Summary of geography doctoral thesis: Studying the spatio-temporal variation of Tien River channel belongs to Dong Thap province for natural disaster adaptive capacity

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Research objectives: Assessing manifestation, finding out reasons, and warning risk of changes in Tien river channel belongs to Dong Thap province; proposing solutions for adaptive capacity with changes in Tien river channel belongs to the study area.

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Nội dung Text: Summary of geography doctoral thesis: Studying the spatio-temporal variation of Tien River channel belongs to Dong Thap province for natural disaster adaptive capacity

MINISTRY OF EDUCATION VIETNAM ACADEMY OF AND TRAINING SCIENCE AND TECHNOLOGY GRADUATE UNIVERSITY SCIENCE AND TECHNOLOGY ……..….***………… TRINH PHI HOANH STUDYING THE SPATIO-TEMPORAL VARIATION OF TIEN RIVER CHANNEL BELONGS TO DONG THAP PROVINCE FOR NATURAL DISASTER ADAPTIVE CAPACITY Major: Natural Resources and Environmental Geography Code: 62 44 02 19 SUMMARY OF GEOGRAPHY DOCTORAL THESIS Ho Chi Minh City - 2017
The study completed at Graduate University of Science and Technology, Vietnam Academy of Science and Technology Supervisor 1: Assoc.Prof. Dr. Nguyen Tham Supervisor 2: Dr. Vu Thi Thu Lan Reviewer 1: Reviewer 2: Reviewer 3: The doctoral thessis will be presented by candidate and commented by the Assessment Council at Graduate University of Science and Technology, Vietnam Academy of Science and Technology . Time: Date…. month …. 2018 Readers con find the thesis at: - The library of Ho Chi Minh City Institute of Resources Geography; - The library of Graduate University of Science and Technology; - National Library of Vietnam.
1 INTRODUCTION 1. THE PROBLEM STATEMENT Mekong River is the longest river in Southeast Asia and is ranked as the 12th longest river over the world. Therefore, it plays a crucial role in the socio - economic development of countries into this area. Tien River is located in the downstream of Mekong River Basin which passes through the upper Mekong River Delta and a part of the river which belongs to Dong Thap province plays vitally important role for developing of socio - economic activities in the province. However, the variation of river channel, includes erosion and deposition in recent years, has considerably happened that is not only frequency, but also space. Against those milieu, there have been a myriad of studies and projects published to implemented on limit the fluctuations, especially in erosion. In fact, some measures have been tried and initially successful in a certain scale but they used to apply an independently, while hydrological modeling methods require a large number of input data. Hence, solutions for the change of Tien River channel have not still enough authentication. It is really necessary to assessing the changing process as well as identifying the agents which cause for them based on the background of general geography. These documents provide a scientific knowledge for both notifying and proposing the measurements to cope with the change of Tien river channel flowing through Dong Thap province. These challenges are so hard to overcome but they are in emergency, which need to solve as soon as possible to prevent the negative effects of Tien river channel changes. Altogether, the study has a potential meaning of scientific research and in practical life. On the other hand, identifying the causes and process of Tien river channel changes can help to apply to conduct research in other river zones belonged to Mekong river system and other rivers that has the same geographic features. Hence, the author/candidate would choose the topic “Studying the spatio-temporal variation of Tien River channel belongs to Dong Thap province for natural disaster adaptive capacity” as do research on doctoral thesis of Geography.
