Spatial analysis of extreme rainfall using hydrological frequency analysis in the cau river basin, vietnam

VIETNAM NATIONAL UNIVERSITY, HANOI VIETNAM JAPAN UNIVERSITY PHAM THI THUY TRANG SPATIAL ANALYSIS OF EXTREME RAINFALL USING HYDROLOGICAL FREQUENCY ANALYSIS IN THE CAU RIVER BASIN VIETNAM MASTER’S THESIS VIETNAM NATIONAL UNIVERSITY, HANOI VIETNAM JAPAN UNIVERSITY PHAM THI THUY TRANG SPATIAL ANALYSIS OF EXTREME RAINFALL USING HYDROLOGICAL FREQUENCY ANALYSIS IN THE CAU RIVER BASIN VIETNAM MAJOR: ENVIRONMENTAL ENGINEERING CODE: 8520320.01 RESEARCH SUPERVISORS Dr NGUYEN VAN QUANG Dr TAISHI YAZAWA Hanoi, 2022 COMMITMENT I declare that I have read and understood the plagiarism violations I pledge with personal honor that this research result is my own and does not violate the Regulation on prevention of plagiarism in academic and scientific research activities at VNU Vietnam Japan University (Issued together with Decision No 700/QD-ĐHVN dated 30/9/2021 by the Rector of Vietnam Japan University) Author of the thesis (Signature) Pham Thi Thuy Trang ACKNOWLEDGEMENT I acknowledge the generous support from the Environmental Engineering Program of Vietnam Japan University I am indebted to my supervisor, Dr Nguyen Van Quang, for his guidance and an endless supply of fascinating information His approach to research and science is a source of inspiration This approach is reflected by his clear mention style, which is something I hope to carry forward throughout my career Thank you to my co-supervisor, Dr Yazawa Taishi, for your patience, guidance, and support I have benefited greatly from your wealth of knowledge and meticulous editing I am extremely grateful that you took me and have faith in me over time Thank you to my advisor, Dr Shen Shang Your encouraging words and thoughtful, detailed feedback have been very important to me Thank you to my classmates for always being there for me and for telling me that I am awesome even when I didn't feel that way Most importantly, I am grateful for my family’s unconditional, unequivocal, and loving support Ha Noi, June 2022 – Pham Thi Thuy Trang TABLE OF CONTENTS LIST OF TABLES i LIST OF FIGURES ii LIST OF ABBREVIATIONS iii CHAPTER INTRODUCTION 1.1 Problem Statement 1.2 Motivation 1.3 Study Area 1.4 Research Objectives 1.5 Scope of Research CHAPTER LITERATURE REVIEW 2.1 Flood and Rainfall Situation in RRD and Cau River Basin .8 2.2 The Probability Distributions to Analyze Rainfall/ Extreme Rainfall in Vietnam 11 2.3 Hydrological Frequency Analysis 12 2.4 Risk Communication Model 18 CHAPTER THEORIES AND METHODOLOGIES 21 3.1 Theory of Hydrological Frequency Analysis 21 3.1.1 Rainfall depths expected for specific probability (𝑋𝑝) .21 3.1.2 Probability of exceedance (𝑃𝑥) 21 3.1.3 Return period (𝑇𝑥) 21 3.2 Application of HFA 21 3.2.1 Data collection 22 3.2.2 Application of probability density functions (PDFs) 24 3.2.3 Goodness-of-fit test and decision of the optimum PDF 27 3.3 Analysis of Rainfall Characteristics .28 3.3.1 Total annual rainfall 28 3.3.2 Number of rainy days 29 3.3.3 The simple precipitation intensity index (SDII) 29 3.4 Spatial Interpolation and Mapping Using GIS .29 3.5 Rainfall Classification 30 3.6 Determining Return Periods for Rainfall 30 CHAPTER FINDING AND DISCUSSION 32 4.1 Estimation of The Probable Rainfall Using HFA 32 4.2 Assessment of The Rainfall Characteristics 36 4.2.1 Analysis of rainfall in the period 2005 – 2019 36 4.2.2 Analysis the probable rainfall 41 4.3 Risk Communication 42 CHAPTER CONCLUSION AND RECOMMENDATION .46 5.1 Conclusion 46 5.1.1 Results 46 5.1.2 Limitation 47 5.2 Recommendation 47 REFERENCES 48 LIST OF TABLES Table 2.1 The major floods on the Cau River (Nhất, 2010) 10 Table 3.1 Rainfall levels according to the General Department of Meteorology and Hydrology…… 30 Table 3.2 Rainfall levels using in thesis 30 Table 4.1 Standard Least-Squares Criterion (SLSC) 32 Table 4.2 The probable rainfall 33 Table 4.3 Return period for each rain level 35 i LIST OF FIGURES Figure 1.1 Map of the Cau River Basin Figure 1.2 Topography of the Cau River basin Figure 3.1 The HFA model application in the Cau River Basin 22 Figure 3.2 Rainfall monitoring points in the Cau river basin 23 Figure 3.3 The Gumbel probability density function 24 Figure 3.4 The GEV probability density function 24 Figure 3.5 Weibull probability density function .26 Figure 3.6 Exponential distribution probability density function 26 Figure 3.7 GP probability density function 27 Figure 3.8 Linear least squares 27 Figure 3.9 Ellips least squares 27 Figure 3.10 Applying GIS in research 29 Figure 4.1 Estimated probable rainfall in the Cau river Basin 34 Figure 4.2 Total annual rainfall and number of rainy days recorded in the Cau River basin for the period 2005 - 2019 