Pham Huy Tien, Tran Duc Thanh, Bui Hong Long, Nguyen Van Cu, 2003 Coastal erosion and sedimentation in Vietnam Collection of works on Marine Environment and Resources, vol X, p.20 - 33 Science and Technics Publishing House Hanoi PRINCIPAL RESEARCH RESULTS ON COASTAL EROSION AND SEDIMENTATION IN VIETNAM Pham Huy Tien (1), Tran Duc Thanh (2) Bui Hong Long (3) & Nguyen Van Cu (4) National Centre for Natural Science and Technology Haiphong Institute of Oceanology, 246 Danang Street, Haiphong City Tel 31-760602; Fax: 31-761521; E-mail: tdthanh@hio.ac.vn Nha Trang Institute of Oceanography Insitute of Geography Abstract The coastal erosion and sedimentation are deeply anxious for many local authorities and communities in Vietnam The erosion has happened in large scale, and its intensity increased with time The sedimentation has been expansive for siltation of shipping channels and filling river mouths and lagoon inlets The deep causes of those are related to coastal evolution, and human impact in both coastal zone and catchment In many cases, the sedimentation are accompanied and caused by sediments supplied from eroded process neighboring These disasters have created heavy consequences such as damages of human beings, properties and land; and degradation of coastal environment and ecosystems; unsustainable development, emigration; and unstable thought in life and production of communities It needs to build national strategy and scientific base for management of these disasters The control of them needs to be in the framework of integrated coastal management, combining with catchment management The priority is given to resolutions combined with other benefits, and to combination of controlling coastal erosion and sedimentation It is also necessary to strengthen legislation and regulations, build database; establish a network of monitoring disasters regularly The international cooperation must be regards as an important resolution responding to coastal erosion and sedimentation Introduction Stretching on the length of 3200km and having 114 small and large rivers, the Vietnam coastal zone is concentrated by many important economical bases and the highest population density, and 24 percent from 77 million people of the whole country distribute in this zone Recently, Coastal erosion and sedimentation disasters have become the deep anxiety of local authorities, communities and economical basements in many cases The strong development of population and society-economy in the coastal zone need to have a sustainable process For this reason, the coastal protection and control of coastal erosion and sedimentation become an urgent demand During the last years, the research for these purposes has been paid an attention Although the limitations research capacity and funding, the obtained results have provided the principal knowledge, proposed the initial resolutions of coastal protection, and created the important scientific basement for the further investigations in coastal erosion and sedimentation disasters in Vietnam 1 Current status 1.1 Coastal Erosion General Estimation The coastal erosion is expansive in all three coastal parts in Vietnam A calculation shows that along the coast there have been 397 eroded sites with the total of 920.21km long From them, the coastal length of weak erosion has occupied 196.82km (21.4%); medium erosion179.90km (19.5%); strong erosion- 260.67km; and very strong erosion- 282.81km (30.7%) For each site, the eroded length could be some hundreds meter or even some tens kilometer The mean eroded rate is of 6-10m/year generally, but can reach 200-250m/year at the special cases in the sort duration Several cases, the coastal erosion has happened in the large scale of some tens kilometer, very strong rate over 10m/year and very long duration approximately one century There have been the cases that coast have eroded suddenly at the very strong rate In some sites, the coast are eroded and accreted alternately with the time However, it is defined that the intensity and the unusualness of coastal erosion have increased obviously and the damages of coastal erosion have been great Coastal Erosion in the North Vietnam Coastal erosion in the North Vietnam from Quang Ninh to Thanh Hoa Provinces, was recorded on the 51 sites with the total length of 113.93km, occupied 34.2% for the total length of coastline, at the mean rate of 6.0m/year, and 68ha land have been eroded per year (Thanh, T.D et all, 2000) The eroded intensity could be divided into degrees The weak degree has been from 0-2.5m/year and occupies 22.4% for the total of eroded; medium degree from 2.5-5m/year and 34.2%; strong degree from 5-10m/year and 16.6%; and very strong degree exceed 10m/year and 26.8% The coastal erosion has happened on the large scale A lot of eroded sites has been at strong and very strong rate, and increased in intensity with the time, for example, Cat Hai site of 6.4km long, Van Ly site of 