2 2. OBJECTIVIES AND MISSIONS OF THESIS 2.1. Research objectives (i) Assessing manifestation, finding out reasons, and warning risk of changes in Tien river channel belongs to Dong Thap province (ii) Proposing solutions for adaptive capacity with changes in Tien river channel belongs to the study area. 2.2. Research tasks To accomplish the goals on top, the doctoral student has to do the mentioned duties below: - Collecting, analyzing, synthesizing related documents to the topic. - Studying literature review based on generally geographic standpoint/ point of view. - Assessing manifestation, developing, and main agents which cause for changing of Tien River channel. - Filtering optimal approaches for proposing adaptive capacity on risk of variation in the study area, includes: preventative solutions, evasive remedies, and protective solutions. 3. SCOPE OF RESEARCH 3.1. Space: A part of Tien River that flows through Dong Thap province by administrative border including the opposite channel’s banks that is owned by An Giang province. 3.2. Time: Assessing the development of Tien River channel for 100 years; of which, the thesis only concentrates during period of 1965 – 2016 and warns about trends in the future. 3.3. Contents: In this dissertation, Tien river’s channel changes are analyzed in both of planar section and cross section. In particular, the doctoral student focus on calculating Tien River channel changes for planar section. The variations in the shore is caused by developing of the river bed in highly influential. 4. DATABASE FOR THESIS This thesis is completed based on many reliable resources. Meteorological and hydrological data, muddy and sandy data are provided by Mekong River Commission (MRC) [10-16] and Southern Regional Hydro-Meteorological Center [17]. Manifestation of river’s channel change is collected by using the result of annual reports from Departments of Agriculture and Rural
3 Development of Dong Thap province and the results of other published studies [9]. Main content of the doctoral thesis has been developed continuously from my master thesis and several projects [18-20]. 5. THESIS STAMENTS 1. The channel of Tien River which flows through Dong Thap province is undertaking on the mechanism of braided river, fluid flow, erosion and accretion often occur along the cross section and along the river. 2. Activities of dwellers have impacted on developing spatio- temporal variation of Tien River belongs to Dong Thap province. 3. It is predicted based on dynamic morphology and testing hypotheses via general geography approach. 6. THE INNOVATION OF THESIS 1. Found out features and reasons which create variations of the channel by applying historical and random approaches. 2. Analyzed in some respect the directly consequences of residents on the Tien River channel process in Dong Thap province. 3. Warning and recommending solutions for reducing effects on socio-economic development in the study area based on synthetic geography approach. It is also scientific foundation for proposing to natural disaster adaptive capacity. 7. SCIENTIFIC AND AUTHENTIC OF THE THESIS 7.1. Scientific significance The results of the thesis is able to contribute to supplement and perfective methodology for river channel’s changes in the upper of tributary and braided river type. This document can be used as reference material for the both studies and teaching. 7.2. Practical implication The research provides evidence for administrative managers in Dong Thap province and they base on this document can plan and propose for reducing as well as adaptation the natural disaster impacts (river channel’s changes). All of them lead to sustainable development and stability. 8. THE STRUCTURE OF THESIS The thesis presented in 148 pages A4, includes: 43 tables, 88 figures and maps, 145 documents, and appendices. Besides, the
4 introduction (5 pages) and conclusion (3 pages), the main contents are structured into three chapters: Chapter 1. Literature review in 46 pages A4 including 7 tables, 15 figures and maps; Chapter 2. Assessing the variation Tien River channel belongs to Dong Thap province in 67 pages A4 including 20 tables, 65 figures and maps; Chapter 3. Warning and proposing for responsive solutions to adaptive with variation Tien River channel belongs to Dong Thap province in 27 pages A4 including 16 tables, 7 figures and maps. Chapter 1. LITERATURE REVIEW OF THE RESEARCH 1.1. DEFINATIONS 1.1.1 River channel There is a difference in several terms relating to components within river basin (figure 1.1) [3], includes: A subterranean stream, also known as the thalweg river, is the line defining the lowest points along the length of a river bed or valley. River bottom (river bed) is a channel occupied (or formerly occupied) by a river. The river channel is the route course (between bed and banks) that a river flows. The flow of the river is often described as channel flow. Riverbank is the land along the edge of a river. In this study, the term of riverbank is included riverbanks and mudflats (alc, river bar, and island). River bar is an elevated region of sediment (such as sand or