in October, 2010 4.3.2.2. Topography data A significant input for hydrodynamic modeling is the correct representation of terrain on which the model will work on. To develop DEM the following dataset were using: 1. Topographic maps of the study area in 1:10,000 scales (measured 2010). Projection is VN 2000 that was transformed to WGS 1984. 2. The total cross section data along the river of Rao Cai and Gia Hoi is 40 cross sections, in which Rao Cai has 25, Gia Hoi has 15. These cross sections were measured in 2011, which are collected from ―Emergency preparedness plans (EPP) 0 0.5 1 1.5 2 2.5 3 3.5 10/4/2010 0:00 10/8/2010 0:00 10/12/2010 0:00 10/16/2010 0:00 W a te r le v e l (m ) Time (hour) Measured water level at Cau Phu and Cau Hoi in October 2010 Phu Bridge Hoi Bridge 39 in emergency case of Ke Go Reservoir - Ha Tinh province‖ project of Thuyloi University. 40 CHAPTER 5: RESULTS AND DISSCUSIONS 5.1. The reasons cause the flooding in downstream area According to analysis of data about hydro-meteorology, statistic data analysis, flood and flooding occur more frequently in recently. In this part, thesis will research reasons caused flooding in downstream of Rao Cai river basin, especially in Ha Tinh city area. 5.1.1. Climate change impacts Along with the status of climate change (CC) on the global recently, Ha Tinh province also has effect of climate change lead to the great disaster suffered more major effect on the development of socio - economic, military and security. According to recent researches were done by the Ha Tinh Department of Natural Resources and Environment, the average temperature in the province increase from 0.1 – 0.2 0 C per decade, the average temperature period 2000-2010 increased more from 0.3 – 0.6 0 C than 10 - 30 years ago, especially, Huong Khe area increased from 0.7 - 1,4 0 C. Meanwhile, annual rainfall tends reduced greater with variation in terms of space, time and intensity. Although the rainfall reduces, the intensive of rainfall caused flooding or flash floods increases. Accordingly, the frequency and regularity of the storm is also changed, which will be described below. Normally, the rainy season in Ha Tinh province is from September to November and the storm season is from July to September. However, recently, the storm season was tends to change clearly, from August to December. (Hoai, 2009) Instability in rainfall would cause more severe floods in rainy season and droughts in dry season. • Increase in frequency and intensity of typhoons, storms would cause high floods & inundation, flash floods, landslide and erosion. • Increasing water shortage and growing water demand threaten water supply, water use conflicts. .To estimate climate change impact cause to flooding in Rai Cai river basin in recent year based on meteorological data in the study area and surrounding area from 1975 to 2005. Some annual rainfall changes are shown below: 41 Figure 5 - 1: Annual rainfall change on the Rao Cai river basin from 1975 – 2005 In the Figure 5 - 1, in general, the average annual rainfall change of Huong Khe and Ha Tinh have decrease trend. The decrease of annual rainfall of Ky Anh station is higher than Ha Tinh ones with 9.42 and 4.03 mm per year period from 1975 to 2005 respectively. This result is suitable with researches of DONRE of Ha Tinh province. However, rainfall intensive (mm/m, mm/day) have opposite trend. The results changing of 1 and 3 days maximum rainfalls are shown the following Figure 5 - 2. y = -4.0289x + 10713 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 1975 1980 1985 1990 1995 2000 2005 X (mm) Year Ha Tinh station y = -9.4216x + 21664 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 1975 1980 1985 1990 1995 2000 2005 X (mm) Year Ky Anh station 42 Figure 5 - 2: Changing of maximum rainfall of Ha Tinh station Based on rainfall of Ha Tinh meteorological station period from 1975 to 2005, daily maximum rainfall changes have stable trend or reduce but not much. The trend of daily maximum rainfall, which indicated various changes (including decrease and increase) in the first years from 1975 to 1995, ending the period with significantly increased. For the maximum rainfall during 3 days shows an increasing trend, this rainfall duration is main reason