30.0km long and Hau Loc site of 5.0km long The system of sea dikes and embankment has reduced the eroded intensity for the mud and sandy-mud coast Coastal Erosion in the Centre Vietnam Coastal erosion in the Centre Vietnam, from Thanh Hoa to Ninh Thuan provinces, was recorded on the 275 sites with the total length of 328.16km, occupied 21.1% for the total length of coastline, at the mean rate of 11.9m/year, and 389.9ha land have been eroded per year (Tien, P.H et all, 2000) The eroded intensity could be divided into degrees The weak degree has been from 0-5m/year and occupies 52.2% for the total of eroded; medium degree from 5-15m/year and 23.4%; strong degree from 15-30m/year and 10%; and very strong degree exceed 30m/year and 13.5% From 1999-2000, the coastal erosion was reaching very high rate from 40-60m/year and maximum 150-250m/year at some sites The sandy coast without embankments and strong wave are reason for the high-eroded rate Coastal Erosion in the South Vietnam Coastal erosion in the South Vietnam from Ninh Thuan to Kien Giang province, was recorded on the 71 sites with the total length of 478.120km, occupied 52% for the total length of coastline, at the rate over 2.5m/year in general (Long, B.H et all, 2000) The eroded intensity could be divided into degrees The medium degree has been from 2.5-5m/year and occupies 12.8 % for the total of eroded; strong degree from 5-10m/year and 43.7%; and very strong degree exceed 10m/year and 43.5% The coastal erosion has happened on the large scale The very strong eroded coast between Soai Rap and Tranh De river mouths has been being longest eroded site in Vietnam The mud and sandy-mud coast without sea dikes and embankments have been a reason for the strong eroded intensity in the South Vietnam Erosion in the Coastal Systems The erosion has occurred in all coastal systems In both the Mekong River Delta and the Red River Delta, which are known to be accreting, erosion has nevertheless occurred along onefourth of the coastline of each delta In the Mekong River Delta, 36 km of the Bo De coast has been eroding at a rate of 30-50 m/year during last century ( Thinh, T.Q., 1992.) In the Red River Delta, 30 km of the Vanly coast was eroding at a rate of 10-15 m/year from 1930-1990, and thanks to strengthening embankments the eroded length has reduced into 17.2 km length, recently At the estuaries such as Dong Nai in the South or Bach Dang in the North, there exist the typical eroded sites For example, Can Gio site has been eroded at the rate of 5-10m/year on the length of 8.5km from long time, and Cat Hai site in Bach Dang estuary has been eroded on the length of 6km, and the eroded rate increased from 3.5m/year up 13m/year during 1930-2000 In the Centre Vietnam, the erosion on the sandy coasts is high in unusualness, so that, very dangerous The coastal erosion, creates the remove of the small river mouths and lagoon inlets, and sometimes they remove in very far distance, some kilometers and even tens kilometer with the moved period of several years, tens years or some hundred years As a typical example, such moves have been recorded at Thuan An and Tu Hien inlets in Tam Giang Lagoon (Thanh, T.D et al.2001b) Coastal erosion can be noticed expansively in the islands, for example, Bach Long Vy Island of clastic rocks in the middle of Tonkin Gulf and Spratly Island of coral reef in the middle of Dong Sea (South China Sea) Generally, coastal erosion and accretion of small islands are alternatively influenced by the northeast and southwest monsoons, and finally the northeastward coast tends to be eroded and the southwestward coast tends to be accreted This is especially obvious in the coral reef Islands, which are deformed with the monsoon (Krempf, A., 1927) Under the influence of this process, the Spratly Island is shorter with a size of 650 x 320m in southwest monsoon and longer with a size of 700 x 30m in the northeast monsoon (Thanh, T.D 1994) 1.2 Sedimentation Accretion in the Deltas Every year, the Vietnam rivers discharge into the sea about 880 billion cub.m water and from 200-250 million ton sediments, and build the Mekong Delta of 39,000km2 in area, Red River Delta of 17,000km2 and many other small deltas In the deltas, the deposition provides the precious resources of land In Mekong River Delta, the strongly coastal deposition has been in Camau Peninsular at the rate of 50 - 80 m/year, maximum 150m/year, and the Peninsular has expanded seaward 130 land/year during the last century Red River Delta has expanded seaward 27 m/year, maximum 120 m/year and 360 land/year have been added in this delta (Thanh, 1995) Sedimentation of Shipping Channels For the water transportation, siltation of shipping channels has become an expansive and heavy disaster (Hoi N.C et Thanh,T.D 1996) Haiphong Port was built from 1876 and over one century, it was the biggest in Vietnam, but recently the big ships have not reached the port due to the heavy