Coastal erosion in red river delta curre

Coastal Erosion in Red River Delta: Current Status and Response Tran Duc Thanh (1), Yosiki Saito (2), Dinh Van Huy (1), Nguyen Huu Cu (1), Do Dinh Chien (1) Haiphong Institute of Oceanology, 246 Danang Street, Haiphong City Tel 31-760602; Fax: 31-761521; E-mail: tdthanh@hio.ac.vn (2) MRE, Geological Survey of Japan, AIST, Tsukuba 305-8567, Japan (1) Introduction Stretching on the length of over 200km, the coastal zone of RRD is concentrated by many important economical bases and the highest population density Recently, coastal erosion disaster has 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 of RRD need to have a sustainable process For this reason, the coastal protection and control of coastal erosion 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 on coastal erosion in RRD Natural factors The rainfall of 1,500mm/year in the whole watershed and of 1,600 – 2,000 mm/year in coastal zone and the mean temperature is 23.50C In NE monsoon during October - April, the average speed of wind is - 4m/s and NE and N wind directions which are parallel to the coast reach a frequency of 70 - 80% from December - January In SW monsoon during May September, the average speed of wind is from to 5m/s with the prevailing wind of SE, S and E directions Every year, the RRE is under the influence of - typhoons happening from June - September, generally with the wind speed reaching 45 - 50m/s Some typhoons occurred in the spring tide combining with typhoon surge have destroyed the coast heavily As a calculation, the surge range reaches 1m every typhoons, 2m every typhoons and maximum 3m (Ninh et all, 1992) When the typhoon surge falling in spring tide, the sea level can rise up - 6m and very strong wave can break out sea dikes and make coastal deformation deeply The diurnal tide is nearly regular with a maximum range of 3.5 - 4m (Thuy, 1984) The tide current is of 20 - 30cm/s in average speed, maximum 60cm/s for ebb tide and 50cm/s for flood tide The coastal circulation is of 25 - 30cm/s and SW ward in dry season, and 15 - 20cm/s and NE ward in rainy season The prevailing wave directions are NE and E in winter and SE and E in summer The mean wave height is of 0.88m, maximum 2m during NE wind season and 5m during typhoons (Huy & Cu 2000) In the NE part of RRE, the sea level rise at rate of 2.24mm/year was measured (Thuy, & Khuoc 1994) With a shoreline length of over 200km, RRE is located on the western coast of the Gulf of Tonkin, which is semi closed in shape, and shallow and gentle sloping The total area of Red River Delta is about 17,000km2 The tidal flats, beaches, beach ridges, mangrove marshes, tributaries and tidal channels are mainly landforms in the coastal area With the seaward boundary reaching up 20m deep, the delta front is a gentle plain, steepness – 0.0015 and its bottom surface sediments are pink - brown fine silt and clay mud The prodelta zone is located from 20 - 30m deep and the pink - brown clay mud covers its bottom surface (Huy, 1994) RRD has developed on the base of Cenozoic depression with Tertiary sediments up to 5,000m in thickness and Quaternary sediments up to 250m in thickness The Holocene sedimentary thickness is generally 30m, maximum 60m (Tiep, 1994) The coastal sediments of RRD consist of different kinds from clay mud to fine sand They belong to marine, marine - marsh and alluvial - marine facies The mangroves play an important role to the deposition of marsh sediments (Thanh et al 1996) Yearly, the Red River discharge about 135 cub Km water, 125 million ton sediments and 70 million ton dissolved matters into coastal zone (Pho, 1984) A great volume of sediments discharged from river are concentrated closely to the coast and deposited on the shallow water that leads to the strong deposition Along shore, the suspended sediments originated from Red River are transported out of RRD in very far distance SW ward, about 24km for the sand drift and 250km for the mud drift The accretion rate in RRD has changed in the coastal parts with time (figure 1) Current status 2.1 General Aspect Every year, the shoreline moves quickly seaward at the mean rate of 25m, maximum 120m However, the deltaic accretion is irregular and first four coastal lengths are eroded with the mean rate of 7m/year maximum 30 m/year Over the last 50 years, the high tidal flats from Do Son to Lach Truong has expanded 386 ha/year, eroded 25 ha/year and balanced an expansion of 361 ha/year (Thanh, et al., 1996; Thanh et al., 1997) The general picture of the coastal accretion and erosion