Doctoral Dissertation summary: Geomorphology of coastal estuaries in Northern central Vietnam

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Practical significance: the scientific outcomes can be used to improve the territorial management capacity of local authorization offices on the rational use of territorial resources and environmental protection of coastal estuaries in northern central Vietnam.

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Nội dung Text: Doctoral Dissertation summary: Geomorphology of coastal estuaries in Northern central Vietnam

VIETNAM ACADEMY OF MINISTRY OF EDUCATION SCIENCE AND AND TRAINING TECHNOLOGY GRADUATE UNIVERSITY OF SCIENCE AND TECHNOLOGY ----------------------------- NGUYỄN CÔNG QUÂN GEOMORPHOLOGY OF COASTAL ESTUARIES IN NORTHERN CENTRAL VIETNAM Major: Geomorphology and Paleogeomorphology Codename: 09 44 02 18 DOCTORAL DISSERTATION SUMMARY Hanoi 2019
The doctoral dissertation was accomplished at the Graduate University of Science and Technology – Vietnam Academy of Science and Technology. Principal supervisor Dr. PHẠM VĂN HÙNG – Institute of Geological Science, VAST Secondary/Other supervisor Dr. PHẠM QUANG SƠN – Institute of Geological Science, VAST Reviewer 1: Reviewer 2: Reviewer 3: The final doctoral dissertation defense will be defended in front of University Dissertation Committee at… (hrs), … 2019 in the Graduate University of Science and Technology, Vietnam Academy of Science and Technology. The dissertation can be accessed in - Graduate University of Science and Technology, VAST library. - National Library of Vietnam.
INTRODUCTION 1. Background Coastal estuaries locate on where the rivers meet the sea, with many favorable physical conditions for human to settle and develop. Large urbans with concentrated residental areas, economic zones, national defense and security regions are gradually expanded on those areas to serve the people such as industrial sites, administrative area, civil works, airports, harbors, services, trades, etc. The topology of the coastal estuaries on Vietnam Northern Central coast has formed and developed under the interplay of numerous external, internal and artificial dynamical processes; furthermore, these are also stage for complex river-ocean interactions. The consequence of those processes has made coastal estuaries prograde toward the sea since Pleistocene epoch to the modern day, formed identical type of landform from shoals, sandbars, sand barriers to deltas. Coastal estuaries are normally favourable places for socio-economic developments, but they are also intimidated as sea level risings which can cause coastal erosion, flooding… and damage coastal construction and lives of local residents. In the context of global climate changes and sea level rise, and under the pressure of the economy, growing socio-economic development, the topology of coastal estuaries on the Northern Central Vietnam has been drastically transformed at many time and spatial scales. This dissertation titled “Geomorphology of coastal estuaries in northern central Vietnam” is the final result of studies to fulfill the necessarirarity of many urgent objectives toward sustainable socio-economic development and environment protection. 2. Objectives - Interpret the geomorphological features of coastal estuaries in 1
northern central Vietnam. - Estimate the course of morphological changes and geomorphological dynamic processes in the study area. 3. Research subjects and Study - Research subjects: Morphology and modern geomorphological dynamic processes of coastal estuaries in northern central Vietnam. - Study area: the main study areas are the coastal estuaries of Ma, Thach Han and Huong rivers (from bifurcation channel to subtidal flats). The interested morphological changes of the study areas are focused during those following phases: late Upper Pleistocene – middle Lower Holocene, middle Lower Holocene – Upper Holocene, and Upper Holocene – present. Figure 1: Map of study area 2
4. Data and methodology Data and methodology of the doctoral dissertation including documents, database was collected and analysed by the author during the course. 5. Research Objectives Research objective 1: The morphology of coastal estuaries in northern central Vietnam is heterogenous, complicated and spatially diverged to latitudinal pattern. Landform of the interested coastal estuaries were classified following morphodynamic genesis approach: the Ma estuary is classified as “delta”, including the alluvial, mixed and marine dynamical processes with the number of morphological unit of 3, 7, and 3 accordingly. The two others are classified as “Liman” type, with respective landform units of alluvial – mixed – marine dynamical processes are 4, 7, 6 units for Thach Han estuary, and 4, 9, 4 units for Huong estuary. Research objective 2: During late Quaternary to present, the morphology of interested coastal estuaries had undergone 3 phases: late Upper Pleistocene – middle Lower Holocene, middle Lower Holocene – Upper Holocene, and Upper Holocene – present, and propagraded at different grades. The present development at each estuary is ranging from progradation (Ma estuary) to severe coastal erosion and retrogradation (Thach Han and Huong estuaries). 