cause to flooding in the downstream area. To have an accurate estimate the data need be updated latest meteorological data. Interestingly, in recent year although daily maximum rainfall of Rao Cai area in 2010 (455.6mm) is higher than 5 time in 2013 (95mm) however peak flow of Ke Go reservoir are estimated as equal. Main reason is rainfall in 2013 occurred a few hours. In addition, maximum rainfall in 2010 and 2013 correspond to frequency 10% and 90%, however, the peak flow of Ke Go reservoir in both year are higher than design peak flow correspond about 0.5%. Rainfall occurred more frequently with higher density, especially in short duration. 0 100 200 300 400 500 600 700 1975 1980 1985 1990 1995 2000 2005 X (mm) Time Changing of maximum total rainfall in a day Rainfall Linear (Rainfall) 0 200 400 600 800 1000 1200 1975 1985 1995 2005 X (mm) Time Changing of maximum total rainfall in 3 days Rainfall Linear (Rainfall) 43 Figure 5 - 3: Inflow of Ke Go reservoir and actual rainfall 2013 These results revealed traditional method to calculate design flow for reservoir need to check again and consider hyetograph or climate change impacts to inflow. In conclusion, with climate change impact inflow into Ke Go reservoir and flow of sub-basin in downstream increase, this cause flooding when happening heavy rainfall. 5.1.2. Infrastructure impacts Further investigating and finding the main reasons cause this situation is due to the drainage system in the province of Ha Tinh was built long ago, lack of synchronized planning, many area seriously degraded, small drainage tunnel cannot get up when heavy rainfall occur during a long time. On other hand, in the process of urbanization, cities are focused on building many projects. Besides, the elevation of some area in HaTinh city is quite low, that might be flooded even when the small rainfall. The roads can be as a river dyke to keep flood in upstream of river, this make water level in upstream increased. This issue increase hazard for resident. 5.2. Flood hazard 5.2.1. Rainfall runoff modeling 5.2.1.1 Model setup Rainfall frequency analysis Determine statistical characteristics of frequency curve 0 100 200 300 4000 1000 2000 3000 4000 5000 0 5 10 15 20 25 X(mm) Q (m3/s) Hour Rainfall and flood at Ke Go reservoir in Oct 16th 2013 Rainfall (mm) Discharge (m3/s) 0 1000 2000 3000 4000 5000 6000 0 2 4 6 8 10 12 14 16 Q (m3/s) Hrs Design Hydrographs of Ke Go reservoir P = 0.5% P = 0.1% P = 0.01% 44 Based on the data of daily rainfall of the meteorological stations: Ha Tinh (1975- 2010), Ky Anh (1975- 2006) and Huong Khe (1975- 2006) in Rao Cai river basin and surrounding, using Pearson type III distribution, daily maximum rainfall corresponding to various return periods of 1000 and 200 years for each station are estimated (see Table 5-1). Table 5 - 1: Result of frequency analysis of maximum daily rainfall Station Avg. Rainfall (mm) Cv Cs Daily maximum rainfall corresponding to return periods (mm) 1000 years 200 years Ha Tinh 304.8 0.43 0.89 882.6 750.9 Ky Anh 292.6 0.36 0.60 711.6 623.6 Huong Khe 246.7 0.37 0.82 639.7 551.7 Frequency curve of maximum rainfall during 1 day and design rainfall hyetography of above these are indicated in Appendix 1 Determining design rainfalls The design rainfall of meteorological stations on Rao Cai river basin are estimated based on the ratio of actual daily rainfall to design daily rainfall responding to different frequencies and the actual time distribution of chosen days. Table 5 - 2: Value of design rainfall distribution of Ha Tinh, Ky Anh and Huong Khe stations during 1 day corresponding to difference frequency (daily rainfall) Stations Frequency Type Day Ratio 1st 2nd 3th H a T in h 0.10% Actual (15 - 17/10/2010) 132 610 147 1.5 Design 191 883 213 0.50% Actual (15 - 17/10/2010) 132 610 147 1.2 Design 163 751 182 H u o n g K h e 0.10% Actual (15 - 17/10/2010) 53 524 86 1.2 Design 65 640 105 45 Stations Frequency Type Day Ratio 1st 2nd 3th 0.50% Actual (15 - 17/10/2010) 53 524 86 1.1 Design 56 552 90 K y A n h 0.10% Actual (13 - 15/10/1984) 85 519 122 1.4 Design 117 712 167 0.50% Actual (13 - 15/10/1984) 85 519 122 1.2 Design 102 624 146 Definition of sub-basin of Rai Cai river basin The MIKE - UHM was used to calculate lateral inflow. In