Phân vùng cho việc đánh giá rủi ro thiên tai liên quan đến nước ở Đồng Bằng Sông Cửu Long (English)

BỘ GIÁO DỤC VÀ ĐÀO TẠO TRƯỜNG ĐẠI HỌC CẦN THƠ KHOA NƠNG NGHIỆP Năm học 2004- 2005 Phân vùng cho việc đánh giá rủi ro thiên tai liên quan đến nước ở Đồng Bằng Sơng Cửu Long (Zoning for risk assessment of water-related natural disasters in the Mekong Delta) Yamashita Akira(山下 晃) Luận văn thạc sỹ chuyên ngành Khoa học Mơi trường Cần Thơ tháng 3 / 2005 BỘ GIÁO DỤC VÀ ĐÀO TẠO TRƯỜNG ĐẠI HỌC CẦN THƠ KHOA NƠNG NGHIỆP Năm học 2004- 2005 Phân vùng cho việc đánh giá

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rủi ro thiên tai liên quan đến nước ở Đồng Bằng Sơng Cửu Long (Zoning for risk assessment of water-related natural disasters in the Mekong Delta) Luận văn thạc sỹ Chuyên Ngành Khoa học Mơi trường Mã số: 60 85 02 Đã bảo vệ và được duyệt Hiệu trưởng: PGs. Ts. Lê Quang Minh Trưởng khoa Cán bộ hướng dẫn PGs. Ts. Nguyễn Bảo Vệ PGs. Ts. Nguyễn Hữu Chiếm Luận văn kèm theo đây , với tựa đề là “Phân vùng cho việc đánh giá rủi ro thiên tai liên quan đến nước ở Đồng Bằng Sơng Cửu Long”, do YAMASHITA AKIRA thực hiện và báo cáo, và đã được Hội đồng chấm luận văn thơng qua. Uỷ viên Uỷ viên Phản biện 1 Phản biện 2 Cần Thơ, ngày 2 tháng 4 năm 2005 Chủ tịch Hội đồng LÝ LỊCH KHOA HỌC I- LÝ LỊCH SƠ LƯỢC Họ và tên khai sinh: Yamashita Akira giới tính: Nam Ngày sinh: 11 tháng 7 năm 1972 Nơi sinh: Nhật Bản Quê quán:.Nhật Bản................................................................................................ Chức vụ, đơn vị cơng tác khi học tập, nghiên cứu: Khơng Chổ ở riêng hoặc địa chỉ liên lạc: 1-184-23-1-412 Miyahara, Kita-ku, Saitama City, Japan Điện thoại cơ quan:................... Điện thoại nhà riêng: 81.486.6657908 Fax:……………………………………….E-mail: songcuulong@hotmail.com II- QUÁ TRÌNH ĐÀO TẠO 1. Trung học chuyên nghiệp: Hệ đào tạo:……………………..Thời gian đào tạo từ……./đến…… Nơi học (trường, thành phố): Ngành học:.. 2. Đại học: Hệ đào tạo: Chính quy …………..Thời gian đào tạo từ 1992/đến 1996 Nơi học (trường, thành phố): Tokyo University of Agriculture and Technology Ngành học: Environment and natural resources, Department of Agriculture Tên đồ án, luận án hoặc mơn thi tốt nghiệp : Study of riverbed formation around fishways at low dams Ngày và nơi bảo vệ đồ án, luận án hoặc mơn thi tốt nghiệp : Tháng 3 Năm 1996 tại Tokyo University of Agriculture and Technology Người hướng dẫn: Dr. Hiroyuki Nakamura 3. Thạc sĩ: Hệ đào tạo:……………………..Thời gian đào tạo từ……./đến…… Nơi học (trường, thành phố): Ngành học:.. Tên đồ án, luận án hoặc mơn thi tốt nghiệp Tên luận văn: Ngày và nơi bảo vệ luận văn Người hướng dẫn: 4.Trình độ ngoại ngữ (biết ngoại ngữ gì, mức độ) - Tiếng Việt (mức độ Trung) - Tiếng Anh (TOEFL CBT 207 điểm bằng TOEFL giấy 550 điểm) 5. Học vị, học hàm, chức vụ kỹ thuật được chính thức cấp; số bằng, ngày và nơi cấp: III. QUÁ TRÌNH CƠNG TÁC CHUYÊN MƠN KỂ TỪ KHI TỐT NGHIỆP ĐẠI HỌC Thời gian Nơi cơng tác Cơng việc đảm nhiệm 1996 – 2002 DIA Consultant Co., Tokyo, Japan Kỹ sư xây dựng và mơi trường IV. CÁC CƠNG TRÌNH KHOA HỌC ĐÃ CƠNG BỐ - Relationship between notches of fishways and waterway continuity in channel works(Thesis for bachelor at TUAT, Chairman's award in the thesis contest of Japan Forestry Technique Association, Paper for journal of Japan erosion control association) - Effects of Ground Sills against Riverbed Shapes and Habitation of Mountin Stream Fishes (Subsidized study by To-kyu Foundation of Environment and Purification) - Values of Agriculture for Commonweal in Japanese paddy fields(Superior prize in the prize essay contest of YANMAR Co.) LỜI CAM ĐOAN Tơi xin cam đoan đây là cơng trình nghiên cứu của bản thân. Các số liệu, kết quả trình bày trong luận án là trung thực và chưa từng được ai cơng bố trong bất kỳ luận văn nào trước đây. Tác giả luận án YAMASHITA AKIRA Acknowledgement First of all the authors thank Can Tho University (CTU), especially faculty of Agriculture, for providing much precious opportunities to know real nature of the Mekong Delta. In addition, DANIDA (Danish International Development Agency) and University of Aarhus, Denmark, also supported our study logistically in the DENRM and CAULES projects, and we owe our study to them very much. During data collecting period, Dr. Hiroshi Hori, a former UN senior adviser, and Mr. Kamoto at Mekong River Commission helped us greatly, especially on collecting hydrological data. At the same time, the staff at Ho Chi Minh Geological Museum assisted us in finding geological material. As a foreign student in CTU, I was very happy because I could work on this study with professors, Dr. Nguyen Huu Chiem, Dr. Le Quang Tri and Dr. Duong Van Ni in CTU with their kind guidance. In addition, it must be impossible for me to study at CTU without great help of Dr. Le Quang Minh and Dr. Do Van Xe who boosted up my enrolment. I also thank Dr. Be and Dr. Anika in the office of CAULES project. And special thanks for friendships of my class mate at the Master course 9, in the Department of Environment and Natural Resources Management. In the end I would like to describe my cordial thanks to all the people who welcomed me, a foreigner from Japan. I hope this thesis will become my small gratitude to nature, people, and all of my beautiful memory in the Mekong Delta of Viet Nam. Thank you very much again. Abstract The purpose of this study is to describe the risk of water-related natural disasters, mainly flooding, in the Mekong Delta by valid zoning based on fundamental conditions. Firstly, basic natural and social conditions in the Mekong Delta are arranged to draw thematic maps; such as Geology, Topography, Meteorology, Flooding, Population and so on. The authors make a point of such fundamental information on the delta to distinguish each phenomenon in the area. For example, "floods" in the Mekong Delta has different meanings according to the location. Upper delta is inundated mainly by riverine factors while lower delta is flooded by tidal regime. Secondly, the difference of flood forms and preliminary hydrological analysis support the basic conditions in terms of floods. Generally, flood in the Mekong Delta is argued as single phenomenon, however, the structure, cause and damages due to the disaster are completely different. After separating the zone by flood forms, statistical analysis by hydrological data was done with some cases of flood years in the Mekong River Basin. Thematic maps drawn by basic conditions and other analysis become raw material to discuss further multi-meaning maps. The main method to examine appropriate zoning is overlaying with thematic maps following specific themes of land conditions, floods, hydrology and social conditions. Consequently, twenty basic zones are carved on the final map as a result of synthesized map of above four factors with a help of basic grid map consisting of horizontal and vertical zoning. This final zoning map can represent the risk for water-related disasters in the Mekong Delta, and the most important function of the map is providing fundamental zoning based on valid background for any further studies and projects at real sites. Tĩm lược Mục tiêu của nghiên cứu này là đánh giá rủi ro về thiên tai liên quan đến nước, đặc biệt là lũ lụt, ở ĐBSCL dựa trên các điều kiện tự nhiên. Thứ nhất sắp xếp tài liệu về những điều kiện tự nhiên và xã hội của ĐBSCL, sau đĩ vẽ các bản đồ cĩ chủ đề (Thematic Maps) như: Địa chât, Địa hình, Khí tượng, Lũ lụt, Dân số v.v. Các thơng tin cơ sở về ĐBSCL trên sẽ là tài liệu rất quan trọng để phân biệt các tai họa. Thí dụ như là "Lụ lũt" ở ĐBSCL cĩ đặc điểm khác nhau theo địa phương; vùng thượng lưu bị ngập do nước lũ của sơng Mê Cơng cịn vùng hạ lưu bị ngập do thủy triều. Mục tiêu thứ hai là việc phân tích hình thức lũ lụt và số liệu thủy văn để phân loại lũ lụt ở ĐBSCL. Nĩi chung là lũ lụt ở ĐBSCL được xem như một hiện tượng đơn giản, tuy nhiên cấu trúc, nguyên nhân và thiệt hại của nĩ rất đa dạng. Để diễn tả tính nhiều dạng của lũ lụt này, Phân vùng được thực hiện bằng cách phân tích thủy văn theo những số liệu của lũ lụt chủ yếu trong các năm lũ thực tế ở lưu vực sơng Mê Cơng. Các bản đồ cĩ sở được phân tích và hợp nhất để phân vùng. Phương pháp chủ yếu để phân vùng thích hợp là chồng lên nhau các bản đồ (Overlaid map) như: đất đai, lũ lụt, thủy văn và điều kiện xã hội. Bản đồ phân vùng cuối cùng được tạo ra này cĩ thể mơ tả được rủi ro thiên tai liên quan đến nước tại các địa phương, và đồng thời cung cấp các thơng tin cơ sở về khoa học tự nhiên và xã hội ở tồn bộ ĐBSCL trên bản đồ. Các kết quả của nghiên cứu này sẽ là cơ sở cho các nghiên cứu hoặc các dự án về mơi trường và tài nguyên thiên nhiên ở ĐBSCL trong tương lai. Contents 1. Introduction ···································································································· 1 1.1 General figure on the Mekong River ······················································ 1 1.2 General figure on the Mekong Delta ······················································ 4 1.3 Background of the study ········································································· 6 2. Problems, Scope, Goals, and Flow ······························································· 8 2.1 Problems ································································································· 8 2.2 Scope ······································································································ 9 2.3 Goals ······································································································ 10 2.3.1 General objective ············································································· 10 2.3.2 Specific objective ············································································ 10 2.4 Research flow ························································································· 10 3. Means and method of the study ··································································· 12 3.1 Basic natural conditions in the Mekong Delta ········································ 12 3.1.1 Geography ······················································································· 13 3.1.2 Topography ······················································································ 15 3.1.3 Geology ···························································································· 17 3.1.4 Soil ···································································································· 18 3.1.5 Meteorology ······················································································ 21 3.1.6 River systems ···················································································· 24 3.1.7 Oceanography ··················································································· 28 3.1.8 Water resources ················································································· 31 3.1.9 Saline water intrusion ······································································· 34 3.1.10 Floods ····························································································· 36 3.1.11 Population and people ····································································· 39 3.1.12 Economy ························································································· 42 3.1.13 Construction ···················································································· 43 3.1.14 Transportation ················································································· 43 3.1.15 Agriculture with Forestry and Fishery ············································ 49 3.2 Specifying water-related disasters in the Mekong Delta ························· 52 3.2.1 Riverine flood ·················································································· 53 3.2.2 Tidal flood ······················································································· 54 3.2.3 Inland flood ····················································································· 55 3.2.4 Urban flood ······················································································ 56 3.2.5 Forms of erosion ·············································································· 59 3.3 Hydrological analysis ············································································· 62 3.3.1 Data collection ················································································· 63 3.3.2 Disaster records and target floods ···················································· 68 3.3.3 Flood characteristics in the Lower Mekong River ··························· 69 3.3.4 Correlation between Mekong Delta and other upstream area ·········· 76 3.3.5 Correlation within the Mekong Delta of Viet Nam ·························· 77 4. Results and discussion ··················································································· 81 4.1 Thematic maps ························································································· 81 4.2 The first-level zoning ················································································ 84 4.3 Land conditions ························································································ 85 4.4 Floods ······································································································· 88 4.5 Hydrology ································································································· 91 4.6 Social conditions ······················································································ 94 5. Conclusion ······································································································· 97 5.1 The second-level zoning ··········································································· 98 5.1.1 Horizontal zoning ········································································ 98 5.1.2 Vertical zoning ············································································· 99 5.1.3 Integrated grid zoning ·································································· 101 5.2 The final zoning and evaluation of disaster potential ······························· 103 5.2.1 Basic grid map ············································································· 103 5.2.2 Land conditions ··········································································· 104 5.2.3 Floods ·························································································· 105 5.2.4 Hydrology ···················································································· 106 5.2.5 Social conditions ·········································································· 107 5.2.6 Map for the risk of water-related disaster and evaluation ············ 108 5.3 Conclusion and challenges ······································································· 113 7. References ······································································································· 115 List of figures Figure 1.1.1 Outline of the Mekong River Basin from headwaters to mouths Figure 1.2.1 Mekong Delta site Figure 1.2.2 Administration map of the Mekong Delta of Viet Nam Figure 2.4.1 Simplified flowchart of the research Figure 3.1.1 Geographic classification of the Mekong Delta Figure 3.1.2 Contour map of the Mekong Delta Figure 3.1.3 Topographic distribution of the Mekong Delta Figure 3.1.4 General geology of the Mekong Delta Figure 3.1.5 Soil distribution map of the Mekong Delta Figure 3.1.6 Typical pressure patterns Figure 3.1.7 Annual rainfall in the Mekong Delta Figure 3.1.8 Annual rainfall distribution at Can Tho City Figure 3.1.9 Major rivers and estuaries in Mekong Delta Figure 3.1.10 Image of longitudinal riverbed Figure 3.1.11 The main canal system in the Mekong Delta Figure 3.1.12 Inshore status around the Mekong Delta Figure 3.1.13 Annual high tide and low tide succession at Ho Chi Minh City Figure 3.1.14 Mean daily water level difference at Ho Chi Minh City and Ha Tien Town Figure 3.1.15 A model of water flow distribution at each tributaries Figure 3.1.16 Groundwater potential in the Mekong Delta Figure 3.1.17 Density of salinity in surface water in the Mekong Delta Figure 3.1.18 Status of saline intrusion in groundwater Figure 3.1.19 An image of flooding area of Cambodia - Viet Nam Figure 3.1.20 Depth of inundation in the Mekong Delta Figure 3.1.21 The successions of birth rate, death rate and growth rate in Can Tho Province Figure 3.1.22 Series of population in HCMC, Cantho and all Vietnam Figure 3.1.23 Population density in the Mekong Delta Figure 3.1.24 National road network in the Mekong Delta Figure 3.1.25 Destinations of local liners from Can Tho City Figure 3.1.26 Rice crop and other crop pattern in the Mekong Delta Figure 3.2.1 Inundation period and depth due to riverine water Figure 3.2.2 Water level of the Tien River in the rainy season, 1982 Figure 3.2.3 Division by flood forms in the Mekong Delta Figure 3.2.4 The cycle of periodical erosion Figure 3.2.5 Cylindrical image of geological status along the Tien River Figure 3.2.6 Division by erosion form in the Mekong Delta Figure 3.3.1 Map of hydrological observatories in the Mekong Delta Figure 3.3.2 Lower Mekong Delta with target observatories without Viet Nam Figure 3.3.3 Lower Mekong Delta with target observatories without Viet Nam Figure 3.3.4 Annual maximum water volume at Kratie Figure 3.3.5 Major flooding events in the Mekong Delta of Viet Nam Figure 3.3.5 Annual hydrograph in 1961 Figure 3.3.6 Annual hydrograph in 1966 Figure 3.3.7 Annual maximum high water level in the Lower Mekong Figure 3.3.8 Flood water movement in the Mekong Delta Figure 3.3.9 Annual maximum water level at Phnom Penh and Chau Doc Figure 3.3.10 Zoning based on hydrological characteristics Figure 3.3.11 A comparison of high water at Tan Chau and Moc Hoa Figure 4.1.1 The image of mapping and overlaying Figure 4.3.1 The first overlaid map (Land Condition) Figure 4.3.2 The second overlaid map (Land Condition) Figure 4.3.3 The third overlaid map (Land Condition) Figure 4.3.4 The final overlaid map of land condition in the Mekong Delta Figure 4.4.1 The first overlaid map (Flood) Figure 4.4.2 The second overlaid map (Flood) Figure 4.4.3 The third overlaid map (Flood) Figure 4.4.4 The final overlaid map of floods in the Mekong Delta Figure 4.5.1 The first overlaid map (Hydrology) Figure 4.5.2 The second overlaid map (Hydrology) Figure 4.5.3 The third overlaid map (Hydrology) Figure 4.5.4 The final overlaid map of hydrology in the Mekong Delta Figure 4.6.1 The first overlaid map (Social Condition) Figure 4.6.2 The second overlaid map(Social Condition) Figure 4.6.3 The third overlaid map(Social Condition) Figure 4.6.4 The final overlaid map of social conditions in the Mekong Delta Figure 5.1.1 The final overlaid map of social conditions in the Mekong Delta Figure 5.1.2 Horizontal zoning of the Mekong Delta Figure 5.1.3 Vertical zoning of the Mekong Delta Figure 5.1.4 Horizontal zoning of the Mekong Delta Figure 5.2.1 The basic grid map of the Mekong Delta Figure 5.2.2 Land conditions on the basic gird Figure 5.2.3 Floods on the basic gird Figure 5.2.4 Hydrology on the basic gird Figure 5.2.5 Social conditions on the basic gird Figure 5.2.6 The final zoning map for risk assessment of water-related disasters Figure 5.2.7 Type of flood in the Mekong Delta List of tables Table 1.1.1 Territory within the catchment of the Mekong River countries Table 3.1.1 The list of natural conditions of the Mekong Delta in this study Table 3.1.2 Appellations of each area in English and Vietnamese Table 3.1.3 Topographic classification of the Mekong Delta Table 3.1.4 Population status in each province of the Mekong Delta in 2001 Table 3.1.5 Distance of major towns from Ho Chi Minh City (unite: km) Table 3.2.1 Classification of floods in the Mekong Delta Table 3.3.1 The list of collected data of water volume and water level Table 3.3.2 Target flood years Table 3.3.3 Target flood years at Chau Doc and My Thuan Table 4.1.1 Matrix of overlaid maps Table 4.3.1 Outline for zones of land condition Table 4.4.1 Outline for zones of floods Table 4.5.1 Outline for zones of hydrology Table 4.6.1 Outline for zones of social conditions Table 5.1.1 Outline for zones of social conditions Table 5.2.1 Outline of Zones in the final zoning map List of photographs Photo 1.1.1 Scenes of Mekong River at each country Photo 1.2.1 Satellite image of the Mekong Delta Photo 1.2.2 Scenes of the Mekong Delta at each Province Photo 3.1.1 Rain clouds in Can Tho City Photo 3.1.2 Separated point of Co Chien River from Tien River at My Thuan Bridge Photo 3.2.1 High water at Can Tho Photo 3.2.2 3/2 Street, Hochiminh Photo 3.2.3 (a) Fallen tree at Can Tho / (b) Eroded riverbank at Chau Doc Photo 3.2.4 Coastal erosion at Bac Lieu Province Photo 3.3.1 Tonlesap Lake near Siem Reup, 2001 Photo 3.3.2 Quarter arm at Phnom Penh, 2003 Photo 3.3.3 Vam Co Basin 1. INTRODUCTION 1.1 General figure on the Mekong River The Mekong River comprises one of the largest river systems in the world as well as Asian region. The drainage area is some 795,000 km2, and stretches approximately 4,800 km across Southeast Asia from Tibetan Plateau to the South China Sea. The Mekong River Basin incorporates areas of six countries, China (Yunnan Province), Myanmar, Lao PDR, Thailand, Cambodia and Viet Nam. These Mekong Countries or regions are generally poor area including the Least Developing Country (LDC) as Cambodia. In the Lower Mekong (LM) area, over 100 ethnic minorities are living, and one-third of them face severe condition in their income of under few dollars per day. Thus the Mekong River is a typical international