36 Figure 4.3 SDII in the Cau River basin in the period 2005 - 2019 37 Figure 4.4 Average rainfall in rainy days in Cau River basin in the period 2005 – 2019…………… 38 Figure 4.5 Seasonal rainfall in the Cau river basin 39 Figure 4.6 Spatial rainfall characteristics in the Cau river basin from 2005 to 2019…………… 40 Figure 4.7 Spatial probable rainfall characteristics in the Cau river basin .42 Figure 4.8 Warning of dangerous levels due to rain in Cau River basin in the period 2005 – 2019 43 Figure 4.9 Warning about the dangerous levels due to rain in the Cau River basin in each return period 44 ii LIST OF ABBREVIATIONS HFA: Hydrological Frequency Analysis RRD: Red River Delta GCMs: Global Climate Models PDF: Probability Density Function GEV: Generalized Extreme Value GP: Generalized Pareto SLSC: Standard Least-Squares Criterion GIS: Geographic Information Systems iii CHAPTER INTRODUCTION 1.1 Problem Statement Climate change is a change of the climate system under the effects of natural factors and human factors However, in recent years, the impacts of climate change are exacerbated by manmade impacts, such as the use of fossil fuels in transportation and industrial production, and the release of greenhouse gases Therefore, climate change is one of the most important issues being forced on the world According to Genamwatch’s report in COP 24, Vietnam stands at the first in ASEAN and the sixth in the world in the rank of the countries vulnerable to climate change The impacts of climate change in Vietnam, including extreme weather events, are increasing in frequency and become difficult to predict The highest monthly rainfall increased from 270 mm in the period of 1901 to 1930 to 281 mm in the period of 1991 to 2015, while the highest monthly temperature increased from 27.1oC to 27.5 oC (WorldBank, 2018) In recent years, new records are still being set every year The words “record heavy rain”, “record hot weather”, and “record of flooding” are become popular in the Vietnamese media It is easy to see the change in the frequency of extreme rain For example, the year 2017 was considered a record year of natural disasters in Vietnam with more than 16 storms and historical floods The changes in water resources (e.g., rainfall and river water level) also increased significantly compared to the average level of the previous years Climate change can affect the intensity and frequency of rainfall Global warming makes the ocean become warmer leading to an increase in the amount of water evaporating into the air When air containing a lot of moisture moves inland or converges into a storm system, it can produce heavy rains that cause flooding and leads to an increase in human, economy, and potential environmental and disease risks Therefore, it is necessary to develop and focus on research methods and models in basinscale flood risk estimation based on changes in rainfall characteristics, and develop multiple risk indicators for management flood management, such as scale, frequency, 4.2 Assessment of The Rainfall Characteristics 4.2.1 Analysis of rainfall in the period 2005 – 2019 Figure 4.2 shows the total annual rainfall and number of rainy days observed in the Cau River basin is plotted for the 2005 -2019 period Annual rainfall varied between 2159 mm (2013) and 1319 mm (2007) and the average of the whole Cau River basin is about 1700 mm The standard deviation of the total rainfall for this 15-year period is respectively 250 mm Number of rainy days varied between 234 days (2012) and 180 days (2006) and the average is 207 days The standard deviation of the total rainy days for this 15-year period is respectively 14 days Figure 4.2 Total annual rainfall and number of rainy days recorded in the Cau River basin for the period 2005 - 2019 In order to see the correlation between the total rainfall and the number of rainy days in the year, the SDII index was calculated Figure 4.3 shows SDII values at 11 monitoring points in the Cau River basin The average rainfall on rainy days in the basin fluctuates from to 16 mm/day in the period 2005 to 2019 However, it also recorded a sudden 36 increase in values such as in Vinh Yen in 2006, the average rainfall value on rainy days is up to nearly 21 mm/day Figure 4.3 SDII in the Cau River basin in the period 2005 - 2019 Figure 4.4 shows the average rainfall in rainy days in the Cau River basin the period 2005 - 2019 In 2005, the area of rainfall from 10 -11 mm/day was the largest, the area of rain from 11-13 mm/day and greater than 13mm concentrated mainly in Vinh Phuc and Bac Giang province In 2010, the average rainfall decreased compared to 2005, the average rainfall fluctuated from to 11 mm/day, the highest average rainfall was still concentrated in Bac Giang province, however, the