6. New findings in the dissertation The detailed geomorphological maps of the three coastal estuaries had been established using high resolution remote sensing images, GIS technology and other traditional research methods, to classify and interpret landforms into groups of similar morphogenesis and age. The geomorphological maps were adapted to understand the morphological changes in the studied areas from late Upper Pleistocene to Holocene. The late Upper Pleistocene shorelines of the Thanh Han and 3
Huong coastal estuaries, and the late Middle Holocene shoreline of the Ma coastal estuary had been reconstructed using geomorphological and geological evidences. Database compiled from topological maps, remote sensing data and GIS was used for reconstruction of temporal shorelines in Thach Han and Huong estuaries for the period from 1952-2017, and shorelines in Ma estuary from 1965-2017. Modern morphology change patterns of the three estuaries had been determined. Dominant process in Ma estuary is progradation to form new seaward landform, meanwhile coastal erosion is dominant in Thach Han and Huong estuaries and causes retrogradation. 7. Scientific and practical significances: Scientific significance: Results of the dissertation provide sufficient database on the morphological changes, estimate the orientation of geomorphological evolution of coastal estuaries in northern central Vietnam. Practical significance: the scientific outcomes can be used to improve the territorial management capacity of local authorization offices on the rational use of territorial resources and environmental protection of coastal estuaries in northern central Vietnam. 8. Dissertation layout Chapter 1: Research methodology on morphological changes of coastal estuaries; Chapter 2: Estuary morphological change characteristics and controlling factors in the northern central Vietnam; Chapter 3: Geomorphology of coastal estuaries in northern central Vietnam; Chapter 4: Morphological changes and the rational use of territorial resources of coastal estuaries in northern central Vietnam. 4
CHAPTER 1: RESEARCH METHODOLOGY ON MORPHOLOGICAL CHANGES OF COASTAL ESTUARIES 1.1. Overview 1.1.1 Definition of estuary River mouth or estuary is the area where the river meets the sea, characterized by the gradually shifting from terrestrial alluvial to marine regime, including massive transformation of physicochemical nature of the water columna, biological properties, morphologic formation and delta initation processes. 1.1.2. Estuary classification in the study area The estuary classification using in the study was based on the synthetic geology-geomorphology criteria: - Convex type (delta) on Hoi river mouth where characterized by front submerged bars. - Straight type (liman) on Viet and Thuan An river mouths, characterized by low sediment concentration and parallel sand barriers and enclosed lagoons. 1.2. Literature review 1.2.1. Global research Globally, the methodology for coastal estuary study becomes more synthetic and completed with the adaption of modern technologies toward quantification outcomes. 1.2.2. Vietnam Most studies on this field in Vietnam implemented modern morpho-dynamic processes evaluation and their correlation to typical natural hazards of coastal estuaries in northern central Vietnam on determine the cause, estimate potential risk and propose mitigation/ damage reduction measures. 1.2.3. Northern central Vietnam Study on the morpho-dynamic issues of coastal estuaries for planning and territorial organization for socio-economic 5
development in the northern central Vietnam is scarce, thus it is necessary to perform more concrete and detail research on each estuary. 1.2 Methodology 1.3.1. Approach 1.3.2. Methodology 1.3.2.1. Field survey and measurement technique 1.3.2.2. Remote sensing method 1.3.2.3. Cartography and GIS 1.3.2.4. Geomorphological method 1.3.2.5. Geological method 1.3.2.6. Synthetic evaluation method CHAPTER 2: ESTUARY MORPHOLOGICAL CHANGE CHARACTERISTICS AND CONTROLLING FACTORS IN THE NORTHERN CENTRAL VIETNAM 2.1 Study area overview Coastal estuaries in the northern central Vietnam are located on the coastal plains of from Thanh Hoa province to Thua Thien – Hue province. Physical settings in the study areas are diverse and favourable for socio-economic development in the northern central Vietnam. 2.2 Geological system 2.2.1 Geological settings The geological settings of the study areas consist of various formations dated from Proterozoic to modern. 2.2.2 Neotectonic structures Weak uplifting activities in the mainland, such as Thanh Hoa – Tan Ky uprising, are favourable for denudation-erosion processes, create abrasion terraces to the west and seaward accumulation of Ma river estuary. Subduction structure develops northwest-southeast, parallel to the modern coast in Quang Tri – Hue province, promotes 6