sedimentation of its channel During 1920-1992, the depth of shipping channel was reducing from 6m to 4m; meanwhile the volume of dredged sediments was increasing from to million on per year The situation of shipping channels, in different degrees, are also recorded in rather many ports and harbors, for example Hai Thinh Harbor in the North, Cua Lo Harbor in the Centre, Saigon Port and Can Tho Harbor in the South Filling River Mouths and Lagoon The coastal floods, which are very often and serious in the Centre Vietnam are relative closely to the coastal sedimentation in the river mouths or inlets A great number of small rivers mouth and lagoon inlets in the Centre Vietnam have been filled or enclosed periodically or temporarily by the sedimentation This phenomenon has noticed for the cases of Tam Giang lagoon in the Thua Thien Hue Province, Ve and My A river mouths in Quang Ngai Province, and Oloan Lagoon and Ban Thach river mouth in Phu Yen Province The most typical case is enclosure of Tu Hien Inlet in the south of Tam Giang Lagoon Fifty last year, this inlet has been enclosed in the times of 1953-1959, 1979-1990, and 19941999 During last five centuries, The Thuan An inlet in the north of this lagoon has replaced on the distance of 7km with the duration from 100-200 years due to combination of both coastal erosion and sedimentation When the new Thuan An inlet is opened, the old Thuan An inlet is enclosed by sedimentation During the terrible historical flood in November 1999, The Thuan An inlet removed from north to south location However, the new Thuan An inlet was embanked for maintaining the road, and the old Thuan An inlet which has been used as an access to Tan My Harbor in lagoon, has been remain by dredging This response has created the drastic argument among the managers and scientists (Hoi, T.D.et al,2001 Thanh et all, 2001b) Not only the enclosure lagoon inlet or small river mouths, the shallow of these coastal waters that concerns the both erosion in the catchment and enclosure of inlets by is a problem In the coastal area of Red River Delta, the river mouths are often remove their directions to northward or southwards that concerns the sedimentary deposition, especially the development sandy bars in front of them Generally, the large river mouths such as Day, Ba Lat and Van Uc are often remove after a duration from 30-40 years, meanwhile the small ones such as Lach Giang and Tra Ly remove after duration from 5-10 years (Thanh et all, 2001a) Moving Sand Dunes On the coast of approximately 1000km long, from Ha Tinh to Binh Thuan Provinces in Vietnam Centre, there exist of sand dunes of 10-30m high, some places 50-80m high In many sites, they move landwards by the processes of " flowing sand" in rainy season and " flying sand" in dry season, and fill the roads, houses, rice fields with the rates of 2-15m/year From dune barriers, the flowing and flying sand contributes to narrow and shallow the lagoons (Thanh, T.D.,1995) Causes and Tendency 2.1 Coastal Erosion The obtained research results show that the intrinsic causes of coastal erosion belong to three groups as natural coastal evolution, included tectonic movement, climate change an human activities in both catchment and coastal zone (Trinh, L.P et al., 2000 Long, B.H.et al, 2001; Thanh,T.D et all, 2001a; Tien,P.H et al., 2001) With the tendency of long time, the coastal erosion from the cause of natural evolution can be noticed in the Bach Dang and Dong Nai estuaries where the tie is high in range, and the subsiding process has been not compensated by the deposition The tectonic movement plays a basement role for the intrinsic cause of coastal erosion In two largest deltas, Mekong and Red River, the tectonic subsidence is dominated, however, the deltas accrete strongly by the compensative deposition In a certain condition of locally deficient sediments, the total subsidence of both tectonic sink and eustatic rise of sea level become the cause of coastal erosion The lack of sediments in coastal zone and estuaries in the present time mainly concerns the water uses in the catchment, for example damming and irrigation This can be demonstrated by influence of Hoa Binh Dam on the upstream of Red River The construction of this dam was completed in 1989 and every year, the volume of some 40 million sediments accounting for 40 percent of total sediment discharge of Red Rive are trapped in the reservoir bottom However, the direct cause of coastal erosion belongs to the meteoro-hydrology factors such as the actions of wave, current, typhoon, and sea level rise, included monsoon and storm surges Recently, the turbulence of these factors which has created by the climate change concerning the global warming, has caused the unusually coastal erosion The observed data in some stations show the sea level rise in Vietnam coastal zone has been at the rate from 1.0-2.2mm/year (Thuy, N.N et Khuoc, B.D., 1994) Ago, there were very rare typhoons, but