Persistent flow-division imbalance in a tidal river network under climate stress and engineering intervention: a case study from the Sai Gon–Dong Nai system

This graphical abstract shows how flow-division imbalance emerges in a tidal river network under climate stress and engineering intervention. Drivers such as tides, reduced discharge, sea-level rise, and hydraulic structures alter boundary conditions and enhance tidal backwater effects at bifurcations, leading to asymmetric flow distribution. The main channel becomes dominant (>90% flow) with increased velocity and erosion, while secondary channels weaken (<10% flow) and undergo sedimentation. This shift is reflected by an increase in the Flow Division Index (FDI) from 0.7 to >0.9. The figure highlights that local interventions cannot restore balance, and that effective management requires adaptive, network-scale approaches. Flow division at river junctions strongly influences flood routing, channel stability, and network resilience in tidal systems, yet it is often treated as a fixed parameter in management. This study investigates flow-division behaviour in the strongly tidal, highly branched Sai Gon–Dong Nai river system under combined climate stress and engineering intervention. A calibrated one-dimensional hydrodynamic network model is applied as a diagnostic tool to analyse discharge partitioning, water levels, and flow velocities across 19 zones during 2014–2023. Results demonstrate that flow-division imbalance is persistent and structurally embedded at the network scale. Main channels convey 75–95% of total discharge under baseline conditions, increasing to >90% under climate stress, with associated water-level rises of 0.07–0.16 m and velocity increases of 5–10%. Engineering interventions reduce peak water levels by approximately 20–30% but do not restore balanced connectivity, particularly in secondary branches subject to strong backwater control. Under combined forcing, intervention effectiveness is non-additive, with climate forcing offsetting much of the hydraulic benefit. These findings reveal a decoupling between flood mitigation and flow redistribution, highlighting flow division as a distinct operational objective for managing climate-stressed tidal river networks.