Seasonal variation of hydrodynamics and sediment dynamics in a shallow subtropical estuary: the Ba Lat River, Vietnam

The Red River is the largest river in the north of Vietnam, transporting 100 million tons of dominantly silty sediment annually. Hydrographic surveys were conducted in the dry and wet seasons of 2000 in its main branch, to determine sub-tidal and intra-tidal river outflow patterns and sediment transport processes, plus their seasonal variation. It was found that stratification is strongly season-dependent resulting in partly stratified conditions in the dry season when the dominant stratifying and de-stratifying processes are tidal straining, tidal advection, and bed-generated turbulent mixing. The wet season is characterized by stratified conditions when estuarine circulation and advection of stratification by tidal currents and river flow are the main stratifying and destratifying mechanisms. A salt-wedge type density-driven circulation was observed in the highly stratified wet season, but the estuary is too shallow for development of classical estuarine circulation in the partly stratified dry season. In the dry season, tidal currents dominate flow patterns and sediment is mainly resuspended locally. The magnitudes of maximum inflow and outflow velocities in the river channel are equal because river discharge is compensated for by flood tidal asymmetry, but net sediment transport is directed seaward because the outflow period is longer. In the wet season, outflow velocities are considerably higher and the period of outflow is longer, whereas inflow velocities are low. Sediment transport is dominated by advection processes, rather than by local resuspension, and the estuary is flushed during each ebb period.