Studying the spatio-Temporal variation of tien river channel belongs to Đong Thap province for natural disaster adaptive capacity

Based on the geographic point of view, the thesis has developed

criteria and warned areas at risk of river channel change at levels

(high, medium, low) as the basis for proposing solutions. Response,

mitigation. The thesis uses three trend-based warning methods,

according to flow dynamics and geographic synthesis based on

dynamic morphometric approach. The results of the general

assessment show that the large fluctuation areas are identified as the

Tien River flowing through Hong Ngu district, Hong Ngu town and

Bac Tay Island in Thanh Binh district and Ho Chi Minh city. Sa Dec

- An Hiep commune, Chau Thanh district.

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n 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- logical characte- ristics Features, causes, evolutions Typical areas 1 Braided river 1.1. The erosion has been strong at top of islets, islands and two opposite sides of the isle. Accumulation has been at the bottom of islets. It is caused by the direct flow dynamics on the top of islets. - Liet Si islet (formerly Tao islet) and Beo islet where are located at Thuong Phuoc village 1 and 2, in Hong Ngu District erosion has been concentrated at the top of islets and the left bank in Thuong Phuoc village 1; deposition at bottom of the Beo islet led to “Drifting” 1.2. Changes in the order of the river branches, includes: erosion at major river branches and accumulation at tributary conversely. River braches at North and South of Long Khanh islet - From 1940s to 2000: erosion occurred mainly at the main river branch which is throughout Hong Ngu town and sedimentation took 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 the river branches, there are often deep holes Binh Hoa hamlet, Binh Thanh village, Thanh Binh district, a part of My Hung B village, Lap Vo district - 16 - 2 The river bends - Erosion at concave bank due to the dynamic of stream flowing forces directly into it combined with turbulent flow, then the river bed often has deep holes deviated to the concave bank. - Sedimentation took place at the opposite bank - A part from Sa Dec to Chau Thanh district which is located at bends of river; erosion mainly occurred at right bank (ward 3 and 4, Sa Dec city, and An Hiep village, Chau Thanh district; convex bank at Binh Tan islet, Linh islet, Binh Thanh village are deposited alluvia - Erosion befalls at Tan Chau district, An Giang province and deposition arises at convex bank of Thuong Phuoc 2 and Thuong Thoi Tien village, Hong Ngu district. 3 The river section is relatively straight Erosion and accretion intertwine but they still retain the characteristic of morphology. The primary reason was caused by the position of river section located at area of tectonic fault line Left bank of West islet, Thanh Binh district, Dong Thap province 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. 17 2.2.1.3. Hydrological The effect of the erosion of the river bed at a fast or slow speed, strong or weak depends on four factors [62], [63]: the magnitude of the flow, the duration of the flow capacity, The direction of the flow of water to the shore, the ability to transport the sediment of the flow. According to the analysis, all four elements in the Tien River in Dong Thap province have facilitated the flow of erosion. In addition, under the influence of climate change has increased the backflow, change the flow structure ... contribute to complicate the channel process. 2.2.1.4. The relationship among morphology and flow dynamics, erosion, accretion The erosion and sedimentation of the river bed is the result of the frequently interaction between the two direct elements, namely flow and river valleys. Flow plays an important role and proactive in regulating mechanism, rate of erosion, deposition of river bed. It contributes to the creation of locations and areas with the risk of erosion and deposition. The conductor represents in geometric form the results of the effects of erosion, accretion, fluid flow, including hydrological factors. It reflects the dynamics of the flow, as well as factors that can change the dynamics axis, directly or indirectly cause erosion and shore accretion. Therefore, studying for prediction of channel change can be considered as the main research object is the river bed [81]. The conductor not only contains the content that reflects the consequence of the interaction between flow and conduction, but also take in the content of the projection. Areas frequently occurring landslides in the Tien River are (i) in the curved section of the river, the force acting on the embankment to create deep holes; (ii) where the flow of the two streams flows; (iii) where the river has narrowed one or both. 2.2.2. Impact of human activities Over the years, local and basin-based mining has increased. In particular, there are exploitation activities that adversely affect the river flow such as deforestation (the rate of forest cover, especially primary forest decreased continuously, average 0.53% per year); Construction of many hydroelectric and irrigation dams (there are 18 currently about 11 mainstream hydropower dams and 125 dams on tributaries already and will be built); Excessive sand mining and unplanned sand mining both in the basin and in the locality. In addition, other local activities such as overloading to the riverbank edge, aquaculture, high speed and high speed trains, building infrastructure systems and enclosed embankments river ...lead to change the river flow, make development of the erosion of the river. Catch operations in the basin and in the local area have continuously increased in scale and scale after 2000. The most worrying is the construction of a system of mainstream hydropower dams, Alluvial as the water flow to the delta caused the phenomenon of "starvation" and the extraction of sand in the river bed excessive, not planned to make the status of channel change complex. The impact of socio-economic development on the river bank is twofold: (i) increasing shoreline load (increasing the river's slip force); (ii) alter the distribution, distribution and structure of water flows, alter the morphology of the river bed, the steepness of the roof, etc., reducing the river slip resistance. CONCLUSION CHAPTER 2 1. In the period of 1965 - 2015, the variation of Tien River channel was complicated but still obeyed the law and mechanism of the branching river (erosion and accretion occurring frequently along the cross sections along the river); Unstable development points concentrated river sections with characteristic morphology (braided rivers: Liet Si, Beo, Long Khanh, Tay, Tre, Dong Dinh; Curved river sections: Ward 3, 4 Sa Dec Town - An Hiep Commune, Chau Thanh District, Tan Chau Town - An Giang, My An Hung - Lap Vo ...). In addition, erosion and sedimentation are two persistent phenomena in which erosion is predominant; Erosion tends to reverse downstream; river channel change complex but river channel change range takes place in ancient riverbed area, the upper riparian area has river channel change level greater than tidal area. 2. Variation of Tien River channel is the result of the combined effect of natural factors and human activities on the relationship between river bed and river flow. The conduction is the result of the 19

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