Mechanisms of riverbank erosion on the Bassac river: Integrating field observations and stability analysis

Abstract

This study investigates the causes of riverbank erosion along the Hau Bassac River in Hoa Lac commune, An Giang province, Vietnamese Mekong Delta (VMD), based on integrating hydrodynamic, sediment, and morphology characteristics. Field data collected in November 2024, representing receding flood season conditions, indicate that high flow velocities, especially on the outer banks of meanders, frequently approached or exceeded calculated permissible non-eroding velocities. Steep bank slopes (greater than 50 degrees), erodible alluvial materials, and tidal fluctuations further contribute to instability. While regional factors like upstream damming and sand mining exacerbate systemic vulnerability, our findings pinpoint the critical role of local concentrated flow energy and geotechnical weaknesses. The analysis reveals a multi-stage failure mechanism, where extreme velocities during peak floods likely act as the primary trigger for basal scour, while sustained, near-threshold flows during the receding season prevent natural recovery and perpetuate bank instability. This study provides a critical snapshot of erosion mechanisms that are likely intensified during peak flood events, offering essential data for local mitigation and emphasizing the need for continued field-based monitoring in this dynamic river system.