DOI: 10.23708/fsqvsz
title: Hydrological and hydrochemical data collected from Van Uc River and estuary (Vietnam), 16-20 June 2022

description: These data were collected in the frame of Juliette Pénicaud’s PhD at LEGOS (Toulouse, France), whose subject is the model-based study of dynamics and hydrosedimentary processes in the riverine estuary and in the plume of the Red River, East Sea of Vietnam. The general objective of the thesis is to develop and deepen the expertise in terms of modelling and observation of the estuary and river plumes in the Haiphong area in order to reproduce their dynamics and to analyse the hydrosedimentary processes on the basis of numerical models, benefiting for their calibration and validation from all archive or future data, in situ or satellite. Supervision of the PhD: Sylvain Ouillon & Marine Herrmann (IRD, LEGOS), with the strong involvement of Vu Duy Vinh (IMER, VAST, Haiphong). The Red River brings to the Gulf of Tonkin considerable quantities of fresh water (3700 m3/s) and suspended matter (45 Mt/year). This region, whose coasts are very densely populated (approx. 30M inhabitants in the Red River delta), is subject to variability factors of different origins (atmospheric, hydrological, oceanic, anthropogenic) and spatio-temporal scales, from extreme events (cyclones) to climate change (sea level rise is particularly sensitive in the delta region). In estuaries, less dense freshwater flows over marine salt water with great variability in space and time. As a continuation of this work, new projects are currently starting on the analysis of river plumes in the Haiphong area (Cam Bach Dang and Van Uc rivers): Plume dispersion analysis based on monitoring of Lagrangian drifters (Alexei Sentchev, LOG) Concomitant measurements of physical parameters (CTD, ADCP) Concomitant measurements of suspended particles (turbidity, MES, grain size distribution) In-situ characterization of the spatial and temporal variability of suspended matter (in-situ measurement of grain size distribution, vertical distribution, long-shore and cross-shore variability) The Van Uc River is a typical distributary of the Red River, with rice crops all along the banks that require water and construction of dikes for flooding protection, and with constant river bed dredging for navigation purposes. The Van Uc estuary is located next to Haiphong city (third largest city of Vietnam). It varies in width from ~350 to ~450 m. The Van Uc River is ranked at the third position of the Red River system in terms of both water (14.5% of the total water discharge from the Red-Thai Binh system to the Gulf of Tonkin) and sediment discharge (14.4% of the total sediment flux of the Red-Thai Binh River to the Red River coastal area. Van Uc river is influenced by a strong seasonal river signal associated with the monsoon. Indeed, the wet summer season (June to September) accounts for 85-95% of the total annual rainfall in the region. In-situ measurement campaigns were carried out to study estuarine processes. Field surveys have been performed in the Van uc estuary and in the close coastal plume at different stages of the wet season (June, August, October) of 2022, always in spring tides. Two sampling strategies have been employed. The first strategy consisted in longitudinal transects of around 15 to 20 km in the estuary performed at various tide levels. Stations were spaced around 2 km and efforts were made to bring closer together the stations where important turbidity was measured and when we discovered rapid change in salinity. The transects were made in around 3h and performed in both senses (from upstream to downstream and opposite). Transect stations extend from 13 km upstream to 9 km downstream the mouth. The second measurement strategy consisted in a fixed station near the mouth of the river of 6h in June, in order to follow the evolution of the same part of the estuary over a specific tide period. During these transects and fixed stations we measured continuously the current velocity Acoustic Doppler Current Profiler (ADCP). At each station, vertical profiles of temperature, salinity, density and turbidity were measured using a profiling CTD Compact. Another CTD, CastAway-CTD® developed by SonTek, was used to perform instant profiles and sharpen the transects strategy, i.e. adapting better the evolution in salinity.