order to develop lateral inflow of sub-basins corresponding to rainfall frequencies, MIKE - UHM (SCS) model is used. The results of this model are input for MIKE 11 HD as point source in downstream area in river network. The sub-catchment delineations and their tributaries (i.e. areas, average surface elevation and slope) were initially calculated from the DEM using the spatial extension tools of ArcGIS software. The UHM model for sub-basin are considered land use, elevation, slopes, and drainage system, also included for schematization as shown in (Figure 5 - 4). Figure 5 - 4: Rao Cai watershed sub-basin schematizations and Thiessen polygon weighting computation of mean rainfall of sub-catchment in Rao Cai river basin Meteorology Station 46 The spatial distribution of rainfall for Ke Go reservoir catchment and sub-catchment in downstream area was calculated based on the Thiessen polygon concept. And the results have shown in Table 5 – 3. Table 5 - 3: Sub-catchment of Rao Cai river basin and weighting factors of meteorological station No Sub- catchment Area (km 2 ) Weighting factor of each station Description Ha Tinh Ky Anh Huong Khe 1. Sub 1 223 0.59 0.20 0.20 Ke Go. Reservoir 2. Sub 2 65.93 1 Downstream Ke Go Reservoir to junction of Ngan Mo 3. Sub 3 70.58 1 Ngan Mo to Cau Ho on Gia Hoi river 4. Sub 4 57.80 0.93 0.07 Quen Catchment 5. Sub 5 255.73 0.08 0.92 Rac Catchment 6. Sub 6 49.39 1 Ngan Mo to Cau Phuon Ngan Mo river 7. Sub 7 80.55 1 Bang river 8. Sub 8 39.66 1 Thach Dong estuary 9. Sub 9 58.55 1 Cam Cua Nhuong 5.2.1.2 Calibration and verification of MIKE_RR UHM model for Ke Go reservoir catchment Calibration The area of Ke Go reservoir catchment is 223 km 2 . In order to calibrate MIKE RR UHM for this catchment, the hourly rainfall and discharge at Ke Go reservoir from Oct 4 th to 11 th Oct 2010 were used to compare between measured and calculated data. The parameters and initial condition are determined (Figure 5 - 5) based on the criteria such as (Coefficient of efficiency (NASH) and volume error, peak time error). 47 Figure 5 - 5: Parameters of UHM in MIKE RR model of Ke Go catchment Table 5 - 4: Parameters of UHM in MIKE RR model of Ke Go catchment Parameters Base flow: BF Loss Curve number: CN Initial AMC: IAMC Time lag: TL Indicators m 3 /s - mm Hours Value 10.3 60 2 0.5 Figure 5 - 6: Observation and simulation of hourly discharge of Ke Go reservoir from 2 Oct to 6 Oct – 2010 for calibration model 48 Table 5 - 5: Different in peaks of observed and simulated discharge for calibration mode at Ke Go reservoir Calibration Q Max (m 3 /s) ΔQ (m 3 /s) Time (hr) Δt (hrs) R 2 Observed 2020 -140 (7%) 22 AM 4/10/2010 0 0.75 Simulated 1880 22 AM 4/10/2010 Verification The discharges of Ke Go reservoir from 14th to 19th of October, 2010 were used to verify parameters of model, which are determined in the calibration step. The calculated and simulated hydrograph at Ke Go reservoir during flood event are illustrated in Figure 5 - 7. The different of peaks of observed and simulated discharge in verification model at Ke Go reservoir is shown in Table 5 - 6. Figure 5 - 7: Observed and simulated hourly discharge of Ke Go reservoir from 14 Oct to 19 Oct – 2010 – Verification Table 5 - 6: Different in peaks of observed and simulated discharge in verification at Ke Go reservoir 49 Calibration Peak of discharge (m 3 /s) Different: ΔQ (m 3 /s) Time of peak hrs Different Δt (hrs) R 2 Observed 3980 190 (5%) 16 AM 16/10/2010 0 0.76 Simulated 4170 16 AM 16/10/2010 In calibration and verification model step, the calculated and measured data are fit well as shown Figure 5 – 6 and Figure 5 – 7. Following Table 5 - 5 and Table 5 - 6, the different of measured and calculated data in the peak flow is trivial, 7% for calibration model and 5% for verification model. The time of peak in the measurement and calculation are coincident. The coefficient of efficient (NASH) of the calibration and verification step is higher than 0.7. Therefore, the determined parameters are acceptable for Ke Go reservoir catchment and surrounding area to simulate discharge. In addition, discharge flow into Ke Go reservoir in October 16 th 2013 is run with the verified rainfall runoff model for and the results are shown below. Figure 5 - 8: Flood at