recently, they have increased in the South Vietnam and became an important cause of coastal erosion, there Human activities have been very important cause for the coastal erosion Not only damming and irrigation in catchment, the activities in the coastal zone such as land reclamation, building dikes, dredging channels, damaging mangroves and coral reefs, quarrying and mineral exploitation have negatively impacted to the coastal erosion The broken of sea dikes is a particular erosion The sea dikes defend many coastal lowland areas, especially Red River Delta Generally, the sea dikes are built in the accreted coastal areas of deltas, However, some of them have been designed in the weak grounds where the coastal accretion has been not sustainable yet, and in this case, the damage of dikes by the action of wave, storm surges or spring tide has caused by the mistake in the human’s knowledge 2.2 Coastal Sedimentation Similar to the eroded cases, the causes of coastal erosion concern to natural evolution, climate change and human activities Filling inlets and river mouths, shallowing estuaries and lagoon, and accretion of deltas are the natural processes However, they become disasters in many cases when these processes happen suddenly and make the damages for the human being and economy The fact has proved that a normally natural sedimentation can be a disaster if there is a wrong decision in planning or arrangement for settlement or economical base The sedimentation cause of shipping channels to access Hai Thinh Harbor in the North, Cua Lo Harbor in the Centre and Can Tho in the South Vietnam appears to be simple that they are located in the strong depositional areas without any effective resolution for controlling The human activities, direct or indirect, have impacted or contributed to the coastal sedimentation disasters at various degrees The sedimentation of Haiphong Port was provided to be related to the coastal Dinh Vu dam built in 1981, and the development of brackish aquaculture and land reclamation for agriculture in the intertidal zone during forty last years that has reduced enormously depositional space in the Bach Dang Estuary The increase of filling river mouths and lagoon inlets in the Vietnam Centre appear to be related to destroying upstream forests that has made the change in temporal distribution and balance of water flowing to the river mouths and lagoon inlets The lengthened exhaustion of stream flow creates the favorable condition for filling river mouths and lagoon inlets by the longshore sand drift under the wave action In addition, this process is strengthened in the El-Nino time, as a manifestation of climate change, and then the heavy rain in the following La Nina time creates the heavy coastal flood A very obvious event for this interaction was the terribly historic flood in Thua Thien Hue Province, including coastal Tam Giang Lagoon in 1999, after El-Nino from 1997-1998 (Thanh T.D et al, 2001b) Some sedimentation cases have occurred due to coastal deformation and change in coastal dynamic equilibrium under the impact of building the neighboring constructions The Ngoc Hai fishing harbor in Haiphong City is a good example This harbor has sedimentated immediately after building the sea dike No.14 In many situations, the sedimentation disasters are accompanied and caused by sedimentary supplied from neighboring coastal erosion disaster in a united coastal dynamic system The examples for this interaction can be noted as erosion in Cat Hai coast and sedimentation in shipping channel to Haiphong Port, erosion in Hai Hau coast and sedimentation in shipping channel to Hai Thinh Harbor, and erosion in Thuan An coast and sedimentation in shipping channel to Tan My Harbor Consequences 3.1 Coastal Erosion The coastal erosion and sedimentation disasters have made the very heavy consequences, included damages of human beings, properties and land; degradation of coastal environment and ecosystems; unsustainable development with the investment of low effectiveness and small scale; emigration; and unstable thought in life and production of communities Every year, a great number of labors and finance are invested for the prevention and control of coastal erosion However, it is still a threatening to many coastal sites (Thanh T.D, Huy D.V, 2000.) In the Red River Delta, North Vietnam, the density inhabitant areas and the important bases are situated in the coastal lowland surrounded by the sea and river dikes which have been built from last thousand years It is very dangerous when the coastal erosion falling in strong typhoons accompanied storm surges and spring tides, creates to break these dikes In this case, inundation and salt intrusion can expand on very large land In history, the damages in human beings and properties in such broken dikes were very enormous, for example, in 1955 and 1996 Thousands people died in the broken dike on 26 th, September 1955 In the Centre