Implication of coastal change due to human activities: A short term perspective in tra vinh province, mekong river delta

Kỷ yếu Hội nghị: Nghiên cứu cơ bản trong “Khoa học Trái đất và Môi trường” DOI: 10.15625/vap.2019.000124 234 IMPLICATION OF COASTAL CHANGE DUE TO HUMAN ACTIVITIES: A SHORT TERM PERSPECTIVE IN TRA VINH PROVINCE, MEKONG RIVER DELTA Nguyen Thi Mong Lan, Nguyen Van Lap * , Ta Thi Kim Oanh, Vo Thi Hong Quyen, Nguyen Minh Tai Ho Chi Minh City Institute of Resources Geography, Vietnam Academy of Science and Technology Vietnam, E-mail: nvlap@hcmig.vast.vn ABSTRACT Tra Vinh coast is approximately 65 km long and extends on NE-SW trend between two main river mouths of the Mekong River, Mekong River Delta. Based on satellite image analyses and field surveys, coastline change is identified since 1966 to present. The coastline change is mainly controlled by natural process showing alternation of accretive and erosive coasts from that accresion coast occurred widespread with rate of 5-10 m/yr, conversely, erossion coast occupied locally with rate of 5-8 m/yr during 1966 to 1989. Since 1990 to present joining together with natural factors, human activities have considerably impacted to coastline change. Accretion rate is 17-33 m/yr at the mangrove restoration areas, meanwhile, coastal erosions are severely with rate of 12-24 m/yr in the coastal works (eg. sea dykes, opening of fairway ), particularly erosive rate is up to 36-45 m/yr in Dan Thanh coastline. Tra Vinh coastline changes since 1966 to present display important and irregular changes in decennium comparison period, caused by natural process and human influence that would be helpful for coastal landuse. Keywords: Tra Vinh, coastline, erosion, accretion, mangrove. 1. INTRODUCTION The Mekong River Delta (MRD) in Vietnam is densely populated over 14 million people living average elevation of the delta plain is around 2 m above present sea level. Currently, MRD is facing severe erosion in its channels, river banks and coastal areas. Many coastal provinces have already experienced serious subsidence and saltwater intrusion, with the impacts of prominently rising sea level in the last decade. Total coverage of mangrove forests on coastal area has decreased by 50% between 1965 and 2001, and considerably destroyed after 1995. Consequently, the mangroves have been severely damaged and fragmented and replaced by aquaculture, mainly shrimp farms or coastline stabilization. The exposure of the coastal area to waves and currents not only impacts the conditions for depositional accumulation and progradation of mangrove, but also complicates coastline evolution on a timescale of decades showing the change in coastline as it responds to temporal variations in sediment supply, deposition and erosion, and sea‐ level rise [2, 3]. It has been significantly protected with sea dykes, infrastuctures from storms, and shrimp farm from saltwater intrusion. Considerable development pressures coastal lowlands, particularly coastal erosion has been increased in intensity and widespread properties. The effects of these developments have been varied the coastal area in various ways. The coastline of Tra Vinh is approximately 65 km long and broadly NE-SW oriented between Co Chien and Dinh An river mouths, is formed by unconsolidated sediments of sand dunes, mangrove marshes, tidal flats [1]. They mainly consist of sand, sandy silt and silt that are easly varried under influences of strong wave, tides, and longshore currents. Subaqueous delta plain has a very gentle gradient and fored in part by a semidiunal tide. The river mouth area is mesotidal with irregular semidiunal tides. The mean tidal range is 2.5±0.1 m and maximum tidal range is 3.2 to 3.8 m. Monthly mean significant wave height is 0.8 to 1.2 m in NE monsoon season, and about 0.5 to Kỷ yếu Hội nghị: Nghiên cứu cơ bản trong “Khoa học Trái đất và Môi trường” 235 0.6 m SW monsoon season. In the recent years, beside natural impacts, the increase of human activities have considerably influenced the coastline change in Tra Vinh province. 