Deltas are low-lying plains which are formed when river sediments deposit in coastal environments. Deltas are nutrient-rich, and productive ecological and agricultural areas with high socio-economic importance. Globally, deltas are home to about 500 million people and are considerably modified by human activities. In addition, they are vulnerable to climate change and natural hazards like changing river flow and sediment supply, coastal flooding by storminess or sea level rise. To encourage better delta management and planning, it is of utmost importance to understand existing delta sediment dynamics. The objective of this study is to investigate the prevailing sediment dynamics and the sediment budget in the Mekong Delta by using a process-based model. Understanding sediment dynamics for the Mekong Delta requires high resolution analysis and detailed data, which is a challenge for managers and scientists. This study introduces such an approach and focuses on modeling the entire system with a process-based approach, Delft3D-4 and Delft3D Flexible Mesh (DFM). The first model is used to explore sediment dynamics at the coastal zone. The latter model allows straightforward coupling of 1D and 2D grids, making it suitable for analysing the complex river and canal network of the Mekong Delta. This study starts by generating trustworthy bathymetries based on limited data availability. It describes a new interpolation method for reproducing the main meandering channel topographies of the Mekong River. The reproduced topographies are validated against high resolution measured data. The proposed method is capable of reproducing the thalweg accurately. Next, this study describes the development of a Delft3D Mekong Delta model. The model is validated for hydrodynamics and sediment dynamics data for several years and focuses on describing near shore sediment dynamics. The model shows that sediment transport changes in the Mekong Delta are strongly modulated by seasonally varying river discharges and monsoons. The nearshore suspended sediment concentration (SSC) is significantly decreased due to a lack of wave-induced stirring when there is no monsoon. 3D Gravitational circulation effects limit the SSC field from expanding seaward in case of high river flow. In addition, the bed composition has an important role in reproducing sediment fluxes which were considerably decreased when a sandy bed layer is included. This happens due to effects of the initially mostly sandy mixing layer, where resuspension of the mud is proportional to the fraction of mud present. It takes time for an equilibrium bed composition to develop. Seasonally, the sediment volumes deposited in the river mouths increase regularly during the high flow season. During October they remain more or less constant and then, as wave action increases and discharges decrease, the deposited material is resuspended and transported southward along the coast. The DFM model explores the hydrodynamics and sediment dynamics in the fluvial reach of the Mekong River including the anthropogenic effect of dyke construction. After an extremely high flood in 2000 which caused huge damages, a dyke system has been built to protect agriculture in the Vietnamese Mekong Delta (VMD). These structures change hydrodynamic characteristics on floodplains by avoiding floodwaters coming into the floodplains. The DFM model shows that the high dykes slightly change hydrodynamics in the VMD downstream. These structures increase daily mean water levels and tidal amplitudes along the mainstreams. Interestingly, the floodplains protected by high dykes in Long Xuyen Quadrangle and Plain of Reeds influence water regimes not only on the directly linked Mekong branch, but also on other branches. Based on the validated hydrodynamic model, the model is validated against sediment data and used to derive a sediment budget for the Mekong Delta. For the first time, this study has computed sediment dynamics over the entire Mekong Delta, considering riverbed sediment exchange. The model suggests that the Mekong Delta receives ~99 Mt/year sediment from the Mekong River This is much lower than the common estimate of 160 Mt/year. Only about 23% of the modelled total sediment load at Kratie is exported to the sea. The remaining portion is trapped in the rivers and floodplains of the Mekong Delta. Located between Kratie and the entrance of the Mekong Delta, the Tonle Sap Lake receives Mekong River flow at increasing flow rates seasonally and returns flow when Mekong River flow rates decay. As a result Tonle Sap Lake traps approximately 3.9 Mt/year of sediments and explains the hysteresis relationship between water discharges and SSC at downstream stations. The VMD receives an amount of 79.1 Mt/year (~80 % of the total sediment supply at Kratie) through the Song Tien, the Song Hau and overflows. The model results suggest that the Mekong mainstream riverbed erodes in Cambodia and accretes in Vietnam. The results of this study advance understanding of sediment dynamics and sediment budget in the Mekong Delta. The model developed is an efficient tool in order to support delta management and planning. The validated model can be used in future studies to explore impact of climate change and human interference in the Mekong Delta. @en