gravel) that has been deposited by the flow. The morphology river is the study of the channel pattern and the channel geometry at several points along a river channel, including the network of tributaries within the drainage basin. Also known as channel morphology; fluviomorphology; stream morphology. The relationship between the river bed and the flow velocity, combining with erosion and decomposition is formed the dynamic morphology. The channel change is a changing process of river bed and structure of the river over time due to the influence of water currents, climatic factors, tectonic factors and economic activities of local residences. According to Brice I.C. Braided rivers are characterized by having a number of alluvial channels with bars and islands between meeting
5 and dividing again, and presenting from the air the intertwining effect of a braid. [26, pp. 47]. Braided rivers have different types of classification. The braided areas often have distinct characteristics in comparison to other types of rivers in terms of hydrology and river bed morphology. There are two basic conditions that made up the formation of the braided river: its river is relatively wide enough for the river bar develop completely; Differences between in the position of the thalweg between flood and dry season. The basic characteristic of the braided river is strong current, thus, the change of river channel takes place frequently (the unstable branches), leading to accumulate of the abundant sediment and the raw material. The braided river usually grows on one side. 1.1.2. Natural Disater According to Point 1, Article 3 of the Law on Natural Disaster Prevention and Control [2], “A natural disaster is a major adverse event resulting from natural processes of the Earth; examples include floods, hurricanes, tornadoes, volcanic eruptions, earthquakes, tsunamis, and other geologic processes. A natural disaster can cause loss of life or property damage, and typically leaves some economic damage in its wake, the severity of which depends on the affected population's resilience, or ability to recover and also on the infrastructure available”. Channel change, especially river bank erosion, are considered as a type of natural disaster that needs to study for finding solutions. 1.2. The history of doing researches relating to river process 1.2.1. In the world 1.2.1.1. The studies approach to channel change Based on analyzing the results of researches on the river channel change, in my opinion, there were a variety of methods and approaches on this stage. Overall, there are three specific approaches: conditions of river morphology, flow dynamic, and integrated approach. 1.2.1.2. Researching on braided river Study of braided river was suggested firstly by Peale A.C., US Geological Survey (1879) with studies in Western Wyoming - tributary of the Green River. Studies of braided river concentrated on a number of countries which they continued to develop studies relating to riverine dynamics in previous period (Western Europe,
6 North America) or the regions where owned many authentic problems with large tributaries (China, India ...). Nowadays, there are three field-researches for doing projects on braided river: (i) hydropower-driven of the river section for projecting variations of braided river; (ii) based on morphological factors to access and predict divergent river morphology, and field of braided river by combination both (i) and (ii). 1.2.2. In Vietnam 1.2.2.1. Channel change Researches on field of river channel changes by Vietnamese scientists also focused on three primary approaches. Of which, the area around morphology of river channel has been interested scientists from the Institute of Geology, Institute of Geography of Vietnamese Academy of Science and Technology and scientist from Faculty of Geography, University of Science - Hanoi National University. The area around flow conditions has been cared by scientists in the field of hydrology such as Institute of Water Resources Research, Hanoi University of Water Resources, University of Civil Engineering,… Scientists from Institute of Geography, Vietnamese Academy of Science and Technology have been interested in the integrated study. 1.2.2.2. Braided river It is initially researched in large river systems, such as: the Red River Delta and the Mekong River Delta. 1.2.3. In Mekong River Delta and particularly Dong Thap province In recent years, the change of river banks, especially riverbank erosion, have been happening quite commonly in the Mekong Delta and have received the attention of many different scientists. Many studies would assess the erosion of the Mekong River Basin in general and Tien River in particular, as well as the application of remote sensing data to assess river bank erosion; all of them generally found out several methods of coastal erosion which enable predict to several major areas, such as river bank in Sa Dec city and in Hong Ngu town. It can be mentioned on several scientists, such as Le Manh Hung, Dinh Cong San [62-65], Hoang Van Huan [36], Truong Thi Nhan, Nguyen Anh Tien [78], To Quang Toan [91-97] ...