river which called the Danube in Asia. In 1957, Mekong River Committee was established to coordinate international development projects in the basin under the UN. Since 1995, after long-term war period in Indo-China, Mekong River Commission (hereafter MRC) has exercised leadership in solving problems as an intergovernmental body (MRC, 2002). MRC consists of Cambodia, Thailand, Lao PDR, and Viet Nam with observer countries of China and Myanmar. MRC's present primary goal is set as "Sustainable Development" of the Mekong River Basin, and MRC mainly deals water management, aquatic resources study, basic human needs, and so on. Figure 1.1.1 shows the outline of the Mekong River Basin from headwaters to the sea, and Table 1.1.1 shows the catchment area in each country followed by some photos taken in the Mekong River Basin. (1) Originated from Tibet to Yun-nan Province in China (2) Runs through the border between Laos PDR and Myanmar (3) Runs through the border between Laos PDR and Thailand (4) Takes tributaries from Thailand and then run into Cambodia (5) Takes Tonle-sap River from Great Lake and begins breaking up (6) Forms Mekong Delta in Viet Nam and run into the South China Sea Figure 1.1.1 Outline of the Mekong River Basin from headwaters to mouths Table 1.1.1 Territory within the catchment of the Mekong River countries Description Area(km2) Catchment as % of country or province Catchment as % of whole basin Yun-nan Province, China 165,000 38 21 Myanmar 24,000 4 3 Lao PDR 202,000 97 25 Thailand 184,000 36 23 Cambodia 155,000 86 20 Viet Nam 65,000 20 8 Whole Basin 795,000 - 100 Source: State of the Basin Report 2003, Mekong River Commission (a) Luang Prabang, Northern Lao (April, 2002) (b) A view from Vientiane, Lao, to Thailand (March, 2004) (c) Tonle Sap River at Phnom Penh, Cambodia (June, 2002) (d) Bassac River at Châu Đốc, Viet Nam (March, 2004) Photo 1.1.1 Scenes of Mekong River at each country 1.2 General figure on the Mekong Delta The Mekong Delta is one of the mega-deltas in the world with the area of approximately 49,520km2 and is located at the lowest segment of the Mekong River (Figure 1.2.1). The top of its deltaic triangle is at Kampong Cham, Cambodia, however the main area of the Mekong Delta is in far South part of Viet Nam. About 26% of the whole delta belong to Cambodia and the rest of 74% is occupied by Viet Nam1) with the area of approximately 39,568km2. Thus the Mekong Delta physically has great influence of the natural and social conditions on whole Indo-China region as well as South Vietnam. Figure 1.2.1 Mekong Delta site Photo 1.2.1 is a satellite photo taken by NASA in the dry season (February, 1996). Near Chau Doc, An Giang Province, is the Vietnam-Cambodia border, and downstream area is colored with green that indicates rich paddy fields of the Mekong Delta. The mainstream of the Mekong River is broke up into two main flows in the Mekong Delta of Vietnam, which called Sơng Tiền and Sơng Hậu in Vietnamese language. The flow running straight from Long Xuyen via Can Tho is Hau River, and Tien River running north side of Hau River repeats breaking up and meanders in its lower segment. These two main streams are divided at Phnom Penh, and produce many branches. Arc-shaped brawn areas in costal areas are sand dune originated from old coastline, and brawn colored water along the coastal sea shows rich alluvium providing from the Mekong River. Figure 1.2.2 shows administration map of the Mekong Delta followed by .pictures taken in the Mekong Delta. Photo 1.2.1 Satellite image of the Mekong Delta Figure 1.2.2 Administration map of the Mekong Delta of Viet Nam * The map includes Hochiminh City for reference Tiền River at Sa Đéc (September, 2003) Cần Thơ River at Ninh Kiều (April, 2002) Mỹ Thuận Bridge (January, 2003) Tiền River at Mỹ Tho (January, 2002) Photo 1.2.2 Scenes of the Mekong Delta at each Province 1.3 Background of the study Among the present issues of the Mekong Delta, water resources management is one of the most urgent subjects for sustainable development of the region. Up to now most of the researches on water issues in the Mekong Delta have had strong relation to agricultural development, because water is closely tied to food security. In addition, some international organizations, such as Mekong River Commission (MRC) and UNDP, have noticed the water-related disasters in the Mekong Delta with the total aim of reducing poverty, while some NGOs, such as the Red Cross, have carried out humanitarian aids to local disaster areas. While floods in the Mekong Delta are annual event and have always been a part of life, severe flooding in the recent history caused millions of dollars worth of damage and often killed local people, especially kids and older people. Some of the causes could be ascribed to unusual meteorological conditions, however, man-made factors such as in land use were implicated (MRC, 2002). In addition, the global climate change can alter hydrological condition in low land areas as the Mekong Delta. Therefore, floods issue in the Mekong Delta is really urgent challenge for sustainable development and protecting human society. To understand water movement as a comprehensive system is essential to studies on water-related disasters in the Mekong Delta. However, the complicated conditions such as tidal phenomenon, inland waterway network and completely flat configuration make the understanding truly difficult. To overcome this difficulty, collecting fundamental data and information and investigating its accuracy have great meaning, because the information concerning the Mekong Delta is still not integrated and often has crucial mistakes even in documents issued by reliable authorities. The main phenomenon as the target of this thesis is flooding complemented by inundation due to saline intrusion, influences of artificial dykes, and riverbank erosion. Floods in the Mekong Delta are caused by different background, although the final result can be always described as inundation. Therefore, the correct understanding of water-related disasters in the Mekong Delta demands very careful discussion based on fundamental natural conditions. A stable society is the most important condition of sustainable development, and natural disasters can destroy the social base such as agriculture, infrastructure and land use in a twinkle. On this standpoint, appropriate evaluation of the risk for natural disasters, especially flooding, in the Mekong Delta is very meaningful not only for disaster prevention planning but also for any kinds of developing project in the region. Thus, issue of flooding in the Mekong Delta is a key for the sustainable development. 2. PROBLEMS, SCOPE, GOALS, and FLOW 2.1 Problems Natural disasters have caused great impacts on the history of mankind everywhere on the earth. Some developed communities were completely destroyed by flood disasters, volcanic disasters, earth quakes, land slide disasters, and so on. To establish stable society, human beings have struggled to mitigate damages caused by such disasters. Therefore the human's community development can be described as strife of disaster management. Meanwhile it is a common thing that ancient civilizations, such as Nile, had been developed by benefit of flood, namely fresh and fertile soil providing from upstream area. In addition, natural disasters are forms of natural phenomenon, therefore, each disaster have meanings in the cycle of Earth's metabolism. For example, longtime flood events have formed fertile deltas at river mouths in the world. Thus the most remarkable characteristic of natural disasters is double-edged potential for human society. Annual flood in the Mekong Delta also have such double-edged aspects. Most part of the present d._.elta was formed by sand and silts that came from upper stream area through flood events. Flood water can flash out degraded soil due to acid-sulfated soil condition, therefore, farmers can grow rice in the Mekong Delta widely. Meanwhile, as the region was developed by human society, the disadvantages of flood in the Mekong Delta became severer. Main problems caused by water-related disasters in the Mekong Delta can be integrated as three focuses, namely agriculture and fishery, infrastructure, life and property. In heavily flooded regions, crop harvest frequency and is limited due to longtime inundation, although farmers have strived to overcome this hard condition by appropriate technology such as introducing of floating rice. Fishery is also vulnerable to flooding because degrees of turbidity and salinity have grate influences upon aquatic cultivation. On the point of infrastructure, the biggest matter is impassable road problem due to inundation. The road blocking has bad influence not only upon regional economy but also upon safety of local people because they often use small unstable wooden boat as their main transport after they cannot use land road. The number of casualties is not so remarkable comparing with flood in other rivers in Viet Nam, however, some cruel flood years such as 1991, 2000, 2001 took a heavy toll of lives in upper area of the Mekong Delta. Local people's properties such as houses and agricultural facilities are also damaged by floods annually. In the case of the flood in 2000, the overall net loss in 54 districts and towns of the Mekong delta was estimated at USD $200 million (Oxfam America, 2000). Thus, the influences of water-related disasters in the Mekong Delta are a big obstacle to sustainable development of the region and are the source of poverty at rural areas. 2.2 Scope Up to now, almost all the international researches on flood in the Mekong River Basin have set the centre of target area in Cambodia. The first reason is that the beginning point of floods in the lower Mekong River is at Kampong Cham. Secondly movement of flood water in the Mekong Delta of Viet Nam is extremely complicated due to flat landform and dense inland waterways, therefore, it has been not easy to study on that issue in the Mekong Delta of Viet Nam. The direct target area of this study is the Mekong Delta of Viet Nam. Although the factors at upstream Cambodia side are essential condition to the Mekong Delta of Viet Nam, this study mainly focuses on the flood issue in Vietnam side. The setting of target area based on a thinking that basic study on the flood issue in the Mekong Delta of Viet Nam is still insufficient and not integrated. Of course, great attention is paid for upstream side conditions in Cambodia, Thailand, Laos, Myanmar, and China during the study time to evaluate flood structure in the Mekong Delta of Viet Nam correctly. 