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PhD Defences 2018


05th October 2018, USTH, Hanoi, Vietnam - Vu Duy Vinh, Researcher IMER

Title: Suspended sediment dynamics in Red River distributaries and along the Red River delta: Focus on estuarine processes and recent balances
Summary:
Estuaries and adjacent coastal area are natural highly dynamic and rapidly changing systems, constrained by a complex combination of riverine and marine processes. Due to their favourable condition for waterway developments like harbors, ports and navigational channels, these regions are attractive places where socio- economics activities grow. However, estuaries and their adjacent coastal areas are sensitive places with influences of human activities as well as land-sea interation processes. Among them, sediment dynamics are attracting increasing attention by coastal researchers. The Red River Delta (RRD) area, located in the western coast of the Gulf of Tonkin, is the second largest delta in Vietnam. RRD region has a very high population density, with about 994 inhabitants per km2 (2016), twice the one of the Mekong river Delta (443 inhabitants per km2). This region encompasses coastal industry zones and ports system, that have constitued the main gate to connect the North Vietnam to the outside world. RRD coastal area is a typical place to study sediment dynamics, with moderate diurnal tidal range (3-4m) as well as a high seasonal variation of fluvial flow. There, deposition in river mouths, siltation on the navigation channels, erosion along some parts of the coastal line, and impacts of modified fluvial fluxes to the coastal zones are occuring. During previous boat trips between Haiphong city and the bay, we observed that the suspended particulate concentration at the surface seemed to increase in the downstream portion of the estuary before decreasing further at sea, which foreshadowed the existence of an Estuarine Turbidity Maximum (ETM) zone in the distributaries of the Red River. The asymmetry of the tide during its propagation in the estuary, studied at three stations, reinforced this hypothesis. In this context, the general aim of this thesis is to improve our knowledge of the sediment processes and sediment fluxes involved, to document and analyze their dynamics, and to improve our capacity to monitor and simulate them, for scientific and management purposes. This study was organized around three main questions: 1) What are the present and recent water and sediment supplies of the Red River basin to the coastal zones? 2) How do the suspended particles transform in the estuary, and is it possible to document the characteristic parameters of Estuarine Turbidity Maxima (ETM) if any in the Cam-Nam Trieu estuary? and 3) Is it possible to estimate the longshore sediment fluxes along the RRD coastal area by numerical simulations?
Committee:

Aldo Sottolichio, EPOC, Univ. Bordeaux
Nguyen Minh Huan, HCMO, VNU-HUS
Dinh Van Manh, Institute of Mechanics, VAST
Marine Herrmann, LEGOS, IRD / USTH, VAST
Bui Hong Long, IO, VAST
Tranh Dinh Lan, IMER, VAST
Alexei Sentchev, LOG, ULCO
Dinh Van Uu, VNU-HUS
Sylvain Ouillon, LEGOS, IRD

18th May 2018, LEGOS, Univ. Toulouse, France - Nguyen Dac Da, LEGOS (IRD ARTS fellowship)

Manuscript available here
Title : The interannual variability of the South Vietnam Upwelling : contributions of atmospheric, oceanic, hydrologic forcing and the ocean intrinsic variability
Summary :
The summer South Vietnam Upwelling (SVU) is a major component of the South China Sea circulation that also influences the ecosystems. The objectives of this thesis are first to quantitatively assess the interannual variability of the SVU in terms of intensity and spatial extent, second to quantify the respective contributions from different factors (atmospheric, river and oceanic forcings; ocean intrinsic variability OIV; El-Nio Southern Oscillation ENSO) to the SVU interannual variability, and third to identify and examine the underlying physical mechanisms. To fulfill these goals we use a set of sensitivity eddy-resolving simulations of the SCS circulation performed with the ROMS_AGRIF ocean regional model at 1/12 resolution for the period 1991-2004. The ability of the model to realistically represent the water masses and dynamics of the circulation in the SCS and SVU regions was first evaluated by comparison with available satellite and in-situ observations. We then defined a group of sea-surface-temperature upwelling indexes to quantify in detail the interannual variability of the SVU in terms of intensity, spatial distribution and duration. Our results reveal that strong SVU years are offshore-dominant with upwelling centers located in the area within 11-12N and 110-112E, whereas weak SVU years are coastal-dominant with upwelling centers located near the coast and over a larger latitude range (10-14N). The first factor that triggers the strength and extent of the SVU is the summer wind curl associated with the summer monsoon. However, its effect is modulated by several factors including first the OIV, whose contribution reaches 50% of the total SVU variability, but also the river discharge and the remote ocean circulation. The coastal upwelling variability is strongly related to the variability of the eastward jet that develops from the coast. The offshore upwelling variability is impacted by the spatio-temporal interactions of the ocean cyclonic eddies with the wind stress curl, which are responsible for the impact of the OIV. The ocean and river forcing also modulate the SVU variability due to their contribution to the eddy field variability. ENSO has a strong influence on the SVU, mainly due to its direct influence on the summer wind. Those results regarding the interannual variability of the SVU are robust to the choice of the surface bias correction method used in the model. We finally present in Appendix-A2 preliminary results about the impacts of tides.
Committee:

John Wilkin, Rutgers University
Vincent Echevin, LOCEAN / IRD
Anne Petrenko, MIO / AMU
Isabelle Dadou, LEGOS, UPS
Rosemary Morrow, LEGOS / UPS
Marine Herrmann, LEGOS / IRD
Nguyen Minh Huan, FHMO, VNU-HUS

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