7 Regarding the braided river is typical forms of river in the Mekong Delta. However, the number of studies which focused significantly on this issues still in limitation. Le Ngoc Bich - a scientist from the Southern Institute of Water Resources Research have several projects on morphological morphology [9]. Furthermore, scientists such as Luong Phuong Hau, Le Manh Hung, Dinh Cong San, Hoang Van Huan and so on with their scientific topics and projects have focused on analyzing the real situation, causes and warning of landslides of river bank in main areas based on mathematical models, physical models and experience formulas based on flow dynamics and conduction morphology. In recent decades, it is necessary to mention on the PhD thesis of Tran Ba Hoang [27] and Ho Viet Cuong [30] and Nguyen Nghia Hung [112]. Tran Ba Hoang. They discovered an overview of the braided river in the Mekong Delta, formed morphological relationships with the distribution of tributaries that offered for the science of applying flow correction equations and distribution rate for Tan Chau - Hong Ngu branch on Tien river. Ho Viet Cuong constructed relationship among flow diversion, sediment separation ratio with hydraulic elements, and river morphology. He also found out the fluctuations of the hydraulic regime caused by sand dredging operations via using mathematical and physical models. Moreover, the results of project applied for the branched area on Hau river pass through the Long Xuyen city (An Giang province). 1.2.4. Evaluation of contributions and gap on researches The results of researches on the river bank in general and riverbank changes in particular showed several accomplishments: - Methods: It showed via analyzing material of measurement; modelling; and application of remote Sensing and GIS. - Regarding the trend of studying the change of river bank: it concentrated on three primary approaches, such as: conditions for river formation; flow formation and synthetic area. - The results of the study focused on assessing the progress of the channel change (mainly erosion), analyzing the causes (mainly natural causes), influenced factors on the variation, and then prediction for the trend of landslide by various methods and particularly modelling.
8 - Proposing solutions for reduce of change as well as damaging consequences caused by riverbank fluctuations in the study area; particularly, focusing on constructing solutions (embankments, soldering iron ...). Until now, besides the actual measurement methods used to identify channel change, studies have focused on evaluating the evolution of individual, sectoral methods. Hence, their results of research are still limited around area; for instance, the erosion and accretion can be prevented in this area but the neighbor areas will be eroded. On the other hand, because of the complicated channel change, especially in international basin all over the world, the problem of evaluating channel change is still considered a part of the world, not thoroughly resolved. In addition, the impacts of local and river basin exploitation (eg watershed destruction, hydropower construction, irrigation, sand mining, etc.) have not been studied. Based on a comprehensive analysis of river basin studies in general, the main content of the dissertation will be considered Tien river basin as the major research subject. Therefore, the thesis will focus on analyzing and clarifying specific issues: - Assessing channel change (vertical, horizontal) of Tien River in Dong Thap province in the period of 1965 - 2015. Of which, concentrate on complicated areas. - Analyzing the causes, agents which impact on channel process, and the relationship among them on the basis of a synthetic and random approach. In particular, focus on the impact of human activities on the basin and the local area at different times. - Warning on the trend of changing channel river based on the morphological analysis of Mekong River's navigation dynamics according to the integrated geographical approach. - Applying the results for assessing, identifying the causes and warning trends to proposing solutions for deal with change of Tien river channel, particularly, focusing on constructing solutions (adaptation and reduce). 1.3. APPROACHES AND METHODS 1.3.1. Approaches: In the process of performing the tasks of research, the author has selected 3 approaches: synthesis geography, history, and random.