2.3 Goals 2.3.1 General objective The primary objective of this study is evaluation for potential of water-related disasters in the Mekong Delta. To achieve the final goal, consequently, the research aims at general objectives below; z Support understanding on water-related disasters in the Mekong Delta as an integrated basic resource z Estimate potential of disasters appropriately and integrally for the sustainable development in the Mekong Delta z Give suggestive provision to such disasters based upon present priority and thinkable future status 2.3.2 Specific objective In addition to the general objectives, process of the research also has grate meaning, such as arrangement and analysis of basic natural conditions in the Mekong Delta. Each method of this study provides useful basic data to any kind of research in the Mekong Delta as below; z Arrangement of basic natural condition z Description on records of water-related disasters z Classification of disaster type z Zoning by each natural condition or tendency z Study on social factors related to natural disasters 2.4 Research flow Below is the rough sketch for items and processes of the research. And following paragraphs shows specific schemes of materials and method at each step. The main part of method is arrangement of the basic natural conditions including some social aspect in the Mekong Delta; at the same time, disaster specification and hydrological analysis supplement the method. The results from above method are re-described in each thematic map, and these maps become essential material for zoning the Mekong Delta in discussion part. After being supplemented by field observation and interview, the risk assessment of water related disasters is discussed by each zoning maps of the Mekong Delta. Figure 2.4.1 shows a simplified flowchart of the research. 1) Collecting materials and data 2) Arrangement of the basic natural conditions 3) Specifying water-related disasters 4) Hydrological analysis 5) Mapping and Zoning 6) Field observation and inquiry 7) Evaluation of potential 8) Total discussion and suggestion 9) Conclusion and following challenge Figure 2.4.1 Simplified flowchart of the research Collecting materials and data Arrangement of the basic natural conditions Specifying water-related disasters Hydrological analysis Mapping and Zoning Evaluation of potential Field observation and inquiry Total discussion and suggestion Conclusion and following challenge 3. MEANS AND METHOD OF THE STUDY 3.1 Basic natural conditions in the Mekong Delta Most of the urgent issues in the Mekong Delta, such as acid sulfate soil, annual flood, and saline intrusion, are strongly associated with basic natural conditions. Besides the pure natural conditions as above, some altered natural forms, or secondary natural conditions, also have great influences on the present situation of the Mekong Delta, namely, artificial waterways system, agricultural ecosystem, coastal reforestation, and so on (Yamashita et al., 2004). Especially in the case of flood disaster, social conditions such as population factors and poverty degree also have to be considered carefully. Therefore, arrangement of basic natural conditions including social aspects is the base method of this study. Table 3.1.1 shows each condition which is picked up in this study. Some core factors of this study such as flood, hydrology, and meteorology are analyzed minutely at following chapters of method or discussion. Table 3.1.1 The list of natural conditions of the Mekong Delta in this study Condition Factor Natural condition Geography Location, Area Topography Altitude, Inclination, Landform Geology Geology, Fault Soil Soil pattern Meteorology Climate, Rainfall, River systems Rivers, Canals Oceanography Tide, Sea current Water resources Surface water, Groundwater Saline water Saline intrusion Floods Flood area Social Condition Population Density, Growth rate, Economy GNP, Poverty Construction Dyke, Riverbanks Transportation National road, water transportation Agriculture, Forestry, Fishery Crops, Farming system 3.1.1 Geography The Mekong Delta of Viet Nam is located at southernmost part of the Indochina Peninsula, from 8°30' to 11°north in latitude and from 104°30' to 106°50' east in longitude (Figure 3.1.1). The Mekong Delta is bound on the east by Thailand Bay, the north by Cambodia, the east by Hồ Chí Minh City, and southeast by South China Sea. The Mekong Delta of Viet Nam is often divided into some regions by Vietnamese habitual way. Figure 3.1.1 shows the classification (Thao, 2001a) with author's supplement to clear each border of the areas. Each area is classified by differences of natural condition. Although the borders and reasons for such classification have some vagueness, the zoning is very useful and important to study on natural science in the Mekong Delta. Table 3.1.2 is the list of geographic areas in English and Vietnamese including another common appellation. Figure 3.1.1 Geographic classification of the Mekong Delta Table 3.1.2 Appellations of each area in English and Vietnamese English Name English Name (2) Vietnamese Name Dong Thap Muoi Plain of reeds Đồng Tháp Mười Upper Delta Thượng Châu Thổ Lower Delta Hạ Châu Thổ Long Xuyen Quadrangle Tư Giác Long Xuyên West Hau Trans Bassac Tây Sơng Hậu Ca Mau Peninsula Bán đảo Cà Mau 3.1.2 Topography Vietnam is a mountainous and hilly country. Its 75% area, equal to 2.5 billion hectare, is recognized as mountain district. As far as the Mekong Delta concerned, however, the topography is completely flat. The Mekong Delta region is belongs to Nam Bo (South) Plain which includes the mouths of Vam Co River and Dong Nai River in Vietnamese regional classification (Ministry of Industry, 1998). As an exception, some hills such as Mount Sam (270m), Mount Co To (258m) is located in An Giang and Kien Giang Province which is next to Cambodian border. (a) Altitude and Inclination The average altitude of the Mekong Delta is about 2m above mean sea level (MSL) of the South China Sea with the average surface inclination of about 0.0085 (Hori, 1996). The highest areas are in Kien Giang and An Giang Province, while the lowest points are in Dong Thap Province, 0.5m below MSL (Sanh et al., 1998). Figure 3.1.2 shows rough contour map of the Mekong Delta with the directions of surface inclination. Figure 3.1.2 Contour map of the Mekong Delta (b) Topographical classification According to Dr. Nguyen Huu Chiem in Cantho University, topographical model in the Mekong Delta could be divide into five major part that include lower sub-divisions. Table 3.1.3 and Figure 3.1.3 are revised from his original paper (Chiem, 1993). Table 3.1.3 Topographic classification of the Mekong Delta Landform Sub-units (A) Sub-units (B) Natural levee Sand bar Back swamp Closed floodplain High flood plain Open floodplain Natural levee Back swamp Flood plain Tide affected floodplain Broad depression floodplain Sand ridge Coastal complex Coastal flat Figure 3.1.3 Topographic distribution of the Mekong Delta 3.1.3 Geology (a) Forming process of the Mekong Delta Mekong delta was formed in the duration from Old-Tertiary Period of Cenozoic (tens of millions of years before) to Pleistocene when the Himalaya Mountains appeared. Thereafter, in the Recent (or Alluvial Epoch: before 5,000 to 6,000 years) of the Quaternary Period, Indochina region was affected with global sea expanding, and consequently maximum 80% of present Mekong Delta was submerged (Hien et al., 1991). After the period, Mekong Delta has been developed by sedimentation of the Mekong River. The depth of alluvial soil due to the sedimentation has very wide range, for example; several meter at Ho Chi Minh City, 20m at Long An, 70m at My Tho, 200m at Bac Lieu, 260m at Ca Mau (Thao, 2001b). (b) General surface geology Although there are some differences in the formation process, most of the Mekong Delta can be defined as young alluvial soil (Figure 3.1.4). As an exception, the area from An Giang Province to Kien Giang Province has some granite hills. In terms of fault, it could be roughly divided into the Northwest-Southeast fault system paralleling with two main rivers and the Northeast-Southwest fault system running from Ca Mau peninsula to Long An Province. Both Tien River and Hau River runs following the faults. In the case of Tien River, the fault is divided into two at Vinh Long. These two faults correspond to My Tho River (North) and Co Chien River (South) respectively, and pinch Ben Tre Province. Figure 3.1.4 General geology of the Mekong Delta 3.1.4 Soil The soils of Mekong delta are mainly constituted of deposited alluvial soil including sand, mud, gravel, and fine debris of shell in coastal area. Generally, surface soils are clayish and middle level in fertility. However, the existence of acid sulfate soil makes the soil status more complicated. The soil patterns in the Mekong Delta can be roughly divided into five groups (Ministry of Industry, 1998). • Alluvium group This soil group is composed of new alluvial soil brought by Tien River and Hau River. Therefore, the group is distributed within a limited area where the two main rivers can provide alluvium. The group covers an area of about 1,100,000 ha (28% of the whole delta). This area is suitable for rice cultivation. • Saline soil group This soil group is distributed from Tien Giang to Kien Giang Province along the coastal zone of the Mekong Delta. Because of the low landform, this area is extremely vulnerable to saline intrusion and is suffered from saline for 5 to 7 month in a year. Saline water can penetrate into base soil layer and come up to surface soil by capillary action in dry season. Under this condition, agricultural activities are still expected with some kinds of improved rice while most of wild plants are very limited salt-roof species. The group covers an area of 808,749 ha (21% of the Mekong Delta). • Acid-Sulfate soil group This soil group is widely shown in the Mekong Delta. The core area is in Dong Thap Muoi, Long Xuyen quadrangle, and Ca Mau Province. There are two types of acid sulfate soils, one is called potential acid sulfate soil (PAS) which has high potential to become actual acid sulfate soil (ASS) and the other is called ASS. Generally, this soil group area cannot enjoy enough fresh water and sediment from main river system. The group occupies an area of 1,590,000 ha (41%) consists of 1,080,236 ha PAS and 510,027 ha actual acid sulfate soil. • Sand group This soil group is distributed through the coastal line of Tien Giang, Ben Tre, Tra Vinh, Soc Trang, and Bac Lieu Province. During the making process, the Mekong Delta has been suffered marine accretions, and eventually, old coastal shores of each marine accretion epoch remained on the present Mekong Delta as sandbars. Therefore, the distribution parallels coastline of the Mekong Delta. Components in this group are mainly sand including shell fragment. • Peat land group This group concentrates around U Minh District in Kien Giang Province and is scattered around Dong Thap Muoi. Soil has high potential for sulfate. Leaf mold, which