One-Line Modelling of Sediment Transport and Shoreline Evolution on a River-Dominated Coast: The Day Estuary, Vietnam

The coastline surrounding the Day estuary in northern Vietnam, extending about 25 km, has experienced rapid shoreline accretion with average rates of approximately 100 m/year, while localized erosion occurs in several sections. Previous studies suggest that long-term shoreline evolution in this region is controlled by sediment supply from the Red River, reduced nearshore wave energy, and sediment trapping by the Hoa Binh Dam. In this study, a numerical shoreline change model based on the one-line theory was applied to investigate large-scale accretion and erosion under the combined influence of riverine sediment supply and nearshore wave dynamics, while tidal currents and river flow were not explicitly simulated. The nearshore wave regime was simulated using a 2D wave transformation model driven by a 30-year deep-water wave dataset from the Global Ocean Wave Analysis (GOWA). The effect of the Hoa Binh Dam was represented by a 30% reduction in riverine sediment supply. The model was calibrated and validated against measured shoreline positions from 1965, 1989, and 2019, showing good agreement with observations and average relative errors of 9.2% and 12.5% during calibration and validation, respectively. Sediment budget analysis indicates that riverine input is the dominant source of shoreline accretion, while longshore sediment transport (LST) plays a secondary role, and part of the sediment is lost offshore. These results should be interpreted within the scope of the simplified one-line shoreline model, which targets large-scale, long-term shoreline evolution.