in RRD is shown in the figure Four development tendencies of eroded sites were discovered: 1) lasting coastal erosion for the last 70 years (Cat Hai – Do Son, Hai Hau and Hau Loc); 2) changing from accretion into erosion (Thuy Xuan, Hai Thinh, Da Loc); 3) changing from erosion into accretion (Bang La, Vinh Quang, Giao Thuy); and 4) interchanging between erosion and accretion for the short duration (Diem Dien, Dong Chau, Dong Long) (Dien et al 2003) The intensity of coastal erosion in RRD was divided into four degrees as weak, medium, strong and verystrong The its scale was also divided into four levels as small, medium, large and very large (Table 1), (Thanh et al 2001) By the aspect of erosion, the coastal zone of RRD can be distinguished into three parts such as Cat Hai – Do Son, Do Son – Ba Lat and Ba Lat – Lach Truong 2.2 Erosion aspect of the coastal parts Coastal Part of Cat Hai – Do Son The coastal part of Cat Hai – Do Son has decreased in eroded scale and intensity unremarkably from 1930 to present By two periods from 1930-1965 and 1965-present, eroded length and rate were determined as 46.6 km and 5.1m/year; and 43.9 km and 4.4m/year, respectively (Table 2) Coastal Part of Do Son – Ba Lat The coastal part of Do Son – Ba Lat has decreased in eroded scale and intensity obviously from 1930 to present By three periods from 1930-1965, 1965-1990 and 1990 to present, eroded length and rate were determined as 25.1 km and 6.8m/year; 19.1 km and 5.1m/year; and 1.8 km and 11.1m/year, respectively (Table 3) Coastal part of Ba Lat – Lach Truong The coastal part of Ba Lat – Lach Truong has decreased in erosion scale a little, but increased in erosion intensity during the last 70 years By three periods from 1930-1965, 1965-1990 and 1990 to present, eroded length and rate were determined as 21.2 km and 7.1m/year; 39.9 km and 9.5m/year; and 27.8 km and 11.4m/year, respectively (Table 4) 2.3 Hot Sites of Coastal Erosion Cat Hai (Cat Hai – Do Son part) and Hai Hau ( Ba Lat – Lach Truong part) have been hot site in coastal erosion in RRD Locating at coastal part of Cat Hai – Do Son, the Cat Hai coast has been eroded on the length of 6km, and the eroded rate increased from 4.5m/year up 13m/year during 1930-2000 (Table 5) The Vanly coast of 30km long included Hai Hau site was eroding in the past, and thanks to strengthening embankments the eroded length has reduced into 17.2 km length, recently (Table 6) Causes With the tendency of long time, the coastal erosion from the cause of natural evolution can be noticed in the Cat Hai – Do Son part (Bach Dang estuary) 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 (Thanh et al 1995) In Red River Delta, 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 (Thanh et al 2004) 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 RRD 2-3mm/year 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 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 Consequences 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 In RRD, 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 26th, September 1955 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 some shipping channels, for example the cases of Haiphong Port and Hai Thinh Harbor In Bach Dang Estuary, Cat Hai – Do Son part, 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 Response From long time, the control of coastal erosion in RRD 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 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 Conclusion The last years, the coastal erosion in RRD happened very complicatedly, heavily damaged human beings, properties and land, and made the bad consequences on economy-society, and environment and ecosystems Natural evolution, climate change and human activities have been the important causes for coastal erosion in RRD The coastal erosion tends to be increased in the future Prevention and the control the coastal erosion in RRD are difficult and long-term tasks, and need put in the framework of integrated coastal management linking to catchment management At first, a national strategy for the coastal erosion in RRD as a basement needs to be built for the activities responding to this disaster 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 this disaster Of course, international cooperation is a very important factors to successful implementation the above mentioned tasks References Dien TV, Thanh, TD, Thao NV, 2003 Monitoring coastal erosion in Red River Delta, Vietnam: a contribution from remote sensing data Asian J Geoinform 3(3):73–78 Huy D.V., 1994 Some main