occupies 50% of total component, is piled up from 1.0 to 1.5m in depth. The sponge-like soil lacks of viscosity, however, wet organic mud can keep good moisture even in dry season. • The gray soil group on old alluvium This soil group is in northern part of the Mekong Delta, Dong Thap and Kien Giang Province. Mud and sand occupies about 70% of whole components. Although nutrients in such soil are poor, the soil fulfills conditions for growing rice or beans. • Mountain group This group is in northwest part of the Mekong Delta, Kien Giang and An Giang Province. Soils are originated from base rocks such as granite, sand stone, and mud stone. This land is poor in nutrient and vulnerable to erosion. Thus the soil distribution in the Mekong Delta is deeply associated with geological land formation process and saline intrusion. Figure3.1.5 is the soil distribution map of the Mekong Delta. The status of partition is very similar to topographic distribution, and this fact also supports the idea of the strong connections between soil and topography in the Mekong Delta. The soil status of the Mekong Delta reflects water characteristics of the area because soils are under influence of the combined actions of rivers and the sea. Figure 3.1.5 Soil distribution map of the Mekong Delta 3.1.5 Meteorology (a) Climate The Mekong delta region belongs to the tropical zone under the influence of Asian Monsoon. There are two clear seasons, the rainy season and dry season, which characterized by monsoon wind system. The southeast wind possessing high moisture is the major direction in the rainy season; on the contrary, relatively cold wind from northeast to southwest is dominant in the dry season. Due to the pressure pattern in the region, the Mekong Delta seldom suffered from Typhoon, however, there are some records such as Linda (1998). Figure 3.1.6 (a)-(c) shows typical pressure patterns in Southeast Asia region. In spring (or transition season from dry to rainy season), isobars are sparse and wind is calm. In summer (or rainy season), vicinity of Yunnan Province, China, is low pressure and isobars are denser than the case of spring. Finally, in autumn and winter, (or dry season), in land China is high pressure, therefore, the primary wind direction is reversed in the dry season. Thus the climate in this area strongly depends on monsoon wind system. Generally, the rainy season lasts 7 months from May to November and the dry season lasts 5 month from December to April. According to the W.P.Kưppen's climatic classification, the Mekong Delta region is defined as the Savannah that has clear rainy season and dry season. L: Low pressure H: High pressure →: Wind direction (a) Spring (or end of the dry season) (b) Summer (or the rainy season) (c) Autumn & Winter (or the dry season) Figure 3.1.6 Typical pressure patterns Revised from Viet Nam National ATLAS, 1996 (b) Rainfall The rainfall distribution in the Mekong Delta varies spatially and seasonally. Firstly, because most of the rainfall is given by Southwest monsoon in the rainy season, west side of the Mekong Delta, such as Kien Giang Province and Ca Mau Province, has the largest annual rainfall with over 2000mm (Figure 3.1.7). In the case of Hau River, lower segment has more rainfall with the value of over 1,800mm while upper segment, Chau Doc, records less than 1,400mm in a year. Meanwhile Tien River is a valley of annual rainfall in Mekong Delta, and most of the segment along the river shows less than 1,400mm annually. The lowest part of the delta such as Go Con Village in Tien Giang Province is an area which has the (a) (b) (c) least rainfall in South Vietnam (less than 1,200mm). The rainfall in the Mekong Delta is same as mean annual rainfall in all over Vietnam, 1960mm/year. In terms of annual rainfall, 2000mm is not an extreme value because some areas in the region of Monsoon Asia have more than 4,000mm. Figure 3.1.7 Annual rainfall in the Mekong Delta Revised from Groundwater of the Nam Bo Plain, 1998 Secondly, the rainfall in the Mekong Delta is uneven in a year. For instance, according to a specific waveform of rainfall at Can Tho City (Figure 3.1.8), the subtotal of rainfall from January to April is less than 10% of overall annual rainfall. The monthly maximum rainfall of 280mm and minimum of 2mm are recorded in October and February respectively. Thus the rain throughout the dry season is very rare case in the Mekong Delta, and approximately 80% of total annual rainfall is caused in the rainy season, from May to November. The annual rainfall distribution is common pattern in all over the Mekong Delta. Figure 3.1.8 Annual rainfall distribution at Can Tho City In the rainy season, there are severe storm mainly in the afternoon and night. The storm can bring heavy rain, much thunder and strong wind in a very short time, generally within an hour due to multi-cell type storm. The range of this air mass thunderstorm might be narrow and the phenomena could be defined as a Meso-scale convective system. Photo 3.1.1 is the form of cumulus and regional shower in the Mekong Delta. Photo 3.1.1 Rain clouds in Can Tho City 3.1.6 River Systems (a) Natural rivers The Mekong Delta has many branches originated from two main streams of Hau River (Bassac River) and Tien River (Mekong River). These streams have carved by tractive force of river flow. Figure 3.1.9 is the location map of major rivers and its estuaries in the Mekong Delta. The head word of each estuary, "Cua" means "door" so could be translated to "mouth" of the river. Although the two mouths, Cua Bat Xac and Cua Ba Lai, are closed due to long-term sedimentation, there are nine mouths in the Mekong Delta, and Vietnamese called the river as Cuu Long (Nine heads dragon) based on the form of river system. Figure 3.1.9 Major rivers and estuaries in Mekong Delta The Hau River meanders again and again from Phnom Penh to An Giang Province, Viet Nam, and the main flow follows a straight fault from Chau Doc to the South China Sea. At Cho Moi, between Chau Doc and Long Xuyen, Hau River is connected with Tien River through the Vam Nao Channel. The Tien River (Mekong River) also comes into the border of An Giang Province and Dong Thap Province from Cambodia territory. From the uppermost delta to My Tho City, the river shows bow-like shape due to a fault which is an imprint of geological joint movement. Near Vinh Long Village, the Tien River is divided into the Co Chien River and the Tien River (Photo 3.1.2). After that Tien River produces Ham Luong River and Ba Lai River which enclose Ben Tre Province like an island. And in the end, Tien River subdivided to Dai River and Tieu River which mean Big River and Small River respectively, and consequently flow into the South China Sea. Photo 3.1.2 Separated point of Co Chien River from Tien River at My Thuan Bridge The regular survey at each river in the Mekong Delta still hasn't been carried out. Figure 3.1.10 shows the image of average longitudinal riverbed shape along the two main streams of the Mekong Delta based on domestic material (Ministry of Industry, 1998). Tien River's inlet from Cambodia is about 240km upper from its mouth, and this uppermost segment has near 40m in maximum depth and the segment is also deepest through the whole Tien River. Tien River loses its depth at downstream section due to separation of branches. Hau River has about 200km length in the Mekong Delta. From the uppermost point, Châu Ðoc City to Long Xuyên City, the water depth changes Figure 3.1.10 Image of longitudinal riverbed many times from 5 to 30m and it turns shallow gradually. Near Can Tho City, 100km point far from the sea is the deepest section. Therefore, the most practical navigation route to international vessels is entering from the mouth of Hau River, passing through Vam Nao Channel to Tien River, and arriving at Phnom Penh City (Hori, 1996). (b) Canals The canal network in the Mekong Delta has been developed widely with the main purpose of land reclamation. It is very difficult to know exact total distance of the canals, but the length is longer than the main stream of the Mekong River from China to Viet Nam. For example, in Can Tho Province, the total distance of canals is about 4, 032 km in the provincial area of 2, 964 km2, i.e., the unit canal density in 1 km2 area is 1.8 km/km2 (Can Tho Province, 2001). Some canals are natural waterway, and others are artificial canal. These canals are concentrated in Long Xuyen quadrangle, Dong Thap Muoi, and Ca Mau peninsula (Sanh et. al, 1998). Figure 3.1.11 is the main canal network in the Mekong Delta. The functions of such canals in the Mekong Delta are mainly described as; 1) Drainage 2) Irrigation 3) Flash out degraded soil 4) Water transportation 5) Eco tourism Thus inland canals in the Mekong Delta have great significance to Agriculture, Fishery, Forestation, Infrastructure, Economy, and so on. However, there are some disadvantages of waterways such as growing mosquito, interception of land road, flood water and saline water intrusion, lack of water in the lower area, and etc. Figure 3.1.11 The main canal system in the Mekong Delta 3.1.7 Oceanography (a) Seabed and Currents The Mekong Delta fronts to the South China Sea, or the East Sea in Vietnamese, and Thailand Bay (Figure 3.1.12). The coastal sea of around the Mekong Delta is relatively shallow for a long distance from shoreline comparing to North Viet Nam. The area of under 20m in depth is extended to about 30km out from coastal line of Bac Lieu and Ca Mau Provinces. More than half of this area satisfies the definition of wetland by Ramsar Wetland Convention, at least 6m depth on the condition of ebb tide. Continental shelf, less than 200m depth, expands 200km out in maximum in the direction of south from the Mekong Delta and this area is most shallow coastal sea around the Indo-china peninsula. However, most of the areas are just the sea and the area of aquatic vegetation such as mangrove forests are very limited. There are some sea currents around the Mekong Delta. Most predominant one is a cold current from north to south. This current is related to the monsoon which has north wind ingredient in the dry season. Due to the current, the Cape of Ca Mau gains on the sea almost 100m every year. These currents originate from west coast of Philippine and down current from west coast of Taiwan isl. There is also a weak warm current from south to north which related to the monsoon, especially in rainy season. Figure 3.1.12 Inshore status around the Mekong Delta (b) Tide The South China Sea, where the Mekong River pours into, is well known with its very dynamic tidal activity, and the maximum tidal amplitude is about 3-4 m. In the case of Co Chien River in Tra Vinh Province, it is said that the coastal line moves about 4km during tidal cycle. This prominent tidal phenomenon causes change of river water level up