average rainfall decreased in the area only from 11 -13 mm/day In 2015, the rain area from 11 to 13 mm/day accounted for the main area, Bac Giang is still a place with high average rainfall (more than 13 mm/day) By 2019, the average rainfall tends to decrease with the rain area from 9-11 mm/day mainly 37 2005 2010 2015 2019 Figure 4.4 Average rainfall in rainy days in Cau River basin in the period 2005 – 2019 Average rainfall on rainy days in the period 2005 - 2019, can be easily predicted through the correlations of total rainfall and the total number of rainy days in the year (Figure 4.2) While 2005, 2010, and 2019 recorded little rainfall but a relatively large number of rainy days, in 2015 the number of rainy days decreased sharply, which recorded a sudden rapid increase in average rainfall in the rain industries in the basin Cau River (Figure 4.4) The cau River basin is located in the heavy rain area (1,500-2,700 mm/year) of Bac Kan and Thai Nguyen provinces Rainfall in the Cau River basin is unevenly distributed Rainfall is concentrated mainly in the rainy season, accounting for nearly 80% of the 38 total rainfall in the whole region in a year Figure 4.5 shows seasonal rainfall in the Cau River basin Figure 4.5 Seasonal rainfall in the Cau river basin Figure 4.6 shows the rainfall whole the Cau river basin in the period 2005 - 2019 Because the minimum value is recorded at 58 mm and the maximum value is 278 mm, thus the rainfall values are divided into levels (figure 4.6) In most areas, daily rainfall of 100 – 150 is the most common and this value is also the representative rainfall for the delta areas While rainfall from 150 – 200 mm occupies a very small area and often occurs in mountainous areas 39 2005 2010 2015 2019 Figure 4.6 Spatial rainfall characteristics in the Cau river basin from 2005 to 2019 In 2005, rainfall from 100 - 150 mm covered the whole area, many places also recorded small rainfall below 100 mm, rainfall from 150 to 200 accounted for a tiny area in the Cau river basin, with rainfall >200 mm is almost not present in the area In 2010, the rainfall increased relatively quickly, the rainfall area from 150-200 mm and more than 200 mm expanded to a large area in the whole area Rainfall was reduced in the following years 2010 and 2019, heavy rain areas are replaced by smaller rainfall rains In general, rainfall characteristics in the Cau River basin are relatively unpredictable and change from year to year Rainfall in the whole region in the period 2005 - 2019 fluctuates in the range of 100 - 150 mm/day mainly However, there are some places 40 where heavy rainfall occurs and it is often concentrated in mountainous areas, and high terrain, especially Tam Dao area located in the monsoon trough (which is the convergence area of two monsoon zones: East - North and South - West) has an axis passing through the North, which is the cause of frequent large-scale rain here It is possible to see the correlation between the past rainfall and the return period of heavy rainfall In Vinh Phuc and Tam Dao, daily rainfall of 150mm is always recorded, thus the time of repetition of extreme rain events in the area also becomes shorter 4.2.2 Analysis the probable rainfall In this study, as mentioned above, rainfall is divided into levels from Spiting rain to violent rain Table 4.7 shows the amount of rainfall for each return period With Tx = 20, with rain levels from spitting rain to pouring rain no longer occurring, the area of heavy rain is predominant, and the area of lashing rain to violent rain has appeared but occupies a very small area in Vinh Phuc province With Tx =40, the rain area from 200 - 300mm has expanded to concentrated areas in densely populated areas such as Thai Nguyen, Bac Kan, Bac Ninh, rainfall greater than 300 mm has begun to appear more present in Vinh Phuc province With Tx =60, heavy rainfall tends to continue to expand and prevail More than 200 mm of rainfall was recorded and expanded in Bac Giang province With Tx =80, Rainfall area >200 mm has become dominant in the entire Cau River basin, when lashing and violent rain are recorded in the whole Thai Nguyen province With Tx =100, rainfall less than 200 mm decreased sharply instead of lashing rain and violent rain throughout the region Tx = 20 Tx = 40 41 Tx = 60 Tx = 80 Tx = 100 Figure 4.7 Spatial probable rainfall characteristics in the Cau river basin 4.3 Risk Communication Based on the division of levels of rainfall, the danger of rain on Cau River basin is divided into levels from very safe to disaster Figure 4.8 describe the levels of danger caused by rain in the Cau River basin in the period 2005 - 2019 In 2005, the whole area was assessed as safe with corresponding rainfall However, by 2010, some areas