These data were collected in the frame of Violaine Piton’s PhD at LEGOS (Toulouse, France), whose subject is to study the dynamics and sediment transport along the estuary-coastal area continuum. The evolution of the Red River delta in terms of siltation, erosion and accretion of its estuaries and coastlines constitute a major concern, as the delta is the most populated region of Vietnam, and is the second-most important rice-producing region of the country. The Red River was ranked as the ninth river in the world in terms of sediment discharge but the construction of two dams across the river in the 1980s caused a reduction of sediment discharge from 145-160 Mt y-1 in the 1970s (Milliman and Meade, 1983) to 40 Mt y-1 nowadays (Le et al., 2007; Vinh et al. 2014; Wei et al., 2019).
In order to explore the seasonal and tidal changes in flow and suspended matter dynamics in the tropical and macrotidal Van Uc river estuary (North Vietnam) and in order to better understand the sediment delivery to the ocean and the processes behind the estuarine siltation, four in-situ measurement campaigns were carried out in 2017.
The campaigns took place during the contrasting high flow and low flow seasons, for each during neap and spring tides. This dataset corresponds to the dry season, spring tide. Water and suspended matter fluxes, salinity, turbidity and particle size distribution (PSD) were measured for 24h at three cross sections along the estuary. These three stations (ST1, ST2 and ST3) are spaced by 10km. ST1 is located near to the river mouth while ST3 is the further upstream. Each station was sampled for 24 hours with cross-river transects measurements every two hours to cover a full diurnal tidal cycle, starting at ST1 and finishing at ST3.
Each series of measurements consisted of four different steps. Firstly, cross river velocity profiles were measured will a hull-mounted, downward-looking, 1200 kHz Acoustic Doppler Current Profiler (ADCP RDI Workhorse in bottom tracking mode). Secondly, vertical profiles of conductivity and temperature were measured for each transect, in the deep channel at the middle of the river, using a CTD Compact-CTD (ASTD687, Alec Electronics Co., Nishinomiya, Japan, now released by JFE Advantech Co., Nishinomiya, Japan, as Rinko-Profiler) at a frequency of 0.1 Hz. Thirdly and parallel to CTD measurements, depth profiles of floc size distribution and concentration were measured using an in-situ laser scattering and transmissometry instrument with a 90% path reduction module (LISST-100X, type B, Sequoia Scientific Inc., Bellevue, WA, USA). Fourthly, in order to study the relationship between turbidity and suspended particulate matter concentration, water samples (~1 L) near subsurface (~1.5 m depth) were collected every four hours with a Niskin bottle. Turbidity of these samples were measured onboard using a Hach 2100Q Turbidimeter.
