Study area
The Red River debouches into the Gulf of Tonkin by several distributaries, of which the largest is the Ba Lat. The Ba lat mouth is flanked by spits, reflecting the importance of wave action (Fig. 1). The present-day spit system has evolved from former river mouth bars within the past 40 years.
Fig. 1
These spits migrate landward by washover processes, while the lagoon between the spit and mainland is infilling. The spits become attached to the mainland, while more seaward river mouthbars are evolving into new spits. Fossil sand ridges on the delta plain are historic evidence of the repetitive occurrence of spit formation and this cyclic pattern of delta growth.
Fig. 2
The balance of river and marine influences is varying along the RRD coastline, resulting in alterations of rapidly accreting and eroding coastal stretches. Past and future changes of the river regime (irrigation, dam construction, climate change) are therefore expected to have severe consequences for coastline development. The aim of this study is to relate delta morphodynamics to river-, wave-, and tidal processes.
Methods
Field surveys with ADCP and CTD in the coastal zone: Cross-shore transects were navigated during various stages of the tide, and 25-hour anchor stations were done in, and seaward of, the river mouth. Modeling: the influence of various hydrodynamic forcings on the morphology will be evaluated by analyzing an idealized model. It is expected that this will reveal the dominant processes shaping the Ba Lat delta in the past 40 years.
Fig. 3
Fig. 4
Conclusions
Buoyant plumes are stably stratified with distinct fronts (Fig. 2) and haloclines (Fig. 3 and 4), separating fresh river water from the surrounding waters. Settling from suspension is rapid. The river plume is transported southward by Coriolis forcing and wind. Near-bottom transport is southward as well by asymmetric tidal currents. The main depositional area is between 0 and 10 km south of the river mouth, within the 20 m isobath. The spit system is symmetric, which indicates that northward- and southward transport by wave action is equal.
Future work
Setup of model in Delft3D, with highly schematized geometry and hydrodynamics (waves and tides). Schematized deposition from river plume based on field surveys. Run different hydrodynamic scenarios (timescale 40 years) and evaluate morphodynamic response. Compare modeling results, field measurements and sedimentary data to relate the cyclic delta growth to physical processes
Links
Please contact Bas van Maren for more info on this project.