2. RESULTS AND DISCUSSION Based on the topographic map of 2001, the coastline changes in 2009, 2014 and 2018 were extracted on the satellite images (Landsat-5 TM and Landsat-8) using Mapinfo software. The Google Earth remote sensing images in 2014, 2015 and 2018 are used to assess coastline changes, particularly in the coastal constructions. Observation of coastal topography was carried out in rainy and dry seasons of 2014, 2015 and 2018. Using theodolite (Total Station), each measuring 5 m long crossing the coast, fixed landmarks in each profiles, coordinates determined from GPS, the landmarks were fixed in measurement processess throughout three year observations. The coastline changes has been identified since 1966 to 2018 (Fig. 1) as follows: 2.1. Coastline change from 1966 to 1989 Coastline change was mainly controlled by natural process displaying alternation of coastal accretion and erosion. Coastal accretion occupied widespread with average rate of 5-10 m/year, particularly, upto 40-50 m/year at the southern coast and Dinh An river mouth where magroves were well growing up (Figure 1). Mangrove trees play important role to reduce unfavourable influences of wave and tidal currents and maintain sediments resulting considerably coastal accreation. Erosion coasts occurred locally on the sandy coast that alternated with depositional coasts, and erosive rate was 5-8 m/yr showing natural process. Figure 1. Changes in coastline of Tra Vinh from 1966 to 2018. 2.2. Coastline change from 1989 to 2018 Since 1990 to present joining together with natural factors, human activities have considerably impacted to coastal change. Mangrove restorations were carried out to protect and maintain the coast areas. Mangrove resorations were carried out to protect coastal erosion at My Long Nam coast Cung Hau river mouth in 1992 and to maintain the coastal wetlands at Dong Hai and Long Vinh in 2001 - 2006 (Table 1). Subsequently, mangroves gradually promoted deposition and coastal accumulation with average rates of 18-20 m/yr in My Long Nam and up to 27-30 m/yr Hồ Chí Minh, tháng 11 năm 2019 236 Dong Hai and Long Vinh areas. In the median coasts of Tra Vinh province, stable coasts alternated with accretive and erosive coasts, and displaying the equivalent rates of 5-12 m/yr and 6-10 m/yr respectively. Table 1. The coastal works in Tra Vinh Year Works, projects Locality 1992 800 ha of mangrove restoration My Long Nam - Cung Hau river mouth (RM). 2001 - 2006 Project "Protection and development of the coastal wetlands” My Long Nam - Cung Hau RM; Dong Hai - southern coast; Long Vinh - Dinh An RM. 2009 - 2013 1320 meters long concreted sea dyke Hiep Thanh- NE coast. 2012 - 2014 750 meters long concreted sea dyke Truong Long Hoa - median coast. 2009 - 2015 Building Thermoelectricity plant and opening of fairway connecting Bassac river and East sea Dan Thanh- median coast 2.3. Coastline change in the typical areas From 1989-2009, coastline continued to erode with average rate of 10-20 m/yr in Hiep Thanh area. After building a sea dyke in 2013, assessment of coastline from 2014 and 2015 indicates a serious erosion with rates of 14 and 27 m/yr in the northern and southern adjacent segments of the sea dyke respectively (Figure 2). After building the sea dyke in 2014, Ba Dong coast Truong Long Hoa area, coastal erosion has been immediately inceased up to 30-38 m/yr during 2014 - 2018 in the northern and southern adjacent segment of the sea dyke. From 1989-2009 coastal erosion was 5-11 m/year, and increasing up to 12-24 m/yr during period of 2009 to 2014 in Dan Thanh area. Assessing coastline change in 2014 to 2018 showing erosive rate is reaching over 36-45 m/year (Figure 3). It is clearly that The sea floor has been Kỷ yếu Hội nghị: Nghiên cứu cơ bản trong “Khoa học Trái đất và Môi trường” 237 lowered down owing to exploitation of sand and mud and opening fairway connecting East sea and Bassac river [3]. Figure 3. Coastline change in Dan Thanh area. 4. CONCLUSION Based on satellite images and field surveys, coastline change is identified since 1966 to 2018 in Tra Vinh province. During 1966-1989 coastline change was mainly controlled by natural process showing alternation of accretive and erosive coasts. Acretion coasts mainly occupied with an average rate of 5-10 m/yr, meanwhile erosion coasts are locally with rate of 5-8 m/yr. Since 1990 to present joining together with natural factors, human activities have considerably impacted to coastline change. Coastal accretion is 17-33 m/yr at the mangrove restoration areas, and severe erosion of 12-45 m/yr occupied in the coastal works. It indicates that the coastlines display important and irregular changes in decennium comparison period, including natural process and human influence that would be helpful for landuse in the coastal areas. It is necessary to assess more detailed the coastline change not only in Tra Vinh province but also anothers in the MRD that is the most rapid development in South Vietnam and further research should continue to study. Acknowledgments This study is partly suppored by project number VAST 05/18-19. We also thank for Ho Chi Minh City Institute of Resources Geography has created favorable conditions. 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