heterogenesis accumulation terrains in the estuaries of Thach Han and Huong rivers. 2.2.3 Modern and neotectonic faults Severe erosion has occurred on river banks and coastal sections where are active faults intersections. 2.3 Morpho-dynamic systems 2.3.1 Holocene sea-level fluctuations Sea-level fluctuations on the northern central Vietnam in Holocene were similar to the general trend in Vietnam and adjacent regions. 2.3.2 Climate regime Study area lies in the monsoon tropical climate, with two separated dry and rainy seasons, under significant influence of marine atmosphere. 2.3.3 River flow and sediment patterns Water and sediment discharge of Ma river are considerably greater than those of Thach Han and Huong rivers, thus the role of Ma river to the formations of its estuary on the initiation and modern morphology changes is more dominant than the other two rivers. 2.3.4 Wave, tidal and nearshore current regimes Wave influence to the coastal estuaries in the study area is dominant in the winter, thus the coast is severe shaped and erosion. Tidal regime in the estuary of Ma river is greater than the other estuaries, thus increase its influence to the estuarine formations. Smaller tidal amplitude in Thach Han and Huong estuaries play less significant role in morphological changes of those areas. 2.3.5 Modern sea level rise and climate changes Sea level rise is a significant factor that has direct influence to the morphological change of coastal estuaries in northern central Vietnam. 2.4 Anthropogenic elements 7
2.4.1 Hydro power plants and irrigation systems The total number of reservoirs and dams in the northern central Vietnam may reach to over a thousand and playing a crucial role as they become sediment traps and reduce natural flow discharge, thus emphasize the influence to the morphological change. 2.4.2 Aquaculture and land reclamation Aquaculture contributes great benefit to the local economy, but the rise of water bodies for aquaculture and practice transformation from rice paddy to fish/shrimp farms on the coastal zone may not only change natural morphology but also increase salt water intrusion in the coastal estuaries. 2.4.3 Construction of residental settlements and industrial areas. Original landforms were artificially modified under the pressure of socio-economic development. New constructions had been built including infrastructures and residental settlements. CHAPTER 3: GEOMORPHOLOGY OF COASTAL ESTUARIES IN NORTHERN CENTRAL VIETNAM 3.1 Overview Major morphological structures of the study areas developed in northwest-southeast and semi-latitudinal directions, including mountainous, hilly, plain and beach landforms. 3.2 Geomorphology of coastal estuaries 3.2.1 Establishment of the geomorphological map of coastal estuaries Methodology: the synthetic geomorphological map was established by using the homogenetic and age uniformity to classify similar features into homogeneous groups for the coastal estuaries of Ma, Thach Han and Huong rivers. Content: morphologic features which have similar origin and age were classified into group. Each morphological group was established according to its age and morphology. 8
3.2.2 Geomorphology of Ma estuary - Karst formations: karst formations in the study area distribute as isolated mounts with height over 30m, scattered to the north and southwest of the Thanh Hoa city and northwest of Ham Rong mountain. - Abrasion formations: those formations distribute scattered in the study area, as isolated mounts and hills over 20m on different kinds of bed rock. - Alluvial formations: deposition formations by alluvial processes distribute along the main channel of Ma river. They are classified into 3 units: Upper Holocene shoals (1-2m); Upper Holocene low sandbars (1-2m); and Upper Holocene high sandbars (1-2m). Figure 3.1. Geomorphological map of Ma estuary - Marine formations: this group includes 3 morphological units: Upper Holocene accumulation terraces widely distribute in Hoang Hoa, Quang Xuong districts and Sam Son city, height of 2-3m; Lower-Middle Holocene accumulation terraces distribute to the west of Hoang Hoa, Dong Son, Thieu Hoa districts and Thanh Hoa city, height of 3-4m; modern beaches lie along the coast of Hau Loc, Hoang Hoa, Quang Xuong districts and Sam Son city. 9