were assessed as dangerous, mainly in Vinh Phuc province and Hanoi In 2015, rainfall tends to decrease, so the whole area is assessed as safe similar to 2019 42 2005 2010 2015 2019 Figure 4.8 Warning of dangerous levels due to rain in Cau River basin in the period 2005 - 2019 However, with a rapid increase in rainfall in return periods The whole area is always alerted from the dangerous level (Figure 4.9) During the Tx= 20 and 40 years periods, the dangerous level prevailed throughout the area However, from Tx=60 to 100 years the warning levels are mostly always disaster level 43 Tx = 20 Tx = 40 Tx = 60 Tx = 80 Tx = 100 Figure 4.9 Warning about the dangerous levels due to rain in the Cau River basin in each return period 44 The results shown in figure 4.9 will be products that are easy to use in communication of risks to raise awareness and are important precautionary components of integrated heavy rain risk management Heavy rains are the main cause of flooding in residential areas, although heavy rains can occur anywhere and can only be warned for a short time, there are dangerous warning maps early on to help the authorities easily to disseminate and explain information from hazard analysis and risk assessment to those at risk And people also have grounds to compare their area with each level of danger through a clearly presented map system Almost the Cau River basin belongs to a dangerous area (Tx= 20) from Tx= 40 to Tx = 100 lies in a disaster area (Figure 4.9) and more than half of the population and capital city are concentrated in this area There are early warning forms and danger zoning for each area that will facilitate the relocation of people and property to minimize harm, in addition, this is also one of the bases for the government to put decision-making regarding water supply and drainage infrastructure investment upgrading The hazard levels represent the extent of the damage rain causes to both people and matter in the area, many people and institutions from numerous professions are affected by the threat of heavy rain Therefore, it is necessary to take measures to upgrade infrastructure and evacuate people in case of necessity to ensure safety and prevent natural disasters and tailor risk communication activities to different target groups 45 CHAPTER CONCLUSION AND RECOMMENDATION 5.1 Conclusion 5.1.1 Results The purpose of this study is estimate the change of rainfall characteristics in Cau River basin The Cau River is one of the main river systems in the RRD, where they are influenced by the monsoon climate divided into two distinct seasons: the rainy season and the dry season The study focused on using rainfall data to include in hydrological frequency analysis, calculating rainfall for each repeating period The general findings obtained are as follows: - In 100 years, the probable rainfall ranges from 150mm to nearly 400mm Extreme rains are concentrated in mountainous areas such as Vinh Yen and Tam Dao with rainfall between more than 200mm and nearly 400mm While the probable rainfall in flat areas such as Hiep Hoa ranges from 150mm to 180mm - Rainfall is less than 100, the frequency repeats extremely quick under 1.5 years, from heavy rain the frequency of repetition is significantly different between monitoring points Tam Dao, Vinh Yen and Son Dong have the fastest frequency of heavy rain (1.5 – years), lashing rain (average 15 years) and violent rain (less 100 years) - The average rainfall on rainy days in the basin fluctuates from to 16 mm/day in the period 2005 to 2019 - Rainfall in the Cau River basin is unevenly distributed Rainfall is concentrated mainly in the rainy season, accounting for nearly 80% of the total rainfall in the whole region in year - In most areas, daily rainfall of 100 – 150 is the most common and this value is also the representative rainfall for the delta areas While rainfall from 150 – 200mm occupies a very small area and often occurs in mountainous areas - In 2005 - 2019, the whole area was assessed as safe with corresponding rainfall - In return periods, with a rapid increase in rainfall, the whole area is always alerted from the dangerous level 46 5.1.2 Limitation During the research process, a limitation was encountered as follows: - The lack of data leads to limitations in the process of assessing and analyzing the characteristics of rainfall, the correlation of rainfall and runoff in the Cau River basin - The risk communication model is only warned based on rainfall, but has not been considered based on other important factors such as infrastructure (dyke, drainage system, etc) or current land use 5.2 Recommendation This study has evaluated the rainfall characteristics and calculated the 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