Ke Go reservoir in 16 October 2013 In general, the results of the simulation on the Ke Go reservoir are quite good. Although some time the observed and calculated data are not fit and water balance 50 error is still high accounting for 28.1%. The results might be affected by local rainfall, hence the pre-process of input data for Rainfall runoff model is necessary. Determining parameters of Rainfall runoff model for sub-basin of Rao Cai river basin. In the downstream area of Ke Go reservoir and its tributaries, there is no hydrological station for discharge data. Therefore, the lateral inflow corresponding to actual flood, design hydrograph was calculated based on the rainfall data UHM model, which rainfall data and parameters obtain from typical sub-basin. In the study area, the Ke Go catchment has similar characteristics with the sub-basins in term of about soil condition, plant cover, land use, geology and meteorology. Thus, the parameters of UHM-SCS model for Ke Go catchment are used to determining parameter for sub-basins in Rai Cao River by adjustment. Besides, the land use condition of each sub-basin is considered. Table 5 - 7: Parameters of UHM - SCS for Rao Cai’s sub-catchments No Sub-catchment Area (km 2 ) Parameter (CN) BF (m 3 /s) CN IAMC (mm) TL (hrs) 1. Sub 2 65.93 3 74 2 1 2. Sub 3 70.58 3 74 3 1 3. Sub 4 57.80 5 74 2 1 4. Sub 5 255.73 10.2 74 3 0.5 5. Sub 6 49.39 3 91 2 1 6. Sub 7 80.55 3 74 2 1 7. Sub 8 39.66 3 74 2 1 8. Sub 9 58.55 3 74 2 1 The discharge of sub-basin is calculated by MIKE – UHM model, are connected to main river based on their location of sub-basins. 5.2.2. Flood modeling 5.2.2.1. Setup MIKE 11 HD model To simulate flooding by MIKE 11 HD model, the following data are required: river network including profiles (shape, roughness, structure, etc.); Digital elevation 51 model (DEM) to determine floodplain; Hydraulic boundary and initial conditions; Measured water level and inflow of sub-catchment. River network Hydraulic diagram is established based on the river network documents containing Gia Hoi, the Rao Cai river behind Ke Go dam. Figure 5 - 9: Hydraulic calculation network in downstream of Ke Go reservoir Topography Cross sections Whole network has 70 cross sections. Cross-sections are specified by a number of x-z coordinates where x is the transverse distance from a fixed point (often left bank top) and z is the corresponding bed elevation. All of cross sections were obtained from Institute of Civil Engineering of Thuy Loi University which measured data in 2011. The missing data were interpolated from measured value river bed, DEM and topography in 10,000 scales. Floodplain Land elevation in the downstream range from 2.5 to 10 m, so when big flood and heavy rainfall, the river can’t stand for big discharge. This issues cause overland along the river. In floodplain during flood event, 1D model is not adequate to 52 describe the spatial variable of overland, water depth, velocity, etc. However, to solve these limitations of the MIKE 11 model, the floodplain via structure or sub- branch can be setup. In this study, floodplain was setup via weir side with storage capacity, the crest of weir was determined by overflow threshold. In downstream have 8 sub- catchments, and each sub-catchment has different overflow threshold. It depends on their topography. Each floodplain is determined by storage capacity, length of weir and overflow threshold. Figure 5 - 10: Storage capacity of floodplain in downstream Determining boundary conditions of the model - Upstream boundary are release of Ke Go reservoir corresponding to actual flood in 2010 and different design floods obtain from the Establishing Ke Go reservoir operation regular in 2012. - Downstream boundaries are hourly water level at the Thach Dong hydrological station (Cua Sot) and the Cam Nhuong hydrological station (Cua Nhuong) - Lateral inflows are calculated by rainfall-runoff models in above part. The downstream of Ke Go reservoir is divided into 8 sub-catchments, which are linked along the major river systems and tributaries. 0 2000 4000 6000 8000 10000 12000 14000 0 1 2 3 4 5 W ( h a) Z (m) Z ~ W Sub9 Sub8 Sub2 Sub3 Sub4 Sub5 Sub6 Sub7 53 Table 5 - 8. Runoff link of sub-catchments into river network in MIKE 11 model No Sub