The The Holocene – Present Shoreline Migration off Thai Binh – Nam Dinh in Relation to Evolution of Deltaic Lobes and History of the So River

THE AGGRADED - ERODED DEVELOPMENT OF THAIBINH - NAMDINH SHORELINE FROM LATE HOLOCENE TO PRESENT IN RELATION TO EVOLUTION OF DELTAIC LOBES AND HISTORYOF SO RIVER Trần Nghi (1), Trần Thị Thanh Nhàn (1), Trần Ngọc Diễn (2), Đinh Xuân Thành (1), Trần Thị Dung (1), Nguyễn Thị Phương Thảo (1) Trần Xuân Trường3, Đỗ Mạnh Tuân3, Doãn Đình Lâm (4)  (1) VNU University of Science (2) Marine Geology and Mineral Resources Division – General department of Geoloy and minerals of VietNam (3) Hanoi university of natural resources and environment (4) Geology of Institue, Vietnam academy of science and technology   The aggraded-eroded development of Thaibinh - Namdinh shoreline from late Holocene to present have occured in deferent situation concerning the evolution of deltaic bobes and history of So River. The shoreline of Balat River mouth is stronglly aggrading, in contrary, the Haihau shoreline have eroded to create serious damage for the people living in coastal area. In Quaternary Geological maps of 1/200000 and 1/50000 scale, cariried out by Hoang Ngoc Ky (1973-1975) and Vu Nhat Thang (1991-1994). The research on lithofacies analysis and sequence stratigraphy was interested by Tran Nghi and Doan Dinh Lam (1991-2014). Vu Cao Minh et al (2006) described in detail based on data of “the geological description of a place” about the change process of Red River mouth from Halan (Nam Dinh) to Balat (Thai Binh) since XIX century.  The evidents of aggraded- eroded shoreline mentioned above have observed by a lot of authors, however nobody has explained what reason is it? This paper should be contributing to clarify the question of why’s the Haihau shoreline rapidly transformed from the aggradation (before 1960) to erosion (since 1960)? These proccess was related to three main evidences: (1) The evolution of deltaic lobes associated with paleoshorelines; (2) The flood in 1787 moved the Red River channel from Nam Dinh to Thai Binh area; (3) The construction of the dam So River in 1960. The process of moving the paleoshorelines and extending from the mainland to the sea remained the remarcable sign of 8 successive deltaic lobes that combined each other in fan form of paleo - shorelines 2500 y. BP, 1000 years. Bp, 500 years. BP and modern shoreline. Before 1787 main Red River channel flowed into the sea through Hai Hau area and then poured into Halan mouth. During that time the distributary flowing though Thai Binh area and pouring into Balat mouth was too small.  However, in 1787 the great flood narrowed the Red River and becoming the So River, in the same time, the main Red River channel transformed from Nam Dinh to Thai Binh area. From 1787 to 1960 Hai Hau shoreline was continuouslly aggradated with velocity of 30m/y. Since the construction of dam So River in 1960 up to now Hai Hau shoreline have eroded for 19.5m/y. Therefore, the cause of erosion in Hai Hau shore line was due to the construction of dam So River in 1960. The simple and effective solution of interpreting on erosion of Hai Hau shoreline is to break the dam So River for returning the natural law of sedimentary balance.