9 1.3.2. Perspective: During the study, the author based on 3 viewpoints to conduct research tasks: synthetic, system, landscape, and sustainable development 1.3.3. Methods: Inherent of literature review, field trip, analysis, synthesis, map and geographic information system, and make interview experts 1.3.4. Steps: Research process of change river channel consists of 4 steps in Figure 1.10 1.4. OVERVIEW THE STUDY AREA 1.4.1. Natural, socio-economic conditions of Mekong river basin The Mekong River is one of the major rivers in the world, with a variety of geological features, geomorphology and natural geography. There were about 65 million persons, who are living in the basin (2015). Of which, there were about 29.6 million people living along the river (about 15 km from the river bank). The average economic growth rate in the basin was 4.3 - 7.0% per year. The main economic activity is agriculture, forestry and fisheries, of which forests play an important role in water resources management and sustainable development. In recent decades, the exploitation of the catchment area has been increased, especially in agricultural, hydropower and irrigation. Basin exploitation activities under the MRC Strategic Environmental M & E Strategic Assessment conducted by the International Center for Environmental Management (ICEM) for the Mekong River Commission in 2010 assessed Vietnam as “having Greatest possible economic loss” due to the potential impact, if mainstream dams were built. The negative results that Vietnam can suffer cannot be calculated, but several consequences can be forecasted including [90, pp. 82]: (i) Reducing the flow in dry season, coupling with the effects of climate change and rising sea levels, leading to increase salt water intrusion and impact on agricultural and aquacultural activities in the Mekong Delta River; (ii) Felling down the account for suspended sediment from the current 26 million tons / year to 7 million tons / year; (iii) Aquaculture will be affected with an estimated loss of at least $ 500 million to $ 1 billion per year.
10 1.4.2. Characteristics of Tien River in Dong Thap province Tien River which its section flowing through Dong Thap province, is located in the upper part of the delta and at the end of the floodplain of the Mekong River. Therefore, this area is characterized by special geological structure, geomorphological characteristics as well as hydrological processes. The flow of Tien River in Dong Thap province is dominated by upstream, tide and rain flow in the locality. Tien River in Dong Thap province is located in the upper part of the delta, located at the end of the floodplain of the Mekong River. Therefore, this area is characterized by geological structure, geomorphological characteristics as well as hydrological processes. The Tien River flow in Dong Thap province is dominated by upstream, tide and rain flow in the locality. The Tien River plays a crucial role in Dong Thap province, the natural boundary between the northern part of the Dong Thap Muoi and the southern part of the province between the Tien and Hau rivers. Currently, large cities (3/3 cities and towns are located along the Tien River), residential areas are concentrated along the river. CONCLUSION OF CHAPTER 1 1. Studies of change river channel focused on three major ways: river morphology, flow formation, and aggregate orientation. In particular, channel change study for proposing on reducing measures is a fundamental research problem of the branches of Geological Sciences in the direction of synthesis. Regarding researches on the braided river was initially studied in Vietnam as well as in the field of thesis implementation. 2. Mekong River is one of the longest river in the world and its role has been approved through the related countries, especially countries located in the downstream. Nowadays, the development of socio- economic activities (construction of hydroelectricity, irrigation, watershed, river bed exploitation ...) has created impacts on the change of both river tide and river bed generally. The section of Tien River belonged to Dong Thap province is located in the upper part of the Mekong River Basin and it has been obviously impacted by the implications of the upstream flow (major
11 factor) and the tide of the East Sea. The Tien River morphology is characterized by braided river. 3. There are three main basic ways which the project is going to apply for solving issues that the proposed objective, including employing the general Geography method as the major one to understand the whole problems that the study needs to conduct, followed by the method of using Historical factor for understanding the whole forming and developing process of Tien river; along with approaching accidentally to determine the unusual factors that impact on the change of river channel. In addition, other methods such as inheritance, remote sensing and GIS, field survey, experts and analyzing are going to use for accomplishing the objectives. Approaches and methods of the study are shown in the content of Chapters 2 and 3, and followed by the diagram in Figure 1.10. Chapter 2. ASSESSING THE VARIATION OF TIEN RIVER CHANNEL BELONGS TO DONG THAP PROVICE 2.1. THE CHANGE OF TIEN RIVER CHANNEL THE PERIOD 1965 - 2015 2.1.1. The vertical change of Tien River channel 2.1.1.1. The change in riverbank During period of 1965 - 2013, Tien riverbank which section across Hong Ngu district continuously fluctuated, especially in the middle island and the riverside. It can be seen the “progress” of the island toward the downstream. Prior to 2003, the process of shifting the banks of the North Branch of the Long Khanh Island which belongs to Hong Ngu Township (now is town) took place rapidly. From 2003 until now, the braided river of Long Khanh Island has been changing more increasingly. The river banks of Long Khanh A, Long Thuan and Phu Thuan B communes have been widely eroded. The change of Tien river bank flowing through Thanh Binh district happened mainly in the Tay island and concave (Binh Thanh commune). The other areas were eroded and decomposition alternatively at slow speed.