- Heterogeneous formations: this group contains 7 formations of various processes including alluvial, marine, bog and aeolian. Scattered Upper Holocene alluvial-bog accumulation surfaces at the height of 1-2m narrowly stretch across some communes of Quang Xuong district and Thanh Hoa city (Ham Rong, Dong Son, Nam Ngan… wards). Upper Holocene alluvial-marine-bog accumulation surfaces at the height of 1-2m distribute as riparian shallow depresses across Hoang Hoa, Quang Xuong, Hau Loc districts and Sam Son city. Upper Holocene marine-bog accumulation surfaces at the height of 1-2m distribute mainly in Quang Xuong district. Semi- longitudinal Upper Holocene alluvial – marine accumulation surfaces at the height of 2-5m distribute in Hoang Hoa district and Sam Son city. Lower-Middle Holocene alluvial-marine accumulation surfaces at the height of 2-5m widely distribute across Hoang Hoa, Quang Xuong, Dong Son districts and Thanh Hoa city. Upper Holocene reformed marine-aeolian sanddunes are small, elongated stripe of sanddunes, distributed from the north of Lach Truong river mouth to the south of Quang Xuong district. 3.2.3 Geomorphology of Thach Han estuary - Volcanic formations: those are denudation-erosion landforms developed on Quaternary basalt hilly surfaces to the north-northwest of the study area on Gio Linh district, at the height of 10-30m. - Abrasion formations: abrasion formations distribute to the south- southwest of the study area, including 2 surfaces of Upper Pleistocene and Lower-Middle Pleistocene at the height of over 20m and 40-80m, respectively. - Alluvial formations: alluvial formations contain 4 geomorphological units: Upper Holocene shoal, at 1-2m; Middle- Upper Holocene low sandbar, at 2-3m; Lower-Middle Holocene high sandbar, at 3-5m; Upper Pleistocene class 1 accumulation terraces, at 10-15m; Middle-Upper Pleistocene class 2 accumulation terraces, at 10
over 20m; Upper Holocene abandoned river channels. Alluvial formations concentrates mainly on the riparian of Cam Lo (Hieu) and Thach Han rivers. Figure 3.10: Geomorphological map of Thach Han estuary. - Marine formations: this group includes modern beaches along the coastline; Lower-Middle Holocene accumulation surfaces distribute across Gio Quang, Gio Viet, Ha Tay and Ai Tu county; Upper Pleistocene and Middle-Upper Pleistocene abrasion terraces, at the height of 12-20m and 25-35m respectively, to the west of the study area and on the margin of the deposition plain. - Heterogeneous formations: this group contain formations which developed by alluvial, marine, bog and aeolian processes. The most widespread formation is Lower-Middle Holocene alluvial-marine accumulation surfaces at the height of 2-6m which accounting major area of Thach Han estuary. Middle-Upper Holocene and Upper Holocene deposition plains narrowly elongate across Gio Mai, Tuong Van, Cao Ly at the height of 2-4m and 1-2m, respectively. Upper Pleistocene alluvial-marine accumulation surfaces spread as small patches in western Gio Quang, Dai Ang at the height of 6-15m. 11
Upper Holocene and Middle-Upper marine-bog accumulation surfaces distribute at the height of 1-2m and 1-3m, respectively and scattered as small patches, flooded year-round. Elongated marine- aeolian coastal sand dunes spread along the modern coastline. 3.2.3 Geomorphology of Huong estuary - Abrasion formations: compiled denudation formations distribute at the height over 20m in Huong Ho, to the south of Kim Ngoc village, Gia Kho, Nguyet Bieu, Duong Xu, An Ha. - Alluvial formations: river channels widely developed in the study area during late Holocene. Late Upper Holocene high sandbars distribute at 3-5m along Huong river in Phu Xuan, beneath railway brigde Trieu Son Dong. Middle-Upper Holocene class I river terraces spread dispersedly along Huong river, Ngoc Ho, Tuan ferry, Nam Hoa at the height of 3-6m. Lower-Middle Holocene class II river terraces distribute as elongated stripes along Huong, Ta Trach, Huu Trach rivers at the height of 10-25m. - Marine formations: modern beaches are formed in Middle-Upper Holocene, at the height of 3-5m, spreading from Thai Duong Thuong to Ke Sung. Lower-Middle Holocene accumulation surfaces widely distribute along the coastline at the height of 4-6m for approximately 15km length. Upper Pleistocene marine abrasion terraces are relatively levered at the height of 15-30m. - Heterogeneous formations: Middle-Upper Holocene alluvial- marine accumulation surfaces distribute along O Lau, Bo and Huong rivers, stretching to the south of the coastal plain. Middle-Upper Holocene alluvial-marine-bog accumulation surfaces distribute in Quang Vinh, Phong Hien, Phu Da. Middle-Upper Holocene marine- bog accumulation surfaces distribute along the shore of Tam Giang lagoon and Ha, Thuy Tu marshes. Lower-Middle Holocene alluvial- marine surfaces distribute in Huong Thuy district. Lower-Middle Holocene alluvial-marine-bog accumulation surfaces distribute as 12