Vietnam, the important settlements and economic areas are situated in the sand coasts very sensitive to the coastal erosion created by the conditions of extreme and unusual weather Some six thousand households belonging to seven provinces have obligated to move by threatening of the coastal erosion (Tiep,N.T et al, 2001) In the Mekong Delta, South Vietnam, the damages of human beings and properties are not as that in Red River Delta, however, the damages of ecosystem land resources created by coastal erosion are very great In term of environment, the erosion reduces the coastal habitats, even destroys mangroves The sediments released from erosion process can make turbid and pollution of organic matters for the waters that impact badly to swimming beaches, coral reefs, and sea grass beds As above mention, the sediments supplied from coastal erosion have contributed significantly to the sedimentation of many shipping channels In Bach Dang Estuary, a calculation showed that every year, the sediment volume of dredging for shipping channel is from 3-5 million tons, meanwhile, the sediment discharge from rivers is some million tons, and the sediment volume is released from coastal erosion is some million tons 3.2 Coastal Sedimentation The economy of water transportation and ports has suffered most severe damages created by the coastal sedimentation disaster The sedimentation of shipping channel to access Haiphong Port is a good example This disaster has impacted very badly to eco-social development of Haiphong City, the third city in Vietnam So far, some responsive resolutions have been proposed, but they are still limited in feasibility Some small harbors, for example Diem Dien and Hai Thinh harbors in the North Vietnam, have been invested tens billion, even hundreds billion Vietnamese dong, but the economical effectiveness is very low due to sedimentation Not only blocking the waterway to sea, filling river mouths and lagoon inlets by sedimentation also strengthen the coastal flood The enclosure of Tu Hien Inlet in Tam Giang Lagoon, Thua Thien Hue Province is a typical example Situating in the region where the rainfall is highest in Vietnam 3,000mm/year on average and the upstream forest has been damaged heavily by wars and landuse for cultivation, the filling process and the close of lagoon inlet have contributed significantly to the strongly coastal floods Falling in the years of enclosed Tu Hien Inlet, the floods are higher in number and intensity, and their consequences are heavier During last 50 years, there have been seven very heavy floods of which six have felt in the enclosed duration of the Tu Hien Inlet The floods have made the serious consequences such as the loss of human life, livestock, crops, damage of infrastructure (houses, roads, gates, bridges, boats etc.), coastal erosion and pollution For example, there ware nearby thousand people died in the flood generated from a typhoon in October 1985 Coastal zone in Vietnam Center usually is flooded and inundated every year The historical flood happened in early of November 1999 with six days long, impacted on seven provinces in Vietnam Center, created very terrible damages including 600 people died, 41100 houses swept, 608718 house submerged 51000 rice field damaged Thua Thien - Hue province was heaviest damaged with 350 people died Not only social economic loss but also environment was polluted and ecosystem was damaged heavily (Thanh,T.D.,et al 2001b) Response From long time, the control of them has been interested However, the implemented resolutions have been passive, responded to concrete situations and lacked definite scientific bases The existing researches have exposed a lot of limitations, especially applied availability (Nga, N.T; et al 1995.) The control of coastal sedimentation and erosion need to be implemented by the synchronous and comprehensive resolutions, from macroscopic to concrete range, direct and indirect, construction and non-construction, and soft and hard constructions, corresponding to each coastal site A comprehensive resolution must be in the framework of integrated coastal management, combining with catchment management, and to have both immediate responses and long-term strategy The priority is given to the construction resolutions combined with other benefits and a special importance is paid to the combination of controlling coastal erosion and sedimentation It needs to be applied modern resolutions, and building the resolutions suitable to Vietnam situation It is also necessary to strengthen legislative basement for coastal protection; to build data base; and to establish a network of observing and monitoring coastal erosion and sedimentation regularly for the purpose of detecting and warning disasters, and having the timely decisions The control of these disasters is a very complicated task, and investment for research needs to be strengthened for responding to the requirements in practice