12 In the Cao Lanh City and Cao Lanh District more broadly: The river bank in this area marked varies in the branched area (Tre Island, Dong Dinh Island, Cao Lanh town) and the confluence of curved rivers (My An Hung commune, Lap Vo district; Binh Hang Trung commune, Binh Hang Tay, Cao Lanh district). The river bank of Tien River in Sa Dec - Chau Thanh area changes quite complicatedly. The left of the river bank of Ward 3, 4 is continuously eroded while the right of the river bank located in the Linh Island changed continuously (it is similar with the result of of Hoang Van Huan, 2008). Look at period of 1969-2002, at An Hiep area in Chau Thanh district was landslide around 1,200m with mean speed at 33m per year. On the other hand , at the opposite side of the river bank at Con Linh (Binh Thanh commune, Cao Lanh district), there was a decomposition at width about 1,400 m and the average deposition rate is 39 m/year. 2.1.1.2. The change of thagwel river The changing process of Tien River channel, the section flows through Hong Ngu district - Hong Ngu town, was associated with the formation and development of islands, middle islands and coastal as well as ”transfer” the flow or the change of main braid river and side braid river of Long Khanh island. This is one of the areas which the changing process of channel has been quite fluctuated over the last century and till now - Vietnam - Cambodia boundary likely Thuong Phuoc 1, 2: channel change accordance with the mechanism of the river's branching. Erosion only concentrated on the tip of the reef, which pushed the river current into the opposite side of the island; the upset occurred at the end of the island. - Long Khanh island: channel change is associated with the transition of the current between the north and south islets. Prior to 2003, the erosion occurred seriously at the riverbanks of Thuong Thoi Tien, Hong Ngu town while decomposition happened in the Southern branch. After 2003, the trend reversed which is completely matched with the main channel - change between two canals.
13 Fluctuations of river banks and deep streams of Tien river in Thanh Binh district are correlated. The area that frequent eroded are often deep in the creek (Binh Thanh commune, Tan Binh commune ...) and vice versa. Deep water creek area. Cao Lanh - Chau Thanh District: - From 1895 to 2004 showed that the erosion holes were growing deeper increasingly in accordance to the development of deep waterways of An Hiep - My Thuan river. The erosion at the meandering Sa Dec river dredged up by 15 m between 1895 and 2000 by the river flow. The deep scour hole in Sa Dec tended to move to the downstream in An Hiep commune and in My Thuan deep 28.45 m (from 1895 to 2003) [78]. Almost all creeks in the river have been lowered. - In the period of 2007 - 2015: changes in thagweg in Sa Dec - Chau Thanh area has been occuring severely. The deep stream continued to deviate from 6 to 10 meters and moved toward the downstream. After analyzing the longitudinal developments (river banks and river bottoms) of Tien River in Dong Thap province during the period 1965 - 2015, it can be mentioned that: - The area with the highest level of fluctuation occured in the river section that runs through Hong Ngu district - Hong Ngu town; Sa Dec - Chau Thanh section and Cao Lanh city. Over the period 1969- 2002, the section river in An Hiep Commune eroded toward the riverbank at 78 m in depth, while the opposite bank deposited by 164 m. The sloping and sloping gully, declining from 6 to 10 m during 2007 - 2015 toward the right bank of Ward 3 and 4 (Sa Dec city) and An Hiep commune, Chau Thanh district. - The river sections where are often fluctuated have typical morphological such as braided river (for examples, Thuong Phuoc 1 and 2, Thuong Thoi Tien, Long Khanh islet...) and the curved of meandering river at Tan Chau town, An Giang province , My An Hung B, Lap Vo district, Ward 3 and 4, Sa Dec, An Hiep, Chau Thanh. These areas, the banks of the islet, and the river bank, were continuously