narrow stripes in Phu Bai at the height of 10-15m, contain greyish blue – bluish black clayey-silty sand. Upper Holocene sand dunes distribute along the coastline as dunes, mounds and sand bars at the height of 10-30m. Figure 3.17. Geomorphological map of Huong estuary. CHAPTER 4: MORPHOLOGICAL CHANGES AND THE RATIONAL USE OF TERRITORIAL RESOURCES OF COASTAL ESTUARIES IN NORTHERN CENTRAL VIETNAM 4.1 Morphological evolution of coastal estuaries in northern central Vietnam 4.1.1 Middle – Late Pleistocene era In Middle-Late Pleistocene era, the tectonic activities became diverged as subsidence to the east, uplifting to the west of coastal estuaries in northern central Vietnam. Marine transgressive boundary shifted inland which was favourable for sedimentation in the east, while abrasion terraces initiated and developed to the west of study 13
area. In the estuary of Ma river, the 15-35m abrasion terraces initiated in Tho Xuan and Trieu Son districts (Thanh Hoa province). Denudation and denudation-erosion on mounts and hills over 40m height took place in Yen Dinh, Tho Xuan and Trieu Son districts. Subsidence occurred beneath and to the east Ma estuary in coherence with marine transgression were the dominant environment for the formations of marine and alluvial-marine landforms. On the coastal estuaries of Thach Han and Huong river, abrasion terraces also distributed to the west of study area, at the height of 15- 35m. Prodominant morphodynamic processes including denudation, denudation-erosion and abrasion occurred in the background of uplifting to the west of study areas, and stepping subsidence and deposition on the major of Thach han and Huong estuaries. Uplift activities promoted surficial denudation, erosion and eluviation to form slope denudation, denudation-eluviation, denudation-deposition surfaces, abrasion marine terraces. Mountains developed on uplifting blocks were the Hoang Gay range to the far southwest, Dong Hoang and Kho Quan mounts to the south, Dong Kieu and Kim Phung mounts to the west and the others such as Ngu Binh, Dong Mieu… Weathering crusts and thin layers of eluvium, deluvium… appeared on slope surfaces, sometimes bedrock outcrops. The low-lying and levered hill series to the south of the 1A highway were classified as abrasion marine terraces at the height of 20-30m. This level was the highest ancient shoreline of the era marked on the topology of study areas. The central bottom of coastal estuaries in the study area was filled by thick layer of alluvium, alluvial-marine sediment dated middle-late Pleistocene. 4.1.2 Early-Middle Holocene era In the coastal estuary of Ma river, sedimentation in the early- middle Holocene had created the Thieu Hoa formation, including alluvial, alluvial-marine and marine landforms widespread in Ma 14
estuary. Weak uplift to the west of the estuary and other morphodynamic processes such as denudation, denudation-erosion took place on former middle-late Pleistocene formations. Marine transgression in Thach Han and Huong estuaries favored alluvial-marine and marine sedimentation as predominat morphodynamic processes. Basalt floods as result of active tectonics covered some locations. In early Holocene, basalt lava occurred to the north of study area in Gio Linh district and formed dome basalt hills at the height of 20-40m. In Huong estuary, alluvial-marine and marine floodplains distributed as northwest-southeast elongated stripes from Huong Phong commune to Phu Luong – Phu Da communes. These formations widely distributed to the south margin of Huong estuary: along 1A Highway, Hue Imperial City, Huong Ho, Huong Long, Huong An, Huong Thuy and northern area of Phu Bai Airport. The surficial height was 5-10m in average, in some places it could reach up to 20m such as Nam Giao Esplanade where was relatively levered, composed from gravely sand mixed up with yellowy red clayey silt, tampered, occasionally laterization. The alluvial-marine-lagoonal deposition plains widespreaded and accounted for the majority of Hue coastal plain in Phu Vang and Huong Thuy districts. 4.1.3 Late Holocent to present In Ma estuary, local subsidence in Ma rivermouth was filled by thick layer of late Holocene sediment, which reached up to 30m as in drillcore LK11.SS in Lach Trao river mouth. Other morphodynamic processes were alluvial, alluvial-marine, marine-bog… Deposition landforms distributed along the river channels, coastal zone and river mouths. The sedimentation determined the modern formation of sand bars, sand barriers, sand ridges and dune systems in Ma estuary. In Len river mouth, tidal flats, sand bars and shoals enlarged continuously and gradually due to abundant sediment supply. Shoreline advance rate in this area reached up to 40-50m per year. Sediment supply in Hoi river mouth was smaller, thus the 15