features in coastal morphology of modern Red River Delta In: “Marine Resources and Environment” T.II Sci & Tech Pub House Hanoi p.30 - 37 Huy DV., N.H.Cu, 2000 Aspect of coastal dynamics in Red River Delta Technical report reserved at Haiphong Institute of Oceanology Ninh PV, Quynh DN, Manh DV, 1992 Typhoon surges in Vietnam and their regime characteristics Proc Nat Sem on Coast Res Manage., Hanoi, pp 81–87 Pho N.V., 1984 The streams in Vietnam Sci & Tech Pub House Hanoi p.1 - 209 Thanh TD, 1995 Coastal morphological changes concerning the management of coastal zone in Vietnam Workshop Report No.105 Supplement UNESCO\IOC, p.451-462 Thanh, T.D., Huy D.V., Lan T.D., 1996 Development of the tidal flood plain in Red River Delta J Sciences of the Earth N0.3 (18) Hanoi p 50 - 59 Thanh TD, Lan TD and Huy DV, 1997 Natural and human impact on the coastal development of Red River Delta LOIZ Meeting Reports No 29 Netherlands p.224-229 Thanh TD, Cu ND, Cu NH and Chien DD, 2001 Research in prediction and control of coastal erosion in North Vietnam Technical report Project KHCN.5a Reserved at Haiphong Institute of Oceanology Thanh TD, Saito Y, Huy DV, Lap NV, Oanh TTK and Tateishi M, 2004 Regimes of human and climate impacts on coastal changes in Vietnam Regional Environment Change Springer – Verlarg No.4 p 49 -62 Thuy NN 1984 Tide in the Vietnam Sea Sci & Tech Pub House Hanoi p - 263 Thuy NN and Khuoc BD, 1994 El - Nino phenomenon, global climate warming, and sea level in Vietnam Sea and South China Sea J Meteorology and Hydrology No.5 Hanoi p 16 – 23 Tien PH, Thanh TD, Long BH, Cu NV, 2003 Coastal Erosion and Sedimentation in Vietnam Marine resources and environment Sci & Tech Pub House Hanoi Tom X p 20 – 33 Tiep NT, 1993 Coastal morphodynamics of Red River Delta (Holocene - Modern) Doctoral report in Geology - Geography Hanoi University Intensity Weak Medium Strong Very strong Erosion rate (m/year) – 2,5 2,5 – 5 - 10 > 10 Scale Small Medium Large Very large Erosion length (km) < 0,5 0,5 - 2-5 >5 Table Intensity and scale of the coastal erosion in RRD period Number of eroded sites Total length of eroded coast (m) Included: Weak Medium Strong Very strong Mean erosion rate (m/year) Maximum erotional rate (m/year) Area of coastal erosion (ha/year) 1930 - 1965 14 46.600 5.300 15.200 22.100 4.000 5,1 14,1 23,7 1965 - 2000 15 43.920 4.950 22.430 10.140 6.400 4,4 50,0 19,5 Table Current of coastal erosion in Cat Hai – Do Son, RRD Area Do Son Ba Lat Length of erosion Weak Medium Strong Very strong Area of erosion 1930-1965 Km % 25,12 0 0 15,52 61,78 9,60 38,22 17,2 ha/year 1965-1990 Km % 19,07 0 0,67 3,5 4,90 25,7 13,5 70,8 9,87 ha/year Table Current of coastal erosion in Do Son – Ba Lat, RRD 10 1990- present Km % 1,8 0 0 0 1,8 100 2,0 ha/year Area Ba Lat Lach Truong Length of erosion Weak Medium Strong Very strong Area of erosion 1930-1965 Km % 21,26 3,3 15,5 2,5 11,8 3,8 17,9 11,66 54,8 15,1 ha/year 1965-1990 Km % 39,9 0 2,0 5,0 8,2 20,6 29,7 74,4 37,98 ha/year 1990- present Km % 22,75 0 0 0,8 3,5 21,95 96,5 26,08 ha/year Table Current of coastal erosion in Ba Lat – Lach Truong, RRD period Number of eroded sites Total length of eroded coast (m) Included: Weak Medium Strong Very strong Mean erosion rate (m/year) Maximum erotional rate (m/year) Area of coastal erosion (ha/year) 1930 - 1965 6.000 1.800 4.200 4,5 8,5 2,7 1965 - 1990 6.200 3.700 0 2.500 5,0 10,0 3,09 1990 - 2000 6.400 3.060 2.040 1.300 12,9 50,0 8,27 Table Current of coastal erosion in Cat Hai Island period Total length of eroded coast (m) 1930 - 1965 1965 - 1991 1991 - 2000 8.600 19.500 17.200 Included: Weak 1.800 1.600 5.200 Medium 4.600 900 Strong 2.200 4.000 Very strong 13.000 12.000 Mean erosion rate (m/year) 3,40 8,6 14,5 Maximum erotional rate (m/year) 6,40 12,0 20,5 Area of coastal erosion (ha/year) 2,95 16,8 25,0 Table Current of coastal erosion in Hai Hau District 11 12 Figure Coastline change by erosion and accretion in Red River Delta (After Dien et al 2003) 13 ... Monitoring coastal erosion in Red River Delta, Vietnam: a contribution from remote sensing data Asian J Geoinform 3(3):73–78 Huy D.V., 1994 Some main features in coastal morphology of modern Red River. .. causes for coastal erosion in RRD The coastal erosion tends to be increased in the future Prevention and the control the coastal erosion in RRD are difficult and long-term tasks, and need put in the... Loc); 2) changing from accretion into erosion (Thuy Xuan, Hai Thinh, Da Loc); 3) changing from erosion into accretion (Bang La, Vinh Quang, Giao Thuy); and 4) interchanging between erosion and