14 eroded. By contrast, the island's tail, the opposite side of the convex bank was deposited and extended the caudal and the shoreline. - Coastal changes are consistent with changes in deep creeks and local erosion pits. The large erosion areas often have extensive offshore creeks associated with local scour holes. This is evidenced at large erosion sites such as An Hiep commune, Chau Thanh district; Tan Chau Town, An Giang; Binh Thanh Commune, Thanh Binh District; Ward 11, Cao Lanh city... 2.1.2. The horizontal change of Tien River channel It can be seen that the general trend of the river channel is consistent with the change in depths and coastlines over the cross sections. Areas along the coastline often have deep creeks (thagweg lines) toward strongly to the shore; The river morphology of the cross-section has a V- shape and its bottom toward the riverbank . In addition, in the cross- sectional area with the middle island (island) the changing process of river channel also takes place strongly. 2.1.3. The relationship between the progression of Tien river toward the vertical and horizontal direction The change of river channel toward the vertical direction is associated with the horizontal direction, for examples: The erosion sites that are seriously eroded at the surface its cross- section is usually V-shaped and its bottom side deflects toward the landslide or the middle-shaped (exists island, the riverbank between the river). If there is considerable erosion at the upper course of middle island, the lower course is deposited. The typical areas which has seriously variation in river channel illustrated for river process of erosion and deposition in recent years such as Thuong Phuoc 1 commune, two districts of Hong Ngu; Long Khanh island area; The west island; Binh Thanh of Thanh Binh district; Tan Thuan Dong island area; Sa Dec - Chau Thanh area The channel change on the site is suitable for deep-water channel evolution (flow dynamics, thalweg close to erosion banks or deep wells close to shore). MRC survey results [14] showed that Tien river in Dong Thap province has 13 deep pools (V001 - V013) where the
15 erosion have occurred seriously inrecent years, including Tan Chau, Binh Thanh, My An Hung B, An Hiep …. 2.1.4. General characteristics of the change in Tien river channel The process of Tien River is quite complicated but it still follows the natural "rules" and has several general features: (i) Tien river channel changed toward the rule of erosion and deposition of braided channel ; (ii) erosion and deposition, two phenomena having a correlated relationship; (iii) river channel changes are affected by the upstream flow greater than those affected by rivertide; (iv) river bed erosion following a downward trend; (v) Channel change is located in the ancient riverbed. The “patterns of development” in the study river is constructed via analyzing the characteristics of the variation of Tien River channel belongs to Dong Thap province and is illustrated in table 2.7. 2.2. THE CAUSES OF THE VARIATION OF TIEN RIVER CHANNEL 2.2.1. Impact of natural factors 2.2.1.1. Geology In general, the river bed of Tien River is composed of several new sediment layers because they have not yet compacted, causing unassembled particles which are very porous. The new alluvial layers are weak soils with average compressive strength of 0.24 - 0.7 kg/cm2, binding force 0.10 - 0.29 kg/cm2. Moreover, this new type of sediment is usually of origined from mixed river-sea sediments containing fine small particles and plenty of soluble salts. On the other hand, the river bed flows through the loose sediments containing two types sediment layer. The upper layer is fine sandy or fine clay mixed with sand which its thickness of 18-20 m, the remain layer is coarse sand which is 13-25 m thickness. Sand layer is a relatively watery, pressure-bearing aquifer with water from Tien River (Figure 2.54). As the flow regime changes, the aquifer in the sand changes, causing a rearrangement of disturbed sand particles. This change can cause drift sand and followed by erosion.