to Cambodian territory through the rivers. On the other hand, west side of the Mekong Delta, Thailand Bay has relatively small tidal amplitude with under 1 m. Therefore water flux from the South China Sea can move to upper region easily through the many canals and rivers. Thus, the characteristics of tidal regimen around the Mekong Delta influences on inland fresh water system such as saline intrusion. In addition, the tide creates large tidal flats, particularly in the low-lying regions as the Mekong Delta (Tang, 2001) Figure 3.1.13 shows annual water level changes due to tidal movement at Phú An observatory in district 2, Hồ Chí Minh City, in 1986. This point shows typical tidal regime of the South China Sea, irregular semi-diurnal tide. Along the coastline from Ho Chi Minh City to the Cape of Ca Mau, flood tide and ebb tide occurs twice in a day generally. Can Gio Distict, a lower area of Phu An observatory, is the area where has largest tidal amplitude in Viet Nam, and its maximum range is 3.6-4.0m from September to January (Ho Chi Minh City, 2002). In the Figure 3.1.13, two approximate curves of high water level in red and low water level in blue are added. Each water level is lower in the rainy season from May to September and higher in the dry season in reverse. One more characteristic is that the amplitude of flood tide level is less than ebb tide's one. It means that high water level in flood tide is almost constant. Figure 3.1.13 Annual high tide and low tide succession at Ho Chi Minh City Figure 3.1.14 is a comparison of daily average water level change at Ho Chi Minh City and Ha Tien Town, typical coastal point of the South China Sea and Thailand Bay respectively. The plot at each month means average of daily water level differential between high water level and low water level. The daily water level differential at Phu An much larger than that one at Ha Tien, i.e. the differential in the South China Sea is three times larger than Thailand Bay. The maximum water level at Ha Tien was 1.7m while Phú An recorded 4.8m. Thus the sealevel is higher in southeast side of the Mekong Delta all year around. Figure 3.1.14 Mean daily water level difference at Ho Chi Minh City and Ha Tien Town 3.1.8 Water Resources The water resources in the Mekong Delta can be divided into two parts, surface water and groundwater. Due to uneven rainfall in the Mekong River Basin and geographical conditions, the water resources in the region is not stable seasonally and spatially. For example, at Crache in Cambodia, the flow coefficient [max flow in a year / min flow in a year] of the Mekong River is 53, i.e., the flow in the flood season is 53 times as much as the flow in the dry season. In addition, increasing of water demands in upper area and development of groundwater exploitation cause severe lack of water in coastal and remote areas. (a) Surface water Main source of surface water is river water coming from upstream Cambodia. However, the Mekong Delta of Viet Nam itself also generates runoff water. The catchment area in Viet Nam of 70, 520 km2, approximately 8.9% of whole the Mekong River Basin, provides 54.9 billion m3 runoff water according with 10.8% of total runoff water from the Mekong River (The World Bank,2002). The Cambodian boarder points, Tan Chau at Tien River and Chau Doc at Hau River, water flow from upstream Mekong River Basin is uneven. Before the two main stream connected by Vam Nao Pass at Cho Moi, Hau River occupies 80% of total flow from Cambodia and the remainder is provide through Hau River (Figure 3.1.15). Actually, some amount of flood water flows into Viet Nam not thorough the two main streams but through canals and paddy fields to Dong Thap Muoi low land area. After Vam Nao Pass, the water flow in Tien River and Hau River became nearly equal amount as well as water level. Figure 3.1.15 A model of water flow distribution at each tributaries (b) Groundwater Available groundwater volume in the Mekong Delta varies widely according to aquifer layers. Under the condition ._. F1 F_Ca Mau 1 None Strong Strong 0.5-1.0 F2 F_Ca Mau 2 None Middle Strong 0.3-0.5 F3 F_Lower delta None Strong Strong less than 0.3 F4 F_Lower Bassac None Middle Strong 0.3-0.5 F5 F_Middle delta Middle Middle Middle 0.5-1.0 F6 F_Upper delta Strong None None 0.5-1.0 F7 F_Top delta Strong None None more than 1.0 F8 F_Tien Giang Middle Middle Middle 0.3-0.5 F9 F_East edge Middle None Middle less than 0.3 * F: Floods F1 F2 F3 F4 F5 F6 F7 F8 F9 104 In the overlaid map of floods, the group of zones F1&F2 and F8&F9 are independent to main river system. Meanwhile, zones from F3 to F7 are divided vertically to the main river system because the characteristics follows upstream, midstream and downstream. Thus it can be said that the zoning due to flood is vertical zoning to the main stream. 4.5 Hydrology The six of thematic maps as below are overlaid: (1) Groundwater (Green line) (2) Canal system (Blue line) (3) Hydrological division (Black line) (4) Annual rainfall (Red line) (5) Erosion (Pink line) (6) Sea Canal system shows direction of main waterways, while others separate the area. Each hydrological phenomenon is in different stage; for example rainfall, groundwater, seawater, and so on, therefore it is a little difficult to overlook the status. Figure 4.5.1 The first overlaid map 105 Firstly, annual rainfall contour lines and groundwater distribution are simplified but the characteristics of rainfall distributions are still remained. Secondly, canal system is revised according to the waterway's direction. Ca Mau Peninsular is independent from main river system, while other waterways are mainly divided into three parts, namely, trans Bassac, the area between two main rivers, and Dong Thap Muoi area. Finally, strong tidal regime in the South China Sea is focused, and creates new belt-like zone along the coastline. Erosion characteristic are taken in other factors. Figure 4.5.2 The second overlaid map Some zoning such as groundwater distribution are unified again, and some duplicated lines are re-drawn by a single line. In this step, hydrological characteristics are described by parallel lines to main river system, while coastal area and middle of the delta are crossed by vertical lines of hydrological analysis. The final overlaid map of floods is described as Figure 4.5.4. According to the overlaid map, the Mekong Delta is divided into ten zones described as table 4.5.1. Figure 4.5.3 The third overlaid map 106 Figure 4.5.4 The final overlaid map of hydrology in the Mekong Delta Table 4.5.1 Outline for zones of hydrology No. Name Canal Rainfall (mm/y) Hydrology Tide Y1 Y_Coastline Independent wide range A' B' C' Strong Y2 Y_Ca Mau Independent more than 2000 A' Middle Y3 Y_Lower Bassac NE-SW 1800 A' Middle Y4 Y_Lower mainstream Connect 1600 B' Middle Y5 Y_Tien Giang West-East 1400 B' Middle Y6 Y_Lower DTM Independent 1400 C' Middle Y7 Y_Upper Bassac NE-SW 2000 A Middle Y8 Y_Upper mainstream Connect 1400 B None Y9 Y_Upper DTM1 West-East 1400 B None Y10 Y_Upper DTM2 Independent 1600 C None * Y: Hydrology Y1 Y2 Y3 Y4 Y5 Y6 Y7 Y8 Y9 Y10 107 In the overlaid map of hydrology, zone Y1, coastal area, is separated independently due to the strong influence of tidal regime. Meanwhile, other areas are basically zoned by drainage direction and distance from the main river system. Y8 and Y4, the area between two main streams of the Tien River and Hau River, are also defined as independent zones because of drainage characteristics. Thus, hydrological characteristics of limnology in the Mekong Delta are strongly controlled by drainage system, namely, natural and artificial canals. 4.6 Social conditions The three of thematic maps as below are overlaid: (1) Agriculture (Green line) (2) Road system (Blue line) (3) Population density (Red line) Road system shows connection of each national road, while other thematic maps separate the area. Population density has strong relation to both the road network and agricultural potential because these conditions are the most basic element to human life. Figure 4.6.1 The first overlaid map 108 Firstly, three main national roads including 1A is focused on in the road network, because these roads are main route to Ho Chi Minh City and two of roads are connected to Cambodia. Secondly, as a representative of human activities, agricultural patterns are summarized into rice crop pattern. The areas where farmers can harvest more than two crops in a year are picked up. Finally, some remote partitions of population density are omitted for simplicity. Figure 4.6.2 The second overlaid map Judged by the distance from Ho Chi Minh City along the National Road 1A, Ca Mau Peninsular is cut out from main deltaic area. Main river system area is reformed to take in other two national roads in it. Other zoning is mainly controlled by distribution of population density after some consideration of other elements such as crop patterns at Ben Tre Province. The final overlaid map of social conditions is described as Figure 4.6.4. According to the overlaid map, the Mekong Delta is divided into seven zones described as table 4.6.1. Figure 4.6.3 The third overlaid map 109 Figure 4.6.4 The final overlaid map of social conditions in the Mekong Delta Table 4.6.1 Outline for zones of social conditions No. Name Population Rice crop Road S1 S_Ca Mau Small No rice Remote S2 S_Coastline Small No rice Remote S3 S_Lower mainstream Middle Double crops Main road S4 S_Ha Tien Middle No rice Remote S5 S_Mainstream Large Double crops Main road S6 S_Lower DTM Middle Double crops Middle S7 S_Upper DTM Small Single crop Middle * S: Social conditions In the overlaid map of social conditions, zone S5, the area including main river system is the central zone. This area is the highest area of population and rice crops, therefore, concentration of investment, infrastructure, and other human activities have been occurred. Zone S3 also have same characteristics but the conditions for such development are worse than S5. Other five areas are separated geographically, especially at Ca Mau Peninsular and far Dong Thap Muoi area. S1 S2 S3 S4 S5 S6 S7 110 5. Conclusion Our most important materials of study, natural and social basic conditions of the Mekong Delta of Viet Nam, are gathered and arranged in four major topics of land conditions, floods, hydrology and social conditions in previous result section. These four basic zoning is defined as the first-level zoning maps as Figure 5.1.1. In this discussion section, these maps were developed to zoning maps to asses the risk for water-related disasters. Figure 5.1.1 The final overlaid map of social conditions in the Mekong Delta (a) Land conditions (b) Floods (c) Hydrology (d) Social conditions 111 Through the first-zoning of overlaying, two major tendency of zoning became noticeable, that is; horizontal zoning and vertical zoning around the direction of main river system. This result from the first-level zoning is very reasonable because the characteristics and risk of water-related disasters changes according to the location from upper stream to lower stream and the distance from the main river. Therefore, these horizontal and vertical zonings can be the basic partition to discus the risk assessment of the disasters in the Mekong Delta, and this step is defined as the second-level zoning. 