Revealing riverbed morphological evolution in river system with complexity: The Vietnam Mekong River case study

Abstract

Riverbed morphology change is the complex evolution of river states and the concern in many river systems due to its relation to riverbank failures. Assessment of the morphological needs to account for natural and human factors in which some of them can be ignored in studies; this limits our understanding of the evolution. The Vietnam Mekong River is such case which received global concern of serious riverbank failures related to sandmining but the understanding of river flow regulation on riverbed morphology has been limited. Therefore, this study aims at disclosing the evolution to complement the studies. The selected case study is Vam Nao confluent area which is tidal-influenced and free from sandmining, and experienced riverbank failures. This study applied numerical models of the Saint-Venant (TELEMAC2D), Navier-Stokes (TELEMAC3D), and Exner (SISYPHE, 2D) equations. We created two (nested) unstructured meshes using inverse distance interpolation: (1) 50 m × 50 m size for TELEMAC2D for hydrologic simulation (timestep of 10 s) to provide boundary conditions for TELEMAC3D and SISYPHE; and (2) 40 m × 40 m for coupled TELEMAC3D and SISYPHE (timestep of 4 s) for riverbed morphology simulation. Duration of simulation was 2009–2018. Data for model assessment were from measurements using an acoustic Doppler current profiler (ADCP) in May 2018 for discharge Q and water elevation H; and in 2017 for riverbed. The applied models performed well with NSE (hourly in 4 days in May 2018) of 0.86 and 0.92 for water elevation (H) and discharge (Q), respectively; and 0.8 (in 2017) for cross and longitudinal sections of riverbed elevation. Morphological simulation reported river flow as the main cause for the riverbed changes and the co-existence of scouring and filling in the region. Study findings indicated that, without the sandmining, river flow can still cause the morphological changes. Results from the morphological change assessment carried out using the nested modeling approach are corroborated by bottom erosion and sedimentation observations at the study area.