Japan Geoscience Union Meeting 2018

Presentation information

[EE] Oral

H (Human Geosciences) » H-CG Complex & General

[H-CG25] Deltas and estuaries: multidisciplinary analyses of complex river-mouth systems

Mon. May 21, 2018 9:00 AM - 10:30 AM 103 (1F International Conference Hall, Makuhari Messe)

convener:Yoshiki Saito(Estuary Research Center, Shimane University), Kazuaki Hori(Department of Geography, Graduate School of Environmental Studies, Nagoya University), Guan-Hong Lee(共同), Qing He(State Key Laboratory of Estuarine and Coastal Research, East China Normal University), Chairperson:Saito Yoshiki(GSJ, AIST / Shimane Univ), Lee Guan-Hong(Inha University), Hori Kazuaki(Nagoya Univ)

9:15 AM - 9:30 AM

[HCG25-02] Fluctuating accretion rates of shoreline compartments of the Mekong delta, Vietnam

*Toru Tamura1, Yoshiki Saito2,1, Marcello Gugliotta2, Van Lap Nguyen3, Thi Kim Oanh Ta3 (1.Institute of Geology and Geoinformation Geological Survey of Japan, AIST, 2.Estuary Research Center, Shimane University, 3.Ho Chi Min City Resource Geography Institute, Vietnam Academy of Science and Technology)

Keywords:delta, shoreline, sediment

Sufficient fluvial sediment supply is required for accreting or even just keeping delta shorelines that are affected by marine-induced sediment reworking processes, such as tides and waves. There are concerns about the long-term effect of the decline in the sediment supply on the Mekong delta shoreline evolution especially since the enhanced human activity in the late 20th century. However, the long-term sediment supply possibly fluctuated much and its quantification helps understanding the ongoing processes and mechanism behind. We present the detailed assessment of the accretion rates of multiple shoreline compartments in the Mekong delta over the last 2500 years based on fossilized shorelines and their OSL and radiocarbon chronologies. The Mekong River bifurcates in the upper delta plain into the main channel and Bassac River. In the northeastern part of the lower delta plain, the main channel further bifurcates three times into four distributaries, which then define sandy inter-distributary shoreline compartments. From the mouth of the Bassac River to Camau Peninsula, southwestern end of the delta, the longest and muddy shoreline compartment is defined. These shoreline compartments are affected by the southwestward longshore drift and fed sediments by their updrift distributaries. The northeastern sandy compartments show minor shifts of depocenter, but their total accretion rate is consistent ranging from 0.7 to 1.2 km2/year in 400–2500 year BP. However, the total rate increased to 2 km2/year in the last 400 years. The accretion rate of the southwestern muddy compartment was from 2 km2 in 1400–2400 year BP and increased to 6 and 8 km2/year at 1400 and 600 year BP, respectively. The Camau Peninsula was formed exclusively in the last 600 years. The accelerated accretion of the southwestern shoreline after 1400 year BP reveals the increase in the mud supply while the rise of the northeastern shoreline after 400 year BP implies additional sand supply. These separate trends in sand and mud may be linked to the difference in the shoreline trend in the northeastern and southwestern shorelines over the last 20 years. Especially, the ongoing remarkable erosion in the southwestern shoreline may reflect a rapid decline in mud supply that was overwhelming over the last 1400 years.