Spatiotemporal dynamics of suspended sediment in coastal Mekong Delta: a hydrodynamic modelling approach under tropical monsoon climate

Suspended sediment concentration (SSC) plays a pivotal role in shaping coastal dynamics, impacting terrestrial and marine ecosystems. This study employed the hydrodynamic model MIKE21 to simulate hydrological runoff and sediment transport within the Mekong River's fluvial-marine continuum, the longest river in Southeast Asia currently challenged with escalating anthropogenic pressures and sea-level rise. By strategically selecting hourly observed data from various locations (river channel, coastal estuary) and periods (dry and rainy seasons) for model calibration and validation, we demonstrated the robust performance of the model simulation of both water levels (RMSE: 0.343 m) and SSC (RMSE: 0.006 kg.m^-3). Spatiotemporal analysis of 2019-2023 revealed the pronounced sensitivity of water level, velocity, and flow direction under tropical monsoon regime. SSC time series decomposition further extracted seasonal amplitudes, while spatial patterns showed distinctly the lowest concentrations occurring in April and the highest in September annually. Furthermore, SSC upward trends were observed during low-flow periods, while downward trends predominated during high-flow periods. Our quantitative analysis offers a comprehensive understanding of hydrological processes within tropical monsoon coastal regions. These findings support the establishment of long-term monitoring frameworks to inform nature-based strategies for sustainable coastal development.