In recent years, in Vietnamese Mekong Delta (VMD) riverbank erosion and collapse have been excessively occurring in many rivers, especially in small rivers, and threatening people living near the riverbank not only their properties but also their future, even their lives. Erosion and collapse are predicted to increase significantly under the influence of tidal range, sea level rise (SLR), and land subsidence. To confront with erosion and riverbank collapse, small rivers should be intensively studied together with large rivers as most recent studies. However, making a research on small rivers in VMD will be very difficult because of the lack of hydraulic data. The main objective of this study is to demonstrate how to conduct a practical flow modelling for a small tidal river in case of only time-series water level at the mouth available. The results of this study are concentrated in three important points. First, a new searching method for convenient interpolation methods was proposed to reproduce the river bathymetry with sparse depth samples. Second, it was found that Riemann boundary condition is very helpful in case of lack of upstream discharge data but need to be modified to be compatible with 2D flow model. Finally, it was demonstrated that flow model of a river can be easily simulated in long period by applying downstream tidal data and Riemann condition. This research will be helpful for other studies with similar field conditions in the future.