It is necessary to be interested in the following issues on the way to complete a process responding to the erosion and sedimentation disasters - Define a national strategy for the controlling the sediment-related disasters, included coastal erosion and sedimentation in the large scale of integrated coastal management and planning - Attach much importance to non-construction resolutions such monitoring, detecting, warming the disasters, and having a plan to move inhabitants out of the dangerous areas - Priority is given to projects, and constructions combining the control of coastal erosion, sedimentation and flood -Apply the modern resolutions with a special attention to the Vietnamese eco-social factors and concrete natural conditions of the disasters -Attach much importance to ecosystem and environment issues in the responsive resolutions to the sediment-related disasters for example protect mangroves and coral reefs -Strengthen the base of legislation and regulation for the management the sediment related disasters Obligate the tasks on the environment impact assessment for all invested projects in both catchment and coastal zone related to the coastal erosion and sedimentation disasters - Establishing a network on observation and monitoring coastal erosion and sedimentation - Building the data bases for the purpose of management of these disasters Conclusion Erosion and sedimentation disasters are related closely to the sedimentary processes in the Vietnam coastal zone The last years, the coastal erosion in all parts of North, Centre and South Vietnam has happened very complicatedly, heavily damaged human beings, properties and land, and made the bad consequences on economy-society, and environment and ecosystems In principal, the coastal deposition brings about the precious accreted lands for the settlements and the development of important sectors of economy However, in many cases, the sedimentation has become serious disaster for the economic development, especially port-water way Sedimentation at the river mouths and lagoon inlets also have contributed to coastal floods, freshening and pollution The results of existing investigations have provided a integrated picture on the current status of coastal erosion and sedimentation with the aspects on their distribution, scale, intensity and occurred duration in order to have an initial estimation on their causes intrinsic and direct, and tendency The obtained research results are as a basement to propose some responsive resolutions for controlling the mentioned disasters The application of some proposed resolutions has brought about the initial successes The prevention and the control coastal erosion and sedimentation disasters are difficult and long-term tasks, and need the more detail investigations At first, a national strategy for the sediment-related disasters as a basement needs to be built for the activities responding to these disasters For further actions, it is necessary to train experts; invest in survey and analysis equipment; apply modeling; build a process for survey, monitoring and applying appropriate technology to control the disasters Of course, international cooperation is a very important factor to successful implementation the above mentioned tasks References 10 11 12 13 14 15 Hoi, N.C; Thanh, T.D., 1996 Argument for the environment protection in the planning project on the sea port system in Vietnam up to 2010 Technical report reserved at Haiphong Institute of Oceanology (HIO) Hoi, T.D.; Cu, N.N.; Phu, H.N., 2001 Study in resolution for the management of Thuan An and Tu Hien inlets in Tam Giang Lagoon Technical report reserved at Science and Technical Institute of Water Resources, Hanoi Krempf, A 1927 La forme des recifs coralliens et regime des vents alternant Memoir de L'Institut Oceanographique de Nha Trang p.1- 29 Long, B.H., Son, T.P.H., 2001 Research in prediction and control of coastal erosion in South Vietnam Technical report Project KHCN.5c Reserved at Nha Trang Institute of Oceanography Nga, N.T; 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Long, B.H., Hieu, T.T., 2000 Research in characteristics and prediction for tendency of sedimentation and erosion in the coastal zone and estuaries in Vietnam Technical report of project KHCN06.08 Reserved at Nha Trang Institute of Oceanography ... all invested projects in both catchment and coastal zone related to the coastal erosion and sedimentation disasters - Establishing a network on observation and monitoring coastal erosion and sedimentation. .. this interaction can be noted as erosion in Cat Hai coast and sedimentation in shipping channel to Haiphong Port, erosion in Hai Hau coast and sedimentation in shipping channel to Hai Thinh Harbor,... channel to Hai Thinh Harbor, and erosion in Thuan An coast and sedimentation in shipping channel to Tan My Harbor Consequences 3.1 Coastal Erosion The coastal erosion and sedimentation disasters have