5.1 The second-level zoning The most different point in the process of between the first-level zoning and the second-level zoning is that the former is geometric simple overlaying while the later is more logical-based rezoning. Naturally the geometric zones in the first-level zoning has great meanings based on basic conditions, however, through the discussion, that zonings are reunified again and are given much logical meanings. In addition, another significant role of the second-level zoning is simplification of the zoning maps to prepare for the final zoning map of risk assessment on water-related disaster. 5.1.1 Horizontal zoning Based upon the four overlaid maps in Figure 5.1.1, the Mekong Delta can be divided into five major zones as Figure 5.1.2 horizontally. The zone H1 has very different conditions and characteristics from other zones where main river system providing freshwater regularly. It can be said that Ca Mau Peninsular is independent from the Mekong Delta freshwater system basically. As Figure 5.1.2 Horizontal zoning of the Mekong H1 H2 H3 H4 H5 112 same as H1, the zone H5 is also independent from the Mekong Delta, and mainly belongs to the Vam Co River Basin. Geology and other conditions are different, too. The zone H2 is the outer zone which is located at the side of Hau River. The shape of the V2 is parallel with the straight river flow of the Hau River. Meanwhile, H4 is the outer zone located at the side of Tien River, therefore the shape also follows the shape of the Tien River that includes many meanders and hooks. Thus in the horizontal zoning, these two outer zones are back marsh of the main river system consists of the Tien River and the Hau River. The zone H3 is sandwiched between H2 and H4 and is the core zone of the Mekong Delta which contains most part of the main river system and major cities in the Mekong Delta such as Can Tho, My Tho and Vinh Long. This area is narrow in upper delta and wider in lower delta as development of tributaries. When the difference of surface water movement in the zone are closed up severely, the area sandwiched between two main streams, the Hau River and Tien River, should be separated from outer zones because the area plays a role of water delivery between the two main rivers. 5.1.2 Vertical zoning Because the vertical zoning follows river flow from upper area to the sea, the areas far from the mainstream does not have accurate meanings except the case of coastal zones. Figure 5.1.3 shows the vertical zoning of the Mekong Delta. The zone V1 corresponds to the coastline facing to the South China Sea. Although there are many different Figure 5.1.3 Vertical zoning of the Mekong V1 V2 V3 V4 V5 113 points in the zone, the most common and strong factor is existence of the sea with great tide. In the micro view, each river mouth should be intruded much deeper than the zone described in the figure because sea influences are diffused through the rivers. The second inner zone of V2 includes some important provincial capital as Tra Vinh, Soc Trang and Ca Mau. This belt-like zone is geographically disadvantageous area, especially in the west part, Ca Mau, Bac Lieu and Soc Trang. The flood form in this area is tidal flooding under the strong influence of tidal regime in the South China Sea. These H1 and H2 is the zone of coastal area. The zone V3 is the core belt of the Mekong Delta of Viet Nam in the case of vertical zoning. As same as the zone H3 in vertical zoning, V3 includes major big cities such as Can Tho, My Tho and Vinh Long. The area corresponds with the buffer zone of fresh riverine water and saline seawater, therefore, high water in this region depends on both floodwater from upper area and floodtide in the sea. The upper boundary of the zone V4 was mainly decided by Vam Nao waterway which connects the Tien River and Hau River in An Giang Province. The waterway provides floodwater from the Tien River to the Hau River, so that the water level becomes even in the lower part of the main two rivers. In this area, riverine flood is much noticeable than tidal flood, and most amount of the flood water is drained or tanked in this area including Dong Thap Muoi, or Plain of reeds. The uppermost zone of V5 borders on Cambodia on the north edge. The area is the most flood-prone area due to riverine flood water from upper stream, especially in east and middle zone. Both flood duration in a year and the degree of damage due to annual flood in V5 are outstanding in other lower parts of the Mekong Delta. 114 5.1.3 Integrated grid zoning As above discussion, the Mekong Delta was firstly zoned by horizontal and vertical partition. Each solo zoning has less meanings because of its too wide range, for example in the case of horizontal zoning, a zone has coast and inner border to Cambodia. Therefore, to complete the second-level zoning, integration of the horizontal and vertical zoning is required. Figure 5.1.4 shows integrated grid zoning based on the horizontal and vertical zoning. Figure 5.1.4 Horizontal zoning of the Mekong Delta The integrated grid map is not a simple overlaid map of the horizontal and vertical zoning map but the grid lines based on those two maps. Some grids in Long An Province are cut for simplification and a extra line is added between the Hau River and the Co Chien River at lower part of the Mekong Delta for intimate partitions. The zones from G1 to G20 in the grid map become basic structure of last zoning map in the next section. Table 5.1.1 shows the relationships between each gritted zone and previous zonings. G1 G2 G3 G4 G5 G6 G7 G8 G20 G10 G11 G12 G13 G14 G15 G16 G17 G18 G19 G9 115 Table 5.1.1 Outline for zones of social conditions No. Horizontal Vertical Land condition Floods Hydrology Social condition G1 H1 V1 L1 F1 Y1 S1 G2 H2 V1 L1 F3 Y1 S2 G3 H3 V1 L1 F3 Y1 S2 G4 H4 V1 L1 F3 Y1 S2 G5 H1 V2 L3 F2 Y2 S1 G6 H2 V2 L2 F3 Y3 S3 G7 H3 V2 L2 F3 Y4 S3 G8 H4 V2 L2 F3 Y4 S3 G9 H4 V2 L2 F8 Y6 S5 G10 H1 V3 L3 F2 Y2 S1 G11 H2 V3 L4 F4 Y3 S3 G12 H3 V3 L6 F5 Y4 S5 G13 H4 V3 L8 F5 Y5 S5 G14 H5 V3 L10 F5 Y6 S6 G15 H2 V4 L5 F4 Y7 S3 G16 H3 V4 L8 F6 Y8 S5 G17 H4 V4 L11 F6 Y10 S7 G18 H2 V5 L5 F4 Y7 S4 G19 H3 V5 L9 F7 Y9 S5 G20 H4 V5 L12 F7 Y10 S7 Underlined bold values in the table means four values in column of "Land condition", "Floods", "Hydrology" and "Social conditions" are equal to other zones. That is, G2, G3 and G4 are very same zones on the point of basic conditions, as well as G7 and G8 / G5 and G10. These zones should be unified in the next step when discuss the risk of water-related disasters based on zoning maps. 116 5.2 The final zoning 5.2.1 Basic grid map The final zoning maps were set by discussion on multi-factors between the first-level map and the second-level map. Firstly, the grid map at Figure 5.1.4 was revised by unifying of some grids mentioned before, and the revised grid map is defined basic grid map based on second-level zoning for every kind of risk assessment on the final zoning map. Figure 5.2.1 is the basic grid map of the Mekong Delta of Viet Nam. Figure 5.2.1 The basic grid map of the Mekong Delta The revised points are in the coastal areas. Coastline facing to the South China Sea is divided into two parts from four parts in the previous map, because only the coastline at Ca Mau area has different characteristics in floods and social conditions. The zone G3 and G5 in the above map is also unified due to similarity of old zoning. G1 G2 G3 G4 G5 G6 G7 G8 G9 G10 G11 G12 G13 G14 G15 G16 117 As a result, the basic grid map for discussing on final zoning maps is composed of sixteen zones. Upon the basic grid map, four first-zoning maps are re-overlaid to examine the distribution and characteristics again. 5.2.2 Land conditions The most important factor of land conditions in the Mekong Delta is the border of saline soil and alluvial soil in terms of water-related disasters in the Mekong Delta. Acid-sulfate soil condition in the Mekong Delta is also an important issue, however, in the case of studying water-related disaster, it is not a dominant condition directly. In this point, the coastline and the blued coastal area is the saline soil area which corresponds to zone G1, G2, G4, G5, and G6. G3 is the area of acid-sulfate soil and saline soil is not so predominant because tidal regime in Thailand Bay is not as strong as the South China Sea. Another important factor on the land condition is elevation. Due to its inclination, contour lines on the Mekong Delta were lined following the direction of NE – SW and NW – SE. Zone G14 is the exception because of its unordinary hilly landform in the flat delta region. In terms of landform, the basic gird can describe the difference of open flood plain, or Long Xuyen Quadrangle, and closed flood plain, Dong Thap Muoi, respectively. Lastly, fresh alluvial area along the main stream is also separated by the basic grid, therefore, the grid is basically effective for land conditions in the Mekong Delta. 