Acceleration and Spatial Reorganization of Bank Erosion under Prolonged Sediment Starvation: Multi-decadal Evidence from the Vam Nao Channel

Nguyen Dam Quoc Huy, Le Thi Thuy Van, Tran Thi Thi Kim 

The reduction in sediment supply in major deltas worldwide is altering the morphological dynamics of river delta systems. However, the way in which this process leads to the spatial reorganization of erosion has not yet been fully quantified. This study analyzes shoreline variations in the Vam Nao channel, Mekong Delta, over the period 1988–2025 in order to clarify the morphological changes of this key distributary reach under conditions of prolonged sediment deficit.
The results show that sediment load decreased at a rate of approximately −2.62 Mt/year at Tan Chau and −0.99 Mt/year at Vam Nao, reflecting a system-wide sediment deficit. At Vam Nao, erosion area increased markedly during the period 1993–2011, with the emergence of continuously eroding segments and large negative LRR. The concurrent increase in the morphological index asymmetry Index AI (up to 0.72–0.88) and the coefficient of variation CV (up to 0.47) indicates that erosion not only intensified in magnitude but also became asymmetric and shifted toward the right bank of the river. In the period after 2012, although AI index remained at a high level, the decrease in CV suggests that erosion zones had been established and became more spatially stable.
These results indicate that the Vam Nao channel system initially shows that this area has undergone a morphological regime shift, from a relatively balanced state to a highly dynamic state and subsequently to a structurally organized erosional state. This study highlights the importance of analyzing the spatial structure of erosion in identifying early signals of morphological transitions in river systems affected by sediment deficit worldwide.