- 15 - Table2.7. The dynamic modelling of the river section at the study area No. Morpho- Features, causes, Typical areas logical evolutions characte- ristics 1 Braided 1.1. The erosion has - Liet Si islet (formerly Tao islet) river been strong at top of and Beo islet where are located at islets, islands and two Thuong Phuoc village 1 and 2, in opposite sides of the Hong Ngu District erosion has isle. Accumulation has been concentrated at the top of been at the bottom of islets and the left bank in Thuong islets. It is caused by the Phuoc village 1; deposition at direct flow dynamics on bottom of the Beo islet led to the top of islets. “Drifting” 1.2. Changes in the River braches at North and South order of the river of Long Khanh islet branches, includes: - From 1940s to 2000: erosion erosion at major river occurred mainly at the main river branches andbranch which is throughout Hong accumulation at Ngu town and sedimentation took tributary conversely. place at Long Khanh river branch. - After 2000 until now: the main branch of river switch to the Long Khanh river branch; therefore, erosion of river channel arose at Long Khanh A and Long Thuan village; accumulation has been a part of the North river branch. 1.3. In the later part of Binh Hoa hamlet, Binh Thanh the river branches, there village, Thanh Binh district, a part of are often deep holes My Hung B village, Lap Vo district…
- 16 - 2 The river - Erosion at concave - A part from Sa Dec to Chau bends bank due to the Thanh district which is located at dynamic of stream bends of river; erosion mainly flowing forces directly occurred at right bank (ward 3 and into it combined with 4, Sa Dec city, and An Hiep turbulent flow, then the village, Chau Thanh district; river bed often has convex bank at Binh Tan islet, deep holes deviated to Linh islet, Binh Thanh village are the concave bank. deposited alluvia - Sedimentation took - Erosion befalls at Tan Chau place at the opposite district, An Giang province and bank deposition arises at convex bank of Thuong Phuoc 2 and Thuong Thoi Tien village, Hong Ngu district. 3 The river Erosion and accretion Left bank of West islet, Thanh section is intertwine but they still Binh district, Dong Thap province relatively retain the characteristic straight of morphology. The primary reason was caused by the position of river section located at area of tectonic fault line
16 When the riverbank is impacted with a relatively large velocity (0.5 - 3.0 m/s), the standard velocity causing without erosion is low, the lower sand layers are washed out faster than the upper clay layer which creates subterranean frogs with a high potential for coastal erosion [63], [77], [78]. In addition, fine clay layer, and clay powders are also of low mechanical nature, leading to lose their bond in the water environment due to high disintegration and are susceptible to external influences because of their high modulus. 2.2.1.2. The morphology of the channel The morpholgy of Tien River belongs to Dong Thap province especially the horizontal and vertical cross section also partly impacts the changing process of river channel. The results of Nguyen Sinh Huy et al. [90], Nguyen Quang My et al. [81], Vu Van Vinh et al [77]. - Clusters along Tien River: On the river bed of Tien River, there is a sudden change in the depth of the river bottom due to the continuous arrangement of deep pools (erosion pits) and shallow fields (rapids). This arrangement is consistent with the morphology and order of the river fluctuations. The greater the degree of variation, the greater the density and size of the rapids. Deep pools are usually found in the main meanders of the meandering section, where the water flow is turbulent and especially when there is flooding. The depth of the river in this area reaches 30-40m, which is one of the direct causes of shoreline erosion at many locations in Tien river such as Thuong Phuoc 1, 2, Thuong Thoi Tien (Hong Ngu district), Hong Ngu town, Sa Dec, My Thuan, Phuong 11 (Cao Lanh city) [9]. According to Le Manh Hung et al. (2008) [64], the Tien River section from Tan Chau to Cho Lach has 12 deep pools, with 12 shallow rapids, the average distance between deep pools is 12.5 km. - The cross-sectional area is of two types: the middle ground type (involving sand dunes, alcoves, islands - typical for braided river beds) and the riparian banks (shallow banks), a deep slope, with local erosion for erosion - erosion. Both horizontal and vertical displays along the river create favorable conditions for the channel changeto occur.