118 Figure 5.2.2 Land conditions on the basic gird 5.2.3 Floods The most characteristics zoning on the floods issue in the Mekong Delta is the difference of flood forms, especially riverine flood and tidal flood. The border of these two different types of floods is at the border between the zone G8 and G9m or a dotted thick line. The important border is not stable and the location is variable due to season, therefore, G7, G8 and G9 is valid zones to describe buffer zone which includes the border of riverine flood and tidal flood. The coastal zones can be integrated to the tidal flood area at both the South China Sea and Thailand Bay. These areas were separated with the shape of a big fan from the border of Ho Chi Minh City to Ha Tien Town near the border between Cambodia and Viet Nam. This shape is very similar to soil distributions of the G1 G2 G3 G4 G5 G6 G7 G8 G9 G10 G11 G12 G13 G14 G15 G16 119 Mekong Delta. Other grid also can describe the characteristics on floods in the Mekong Delta basically. However, there are some lines which should be reformed as two dotted thick lines located at upper delta. Duration and degree of riverine flood decrease as distance from the axis of main river system, therefore the lines become as semi-circle. Figure 5.2.3 Floods on the basic gird 5.2.4 Hydrology The zoning on hydrology in the Mekong Delta is very geometric and similar to the basic grid. The basic grid partitions off the delta more intimate, so the grid map can describe hydrological characteristics completely. A dotted line in Dong Thap G1 G2 G3 G4 G5 G6 G7 G8 G9 G10 G11 G12 G13 G14 G15 G16 120 Muoi area is the only a lack of zoning in the basic grid map. Figure 5.2.4 Hydrology on the basic gird 5.2.5 Social conditions All the elements associated to social aspects finally express human activities in the area, therefore, population density is the strongest index to rank the conditions. Other factors as agricultural potential and infrastructure follows the population density. The most active area of human activities is started from Cambodian border to the middle delta along the Tien River and Hau River, then change the direction to Ho Chi Minh City following the national road No.1A. Outer zones follow the fan-like shape from the Northwest to the East. As the case of hydrology, one additional dotted line in Dong Thap Muoi area was suggested to clear the border. G1 G2 G3 G4 G5 G6 G7 G8 G9 G10 G11 G12 G13 G14 G15 G16 121 Figure 5.2.5 Social conditions on the basic gird 5.2.6 Map for the risk of water-related disaster and evaluation After the calibration by four topics of land conditions, floods, hydrology and social conditions, the basic grid finally became the risk assessment map of water-related disasters in the Mekong Delta. In this final map, the Mekong Delta is divided into 20 zones described as Figure 5.2.6. Based on the basic grid map, the final zoning map was revised by four themes in the first-zoning, therefore, lines separating the delta became smooth shape from straight line in the basic grid map. Z1 and Z2 compose the coastline facing to the South China Sea. Z3 occupies almost all the part of Ca Mau Peninsular. The upper line of Z4 and Z5 were lifted up to up-stream side but the basic zoning was not changed. Z5 occupies pretty G1 G2 G3 G4 G5 G6 G7 G8 G9 G10 G11 G12 G13 G14 G15 G16 122 large area of the lower Mekong Delta. Z6 is the mouth of Vam Co river system with some part of left bank of the Tien River including My Tho City. Figure 5.2.6 The final zoning map for risk assessment of water-related disasters Z7 and Z8 covers lower trans Bassac area including Can Tho Province, or Can Tho City and Hau Giang Province since 2004. Z9 is the new zoning in this final step located between the Tien River and Hau River including Vinh Long Town. Z10 is almost in Tien Giang Province. Z11 is located at lower Dong Thap Muoi, or Plain of reeds. Z12 lies along the coastline to the Thailand Bay from Ha Tien to Rach Gia. Z13 plays a role of central Long Xuyen Quadrangle. Z14 is a zone which connecting the Tien River and Hau River after Vam Nao waterway at Cho Moi. Z15, Z16 and Z20 occupies main area of Dong Thap Muoi lowland. Z17 is a unique hilly area including some low mountains. Z18 and Z19 is the most Z1 Z2 Z3 Z4 Z5 Z6 Z7 Z8 Z9 Z10 Z11 Z12 Z13 Z14 Z15 Z16 Z17 Z18 Z19 Z20 123 flood-prone area including Chau Doc town. Above is the outline of each zone in the final map. To make the characteristics clearer, matrix on each zone with different factors was discussed as Table 5.2.1. 124 Table 5.2.1 Outline of Zones in the final zoning map No. Name Land condition Flood form Flood degree Hydrology Social condition Landmarks Z1 Mui Ca Mau Saline Tidal High Maritime Inactive Z2 Coastline Saline Tidal High Maritime Inactive Z3 Ca Mau Peninsula Saline Tidal High Isolated Inactive Ca Mau Z4 Soc Trang Saline Tidal Low River system Middle Z5 Lower Delta Saline Tidal Low River system Middle Ben Tre, Tra Vinh Z6 Lower Vam Co Saline Tidal Middle River system Active Tan An Z7 Long My Alluvial Buffer High Isolated Middle Z8 Can Tho Alluvial Buffer Middle River system Active Z9 Vinh Long Alluvial Buffer Middle River system Active Z10 My Thuan Alluvial Buffer Middle River system Active Z11 Vam Co Basin Alluvial Riverine Middle Vam Co River Middle Z12 Thailand Bay Alluvial Buffer Low Maritime Middle Ha Tien, Rach Gia Z13 Long Xuyen Alluvial Riverine High Hau River Active Z14 Cho Moi Alluvial Riverine High River system Active Z15 Lower DTM Alluvial Riverine High River system Middle Z16 Cambodian boder Old alluvial Riverine High Isolated Inactive Z17 Hill Granite rock Flush Low Isolated Inactive Z18 Chau Doc Alluvial Riverine High River system Active Z19 Upper delta Alluvial Riverine High River system Active Z20 Upper DTM Alluvial Riverine High River system Active 125 Main purpose of the final evaluation for risk of water-related disasters in the Mekong Delta is qualitative rather than quantitative, although some zones are classified clearly by the degree of disaster or damages. In this final evaluation, all the elements in the Table 5.2.1 is reviewed to make a risk map finally. Among the factors, actually land condition and flood forms express a just identical result in terms of zoning, therefore, flood forms, flood degree, hydrology and social conditions are considered indeed. For example, the figure 5.2.7 shows the pattern of flood forms in the Mekong Delta following final zoning. As the figure shows, general saying, "flood" in the Mekong Delta must be identified according to basic natural conditions. The tidal flood zone is not only the zones where the main cause of flood is tide but also the area of other factors such as saline soil. Thus each zone based on much natural and social condition studied in the previous part, therefore, this final zoning can describe any difference of risk factors or composition of factors as minimum assessment cells. 126 Figure 5.2.7 Type of flood in the Mekong Delta 5.3 Conclusion and challenges The final zoning map and its matrix, Figure 5.2.6 and Table 5.2.1, display the result of this study briefly. Each zone is very simple in the final zoning, however, background of the way of zoning supports the result logically in each step. Firstly, full basic natural conditions and some parts of social conditions are collected to draw thematic maps of the Mekong Delta with an aim of classifying water-related natural disasters. Such basic information in the Mekong Delta is very precious and the meaning of arranging such conditions is as important as the result of this study, because the raw data is effective to all kinds of research and project in the future, especially when foreigners discuss with local counter parts in Viet Nam. Z1 Z2 Z3 Z4 Z5 Z6 Z7 Z8 Z9 Z10 Z11 Z12 Z13 Z14 Z15 Z16 Z17 Z18 Z19 Z20 127 Secondly, examination of floods form in the Mekong Delta gave significant zoning which is supported by other natural conditions such as tidal regime and soil status. The classification of floods declared three areas of riverine flood area, tidal flood area and buffer area. Such rough classification is very meaningful because the flood maps on the Mekong Delta generally have not made a thing of the origin of floods. Thirdly, hydrological analysis in some flood years of the Mekong River Basin provided the correlation among different areas in the Mekong Delta. The correlations describe complicated surface water movement, and some of results from the statistic analysis support the difference of basic natural conditions in the Mekong Delta. And finally, overlaying of thematic maps and the basic grid maps led the study into the final zoning as above. The study on water-related disasters in the Mekong Delta is very huge and complicated matter as previous studies. Therefore, the basic information in this study and background of final zoning is not enough to describe the risk of water-related disaster. To compensate this week point, GIS method would be favorable, however, such digital technology overlook the quality of data source in general. Mass-disposal in digital technology strictly needs careful choice and use of original data, and on the viewpoint, each method in the study would be helpful to understand the conditions in the Mekong Delta in terms of water-related natural disasters. We really hope further studies on the basic natural condition in the Mekong Delta to provide correct information to other applied researches and difficult development project in the area. 128 References Tiếng Việt (Vietnamese materials) 1. Bộ cơng nghiệp. 1998. Nước dưới đất Đồng Bằng Nam Bộ. Hà Nội 2. Buu Ngon. 2001. Đất phương nam. Nhà xuất bản trẻ. TP. HCM 3. Nguyễn Ngoc Son. 2003. Du Lịch Đồng Bằng Sơng Cửu Long. Nhà xuất bản mũi Cà Mau 4. Le Ba Thao. 2001a. Thiên nhiên Việt Nam. Nhà xuất bản giáo dục. Hà Nội 5. Le Ba Thao. 2001b. Lãnh thổ và địa lý Việt Nam. Nhà xuất bản thế giới. Hà Nội 6. Viet Nam. 1996. Viet Nam National ATLAS. Hà Nội 7. Tỉnh Cần Thơ. 2001. Địa lý Cần Thơ. Sở giáo dục và đào tạo. Tỉnh Cần Thơ Tiếng Anh (English materials) 1. Nguyen Huu Chiem. 1993. Geo-Pedological Study of the Mekong Delta. Southeast Asian Studies, Vol.31, No.2, September 1993. Kyoto University, Japan 2. Nguyen Huu Chiem 1994. Former and present cropping patterns in the Mekong Delta. Southeast Asian Studies, Vol.31, No.4, March 1994. Kyoto University, Japan 3. Pham The Hien, Nguyen Ngoc Hoa, Le Van Lon. 1991. On the Paleography of 129 the periods of development of Quaternary deposits in Nam Bo Plain. Geography Geology Environments, No.1, June 1991. Geological - Geographical Association of Ho Chi Minh City 4. Ho Chi Minh City. 2002. Can Gio Mangrove Biosphere Reserve. Agriculture Publishing House 5. Hori Hiroshi. 1996. Mekong Gawa (THE MEKONG): The Development and Its Environmental Effects p.23. KOKON-SHOIN Publishing. Japan 6. MRC [Mekong River Commission]. 2002. Annual Report 2002. Phnom Penh 7. MRC [Mekong River Commission]. 2003. State of the Basin Report 2003. Phnom Penh 8. MRC [Mekong River Commission]. 2002. The story of Mekong Cooperation. Phnom Penh 9. Nguyen Dinh Phuoc. 1992. Some problems on the flood in 1991 in the Mekong River Delta. Geography-Geology-Environments No.3 June 1992. Ho Chi Minh City 10. Nguyen Van Sanh, Vo Tong Xuan, Tran An Phong. 1998. History and future of farming systems in the Mekong Delta. Development of Farming Systems in the Mekong Delta of Vietnam p.19. JIRCAS, CTU, CLRRI. Ho Chi Minh City Publishing House 11. Vu Trung Tang. 2001. The Eastern Sea: Resources and Environment p.31. World Publishing House, Ha Noi 12. Nguyen Van Tho. 1991. Special forms of the Mekong River bank-slide on 130 Holocene sediment bed and method of bank protection. Geography-Geology-Environments No.1 June 1991, Ho Chi Minh City 13. Le Quang Tri, Vo Tong Xuan. 1991. Farmer's experiences in using acid sulphate and saline soils in the Mekong Delta (Mekong Delta Master Plan VIE/87/031). Vietnam, SPC, WB, MRC, UNDP 14. Vietnamese Government. 1988. Feasibility study for Cantho Bridge Project – Annex, 1988, Hanoi 15. The World Bank. 2002. Vietnam Environment Monitor 2002. Ha Noi 16. Yamashita Akira, Nguyen Huu Chiem. 2004. The zoning of the Mekong Delta based on the natural conditions. Summary of 5th International Conference of Asian Marine Geology ._.

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