Author(s): Menna Ahmed; Sameh Kantoush; Tetsuya Sumi; Doan Binh; Nguyen Mai; Binh Nguyen; Nguyen Doan; Luc Tuan; Nguyen Vuong; Huong Vu; Le Quyen; Ha Thang
Linked Author(s): Binh Nguyen, SAMEH KANTOUSH, Tetsuya Sumi, Nguyen Mai
Keywords: Estuarine circulation Riverbed incision Riverbank erosion Mekong delt
Abstract: Classical estuaries typically exhibit a net near‐bottom landward inflow that transports seawater into the estuary. This seawater is then transformed by estuarine mixing processes into a net near‐surface seaward outflow, creating a bidirectional exchange flow known as estuarine circulation, which serves as a key mechanism in defining estuarine physical, chemical, and biological dynamics (Geyer & MacCready, 2014). While previous studies have examined the circulation dynamics for different distributions in the Vietnamese Mekong Delta (VMD), a comprehensive analysis of the interrelation between estuarine circulation dynamics and morphological alterations in the delta is lacking. Our study aims to assess short-term spatiotemporal variations in estuarine circulations in the Dinh An and Co Chien distributaries during the dry season of 2016, a year marked by one of the worst droughts in the region. A one-dimensional hydrodynamic and advection-dispersion model was employed to analyze estuarine circulation dynamics based on a nondimensional parameter space. This estuarine parameter space was used to classify circulation dynamics based on the freshwater Froude number (F_ (r_f) ) and the mixing number (M) (Geyer &MacCready, 2014). Additionally, this study investigated the relevance between estuarine circulation dynamics and morphological changes in the VMD, utilizing bathymetric datasets from 2017 and 2020 coupled with remote sensing techniques. The bathymetric data were interpolated employing the geographic information system (GIS) (Binh et al., 2021). Our subsequent analysis delved into the annual mean morphological variations in depth (m) and volume (m3) across the 2017-2020 interval. To identify bank erosion in our study area, we began by leveraging historical satellite data sourced from the U. S. Geological Survey (USGS). We applied an atmospheric correction to ensure accurate subsequent analyses (Vanhellemont, 2019). Subsequently, to distinguish between water and non-water classes, we harnessed the capabilities of the Deep Forest (DF) model (Zhou & Feng, 2019), generating binary maps. The water boundaries were then transformed into vector format, i. e., polyline format, facilitating a more detailed analysis of temporal shoreline changes at the interval of 2017-2020. The linear regression rate (LRR) was estimated as the average annual change in the bank migration rate in m. The analysis revealed that the semidiurnal cycle of neap and spring tides significantly impacted the circulation patterns of both the Dinh An and Co Chien distributaries (Fig. 1). Spatially, the circulation at 0 km and 25 km from the river mouth displayed distinct behavior: Co Chien’s circulation pattern was influenced by severe morphological alterations in the riverbed elevations, whereas Dinh An maintained a relatively stable circulation pattern due to a stabilized riverbed along its length (Fig. 2). Moreover, during this dry season, Dinh An was classified as a salt wedge estuary, experiencing a localized riverbank erosion intensified around the wedge areas where the suspended sediment concentration was substantially lower than that in the freshwater zone. In contrast, Co Chien, classified as a strongly stratified estuary, exhibited more widespread riverbank erosion due to the shear stresses caused by stratified water layers across that segment (Fig. 3). This study provides critical insights into the interaction between estuarine circulation and morphological changes in the VMD, providing a deeper understanding of how hydrodynamic processes in estuaries influence riverbed incision and riverbank stability in large tropical river systems.
Year: 2025