sedimentation was also weaker, accumulation occurred to both north and south sides of the river mouth, shoreline advance rate was 5-10m annually. In Thach Han and Huong estuaries, tectonic activity was diverged with prominently subsidence to form floodplains, marshes and lagoons as narrow strips along the coastline from Duong Thuong to Ke Sung. Beside, alluvial formations also emerged on the riparian of Thach Han and Huong rivers as multiple level sand bars; alluvial- marine-lagoonal deposition formations had been dried out gradually and distributed dispersedly in Thach Han and Huong estuaries. Morphodynamic factors also caused coastal erosion and aggradation in the study areas. 4.2 Morphological change evaluation The evaluation of morphological changes, especially modern morphology, is the major component of applied geomorphology. In order to determine the morphological evolution tendency of estuary, it is necessary to analyze geomorphological characteristics, morphological evolution history and specify ancient shorelines in the study areas. 4.2.1 Morphological changes in the Ma estuary The morphological features of Ma estuary had been changed spatially and temporally. Evidences of late Pleistocene ancient coastal zone existed in Yen Thai, Yen Lac communes (Yen Dinh district) and Xuan Lai, Xuan Yen communes (Tho Xuan district). According to those evidences, mainland of Thanh Hoa floodplain was landward further to the modern coast and lied in Tho Xuan, Yen Dinh and Trieu Son districts. During middle Holocene, ancient shoreline stretched from Van Loc commune (Hau Loc district) to Hoang Dat, Hoang Duc, Hoang Dai communes (Hoang Hoa district), Quang Phu, Quang Nhan communes (Quang Xuong district). In the modern age from 1965 to present, the shoreline has changed relatively complicated (Table 4.2). 16
Figure 4.5. Shoreline changes of Ma estuary Table 4.2. Coast status of Ma estuary North of Hoi rivermouth South of Hoi rivermouth Rate of change Rate of change Period Status Status (m/y) Conclusion (m/y) Conclusion Maximum Average Maximum Average 1965- Slightly slightly (-) 10.8 3.1 (+) 12.1 4 1975 retreat advance 1975- Moderate Slightly (-) 18.7 4.2 (-) 10.5 3.5 1990 retreat retreat 1990- slightly slightly (+) 6.2 3.2 (+) 10.7 2 2001 advance advance 2001- Slightly slightly (-) 7.23 3.75 (+) 14.2 1 2017 retreat advance Note: advance/retreat/mixed Generally, the shoreline of Ma estuary had advanced and move seaward at relatively high rate. During late Pleistocene to middle Holocene, the estuary advanced seaward up to few tens’ kilometers. From late Holocene, the estuary gained more than 20km seaward. Evaluation of modern morphological changes of Hoi and Lach Truong rivermouths were energetic, thus showing the movement trend of the shoreline in near future (Figure 4.5). 17
4.2.2 Morphological changes of Thach Han estuary The estuary of Thach Han river was initiated since late Pleistocene according to research results on local geomorphology and geological settings. In late Pleistocene, the mainland lied distant from modern shoreline, while local subsidence promoted marine transgression deep into the mainland and the majority of Thach Han estuary submerged. In late Holocene, uplifting activities and marine regression took place and formed coastal plains. From 1952 to present, the estuary has changed diversely as shown in Table 4.4. Table 4.4.. Coast status of Thach Han estuary Cua Viet coastal zone Interior of Cua Period Northern Southern Morphological evolution Viet coast coast rivermouth Shoreline retreat/advance alternatively, tend to be stable 1952-1965 (+) (-) (+/-) southward. Minor change in rivermouth. Prodominent shoreline retreat on northern coast, alternative retreat/advance on southern 1965-1979 (-) (+) (+/-) coast, relatively stable. Change reduced southward. Prodominent erosion inside channel. Minor change in rivermouth Prodominent shoreline retreat, slight alternative retreat/advance 1979-1989 (-) (-) (+/-) inside channel. Significant change in rivermouth. Advanced shoreline, significant 1989-1999 (+) (+) (+/-) change inside channel. Unstable rivermouth. Moderage shoreline advance, alternative shoreline 1999-2017 (+) (+) (+/-) advance/retreat inside channel. Significant change in rivermouth. Unstable Unstable Northern coast had experienced Unstable coast, coast, significant changes, exceed channel, alternative alternative southern coast. Coastal stability Overview alternative shoreline shoreline enhanced southward. Significant shoreline retreat/ retreat/ change retreat/ advance advance advance in-channel in rivermouth. Note: advance/retreat/mixed 18