Bathymetric data plays a major role in obtaining accurate results in hydrodynamic modeling of rivers, estuaries, and coasts. Bathymetries are commonly generated by spatial interpolation methods of data on a model grid. Sparse and limited data will impact the quality of the interpolated bathymetry. This study proposes an efficient spatial interpolation framework for producing a channel bathymetry from sparse, cross-sectional data. The proposed approach consists of three steps: (1) anisotropic bed topography data locations transformed to an orthogonal and smooth grid coordinate system that is aligned with its riverbanks and thalweg; (2) sample data are linearly interpolated to generate river bathymetry; and (3) the generated river bathymetry is converted into its original coordinates. The proposed approach was validated with a high spatial resolution topography of the Tieu estuarine branch. In addition, the proposed approach is compared with other spatial interpolation methods such as ordinary kriging, inverse distance weighting, and kriging with external drift. The proposed approach gives a nearly unbiased topography and a strongly reduced RMSE compared with the other methods. In addition, it accurately reproduces the thalweg. The proposed approach appears to be efficiently applicable for regions with sparse cross-sections. Moreover, river topography generated by the proposed approach is smooth including important morphologic features, making it suitable for two- and three-dimensional hydrodynamic modeling.

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Building high dykes is a common measure of coping with floods and plays an important role in agricultural management in the Vietnamese Mekong Delta. However, the construction of high dykes causes considerable changes in hydrodynamics of the Mekong River. This paper aims to assess the impact of the high-dyke system on water level fluctuations and tidal propagation in the Mekong River branches. We developed a coupled 1-D to 2-D unstructured grid using Delft3D Flexible Mesh software. The model domain covered the Mekong Delta extending to the East (South China Sea) and West (Gulf of Thailand) seas, while the scenarios included the presence of high dykes in the Long Xuyen Quadrangle (LXQ), the Plain of Reeds (PoR) and the Trans-Bassac regions. The model was calibrated for the year 2000 high-flow season. Results show that the inclusion of high dykes changes the percentages of seaward outflow through the different Mekong branches and slightly redistributes flow over the low-flow and high-flow seasons. The LXQ and PoR high dykes result in an increase in the daily mean water levels and a decrease in the tidal amplitudes in their adjacent river branches. Moreover, the different high-dyke systems not only have an influence on the hydrodynamics in their own branch, but also influence other branches due to the Vam Nao connecting channel. These conclusions also hold for the extreme flood scenarios of 1981 and 1991 that had larger peak flows but smaller flood volumes. Peak flood water levels in the Mekong Delta in 1981 and 1991 are comparable to the 2000 flood as peak floods decrease and elongate due to upstream flooding in Cambodia. Future studies will focus on sediment pathways and distribution as well as climate change impact assessment.

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Fluvial sediment is the major source for the formation and development of the Mekong Delta. This paper aims to analyse the dynamics of suspended sediment and to investigate the roles of different processes in order to explore flux pattern changes. We applied modelling on two scales, comprising a large-scale model (the whole delta) to consider the upstream characteristics, particularly the Tonle Sap Lake's flood regulation, and a smaller-scale model (tidal rivers and shelf) to understand the sediment processes on the subaqueous delta. A comprehensive comparison to in-situ measurements and remote sensing data demonstrated that the model is capable of qualitatively simulating sediment dynamics on the subaqueous delta. It estimates that the Mekong River supplied an amount of 41.5 mil tons from April 2014 to April 2015. A substantial amount of sediment delivered by the Mekong River is deposited in front of the river mouths in the high flow season and resuspended in the low flow season. A sensitivity analysis shows that waves, baroclinic effects and bed composition strongly influence suspended sediment distribution and transport on the shelf. Waves in particular play an essential role in sediment resuspension. The development of this model is an important step towards an operational model for scientific and engineering applications, since the model is capable of predicting tidal propagation and discharge distribution through the main branches, and in predicting the seasonal SSC and erosion/deposition patterns on the shelf, while it is forced by readily available inputs: discharge at Kratie (Cambodia), GFS winds, ERA40 reanalysis waves, and TPXO 8v1 HR tidal forcing.

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