Assessment of riverbed evolution in the Vam Nao River under the influence of sand mining using a numerical model

Nguyen Dam Quoc Huy, Tran Thi Thi Kim 

Riverbed sand mining is a major anthropogenic driver of sediment imbalance and riverbank erosion in the Mekong Delta. This study investigates its morphological impacts using the hydrodynamic–morphological model HYDIST, coupled with a sand mining source function (Ssm), to directly simulate unsustainable sand mining. The model is applied to the Vam Nao River, a confluence connecting the Tien and Hau rivers. Unlike conventional approaches that assume static mining pits, sand mining is represented here as a dynamic process by incorporating the source function directly into the bed evolution equation, allowing a more realistic description of mining–morphology interactions.
Simulation results show that sand mining markedly intensifies riverbed erosion and disrupts the natural erosion–deposition balance. In the Vam Nao River reach (Zone 1), the proportion of deposition declines from 27.1% to 12.9%, while erosion increases to 87.1% of total morphological change, indicating a transition toward an erosion-dominated regime. In the Hau River reach (Zone 2), which is naturally depositional with deposition accounting for up to 83.77% under non-mining conditions, sand mining completely reverses the morphological pattern. The total erosion volume becomes 8.4 times greater than that in the non-mining scenario. At the Vam Nao–Hau River confluence (Zone 3), deposition volume decreases by 30.2%, and erosion represents 69.56% of total morphological change, highlighting a strong downstream propagation of mining impacts. The results demonstrate that sand mining substantially enhances morphological instability, particularly in river confluence zones, and that these zones play a critical role in amplifying the effects of upstream sand mining.